fed-std-h28 screw-thread standards for federal...

FED-STD-H28 31 March 1978 Superseding NBS Handbook H28 PART I (1969) PART II (1957) PART III (1957)

FEDERAL STANDARD

SCREW-THREAD STANDARDS FOR FEDERAL SERVICES

This standard was approved by the Commissioner, Federal Supply Service,General Services Administration, for the use of all Federal agencies.

U.S. GOVERNMENT PRINTING OFFICE : 1978 - 261-423/1075

Orders for this publication are to be placed with General ServicesAdministration, acting as an agent for the Superintendent of Documents.Single copies of this standard are available at the GSA Business ServiceCenters in Boston, New York, Philadelphia, Atlanta, Chicago, Kansas City, MO,Fort Worth, Houston, Denver, San Francisco, Los Angeles, and Seattle, WA. orfrom the General Services Administration, Specification and Consumer Information Distribution Branch, Bldg. 197, Washington Navy Yard, Washington,DC 20407. Price 2.50 cents each.

FSC THDS

FED-STD-H28 31 March 1978

INFORMATION SHEET ON FEDERAL STANDARDS

This Federal Standard is issued in loose leaf form to permit the insertionor removal of new or revised pages and sections.

All Users of Federal Standards should keep them up to date by insertingrevised or new pages as issued and removing superseded and cancelled pages.

New and revised pages will be issued under Change Notices which will benumbered consecutively and will bear the date of issuance. Change Noticesshould be retained and filed in front of the Standard until such time as theyare superseded by a reissue of the entire Standard.

NOTICE

From 1939, the Interdepartmental Screw-Thread Committee (ISTC), under theChairmanship of the National Bureau of Standards (NBS), Department ofCommerce had developed and published NBS Handbook H28, Screw-Thread Standardsfor Federal Services.

Section 487 of Title 40 of the U.S. Code states that the authority fordevelopment of Federal Standards for procurement purposes rests with theGeneral Services Administration (GSA).

In November 1976, the ISTC was terminated, and the General Services Administration (GSA) accepted the responsibility for NBS Handbook H28 andagreed to convert it and maintain it as a Federal Standard.

The standards which had been published as NBS Handbook H28, Part 1, PartII and Part III will now be promulgated as a fully coordinated FED-STD-H28,maintaining the existing sections and identifying them with slant lines. Forexample, NBS Handbook H28, Part 1, Section 3 will be detailed standard FED-S-TD-H28/3 which must be procured individually.

Military Custodians Preparing Activity

ARMY - AR DLA-ISNAVY - ASAIR FORCE - 11 (Project No. THDS-0003)

Civil Agency Coordinating Activity

ACO FPI MSFAFS FRA NBSBPA FSS PCDFHW JFK RDSFIS LRC TCS

ii


CONTENTS

Page

1. PURPOSE AND SCOPE...................................... 11.1 PURPOSE............................................... 11.2 SCOPE................................................. 1

2. APPLICATION........................................... 1

3. DETAILED STANDARDS.................................... 1

4. APPENDIX.............................................. 2

5. COMPARABLE INDUSTRY STANDARDS......................... 3

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿³ The text of this section is reprinted from the NBS HANDBOOK H28 with ³³ minor editorial corrections. Pages 44, 48, 55 thru 63, 65 thru 68, 71, ³³ 72, 73, 74. 75 and 80 thru 86 contain corrections indicated by an ³³ asterisk. ³³ ³³ Reorganization of the document from NBS HANDBOOK H28 to FED-STD-H28 ³³ creates an editorial inconvenience, when maintaining continuity of cross ³³ references amongst the pages, paragraphs, tables and figures of the ³³ different sections. For this standard individual sections will be ³³ numbered sequentially starting with (1) one. If the reprinted text ³³ refers to another page, such as Page 6.3, this will be understood to mean ³³ section 6 page 3. All figures and tables will maintain the established ³³ designations, prefixed with the section; e.g. Table 3.1 and Figure 2.5 to ³³ identify their location in this standard. All appendices will be ³³ incorporated in the basic document FED-STD-H28 with other general ³³ information and will continue to be identified with the prefix A. ³ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

iii

FED-STD-H28 31 March 1978 1. PURPOSE AND SCOPE

1.1 Purpose.

This standard establishes complete dimensional data requirements for screwthreads including, as necessary, recommendations on gages, dies and taps andother items associated with the purchase/use of interchangeable threadedparts by Federal Government Agencies. So far as practicable, these data areintended to conform to generally accepted commercial practice, althoughcertain special requirements of the Federal Services necessitate the inclusion of some standards not generally applicable outside the Government. References are cited throughout the text to the standards promulgated by The American Standards Association (ASA) and the United States of America Standards Institute (USASI), now called The American National Standards Institute (ANSI) and to such other published standards as are in agreement with the specifications herein.

1.2 Scope.

This standard includes the basic dimensional requirements for assembly,interchangeability, tensile strength, shear strength, fatigue strength andinstallation and removal torque of threaded products. Specific requirementsfor particular Screw-Thread Standards are covered by the Detail Standards(see Para 3). Supplements to the Detail Standards are covered in the Appendixof this document. (See Para 4).

2. APPLICATION

This standard is for use by all Federal Agencies for procurement of threadedparts.

3. DETAILED STANDARDS. Detailed Standards for Screw-Threadsare identified as:

FED-STD-H28/1 - Nomenclature, Definitions, and Letter Symbols for Screw Threads

FED-STD-H28/2 - Unified Thread Form and Thread Series for Bolts, Screws, Nuts, Tapped Holes and General Applications

FED-STD-H28/3 - Unified Threads of Special Diameter, Pitches, and Lengths of Engagement

FED-STD-H28/4 - Controlled Radius Root Screw Threads, UNJ Symbol

1


FED-STD-H28/5 - Unified Miniature Screw Threads FED-STD-H28/6 - Gages and Gaging for Unified Screw Threads FED-STD-H28/7 - American Standard Pipe Threads (except Dryseal and Hose Coupling Types) FED-STD-H28/8 - Dryseal American Standard Pipe Threads FED-STD-H28/9 - Gas Cylinder Valve Outlet and Inlet Threads FED-STD-H28/10 - American National Hose Coupling and Fire-Hose Coupling Threads FED-STD-H28/11 - Hose Connections For Welding and Cutting Equipment FED-STD-H28/12 - Acme Threads FED-STD-H28/13 - Stub Acme Threads FED-STD-H28/14 - National Buttress Threads FED-STD-H28/15 - American Standard Rolled Threads For Screw Shells of Electric Lamp Holders and For Screw Shells of Unassembled Lamp Bases FED-STD-H28/16 - Microscope Objective and Nosepiece Threads, 0.800 - 36AMO FED-STD-H28/17 - Surveying Instrument Mounting Threads FED-STD-H28/18 - Photographic Equipment Threads FED-STD-H28/19 - Miscellaneous Threads

(a) FED-STD-H28/20 -Inspection Methods For Acceptability of UN, UNR, UNJ,M and MJ Screw-Threads

(b) FED-STD-H28/21 -Metric Screw-Threads

(a) FED-STD-H28/22 -Metric Screw-Thread Gages

4. APPENDIX - The following supplements to the detailedstandards are included in this document.

A1 - American National Form of Thread Series for Bolts, Machine Screws, Nuts, Tapped Holes and General Applications (Inactive for new design)

(a) Not issued as of 31 March 1978(b) Initial release as INTERIM FED-STD-00H28/21 (DLA-IS) dated 31 May 1977

2


A2 - American National Screw Threads of Special Diameters, Pitches, and Lengths of Engagement (Deleted from 1969 issue of Hand book H28)

A3 - Recommended Hole Size Limits Before Threading and Tap Drill Sizes

A4 - Methods of Wire Measurements of Pitch Diameter of 600 Threads

A5 - Design of Special Threads

A6 - References (Deleted from 1969 issue of Handbook H28)

A7 - Supplementary Pipe Threads Information

A8 - Geometry of Taper Screw Threads

A9 - Extent of Usage of the American National Fire-Hose Coupling Threads on Couplings and Nipples used with 211 inch nominal Size Fire-Hose

A10 - Wrench Openings

A11 - Class 5 Interference - Fit Threads

A12 - The Tightening of Threaded Fasteners to Proper Tension

A13 - Three Wire method of measurement of Pitch Diameter of 290 Acme, 29 Stub Acme, and Buttress Threads

A14 - Metric Screw Thread Standards (Superseded by FED-STD-00H28/21 (DLA-IS))

5. COMPARABLE INDUSTRY STANDARDS: but not Identical

FED-STD-H28/1 - ANSI B1.7


FED-STD-H28/3 - ANSI B1.1 (a) FED-STD-H28/4 - ANSIB1.15

FED-STD-H28/5 - ANSIB1.10


3

FED-STD-H2831 March 1978


FED-STD-H28/8 - ANSI B1.20.3/B1.20.5

FED-STD-H28/9 - ANSI 85.7.1/AGA Std V-1

FED-STD-H28/10 - ANSI B2.4/B1.81.1 or NFPA No. 194

FED-STD-H28/11 - CGA

FED-STD-H28/12 - ANSI B1-5


FED-STD-H28/14 - ANSI B1-9

FED-STD-H28/15 - ANSI C8.1


FED-STD-H28/17 -

FED-STD-H28/18 - ANSI PH3-7, 10, 12, 24

FED-STD-H28/19 -

FED-STD-H28/20 - ANSI B1.3 (a) (a) FED-STD-H28/21 - ANSI B1.13 , B1.22

Appendix A4 - ANSI B1.2

Appendix A7 - ANSI B1.3, B1.5, B2.1



Appendix A13 - ANSI B1.5, B1.8, B1.9

(a) Not Issued as of 31 March 1978

4


ANSI, USASI, ASA, NFPA Publications

American National Standards Institute 1430 Broadway New York, NY 10018

National Fire Protection Association 470 Atlantic Avenue Boston, MA 02210

CGA Publications

Compressed Gas Association, Inc. 500 5th Avenue New York, NY 10036

5


APPENDIX A1

American National Form of Thread and Thread Series for Bolts, MachineScrews, Nuts, Tapped Holes and General Applications.

Since the American National threads have been superseded by the Unified threads, most of appendix 1, as shown in the previous (1957) issue of Part 1,has been deleted. Shown herein is data on the class 3 internal threads in theCoarse Thread Series in nominal sizes from 0.25 to one inch as there is still a need for this information. Data shown is from tables 1.2, 1.8, 1.16, and 1.17 of the 1957 issue. (Appendix number and table numbers now preceded by an A.)

6


7


8


9


10


11


APPENDIX A2

AMERICAN NATIONAL SCREW

THREADS OF SPECIAL DIAMETERS, PITCHES, AND LENGTHS OF ENGAGEMENT

(APPENDIX A2 DELETED FROM 1969 ISSUE OF HANDBOOK H28)

12


APPENDIX A3

Tap Drill Sizes for Unified Screw Threads and Recommended Hole Size Limits Before Threading.

1. TAP DRILL SIZES FOR UNIFIED SCREW THREADS

When it is important that the minor diameter of an internal thread conformto specified limits it may be necessary to use a reamer to finish the hole. However, a drill often can be made to cut with a sufficient accuracy for thisrequirement. A variety of factors enter into the production of a clean,round, straight hole of the correct diameter. For a discussion of these andother data on drilling and tapping, reference should be made to "DrilledHoles for Tapping," published by the Drill and Reamer Division and the Tapand Die Division of the Metal Cutting Tool Institute, 405 Lexington Avenue,New York, N.Y. 10017. Table A3.1, gives minor diameter limits and corresponding percentages ofbasic thread height, 0.75H, for all standard series threads up to andincluding 3.75 inch-diameter for classes 1B and 2B. Table A3.2 is a similartable for class 3B. These tables also list sizes of drills that may be expected to drill holes within or near the specified minor diameter limits. The diameter of the drill, the probable hole size, and the corresponding percentages of basic thread height are tabulated. As a drill may normally be expected to cut oversize, probable hole sizesare tabulated that are derived from probable mean oversizes, also tabulated. The following is quoted from the above-mentioned report: "These oversizeswere determined from a series of tests conducted by a number of drillmanufacturers. Using six sizes of drills ranging from 1/16 to 1 in. a totalof 2,808 holes were drilled in cast iron and steel. Commercial high speeddrills were used and the drilling equipment was of the same type andcondition that is normally encountered in metal working shops. The averagedepth of hole drilled was equal to 1.5 times the drill diameter and themeasurement of the hole was made at, the midpoint of the depth drilled.....With good drilling practices and with reasonable care in the resharpening ofdrills the average user may expect to drill oversize in the same manner."

2. RECOMMENDED HOLE SIZE LIMITS BEFORE THREADING Recommended hole size limits before threading and the correspondingtolerances are derived from the minimum and maximum minor diameters of theinternal thread to provide for optimum strength of fastenings and tappingconditions. The following rules as illustrated in figure A3.3 are used. For the range to and including 0.33D, the minimum hole size is equal tothe minimum minor diameter of the internal thread and the maximum hole sizeis larger by half the minor diameter tolerance. For the range from 0.33D to 0.67D, the minimum and maximum hole sizes aneach one quarter of the minor diameter tolerance larger than thecorresponding limits for the length of engagement to and including 0.33D. For the range from 0.67D to 1.5D, the minimum hole size is larger than theminimum minor diameter of the internal thread by half the minor diametertolerance and the maximum hole size is equal to the maximum minor diameter. For the range from 1.5D to 3D, the minimum and maximum hole sizes are eachone quarter of the minor diameter tolerance of the internal thread largerthan the corresponding limits for the 0.67D to 1.5D length of engagement. From the foregoing it will be seen that the difference between limits ineach range is the same and equal to half of the minor diameter tolerance. This is a general rule. However, the minimum differences for sizes below 0.25in are equal to the minor diameter tolerances given in tables 3.9 and 3.10for lengths of engagement to and including 0.33D. For lengths of engagement

greater than 0.33D for sizes 0.25 in and larger the values are adjusted so that the difference between limits is never less than 0.0040 in. 2.1. RECOMMENDED HOLE SiZE LIMITS FOR STANDARD UNIFIED THREADS AND SOMEUNS THREADS ARE GIVEN IN TABLES A3.5 AND A3.6.-For diameter-pitchcombinations other than those given in these tables, the tolerances given intable 2.21 or the tolerance derived from the formula, should be similarlyapplied to determine the hole size limits. Internal threads requiring modified minor diameters for lengths ofengagement less than 0.67D to develop the optimum strength of the fastening,or longer than 1.5D to reduce tapping difficulties, should be designated asspecified in section 2. (See under "Designating threads having modifiedcrests" in that section.) 2.2. FOR UNIFIED Miniature threads, the distribution of hole size limits,differs, from the above, to accord with conditions peculiar to miniaturethreads and is shown in figure A3.4. The maximum limits are based onproviding a functionally adequate fastening for the most common applications,where the material of the externally threaded member is of a strength essentially equal to or greater than that of its mating part. In applicationswhere, because of considerations other than the fastening, the screw is madeof an appreciably weaker material, the use of smaller hole sizes is usuallynecessary to extend thread engagement to a greater depth on the externalthread. However, hole sizes down to the minimum limit of the minor diametersmust be avoided to allow for the spin-up developed as the result of thenegative rake with which these small taps are ground. Recommended hole size limits, for these threads are tabulated in table A3.7.

13


14


23


24


25


26


27


28


29


30


31


32


33


34


APPENDIX A4

Methods of Wire Measurements of Pitch Diameter of 600 deg. Threads.

On a straight thread, the pitch diameter is the diameter of the cylinderwhose surface passes through the thread profiles at such points as to makethe widths of thread groove arid thread ridge equal. Oil a taper thread, the pitch diameter at a given position oil the threadaxis is the diameter of the pitch cone at that position. The degree of accuracy, to which the pitch diameter call be measured willdepend on the accuracy of lead, helix, and form of thread. As, thread pluggages and thread setting plug gages have highly accurate threads, their pitchdiameters, may be measured to a correspondingly high degree of accuracy byapplying the methods described in this appendix. In turn, the virtual diameters (or effective sizes) of thread ring, most snap, and most indicating gagesmay be determined by fitting or comparison with such setting plug gages. Those snap and indicating gages which utilize elements with curved contactshave a pitch (simple effective) diameter determined by comparison to theapplicable setting plug gages. As, most thread of mechanical fastener and components are made to a lesserdegree of accuracy than gage threads, their pitch diameters are not susceptible to accurate determination by direct measuring methods. Therefore, it isnot recommended that such threads be measured by the use of wires. On suchthreads, the pitch diameter is to be regarded as the pitch cylinder or conewhich would bound, on the maximum-material side, the approximately cylindrical or conical surface which would pass through the thread profiles at allpoints such that the widths of the thread and groove are equal. Accordingly,the conformity of such threads with specified pitch diameter limits is determined by gaging means and methods specified in section 6. The accurate measurement of pitch diameter of a thread, which may beperfect as to form and lead, present certain difficulties which result insome uncertainty as to its, true value. The adoption of a standard uniformpractice in making such measurements is, therefore, desirable in order toreduce such uncertainty of measurement to a minimum. The so-called "three- wire method" of measuring pitch diameter of straight thread plug gages, asoutlined herein, has been found to be the most generally satisfactory methodwhen properly carried out, and is recommended for universal use in the directmeasurement of thread plug gages. (See fig. A4.1.)

1. SIZE OF WIRES

In the three-wire method of measuring pitch diameter, small hardened steelcylinders or wires of correct size are placed in the thread groove, two onone side of the screw and one on the opposite side, as shown in figure A4.1.The contact face of the comparator, measuring machine, or micrometer anvil orspindle over the two wires must be sufficiently large in diameter to touchboth wires; that is, the diameter must be greater than the pitch of thethread. It is best to select wires of such size that then, touch the flanksof the thread at the midslope since the measurement of pitch diameter isleast affected by any deviation in thread angle that may be present when suchsize is used. The size of wire that touches exactly at the midslope of aperfect thread of a given pitch is termed the "best-size" wire for thatpitch. Any size, however, may be used that will permit the wires to rest onthe flanks of the thread and also project above the crest of the thread. The depth sit which a wire of given diameter will rest in a thread groovedepends primarily on the pitch and included angle of the thread and, secondarily, on the angle made by the helix at the point of contact of the wire find the thread, with a plane perpendicular to the axis of the screw. Inasmuch as variation in the lead angle has a very small effect in determiningthe diameter of the wire that touches at the midslope of the thread, and asit is desirable to use one size of wire to measure all threads of a given

35


pitch and included angle, the best-size wire is taken as that size which willtouch at the midslope of a groove cut wound a cylinder perpendicular to theaxis of the cylinder, and of the same angle and depth as the thread of thegiven pitch. This is equivalent to a thread of zero lead angle. The size ofwire touching at the midslope, "best-size" wire, is given by the formula:

W = p ÄÄ sec [alpha] 2in which

W = diameter of wire

p = pitch

[alpha] = half included angle of thread.

This formula reduces to--

W = 0.57735p, for 60 deg. threads.

It is frequently desirable, as, for example, when a best-size wire is not available, to measure pitch diameter by means of wire of other than the bestsize. The minimum size that may be used is limited to that permitting thewire to project above the crest of the thread, and the maximum to that permitting the wire to rest on the flanks of the thread just below the crest,and not ride on the crest of the thread. The diameters of the best size,maximum, and minimum wires for all USA Standard 60 deg. threads are given intables A4.2 and A4.3. When using wires of other than the best-size, precautions must be observedin the calculation of pitch diameter. Actual measured values for half-angles and the angles between the axis of the wire and a plane perpendicular to theaxis of the thread must be used for the calculation of pitch diameter whenusing wires other than best-size. The uncertainties of the values used andthe different wire contact conditions will increase the uncertainty of thepitch diameter measurement.

2. METHODS OF MEASURING AND USING WIRES

The computed value for the pitch diameter of a screw thread gage obtainedfrom readings over wire will depend upon the accuracy of the measuringinstrument used, the contact force, and the value of the diameter of thewires used in the computations. In order to measure the pitch diameter of a60 deg. screw-thread gage to an accuracy within 0.0001 in by means of wires,it is necessary to know the wire diameters, to within 0.00002 in. Accordingly, it is necessary to use a measuring instrument that accurately to0.00001 in. Variations in diameter around the wire should be determined by rotating thewire between a measuring contact and an anvil having the form of a 60 deg. V-groove. Variations in diameter along the wire should be determined bymeasuring between a flat contact and a cylindrical anvil. A wire presses on the flanks of a 60 deg. thread with the force that isapplied to the wire by the measuring instrument. Inasmuch as the wire andthread deform at the contact areas, it is desirable to determine the size ofthe wire under conditions which will compensate for this deformation. It inrecommended for standard practice that diameters of wires be measured betweena flat contact and a hardened and accurately ground and lapped steel cylinderhaving a diameter of 0.125 in. for wires used on threads having more then 40up to and including 80 tpi and 0.750 in. for wires used on thread having 40and fewer tpi with the force used in measuring the pitch diameter of thegage. The plane of the flat contact should be parallel to the contactelement of the cylinder within 0.000005 in.

36


To avoid a permanent deformation of the material of the wires or gages, itis necessary to limit the contact force and, for consistent results, a uniform practice as to contact force in making wire measurements of hardened screwthreads gages is necessary. The practice recommended is to use the followingforces:

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Thread per inch ³ Measuring force (+/-10%)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 20 or less - - - - - - -³2.5 poundsAbove 20 thru 40 - - - - - - -³1 poundAbove 40 thru 90 - - - - - - -³8 ouncesAbove 80 thru 140 - - - - - - ³4 ouncesAbove 140 - - - - - - - - - - ³2 ouncesÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The use of other contact forces will cause a difference in the reading overthe wires and to completely compensate for such errors is impractical. The practice of using holding means, such as rubber bands, which has atendency to prevent the wires from adjusting themselves to the proper position in the thread grooves, will result in false measurements. In some cases it has also been the practice to support the gage being measured on two wires, which am in turn supported on a horizontal surface, and measuring fromthis surface to the top of a wire placed over and between the other wires. Ifthe gage is of large diameter, its weight causes an increase in the elasticdeformation at the contact points, and an inaccurate reading is obtained.

Tests on a 1-12 UNF setting plug gage showed a 0.00001 in. error when measured in this manner. This practice should therefore be avoided for gages ofsuch size and larger. Wires from different sets of the same nominal diametershould not be mixed unless calibrated because thread wires in different setmay not have the same diameter. (See par. 3.2.) In order to minimize the deviation of the measured pitch diameter from thetrue pitch diameter (neglecting the effect of lead angle) and reduce thechance of permanently deforming the gages and wires, this revision contains achange in the recommended measuring practice for threads and wires for threads having more than 40 up to and including 80 tpi. The new recommended practice reduces the force for measuring gages and wires from one to 0.5 lb and the size of the cylinder over which the wires are measured from 0.750 to 0.125 in. As a result of this change, the measured pitch diameters of threads in this range will be approximately 0.00005 in. larger than they were under the previous recommended practice. The measured value will be much closer to the true pitch diameter,however. Plug gages manufactured prior to this revision and within tolerance

37


when measured under the previous recommended practice but not within tolerance when measured under the new recommended practice should be considered as within tolerance for a transition period. With the new recommended practice, it can be shown that for all sizes of threads up to 1.500 in. in the fine thread series (UNF) and all sizes up to 2.000 in. in the series (UNC), the measured pitch will not differ from the true pitch diameter (neglecting the effect of lead angle) in excess of 0.000035 in. Slightly larger discrepancies in the 2 to 4 in. size range are relatively unimportant because these size have larger tolerances. The measured diameter of the thread wires for threads having more than 40 up to and including 90 tpi under the new recommended practice differ by less than two micro inches from the measured diameter under the previous recommended practice. Therefore, neither wire diameters nor corrections for computing pitch diameter need be changed.

Measurements of a thread plug gage made in accordance with theseinstructions, with wires that conform to the following specifications, shouldbe accurate too within 0.0001 in.

3. STANDARD SPECIFICATION FOR WIRES AND STANDARD PRACTICE IN MEASUREMENT OF WIRES

The following specifications represent present practice relative to threadmeasuring wires: 3.1. Composition.-The wires shall be accurately finished, hardened steelcylinders of the maximum possible hardness without being brittle. The shallnot be less than that corresponding to a Knoop indentation number of 630. Awire of this hardness can be cut with a file only with difficulty. The surface shall not be rougher than the equivalent of one having a surface roughness rating of 2 microinches arithmetical average. 3.2. DIAMETER OF WIRES.-One set of wires shall consist of three wires thatshall have the same diameter within 0.00001 in., and this common diametershall be within 0.00002 in of that corresponding to the best size for the tpifor which the wire is to be used. Wires shall be measured between a flatcontact and a hardened and accurately finished cylinder having a surfaceroughness rating not in excess of 2 microinches arithmetical average. Themeasuring forces and cylinder diameters shall be as follows:

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ Measuring ³ Cylinder Thread per inch ³ force ³ diameter ³ (+/-10%) ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 20 or less- - - - - -³2.5 pounds ³ 0.750 inch Above 20 thru 40- - - - - -³1 pound ³ 0.750 inch Above 40 thru 80- - - - - -³8 ounces ³ 0.125 inch Above 80 thru 140- - - - - ³4 ounces ³ 0.050 inch Above 140- - - - - - - - - ³2 ounces ³ 0.020 inch ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Using these conditions, the uncertainties of the wire diameter measurementdue to other metrological considerations should be limited and not exceed0.000010 in. An acceptable technique for the measurement of the diameter of each set ofthread measuring wires is to compare them to a reference master wire with asuitable comparison measuring instrument having any anvil shape or measuringforce consistent with good metrological practice. The diameter of each reference master wire, however, must be calibrated by the specified technique with an uncertainty not in excess of 0.000005 in. Wires which are to be used where the contact of the wire is a line contact,

such as in gear wires, should not be used for measuring thread gages. The recommended practice for measuring such wires is between flat parallelcontacts with a one pound force. 3.3. VARIATIONS IN DIAMETER.-Variations in diameter along a wire (taper)over the 1 in. interval at the center of its length shall not exceed 0.000010in as determined by measuring between a flat contact and a cylindrical contact. Variations from true cylindrical contour of a wire (out-of-roundness ornoncircular cross section) over its 1 in. central interval shall not exceed0.000010 in as determined by measuring between a flat measuring contact and awell finished 60 deg.V-groove. Tests for compliance of thread measuring wires with the above specifications are made by the National Bureau of Standards for a stated fee.

4. GENERAL FORMULA FOR MEASUREMENT OF PITCH DIAMETER

The general formula for determining the pitch diameter of any thread whoseflanks are symmetrical with respect to a line drawn through the vertex andperpendicular to the axis of the thread, in which the slight effect of leadangle is taken into account, is

[Epsilon]=MÚw¿+cot[alpha]-w[1+(cosecÀ2Ù[alpha]+cotÀ2Ù[alpha]tanÀ2Ù[lambda]')À1/2Ù], ÄÄÄÄÄ 2n (1)

in which

[Epsilon] = pitch diameter MÚw¿ = measurement over wires [alpha] = half angle of thread n = number of threads per inch - 1/p w = mean diameter of wires[lambda]' = angle between axis of wire and plane perpendicular to axis of thread.

38


This formula is a very close approximation, being based on certainassumptions regarding the positions of the point of contract between the wireand the thread. Formula (1) can be converted to the following simplified form, which isparticularly useful when measuring threads of large lead angle:

[Epsilon] = MÚw¿ + cot [alpha] - w (1 + cosec [alpha]'), (2) ÄÄÄÄÄ 2n in which [alpha]' = the angle whose tangent = tan [alpha] cos [lambda]'.

When formula (1) is used, the usual practice is to expand the square rootterm as a series, retaining only the first and second terms, which gives thefollowing: ÚÄÄ ÄÄÄ¿[Epsilon] = MÚw¿ + cot [alpha] - w³(1 + cosec [alpha] + tanÀ2Ù [lambda]'cos[alpha] cot [alpha] ³ ÄÄÄÄÄ ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ 2n ÀÄÄ 2 ÄÄÄÙ (3)

For large lead angles it is necessary to measure the wire angle, [lambda]', but for lead angles of 5 deg. or less, if the "best-size" wire is used, this angle of the thread at the pitch line, [lambda]. The value of tan [lambda], the tangent of the lead angle, is given by the formula tan [lambda] = L = 1 ÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄ 3.1416E 3.1416NE in which L = lead N = number of turns per inch E = nominal pitch diameter, or an approximation of the measured pitch diameter. 5. MEASUREMENT OF PITCH DIAMETER OF ALL USA STANDARD 60 deg. STRAIGHT THREADS (UNIFIED, HOSE-COUPLING, AND PIPE) For threads of the Unified standard series, the term w tanÀ2Ù [lambda]' cos [alpha] cot [alpha] ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 2 is neglected, as its, value is small, being in all cases less than 0.00015 infor standard fastening screws when the best-size wire is; used, and the aboveformula (3) takes the simplified form [Epsilon] = MÚw¿ + cot [alpha] - w(1 + cosec [alpha]). (4) ÄÄÄÄÄ 2n

This practice is permissible provided that it is uniformly followed, and inorder to maintain uniformity of practice, and thus avoid confusion, theNational Bureau of Standards uses formula (4) for such threads. The Bureaualso uses formula (4) for special 60 deg. threads, except when the value ofthe term w tanÀ2Ù [lambda]' cos [alpha] cot [alpha] ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 2 exceeds 0.00015 in. as in the case of multiple threads, or other threadshaving exceptionally large lead angles. For 60 deg. threads this term exceeds0.00015 when NE [SQRT]n is less than 17.1. For 60 deg. thread of correct angle and thread form formula (4) simplifiesto [Epsilon] = MÚw¿ + 0.86603 - 3w ÄÄÄÄÄÄÄ n (5) For a given act of best-size wires [Epsilon] = MÚw¿ - C when C = w(1 + cosec [alpha]) - cot [alpha] ÄÄÄÄÄ 2n

The quantity C is a constant for a given thread angle, and, when the wiresare, used for measuring threads of the pitch and angle for which they are thebest size, the pitch diameter is obtained by the simple operation of subtracting this constant from the measurement taken over the wires. In fact,when best-size wires am used, this constant is changed very little by amoderate deviation in the angle of the thread. Consequently, the constantsfor the various sets, of wires in use may be tabulated, thus saving a considerable amount of time in the inspection of gages. However, when wires ofother than the best size are used, this constant changes appreciably with atdeviation in the angle of the thread. It has been shown that, with the exception of coarse pitch screws, variation in angle from the basic size causes no appreciable change in the quantity C for the best-size wires. On the other hand, when a wire near the maximum or minimum allowable size is used, a considerable change occurs, and the values of the cotangent and cosecant of the actual measured half angle are to be used. It is, apparent, therefore, that there is a great advantage in using wires very closely approximating the best size. For convenience in carrying out computations, the values of cot a/2n for standard pitches are given in table A4.2. 39


When the value of the term

(w tan À2Ù [lambda]' cos [alpha] cot [alpha]) ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 2

exceeds 0.00015 in., the following pitch diameter formula should be used:

[Epsilon] = MÚw¿ - C + c)

Tabular values for (C+c)Ú1¿, for a 1-in axial pitch screw for 60 deg. threads are given in table A4.4 which values should be divided by the threads per inchfor a given case. (See appendix in Part III, titled "Three-wire method ofmeasurement of pitch diameter of 29 deg. Acme, 29 deg. Stub Acme, andButtress threads," for further details.)


41


6. MEASUREMENT OF PITCH DIAMETER OF USA STANDARD TAPER THREADS

The pitch diameter of the taper thread plug gage is measured in much thesame manner as that of a straight, thread gage, except that a definiteposition at which the measurement is to be made must be located. A point at aknown distance, L, from the reference end of the gage is located by means ofthe gage is located by means of a combination of precision gage blocks, andthe cone point furnished its as an accessory with these, blocks, as shown inthe inset. in figure A4.5. The gage is set vertically on a surface plate, thecone point is placed with its axis horizontal at the desired height, and theplug is turned until the point fits accurately into the thread. The positionof this point is marked carefully with a pencil or a bit of prussian blue. 6.1. TWO-WIRE METHOD.-Assuming that the measurement is to be with a horizontal comparator, the gage is set in the comparator with its axis

perpendicular to each other. The measurement is made with two wires, as shown in figure A4.5, one of which is place in the thread to make contact at same axial section of the thread as was touched by the cone point. This wire is

thread groove next below the fixed wire, and a second measurement is made. The average of these two measurements is Mu omega the measurement over the wires at the position of the fixed wire. The general formula for a taper thread, corresponding to formula (3) is

[Epsilon] = MÚw¿ + cot [alpha] - tanÀ2Ù [beta] tan [alpha] ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 2n ÚÄÄ ÄÄ¿ ³ ³ -w³1 + cosec [alpha] + tanÀ2Ù [lambda]'cos [alpha] cot [alpha] ³ ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÀÄÄ 2 ÄÄÙ (6)

in which

[Epsilon] = pitch diameter MÚw¿ = measurement over wire [beta] = half angle of taper of thread n = number of threads per inch = 1/p [alpha] = half angle of thread w = mean diameter of wires [lambda]'= wire angle

42

FED-STD-H28

The term

cot [alpha] - tanÀ2Ù [beta] tan [alpha]

2n

thread, which is less than that of the same-pitch thread cut on a cylinder. For steep-tapered thread gages, having an included taper larger than 0.75

a small lead angle, formula (6) takes the form

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Otherwise, as for USA Standard taper pipe threads having an included taper of0.75) in/ft, the simplified formula (5) [Epsilon] = MÚw¿ + 0.86603 - 3w ÄÄÄÄÄÄÄ n

for 60 deg. threads may be used. This simplified formula gives a value of[Epsilon] that is 0.0000.5 in larger than that given by the above generalformula (6) for the 2.5-8 USA Standard taper pipe thread, the worst case inthis thread series. The pitch diameter at any other point along the thread, as at the gagingnotch, is obtained by multiplying the distance parallel to the axis of thethread, between this point and the point at which the measurement was taken,by the taper per inch, then adding the product to or subtracting it from themeasured pitch diameter according to the direction in which the second pointis located with respect to the first. 6.2. THREE-WIRE METHOD.-Depending on the measuring facilities available orother circumstances, it is sometimes more convenient to use three wires.In such case measurement is made in the usual manner, but cam must be takenthat the measuring contacts touch all three wires, as the line of measurementis not perpendicular to the axis of the screw when them is proper contact.(See fig. A4.6.) On account of this inclination, the measured distance between the axes ofthe wires must be multiplied by the secant of the half angle of the taper ofthe thread. The formula for the pitch diameter of any taper thread plug gage,the threads of which are symmetrical with respect to a line perpendicular tothe axis, then has the form corresponding to formula (4):

31 March 1978

E = (MÚw¿-w) sec [beta] + cot [alpha] - w cosec [alpha]

2n (8)

in which Beta = half-angle of taper of thread. Thus the pitch diameter of a

in/ft) is then given by the formula

E = 1.000 49(MÚw¿ - w) + 0.866 03p - 2w (9)

time required when the pitch diameter of a number of gages of the same size is to be measured. Only light gages, up to about 1 in nominal

on two wires placed several threads apart, which are, in turn supported * on a taper thread testing fixture. The third wire is placed in the

this wire to the bottom of the fixture with a vertical comparator having a flat anvil, using a gage block combination as the standard. The fixture consists of a block, the upper surface of which is at an angle to the base plane equal to the nominal angle of taper of the thread, 2[beta]. Thus the element of the cone at the top of the thread gage is made parallel to the base of the instrument. The direction of measurement is not perpendicular to the axis of the gage but at an angle, [beta], from perpendicularity. A stop is provided at the thick end of the block with respect to which the gage is positioned on the fixture. As the plane of the end of the gage may not be perpendicular to the axis, a roll approximately equal to the diameter of the gage should be inserted between the stop and the gage to assure contact at the axis of the gage. For a given fixture and roll, a constant is computed which, when subtracted from the measured distance from the top of the upper wire to the base plane, gives M corresponding to the pitch diameter, EÚo¿, at the small end of the gage. EÚo¿ is then determined by applying formula (8) or (9). 6.3. FOUR-WIRE METHOD.-A four-wire method of measurement that yields measurements of the pitch diameter, EÚo¿, at the small end of the gage, and the half-angle of taper, [beta], is also sometimes used. This method is illustrated in figure A4.7 and requires four thread wires of equal diameter, a pair of gage blocks of equal thickness, and two pairs of rolls of different diameters, the rolls of each pair being equal in diameter. Two measurements, MÚ1¿ and MÚ2¿, are made over the rolls and formulas are applied as follows:

90-[beta] = MÚ2¿-MÚ1¿+dÚ1¿-dÚ2¿ (10) cot ÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 2 dÚ2¿-dÚ1¿ ÚÄ Ä¿ MÚw¿ = MÚ2¿-dÚ2¿ ³1 + cot 90 deg.- [beta])³-2g sec [beta] (11) ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÀÄ 2 ÄÙ

in which

MÚ2¿ = measurement over larger rolls MÚ1¿ = measurement over smaller rolls dÚ1¿ = diameter of larger rolls dÚ2¿ = diameter of smaller rolls [beta] = actual half-angle of taper of thread g = thickness of each gage block.

To determine EÚo¿, the pitch diameter at the small end of the gage, MÚw¿, as

determined from formula (11), is substituted in formula (6) or (7). The errors of measurement by this method may be slightly but not

elastic deformations of the rolls and gage blocks under the measuring force, and differing conditions of loading of the thread wires.

The application of direct methods of measurement to determine the pitchdiameter of thread ring gages presents serious difficulties particularly in

The usual practice is to fit the ring gage to a threaded setting plug. Whenthe thread ring gage is of correct lead, angle, and thread form, within close

United States. It is the only method available for small size of threads. For the larger sizes, various more or less satisfactory, methods have been

44

FED-STD-H28

APPENDIX A5

Design of Special Threads.

In general, any given problem in thread design may be susceptible to several more or less satisfactory solutions based on the preliminary selection of

In other words, thread design is to a large extent empirical and is partially based on previous experience with similar designs and the judgment of the

approach to the design of a threaded assembly but merely to present a discussion of various design factors.

external thread, maximum minor diameter of the internal thread, and thestrength of the assembled thread needs to be understood and carefully

not economical to use either a length of thread engagement which is longer thanrequired or shorter than that which will develop the full strength of theexternally threaded member. Other factors, such as control of tap breakage,proper seating of a threaded part on a shoulder, the prevention of crossthreading, conditions of loading when the assembled parts are not concentric,and possible collapse of a hollow externally threaded member, require carefulanalysis and adjustment of the design with respect to selection of the diameter-pitch combination, the class of thread, length of engagement, and major and minor diameter tolerances. In redesigning threads from American National to Unified standards, itshould be remembered that exact correspondence between the old and new classnumbers does not exist. For most, but not all, diameter-pitch combinations, the combined tolerances and allowances of the Unified classes are some what larger than American National classes of corresponding number. Recommended procedure is to convert the thread to the Corresponding class of Unifiedthread, compare the new major, pitch, and minor diameter tolerances with theold tolerances, and then give careful consideration to the desirability ofthe new limits of size. Taking, for example, the conversion of a class 1 thread to classes 1A and1B: Under ordinary conditions where the thread is being used only as a simplefastener and the length of engagement is normal, such substitution may bemade. If, for any reason, the previously specified tolerances may not beexceeded, it may be necessary to specify class 2A or 2B or both. Also, if thethread must carry a high axial strew or if concentricity of the two matingparts is a factor, the conversion should be from claw 1 to classes 2A and 2B. A close fitting thread assembly under some conditions may failure, whereasthe cause of failure may be eliminated by providing a looser fit. A cap screwthat seat only on one side of the bearing surface under the head may breakoff when the is tightened. When a screw has a large bearing surface under the head or when the head must be square with a projecting pin, sufficientpitch diameter clearance must be provided to allow for any out-of-squarenessof the screw axis with the bearing surface under the head. Thus, as large a pitch diameter tolerance as possible, together with providing proper tolerances on squareness of face with the thread axis where seating is required, may avoid the necessity for specifying a heat treated bolt.

2. ECCENTRICITY OF ASSEMBLY AND CROSS THREADING

In assembly, and use, the combined tolerances and allowances. on bothmating parts, should not allow threads to disengage on one side when assemblyis eccentric. The axis of the internal thread can be displaced radially fromcoincidence with the axis of the external thread by an amount equal to the

displacement may be sufficient so that the flank contact is entirely on one side and an the opposite side the crest of the external thread will be in line

screw is constrained in such a position in a tapped hole: (1) There will bedanger of crossing the threads in starting, and (2) the screw may pull out of

amount of overlap is arbitrary and controversial, but the following generalrule can be used in lieu of more specific data:

assembly is concentric, the difference between the minimum major diameter ofthe external thread and the maximum minor diameter of the internal thread

table 2.1). Otherwise stated, the sum of the major-diameter tolerance andallowance, if any, of the external thread and the minor-diameter tolerance of

thread, 0.5Eta, table 2.1. This provides for a minimum of 50 percent thread engagement. As the second step, to secure the minimum safe overlap on one side when the assembly is eccentric, the difference between the maximum pitch

thread should not be greater than twice the addendum of the external

45


thread (0.75H, table 2.1). Other, the stated, the sum the pitch-diametertolerances of both threads and the allowance, if any, should not be greaterthan twice the addendum of the external threads, (0.75H, table 2.1). Thisprovides for an eccentric assembly condition equal to the addendum ofexternal thread (0.375H, table 2.1) and zero minimum overlap on one side. Ifthe results from the limits of size selected violate the above rules, thetolerances should be reduced by using a closer class of tolerance, assumingconsistent with manufacturing possibility, or a coarser pitch should be usedto increase the amount of overlap. The major-diameter tolerance of the external thread or minor-diameter tolerance of the internal thread should not be less than the pitch-diameter tolerance of the respective thread to maintain thread form. It should be noted that, if the tolerance on the minor diameter of theinternal thread must necessary be large, the major diameter of the externalthread must be held close to the maximum major diameter and vice versa.

3. STRENGTH FACTORS

CRITICAL AREAS--The critical area of mating threads, as related to the tensile strength of the thread assembly, are: The effective cross-sectional area, or stress area, of the external thread, (2) the shear area of the external thread that depends principally on the minor diameter of the tapped hole, and (3) the shear area of the internal thread that depends principally on the major diameter of the external thread. The formulas for tensile stress area and thread shear area are shown below. These areas are indicated in figure A5.1. Tensile Stress Area.-The tensile stress area is the assumed area of anexternal threaded part that is used for the purpose of computing the tensilestrength. Direct Tensile Stress.-When parts are subjected only to a direct tensilestress the assumed area applicable to steel parts up to 180,00 psi used incalculating the ultimate strength is computed from the following formula: ÚÄÄ ÄÄ¿ AÚs¿ = 3.1416³(E - 3H ³À2Ù ³ ÄÄ ÄÄ ³ ³ 2 16 ³ ÀÄÄ ÄÄÙor AÚs¿ = 0.7854(D-0.9743/n)À2Ù

where

E = basic pitch diameter D = basic pitch diameter n = threads per inch

for 3H/16, see table 2.1. tabulated stress areas are listed in tables 2.8through 2.18.


Combined Tensile Stress.-When parts are subject to a direct tensile stressplus a torsional stress due to tightening the nut or bolt head, it is necessary to consider the combined shear and tensile stresses when calculating thestrength of the externally threaded part. It is recommended that the combinedbe computed on the basis of the section at the minimum minor diameter of theexternal thread. The direct tensile is given by the formulas:

SÚt¿ = F/A AÚr¿ = 0.7854[(KÚs¿ min)À2Ù-dÀ2]

where

AÚr¿ = area in sq in at the minimum minor diameter. F = axial load on externally threaded parts in lb.

The direct torsional stress is given by the formulas:

SÚt¿ = TÚ1¿/ZÚp¿

ÚÄÄ ÄÄÄ¿

³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ³ KÚ2¿ min ³

where

approximately equal to half of the total wrench torque in lb-in. ZÚp¿ = polar section modules inÀ3Ù

d = inside diameter of externally threaded part in; if part is solid, d = zero.

The combined shear stress in psi is given by the formula: ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

SÚt¿' = ³ ³ (SÚt¿³À2Ù+(SÚ2¿)2 Ä¿³ ³ ÄÄÄÄ ³

ÀÙ ÀÄÄ ÄÄÙ The combined tensile stress in psi the formula:

SÚt¿' = SÚs¿' + SÚt¿/2

for wrench torque and coefficient of friction, other combined stresses willbe directly proportional to the wrench torque.

FED-STD-H28 31 March 1978 Thread Shear Area.-The diameter corresponding to the effective thread sheararea will vary with the relative unit tensile strengths of the materials ofthe internal and external threads. When the external and internal threadsare manufactured from materials of equal unit tensile strength, failure winusually take place simultaneously in both thread at or near a diameter equalto the basic pitch diameter. The shear area (AS) for external and internalthreads made of such materials can be computed from the following formula: AS = 3.1416E LÚs¿ ÄÄÄÄ 2 where E = basic pitch diameter LÚs¿ = length of engagement at basic pitch diameter. When the unit tensile strength of the external thread material greatly exceeds that of the internal thread material, as in the case of a threaded holein a cast aluminum block mated with a 100,000 psi ultimate strength materialbolt, the shear area of the internal thread (ASÚn¿) can be computed from thefollowing formulas: (1) For simplified calculations that will provide shear areas within about 5percent of these given by the precise formula shown below, the shear area ofthe internal thread may be computed as follows: 3LÚs¿ ASÚn¿ = 3.1416E ÄÄÄÄÄ 4 where LÚs¿ = length of engagement at the basic pitch diameter. (2) The precise equation for shear area of the internal thread at a diameter equal to the minimum major diameter of the external thread is as follows: ÚÄÄ ÄÄ¿ ASÚn¿ = 3.1416nLÚs¿DÚs¿ min³ 1 + 0.57735 (DÚs¿ min-EÚn¿ max)³ ³ ÄÄ ³ ³ 2n ³ ÀÄÄ ÄÄÙwhere n = number of threads per inchDÚs¿ min = minimum major diameter of external threadEÚn¿ max = maximum pitch diameter of internal thread LÚs¿ = length of engagement at minimum major diameter of external thread. (Use L. at basic pitch diameter for simplicity this is conservative.) When the unit tensile strength of the internal thread material greatly exceeds that of the external thread material, the shear area of the externalthread (AS) can be computed from the following formulas: (1) For simplified calculations for diameter 0.250 of external of internalin and larger, that will provide shear area within about 5 percent of thosegiven by the precise formula shown below, the shear am of the may be computedas follows: 5LÚs¿ ASÚs¿ = 3.1416E ÄÄÄÄÄ 8 where LÚs¿ = length of engagement at the basic pitch diameter.

diameter equal to the minor diameter of the int=W thread is as follows: ÚÄ Ä¿

³ÄÄÄ ³ ³ 2n ³

where KÚn¿ max = maximum minor diameter of internal thread.

If failure of a thread assembly should occur it is desirable that the

thread will strip. In other words, the length of thread engagement shall besufficient to develop the full strength of the screw. Thus, the length of

diameter, should be such that, taking into account a possible difference in strength of material of the internal and external threads, the threadedportion of the external thread will break before either the external orinternal thread strip. LENGTH OF THREAD ENGAGEMENT-The length of engagement of a thread unit thatwill develop maximum strength of an assembly threaded with external andinternal threads manufactured from materials of near or equal unit tensilestrength may be computed from the following formula, with incorporates thefactor "half" relation of unit shearing strength to unit tensile strength: LÚs¿ = 4AÚs¿/3.1416E where ÚÄÄ ÄÄ¿ AÚs¿ = 3.1416³ E 3H³ À2Ù ³ÄÄ - ÄÄ³ ³ 2 16³ ÀÄÄ ÄÄÙ When the unit tensile strength of the external thread materially exceeds thatof the internal thread, the required length of engagement to develop maximumstrength may be computed from the following formula, which is also based onthe shear area being twice the tensile stress area: 2AÚs¿ LÚs¿ = ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÚÄ Ä¿ 3.1416nDÚs¿ min ³ 1 + 0.57735(DÚs¿ min - EÚn¿ max)³ ³ÄÄÄ ³ ³ 2n ³ ÀÄ ÄÙ 48


Likewise, when the unit tensile strength of the internal thread materiallyexceeds that of the external thread, the following formula should be used:

2AÚs¿ LÚs¿ = ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

3.1416n KÚn¿max³1 + 0.57735(EÚs¿ min - KÚn¿ max)³ ³Ä ³

ÀÄ ÄÙ

The factor 2 used in the numerator of this formula mean that it is assumedthat the area in shear must be twice the tensile stress area to develop the

the National Bureau of Standards in 1929, in which it was found that forhot-rolled and cold-rolled steel, and brass screws and nuts, this factorvaried from 1.7 to 2.0. Taking the factor as 2 provides in general a small

To facilitate the application of this formula various notations, constants,and formulas applicable to the determination of the relation of criticalareas to thread dimensions are given in table A5.2 and are discussed below.

Formula 8, table A5.2, gives the length of engagement required to develop thefull strength of the screw when the strength of the material in which thehole is tapped is the same as, or slightly less than, the strength of the

permanently-fastened connection. If, however, the screw is an adjusting orlead screw, or if the connection will be frequently unscrewed, L, should beincreased to allow for the expected wear on the flanks of the threads during

For tapped holes in sheet metal, the maximum size of the screw to be specified should be such that the thickness of sheet equals the LÚe¿, required to develop full strength. In order to use the largest possible screw, it is

be the practical minimum. If it should prove to be impracticable to reduce theminor diameter tolerance to such a value, it may be necessary to decrease theminimum minor diameter of the internal thread and to increase the minor

diameter of the screw must be reduced by the same amount to prevent interference, and the minor diameter of the "go" thread ring, gage must likewise be decreased, as this is the only control of the minor diameter of the

according to the standard, the threads should be designated as specified in section 2. (See under "Designating threads having modified crests" in that section.) (b) Length of engagement determined by shear area of internal thread.-Theratio of the area in shear in the screw and the area in shear in the tappedhole is given by formula 12, table A5.2. This ratio, R will usually be lessthan 1 and the strength of the material of the tapped hole can be less thanthe strength of the material of the screw by this ratio with no indicatedincrease in LÚe¿, by formula 8. If, however, the ratio

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ultimate tensile strength of screw material

is less that RÚ1¿ then LÚe¿ should be multiplied by RÚ1¿/RÚ2¿ to provide

hole.

For retaining collars on shafts where the expected axial force resisted bythe collar is appreciably less than the tensile force hat the shaft itself iscapable of resisting, L, need only be long enough to withstand the expectedaxial force on the collar. If F is the axial force to be carried by thecollar and uts is the tensile strength of the material of the shaft in poundsper square inch, then the length of thread engagement required on the shaftis equal to 2FÚe¿/(uts x SÚs¿ min), where SÚs¿ min is given by formula 7 when the strength of material of the collar is the same or slightly less than thestrength of material of the shaft. Ratios RÚ1¿ and RÚ2¿ should be computedas previously explained to determine whether or not a greater length is

solid. For this condition, formula becomes

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ SÚs¿ min per inch

where AÚn¿ is the cross-sectional area of the hole. However, as the wall thickness of either or both the internal and external numbers becomes thin, the tendency of the external member to enlarge and the internal member to neckdown in the thread means that an LÚe¿ great than given by the above formula must be used, also that the tolerance on minor diameter of the internal thread and major diameter of the external thread, TÚK¿Ún¿ and TÚD¿Ús¿ must be small to obtain the maximum practicable depth of thread engagement. For components having

made to determine proper selection of wall thicknesses, length of engagement,and pitch of thread.


4. THREAD PROPORTIONS IN RELATION TO TAPPING

In the production of threads it is considered impractical to tap a threadunless its diameter is greater than six times the basic thread height; therefore, when the ratio of D to H is less, than 4.5, the use of a larger diameter, a finer pitch of thread, or both, should be considered. The size ofKappa is a factor in controlling tap breakage. Tap breakage is infrequent ifthe diameter of the tap is over 0.5 in or if the length of thread to betapped is less than 0.5D. For sizes less than 0.5 in and length of threadover 0.5D, tap b can be minimized by use of a large KÚn¿, that is TÚK¿Ún¿maximum. However, this means that LÚs¿ may have to he increased to develop thefull strength of the screw.

5. EXAMPLES OF THREAD DESIGN

The design of special threads for particular purpose is illustrated by thefollowing examples: Example: A gun barrel is subjected to an internal explosive pressure that produces a tensile stress in the threaded end. Thelength of engagement of the threads should be sufficient to produce a minimumarea in shear or the threads of the screw in line with the minor diameter ofthe tapped hole threads equal to twice the maximum stress area of the threaded portion of the barrel. Assume that the thread on the barrel is 1.500-8UN-2A and the minimum internal diameter of the barrel at the threaded end is0.792 in. In table 2.21 will be found the following maximum dimensions ofthe external thread:

DÚs¿ max = 1.4978 in EÚs¿ max = 1.4166 in KÚs¿ max = 1.3444 in.

From table 2.21, KÚn¿ min = 1.365 in. If we select the tolerance for minordiameter of hole TÚK¿Ún¿ = 0.0250 in, KÚn¿ max will equal 1.365 + 0.025 =1.390,which will permit the use of a 1.375 in tap drill. The minimum area in shear per inch can be computed, using formula 7, tableA5.2:

SÚs¿ min = KÚn¿ max (CÚ1¿ - CÚ5¿TÚK¿Ún¿) = 1.390 (2.3,56 - 14.51 X 0.025) = 2.7706 in.À2Ù

The maximum stress area of the external thread, if solid, using formula 5,table A5.2, is AÚs¿ max =0.5 (CÚ1¿KÚn¿ min x LÚe¿ x DÚs¿ max ÄÄÄÄ D LÚe¿ from chart, fig A5.3 = 0.6185, ÄÄÄÄÄ D = 0.5(2.356 X 1.365 X 0.6195 X 1.4978) = 1,4896 Area of minimum center hole = ([pi]/4) X 0.792À2Ù = 0.4926 Max stress area of external threaded member

1.4896-0.4926=0.9970

2 X max AÚs¿

SÚs¿ min

2 X 0.997 = ÄÄÄÄÄÄÄÄÄ

=0.7197 in.

If a length of engagement of 0.72 in cannot be obtained, the tolerance on

for a longer length of engagement is available, TÚK¿Ún¿ can be increased. Example: The dimension is required of the largest steel cap screw that can

steel is 60,000 psi, the tensile strength of the cast iron 20,000 psi, and thethickness of the cast iron is such that the length of thread engagementcannot exceed 1.750 in. The screws on the top side of the bracket will be intension. From the ratio of the tensile strengths of the two materials, RÚ2¿=20,000/60,000=0.333, it is evident that the length of the tapped hole threadmust be considerably longer than the length of thread engagement required todevelop the full strength of the screw. RÚ1¿ will be of the order of 0.85and the length of thread in the tapped hole will be approximately RÚ1¿/RÚ2¿ -0.85/0.333 = 2.55 times as, long as the length required to develop the fullstrength of the screw. LÚe¿ required to develop the full strength of the screwmust be of the order of 1.750/2.55=0.686 in. Inasmuch as the hole is tapped in cast iron, a relatively coarse thread would be required, that is UNC or coarser. For such threads LÚe¿/D, as shown on the chart, figure A5.3, varies between 0.57 and 0.61. Taking LÚe¿/D=0.59, the approximate diameter required is 0.686/0.59 = 1.163. Try D = 11/16 = 1.0625 in. The selected pitch could be either 10 or 8 threads per inch with 8 threads per inch preferred. For a bracket screw, class 2A would be the preferred class.

50


Next, read the dimensions of the screw and hole from table 2.21 to determine whether or not the above selection is correct.

Min major diameter of screw, DÚs¿ min = 1.0435 Min minor diameter of tapped hole, KÚn¿ min = 0.927

develop on the bracket that will produce tension in the screws. It should bepossible to tighten these screws to the yield strength of the steel without

The complete table of dimensions of the tapped hole and screw is (Fromtable 2.21)

Min major diameter = 1.0623 Min pitch diameter = 0.9813

Min minor diameter = 0.927 Max minor diameter = 0.952

Max major diameter = 1.0605 Min major diameter = 1.0455 Max pitch diameter = 0.9793 Min pitch diameter = 0.9725 Max minor diameter = 0.9071

LÚe¿/D from chart, figure A5.3 = 0.5990

LÚe¿ min = LÚe¿/D X DÚ2¿ max = 0.5990 x 1.0605 = 0.6352

TÚE¿Ún¿(table 2.21) = 0.0089

RÚ1¿, table A5.2, formula 12 0.75 KÚn¿ min = ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ DÚs¿ min [0.875-CÚ4¿ (TÚE¿Ún¿ + TÚD¿Ús¿ + G)]

= 0.75 X 0.927 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1.0455[0.875-4.619(0.0089+0.0150+0.0020)]

= 0.8803

LÚe¿ required in hole

=LÚe¿ min X RÚ1¿ = 0.6352 X 0.8803 / 0.3333 = 1.6777 in, ÄÄÄÄ RÚ2¿

which is less than the LÚe¿ (1.750 in) permitted.

31 March 1978

REFERENCES

(APPENDIX A6 DELETED FROM 1969 ISSUE OF HANDBOOK H28)


Supplementary Pipe Thread Information.

The information contained herein supplements sections VII and VIII.

1. DEFINITIONS.- Terms relating only to taper pipe threads are defined asfollows:

through the thread profiles at such points as to make the width of the grooveequal to one half of the basic pitch. On a perfect thread this occurs at the

2. Major cone -The major cone is a cone having an apex angle equal to that of the pitch cone, the surface of which would bound the crest of An external

3. Sharp major cone.-The sharp major cone is a cone having an apex angleequal to that of the pitch cone, the surface of which would pan through the

4. Minor cone.-The minor cone is a cone having an apex angle equal to that of the pitch cone, the surface of which would bound the root of an external

5. Sharp minor cone.-The sharp minor cone is a cone having an apex angleequal to that of the pitch cone, the surface of which would pass through the

points on external and internal taper threaded members or gages, when assembled with a specified torque or under other specified condition. 7. Bottom of chamfer.-On a chamfered in thread the bottom of the chamfer is

thread.

2. LETTER SYMBOLS.-Standard letter symbols used to designate the dimensions

symbols are shown in figure A7.1.

2. SUGGESTED TWIST DRILL DIAMETERS FOR DRILLED HOLE SIZES FOR PIPE THREADS

The drill diameters given in table A7.2 for the drilled hole for taper andstraight internal pipe threads are the diameters of the standard and stock

VII.2, column 22. They represent the diameters of the holes which would be cut with a twist

metal. This is approximately the condition that exists when a correctlysharpened twist drill is cutting a hole in a homogeneous block of cast iron.

to produce a hole of the required diameter. When nonferrous metals and other similar materials to be drilled and tapped,it may be found necessary to use a drill of slightly larger or smaller diameter

acceptable pipe thread with the required thread height. It should be understood that this table of twist drill diameters is intended to help only the occasional user of drills in the application of this standard.

type of material and with specially designed machinery be found to be more

advantageous to use a drill size not given in the table, even one having a nonstandard diameter.

54


3. SUGGESTED TWIST DRILL DIAMETERS FOR DRILLED HOLE SIZES FOR DRYSEAL PIPE

The drill diameters given in table A7.3 are for taper and straight internalpipe thread and will usually permit the tapping of acceptable threads in

When hard metals or other similar materials are to be drilled and trapped, itmay be necessary to use a drill of slightly smaller diameter whereas soft

Taper pipe threads of improved quality are obtained when the holes are.taper reamed after drilling and before tapping. Standard taper pipe reamers

material and is best determined by trial.

4. THREADING OF PIPE FOR AMERICAN STANDARD THREADED STEEL FLANGES

The length of the effective external taper thread of American Standard pipethread provides a sufficient number of threads on the pipe to insure a satisfactory joint with the ordinary weight of fitting or flange.

56

FED-STD-H28 March 31 1978

The American Standard Steel Flanges for high pressure temperature service (ASAB16.5) calls for thread lengths in the flanges in proportion to the thickness of the flange. This means that the thread lengths in the flanges intendedfor higher pressures in a given size are longer than the thread lengths inthe flanges intended for the lower pressures. Table A7.4 provides for a length of effective thread oil pipe for sizes and weights of flanges where the regular American Standard length of effectivethread is too short to bring the end of the pipe reasonably close to the faceof the Range when both parts are assembled by power. As the thread in allflange as well as on the pipe are gaged with a tolerance of one thread largeand one thread small there will naturally be some difference in distancebetween the end of the pipe and face of the flange in the various assembliesfor the different sizes and weights of flanges. In table A7.4 the additional number of threads are added to the small end ofthe standard pipe thread. The pitch diameter at the end of the externalthread is, therefore, smaller than that of the regular standard pipe. Inother words, the small end of the ring gage will pass over the end of thepipe the number of turns or the length in inches equal to the values given intable A7.4.

5. INTERNAL STRAIGHT PIPE THREADS IN FINISHED DRUMS AND EXTERNAL THREADS ONPLUGS

The screw threads which have been used for some years to hold the bung plugsin steel barrels or drums are another application of straight pipe threads.À12Ù The flanges of the bung and vent are tapped respectively with 2 in. and 3/4in. American Standard form straight pipe thread having dimensions inaccordance with Table A7.5.

NOTE (12)]

Large tolerances in addition to the allowance have been provided to ensureeasy seating of the plug in the flange when making up the joint with a propergasket.

6. TAPER AND STRAIGHT THREADS FOR RIGID STEEL ELECTRICAL CONDUIT AND FITTINGS

1. GENERAL--Tables A7.6 and A7.7 give the principal thread data used in theproduction of rigid steel electrical conduit and fittings. These data weretaken from the publications of the conduit manufacturers, the UnderwritersLaboratories, and the National Electrical Manufacturers Association. Theyare also published in American Standards ASA C80.1-1959 Rigid Steel Conduit,Zinc coated and C80.2-1959 Rigid Steel Conduit, Enameled. In certain placesslight adjustments have been made to bring the dimensions in line with thelong established pipe thread practice. In every case these adjustments havebeen discussed with the interested group. The sole purpose of the printing ofthese data is to show their relation to the original standard and to makethem generally available. 2. TAPER THREADS FOR CONDUIT-The taper threads on rigid steel conduit shownin table A7.6 are generally made in accordance with table VII.2 Table A7.6records the dimensions commonly referred to for the conduit thread. Whenscrew threads are cut by hand on rigid-steel conduit at the job, regular pipefitter's stocks and dies (pipe threading tools) are used. 3. STRAIGHT EXTERNAL RUNNING THREADS.-The straight external running threadsfor conduit as used for fixture stems and conduit fittings are made in accordance with the dimensions given in tableA7.7 columns 3 and 4. 4. STRAIGHT INTERNAL THREADS.-The straight internal threads used in conduitfittings, are shown in table A7.7, these threads are made with the AmericanStandard pipe thread form.

7. PITCH DIAMETERS OF TAPER PIPE THREADS SHOWN IN THEIR RELATION TO EÚ1¿

Pitch diameters of taper pipe threads are shown in their relation to EÚ1¿,basic pitch diameter, in table A7.8.

8. SPECIAL SHORT, PTF-SPL SHORT; SPECIAL EXTRA SHORT, PTF-SPL EXTRA SHORT;FINE THREAD, F-PTF: AND SPECIAL DIAMETER-PITCH COMBINATION, SPL-PTF, DRYSEALPIPE THREADS 1. GENERAL. -Included in this portion of the appendix are data on the following threads:DRYSEAL SPECIAL SHORT TAPER PIPE THREAD PTF-SPL SHORT. (Par. 2)DRYSEAL SPECIAL EXTRA SHORT TAPER PIPE THREAD PTP-SPL EXTRA SHORT (Par. 3)DRYSEAL FINE THREAD SERIES, F-PTF (Par. 6)DRYSEAL SPECIAL DIAMETER-PITCH COMBINATION SERIES, 27 threads per inch, SPL-PTF (Par. 7). The SAE Dryseal pipe thread series are based on thread length. Full threadlengths and clearance for Dryseal standard and SAE SHORT series are shown intables VIII.4, VIII.5, and VIII.6. These full thread lengths and clearancesshould be used in design applications wherever possible. Design limitations, economy of material, permanent installation, or otherlimiting conditions may not permit the use of either of the full threadlengths and shoulder lengths in the proceeding tables for the above threadseries. To meet these conditions two special thread series have been established as shown in figure A7.2. The deviations from standard practice aredescribed below. 2. DRYSEAL SPECIAL SHORT TAPER PIPE THREAD, PTF-SPL SHORT.-Thread of this,series conform in all respects to the PTF-SAE SHORT threads except that thefull thread length has been further shortened by eliminating one thread atthe large end of external threads or eliminating one thread at the small endof internal threads.

57


Gaging is the same as for PTF-SAE SHORT except the LÚs¿ ring thread gage forexternal thread length and taper or the LÚs¿ plug thread gage for internalthread length and taper cannot be used. The tolerance must be altered asdescribed in paragraph 4 below on Limitations of Assembly. For interchangeability, see table A7.9. This thread shall be designated as follows:

1/2-27 DRYSEAL PTF-SPL SHORT.

3. DRYSEAL SPECIAL EXTRA SHORT TAPER PIPE THREAD, PTF-SPL EXTRA SHORT.--Threads of this series conform in all respects to the PTF-SAE SHORT threadsexcept that the full thread length has been further shortened by eliminatingtwo threads at the large end of external threads or eliminating two threadsat the small end of internal threads. Gaging is the same as for PTF-SAESHORT except the LÚs¿ ring thread gage for external thread length and taperor the LÚs¿ ring thread gage for internal thread length and taper cannot beused. The tolerance must be altered as described in paragraph 4 below onLimitations of Assembly. For interchangeability, see table A7.9. This threadshall be designated as follows:

1/2 DRYSEAL PTF-SPL EXTRA SHORT.

4. LIMITATIONS OF ASSEMBLY FOR DRYSEAL PTF-SPL SHORT AND PTF-SPL EXTRA SHORTTHREADS.-Combinations of the standard Dryseal pipe threads are given in tableVIII.3, page 21. However, where special combinations are used, additionalconsiderations must be observed. In addition to "SPL" in the designation, thegaging tolerance should be specified. 5. INTERCHANGEABILITY BETWEEN DRYSEAL SPECIAL AND DRYSEAL STANDARDTHREADS.- Interchangeability between Dryseal special and Dryseal standardthreads of section VIII, is given in table A7.9. 6. DRYSEAL FINE THREAD SERIES, F-PTF.- The need for finer pitches fornominal pipe sizes has brought into use applications of 27 threads per inchto 1/4 and 1/2 in. pipe size. There may be other need which require finerpitches for larger pipe sizes. It is recommended that the existing threadsper inch be applied to next size larger pipe size for a fine thread seriessuch as are shown in table A7.10. This series applies to external andinternal threads of full length and is suitable for applications wherethreads finer than NPTF are required. The designation for this thread shouldinclude the letter F and omit the letter N as follows:

1/4-27 DRYSEAL F-PTF

7. DRYSEAL SPECIAL DIAMETER-PITCH COMBINATION SERIES, SPL-PTF.-Otherapplications of diameter-pitch combinations have also come into use wheretaper pipe threads are applied to nominal size thin wall tubing such as areshown in table A7.11. This series applies to external and internal threads offull length and is applicable to thin wall nominal outside diameter tubing.The number of threads is uniform at 27 per inch. The designation for thisspecial and omit the letter N. Also, the outside diameter of the tubingshould be given as follows:

1/2-27 DRYSEAL SPL-PTF, O.D. 0.500

8. FORMULAS FOR DIAMETER AND LENGTH OF THREAD.-Basic diameter and length ofthread for sizes of Dryseal fine taper pipe thread, F-PTF, and Dryseal special taper pipe thread, SPL-FTF, given in tables A7.10 and A7.11 are on thefollowing formulas:

D = outside diameter of pipe or tubing in inches p = pitch of thread in inches

Diametral taper = 0.75 in. per 12.00 in. of length

60


Tolerance shall be equal to plus or minus the taper of one thread on thediameter.

*9. SUPERSEDED GAGE DIMENSIONS AND GAGING PRACTICE FOR 1/6 AND 1/4 SIZEDRYSEAL PIPE THREADS

* In this standard, the LÚ1¿ dimensions for the 1/2-27 and 1/4-18 sizeswere revised to correct for a disproportionate number of threads for handengagement. The LÚ1¿ hand engagement dimensions affecting gages in tablesVII.2, VII.9, VII.15, VIII.16, and VIII.17 were revised to agree with theproduct dimensions for future gage procurement. Therefore, it should be noted that where basic-notch thread gages havingsuperseded dimensions (see table 7.12) are being used for gaging the 1/2/-27and 1/4 turn larger for the 1/4-18 size that the specified PD gaging steps. External threads gaged by the Turns Engagement Method should be 1/2 turngreater for the 1/2-27 size and 1/2 turn less for the 1/4-18 size than thebasic turns specified. Table 7.12 lists the dimensions related to the superseded LÚ1¿ dimensions of 0.1800 for the 1/2-27 size and 0.2000 for the 1/4-18 size.


63


APPENDIX A8

Geometry of Taper Screw Threads.

1. INTRODUCTION

This appendix presents several geometrical relationships relative to theconical spiral, which is the curve of generation of the taper screw thread,and also briefly discusses the conical helix. With reference to these curves,the formulas include the parametric equations, the projection, the development, the lead angle, and length of an arc. The geometry of taper screw thread has, in practice, developed by modification of the geometry of straight screw threads, with the result that formulas commonly used for taper screw threads are often approximations insteadof being exact. That such approximations have been satisfactory in practicearises from the fact that the angle of taper, or cone angle, of standardtapes threads has been small. The more recent use of larger taper anglestogether with the higher precision of measurement of screw thread gages nowdemanded, sometimes requires the availability of exact, formulas to be substituted for the approximate formulas or used to determine the magnitude oferrors introduced by the usual approximations. It is convenient to approach the subject by considering the nature of thecurves of generation of straight screw threads and of taper screw threads,respectively, namely the cylindrical helix and the conical spiral. A cylindrical helix may be defined in various ways. First it is a curve onthe surface of a circular cylinder which cuts the elements of the cylinder ata moment angle. The same curve may also be defined as the curve generated bya point moving at a uniform rate along a straight line while the line revolves uniformly about an axis parallel to itself, so that successive intersections of the curve and an element of the cylinder are equally spaced. These definition establish the fact that the cylindrical helix in both loxodromic and isometric. There is no corresponding curve on the surface of a cone which simultaneously answers to both methods of generation. Thus there are two differentspiral-shaped curves lying on the surface of a cone which are analogous tothe helix, one of which is loxodromic and the other isometric. Mathematicianshave agreed (6)À12Ù that the loxodromic curve corresponds to the definitionof a general helix, and that it should property be termed a conical helix.The isometric curve has been called the conical spiral. Loria (6) gives abrief history this curve, stating that it is found in a work by B. Pascal andciting several 18th century references, one of which points out that thecurve was known to ancient Greek geometries Thus there we the followingdefinitions: A conical spiral is generated if a point travels on the surface of a rightcircular cone so as to combine a uniform angular motion around the axis ofthe cone with a uniform linear motion along a generator toward or from thevertex. It is characterized by uniformity of pitch, that is, successiveintersections of the curve and an element of the cone are equally spaced, andby the fact that it passes through the vertex of the cone. The conical spiraloccurs in such mechanical applications as the taper screw thread, the spiralbevel gear (11), and the conical spring (12). A conical helix is generated if a point travels on the surface of a rightcircular cone in such a way that the curve produced interests the elements ofthe cone at a constant angle. The pitch of this curve varies from point topoint and it approaches the vertex of the cone as an asymptote. It is appliedmechanically in the conical spring (1,7,9) as sometimes made. (For conicalsprings with coils of constant slope it is desirable that the projection ofthe neutral axis of the spring be an Archimedes spiral. Such a spring is nottruly conical but is wound on a paraboloid of revolution.) [10]

The cylindrical helix is the curve of intersection of a helicoid and a coaxial cylinder, and the conical spiral is the intersection of a helicoid and a coaxial cone. accordingly, although the geometry of the conical spiral differs from that of the helix, there is but one geometry of helicoid. A screwhelicoid, for example, remains a screw helicoid, whether the ends of itsgeneratrix are determined by coaxial cylinders, as in straight screw threads,or by coaxial cones, as in taper screw threads. These different boundaryconditions give rise, however, to certain different geometrical relations.

2. PARAMETRIC EQUATIONS OF THE CONICAL SPIRAL; THE PROJECTION, DEVELOPMENT, LEAD ANGLE, AND LENGTH OF AN ARC

The parametric equations of the conical spiral, with the vertex of the coneat the origin and the axis of the cone coinciding with the axis, as shown infigure A8.1, are;

x = L [theta] tan [alpha] cos [theta] ÄÄÄÄÄ¿ ÄÄÄÄÄÄÄ ³ 2[pi] ³ ³ y = L [theta] tan [alpha] sin [theta] ³ (1) ÄÄÄÄÄÄÄ ³ 2[pi] ³ ³ z = L [theta]Ú1¿ ³ ÄÄÄÄÄÄÄ ³ 2[pi] ÄÄÄÄÄÙ where

[alpha] = 1/2 included angle between opposite elements of the cone,

[theta] = the variable parameter, and is the angle which the projection of the radius vector of the point on the conical spiral makes with the x-axis on the xy=plane.

L = lead of spiral, or advances, parallel to the axis, in one revolution.

The length, r, of the radius vector at any point an the conical spiral isgiven by the relation:

r = L [theta] sec [alpha] ÄÄÄÄÄ 2[pi] (2) 64

The project, r', of the radius vector, r, on the xy-plane is given by therelation: r'= L [theta] tan [alpha] ÄÄÄÄÄ 2[pi] (3)

This is the equation of a spiral of Archimedes, which is the projection of theconical spiral on the xy-plane.The developed cylindrical helix is a straight line, which makes an angle, s,with the line perpendicular to the axis such that.

tan s = L ÄÄÄÄÄ 2[pi] (4)

The developed conical spiral is an Archimedes spiral derived form the equations for the conical spiral and represented by the equation:

[p] = L[theta] csc 2[alpha] ÄÄÄÄÄÄÄÄ [pi] (5)

wherep = radius vector = x sec [theta] = L [theta] sec [alpha] ÄÄÄ 2[pi]

[theta] = vectorial angle = [theta] sin [alpha].

The lead angle, s defined as the angle made by the conical spiral at a givenpoint with a plane perpendicular to the axis, is determined from the formulafor the tangent line:

tan s = cot [alpha] ÄÄÄÄÄÄÄÄÄ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Ä¿³[theta]À2Ù + 1 (6) ÀÙThis expression is of interest because the lead, L, is not directly involved.It shows that all isometric conical spirals at a given number of revolutionsfrom the apex out an element of the cone at the same angle, regardless of thepitch. Also, a conical spiral is tangent to an element of the cone at the apex. The exact length, S, of the arc of a conical spiral subtended by the vectorial angle ([theta]Ú1¿ - [theta]Ú2¿), is given by the expression:

ÚÄÄÄÄ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄSÚ[alpha]¿ = L tan [alpha]³[theta]Ú2¿ Ä¿³[theta]À2ÙÚ2¿+cscÀ2Ù[alpha] - ÄÄÄÄ ³ ÀÙ 4 [pi] ³ ÀÄÄÄÄ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ [theta]Ú1¿ Ä¿³ [theta]À2ÙÚ1¿ + cscÀ2Ù[alpha] + ÀÙ ÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ [theta]Ú2¿+ Ä¿³ [theta]À2ÙÚ2¿+cscÀ2Ù[alpha] ³ ÀÙ ³ c[alpha]cÀ2Ù[alpha] log ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ [theta]Ú1¿+ Ä¿³ [theta]À2ÙÚ1¿+cscÀ2Ù[alpha] ³ ÀÙ ÄÄÄÄÄÄÙ

(7)

An approximation of the value of S[alpha], which is exact for the cylindrical helix and sufficiently close to the exact value for the conical spiral for mostpractical purposes, is given by the relation:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄS[alpha] = Ä¿³[pi]À3Ù (r'Ú1¿ + r'Ú2¿)À2Ù + LÀ2Ù (8) ÀÙWhere r'Ú1¿ and r'Ú2¿ are the projections of the radii vectors corresponding to [theta]Ú1¿ and [theta]Ú2¿ .

3. PARAMETRIC EQUATIONS OF THE CONICAL HELIX: THE PROJECTION, DEVELOPMENT,LEAD ANGLE, AND LENGTH OF AN ARC

The properties of the conical helix, which is defined above have been discussed by Dieu [5], Resal [8], and others [1,2,6]. Some of the more important analytical

relations are here presented. Taking as for the conical spiral the vertex of the cone as the origin, andthe axis of the cone as the z-axis, figure A8,2, the general parametric equations, and the equation of the radius vector of the conical helix, are:

x = c eÀaÙÀrÙ cos [theta] ÄÄÄ¿ ³ y = c eÀaÙÀrÙ sin [theta] ³ (9) ³ z = b eÀaÙÀrÙ ÄÄÄÙ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ r = Ä¿³bÀ2Ù+cÀ2ÙeÀaÙÀrÙ (10) ÀÙ

where a,b, and c are constants, e is the natural logarithmic base, 2.71828,[theta] is the variable parameter, and r is the radius vector. This curve, unlike the conical spiral, approaches the origin as an asympotoicpoint, the successive turns about the cone being closer together toward thevertex. To simplify the mathematics involved in dealing with this curve, it is convenient so to locate the curve with respect to the origin that [theta]=0 when y=0 and r=1. Under these conditions.

b = cos [alpha] c = sin [alpha]

where

[alpha] = 1/2 included angle between opposite elements of cone.

Also

[alpha] = sin [alpha] cot [beta], where [beta]<90 deg.,

= the constant angle with which the curve intersects the elements of the cone.

65

Thus

x = sin [alpha] cos [theta] eÀaÙ Àsin [alpha] cot [beta]Ù ÄÄ¿ ³ y = sin [alpha] sin [theta] eÀaÙ Àsin [alpha] cot [theta]Ù ³ (11) ³ z = cos [alpha] eÀaÙ Àsin [alpha] cot [beta]Ù ÄÄÙ

r = eÀaÙ Àsin [alpha] cot [beta]Ù (12)

when

r > 1, [theta] is plus, and when r < 1, [theta] is minus.

The expression for r, equation (12), completely represents the curve in polarcoordinates, a third coordinates being unnecessary because the radius vectormakes a constant angle [alpha] with the x-axis.If, instead of [beta], the angle [gamma] which the curve makes with the xy-plane is given, cot [beta] = sec [alpha] tan [gamma], and the above exponent [theta] sin [alpha] cot [beta] is replaced by [theta] tan [alpha] tan [gamma].

The lead angle s (= [gamma]) is accordingly given by

tan s = cos [alpha] cot [beta]

= [alpha] cot [alpha]

The projection r' of the radius vector, r, on the xy-plane is given by

r' = sin [alpha] eÀaÙÀsin [alpha] cot [beta]Ù (13)

This is a logarithmic spiral, and the equation represents the projection ofthe conical helix on the xy-plane. The developed conical helix is a logarithmic spiral, derived from the parametric equations of the curve, which is represented by

[rho] = eÀaÙ Àcot [beta]Ù (14)

where [rho] = radius vector = [beta] sec [alpha]

= cos [alpha] eÀaÙÀsin [alpha] cot [beta]Ù sec [alpha]

[theta] = vectorial angle = [theta] sin [alpha].

The length, SÚo¿, of the arc of a conical helix subtended by the vectorialangle, [theta]Ú2¿ - [theta]Ú1¿, is :

SÚo¿ = sin [alpha] csc [beta] eÀaÙ Àsin [alpha] cot [beta]Ù (15)

4. GEOMETRICAL PROPERTIES OF TAPER SCREW THREADS

Taper screw threads are generally produced commercially with the bisector ofthe thread angle perpendicular to the axis of the thread. The pitch, [rho],is the distance, measured parallel to the axis of the thread, between any twocorresponding points in an axial plane on parallel sides of adjacent threads.If [rho]Úe¿ is the length of the line joining the two points, then, [rho] = [rho]Úe¿ cos y (16)where

y = half-angle of taper.

The same is true of any taper thread, the sides of which are unsymmetricalwith respect to a line which is perpendicular to the axis and passes througha vertex of the thread. The lead or pitch of the generatrix of the helicoid forming the side of ataper thread differs from the lead or pitch of the thread [3, 4]. The lead ofthe generatrix of the following side of the thread is 9 see fig. A8.3):

LÚf¿ = ([rho] + n) - L(1 + tan [alpha]Ú1¿ tan y). (17)

The lead of the generatrix of the leading side of the thread is:

LÚe¿ = ([rho] - m) = L(1 - tan [alpha]Ú2¿ tan y) (18)

where

L = lead of thread

[alpha]Ú1¿ = angle between line perpendicular to axis of thread and the following side of thread.

[alpha]Ú2¿ = angle between line perpendicular to axis of thread and the leading side of thread.

For a symmetrical thread, [alpha]Ú1¿ = [alpha]Ú2¿ = [alpha], and

LÚf¿ = L(1 + tan y tan [alpha]) (19)

LÚe¿ = L(1 - tan y tan [alpha]) (20)

66


The depth, H, of an unsymmetrical sharp-V taper thread is the distance, perpendicular to the axis, between the cones enveloping the thread at crest androot, and is given by the relation:

H = [rho] secÀ2Ù y csc A cos ([alpha]Ú2¿ + y) cos ([alpha]Ú1¿ - y) (21)

where A = [alpha]Ú1¿ + [alpha]Ú2¿

If a thread is symmetrical about the perpendicular to the axis,

[alpha]Ú1¿=[alpha]Ú2¿=[alpha],and H = [rho] (cot [alpha] - tanÀ2Ù y tan [alpha]). (22) ÄÄÄÄÄ 2

In a taper screw thread the crest of the thread is not exactly opposite the

root of the thread at points located 180 deg. apart, as in a straight threadbut there, but there is an axial displacement between these positions. Thisdisplacement is given by the formula:

S = L (1 - 2 sec y csc A cos ([alpha]Ú2¿ - y) sin [alpha]Ú2¿) (23) Ä 2

for an unsymmetrical thread. For a symmetrical thread this formula reduces to[3]: S = L tan [alpha] y. (24) Ä 2

For an unsymmetrical thread having the bisector of the thread angle perpendicular to the element of the cone [3],

S = L cot [alpha] tan y. (25) Ä 2

5. DIAMETER OF AN ARCHIMEDES SPIRAL AS MEASURED BETWEEN FLAT, PARALLEL SURFACES

As previously stated, the projection of the pitch line of a taper thread gageon a plane perpendicular to the axis of the gage is an Archimedes' spiral. Ifparallel lines are drawn tangent to opposite sides of a segment of such aspiral, figureA8.4, the radii vectors to the points of contact subtend anangle of slightly more than 180 deg., and the distance between the lines isslightly greater than the length of the line, intercepted by the spiral,which is drawn perpendicular to these tangents through the axis of the spiral. In measuring the diameter of a taper thread gage between flat parallel anvils AA and BB, figure 8.4 the measurement obtained corresponds top the distance between the parallel lines, whereas the true diameter corresponds to the length of the intercepted perpendicular line. For commercial taper screw threads the difference, [delta]M, between these lengths is given very nearly by the formula:

[delta]M = [Epsilon] (cos [phi] - 1) + L [phi] tan [alpha] cos [phi] (26) Ä [pi]

67


where

[Epsilon] = true diameter, or length of intercepted line

[theta] = approximate mean vectorial angle at points of measurement

[phi] = cot À -1 Ù [theta].

Example: Solve for [delta]M for presumably the worst case encountered incommercial practice, namely the A.P.I. standard 2-3/8 in. rotary joint thread.

L = 0.20 in.

[Epsilon] = 2.36537 in.

taper = 0.25 in./in.

[theta] = 2.36537 ÄÄÄÄÄÄÄ X 2 [pi] + [pi] 0.25 X 0.20

= 300.38276 radians

cot [phi] = [theta] = 300.38276

[phi] = 0 deg. 11 '28"

= 0.00334 radians

[Delta]M = 2.36537 (0.9999944 - 1) + 0.20 X 0.125 X 0.00334 X 0.9999944 ÄÄÄÄ [pi] = + 0.00013 in.

6. REFERENCES

[1] O. B. Ader, Extension and period of a conically wound spring, Phys. Rev., 34, 656 (1929).[2] Robert J. T. Bell, Coordinate geometry of three dimensions. MacMillan and Co. Ltd. 205 (1928).[3] G. Berndt, Die Gewinde, Julius Springer, Berlin, p.6 (1925).[4] Earle Buckingham, The design of hobs for taperthreaded joints, Am. Machinist, 69, 759,801 (1928).[5] M. Dieu, Concous d'agregation aux Lyeees Annee 1845, Nouvelles Annales de Mathematiques, 12, 373 (1853).[6] Gino Loria, Curve sghembe speciali algebriche F. transcendenti, vol. 2, Nicola Zanichelli, Bologne, 146, 166 (1925).[7] E. R. Morrison, The design conical helical springs, Manchinery (N.Y.)., 18, 681 (1912).[8] H. Resal, Sur les proprietes de la loxodromie d'un cone de revolution et leur application au ressort conique, Compt Rend, 114, 147 (1892).[9] J. B. Reynolds and investigation of conical springs with coils of constant slope. A.S.M.E. Advance Paper, Dec.1931, 24 pp.[10] J. B. Reynolds and O. B. Schier (Lehigh University), The design and investigation of a spring in which all coils nest simultaneously. A.S.M.E.

Advance Paper, Dec. 1931, 16 pp.[11] Reginald Trautschold, Spiral type beval gears, Machinery (N.Y.), 23, 199 (1916).

[12] Joseph Kaye Wood, Conical spring design, some new facts and formulas, Am. Machinist, 71, 620 (1929).

68


APPENDIX A9. EXTENT OF USAGE OF THE AMERICAN NATIONAL FIRE-HOSE COUPLINGTHREADS ON COUPLINGS AND NIPPLES USED WITH 2 frac12 INCH NOMINAL SIZE FIRE HOSE Listed below are the cities in the United States which had a population of25,000 or more according to the 1950 census, and which have not standardizedon the American National fire-hose coupling thread on hydrants, couplings,and nipples used with 2-1/2 in. nominal size fire hose. The outside diameter of the nipple (external) thread is 3-1/16 in. and the pitch is 7-1/2 threads per inch for this American National thread. If all cities of 25,000 population or over in a state have adopted the state is shown in the tabulation as being 100 percent Standard. The tabulation shows the outside diameter of the nipple (external) threadin inches and the number of threads per inch, the number preceding the dashbeing the outside diameters are given to the nearest 1/64 in.). Special types of snap, clutch, or other patent couplings are designated bytheir trade names. In some instances, the dimensions were not available and were left blank.* In some instances it may appear to be feasible to make cross connectionswith the American National standard thread.À14Ù However, where there aredifferences in pitch of more than one-half a thread between the mating parts,or differences in pitch diameter in excess of 1/22 in., there will not bewasher tight fits. Coupling parts more than 1/16 in. larger in pitch diameterthan the nipple will blow off when subjected to high pressures.

ALABAMA (100 percent Standard)ARIZONA Phoenix (Adapters), 2 1/16 to 2 1/22--6ARKANSAS (100 percent StandardCALIFORNIA BurbankCOLORADO Colorado Springs, 3 & 3 1/22 -- 7 1/2 Note: Colorado Springs is in the process of changing over to the National Standard Denver, 3 3/22--8 Pueblo, 3 1/4--6CONNECTICUT Bridgeport, 3 11/34 & 3 1/22--8 Norwich, 3 2/34--7 1/2 and National StandardDELAWARE (100 percent Standard)FLORIDA Gainesville Key West,2 31/32 to 3--8 Lakeland, 3 1/16--6 Tallahassee, 3--6GEORGIA Macon, 3--8 Rome, 3 3/16--7IDAHO (100 percent StandardILLINOIS Alton, 3 1/22 and 3 1/34--7 Aurora, 3 1/22--7 and National Standard Berwyn, 3--7 1/2 Bloomington Champaign 3 1/22 and 3 1/16--8 Chicago, 1/34--7 1/2 Cleero, 3 1/22 to 3 1/34--7 1/2 Danville, 3 1/16--8 Decatur, 3 2/22--7

East St. Louis, 3 3/34 & 3 1/3--6 Elgin, 3 1/2--7 Galesburg, 3 1/16--7 Granite City, 3 3/32--6 Joliet, 3--8 Kankakee, 2 31/32 to 3--7 1/2 Maywood, 3--7 1/2 Moline, 3 1/16--7 Oak Park, 3 1/16--7 Peoria, 3 1/16--7 Rock Island, 3 2/34 & 3 1/16 Springfield, 3 1/16--7 Waukegan, 3 1/16--7INDIANA Bloomington Marion, 3 7/22--6IOWA Burlington, 3 1/4--6 Cedar Rapids, 3 1/16--6 Clinton Council Bluffs, 3 2/22--8 Fort Dodges, 3 1/4--6 Sioux City, 3 16/34--6KANSAS (100 percent Standard)KENTUCKY Covington, 3 3/34 & 3 2/22--6 Louisville, 3 1/2--6 Newport Paducah, 3 7/22--6LOUISIANA Alexandria, 3 2/22--8 Baton Rouge, 3 1/12--8 Lafayette, 3 1/22--8 Lake Charles, 3 1/16--8 Monroe, 3 1/16--8 New Orleans Hose, 3 1/34-7 1/2 Hydrants, 3 2/22--6MAINE (100 percent Standard)MARYLAND Hagerstown Hose, 3 1/16--8 Hydrants, 3 1/8--7MASSACHUSSETTS (100 percent Standard)MICHIGAN Dearborn, 3 1/8--7 1/2 Detroit, 3 1/8--7 1/2 Ferndale, 3 1/8--7 1/2 Hamtramck, 3 1/8--7 1/2 Highland Park, 3 1/8--7 1/2 Lincoln Park, 3 2/16--7 Muskegon, 3 7/22--6 Pontiac, 3 1/8--7 1/2 Royal Oak, 3 2/16--9 Wyandotte, 3 1/8--7 1/2MINNESOTA (100 percent Standard)MISSISSIPPI Biloxi Greenville Laurel, Iron pipe threads--8 Meridian, 3 1/16--8 and National StandardMISSOURI Joplin, "Anderson" & 3 7/22--6 St. Joseph, 3 7/22--6

MONTANA (100 percent Standard)NEBRASKA Lincoln 3 1/22--8 and National Standard Omaha, 3 1/16--8NEVADA (100 percent Standard)NEW HAMPSHIRE (100 percent Standard)NEW JERSEY Bavonne, 3--8 Belleville, 3--8 Bloomfield, 2 18/16 to 3--8 Camden Hose, "Jones Hydrants 3 1/22--6 Clifton, 3--8 East Orange, 3--8 Elizabeth, 3--8 Garfield Hoboken, 3--8 Irvington, 3 & 3 1/64--8 Jersey City, 3--8 Kearny, 3--8

69


NEW JERSEY--Continued Linden, 3 1/22--8 Montclair, 3--8 Newark, 3--8 Orange Passaic, 3 1/16--8 Paterson, 3--8 Plainfield, 3--8 West Orange, 3--8NEW MEXICO Roswell Santa Fe, 3 1/16--6NEW YORK Amsterdam, 3 1/22 & 3 1/16--6 Auburn, 3 1/22--7 1/2 and National Standard Buffalo, 3 1/16--8 Elmira, 3 1/22--8 Hempstead, 3 1/22--8 Ithaca, 3 1/64--7 1/2 Jamestown, 3 1/16--8 Lockport, 3 1/16--8 Mount Vernon, 3--8 New Rochelle, 3 1/22--8 New York City, 3 1/22--8 Rochester, 3 1/22--7 Rome, 3 1/16--8 Schenectady, Dbl. 3 1/8--6 Syracuse, 3 1/64--8 Utica, 3 11/64--8 Valley Stream, 3 1/22--6 Yonkers, Adapters & 3--8NORTH CAROLINA Asheville, 3 1/16--6 Raleigh, 3 1/16--6 Wilmington, 3 1/4--6 Winston-Salem, 2 22/54 & 2 31/32--7 1/2NORTH DAKOTA (100 percent Standard)OHIO Akron, 3 1/4--6 Alliance, 3 18/64--6 Barberton, 3 18/64--6 Cincinnati Cleveland, 3 5/64--8 Cleveland Heights, 3 1/16--8 Cuyahoga Falls, 3 1/4--6 Dayton, 3 18/64--6 East Cleveland, 3 1/16--8 Euclid Hamilton, 3 3/16--7 Lakewood, 3 1/16--8 Middletown, 3 1/4--6 Parma, 3 5/64--8 Shaker Heights Springfield, 3 5/16--6 Steubenvillem 3 1/18--6 Toledo, 3 & 3 1/64--8OKLAHOMA Muskogee Oklahoma City, 3 5/22--6

OkmulgeeOREGON (100 percent Standard)PENNSYLVANIA Aliquipps, 3--8 Allentown, 3 1/4--6 & 3--8 Altoona, 3 1/16--6 Bethlehem, 3 3/16--6 Chester, "Jones" & 2 15/16 & 2 61/64--7 1/2 Easton, 3 & 3 1/22--6 Erie Harrisburg, 3 1/22--8 Hazelton, 3 1/22--6 Johnstown, 3 1/14--7 Lancaster, 3 1/16--7 Lebanon, 3--8 McKeesport, 3 1/22--6 New Kensington Norristown, "Jones Snap" Philadelphia, "Jones Snap" Pittsburgh, 3 1/16--6 Reading, 3 7/22--6 Scranton Sharon Washington, 3 1/64--6 Wilkes-Barre, 3 1/16--6 Wilkinsburg, 3 1/8--6 Williamsport, 3 1/16--6 York, 3 9/64--7RHODE ISLAND Newport, 3 11/64--6SOUTH CAROLINA (100 percent Standard)SOUTH DAKOTA Aberdeen, 3 12/64--6 Rapid City, 3--8TENNESSEE (100 percent Standard)TEXAS San Angelo TempleUTAH Salt Lake City, 3 11/64--6VERMONT Burlington, 3 1/8--6VIRGINIA Charlottesville, 3 5/16--8 Petersburg, 2 15/16--8 Richmond, 3 5/16--8 Roanoke, "Clay Snap"WASHINGTON (100 percent Standard)WEST VIRGINIA Clarksburg, 3 5/64--6 Fairmont, 3--8 Huntington, 3 1/16 & 3 3/22--6 Morgantown, 3 1/16--6 Wheeling, 3 1/16--6WISCONSIN (100 percent Standard)WYOMING Casper, 3 5/16--7 and National Standard Cheyenne, 3 1/4--6

70


APPENDIX A11. CLASS 5 INTERFERENCE-FIT THREADS**

1. INTRODUCTION

Interference-fit threads are threads in which the externally threaded member is larger than the internally member when both members am in a free state and which, when assembled, become the same size and develop a high resistance to any applied unscrewing torque through elastic compression, plastic movement of material, or both. By custom, theme threads am designated class 5. The standards previously published in this handbook were helpful in stabilizing design; however, in spite of restrictive tolerance, loosening or breakage of externally threaded members has been all too frequent. They alsoestablished minimum and maximum torque values, the validity of which has beengenerally accepted in service for the past 20 years. This standard À1Ù is based on 10 years of research, testing, and fieldstudy, and represents the first attempt to establish an American standard forinterference fit threads. It is predicated on the following conclusions whichhave been drawn from the research and field experience: (1) Materials of the external and internal interference fit threads compress elastically during assembly and when assembled. (2) During driving, plastic flow of materials occurs, resulting in eitheran increase of the external thread minor diameter, or a decrease in the internal thread minor diameter, or both. (3) Relieving the external thread major diameter and the internal threadminor diameter to make allowance for plastic flow eliminates the main causesof seizing, galling and abnormally high and erratic driving torques. (4) Such reliefs require an increase in the pitch diameter interference inorder to obtain driving torques with it the range previously established. (Indriving studs, it was found that the minimum driving torque-should be about50 percent of the torque required to break loose a property tightened nut.) (5) Lubricating only the internal thread results in more uniform torques than lubricating only the external thread and is almost as beneficial as lubricating both external and internal threads. (6) For threads having truncated profile, torques increase directly as thepitch diameter interference for low interferences, but torques soon becomepractically constant and increase little, if at all, with increases of interference. Obviously, for uniformity of driving torques, it, is desirable to work with greater interferences. (7) Comparatively large pitch diameter interferences can be tolerated provided that the external thread major diameter and internal thread minordiameter are adequately relieved, and proper lubrication is used duringassembly. (8) Driving torque increases directly with turns of engagement. (For thinwall applications, it may be desirable to use longer engagement rather than large pitch diameter interference to obtain desired driving torque.) (9) Studs should be driven to a predetermined depth. "Bottoming" or "shouldering" should be avoided. "Bottoming," which is engagement of the threads of the stud with the incomplete threads at the bottom of a shallow drilled and tapped hole causes the stud to stop suddenly thus inviting failure in torsional shear. "Shouldering," which is the practice of driving the studuntil the thread runout engages with the top threads of the hole, creates radial compressive stresses and upward bulging of the material at the top of the hole. This results in erratic variations in free stud length after driving. As application experience is gained by users of this standard, it is urgedthat results, good or bad, be reported to the Industrial Fasteners Institute,1505 East Ohio Building, 1717 East 9th Street, Cleveland, Ohio 44114, withcopy to Standards Department, The American Society of Mechanical Engineers, United Engineering Center, 345 East 47th Street, New York, N.Y. 10017. Future

adjustments to the standard will be based largely on such field reports.2. SCOPE

This standard [1] provides dimensional tables for external and internalinterference fit (class 5) threat, of modified Unified form in the coarsethread series, size, 1/4 to 1 1/2 in. It is intended that designs conforming with this standard will provide adequate torque conditions which fall within the limits shown in table A 11.3 These torque limits are the same as those in H28(1944) and the 1950 Supplement. The minimum torques are intended to be sufficient to ensure that externally threaded members will not loosen in service; the maximum torques establish alpha limit below which seizing, galling or torsional failure of the externally threaded components is unlikely. See figure A.11.1 for conditions of fit.

3. DESIGN AND APPLICATION DATA

Following are conditions of usage and inspection on which satisfactoryapplication of products made to dimensions in tables A 11.1, A11.2, and A11.3are predicated. 1. THREAD DESIGNATIONS--(a) The following thread designations provide ameans of distinguishing the American Standard class 5 threads of this standard from the tentative class 5 and alternate class 5 threads specified previously in Handbook H28. It also distinguishes between external and internal American Standard class 5 threads.(b) Class. 5 external threads are designated as follows: NC5 HF --For driving in hard ferrous material of hardness over 160 BH N. NC5 CSF--For driving in copper alloy and soft ferrous material of 160 BHN or less. NC5 ONF--For driving in other nonferrous material (nonferrous materials other than copper alloys), any hardness.(c) Class 5 internal threads are designated as follows. NC 5 IF--Entire ferrous material range. NC5 INF--Entire nonferrous material range.

2. STUDS.--(a) inspection.--Since angle and lead deviations are not ascritical factors as in free fitting screw threads, the controlling elementfor class 5 threaded products is pitch diameter. This element can be satisfactorily checked by an optical comparator, a thread micrometer, or threadsnap gages having anvils that are not affected by lead or angle. For rapidand convenient control in mass production, the use of "go" and "not go" snapgages is recommended. Ring gages may be used, but their use is not primarilyrecommended. The "not go" ring gage shall stop at 1 frac12 turns or lessengagement in order to maintain minimum pitch diameter interference. W thread setting plugs shall be used for all gages, and tolerances shall be applied within the product limits. The maximum major diameter of the truncated portion of the truncated setting plug should be equal to the minimum major diameter of the stud thread. If the threads are zinc, cadmium, or copper plated, limits are applicable before plating. (b) Points.--Points of externally threaded components should be chamferedor otherwise reduced to a diameter below the minimum, minor diameter of thethread. (c) Workmanship.--Studs should be free from excessive nicks, burrs, chips,grit, or other extraneous material before driving.

3. STUD MATERIALS.--The length of engagement, depth of thread engagementand pitch diameter limits in tables, A11.1, A11.2, and A11.3 are designed toproduce adequate torque conditions when heat-treated medium-carbon steelstuds, ASTM A-325 (SAE grade 5) or better, are used. In many applications,case-carburized studs and unheattreated medium-carbon steel studs, SAE grade4, are satisfactory.

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ** See page II.

[1] This Section is in technical agreement with U.S. Standards Institute(Formerly the American Standards Association, ASA) publication A.S.A. B1.12-1963 Interference-Fit Thread, which is published by the American Societyof Mechanical Engineers, United Engineering Center, 345 East 47th Street, NewYork, N.Y. 10017. 72


74


SAE grades 1, 2, and 8 may be desirable under certain conditions. This standard is not intended to cover the use of studs made of stainless steel, silicon bronze, brass or similar materials. Where such materials are used, the dimensions listed herein will probably require adjustment based on pilot experimental work with the combination of materials involved.

4. HOLES.--(a) Inspection.--Gages in accordance with Part I, section VI,shall be used. "Go" plain plug and "go" thread plug gages should be insertedto full depth in order to detect the effect of excessive drill or tap wear atthe bottom of the hole. "Not go" thread plug gages should not enter more than1 1/2 threads. Holes shall be clean from grit, chips, oil, or other extraneous material prior to gaging.(b) Countersinks.--Holes shall be countersunk to a diameter greater than themajor diameter in order to facilitate starting of the studs and to preventraising a lip around the hole after the stud is driven.(c) Cleanliness.--Holes shall be free from chips grit, or other foreignmaterial before driving studs. 5. LEAD AND ANGLE DEVIATIONS.--This standard does not provide control forlead and angle deviations. Angle and lead deviations are not normally objectionable, since they contribute to interference and this is the purpose ofthe class 5 thread. Experience may dictate the need for imposing some limitsunder certain conditions. 6. LUBRICATION.--(a) For driving in ferrous material, a good lubricantsealer should be used, particularly in the hole. A noncarbonizing type oflubricant (such as a rubber-in-water dispersion), is suggested. The lubricantshall be applied to the hole and it may also be applied to the hole and itmay also be applied to the stud. In applying it to the hole, care must betaken so that an excess amount of lubricant will not cause the stud to beimpeded by hydraulic pressure in a blind hole.(b) When class 5 threaded products are driven in nonferrous materials,lubrication may not be needed. Recent British research recommends the use ofmedium gear oil for driving in aluminum. In American research it has beenobserved that the minor diameter of lubricated tapped holes in nonferrousmaterials may tend to close in, that is be reduced in driving: whereas withan unlubricated hole the minor diameter may tend to open up in some cases.(c) Where sealing is involved, a lubricant should be selected which isinsoluble in the medium being sealed. 7. DRIVING SPEED.--This standard makes no recommendation for driving speed.Some opinion has been advanced that careful selection and control of drivingspeed is desirable to obtain optimum results with various combinations ofsurface hardness and roughness. Field experience with threads made to thisstandard may indicate what limitations should be placed on driving speeds. 8. RELATION OF DRIVING TORQUE TO LENGTH OF ENGAGEMENT.--Torques increasedirectly as the length of engagement. American research indicates that thisincrease is proportionately more rapid as size increases. 9. BREAKLOOSE TORQUES AFTER REAPPLICATION.--This standard does not establish recommended reapplication breakloose torques increases where repeated usage is involved. Field experience with a large variety of sizes and material will be needed to establish adequate values. 10. ASSEMBLY TORQUES FOR REAPPLICATION.--This standard does not establishassembly torques for reapplication. Field experience with a large variety ofsizes and materials will be necessary to determine the torques which willinsure the same performance where repeated usage is inviolate.


4. TABLES OF DIMENSIONS, TORQUES, AND INTERFERENCES

Table A11.1 and A11.2 of the standard are based on engagement lengths,external thread lengths, and tapping hole depths specified in table A11.3 andin compliance with the above design aid application data. Table A11. 1contains the limits of size for external threads. (a) For each size, it contains one set of pitch diameter limits regardless ofmaterial involved. The minimum pitch diameter is larger than the basic pitchdiameter of comparable UNC series threads. (b) For driving into brass and into ferrous materials having hardnessunder 160 Bhn, the length of engagement is 1 frac14). For driving into othernonferrous materials, the length of engagement is 2 frac12 D. In both cases,the minimum major diameter is approximately that of the minimum major diameter for class 2A. (c) For driving into ferrous material of 160 Bhn and harder, the lengthof engagement is 1 frac14 D; however, the maximum and minimum major diameterlimits are reduced to permit plastic flow and to reduce and stabilize drivingtorque. Table A11.2 contains the limits of size for internal threads. (a) One set of pitch diameter limits is maintained for each size regardlessof material. (b) The hole minor diameter limits are the same as those of class 3 forall sizes in nonferrous materials and for sizes up to and including 3/8 in. in ferrous materials. For 7/16 in. and larger sizes in ferrous materials, the minor diametershave been enlarged slightly in order to reduce driving torques, and tolerancehave been adjusted. Table A11.3 gives interferences and engagement lengths. For lengths ofengagement of 1 1/4 D, the external thread length and depth of full formthreads in tapped holes are set at 1 1/2 D with a tolerance of plus 2 1/2 [rho], minus 0. For lengths of engagement of 2 1/2 D, the length of external thread and depth of full form thread in the tapped hole are set at 2 1/4 D with a tolerance of plus 2 1/2 pitches, minus 0.

5. EXTENSION OF THE STANDARD

1. SMALL SIZES (UNDER 1/4 IN.).-- By using the new principles upon whichthis standard is based, stud sizes may be extended downward. However, adequate data are not now (1958) available to permit setting a standard. American research indicates that on smaller sizes the main reliance for producing adequate breakloose torques should be placed on pitch diameter interferenceand not on increasing the length of engagement. Extension of the standard isbeing investigated further. 2. LARGE SIZES (OVER 1 1/2 IN.).-- Although there is some current usage ofinterference fits on large size threads, adequate data is not now, (1958)available to permit setting a standard on larger sizes. 3. FINE THREAD SERIES.-Use of the coarse thread Series is urged unlessrequirements for strength of the stud make a finer pitch necessary. No research data are available now (1958) to enable the setting of a trial standard for fine thread studs having reduced major diameters. Indications are, however, that the product of the ratio:

Class 2A UNF PD tolerance ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Class 2A UNC PD tolerance

and the following coarse thread characteristics will probably work: (a) stud major diameter tolerance, (b) stud pitch diameter tolerance,

(c) minimum interference. Similarly, the above principles observed in setting the pitch and majordiameter limits on the fine series class 5 external thread may be followed inderiving the pitch and minor diameters of the internal thread for the fineseries. 4. 8-THREAD SERIES. The 8-thread series is now (1958) being investigated.

76


APPENDIX A12. THE TIGHTENING OF THREADED FASTENERS TO PROPER TENSION

The effectiveness of a threaded fastener usually depends on the degree towhich it is initially tightened, and in some applications the amount ofprestressing within a narrow range of tension is critical. For example, sufficient tension must be produced in pipe flange bolts to exceed the longitudinal force caused by the pressure in the piping, so that the flanged connection does not leak. The same problem is faced in tightening the nuts onthe cylinder head of an engine block, so that the studs are all stressed equally and to a tension that precludes leakage. In statically loaded structures in which there is a clearance between the bolt and the members heldtogether the clamping tension is important where rigidity of joints is desired to prevent relative motion of such members. In structures subjected to varying or alternating stresses, the range of the dynamic stress in the members varies with the bolt tension, and consequently the fatigue strength varies with the bolt tension. Factors affecting the maintenance of bolt tension are the proportion ofseating area to thread cross-section, elastic properties of the statingmaterial, stretch of the bolt, or creep of the bolt under load. The use ofwashers or other springy members in a fastener assembly tends to reduce theamount of external load that can be applied to a prestressed fastener beforethe load becomes additive to the initial bolt tension. In the design of bolted connections, enough experience is generally available to determine the amount of the required tension. To assure that such tension is actually induced in the bolt, screw, or stud when the joint is assembled requires a method that either directly or indirectly measures or determines the amount of tension. In the laboratory the tension induced in a bolt by tightening nut can beaccurately determined in a tensile testing machine. In the practical application of fasteners there are five generally used methods for setting bolttension, as follows: 1. Micrometer method, in which both ends of the bolt must be accessible tomeasure the change in the overall length of the bolt. 2. "Feel" method, applicable only when the desired tensile stress is justbeyond the yield point of the bolt material. 3. Torque measurement methods, which require that the torque-tension relationship be established for the specific conditions of assembly. 4. Angular turn-of-the-nut method. 5. Use of special devices for controlling tension.

1. MICROMETER METHOD

When a bolt is tightened, it elongates as the tension in the bolt is increased. Since the modulus of elasticity is practically constant at 29,500,000 psi for all steels at room temperature, the following formula applies:

Desired stress is bolt in psi = elongation in inches per inchÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 29,500,000 * of effective length, LÚe¿ (see * fig. 12.1.)

Example: For a length LÚe¿ of 5 in. and a desired stress of 45,000 psi,

Elongation = 45,000 X 5 = 0.0076 in. ÄÄÄÄÄÄÄÄÄÄ 29,500,000

To apply this method, the length of the bolt is measured by a micrometerbefore tightening. The bolt is then tightened until it has elongated the

required amount. 77


The micrometer method is applicable for bolts that are threaded theirentire length or for bolts that are so designed that the elongation will beuniform throughout the length. This method is not practical for general onebut may be used for spot checking. It may also be applicable in establishingtorque-tension relationships when a tensile testing machine is not available.

2. "FEEL" METHOD

Authorities agree that when in assembly has been properly designed, theyield point of the bolt may be slightly exceeded without harmful results. When a skilled workman is tightening a nut, he can "feet" a very slight yieldin the bolt when the yield point line been reached, and he stops tighteningwhen he feels this yield.

3. TORQUE MEASUREMENT METHOD

In most applications of threaded fasteners, it is not it is not practicableto measure directly the tension produced in each fastener during assembly.Fortunately, for many applications the tension may be controlled withinsatisfactory limits by applying known torques in tightening the nuts on thebolts or studs. Tests in numerous laboratories have shown that satisfactorytorque-tension relationships may be established for a given set of conditions, but that the change of any one variable may alter the of the fact that most of indeterminate friction, a change in the surface roughness of the bearing surfaces or of the threads, or a change in lubrication will drastically affect

the friction and thus the torque-tension relationship. Thus, it must be recognized that a given torque will not always produce a definite stress in the bolt but will probably induce a stress that lies in a stress range that is satisfactory. The torque-tension relationship for a given set of conditions may he established by means of a torque-wrench in combination with a tensile testingmachine or by the micrometer method described above. When both ends of afastener are not accessible for measurement, if the diameter of the bolt orstud is sufficiently large an axial hole may be drilled in it, see figure A12.2. By applying a micrometer depth gage to determine the change in depthof the hole during tightening of the fastener the tension can be determined.

4. ANGULAR TURN OF NUT METHOD

A procedure that is consistently being used in the installation of highstrength bolts in structures is based on the turn-of-the-nut method. The nut,is first tightened to seat the contacting surfaces firmly. It is then loosened sufficiently, if deemed necessary, to just release the bolt tension. Thisnut is then tightened through a specified fraction of a turn to produce therequired bolt tension. The angle through which the nut should be turned willbe different for each bolt size, length, material, threads per inch, and willalso vary with the elastic properties of the abutting material.

5. USE OF SPECIAL DEVICES FOR CONTROLLING TENSION

There are some specialized proprietary devices available whose function isaccurately to control the tension induced in the bolt. These devices areoperative even when both ends of the fastener an not available for measurement. They are known as preload indicating washers, load sensitive screws, and tru-load bolts. (a) Preload indicating washer.--This device consists of two concentricsteel rings sandwiched between two close tolerance, hardened steel washers. The inner ring is smaller in diameter and higher than the outer by a predetermined amount. A known preload in the bolt is indicated when the innerring is compressed to the point where the outer ring can no longer be movedfreely by means of a pin inserted into one of the peripheral holes. (b) Load sensitive screw.--A screw is made load sensitive by having aspecial resistance-type strain gage potted axially at its center. The changein resistance of the strain gage is read on a calibrated potentiometer asactual bolt tension. (c) Tru-load bolt.--The "tru-load" bolt provides a positive means forindicating the actual tensile loading on a bolt by the amount of elongation.It consists of almost any kind of bolt modified to contain a pin insertedalong the axis of the bolt. The pin is in contact with the bolt only at theinner end. The pin usually is made to be flush with the bolt head surfacebefore loading. As the bolt is loaded, the elongation produced in the bolt

causes the pin surface to move below the reference surface. This change isdistance is converted directly into unit stress by gaging with a calibrateddial gage. For some applications, it may be desirable to have the indicating pin extendabove the top of the bolt before tightening. When the load is applied, thepin withdraws into the bolt. The length of the pin is such that when the fullload has been applied, the pin will be drawn in until it is flush with thetop of the bolt. A dial depth gage reading of zero then indicates full preload.

6. BIBLIOGRAPHY

[1] J. V. Poncelet, Frottement des vis et des ecroux, Crelles Jour., 2,293 (1827).[2] F. Grashof, Theoretische Machinenlebre, 2,265 (1883). (Voss, leipzig.)[3] Melvin D. Casler, Effect of initial tension in flange bolts. Am. Mach. 60,369 (1924).[4] H. L. Whittemore, C. W. Nusbaum, and E. O. Seaquist, The relation of torque to tension for threadlocking devices. B. S. Jour., Res., 7, 945 (1931), R P 386.[5] J. N. Goodier, The distribution of load on the threads of screws. Trans. ASME, 62, A-10 (1040).[6] J. 0. Almen, How tight should a bolt be? Fasteners, 1, 1. 16 (1944), 1, 2, 14 (1944).[7] W. C. Stewart, What torque? Fasteners, 1, 4, 8 (1940).[8] L. H. Carr, Draw up flange bolts more uniformly. Fasteners, 3, 4, 7 (1946); Power, Feb. 1946.[9] G. A. Manley, Predicting bolt tension. Fasteners. 3, 5, 16 (1946).[10] V. E. Hillman, Engineering aspects of bolt variables. Iron Age, 160, 62 (1947). 78


[11] A. W. Brunot and W. G. Schmittner, Studies of the bolting of a fabricated steam chest. Fasteners, 3, 5,16 (1947).[12] K. H. Lenzen, Strength and clamping force of bolts. Prod. Eng., 18, 130 (1947).[13] M. P. Milliken, How tight is too tight? Fasteners, 6, 1, 14 (1949).[14] W. F. Pickel, Tightening characteristics of nut and stud Assemblies, Fasteners, 6. 1, 10 (1949).[15] W. C. Stewart, Torque for bolts and nuts. Fasteners, 10, 1, 12 (1955).[16] Walter M. Hanneman, The simultaneous measurement of torque and tension in machine screws. Fasteners, 12, 2 and 3, 4 (1957).[17] Richard B. Skidmore, New bolt tension calibrator. Fasteners, 13, 2, 7 (1958).[18] Richard B. Belford, A review of the use of structural fasteners for greatest economy, safety, and dependability. Fasteners, 14, 2 and 3, 5 (1959).

79


APPENDIX A13. THREE-WIRE METHOD OF MEASUREMENT OF PITCH DIAMETER OF 29 deg.ACME, 29 deg. STUB ACME AND BUTTRESS THREADS[2]

The computed value for the pitch diameter of a screw thread obtained fromreadings over wires will depend upon the accuracy of the measuring instrumentused, the contact load, and the value of the diameter of the wires used inthe computations. In order to measure the pitch diameter of a screw-threadgage to an accuracy of 0.0001 inch by means of wires, it is necessary toknow the wire diameters to 0.00002 in. Accordingly, it in necessary to use ameasuring instrument which read accurately to 0.00001 in. Variations in diameter around the wire should be determined by rotating thewire between a measuring contact and an anvil having the form of a V-groovecut on a cylinder and having the same flank angles, 14 deg. 30', as thethread to be measured. As thus measured the limit on roundness deviationshall be 0.00005 in. To avoid a permanent deformation of the material of the wires and gages itis necessary to limit the contact load, and for consistent results a standardpractice as to contact load in making wire measurements of hardened screwthread gages is necessary. In the case of Acme threads the wire presses against the sides of the threadwith a pressure of approximately twice that of the measuring instrument. This would indicate that the diameter of the wires should be measured againsta hardened cylinder having a radius equal to the radius of curvature of thehelical surface of the, thread at the point of contact, using approximatelytwice the load to be used in making pitch diameter readings. As with 60 deg.threads it is not practical to use such a variety of sizes, and it is recommended that the measurements of wire diameter be made between a flat contact and a 0.750-in. hardened and accurately finished steel cylinder. To limit the tendency of the wires to wedge in and deform the sides of an Acme thread, it is recommended that pitch diameter measurements on 8 tpi and finer be made at 1 lb. For coarser pitches and larger wires the deformation of wires andthreads is less than for finer pitches. Furthermore, the coarser pitches areused on larger and heavier product, on which pitch diameter tolerance isgreater and a larger measuring load may be required to make satisfactory measurements. It is, therefore, recommended that for tpi coarser than 8, thepitch diameter be measured at 2 1/2 lb. The standard specification for wires and standard practice in the measurement of wires stated in H28 (1957) Part I, Appendix 4, p. 196, are applicable to wires for Acme, Stud Acme, and Buttress threads, with the above stated exceptions as to angle of V-groove and limit on roundness.

1. ACME AND STUB ACME THREADS (29 deg.)

The combination of small flank angle and large lead angle that is characteristic of Acme threads results in a relatively large lead-angle correction to be applied in wire measurements of pitch diameter of such threads. In the case of multiple-start threads the geometry is such that it is no longer feasible to make the usual simplifying assumptions as to the positions of contact of the wire the thread. Accordingly, in this appendix measurement of single-start threads (with lead angles generally less than 5 deg.) are treated as they were in the 1950 Supplement to H28 (1944), whereas for threads having led angles greater than 5 deg. the necessary refinements in the calculations are presented.

(a) SINGLE START EXTERNAL THREADS

The general formula[3] is:

[Epsilon] = MÚw¿ + cot [alpha] - w (1 + cosec [alpha]') (1)

ÄÄÄÄÄÄÄÄÄÄÄ 2n

in which

[Epsilon] = pitch diameter, MÚw¿ = measurement over wires, [alpha] = half-angle of thread, [n] = threads per inch = 1/pitch, w = wire diameter, [alpha]' = tanÀ -1 Ù (tan [alpha] cos [lambda]) [lambda] = lead angle at pitch diameter.

For a half-angle of 14 deg. 30', formula (1) takes the form

[Epsilon] = MÚw¿ + 1.933 357 - w(1 + cosec [alpha]') (2) ÄÄÄÄÄÄÄÄÄ 2n

The diameter, w, of the wires used should be as close as practicable tothe size that will contact the flanks of the thread at the pitch line, tominimize errors caused by deviations of the flank angle from nominal value.The best-size wire, to be applied only where the lead angle does not exceedapproximately 3 deg., may be taken as

wÚ5¿ = sec [alpha] = 0.516450 ÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄ 2n n (3)

for which values are tabulated in table A13.1.

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ[2] See Appendix 4, parts of which are applicable to this appendix.


For standard diameter-pitch combinations of Acme or Stud Acme threads, andif the best-size wire is used, the computations are simplified by the use oftable A13.2 or A13.3, thus

[Epsilon] = MÚw¿ - col. 7, (4)

or if [Epsilon] differs appreciably from the basic value given in column 3,

[Epsilon] = MÚw¿ - col. 7 - 100 (col. 3 - [Epsilon]Ú1¿) col. 8, (5)

where [Epsilon]Ú1¿ = MÚw¿ - col. 7

If the measured wire diameter, w', differs slightly (not more than 0.0003 in.) from the best size, w, shown in column 4

[Epsilon] = MÚw¿ - col. 7 - 5(w' - w) - 100(col. 3 - [Epsilon]Ú1¿)col 8. (6)

However, the correction derived from column 8 is seldom significant in amountfor standard diameter-pitch combinations.

Values of the term (1 + cosec [alpha]') are given in table A13.4 for use whenthreads of other than standard diameter-pitch combinations are to be measured.

Values for intermediate lead angles may be determined by determined byinterpolation. The three-wire measurement of Stub Acme threads corresponds to that of 29deg. Acme threads. However, because of the shallower root on the Stub, Acmethreads, no smaller wire than the best-size wire given in table A13.3 shall beused. There can be instances when the best-size wire will touch the threadroot. Hence, a check should always be made to ensure that the wires do nottouch the thread root.

(b) MULTIPLE-START EXTERNAL THREADS

Multiple-start threads commonly have lead angles greater than 5 deg. Inthose exceptional cases that have smaller lead angles the procedures described above may be applied.


For larger lead angles there are two procedures available that give almost identical results; that is the discrepancy between the values obtained for the lead angle correction, c, is well within the possible observational error in making the measurement of pitch diameter. The methods are those of Marriner and Wood [26],À4Ù based on the analytical approach of Gary [22] and of Vogel [21]. It is necessary to determine the best-wire size for the individual thread, as this size is dependent on the lead angle of the thread. This determination is simplified by extracting from table A13.5 the wire diameter (interpolating if necessary) for a 1-in. axial pitch screw and dividing by the threads per inch [15]. Thus,

w = wÚ1¿ / n (7)

The pitch diameter is given by formulas, as follows:

[Epsilon] = MÚw¿ - (C+c) (8)

where

MÚw¿ = measurement over wires C = w(1 + cosec [alpha]) - (cot [alpha]) / 2n = 4.993929 w - 1.933357 / n (9)

c = 2(OP-OQ) of figure A13.1 (10)

Tabular values for (C+c)Ú1¿ for a 1-in. axial pitch screw are also given intable A13.5 and references [15] and [21], which should be divided by the threads per inch for a given case. In figureA13.1 the actual points of contact of the wire with the thread flanks are at A and B. Under certain conditions a

wire may contact one flank at two points, in which case it is advisable touse a ball, equal in diameter to the wire. The value of c is the same for aball as for a wire. the conditions determining single or double contact aredealt with below.

FED-STD-H28 31 March 1978To evaluate c

OP = [gamma] cos [alpha] cos [beta] +

ÚÄÄ ÄÄ¿ w ³ 1 ³ Ä ³ ÄÄÄÄÄ ³ 2 ³ 2[pi] sin [beta] + [gamma] sin [alpha] cos [beta]³ ÀÄÄ ÄÄÙ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÚÄ Ä¿ ³ ³ 1 ³ ³ ³ ÄÄÄÄÄ ³ ÄÄ¿³ [gamma]À3Ù + ³ 2[pi] ³À2Ù ³³ ÀÄ ÄÙ ÀÙ (11)

w Ä OQ = R + 2 cosec [alpha]

[gamma] = distance from contact point Alpha to a point L on the thread axis,measured parallel to an element of the thread flank, in the axial planecontaining LA.

[beta] = (designated the "key angle" by Vogel) angle in a plane perpendicularto the thread axis between lines connecting the point O on the thread axis,to the axis of the wire (or center of the ball) and to the point of contractof the wire and thread flank, respectively.

The values of [beta] and [gamma] are determined by the following equations: ÚÄÄ ÄÄ¿ w ³ 1 cos [beta] ³ Ä ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³sin [beta] = 2 ³ 2[pi][lambda] cos [alpha] - tan [alpha] sin [beta] ³ ÀÄÄ ÄÄÙ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÚÄ Ä¿ ³ ³ 1 ³ ³ ³ ÄÄÄÄÄ ³ ÄÄ¿³ [gamma]À2Ù + ³ 2[pi] ³À2Ù ³³ ÀÄ ÄÙ ÀÙ

ÚÄÄ ÄÄ¿ (13) ³ w ³ ³ Ä ³[gamma] = R + ³ 2 [gamma] cot [alpha] ³ + l[beta] (14) ÀÄÄ ÄÄÙ ÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄ cos[alpha] ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 2[pi] sin [alpha] ³ ÚÄ 1 Ä¿ ³ ³ ÄÄÄÄÄ ³ ÄÄ¿³[gamma]À2Ù + ³ 2[pi] ³À2Ù ÀÙ ÀÄ ÄÙ

These are simultaneous equations in beta and gamma which cannot be solveddirectly but can be solved by iteration. Letting [beta]=0, the firstapproximation for [gamma] is

[gamma]Ú0¿ = R sec [alpha] + w cot [alpha] Ä 2 (15)

83


This approximate value of [gamma] is entered in the right-hand side of eq (13) to obtain a new value of [beta]=[beta]Ú1¿. Then this new value of [beta] is entered in the right-hand side of eq (14), together with the first approximation of [gamma] to obtain a new value of [gamma]=[gamma]Ú1¿. Then [gamma]Ú1¿ and [beta]Ú1¿ are entered in eq (13) to obtain a new [beta]=[beta]Ú2¿. This process is repeated until the values of [beta] and [gamma] repeat themselves to the required degree of accuracy. Their final values are then entered in eq (11) and (12) to obtain the lead angle correction given byeq.(10).The following calculation exemplifies the process, and the result may becompared with that obtained for the same example by the Vogel method [21] orthe Van Keuren method utilizing tables [15,21].1 1/8"- 5, 4 start 29 deg. Acme screw thread [Epsilon] = 1.025, nominal, l = 0.800, [rho] = 0.200, [lambda] = 13.951927 deg., w = 0.10020 (from table A13.5 p.57,[15,21], [alpha] = 14.5deg. sin [alpha] = 0.25038 00041, cos [alpha] = .96814 76404, tan [alpha] = .25861 75844, cot [alpha] = 3.86671 30949, sec [alpha] = 3.99392 91629, 1/[pi] = .31830 98862, R = .31916 43455, l/2[pi] = .12732 39545, (l/2[pi])À2Ù = .01621 13939,l/(2[pi] sin [alpha]) = .50852 28550, l/(2 cos [alpha]) = .13151 29523, R/cos [alpha] = .32966 49520, gammaÚ0¿ = .27393 42429.


ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ³ ³ ³Ä¿³ [gamma]À2Ù + ³ ³ ³ ³ ÀÙ (1/2w)À2Ù ³ sin [beta] ³[beta] (radians)³ cos [beta] ³ [gamma] ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄ 0.53665.168 ³0.02337 088 ³ 0.2337 301 ³0.99972 686 ³0.52978 325 .54486 854 ³ .02226 331 ³ .02226 515 ³ .99975 214 ³ .52934 621 .54444 357 ³ .02232 647 ³ .02232 833 ³ .99975 073 ³ .52936 984 .54446 685 ³ .02232 298 ³ .02232 483 ³ .99975 081 ³ .52936 853 .54446 526 ³ .02232 317 ³ .02232 802 ³ .99975 081 ³ .52936 860 .54446 5336 ³ .02232 3160³ .02232 8012 ³ .99975 0807³ .52936 8598 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

OP = 0.52483 3962 OQ = 0.31926 0196 c = 0.01114 8 2 OP = 1.04966 79 = nominal measurement between centers of wires MÚw¿ = 2 OP + w = 1.149 868 in. = nominal measurement over wires MÚw¿ = 1.149 868 = actual measurement over wires. [Epsilon] = 1.149 868 - (C + c). (see equations 8 and 9) C = 4.993 929 X 0.100 20 - 1.933 35775 = 0.113 720 C+c = 0.113 720 + 0.011 148 = 0.124 868 [Epsilon] = 1.149 866 - 0.124 808 = 1.023 000 (as measured)

If instead of the Marriner and Wood equations those of Vogel are applied wehave

[sigma] - [beta] = cotÀ2Ù [gamma]

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ tan [alpha] ÄÄÄÄÄÄÄÄÄÄ cot [beta] - tan [gamma] (16)where

[sigma] = [pi] ÄÄÄÄ 2NÚa¿

NÚa¿ = number of starts[lambda] = lead angle at pitch line [alpha] = half angle of thread in axial plane.

This equation may likewise be solved for [beta] by iteration, but variousshort cuts are presented in reference [21], including a short, highly accurate, and nontranscendent formula for [beta]. The value of [beta] in the above example which satisfies this equation is 0.02232 480 radian, as compared with 0.02232 501 obtained with the Marriner Wood formulas. The measurement to thecenter of the wires is given by the Vogel formula

2 OP = [Epsilon] tanÀ2Ù [lambda] ([sigma] - [beta]) cosec [beta] =

1.0496 522 in.,

which is 0.0000 157 smaller than the value (1.0496 679) obtained by theMarriner and Wood formulas. As this discrepancy is small compared with thepossible error in measurement of Mu omega, either set of formulas is applicable. Also, the discrepancy between the value of (C+c) by the Marrinerand Wood formulas and that extracted from table A13.5 is only 0.000 018 in.

(c) LIMITATIONS ON THREE-WIRE MEASUREMENT OF EXTERNAL THREADS When the lead angle and diameter of a thread are such that double contractof the measuring wire occurs, it will be necessary to check the pitch diameter by means of balls rather than wires. For accurate measurement with wires single contact on each flank mustoccur. Measuring wires can be used if the following formula [26] is satisfied for a specific thread:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ 1 tan [alpha] > 1 ÄÄ¿³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Ä ³³ (R + w cos [alpha] cot [alpha])À2Ù - 4/DÀ2Ù (17) [pi] ³³ Ä ÀÙ 2

in which

[alpha] = half angle of thread in an axial plane l = lead R = distance from thread axis to sharp root (see fig. 13.1) w = diameter of measuring wires D = major diameter of thread

If best-size wires are used, so that contact is near the pitch line, thecondition for single contact simplifies to: ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄtan [alpha] > 2l ³ 1 - 1 ÄÄ Ä¿³ ÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄ [pi] ÀÙ [Epsilon]À2Ù DÀ2Ù (18)

On account of the approximate nature of the above formulas, double contactdoes not necessarily occur when these formulas are not satisfied. If not

satisfied the following formula can be used for a more precise determination:

D tan [alpha] - [gamma]ÚA¿ sin [alpha] + l ([beta]ÚA¿ - [beta]ÚP¿) + Ä ÄÄÄÄÄ2 2[pi]

ÚÄÄ ÄÄ¿ ³ [gamma]ÚA¿ sin [alpha] sin [beta]ÚA¿ - l cos [beta]ÚA¿³ x ³ ÄÄÄÄÄ ³ ÀÄÄ 2[pi] ÄÄÙ

sec [beta]ÚP¿ sin ([beta]ÚA¿ - [beta]ÚP¿) > 0 (19)

in which,

[gama]ÚA¿ = final value for [gamma] in the correction calculation (0.52936 8598 would be the [gamma]ÚA¿ for sample calculation, the results of which are shown above.[beta]ÚA¿ = final value for beta in the correction calculation.[beta]ÚP¿ = cos À -1 Ù (2 [gamma]ÚA¿ cos [alpha] cos [beta]ÚA¿/D) and is a negative angle.

If the formula is satisfied, double contact does not occur.

2. BUTTRESS THREADS

Two optional procedures are used in determining the pitch diameter ofexternal threads, from the reading over the wires, MÚw¿. The comparator reading MÚw¿ over the wires is checked using a gage block or combination as amaster. Then, using the average diameter of the wires, w, the pitch diameter, [Epsilon], is computed using the formula

* [Epsilon] = MÚw¿ + [rho] - ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ tan[alpha]Ú1¿+tan[alpha]Ú2¿

ÚÄÄ ÄÄ¿ - w ³1+cosec([alpha]Ú1¿+[alpha]Ú2¿)cos([alpha]Ú1¿-[alpha]Ú2¿)³ - c ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÀÄÄ 2 2 ÄÄÙ (20)

when [alpha]Ú1¿ = 45 deg. and [alpha]Ú2¿= 7 deg., this formula reduces to

[Epsilon] = MÚw¿ + 0.89064 [rho] - 3.15689 w - c

In the optional method, a reading MÚrho¿ is taken over the wires placedover placed on either side of a plain cylindrical plug gage of known diameterD. Then, the distance T between the wires as seated in the threads of thethread plug is computed by formula

T = D - MÚD¿ + MÚw¿

and the formula for pitch diameter [Epsilon] becomes

[Epsilon] = MÚw¿ + [rho] - ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ tan[alpha]Ú1¿+tan[alpha]Ú2¿

ÚÄÄ ÄÄ¿ - w ³cosec([alpha]Ú1¿+[alpha]Ú2¿)cos([alpha]Ú1¿-[alpha]Ú2¿) - 1³ - c ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÀÄÄ 2 2 ÄÄÙ (21)

or [Epsilon] = T + 0.89064 [rho] - 1.15689 w - c

D should be slightly smaller then the major diameter of the thread plug gageto be measured. In both formulas 20 and 21, c is a correction depending on the angle thewires make with a plane perpendicular to the axis of the thread plug gage. For all possible single-start combinations of diameters and pitches listed intables XIV.2, XIV.3, and XIV.4, c is less than 0.0004 in. As Buttress threadsare designed to avoid metal-to-metal fits in all cases, it is not essentialthat the absolute value of the pitch diameter be accurately determined by

85


applying the correction c. Accordingly, it is recommended that the wire anglecorrection be neglected for these combinations and all other single-startbuttress thread plug gages.

However, if it is desired to take the lead-angle correction into account,the following formula to determine pitch diameter derived in reference [13] may be applied where the lead angle does not exceed 5 deg.:

[Epsilon] = MÚw¿ + [rho] cos [alpha]Ú1¿ cos [alpha]Ú2¿ - ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sin ([alpha]Ú1¿ + [alpha] Ú2¿) ÚÄÄÄ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ w ³1 + Ä¿³ (1 + tan À2Ù [lambda]) cot À2Ù [alpha] Ú1¿ + 1 - ³ ÀÙ ÀÄÄÄ ÚÄÄ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄcos[alpha]Ú1¿sin[alpha]Ú2¿ ³ Ä¿³(1 + tanÀ2Ù [lambda]) cotÀ2Ù [alpha]Ú1¿ + 1 -ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÀÙsin ([alpha]Ú1¿+[alpha]Ú2¿)ÀÄÄ ÄÄÄÄÄ¿ ÄÄÄ¿ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ³ Ä¿³ (1 + tan À2Ù [lambda]) cot À2Ù [alpha] Ú1¿ + 1 ³ ³ ÀÙ ³ ³ ÄÄÄÙ ³ ÄÄÄÄÄÙ (22)where

[alpha]Ú1¿ = flank angle of pressure flank,[alpha]Ú2¿ = flank angle of trailing flank, [lambda] = lead angle at pitch line.

For the 7 deg., 45deg. Buttress thread this formula becomes

[Epsilon] = MÚw¿ + 0.890643 [rho] -

ÚÄÄÄ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ w ³1 + Ä¿³ (66.330378 (1 + tan À2Ù [lambda] + 1 - ³ ÀÙ ÀÄÄÄ ÚÄÄ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 0.890643 ³ Ä¿³(66.330378 (1 + tan À2Ù [lambda] + 1 - ³ ÀÙ ÀÄÄ ÄÄÄÄÄ¿ ÄÄÄ¿ ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ³ Ä¿³ tan À2Ù [lambda] + 2 ³ ³ ÀÙ ³ ³ ÄÄÄÙ ³ ÄÄÄÄÄÙ (23)

For larger lead angles formulas may be applied that are derived in reference.

[22]. 1. WIRE SIZES.--In order to eliminate the effect of deviation of the threadform on the calculated pitch diameter, the "best size" wires, for symmetricalthreads, should contact the flanks of the thread at the pitch line. Becauseof the wide difference in the flank angles of a buttress thread it is impossible for the thread measuring wires to contact both flanks simultaneously at the pitch line.A deviation in the angle [alpha]Ú1¿ of the trailing flank has approximatelytwice the effect on the pitch diameter calculated from reading over wiresthan the same angle deviation on the pressure flank angle, [alpha]Ú2¿. Forthis reason it was decided that the diameter of the "best size" wire shouldbe such that it will contact the pressure flank at a point twice the distanceabove the pitch line that the contact point on the trailing flank is belowthe pitch line. This wire diameter for flank angle 7 deg., 45 deg. is given by wÚb¿ = 0.54147[rho]. (24)

As shown in figure A13.2, the "best" size wire will contact the pressureflank of a thread of basic form 0.1944[rho] below the thread crest, and thewire will project above the crest 0.1094[rho]. If this wire fails to projectabove the crest of the thread in an actual case, a larger wire, having adiameter of 0.61433[rho], which contacts the trailing flank at the pitch shouldbe used. The relation of the "best" and "max" size wire to the flanks andcrests of the 7 deg.,45 deg. Buttress thread is shown in figure A13.2. Thediameter of "best" and "max" wires and the protection above the crest of thethread are shown in table A13.6. Because of the small pressure flank angle of 7 deg. there may be doublecontact of the wire on this flank if the lead angle is more than a few degrees. Such double contact will introduce * an error into the measurement ofpitch diameter. Double contact is less likely with the "max" wire than withthe "best" wire, as the former contacts this flank nearer the thread crest. Therefore, it is desirable in such cases to check the pitch diameter measurement obtained with "best" wires by measurement with "max" wires also. With large lead angles a further check should be made using balls instead ofwires. Inconsistencies in results may indicate double contacts of wires. Ifdouble contact occurs with max wires it will be necessary to make pitchdiameter measurements by means of by means of balls. An alternative method for determining whether or not single contact occursis to apply the Marriner and Wood [26] formula 19, p. 59, for the exact condition for single contact.

TABLE A13.6 -- WIRE SIZES AND CONSTANTS, SINGLE-START BUTTRESS THREADS (7 DEGREES, 45 DEGREES)ÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄThreads³ ³ "Best" wire ³ ³ "Max" wire ³ per ³ Pitch, ³ diameter, ³ Projection, ³ diameter, ³ Projection, inch ³ [rho] ³w=0.54147[rho] ³ a=0.1094[rho] ³w=0.61433[rho] ³a'=0.2244[rho]ÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ in. ³ in. ³ in. ³ in. ³ in.20-----³0.05000 ³ 0.02707 ³ 0.0055 ³ 0.03072 ³ 0.0112 16-----³ .06250 ³ .03384 ³ .0068 ³ .03840 ³ .014012-----³ .06333 ³ .04512 ³ .0091 ³ .05119 ³ .018710-----³ .10000 ³ .05415 ³ .0109 ³ .06143 ³ .02248------³ .12500 ³ .06768 ³ .0137 ³ .07679 ³ .0280 ³ ³ ³ ³ ³6------³ .16667 ³ .09024 ³ .0182 ³ .10239 ³ .03745------³ .20000 ³ .10829 ³ .0219 ³ .12287 ³ .04494------³ .25000 ³ .13537 ³ .0274 ³ .15358 ³ .05613------³ .33333 ³ .18049 ³ .0304 ³ .20478 ³ .07472-1/2--³ .40000 ³ .21059 ³ .0438 ³ .24573 ³ .0898 ³ ³ ³ ³ ³2------³ .50000 ³ .27074 ³ .0547 ³ .30716 ³ .11221-1/2--³ .66667 ³ .36098 ³ .0729 ³ .40955 ³ .14961-1/4--³ .80000 ³ .43318 ³ .0875 ³ .49146 ³ .17951------³1.00000 ³ .54147 ³ .1094 ³ .61433 ³ .2244ÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

3. BIBLIOGRAPHY ON MEASUREMENT OF PITCH DIAMETER BY MEANS OF WIRES

[1] H.H. Jeffcott. Notes on screw threads, Proc. Inst. Mechanical Engineers, December 1907, 1067-1108. Reprinted in Collected Researches, The National Physical Laboratory, Vol.5, (1909).[2] O.Eppenstein, Die optische Messung des Flankendurchmessers von Gewinden. Prazision, No. 25, 1922.[3] Derivation of the general formula for pitch diameter measured by the three-wire method. NBS Letter Circular LC 23, Appendix 1,(1923).[4] E. A. Limming, Normal thickness of a worm thread. Machinery (London) 21, 360 and 759 (1922); 22, 114 (1923).[5] E. A. Limming, Cylinders and spheres for gauging worm or screw threads. Machinery (N.Y.), 30, 466 (1924).[6] G.A. Tomlinson, Correction for elastic compression in the measurement of screw with small cylinders. Machinery (London) 28,616 (1926).[7] G.A. Tomlinson, Correction for rake in screw thread measurement, Proc. Inst. Mechanical Engineers, December 1927,pp. 1031-1036.[8] G. Berndt, Die Messung konisher Gewinde. Neuss (1937).[9] Earle Buckingham, Pin method of measuring worms and helical gears, Machinery (N.Y.), 45,1 (1938).[10] G. Bernt, Die Bestimmung des Flankendurchmessers von Gewinden mit sym metrischem Profil nach der Deridrahtmethode, Zeit fur Instrumentenkunde. 59,439(Nov. 1939).

86


[11] N. Gunther and H. Zollner,Die Messung der Flankendurchmesser mehrgagiger Gewinde mit drei Drahten. Feinmech, und Praz.,47,129 (1939).[12] G. Berndt, Die Anlagekorrekturnen bei der Bestimmung des Flankendurchmessers von symmetrischen und unsymmetrischen Assen-und Innengewinden nach der Dreidrahtmethode oder mittels sweier Kugeln. Zeit. fur Instrumentenkunde, 60, 14, 141, 177, 209, 237, 272, (1940); Werkstattstechnik und Werksleiter, 34, 277 (1940).[13] D.E. Williamson, Effect of elastic modulus on measurement of thread wires, Production Engineering and Management,18, 1,(Aug. 1946).[14] Werner F. Vogel, The best wire for over wire measurement of general screws. The Van Keuren Co., Watertown, Mass. (Unpublished.)[15] H. L. Van Keuren, Tables for precise measurement of screws. Catalog and Handbook No. 34, The Van Keuren Co. (1948).[16] F. L. Litvin, Determination of the thickness of the teeth of worms and spiral-tooth wheels with the help of rollers and balls,(Unidentified Russian publication, July 6, 1950. Translation available at NBS and Van Keuren Co.).[17] M. Ogawa, precision screw threads. Report of the Institute of Industrial Science, Univ. of Tokyo, 2, No. 1, (April 1951).[18] Gauging and Measuring Screw Threads, Section VII, Formulae and correction for use in the measurement of the effective diameter of parallel screw plug gauges by means of small cylinders. National Physical Laboratory, 1951.[19] F. Wolf, Prufen und Messen von Gewinden. Munich (1952).[20] W. H. Harrison, Analysis of screw thread measurement. Machinist, 96, 602 (1952).[21] Werner F. Vogel, New thread measuring formulas. Catalog and Handbook No. 36, Appendix D, The Van Keuren Co. (1955).[22] M. Gary, Die Berechnung der Gewinde-Anlagekorrekturn. Physikalisch- Technischen Bundesanstalt, Braunschweig, 21,No. 4 (1955).[23] M. Gary, Geometrishe Probleme bei der Vermessung von zylindrischen Evolventen-Schnecken und Evolventen-Schragstrinradern. Konstruktion 8, no. 10,412 (1956).[24] E. C. Varnum and S. J. Johnson, Precise formulas for overpin measurements of helical forms, Barber-Colman Co. (1957).[25] L. W. Nickolos, Effect of differences between U.S. and British practices in measuring screw gages for Unified threads, Machinery (Lond.) 90, 723 (1957).[26] R. S. Marriner and Mrs. J. G. Wood, Rake correction in the measurement of parallel external and internal screw threads. Inst. of Mechanical Engineers, London, (July 1958).

87


APPENDIX A14 METRIC SCREW THREAD STANDARDS (SUPERSEDED BY FED-STD-00H28/21 (DLA-IS) DATED 31 MAY 1977)

88

CHANGE NOTICES ARE NOT CUMULATIVE AND FED-STD-H28SHALL BE RETAINED UNTIL SUCH TIME AS CHANGE NOTICE 1THE STANDARD IS REVISED 28 May 1986

FEDERAL STANDARD


The following changes, which form a part of FED-STD-H28 dated March 1978, areapproved by the Assistant Administrator, Office of Federal Supply and Services, General Services Administration, for the use of all Federal Agencies.

REMOVE: Supplement 1A dated 31 August 1978 REMOVE: Pages 1 through 6 of 31 March 1978 ADD: Pages 1 through 6 of 28 May 1986 (Page 6 reprinted without change)

REMOVE: Pages 13 through 34 of 31 March 1978 ADD: Page 13 - 34 inclusive of 28 May 1986



REMOVE: Pages 54 through 64 of 31 March 1978 ADD: Page 54 - 64 inclusive of 28 May 1986 (Page 64 reprinted without change) EXPLANATION: The purpose of this Change Notice is to incorporatesupersession data for Appendices already superseded and to update referencesto FED-STD-H28 sections and their related industry standards.

RETAIN THIS CHANGE NOTICE AND PLACE IT BEFORE THE FIRST PAGE OF THE STANDARD

MILITARY INTERESTS: CIVIL AGENCY COORDINATING ACTIVITY: GSA - 7FCE

Custodians: Army - AR PREPARING ACTIVITY Navy - AS DLA - IS Air Force - 11 (Project THDS-0062)

NO DELIVERABLE DATA REQUIRED BY THIS DOCUMENT AREA THDSDISTRIBUTION STATEMENT A. Approved for public release; distribution isunlimited.

FED-STD-H28 31 March 1978 CHANGE NOTICE 1 28 May 1986

1. PURPOSE AND SCOPE

1.1 Purpose.

This standard establishes complete dimensional data requirements for screwthreads including, as necessary, recommendations on gages, dies and taps andother items associated with the purchase/use of interchangeable threadedparts by Federal Government Agencies. So far as practicable, these data areintended to conform to generally accepted commercial practice, although certain special requirements of the Federal Services necessitate the inclusionof some standards not generally applicable outside the Government. References are cited throughout the text to the standards promulgated by The American Standards Association (ASA) and the United States of America StandardsInstitute (USASI), now called The American National Standards Institute (ANSI) and to such other published standards as are in agreement with the specifications herein.

1.2 Scope.

This standard includes the basic dimensional requirements for assembly,interchangeability, tensile strength, shear strength, fatigue strength andinstallation and removal torque of threaded products. Specific requirementsfor particular Screw-Thread Standards are covered by the Detail Standards(see Para 3). Supplements to the Detail Standards are covered in the Appendixof this document. (See Para 4).

2. APPLICATION


3. Detailed Standards for Screw-Threads are identified as:


FM-STD-H28/2 - Unified Inch Screw Threads - UN and UNR Thread Form

FED-STD-H28/3 - CANCELLED AND SUPERSEDED by FED-STD-H28/2


1Supersedes page 1 of 31 March 1978

FED-STD-H2831 March 1978CHANGE NOTICE 128 May 1986

FED-STD-H28/5 - Unified Miniature Screw Threads

FED-STD-H28/6 - Gages and Gaging for Unified Screw Threads - UN and UNR Thread Forms

FED-STD-H28/7 - Pipe Threads, General Purpose

FED-STD-H28/8 - Dryseal Pipe Threads

FED-STD-H28/9 - Gas Cylinder Valve Outlet and Inlet Threads

FED-STD-H28/10 - American National Hose Coupling and Fire-Hose Coupling Threads

FED-STD-H28/11 - Hose Connections For welding and cutting Equipment

FED-STD-H28/12 - Acme Threads

FED-STD-H28/13 - Stub Acme Threads

FED-STD-H28/14 - Buttress Screw Threads - 7 deg./45 deg. Flank Angles

FED-STD-H28/15 - Rolled Threads For Screw-Shells for Electric Lampholders and Lamp Bases

FED-STD-H28/16 - Microscope Objective and Nosepiece Threads, 0.800 - 36AMO

FED-STD-H28/17 - Surveying Instrument Mounting Threads

FED-STD-H28/18 - Photographic Equipment Threads

FED-STD-H28/19 - Miscellaneous Threads

FED-STD-H28/20 - Inspection Methods For Acceptability of UN, UNR, UNJ, M and MJ Screw-Threads

FED-STD-H28/21 - Metric Screw-Threads

FED-STD-H28/22 - Metric Screw-Thread Gages

4. APPENDIX - The following supplements to the detailed standards are included in this document.

A1 - American National Form of Thread Series for Bolts, Machine Screws, Nuts, Tapped Holes and General Applications (Inactive for new design)



A-2 - American National Screw Threads of Special Diameters, Pitches, and Lengths of Engagement (Deleted from 1969 issue of Handbook H28)

A-3 - Recommended Hole Size Limits Before Threading and Tap Drill Sizes (Superseded by FED-STD-H28/2)

A-4 - Methods of Wire Measurements of Pitch Diameter of 60deg. Threads (Superseded by FED-STD-H28/6, /7, /8)

A-5 - Design of Special Threads (Superseded by FED-STD-H28/2)

A-6 - References (Deleted from 1969 issue of Handbook H28)

A-7 - Supplementary Pipe Threads Information (Superseded by FED-STD-H28 /7,/8)

A-8 - Geometry of Taper Screw Threads

A-9 - Extent of Usage of the American National Fire-Hose Coupling Threads on Couplings and Nipples used with 2 1/2 inch nominal Size Fire-Hose

A-10 - Wrench Openings

A-11 - Class 5 Interference - Fit Threads

A-12 - The Tightening of Threaded Fasteners to Proper Tension

A-13 - Three Wire method of measurement of Pitch Diameter of 29 deg. Acme, 29 deg. Stub Acme, and Buttress Threads

A-14 - Metric Screw Thread Standards (Superseded by FED-STD-00H28/21 (DLA-IS) See FED-STD-H28/21)


FED-STD-H28/1 - ANSI/ASME B1.7M


FED-STD-H28/4 - ANSI B1.15 (a)


FED-STD-H28/6 - ANSI/AMSE B1.2



FED-STD-H28/7 - ANSI/ASME B1.20.1

FED-STD-H28/8 - ANSI B1.20.3/B1.20.5

FED-STD-H28/9 - ANSI B57.1/CGA Std V-1

FED-STD-H28/10 - ANSI/ASME B1.20.7 and NFPA 1963

FED-STD-H28/11 - CGA E-1






FED-STD-H28/17 -

FED-STD-H28/18 - ANSI PH3.101, 102, 103, 107 and ANSI PH22.76

FED-STD-H28/19 -


FED-STD-H28/21 - ANSI/ASME B1.13M and ANSI B1.21M

FED-STD-H28/22 - ANSI/ASME B1.16M, B1.22M

Appendix A10- ANSI B18.2.1, B18.2.2, B18.6.3

Appendix All- ANSI B1.12

Appendix A13- ANSI B1.5, B1.8, B1.9

(a) Not issued as of 28 May 1986

4

Supersedes page 4 of 31 March 1978


ANSI, USASI, ASA PublicationsÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄAmerican National Standards Institute 1430 Broadway New York, NY 10018 NFPA Publications ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ National Fire Protection AssociationBattery March ParkQuincy, MN 02269 CGA PublicationsÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄCompressed Gas Association, Inc.1235 Jefferson Davis HighwayArlington, VA 22202

ANSI/ASME, ASME PublicationsÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄAmerican Society of Mechanical EngineersOrder Department 22 Law Drive, Box 2300 Fairfield, NJ 07007-2300

5

supersedes page 5 of 31 March 1978

FED-STD-H28 31 March 1978APPENDIX A1

American National Form of Thread and Thread Series for Bolts, MachineScrews, Nuts, Tapped Holes and General Applications.

Since the American National threads have been superseded by the Unifiedthreads, most of appendix 1, as shown in the previous (1957) issue of Part I,has bow deleted. Shown herein is data on the class 3 internal threads in theCoarse Thread Series in nominal size from 0.25 to one inch as there is stilla need for this information. Data shown is from tables 1.2, 1.8, 1.16, and 1.17 of the 1957 issue. (Appendix number and table numbers now preceded by an A.)

6

FED-STD-H28 31 March 1978 NOTICE 1 28 May 1986 APPENDIX A3

Tap Drill Sizes for Unified Screw Threads and

Recommended Hole Size Limits Before Threading

(SUPERSEDED BY FED-STD-H28/2)

13 - 34 inclusive

Supersedes pages 13 - 34 of 31 March 1978


APPENDIX A4

Methods of Wire Measurements of Pitch Diameter of 60 deg. Threads

(SUPERSEDED BY FED-STD-H28/6,/7,/8)

35 - 44 inclusive



APPENDIX A5

Design of Special Threads


45 - 52 inclusive



APPENDIX A7

Supplementary Pipe Thread Information

(SUPERSEDED BY FED-STD-H28/7,/8)

54 - 63 inclusive



APPENDIX A8

Geometry of Taper Screw Threads.

1. INTRODUCTION

This appendix present several geometrical relationships relative to theconical spiral, which is the curve of generation of the taper screw thread,and also briefly discusses the conical helix. With reference to these curves,the formulas include the parametric equations, the projection, the development, the lead angle, and length of an arc. The geometry of taper screw threads has, in practice, develop by modification of the geometry of straight screw threads, with the result that formula commonly used for taper screw threads are often approximations instead of being exact. That such approximations have been satisfactory in practice arises from the fact that the angle of taper or cone angle, of standard taper threads has been small. The recent use of larger taper angle together with the higher precision of measurement of screw thread gages now demanded, sometimes requires the availability of exact, or more nearly exact, or more nearly exact, formulas to be substituted for the approximate formulas or used to determine the magnitude of errors introduced by the usual approximations. It is convenient to approach the subject by considering the nature of thecurves of generation of straight screw threads and of taper screw threads,respectively, namely the cylindrical helix and the conical spiral. A cylindrical helix may be defined in various ways. First it is a curve onthe surface of a circular cylinder which cuts the elements of the cylinder ata constant angle. The same curve may also be defended as the curve generatedby a point moving at a uniform rate along a straight line while the linerevolves uniformly about an axis parallel to itself, so that successiveintersections of the curve and an element of the cylinder are equally spaced.These definitions establish the fact that the cylindrical helix is bothloxodromic and isometric. There is no corresponding curve on the surface of a cone which simultaneously answers to both methods of generation. Thus there are two different spiral-shaped curves lying on the surface of a cons which are analogous to the circular helix, one of which is loxodromic and the other isometric. Mathematicians have agreed [6]À12Ù that the loxodromic curve corresponds to thedefinition of a general helix, and that it should properly be termed a conical helix. The isometric curve has been called the conical spiral. Loria [6] gives a brief history of this curve, starting that it is found in a workby B. Pascal and citing several 18th century references, one of which pointsout that the curve was known to ancient Greek geometricians. Thus there arethe following definitions. A conical spiral is generated if a point travels on the surface of a rightcircular cone so as to combine a uniform angular motion around the axis ofthe cone with a uniform linear motion along a generator toward or from thevertex. It is characterized by uniformity of pitch, that is, successiveinteractions of the curve and an element of the cone are equally spaced, and by the fact that it passes through the vertex of the cone. The conicalspiral occurs in such mechanical applications as the taper screw thread, thespiral bevel gear [11], and the conical spring [12]. A conical helix is generated if a point travels on the surface of a rightcircular some in such a way that the curve produce interests the elements ofthe cone at a constant angle. The pitch of this curve varies from point toand it approaches the vertex of the cone as an asymptote. It is applied mechanically in the conical spring [1,7,9] as sometimes made. (For conicalsprings with coils of constant slope it is desirable that the projection ofthe neutral axis of the spring be an Archimedes spiral. Such a spring is nottruly conical but is wound on a paraboloid of revolution.)[10]

The cylindrical helix is the curve of intersection of a helicoid and acoaxial cylinder, and the conical spiral is the intersection of a helicoid and a coaxial cone. According, although the geometry of the conical spiral differs from that of the helix, there is but one geometry of helicoids. A screw helicoid, for example, remains a screw helicoid, whether the ends of its generatrix are determined by coaxial cylinders, as in straight screw threads or by coaxial cones, as in taper screw threads. These different boundary conditions give rise, however, to certain different geometrical relations.

3. PARAMETRIC EQUATIONS OF THE CONICAL SPIRAL; THE PROJECTION, DEVELOPMENT,LEAD ANGLE, AND LENGTH OF AN ARC

The parametric equations of the conical spiral, with the vertex of thecone at the origin and the axis of the cone coinciding with the z-axis, asshown if figure A8.1, are: ÄÄÄÄÄÄÄÄÄÄ¿x = L [theta] tan [alpha] cos [theta] ³ ÄÄÄ ³ 2[pi] ³ ³y = L [theta] tan [alpha] sin [theta] ³ (1) ÄÄÄ ³ 2[pi] ³ ³z = L [theta] Ú1¿ ³ ÄÄÄ ³ 2[pi] ÄÄÄÄÄÄÄÄÄÙ

where

[alpha] = 1/2 included angle between opposite elements of the cone,

[theta] = the variable parameter, and is the angle which the projection of the radius vector of the point on the conical spiral makes with the x-axis on the xy-plane.

L = lead of spiral, or advances, parallel to the axis in one revolution.

The length, r, of the radius vector at any point on the conical spiral isgiven by the relation:

r = L [theta] sec [alpha]. (2) ÄÄÄ 2[pi]ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ[12] Numbers in brackets refer to similarity membered items in the bibliographyat the end of the appendix.

64

CHANGE NOTICES ARE NOT CUMULATIVE AND FED-STD-H28SHALL BE RETAINED UNTIL SUCH TIME AS CHANGE NOTICE 2THE STANDARD IS REVISED. 29 January 1989

FEDERAL STANDARD


The following changes, which form a part of FED-STD-H28 dated 31 March 1978,are approved by the Commissioner, Federal Supply Service, General ServicesAdministration, for the use of all Federal Agencies.

REMOVE: Pages 1 through 4 of 28 May 1986 ADD: Pages 1 through 4 of 20 January 1989 (Page 1 reprinted without change)

REMOVE: Pages 71 through 76 of 31 March 1978 ADD: Pages 71 and 72 through 76 inclusive of 20 January 1989


MILITARY INTERESTS: CIVIL AGENCY COORDINATING ACTIVITIES:

Custodians: DOT - ACO GSA - 7FXE Army - AR NASA - JFK Navy - AS Air Force - 99 PREPARING ACTIVITY:

Review Activities: DLA - IS

Army - AT, AV, ME, MI (DoD Project THDS-0071 Navy - EC Air Force - 15

User Activities:

Army - CR, ER

ÚÄÄÄÄÄÄ¿AMSC N/A ³ THDS ³ ÀÄÄÄÄÄÄÙ DISTRIBUTION STATEMENT A. Approved for public release; distribution isunlimited.

FED-STD-H2831 March 1978CHANGE NOTICE 2


FED-STD-H28/6 - Gages and Gaging for Unified Screw Threads - UN and UNR Thread Forms FED-STD-H28/7 - Pipe Threads, General Purpose




FED-STD-H28/11 - Hose Connections For Welding and Cutting Equipment



FED-STD-H28/14 - Buttress Screw Threads - 7deg./45deg. Flank Angles

FED-STD-H28/15 - Threads For Screw-Shells for Electric Lampholders and Lamp Bases






FED-STD-H28/22 - Metric Screw Thread Gages

FED-STD-H28/23 - Class 5 Interference-Fit Screw Threads

4. APPENDIX - The following supplement to the detailed standards areincluded in this document.

A1 - American National Form of Thread Series for Bolts, Machine Screws, Nuts,Tapped Holes and General Applications (Inactive for new design)

2Supersedes page 2 of 28 May 1986

FED-STD-H28 31 March 1978 CHANGE NOTICE 2

A2 - American National Screw Threads of Special Diameters, Pitches, and Lengths of Engagement (Deleted from 1969 issue of Handbook H28)

A3 - Recommended Hole Size Limits Before Threading and Tap Drill Sizes (Superseded by FED-STD-H28/2)

A4 - Methods of Wire Measurements of Pitch Diameter of 60deg. Threads (Superseded By FED-STD-H28/6,/7,/8)

A5 - Design of Special Threads (Superseded by FED-STD-H28/2)


A7 - Supplementary Pipe Threads Information (Superseded by FED-STD-H28/7,/8)


A9 - Extent of Usage of the American National Fire-Hose Coupling Threads on Couplings and Nipples used with 2 frac12 inch nominal Size Fire-Hose

A10 - Wrench Openings (Superseded by ANSI B18.2.1, ASME/ANSI B18.2.2 ANSI B18.6.2, ANSI B18.6.3, ANSI B18.6.4)

A11 - 5 Interference-Fit Threads (Superseded by Fed-STD-H28/23 A12 - The Tightening of Threaded Fasteners to Proper Tension

A13 - Three Wire Method of Measurement of Pitch Diameter of 29deg. Acme, 29deg. Stub Acme, and Buttress Threads

A14 - Metric Screw Thread Standards (Superseded by FED-STD-00H28/21 (DLA-IS) - See FED-STD-H28/21)




FED-STD-H28/4 - ANSI B1. 15 (a)


FED-STD-H28/6 - ANSI/AMSE B1.2

3

Supersedes page 3 of 28 May 1986



FED-STD-H28/8 - ANSI B1.20.3/B1.20.5

FED-STD-H28/9 - ANSI B57.1/CGA Std V-1

FED-STD-H28/10 - ANSI/ASME B1.20.7 and NFPA 1963







FED-STD-H28/17 -


FED-STD-H28/19 -


FED-STD-H28/21 - ANSI/ASME B1.13M ad and B1.12M


FED-STD-H28/23 - ASME/ANSI B1.12

APPENDIX A13 - ANSI B1.5, B1.8 B1.9

(a) Not issued as of

4



APPENDIX A10

Wrench Openings

[SUPERSEDED by appendices in the following American National Standards:

ANSI B18.2.1 Square and Hex Bolts and Screws, Inch Series

ANSI/ANSI B18.2.2 Square and Hex Nuts (Inch Series)

ANSI B18.6.2 Slotted Head Cap Screws, Square Head Set Screws and Slotted Headless Set Screws

ANSI B18.6.3 Machine Screws and Machine Screw Nuts

ANSI B18.6.4 Thread Forming and Thread Cutting Tapping Screws and Metallic Drive Screws (Inch Series )]

71



APPENDIX A11

Class 5 interference-Fit Threads

(SUPERSEDED BY FED-H28/23)

72 - 76 inclusive


CHANGE NOTICES ARE NOT CUMULATIVE AND FED-STD-H28SHALL BE RETAINED UNTIL SUCH TIME AS CHANGE NOTICE 3THE STANDARD IS REVISED. 12 March 1990

FEDERAL STANDARD


The follow changes, which form a part of FED-STD-H28 dated 31 March 1978, areapproved by the Commissioner, Federal Supply Service, General ServicesAdministration, for the use of all Federal Agencies.

REMOVE: Pages 3, 4 of Change Notice 2, 20 January 1989 ADD: Pages 3, 4 of Change Notice 3, 12 March 1990

REMOVE: Pages 77-88 of 31 March 1978 ADD: Pages 77-79 of Change Notice 3, 12 March 1990 Pages 80-67 of Change Notice 3, 12 March 1990 Page 88 of Change Notice 3, 12 March 1990

NOTE: A reprint of pages 1 and 2 is included with this Notice to correct omission of page 1 in Change Notice 2.


MILITARY INTERESTS: CIVIL AGENCY COORDINATING ACTIVITIES:

Custodians: DOT - ACO GSA - 7FXE Army - AR NASA - JFK Navy - AS Air Force - 99 PREPARING ACTIVITY:

Review Activities: DLA - IS

Army - AT, AV, CR, ME,, MI (DoD Project THDS-0079) Navy - EC Air Force - 15

User Activities:

Army - ER

AMSC N/A THDS

DISTRIBUTION STATEMENT A. Approved for public release; distribution isunlimited.


1. PURPOSE AND SCOPE

1.1 Purpose.

This standard establishes complete dimensional data requirements for screwthreads including, as necessary, recommendations an gages, dies and taps andother items associated with the purchase/use of interchangeable threadedparts by Federal Government Agencies. So far as practicable these data areintended to conform to generally accepted commercial practice, although certain special requirements of the Federal Services necessitate the inclusion of some standards not generally applicable outside the Government. References are cited throughout the text to the standards promulgated by The American Standards Association (ASA) and the United States of America Standards Institute (USASI), now called The American National Standards Institute (ANSI) and to such other published standards as are in agreement with the specifications herein.

1.2 Scope.

This standard includes the basic dimensional requirements for assembly,interchangeability, tensile strength, shear strengths, fatigue strength andinstallation and removal torque of threaded products. Specific requirements for particular Screw-Thread Standards are covered by the Detail Standards (see Para 3). Supplements to the Detail Standards are covered in the Appendix of this document. (See Para 4).

2. APPLICATION


3. DETAILED STANDARDS. Detailed Standards for Screw-Threads are identified as:


FED-STD-H28/2 - Unified Inch Screw Threads - UN and UNR Thread Forms

FED-STD-H28/3 - CANCELLED AND SUPERSEDED by FED-STD-H28/2


1



FED-STD-H28/5 - Unified Miniature Screw Threads FED-STD-H28/6 - Gages and Gaging for Unified Screw Thread - UN and UNR Thread Form

FED-STD-H28/7 - Pipe Threads, General Purpose




FED-STD-H28/11 - Hose Connections for Welding and Cutting Equipment



FED-STD-H28/14 - Buttress Screw Threads - 7deg./45deg. Flank Angles

FED-STD-H28/15 - Rolled Threads For Screw-Shells For Electric Lampholders and Lamp Bases







FED-STD-H28/22 - Metric Screw-Thread

FED-STD-H28/23 - Class 5 Interference-Fit Screw Threads

4. APPENDIX - The following supplements to the detailed standards are included in this document.

A1 - American National Form of Thread Series for Bolts, Machine Screws, Nuts, Tapped Holes and General Applications (inactive for new design)

2


FED-STD-H28 31 March 1978 CHANGE NOTICE 3 12 March 1990

A2 - American National Screw Thread of Special Diameters, Pitches, and Lengths of Engagement (Deleted from 1969 issue of Handbook H28)

A3 - Recommended Hole Size Limits Before Threading and Tap Drill Sizes (Superseded by FED-STD-H28/2)

A4 - Methods of Wire Measurements of Pitch Diameter of 60 deg. Threads (Superseded By FED-STD-H28/6,/7,/8)

A5 - Design of Special Threads (Superseded by FED-STD-H28/2)


A7 - Supplementary Pipe Threads Information (Superseded by FED-STD-H28/7,/8)


A9 - Extent of Usage of the American National Fire-Hose Coupling Threads on Couplings and Nipples used with 2 frac12 inch Size Fire-Hose

A10 - Wrench Openings (Superseded by ANSI B18.2.1, ASME/ANSI B18.2.2, ANSI B18.6.2, ANSI B18.6.3, ANSI B18.6.4)

A11 - Class 5 Interference-Fit Threads (Superseded by FED-STD-H28/23)

A12 - The Tightening of Threaded Fasteners to Proper Tension (Superseded by MIL-HDBK-60)

A13 - Three Wire Method of Measurement of Pitch Diameter of 29 deg. Acme, 29 deg. Stub Acme, and Buttress Threads (Superseded by FED-STD-H28/12,/13, /14)

A14 - Metric Screw Thread Standards (Superseded by FED-STD-H28/21)



FED-STD-H28/2 - ASME B1.1

FED-STD-H28/4 - ASME B1.15 (a)


FED-STD-H28/6 - ANSI/ASME B1.2

(a) Not issued as of 12 March 1990

3

Supersedes page 3 of Change Notice 2, 20 January 1989

FED-STD-H2831 March 1978 CHANGE NOTICE 312 March 1990


FED-STD-H28/8 - ANSI B1.20.3/B1.20.5

FED-STD-H28/9 - ANSI/CSA/CGA V-1

FED-STD-H28/10 - ANSI/ASME B1.20.7 AND NFPA 1963







FED-STD-H28/17 -


FED-STD-H28/19 -


FED-STD-H28/21 - ANSI/ASME B1.13M and ANSI B1.21M



4

Supersedes page 4 of Change Notice 2, 20 January 1989


APPENDIX A12

The Tightening of Threaded Fasteners to Proper Tension

(SUPERSEDED BY MIL-HDBK-60)

77 - 79 inclusive


FED-STD-H2831 March 1978CHANGE NOTICE 312 March 1990

APPENDIX A13

Three-Wire Method of Measurement of Pitch Diameter of 29 deg. Acme, 29 deg. Stub Acme and Buttress Threads

(SUPERSEDED BY FED-STD-H28/12,/13,/14)

80 - 87 inclusive



APPENDIX A 14

Metric Screw Thread Standards


88


FED-STD-H28 SUPPLEMENT 1A 31 August 1978

FEDERAL STANDARD


This supplement form part of Federal Standard FED-STD-H28, dated 31 March1978.

DETAIL STANDARDS


FED-STD-H28/2 - Unified Thread Form and Thread Series for Bolts, Screws, Nuts, Tapped Holes and General Applications

FED-STD-H28/3 - Unified Threads-of Special Diameter, Pitches, and Lengths of Engagement



FED-STD-H28/6 - Gages and Gaging for Unified Screw Threads

FED-STD-H28/7 - American Standard Pipe Threads (except Dryseal and Hose Coupling Types)

FED-STD-H28/8 - Dryseal American Standard Pipe Threads

U.S. GOVERNMENT PRINTING OFFICE : 1979 - 281-173/2137 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄOrders for this publication are to be placed with General Services Administration, acting as an agent for the Superintendent of Documents. Single copies of this standard are available at the GSA Business Service Centers in Boston, New York, Philadelphia, Atlanta, Chicago, Kansas City, MO, Fort Worth, Houston, Denver, San Francisco, Los Angeles, and Seattle, WA, or from the General Services Administration, Specification and Consumer Information Distribution Branch, Bldg. 197, Washington Navy Yard, Washington, DC 20407.)

FSC THDS

FED-STD-H28SUPPLEMENT 1A31 August 1978



FED-STD-H28/11 - Hose Connections for Welding and Cutting Equipment



FED-STD-H28/14 - National Buttress Threads

FED-STD-H28/15 - American Standard Rolled Threads For Screw Shells of Electric Lamp Holders and For Screw Shells of Unassembled Lamp Bases





FED-STD-00H28/21 - Metric Screw-Threads

Military Custodians Preparing Activity

ARMY - AR DLA-ISNAVY - ASAIR FORCE - 11

Civil Agency Coordinating Activity

ACO FPI MSFAFS FRA NBSBPA FSS PCDFHW JFK RDSFIS LRC TCS