2-lectures lec 33

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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307MachineDesign I


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8-1 Thread Standards and Definitions

8-2 The Mechanics of Power Screws8-3 Strength Constraints8-4 Joints-Fasteners Stiffness8-5 Joints-Member Stiffness8-6 Bolt Strength

8-7 Tension Joints-The External Load8-8 Relating Bolt Torque to Bolt Tension8-9 Statically Loaded Tension Joint with Preload8-10 Gasketed Joints8-11 Fatigue Loading of Tension Joints

8-12 Shear Joints8-13 Setscrews8-14 Keys and Pins8-15 Stochastic Considerations

CH-8 LEC 33 Slide 2


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ME 307MachineDesign I Announcements

HW #5 Ch 18, on WebCT

Due Date for HW #5 is Mon. DEC. 31, 2007

Quiz on Ch. 18, Mon. DEC. 31, 2007

CH-8 LEC 33 Slide 3


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CH-8 LEC 33 Slide 4

8-1 Thread Standards and Definitions


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Introduction

The fundamental operation in manufacture is the creation of shape

- this includes assembly, where a number of components are

fastened or joined together either permanently by welding (Ch9)

for example or detachably(nonpermanent) by screws,nuts and

bolts and so on. Since there is such a variety of shapes in

engineering to be assembled, it is hardly surprising that there is

more variety in demountable fasteners than in any other machine

element. Fasteners based upon screw threads are the most

common, so it is important that their performance is understood,

and the limitations of the fastened assemblies appreciated.CH-8 LEC 33 Slide 5


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ME 307MachineDesign I Introduction

There are two distinct

uses for screw threads andthey usually demand

different behavior from

the threads :

a power screwsuch asa lathe leadscrew or the

screw in a car lifting jack

which transforms rotary

motion into substantial

linear motion (or viceversa in certain

applications), and

CH-8 LEC 33 Slide 6


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Fasteners

a Threaded Fastenersimilar to a nut and bolt which joins a

number of components together again by transforming rotarymotion into linear motion, though in this case the translation issmall.

CH-8 LEC 33 Slide 7


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Thread Profiles

Thread profiles.

a) Square (b) ACME; (c) UN, ISO

(c)

CH-8 LEC 33 Slide 8


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ME 307MachineDesign I Thread Profile Parameters

Lead=L=n p

Figure 8-1Terminology of

screw threads.Sharp Vee Threads

shown for clarity;the crests and roots

are actually

flattened or

rounded during the

forming operation

CH-8 LEC 33 Slide 9


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Threads

(a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws.

Text Reference: Figure 15.2, page 667 CH-8 LEC 33 Slide 10

ME 307


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ME 307MachineDesign I Thread Systems

A thread 'system' is a set of basic thread proportions which

is scaled to different screw sizes to define the thread

geometry. Whitworth, Sellers, British Standard Pipe (BSP)

are just three of the many systems which proliferated beforethe adoption of the

1. ISO Metric thread system.

2. The American National (Unified) Thread standards isused mainly in the US.

3. Square and ACME

CH-8 LEC 33 Slide 11

ME 307


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ME 307MachineDesign I Thread geometry

Thebasic profile of ISO Metric threadsis built upfrom contiguous equiangular triangles of height H disposed

symmetrically about a pitch line which becomes the pitchcylinder of diameter d

2when the profile is rotated about the

axis to form the thread. The distance between adjacent

triangles - the pitch - is p = 2 H /3. The tips of the trianglesare truncated by h/8 to form the major diameter ( size ) d of

the thread, and the bases are truncated by h/4 to form theminor diameter d1 . It follows that d1 = d - 5 h/4 = d - 1.08 p.

This leads to the rule of thumb for suitable tapping size


ME 307

Thread Profile For Metric System (M MJ)


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Thread Profile For Metric System (M, MJ)

pitch

minor

major

H= 0.5(3)1/2 p

US N= # threads/in

a ISO 68= aAmerican National (Unified) thread standard= 60CH-8 LEC 33 Slide 13

ME 307

h d S d d


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ME 307MachineDesign I Thread Standards

1) ISO Metric thread system: Table 8-1

Major diameter (mm),

2a= 60 Standard thread is RHSpecifications: e.g.: M12x1.75 orM = Basic Metric, J = round root; 12 = nominal major

diameter (mm); 1.75 = pitch (mm)

Tensile stress area At =p/4(dm+dr)/2 (see footnote T.8.1)

MJ12x1.75


ME 307


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ME 307

Th d St d d


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ME 307MachineDesign I Thread Standards

2) The American National (Unified)

Thread standards is used mainly in the

US: Table-8-2 (Size designation) use dUN=regular thread, UNR=round root (use

root radius)Specifications: 5/8-18 UN, UNC, UNF

UNR, UNRC, UNRF

5/8=d 18 = N (thread size)UN = Unified, F=fine, C=Coarse, R =Round Root


ME 307

Thread Standards


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MachineDesign I

Thread Standards

3. Square (a) and The ACME Threads (b)-usedmainly in power screws Table 8.3 gives preferred

pitches for ACME threads


ME 307


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MachineDesign I

Power screw thread forms. [Note: All threads shown are external (i.e., on thescrew, not on the nut); dm is the mean diameter of the thread contact and isapproximately equal to (d+ dr)/2.]


ME 307


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MachineDesign I


ME 307h ACME Th d P fil


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MachineDesign I ACME Thread Profile

Figure 15.5 Details of ACME thread profile. (All

dimensions are in inches.)

Text Reference: Figure 15.5, page 670 CH-8 LEC 33 Slide 20

ME 307M hi ACME Thread Properties


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MachineDesign I ACME Thread Properties

Crest diameter,d

c, in.

Number ofthreads per inch, a

n

Tensile stressarea, A

t, in2

Shear stressarea, A

s, in2

1/4

5/16

3/8

7/16

1/2

5/8

3/4

7/8

1

1 1/8

1 1/4

1 3/8

1 1/2

1 3/4

2

2 1/4

2 1/2

2 3/4

3

3 1/2

4

4 1/2

5

16

14

12

12

10

8

6

6

5

5

5

4

4

4

4

3

3

3

2

2

2

2

2

0.02663

0.04438

0.06589

0.09720

0.1225

0.1955

0.2732

0.4003

0.5175

0.6881

0.8831

1.030

1.266

1.811

2.454

2.982

3.802

4.711

5.181

7.338

9.985

12.972

16.351

0.3355

0.4344

0.5276

0.6396

0.7278

0.9180

1.084

1.313

1.493

1.722

1.952

2.110

2.341

2.803

3.262

3.610

4.075

4.538

4.757

5.700

6.640

7.577

8.511

Crest diameters, threads per inch, and stresses for Acme thread.

N


ME 307Machine

Mechanics of Power Screws


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MachineDesign I Mechanics of Power Screws

A power screw is a device thatis common to tools or

machinery that are used to

change angular motion intotranslation. It is also capable of

developing a large amount of

mechanical advantage. Familiar

applications include clamps or

vises, presses, lathes lead

screws, and jacks.

The joyce

warm-gear

screw jack


ME 307Machine Mechanics of Power Screws


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MachineDesign I Mechanics of Power Screws

Weight supported by three screw jacks. In each screw jack, only theshaded member rotates.


ME 307Machine Mechanics of Power Screws


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MachineDesign I

Example of catalogue:

http://www.roton.com/web/quick.power_screws.jsp

Mechanics of Power Screws


ME 307Machine Vehicle Screw Jack


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MachineDesign I

Vehicle Screw Jack


ME 307Machine


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Design I


ME 307Machine

Power Screw with collar


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Design I

Different types of collars

Power Screw with collar


2-lectures lec 33

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