me 312 mechanical machine design [screws, bolts, nuts]

05/01/2023Spring 312 (c) Bilal Siddiqui 2016

1

ME 312 – Machine DesignDr. Bilal A. Siddiqui,Asst. Prof (ME)


2 Screw, Fastners and Non-permanent Joints

The helical thread screw is one the most important mechanical device. It forms the basis of

Power screws: change angular motion to linear motion (presses, jacks, etc.) threaded fasteners for non-permanent joints.

Typical methods for non-permanently joining parts: Bolts Nuts Screws Rivets retainers, locking devices, pins, keys, adhesives etc.


3 Can something like a nut and screw be important?

Fasteners will always be need to facilitate disassembly for maintenance etc. One of the key targets of current design for manufacture is to reduce the number of

fasteners. Boeing’s 747 airliner requires 2.5 million fasteners, some of which cost several

dollars each. To keep costs down manufacturers constantly review new fastener designs,

installation techniques, and tooling. Design and Manufacture of fasteners is a huge industry globally Methods of joining parts are extremely important in the engineering of a quality

design


4 Nomenclature

Pitch is the distance between adjacent thread turns parallel to thread axis. The pitch in B.E. units is the reciprocal of the number of thread turns per inch (N).

The major diameter d is the largest diameter of a screw thread. The minor (or root) diameter dr is the smallest diameter of a screw thread. Pitch diameter dp is a theoretical diameter between major & minor diameters. The lead l, not shown, is the distance the nut moves parallel to screw axis when the nut

is given one turn. For a single thread, the lead is the same as the pitch. A multiple-threaded screw is one having two or more threads cut beside each other. Most screws, bolts, and nuts all have single threads A double-threaded screw has a lead equal to twice the pitch If the bolt is turned clockwise, the bolt advances toward the nut. I.e. it closes. The thread angle is 60◦ and the crests of the thread may be either flat or rounded.

Sharp vee threads shown for clarity; the crests and roots are actually flattened or rounded during the forming operation.

5 Unified classes of threads

The UN (unified common) class of screw is the most commonly used series in common applications. It is The UNRF (unified reduced fatigue) profile has a rounded fillet at the root of the thread. Due to reduced

stress concentration, they are used where high fatigue strength is required. Unified (UN and UNRF) threads are specified by the nominal major diameter, no of threads per inch, and

thread series, for example, 0.58 in-18 UNRF or 0.625 in-18 UNRF. Metric threads are specified by writing the diameter and pitch in millimeters M12 × 1.75 is a thread having a nominal major diameter of 12 mm and a pitch of 1.75 mm. [Note that

the letter M here means it is metric unit designation] Many tensile tests of threaded rods have shown that an unthreaded rod having a diameter equal to the

mean of the pitch diameter and minor diameter will have the same tensile strength as the threaded rod. The area of this unthreaded rod is called the tensile-stress area At of the threaded rod. See tables 8-1 and 8-2



6

Table 8-1 Table 8-2


7 Thread for Power Screws Square and Acme threads are used on screws when power is to

be transmitted. Preferred pitches are listed. However, other pitches can be and

often are used, since the need for a standard for such threads is not great.


8Threaded Fasteners – Bolts and Screws

Screws, nuts and bolts Bolts are for the assembly of two unthreaded components, with the aid of a nut.

Screws in contrast are used with components, at least one of which contains its own internal thread

The purpose of a bolt is to clamp two or more parts together. The clamping load stretches or elongates the bolt; the load is obtained by twisting

the nut until the bolt has elongated almost to the elastic limit. hold the bolt head stationary and twist the nut; in this way the bolt shank will not

feel the thread-friction torque. Points of stress concentration are at the fillet, at the start of the threads (runout),

and at the thread-root fillet in the plane of the nut ( when present) The diameter of the washer face is the same as the width W of hexagon.


9 Threaded Fasteners - Nuts

In applications where vibration or rotation may work a nut loose, various locking mechanisms may be employed: Adhesives, safety pins or lockwire, nylon inserts, or slightly oval-shaped threads.

A rule of thumb is that preloads of 60 %of proof load rarely loosen. Sometimes we use two nuts to prevent loosening. In local jargon it is called “check-

nut”A car hub, with the central nut hidden behind a castellated nut cover that is locked against rotation using a cotter pin. The effect is similar to using a castellated nut.

Spring 312 (c) Bilal Siddiqui 2016

10 Why Hexagonal? The most common shape for bolt-heads and nuts is hexagonal 6 sides give a good granularity of angles for a tool to approach from

(good in tight spots) More (and smaller) corners would be vulnerable to being rounded off. It takes only 1/6th of a rotation to obtain the next side of the hexagon

and grip is optimal. Polygons with more than 6 sides do not give the requisite grip and

polygons with less than 6 sides take more time to be given the next grip position.

Other specialized shapes exist for certain needs, such as wings for finger adjustment and captive nuts for inaccessible areas

05/01/2023


Threaded Fasteners - Washers The ideal bolt length is one in which only one or two threads project from

the nut after it is tightened. Bolt holes may have burrs or sharp edges after drilling. These could bite into the fillet and increase stress concentration. Therefore, washers must always be used under the bolt head to prevent

this. They should be of hardened steel and loaded onto the bolt so that the

rounded edge of the stamped hole faces the washer face of the bolt. Sometimes it is necessary to use washers under the nut too.

11

Plain (left) and split (right)washers Belleville or conical washer Curved disc spring washer Toothed lock washer with external teeth


12Threaded Fasteners - Gaskets A gasket is a mechanical seal which fills the space between two or more

mating surfaces, generally to prevent leakage from or into the joined objects while under compression.

Gaskets allow "less-than-perfect" mating surfaces on machine parts where they can fill irregularities.

Gaskets are commonly produced by cutting from sheet materials. It is usually desirable that the gasket is able to deform and tightly fills the

space it is designed for, including any slight irregularities. A few gaskets require an application of sealant directly to the gasket surface to function properly.

Gaskets are normally made paper, rubber, silicone, metal, cork, felt, neoprene, nitrile rubber, fiberglass, Teflon) or a plastic polymer.

Most industrial gasket applications involve bolts exerting compression >2000 psi

The more compressive load exerted on the gasket, the longer it will last!

Some seals and gaskets1. O-ring2. fiber washer3. paper gaskets4. cylinder head gasket


13 O-rings

An O-ring is a gasket in the shape of a torus made of an elastomer It is designed to be seated in a groove and compressed during assembly

between two or more parts, creating a seal at the interface. O-rings are one of the most common seals used in machine design

because they are inexpensive, easy to make, reliable, and have simple mounting requirements.

They can seal tens of MPa of pressure There are O-ring materials which can tolerate temperatures as low as -

200 C or as high as 250+ C.


14Washers and Gaskets

Section of cylindrical pressurevessel. Hexagon-head capscrews are used to fasten thecylinder head to the body.Note the use of an O-ring seal.

A bolted connection loaded intension by the forces P. Notethe use of two washers. Notehow the threads extend into thebody of the connection.

When a connection is desired that can be disassembled without destructive methodsand that is strong enough to resist external tensile loads, moment loads, and shearloads, or a combination of these, then the simple bolted joint using hardened-steelwashers is a good solution. Such a joint can also be dangerous unless it is properlydesigned and assembled by a trained mechanic.


15 Cap-screws: bolting without nut


16 Stud Screw – Alternate to not using nut A stud is a rod threaded on both ends. A stud is screwed into the lower member first then the top member is positioned and fastened down with hardened washers

and nuts. Studs are regarded as permanent Joint can be disassembled merely by removing the nut and washer Threaded part of lower member not damaged by reusing threads (unlike

screws)


17 Head studs

Sometimes, both ends are tightened with nuts. This joint is non-permanent. In engines, an important decision to choose b/w head bolt or head stud. Head bolts are also twisted during torqueing, which means not all torque

is converted into clamping force. Stud bolt is more efficient in torqueing. No torque lost in twisting. Head bolts more convenient for disassembly though

head studs are better suited for high-performance vehicles with greater power requirements, while head bolts are more practical for personal, everyday automobiles.


18 Grip of the Fastener The thread length of inch-series bolts, where d is the nominal diameter,

is

For metric bolts it is

The ideal bolt length is one in which only one or two threads project from the nut after it is tightened.

During tightening, the 1st thread of the nut tends to take the entire load But yielding occurs & load is eventually divided over about 3 nut

threads. The grip l of a connection is the total thickness of the clamped material. Due to a few threads taking the load,


19Fastener and Grip Length

Find H and t from tables A-31 and A-32


20 Nut and Washer Dimensions


21Stiffness of the Bolt

Twisting the nut stretches the bolt to produce the clamping force. This clamping force is called the pretension or bolt preload.

Since the members are being clamped together, the clamping force that produces tension in the bolt induces compression in the members.

The spring rate is the ratio between the force applied to the member and the deflection produced by that force. ()

Find E from Table 8-8

Stiffness of portion of a bolt or screw clamped zone consistsa) Unthreaded

portion

b) Threaded portion

Two springs in series

05/01/2023

22Stiffness of the Joint

Since the members are being clamped together, the clamping force that produces tension in the bolt induces compression in the members.

Clamped members also act like springs in series The spring rate of the members being joint by the fastener is

Where E is the modulus of elasticity of the material of the members. In this formula we assume all the members being jointed are of the same material


Find E from Table 8-8

If one of the members is a soft gasket, its stiffness relative to the other members isusually so small that for all practical purposes the others can be neglected and onlythe gasket stiffness used.


23 An Example

As shown above, two steel plates are clamped by washer-faced in-20 UNF × in SAE grade 5 bolts each with a standard N steel plain washer.

(a) Determine the bolt spring rate kb. (b) Determine the member spring rate km (c) Determine the length of the bolt.

05/01/2023

24 Answer From Table A–32, the thickness of a standard 1/2N plain

washer is 0.095 in. From Table A-31, height for 0.5” nut is H=0.4375”

d=0.5”, , For all materials Grip length, Length of bolt is Threaded length of bolt is Length of unthreaded portion in grip Threaded portion in the grip From Table 8-1, The bolt stiffness is Member stiffness is



25 Bolt Strength The proof load is the maximum load (force) that a bolt can withstand without

acquiring a permanent set. The proof strength Sp is the ratio of proof load and tensile-stress area. In the specification standards for bolts, the strength is specified by stating SAE

or ASTM minimum quantities, the minimum proof strength, or minimum proof load, and the minimum tensile strength.

Min proof load is load at which 1% fasteners fail. 99% fasteners exceed it The bolt grades are numbered according to the tensile strengths. The grade of the nut should be the grade of the bolt. Refer to Tables 8-9, 8-10 and 8-11 for minimum strength of steel bolts ASTM threads are shorter because ASTM deals mostly with structures;

structural connections are generally loaded in shear, and the decreased thread length provides more unthreaded shank area (remember ).

Typical stress-strain diagramfor bolt materials


26 Bolt Head Markings The bolt grades are numbered according to the tensile

strengths. Grades are printed (grades or some geometric code) along

with manufacturer logo on the bolt head. Unmarked bolts should be avoided as they may be

unstandardized.


27


28


29Standard Bolt Lengths


30 External Loads

The clamping force, aka preload Fi , has been correctly applied by tightening the nut before external load P is applied.

Fi = preloadPtotal = Total external tensile load applied to the jointP = external tensile load per boltPb = portion of P taken by boltPm = portion of P taken by membersFb = Pb + Fi = resultant bolt loadFm = Pm − Fi = resultant load on membersC = fraction of external load P carried by bolt1 − C = fraction of external load P carried by membersN = Number of bolts in the joint The bolt takes only 20% of the load, the rest is borne by the members

𝜎 𝑏=𝐹 𝑏

𝐴𝑡=𝐶𝑃+𝐹 𝑖

𝐴𝑡,𝜎 𝑖=

𝐹 𝑖

𝐴𝑡

The entire joint behaves like two springs in parallel, since they deflect by the same amount. Therefore, and hence


31 External Loads

Load is equally shared by bolts, P is tension load It causes both bolt and member to deform by This is like springs in parallel. and Since Let be the “stiffness of the joint” Hence, we can write and Resultant loads are therefore and


32 Factors of Safety in Static Loads As a rule, a bolt will either fracture during tightening, or not at all. For non-permanent joints, it is recommended that preload Fi is of 75 %

of proof load Fp=SPAt

For permanent joints, Fi=0.9 Fp The yielding FoS against the static stress exceeding proof strength is Another indicator of yielding that is sometimes used is a load factor nL

which is applied only to the load P as a guard against overloading.

It is essential for a safe joint that external load be smaller than that needed to cause the joint to separate. If separation occurs, then the entire external load is borne by the bolt (remember bolt bears only 20-40% load in a joint)!

The load factor guarding against joint separation is


33 Bolt Tension and Bolt Torque

High preload is very desirable in important bolted connections. The preload is the “muscle” of the joint, and its magnitude is determined by

the bolt strength. If the full bolt strength is not used in developing the pretension, then money is wasted and the joint is weaker.

In such cases the wrench torque required to develop the specified preload must be estimated.

The torque wrench has a built-in dial that indicates the proper torque. Defining a torque coefficient K, we can relate preload with torque required

K depends on the type of surfaces as well as lubrication If not mentioned, you can take K=0.2


34 Another Example

A 3/4 in-16 UNF × in SAE grade 5 bolt is subjected to a load P of 6 kip in a tension joint. The initial bolt tension is Fi = 25 kip. The bolt and joint stiffnesses are kb = 6.50 and km = 13.8 Mlbf/in, respectively.

(a) Determine the preload and service load stresses in the bolt. Compare these to the SAE minimum proof strength of the bolt.

(b) Specify the torque necessary to develop the preload Calculate the factors of safety and load factors.Answer:

Done in class.


35 Yet another example

cross section of a grade 25 cast-iron pressure vessel. N bolts are to be used to resist a separating force of 36 kip.

(a)Determine kb, km, and C.(b)Find number of bolts required for a load factor of 2 where the bolts may

be reused when the joint is taken apart.(c) With the number of bolts obtained in part (b), determine actual load

factor for overload, the yielding factor of safety, and the load factor for joint separation.

Answer:Done in class

me 312 mechanical machine design [screws, bolts, nuts]

Education