Mechanical+Fasteners

8/19/2019 Mechanical+Fasteners

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MENG 204 - Mechanical Drawing Lecture Notes by: Dr. Ala Hijazi

Mechanical Fasteners Page 1 of 15

Mechanical Fasteners

A mechanical fastener is a device that is

used to mechanically join (or fasten) two

or more objects together. There are manydifferent types of mechanical fasteners,

but, in general, fasteners can be divided

into two main categories; non-permanent

and permanent fasteners.

Non-permanent fasteners (they enable

parts to be assembled and disassembled

repeatedly ) can further be divided into

two groups:

General fasteners such as keys, pins, retaining rings, etc. (they are often

associated with shafts)

Threaded fasteners such as bolts, screws, studs, setscrews, etc. They are the

most widely used type of non-permanent fasteners since they can easily be

removed then reused.

On the other hand, rivets are considered to be permanent fasteners since removing

rivets will destroy them.

Threaded Fasteners

Threaded fasteners are the principal devices used for assembling components and

they are usually grouped into three main categories as shown in the figure:

Bolts: A bolt has a head on one end and threads on the other end and it is

paired with a nut.

Screws: Screws are used to join two mating parts together and similar to

bolts, they have head on one end and threads on the other end. However,

screws usually have longer threads than bolts, also they can be made withslotted heads.

Screws are sometimes divided into two sub-categories; Cap Screws and

Machine Screws. Machine screws are generally smaller in size than cap

screws and they are used for screwing into thin materials.

Studs: A stud is a rod that is threaded on both ends and joins two mating

parts. A nut may be used on one end.


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The terms bolt and screw are sometimes used interchangeably and they can refer to

the same element. In practice, the basic difference between a bolt and a screw is

that a bolt is usually intended to be used in conjunction with a nut where it will be

tightened or loosened using the nut, while a screw is usually intended to be matedwith an internally threaded hole and the screw head is used for tightening or

loosening.

o All aspects related to mechanical fasteners are standardized. The ISO standards

are collected in a handbook:

ISO Standards Handbook: Fasteners and screw threads.

Thread Standards and Definitions

The terminology of screw threads is illustrated in the figure:Major diameter (, ): thelargest diameter of the screw

thread.

Minor diameter (, ): alsocalled “root diameter ”, is the

smallest diameter of the screw

thread.Mean diameter (, ): alsocalled “ pitch diameter ”, the

average diameter of the screw thread (considering the theoretical full height of

the threads).

Pitch (): the distance between adjacent threads measured parallel to threadaxis.

Thread angle (2): the angle between the mating faces of two adjacentthreads.


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The table gives the preferred standard sizes (1st

choice) for Metric bolts and

screws (ISO 262).

Note that there is Coarse-pitch and Fine-pitch (more threads) where the fine-

pitch has better tensile strength.

If the thread being specified is "coarse" pitch thread (which is considered the

default ), the pitch can be omitted from the designation:

10×1.5 or 10 External and Internal Threads

Threads can be either external or internal:

External Thread : External threads are on the outside of a member (such as the

treads of bolts and screws). A chamfer on the end of the screw thread makes it

easier to engage it into a hole or a nut.

An external thread is usually cut using a die (such as seen in the

figure) or a lathe.

Internal Thread : Internal threads are on the inside of a member

(such as the threads of nuts and holes). Usually, threaded holes

have a chamfer on the side from which the screw will enter to make its

engagement easier.

An internal thread is usually cut using a tap.


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Drawing Thread Symbols and Notes

In technical drawings, threads are not shown explicitly. Usually, simplified thread

symbols are used to indicate the presence and extent of external or internal

threads. In the simplified thread symbols, thin dashed lines (similar to the hidden

lines used in drawings) parallel to the axis at the approximate depth of the threadare used to indicate the threaded portion of a bolt or screw. The figure shows

examples of using the simplified thread symbols for indicating external and internal

threads.

Threads are usually dimensioned by giving a thread note in the drawing as seen

in the figure.

It should be noted that threaded holes are first drilled at a diameter smaller

than the nominal size of the bolt that will fit into it, then a tap is used to cut

the desired thread (see the internal thread note shown in the figure).

o For blind threaded holes, standards recommend that the depth of the thread istwice the major diameter.


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Types of Bolts and Screws

There are many different types of bolts and screws where each is suitable for

different types of applications. The figure shows some of the most commonly used

types of bolts and screws.

There are standards and common practices related to all aspects of the form and

geometry of threaded fasteners.

While bolts mostly have Hexagonal Heads, there are several head styles that are

being used for cap screws. The figure shows some of the common head styles

that are used for cap screws.

All head dimensions are standardized and they are usually given in tables

according to the nominal size.


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The table shown gives the standard head dimensions for regular hexagonal

head bolts.

The length of a bolt () is usuallychosen from the preferred ISO sizes.

The length of the threaded portion

of a bolt () is usually determinedaccording to the length of the bolt

using the relation:

= {2 + 6 ≤ 125 , ≤ 48 2 + 12 125 < ≤ 200 2 + 25 > 200 The length () and the threaded length () of a bolt are used in size designation.Example: 14×1.5 ×80 ×34 Some of the common head styles used for Machine Screws are shown in the

figure.


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Setscrews

Setscrews are a special type of screws that usually does not

have a head and they are used to prevent rotation or

movement between parts (such as a shaft and a collar ). The

main difference between a setscrew and other type of screws

is that a setscrew is driven through one member such that it

will apply a compressive force on the other member while a

screw or a bolt clamps the two members together.In most cases, setscrews does not have heads such that they can be

screwed all the way into threaded holes using a socket or a slot in

the screw shank itself as seen in the figure. But, sometimes set

screws may have heads such as the thumbscrew shown in the figure.

Nuts

A nut is the counterpart of a bolt. Bolts and nuts are used together to clamp two or

more parts having unthreaded holes together. While the most commonly used typeof nuts are the regular hexagonal nuts, there are different types of nuts where each

is more suitable for a different type of applications. The figure shows some of the

common types of nuts.

What is Jam, slotted or castle nuts used for?

One of the other common types of "lock" nuts is the "Prevailing

Torque Nut " which differs from a free spinning nut in that a

torque is required during installation. A typical example of this

type is the nylon-insert nut, shown in the figure, where a torque

is needed to overcome the resistance of the nylon draggingacross the mating thread.


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Also, some of the other miscellaneous types of internally threaded elements are

shown in the figure.

Washers

Washers are commonly used under the heads of bolts and screws and they are used

under nuts as well. The most common type of washers is the Flat Washer, such as

that seen in the figure, where it is used to increase the bearing areas of the head,

and protect the material finish during assembly. The use of

washers also prevents the sharp corner of the hole into whichthe bolt or screw is being inserted from biting into bolt head

fillet where that can cause the failure of the bolts during service.

The other common type of washers is the Lock Washers where they are used to

prevent loosening of screws and nuts by providing continuous friction due to their

spring action. Some of the common types of lock washers are shown in the figure.

Fastener's Holes

The holes into which threaded fasteners are inserted are of two basic types;

clearance holes (unthreaded ) and threaded holes.

Clearance holes are larger than the nominal diameter of the bolt or screw and

the amount of clearance depends on the desired type of fit.

The table gives the diameter of clearance holes for the different types of fits.

Thread Close Fit Medium Fit Free Fit Thread Close Fit Medium Fit Free Fit

M1.6 1.7 1.8 2 M8 8.4 9 10

M2 2.2 2.4 2.6 M10 10.5 11 12

M2.5 2.7 2.9 3.1 M12 13 14 15

M3 3.2 3.4 3.6 M14 15 16 17

M3.5 3.7 3.9 4.2 M16 17 18 19

M4 4.3 4.5 4.8 M20 21 22 24

M5 5.3 5.5 5.8 M24 25 26 28

M6 6.4 6.6 7 M30 31 33 35


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Threaded holes are drilled at a diameter smaller than the nominal diameter of

the screw that will go into it (almost equal to the root diameter of the bolt ), then

a tap is used to cut the thread.

The table gives the drill size to be used for the different sizes of threaded

holes.

Thread Drill size

Coarse Pitch

Drill size

Fine PitchThread

Drill size

Coarse Pitch

Drill size

Fine Pitch

M1.6 1.5 - M8 6.8 7

M2 1.6 - M10 8.5 8.8

M2.5 2 - M12 10.2 10.8

M3 2.5 - M14 12 12.5

M3.5 2.9 - M16 14 14.5

M4 3.3 - M20 17.5 18.5

M5 4.2 - M24 21 22

M6 5 - M30 26.5 28

How many threaded holes are needed to connect several parts using a bolt

and a nut or using a cap screw?

In most cases, cap screw holes have a constant diameter (Plain holes), however,

in some cases it is desired that the screw head is not sticking-out of the surface.

For such cases, Counter-bored or Countersunk holes may be used. The figureshows the shape of counter-bored or Countersunk holes as well as the symbols

used to give their specifications in drawings. Countersunk holes are used for flat

head screws, while counter-bored holes may be used with almost any type of

screw heads (socket head screws are the most commonly used since they require

small counter-bore diameter ).


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The size details of fastener holes are usually given in technical drawings using a Hole

Callout such as shown in the previous figure. However, when a part has many holes

with different sizes, it becomes more

convenient to include a Hole Table in

the drawing. When a hole table is usedin a drawing, each hole is identified

using a tag, and the location and size

details for each of the holes are listed

in the table, such as seen in the hole

table shown.

Thread Fits

In some cases, the required looseness or tightness of fit between the internal and

external threads may vary.

There are two classes of metric thread fits that are generally used:

General purpose fit (6H/6g). A tolerance class of 6H/6g is assumed if it is not

specified .

Closer fit (6H/5g6g).

Letters: specify the amount of allowance.

Upper case letters: specify internal threads.

Lower case letters: specify external threads.Numbers: specify tolerance grade (smaller numbers indicate a tighter fit )

Bolts and Screws Tightening

Bolts and screws are used to clamp two, or more, parts together where these parts

are subjected to an external force trying to separate them. When a bolt and a nut

are used to make a joint, the nut is usually tightened to grip the joint firmly.

Applying preload to the bolt (by tightening) is very important where it increase the

strength of the joint by preventing separation of the members.

It is important to relate the torque used in tightening the bolt to the amount of

the preload () developed in the bolt in order to ensure that the preload issufficient.

The relation between torque and preload is given as:

= Where “” is the Torque Coefficient and “” is the bolt or screw nominaldiameter.


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The table gives the Torque Coefficient values for different bolt conditions.

Plating is typically used for bolts and screws

for corrosion protection. The figure shows

non-plated and plated screws.

How the tightening torque is controlled during assembly?

By using a Torque Wrench as shown in the figure.

Usually, assembly drawings include a Torque Table that

specifies the required tightening torque for all threaded

fasteners in the assembly.

Example: A zinc-plated cap screw of 8 size was tightened with 12 . torque.Find the preload in the screw.

Solution: = = = (.) ×− = 8333.3

Power Screws

Power screws are NOT used as mechanical

fasteners, but their work principle is very similar to

screws and bolts. Power screws are used to change

rotational motion to linear motion (they are

commonly used in vices, jacks, lathes, etc.). Because

of their design, they are capable of providing very

high force from a small value of applied torque.

The terminology used for defining the thread

profile of power screws is very similar to that usedfor screws:


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Major diameter, Minor diameter , Mean diameter, and Pitch have the same

definitions.

Lead (): is the distance the screw moves parallelto its axis when it rotates one full turn.

For a single thread screw the lead is same as

the pitch.

For multiple thread screws (two or more

threads run beside each other ) the lead equals

the pitch multiplied by the number of threads. Single and multiple thread

power screws are illustrated in the figure.

The thread profiles used for power screws are different from those used for bolts

and screws. There are four types of thread profiles used for power screws and

these are: Square, Acme, Knuckle and Buttress as shown in the figure.

o Square and Acme threads are more commonly used than other types.

Square threads give better efficiency than Acme threads.

However, Acme threads are more commonly used because they areeasier to machine and split-nuts (to compensate for wear ) can be used.


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For metric sized power screws, standard values of diameters and associated

pitch values usually follow the ISO coarse pitch thread standard (see the table

given earlier ).

Ball Screws

Ball screws are very similar to power screws but

they provide much higher precision. Instead of

using a friction nut such as in regular power

screws, ball screws use a nut filled with balls and

that results in better precision (no backlash) and

higher efficiency (due to the reduced friction).

Such screws are commonly used in machines

requiring high precession such as CNC machines.

Rivets

Rivets are regarded as permanent fasteners, unlike removable (or nonpermanent )

fasteners, such as bolts and screws. Rivets are generally used to hold plates or sheet

metal together.

Rivets are used in a very wide variety of applications ranging from aircraft structures

to civil steel structures to automobiles to boats and ships to clothing, etc.

There are many different types of rivets where

each type is more suitable for certain types of

applications. The most common and distinct two

types of rivets are the Solid rivets and the Pop

rivets.

o Solid rivets are one of the oldest and most

reliable types of rivets. They consist simply of a

cylindrical shank with a single head. When the

shank it is inserted inside the hole, the otherend of the rivet is deformed with a hammer or

rivet gun as illustrated in the figure. Such type

of rivets are used in civil steel structures such as

bridges and they usually require heating in

order to make it easy to deform the other end.

A somehow similar type of rivets is used in

aircraft structures, but it does not require

heating.


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o Pop rivets (also called blind rivets) are the most widely used type of rivets for

general purpose applications. The installation of pop rivets is very easy and

fast, and one of the major advantages of this type of rivets is that the

installation does not require access to both sides of the assembly. Pop rivets

are tubular and are supplied with a mandrel through the center. After the

rivet is inserted into a hole drilled through the parts to be joined, a specially

designed tool is used to draw the mandrel into the rivet. This expands the

blind end of the rivet and then the mandrel snaps off, as illustrated in the

figure. However such rivets are not very reliable and they are not capable of

supporting large loads.

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