mechanical fasteners - bajkulcollegeonlinestudy.in

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

Mechanical Fasteners 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 many different 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 with slotted 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.



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 mated with 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 (𝐷, 𝑑): the

largest diameter of the screw

thread.

Minor diameter (𝐷1, 𝑑1): also

called “root diameter”, is the

smallest diameter of the screw

thread.

Mean diameter (𝐷2, 𝑑2): also

called “pitch diameter”, the

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

the threads).

Pitch (𝑝): the distance between adjacent threads measured parallel to thread

axis.

Thread angle (2𝛼): the angle between the mating faces of two adjacent

threads.



All threads are usually right-handed unless otherwise is indicated (screws with

right-handed threads advance forward when the screw is rotated in the direction

of the curled right-hand fingers).

Bolt and screw threads are standardized and there are two major standards: (ISO)

Metric and (ANSI) Unified. In both standards the thread angle is 60°.

Metric (ISO):

There are two standard profiles M and MJ where both have a similar geometry

but the MJ has a rounded fillet at the root and a larger minor diameter (it is

better in resisting fatigue loading).

Metric bolts are specified by the major diameter and the pitch (both in mm).

Example: 𝑀10 × 1.25 (10 mm major diameter and 1.25 mm pitch).

Unified (ANSI):

There are two standard profiles UN and UNR where the UNR has a filleted root.

Unified threads are specified by the major diameter (in inch) and the number of

threads per inch (𝑁).

Example: 1 4⁄ − 20 𝑈𝑁𝐶

Profile

Coarse or F (Fine)

(𝑁) Diameter Profile



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:

𝑀10 × 1.5 or 𝑀10

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.



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 thread are 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 is

twice the major diameter.



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.



The table shown gives the standard head dimensions for regular hexagonal head bolts.

The length of a bolt (𝐿) is usually chosen from the preferred ISO sizes.

The length of the threaded portion of a bolt (𝐿𝑇) is usually determined according to the length of the bolt using the relation:

𝐿𝑇 = {2 𝑑 + 6 𝑚𝑚 𝐿 ≤ 125 , 𝑑 ≤ 48 𝑚𝑚2 𝑑 + 12 𝑚𝑚 125 < 𝐿 ≤ 200 𝑚𝑚 2 𝑑 + 25 𝑚𝑚 𝐿 > 200 𝑚𝑚

The length (𝐿) and the threaded length (𝐿𝑇) of a bolt are used in size designation.

Example: 𝑀14 × 1.5 × 80 × 34

Some of the common head styles used for Machine Screws are shown in the figure.



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 type of 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 dragging across the mating thread.



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 which the 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



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 Pitch

Thread 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 figure shows 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).



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 used in 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 (𝐹𝑖) developed in the bolt in order to ensure that the preload is sufficient.

The relation between torque and preload is given as:

𝑇 = 𝐾 𝐹𝑖𝑑

Where “𝐾” is the Torque Coefficient and “𝑑” is the bolt or screw nominal diameter.



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 𝑀8 size was tightened with 12 𝑁. 𝑚 torque. Find the preload in the screw.

Solution:

𝑇 = 𝐾 𝐹𝑖𝑑 𝐹𝑖 =𝑇

𝐾 𝑑=

12

(0.18) 8×10−3

𝐹𝑖 = 8333.3 𝑁

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 used for screws:



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

Lead (𝑙): is the distance the screw moves parallel to 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 are easier to machine and split-nuts (to compensate for wear) can be used.



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 other end 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.



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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