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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 1 of 15
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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 2 of 15
 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).
Profile
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
Coarse or F (Fine)
Diameter
(𝑁) Profile
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 3 of 15
 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 finepitch 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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 4 of 15
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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 5 of 15
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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 6 of 15
 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 (𝐿 𝑇 ) is usually determined
according to the length of the bolt
using the relation:
2 𝑑 + 6 𝑚𝑚
𝐿 𝑇 = { 2 𝑑 + 12 𝑚𝑚
2 𝑑 + 25 𝑚𝑚
𝐿 ≤ 125 , 𝑑 ≤ 48 𝑚𝑚
125 < 𝐿 ≤ 200 𝑚𝑚
𝐿 > 200 𝑚𝑚
 The length (𝐿) and the threaded length (𝐿 𝑇 ) 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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 7 of 15
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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 8 of 15
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
Medium Fit
1.8
Free Fit
2
Thread
M1.6
Close Fit
1.7
M2
2.2
2.4
2.6
M2.5
2.7
2.9
M3
3.2
M3.5
M8
Close Fit
8.4
Medium Fit
9
Free Fit
10
M10
10.5
11
12
3.1
M12
13
14
15
3.4
3.6
M14
15
16
17
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
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 9 of 15
 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).
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 10 of 15
 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 𝐹𝑖 𝑑
Where “𝐾” is the Torque Coefficient and “𝑑” is the bolt or screw nominal
diameter.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 11 of 15
 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 𝐹𝑖 𝑑


𝐹𝑖 =
𝑇
𝐾𝑑
=
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:
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 12 of 15
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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 13 of 15
 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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 14 of 15
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.
MENG 204 - Mechanical Drawing
Mechanical Fasteners
Lecture Notes by: Dr. Ala Hijazi
Page 15 of 15