Screwed Joints
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381
7. Knuckle thread. It is also a modification of
square thread. It has rounded top and bottom. It can
be cast or rolled easily and can not economically be
made on a machine. These threads are used for rough
and ready work. They are usually found on railway
carriage couplings, hydrants, necks of glass bottles
and large moulded insulators used in electrical trade.
8. Buttress thread. It is used for transmission
of power in one direction only. The force is
transmitted almost parallel to the axis. This thread
units the advantage of both square and V-threads. It
has a low frictional resistance characteristics of the square thread and have the same strength as that
of V-thread. The spindles of bench vices are usually provided with buttress thread. The various
proportions of buttress thread are shown in Fig. 11.9.
Fig. 11.9. Buttress thread.
9. Metric thread. It is an Indian standard thread and is similar to B.S.W. threads. It has an
included angle of 60° instead of 55°. The basic profile of the thread is shown in Fig. 11.10 and the
design profile of the nut and bolt is shown in Fig. 11.11.
Fig. 11.8. Knuckle thread.
Simple Machine Tools.
Note : This picture is given as additional information and is not a direct example of the current chapter.
Washer
Nut
Crinkle
washer
Chromium-plated wood
screw
Black painted wood screw
Brass wood screw
Nail plate for joining two
pieces of wood
Angle plate
Corrugated
fasteners for
joining corners
Zinc-plated machine screw
Wall plug holds screws in walls
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Fig. 11.10. Basic profile of the thread.
d = Diameter of nut; D = Diameter of bolt.
Fig. 11.11. Design profile of the nut and bolt.
11.511.5
11.511.5
11.5
Location of Screwed JointsLocation of Screwed Joints
Location of Screwed JointsLocation of Screwed Joints
Location of Screwed Joints
The choice of type of fastenings and its
location are very important. The fastenings
should be located in such a way so that they will
be subjected to tensile and/or shear loads and
bending of the fastening should be reduced to a
minimum. The bending of the fastening due to
misalignment, tightening up loads, or external
loads are responsible for many failures. In order
to relieve fastenings of bending stresses, the use
of clearance spaces, spherical seat washers, or
other devices may be used.
Beech wood side of
drawer
Dovetail joint
Cherry wood drawer front
Bolt
Screwed Joints
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383
11.611.6
11.611.6
11.6
Common Types of Screw FasteningsCommon Types of Screw Fastenings
Common Types of Screw FasteningsCommon Types of Screw Fastenings
Common Types of Screw Fastenings
Following are the common types of screw fastenings :
1. Through bolts. A through bolt (or simply a bolt) is shown in Fig. 11.12 (a). It is a cylindrical
bar with threads for the nut at one end and head at the other end. The cylindrical part of the bolt is
known as shank. It is passed through drilled holes in the two parts to be fastened together and clamped
them securely to each other as the nut is screwed on to the threaded end. The through bolts may or
may not have a machined finish and are made with either hexagonal or square heads. A through bolt
should pass easily in the holes, when put under tension by a load along its axis. If the load acts
perpendicular to the axis, tending to slide one of the connected parts along the other end thus subject-
ing it to shear, the holes should be reamed so that the bolt shank fits snugly there in. The through bolts
according to their usage may be known as machine bolts, carriage bolts, automobile bolts, eye bolts
etc.
Fig. 11.12
2. Tap bolts. A tap bolt or screw differs from a bolt. It is screwed into a tapped hole of one of
the parts to be fastened without the nut, as shown in Fig. 11.12 (b).
3. Studs. A stud is a round bar threaded at both ends. One end of the stud is screwed into a
tapped hole of the parts to be fastened, while the other end receives a nut on it, as shown in Fig. 11.12
(c). Studs are chiefly used instead of tap bolts for securing various kinds of covers e.g. covers of
engine and pump cylinders, valves, chests etc.
Deck-handler crane is used on ships to move loads
Note : This picture is given as additional information and is not a direct example of the current chapter.
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This is due to the fact that when tap bolts are unscrewed or replaced, they have a tendency to
break the threads in the hole. This disadvantage is overcome by the use of studs.
4. Cap screws. The cap screws are similar to tap bolts except that they are of small size and a
variety of shapes of heads are available as shown in Fig. 11.13.
Fig. 11.13. Types of cap screws.
5. Machine screws. These are similar to cap screws with the head slotted for a screw driver.
These are generally used with a nut.
6. Set screws. The set screws are shown in Fig. 11.14. These are used to prevent relative
motion between the two parts. A set screw is screwed through a threaded hole in one part so that its
point (i.e. end of the screw) presses against the other part. This resists the relative motion between the
two parts by means of friction between the point of the screw and one of the parts. They may be used
instead of key to prevent relative motion between a hub and a shaft in light power transmission
members. They may also be used in connection with a key, where they prevent relative axial motion
of the shaft, key and hub assembly.
Fig. 11.14. Set screws.
The diameter of the set screw (d) may be obtained from the following expression:
d = 0.125 D + 8 mm
where D is the diameter of the shaft (in mm) on which the set screw is pressed.
The tangential force (in newtons) at the surface of the shaft is given by
F = 6.6 (d )
2.3
Screwed Joints
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385
!Torque transmitted by a set screw,
T =
N-m
2
D
F
∀
(D is in metres)
and power transmitted (in watts), P =
2.
60
NT
#
, where N is the speed in r.p.m.
11.711.7
11.711.7
11.7
Locking DevicesLocking Devices
Locking DevicesLocking Devices
Locking Devices
Ordinary thread fastenings, generally, remain tight under static loads, but many of these
fastenings become loose under the action of variable loads or when machine is subjected to vibra-
tions. The loosening of fastening is very dangerous and must be prevented. In order to prevent this, a
large number of locking devices are available, some of which are discussed below :
1. Jam nut or lock nut. A most common locking device is a jam, lock or check nut. It has about
one-half to two-third thickness of the standard nut. The thin lock nut is first tightened down with
ordinary force, and then the upper nut (i.e. thicker nut) is tightened down upon it, as shown in Fig.
11.15 (a). The upper nut is then held tightly while the lower one is slackened back against it.
Fig. 11.15. Jam nut or lock nut.
In slackening back the lock nut, a thin spanner is required which is difficult to find in many
shops. Therefore to overcome this difficulty, a thin nut is placed on the top as shown in Fig. 11.15 (b).
If the nuts are really tightened down as they should be, the upper nut carries a greater tensile
load than the bottom one. Therefore, the top nut should be thicker one with a thin nut below it because
it is desirable to put whole of the load on the thin nut. In order to overcome both the difficulties, both
the nuts are made of the same thickness as shown in Fig. 11.15 (c).
2. Castle nut. It consists of a hexagonal portion with a cylindrical upper part which is slotted in
line with the centre of each face, as shown in Fig. 11.16. The split pin passes through two slots in the
nut and a hole in the bolt, so that a positive lock is obtained unless the pin shears. It is extensively used
on jobs subjected to sudden shocks and considerable vibration such as in automobile industry.
3. Sawn nut. It has a slot sawed about half way through, as shown in Fig. 11.17. After the nut
is screwed down, the small screw is tightened which produces more friction between the nut and the
bolt. This prevents the loosening of nut.
4. Penn, ring or grooved nut. It has a upper portion hexagonal and a lower part cylindrical as
shown in Fig. 11.18. It is largely used where bolts pass through connected pieces reasonably near
their edges such as in marine type connecting rod ends. The bottom portion is cylindrical and is
recessed to receive the tip of the locking set screw. The bolt hole requires counter-boring to receive
the cylindrical portion of the nut. In order to prevent bruising of the latter by the case hardened tip of
the set screw, it is recessed.
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Fig. 11.16. Castle nut. Fig. 11.17. Sawn nut. Fig. 11.18. Penn, ring or grooved nut.
5. Locking with pin. The nuts may be locked by means of a taper pin or cotter pin passing
through the middle of the nut as shown in Fig. 11.19 (a). But a split pin is often driven through the bolt
above the nut, as shown in Fig. 11.19 (b).
Fig. 11.19. Locking with pin.
6. Locking with plate. A form of stop plate or locking plate is shown in Fig. 11.20. The nut can
be adjusted and subsequently locked through angular intervals of 30° by using these plates.
Fig. 11.20. Locking with plate. Fig. 11.21. Locking with washer.
7. Spring lock washer. A spring lock washer is shown in Fig. 11.21. As the nut tightens the
washer against the piece below, one edge of the washer is caused to dig itself into that piece, thus
increasing the resistance so that the nut will not loosen so easily. There are many kinds of spring lock
washers manufactured, some of which are fairly effective.
11.811.8
11.811.8
11.8
Designation of Screw ThreadsDesignation of Screw Threads
Designation of Screw ThreadsDesignation of Screw Threads
Designation of Screw Threads
According to Indian standards, IS : 4218 (Part IV) 1976 (Reaffirmed 1996), the complete
designation of the screw thread shall include
Screwed Joints
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387
1. Size designation. The size of the screw thread is designated by the letter `M' followed by the
diameter and pitch, the two being separated by the sign ×. When there is no indication of the pitch, it
shall mean that a coarse pitch is implied.
2. Tolerance designation. This shall include
(a) A figure designating tolerance grade as indicated below:
‘7’ for fine grade, ‘8’ for normal (medium) grade, and ‘9’ for coarse grade.
(b) A letter designating the tolerance position as indicated below :
‘H’ for unit thread, ‘d’ for bolt thread with allowance, and ‘h’ for bolt thread without
allowance.
For example, A bolt thread of 6 mm size of coarse pitch and with allowance on the threads and
normal (medium) tolerance grade is designated as M6-8d.
11.911.9
11.911.9
11.9
Standard Dimensions of Screw ThreadsStandard Dimensions of Screw Threads
Standard Dimensions of Screw ThreadsStandard Dimensions of Screw Threads
Standard Dimensions of Screw Threads
The design dimensions of I.S.O. screw threads for screws, bolts and nuts of coarse and fine
series are shown in Table 11.1.
Table 11.1. Design dimensions of screw threads, bolts and nuts accordingTable 11.1. Design dimensions of screw threads, bolts and nuts according
Table 11.1. Design dimensions of screw threads, bolts and nuts accordingTable 11.1. Design dimensions of screw threads, bolts and nuts according
Table 11.1. Design dimensions of screw threads, bolts and nuts according
to IS : 4218 (Part III) 1976 (Reaffirmed 1996) (Refer Fig. 11.1)to IS : 4218 (Part III) 1976 (Reaffirmed 1996) (Refer Fig. 11.1)
to IS : 4218 (Part III) 1976 (Reaffirmed 1996) (Refer Fig. 11.1)to IS : 4218 (Part III) 1976 (Reaffirmed 1996) (Refer Fig. 11.1)
to IS : 4218 (Part III) 1976 (Reaffirmed 1996) (Refer Fig. 11.1)
Designation Pitch Major Effective Minor or core Depth of Stress
mm or or pitch diameter thread area
nominal diameter (d
c
) mm (bolt) mm
2
diameter Nut and mm
Nut and Bolt
Bolt (d
p
) mm Bolt Nut
(d = D)
mm
(1) (2) (3) (4) (5) (6) (7) (8)
Coarse series
M 0.4 0.1 0.400 0.335 0.277 0.292 0.061 0.074
M 0.6 0.15 0.600 0.503 0.416 0.438 0.092 0.166
M 0.8 0.2 0.800 0.670 0.555 0.584 0.123 0.295
M 1 0.25 1.000 0.838 0.693 0.729 0.153 0.460
M 1.2 0.25 1.200 1.038 0.893 0.929 0.158 0.732
M 1.4 0.3 1.400 1.205 1.032 1.075 0.184 0.983
M 1.6 0.35 1.600 1.373 1.171 1.221 0.215 1.27
M 1.8 0.35 1.800 1.573 1.371 1.421 0.215 1.70
M 2 0.4 2.000 1.740 1.509 1.567 0.245 2.07
M 2.2 0.45 2.200 1.908 1.648 1.713 0.276 2.48
M 2.5 0.45 2.500 2.208 1.948 2.013 0.276 3.39
M 3 0.5 3.000 2.675 2.387 2.459 0.307 5.03
M 3.5 0.6 3.500 3.110 2.764 2.850 0.368 6.78
M 4 0.7 4.000 3.545 3.141 3.242 0.429 8.78
M 4.5 0.75 4.500 4.013 3.580 3.688 0.460 11.3
M 5 0.8 5.000 4.480 4.019 4.134 0.491 14.2
M 6 1 6.000 5.350 4.773 4.918 0.613 20.1
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A Textbook of Machine Design
(1) (2) (3) (4) (5) (6) (7) (8)
M 7 1 7.000 6.350 5.773 5.918 0.613 28.9
M 8 1.25 8.000 7.188 6.466 6.647 0.767 36.6
M 10 1.5 10.000 9.026 8.160 8.876 0.920 58.3
M 12 1.75 12.000 10.863 9.858 10.106 1.074 84.0
M 14 2 14.000 12.701 11.546 11.835 1.227 115
M 16 2 16.000 14.701 13.546 13.835 1.227 157
M 18 2.5 18.000 16.376 14.933 15.294 1.534 192
M 20 2.5 20.000 18.376 16.933 17.294 1.534 245
M 22 2.5 22.000 20.376 18.933 19.294 1.534 303
M 24 3 24.000 22.051 20.320 20.752 1.840 353
M 27 3 27.000 25.051 23.320 23.752 1.840 459
M 30 3.5 30.000 27.727 25.706 26.211 2.147 561
M 33 3.5 33.000 30.727 28.706 29.211 2.147 694
M 36 4 36.000 33.402 31.093 31.670 2.454 817
M 39 4 39.000 36.402 34.093 34.670 2.454 976
M 42 4.5 42.000 39.077 36.416 37.129 2.760 1104
M 45 4.5 45.000 42.077 39.416 40.129 2.760 1300
M 48 5 48.000 44.752 41.795 42.587 3.067 1465
M 52 5 52.000 48.752 45.795 46.587 3.067 1755
M 56 5.5 56.000 52.428 49.177 50.046 3.067 2022
M 60 5.5 60.000 56.428 53.177 54.046 3.374 2360
Fine series
M 8 × 1 1 8.000 7.350 6.773 6.918 0.613 39.2
M 10 × 1.25 1.25 10.000 9.188 8.466 8.647 0.767 61.6
M 12 × 1.25 1.25 12.000 11.184 10.466 10.647 0.767 92.1
M 14 × 1.5 1.5 14.000 13.026 12.160 12.376 0.920 125
M 16 × 1.5 1.5 16.000 15.026 14.160 14.376 0.920 167
M 18 × 1.5 1.5 18.000 17.026 16.160 16.376 0.920 216
M 20 × 1.5 1.5 20.000 19.026 18.160 18.376 0.920 272
M 22 × 1.5 1.5 22.000 21.026 20.160 20.376 0.920 333
M 24 × 2 2 24.000 22.701 21.546 21.835 1.227 384
M 27 × 2 2 27.000 25.701 24.546 24.835 1.227 496
M 30 × 2 2 30.000 28.701 27.546 27.835 1.227 621
M 33 × 2 2 33.000 31.701 30.546 30.835 1.227 761
M 36 × 3 3 36.000 34.051 32.319 32.752 1.840 865
M 39 × 3 3 39.000 37.051 35.319 35.752 1.840 1028
Note : In case the table is not available, then the core diameter (d
c
) may be taken as 0.84 d, where d is the major
diameter.
Screwed Joints
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389
11.1011.10
11.1011.10
11.10
Stresses in Screwed Fastening due to Static LoadingStresses in Screwed Fastening due to Static Loading
Stresses in Screwed Fastening due to Static LoadingStresses in Screwed Fastening due to Static Loading
Stresses in Screwed Fastening due to Static Loading
The following stresses in screwed fastening due to static loading are important from the subject
point of view :
1. Internal stresses due to screwing up forces,
2. Stresses due to external forces, and
3. Stress due to combination of stresses at (1) and (2).
We shall now discuss these stresses, in detail, in the following articles.
11.1111.11
11.1111.11
11.11
Initial Stresses due to Screwing up ForcesInitial Stresses due to Screwing up Forces
Initial Stresses due to Screwing up ForcesInitial Stresses due to Screwing up Forces
Initial Stresses due to Screwing up Forces
The following stresses are induced in a bolt, screw or stud when it is screwed up tightly.
1. Tensile stress due to stretching of bolt. Since none of the above mentioned stresses are
accurately determined, therefore bolts are designed on the basis of direct tensile stress with a large
factor of safety in order to account for the indeterminate stresses. The initial tension in a bolt, based
on experiments, may be found by the relation
P
i
= 2840 d N
where P
i
= Initial tension in a bolt, and
d = Nominal diameter of bolt, in mm.
The above relation is used for making a joint fluid tight like steam engine cylinder cover joints
etc. When the joint is not required as tight as fluid-tight joint, then the initial tension in a bolt may be
reduced to half of the above value. In such cases
P
i
= 1420 d N
The small diameter bolts may fail during tightening, therefore bolts of smaller diameter (less
than M 16 or M 18) are not permitted in making fluid tight joints.
If the bolt is not initially stressed, then the maximum safe axial load which may be applied to it,
is given by
P = Permissible stress × Cross-sectional area at bottom of the thread
(i.e. stress area)
The stress area may be obtained from Table 11.1 or it may be found by using the relation
Stress area =
2
42
pc
dd
∃
%&
#
∋(
)∗
where d
p
= Pitch diameter, and
d
c
= Core or minor diameter.
Note : This picture is given as additional information and is not a direct example of the current chapter.
Simple machine tools.
ball-peen hammer
for shaping metal
wooden mallet for
tapping chisels
claw hammer for
driving in nails and
pulling them out
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2. Torsional shear stress caused by the frictional resistance of the threads during its tighten-
ing. The torsional shear stress caused by the frictional resistance of the threads during its tightening
may be obtained by using the torsion equation. We know that
T
J
=
r
+
!+=
3
4
16
2
()
()
32
c
c
c
d
TT T
r
J
d
d
∀, ∀ ,
#
#
where + = Torsional shear stress,
T = Torque applied, and
d
c
= Minor or core diameter of the thread.
It has been shown during experiments that due to repeated unscrewing and tightening of the nut,
there is a gradual scoring of the threads, which increases the torsional twisting moment (T).
3. Shear stress across the threads. The average thread shearing stress for the screw (+
s
) is
obtained by using the relation :
+
s
=
c
P
dbn
#∀∀
where b = Width of the thread section at the root.
The average thread shearing stress for the nut is
+
n
=
P
dbn
#∀∀
where d = Major diameter.
4. Compression or crushing stress on threads. The compression or crushing stress between
the threads (−
c
) may be obtained by using the relation :
−
c
=
22
[–()]
c
P
ddn
#
where d = Major diameter,
d
c
= Minor diameter, and
n = Number of threads in engagement.
5. Bending stress if the surfaces under the head or nut are not perfectly parallel to the bolt
axis. When the outside surfaces of the parts to be connected are not parallel to each other, then the
bolt will be subjected to bending action. The bending stress (−
b
) induced in the shank of the bolt is
given by
−
b
=
.
2
xE
l
where x = Difference in height between the extreme corners of the nut or
head,
l = Length of the shank of the bolt, and
E = Young’s modulus for the material of the bolt.
Example 11.1. Determine the safe tensile load for a bolt of M 30, assuming a safe tensile stress
of 42 MPa.
Solution. Given : d = 30 mm ; −
t
= 42 MPa = 42 N/mm
2
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