ME331-DESIGN OF MACHINE

ELEMENTS
UNIT III
DESIGN OF FASTENERS AND JOINTS
SCREWED FASTENERS

INTRODUCTION

The screw thread is a form obtained b cutting a continuous helical groove on the ideal
surface. The threaded portion engages vital corresponding threaded hole in the nut or
machine part. The elements is called a Screw pair

a) To act as fastening to secure one member to other member

b) To transmit power.

Thread joints are used in mechanical assemblies

The advantages of threaded joints are

i) High clamping force due to wedge action of threads

ii) Small tightening force is sufficient
iii) Simple design

FORMS OF SCREW THREADS

Commonly used thread forms are as follows

.
i) ISO metric thread
ii) Acme thread
iii) Square thread
iv) Buttress thread
v) BSW thread
vi) BA thread
vii) ANS thread
viii) Lowenherz thread
Fig. Various kinds of screw threads

APPLICATION AREAS OF THREAD FORMS

Thread forms Applications

Square Screw jack, Presie

Clamping device
Acme & Trapezoidal Lead screw, Power transmission devices
Buttress Vices

SCREW FASTENINGS

The following important screw fastenings arc met with in construction of machines
a) Through bolts
b) Tap bolts and cap screws
c) Machine screws
d) Set screws
e) Studs

Fig Various kinds of screwed fastenings

EFFICIENCY OF THREADS

When we consider thread friction only, the efficiency of V threaded screw is given by the
expression,
Where,

The efficiency expression simplifies to

SELF-LOCKING SCREW

When the torque required to lower the load is negative.

Because of this, the load begins to come down by itself.

When some torque is required to lower the load. This is

called as self-locking condition.

A screw for which is called as self-locking screw.

STRESSES IN SCREW FASTENINGS

In order to determine the dimensions of screwed fastenings, we should consider the

stresses due to the static loading as well as dynamic loading. The following stresses
should be considered in screw fastenings with static loading:
a) Initial stresses due to screwing up.
b) Stresses due to the external forces.
c) Stresses due to a combination of forces of types (a) and (b)

. INITIAL STRESSES

The stresses in a bolt, stud or screw when it is screwed up tightly are:

(i) Tensile stress due to stretching of the bolt.
(ii) Compression or bearing stresses on the thread.
(iii) Shearing stresses across threads.
(iv) Torsional shear stresses caused by the frictional resistance of the threads, and
(v) Bending stresses
If the surfaces under the head or nut are not perfectly normal to the bolt axis. None of
these above-mentioned stresses can be accurately determined. So the bolts are designed
on the basis of direct tensile stress with a very high factor of safety which will allow for
tightening stresses.
The torsional shear stress can be calculated by the formula
STRESSES DUE TO EXTERNAL FORCES:

Bolts, studs and screws are subjected to tensile stresses by the external forces acting on
them, but occasionally the bolts are subjected to shear loads also, the common example
being the bolts of the flange coupling. When a bolt is subjected to an axial tensile load,
the weakest section will be at the root of the thread.
If d be the diameter at the root of the thread, then

DESIGN OF BOLTS FOR EASY SITUATION

ECCENTRICALLY LOADED BOLTED JOINTS

Taking moments about 0,

It can be seen from the above that bolts in the second two stresses due to F and F are, are
put into higher stress.
Problem.
The cylinder head of a steam engine with 250mm bore is fastened by eight stud bolts
made of 30C8 steel Maximum pressure inside the cylinder is 1 Mpa. Determine the bolt
size and approximate tightening torque. Take 20% overload. Assume = 300Mpa for
bolt material
Given data:

To find:
i) Bolt size ii) Tightening torque
Total load on the head
(With 20% overload)
Solution:
Total load on the head (with 20% over load)

This is shared by 8 bolts.

Results:
Bolt chosen inMl6
Tightening torque = 146432N-m

Problem .
A bolted assembly of two components is shown in fig. 3.1.11. Initially, the nut is
tightened by means of a spanner so as to induce a pre-load of 2.SkN in the bolt the
external force P acting on the assembly is SkN. The bolt with coarse threads is made of
plain carbon steeI3OC8 ( = 400N/mm and the factor of safety is 2.5.) The effective
stiffness of the parts held together by the Bolt is 2.5 times the stiffness of the bolL
Specify the size of the boll

Given data:

To find:
Bolt size
Bolt load,
Solution:

The tensile stress area A of the bolt is given by

From PSGDB page 5.42 a bolt with threads M8 is chosen Ans.

RIVETED JOINTS

INTRODUCTION

A rivet is a round bar provided with a head on one side and a tail on t other side as shown
in fig. various parts of the rivet also described in this fig
It is general made of mild
Steel or iron. Sometimes it may be made of copper and Aluminum where the corrosion
Resistance and lightweight .The material used should be strong and ductile.
Riveted joints arc used for connecting two parts in high strength necessary in structural
connections, strength and rigidity are required
Pressure vessel work. Strength. Rigidity and leakage are the essentialities of the joint. It
is also used for general-purpose: turbines break bands etc.

fig: Rivet

EFFICIENCY OF A RIVETED JOINT:

The efficiency of a riveted joint is defined, as the ratio of strength of the joint at the
weakest made of failure to the strength of unpunched plate in one pitch length of the
joint.

Table indicates the a efficiency and maximum pitch for some types of commercial boiler
joints.

THICKNESS OF COVER PLATES

According to I. B. R. the thickness of cover plate (ti) butt joint is as

Follows.
row having alternate rivets.

for double cover with the outer most row having alternate rivets.
For unequal width of cover plates:

SPACING BETWEEN ADJACENT ROWS OF RIVETS OR BACK PITCH (Pb):

(i) For lap and butt joints with equal number of rivets all rows

(ii) For joints having number of rivets in outer row is half the number in the inner row

For general purpose

DIAMETER OF RIVET (D) AND PITCH (P)

Diameter of rivet. When the diameter of rivet (d)

can be obtained by equating crushing and shearing strengths.
The pitch is calculated by equating tearing strength of plate to shearing strength or
crushing strength of rivets which even is less. But maximum pitch should not be greater
than three times the diameter of rivet where as minimum pitch should be two times the
diameter of rivet.
Maximum pitch.

Minimum pitch.

Eccentrically loaded riveted joints

In the previous cases of riveted joints, it us designed with the assumption that all the rivet
of the joints are subjected to shear force i.e.. line of action of force passes through the
centre of gravity. But in some practical cases of structural joints the line of action of force
does not pass through the centre of gravity. It is offset by a distance ‘ e as shown in fig
Such type of joint is called eccentrically loaded riveted joint.
The eccentric loading produces an additional shear force on the rivet system that tends to
rotate the structural joint as shown in fig.

Fig. ( Di.%trihntion of direct and secondary shear force

The following procedure should be adopted for force analysis of above system.

1. Determine centre of gravity of the joint by taking moment about OX and OY

2. The eccentric load will produce two types of stresses on the

Joint.
(i) Direct shear stress due to external for P
(ii) Moment P x e which rises to rotate the joint .

Primary or direct shear force,

Acting parallel to the load P
The secondary shear force on each rivet can be determined from
The fact that the amount of shear force shared by each rivet is
5. Calculate the resultant force on all rivets. The resultant force on
the rivet
Where, 8 = Angle between the direct and the secondary shear force.
6. Determine the diameter of the rivet by considering the shear failure of the rivet

Maximum resultant shear load.

DESIGN OF RIVETED JOINT FOR PRESSURE VESSELS

In a pressure vessel i.e. Boiler of cylindrical shape, there are two types of joints.
(1,) Longitudinal joint:
It is used to join the ends of the plate to get the required diameter of a boiler, as shown in
fig Generally. The double-strap bull joint is used for this purpose.

(ii) Circumferential joint

It is used to get the required length of the shell and to close its ends. A lap joint is most
widely used for this purpose. Refer fig 3.4.14.

DESIGN OF LONGITUDIONAL BUTT JOINT FOR

PRESSURE VESSELS

According to I.B.R the following procedure should be adopted for design of longitudinal
butt joint for a pressure vessel.
1. Select a suitable butt joint according to the diameter of the boiler shell and desired
efficiency from table 3.4.3.
2. The thickness of the boiler shell plate is determined by the following formula

As corrosion allowable
Where,
p- Stream pressure, N/mm
D-Diameter of the shell, mm.
-Allowable tensile strength of the material, N/mm
-Efficiency of the joint (refer table 3.4.3).

The thickness of the boiler shell should not be less than 7mm.
 3. Calculate diameter of the rivet by the Unwin formula

4. If t < 1 then the diameter of the rivet may be calculated by equating the shearing
and crushing strength of the rivet. The diameter of rivet should not be less than
the thickness of the plate.
5. The pitch of the riveted joint is determined by equating the shear strength to the
tearing strength of the joint It may be noted that,

6. The transverse or back pitch may be selected from the equation

given under topic 3.4.9.
7. Thickness of the cover plate for butt joint can be calculated from the relations
given under the topic 3.4.S.
8. . The margin or marginal pitch, m = 1.5 d.

DESIGN OF CIRCUMFERENTIAL JOINT FOR PRESSURE VESSELS

1. The multi row lap joint is commonly used for circumferential joint of the boiler.
2. According to I.B.R, the following steps should be followed for the design of
circumferential joint.
The diameter of the rivet and thickness of the plate (1) should be
Same as that of longitudinal joint. Refer step 2 and step 3 of longitudinal joint.
1. Total number of rivets required for the circumferential joint

1. The pitch of the circumferential joint is calculated from the tearing efficiency ol the
joint.

2. 4. The transverse pitch is selected in a similar way as in longitudinal joint. Refer topic
3.4.9.
3. . Number of rows required can be calculated by No. of row.

PROBLEMS
Problem
 A double riveted lap joint is to be made between 6mm plates. The safe working stresses
for plates and rivet materials are . = 80N/,mm2 = 50N/mm2 Design the joint
Given data:
Double riveted lap joint
Thickness of the plate t = 6mm.
Working stresses

To find:
Design of the joint.
(i) Thickness of the cover plate.
(ii) Diameter of the rivet, d.
(iii)Margin of the rivet, e.
(iv) Distance between two rows, ph.
(v) Pitch of the rivet, p.
(vi) Efficiency of the riveted joint.
Solution:
(i) Thickness of the cover plate, t,:

(ii) Diameter of the rivet, d:

Results:
Design of the joint.
(i) Thickness of the cover plate, Ii = 3.75mm
(ii) Diameter of the rivet, d= 13mm
(in) Margin of the rivet, e = 19.5mm
(iv) Distance between two rows, Pb = 39mm
(v) Pitch of the rivet, p = 50mm
(vi)Efficiency of the riveted joint 1 = 69.33%
problem .
A single riveted lap joint is to be made of 10mm plates. Find the diameter of rivet
their pitch and efficiency of the joint Take f = 64N/mm2 tensile = 80N/mm and so
design the joint that its strength to withstand shear of rivets equals its strength to
withstand tearing of the plate across the line of rivet holes.
Given data:
Thickness of the plate, t = 10mm
Shear = 64N/mm
To find:

(i) Diameter of the rivet, d.

(ii) Pitch of the rivet, p.
(iii) Efficiency of the joint.
(iii) Design of joint
Solution:
TWO MARK QUESTIONS
Screw fasteners
1 what are the various types of screwed fasteners used in machine construction?
2 Classify the riveted joints.
3. List out the application areas of square, buttress and trapezoidal threads.
4. Explain the use of a taper pin.Welded joints
5. Define the term efficiency of a riveted joint.
6. Want is a self-locking screw?
7. What are the methods of riveting? and explain briefly.
8 Define. Welding and what are the main types of welding?
9. What are the important general applications, advantages and limitations of
welding?
10 List out all the rivet terminologies and state its significance.

PROBLEMS

1 .A bolted joint is used to connect two components. The combined stiffness of the
two components is twice the bolt stiffness. Initial tightening load is 5kN. The external
force of I0kN creates further tension in the bolt. The bolt is made of plain carbon steel
30C8 for which yield strength in tension is 400N/mm Using a factor of safety of 3
and assuming coarse threads, select a suitable bolt size.

2.A steel plate subjected to a force of3kNand fixed to a vertical

channel by means of four identical bolts is shown in Fig . The bolts are made of plain
carbon steel 45C8 (Yield strength = 380N/mm and the FOS is 2. Determine the
nominal diameter of the bolt.
3. Find the site of the weld for the conversion shown in flg. If the tensile load acting
of the connection is 11 kN. Assume permissible shear stress on the weld is 9OKpa.

4. A bracket shown in figure carries a load of l35kN. Calculate the size of the weld, if
the allowable stress is not to exceed 70N/mm

5. A plate 120mm wide and 12.5mm thick is to be welded to another plate by means
of single transverse and double parallel fillet welds. Determine the length of weld run
in each case, if the joint is subjected to varying loads. The recommended design stress
in tension is not exceed 90N and in shear 64N/mm2 for static loading.