DME unit 1 notes 2021 rev for diploma mechanical engineering

NSS PTC , Pandalam, DME unit 1 Page 1
Design of Machine Elements
Unit 1
1. Introduction
A machine is an assemblage of rigid bodies or elements, with successfully constrained relative motion, used
for transmitting or transforming energy. The components of a machine such as bolt, key, shaft, gear, cam,
pulley, belt, spring etc are called as machine elements.
2. Classification of machine design
Machine design can be broadly classified into three. They are
a) Adaptive design
b) Development design
c) New design
2.1Adaptive Design
In this design an existing design is adapted without much modifications. Special skills are not required for
this type of design. Eg: design of a bi-cycle or an IC engine.
2.2 Development design
An existing design is modified to get a better machine by applying science principles and technical
knowledge. The final product may much differ from the existing design.
2.3 New design
New machines are created from a scratch through the application of scientific laws, principles of mechanics,
technical knowhow and creative thinking. It is also called as an invention.
3. Design of machine elements
Machine elements can be designed either by
a) Analytical or rational design
b) Empirical design
3.1 Analytical design
It is carried out purely by mathematical formulae based on the strength and principles of mechanics.
3.2 Empirical design
Empirical designs are carried out by empirical relations derived on the past experience and existing practice.
Eg. Design of a flange coupling, flywheel etc
4. Design considerations
The factors to be considered for the design of a machine or machine elements are
1 Strength 8 Mechanism 15 Ergonomics
2 Cost 9 Weight 16 Processability
3 Reliability 10 Standard parts 17 Thermal considerations
4 Service life 11 Shipment 18 Rigidity
5 Volume 12 Corrosion 19 Surface finish and style
6 Existing products 13 Lubrication 20 Maintenance
7 Size and shape 14 Utility 21 Safety

5. General procedure for Machine design
Following steps are generally followed for the design of a machine or machine element.
1. Specifying the problem or recognizing the need
Design is started with a written statement about the need and all aspects of the problem.
2. Selection of proper mechanism
A suitable mechanism is designed based on the available input power motion and the desired output
movement.
3. Analysis of forces
Analysis of forces and power to be transmitted by each element of the mechanism is carried out and sketches
showing the magnitude, direction and point of application are drawn.
4. Selection of material
Material is selected for each element of machine by considering cost, strength, wear, service conditions,
corrosion etc.
5. Selection of Factor of safety
A suitable factor of safety is assigned to the element and based on this design stress is calculated.
6. Calculation of cross sectional dimensions
Limiting to the design stress, cross-sectional area of each element and its shape is designed.
7. Modifications
Modifications can be done at any stage to suite the strength, market considerations and cost.
8. Detailed drawing
Detailed drawings are made with complete specifications, method of manufacture, accuracy and surface
finish.
6. Units for Design
The SI unit of stress is N/m2
or Pascal , which is a very small unit when compared to the strength of machine
element. Hence the stress values of machine elements are usually expressed in MPa.
1 M Pa = 106
N/m2
= 1N/mm2
By using the stress value in M Pa, the dimensions of elements can be substituted in mm directly.
7. Working stress
The actual stress induced in a machine element under its working is called working stress.
8. Design stress
The maximum stress value permitted to a machine element is called design stress. It is the maximum working
stress that an element can hold at any condition. The designer is setting this maximum allowable stress value
by considering all possible modes of failures.

9. Factor of Safety
To avoid the failure of a machine element due to unforeseen factors, the design stress permitted to a machine
element is kept well below the yield stress or ultimate stress depending upon the nature of material whether
ductile or brittle respectively.
For ductile material F.S =
For Brittle material F.S =
9.1 Selection of Magnitude of factor of safety
Magnitude of factor of safety is selected based on the extent of uncertainties and unknowns involved in the
design and the reliability needed for the machine. Selection of factor of safety depends upon the following
factors.
1. Type of loading – static or dynamic
2. Type of material
3. Effect of manufacturing process
4. Level of reliability required
5. Effect of heat treatment
6. Effect of quality control
7. Effect of time and environment
8. Market segment.

10. Simple mechanisms
10.1. Kinematic link (Link)
Each part of a machine, which moves relative to some other part, is known as a kinematic link (or simply link) or
element. A link or element need not to be a rigid body, but it must be a resistant body. A body is said to be a resistant
body if it is capable of transmitting the required forces with negligible deformation. Thus a link should have the
following two characteristics:
1. It should have relative motion, and
2. It must be a resistant body.
10.1.1. Types of Links
There are three types of links :
1. Rigid link. A rigid link is one which does not undergo any deformation while transmitting motion. Strictly speaking,
rigid links do not exist. However, as the deformation of a connecting rod, crank etc. of a reciprocating steam engine is
not appreciable, they can be considered as rigid links.
2. Flexible link. A flexible link is one which is partly deformed in a manner not to affect the transmission of motion.
For example, belts, ropes, chains and wires are flexible links and transmit tensile forces only.
3. Fluid link. A fluid link is one which is formed by having a fluid in a receptacle and the motion is transmitted through
the fluid by pressure or compression only, as in the case of hydraulic presses, jacks and brakes.
10.2. Kinematic Pair
The two links or elements of a machine, when in contact with each other, are said to form a pair. If the relative motion
between them is completely or successfully constrained (i.e. in a definite direction), the pair is known as kinematic
pair.
10.2.1. Classification of Kinematic Pairs
The kinematic pairs may be classified according to the following considerations :
1. According to the type of relative motion between the elements. The kinematic pairs according to type of relative
motion between the elements may be classified as discussed below:
(a) Sliding pair. When the two elements of a pair are connected in such a way that one can only slide relative to the
other, the pair is known as a sliding pair. The piston and cylinder, cross-head and guides of a reciprocating steam
engine, ram and its guides in shaper, tail stock on the lathe bed etc. are the examples of a sliding pair.
(b) Turning pair. When the two elements of a pair are connected in such a way that one can only turn or revolve
about a fixed axis of another link, the pair is known as turning pair. A shaft with collars at both ends fitted into a
circular hole, the crankshaft in a journal bearing in an engine, lathe spindle supported in head stock, cycle wheels
turning over their axles etc. are the examples of a turning pair. A turning pair also has a completely constrained
motion.
(c) Rolling pair. When the two elements of a pair are connected in such a way that one rolls over another fixed link,
the pair is known as rolling pair. Ball and roller bearings are examples of rolling pair.
(d) Screw pair. When the two elements of a pair are connected in such a way that one element can turn about the
other by screw threads, the pair is known as screw pair. The lead screw of a lathe with nut, and bolt with a nut are
examples of a screw pair.
(e) Spherical pair. When the two elements of a pair are connected in such a way that one element (with spherical
shape) turns or swivels about the other fixed element, the pair formed is called a spherical pair. The ball and socket
joint, attachment of a car mirror, pen stand etc., are the examples of a spherical pair.
2. According to the type of contact between the elements. The kinematic pairs according to the type of contact between
the elements may be classified as discussed below :
(a) Lower pair. When the two elements of a pair have a surface contact when relative motion takes place and the
surface of one element slides over the surface of the other, the pair formed is known as lower pair. It will be seen
that sliding pairs, turning pairs and screw pairs form lower pairs.

(b) Higher pair. When the two elements of a pair have a line or point contact when relative motion takes place and
the motion between the two elements is partly turning and partly sliding, then the pair is known as higher pair. A
pair of friction discs, toothed gearing, belt and rope drives, ball and roller bearings and cam and follower are the
examples of higher pairs.
3. According to the type of closure. The kinematic pairs according to the type of closure between the elements may be
classified as discussed below :
(a) Self closed pair. When the two elements of a pair are connected together mechanically in such a way that only
required kind of relative motion occurs, it is then known as self closed pair. The lower pairs are self closed pair.
(b) Force - closed pair. When the two elements of a pair are not connected mechanically but are kept in contact by
the action of external forces, the pair is said to be a force-closed pair. The cam and follower is an example of force
closed pair, as it is kept in contact by the forces exerted by spring and gravity.
10.3.Types of Constrained Motions
Following are the three types of constrained motions :
10.3.1. Completely constrained motion.
When the motion between a pair is limited to a definite direction irrespective of the direction of force applied, then the
motion is said to be a completely constrained motion. For example, the piston and cylinder (in a steam engine) form a
pair and the motion of the piston is limited to a definite direction (i.e. it will only reciprocate) relative to the cylinder
irrespective of the direction of motion of the crank, as shown in Fig.
Fig.1 Square bar in a square hole. Fig.2 Shaft with collars in a circular hole.
The motion of a square bar in a square hole, as shown in Fig. 1, and the motion of a shaft with collars at each end in a
circular hole, as shown in Fig. 2, are also examples of completely constrained motion.
10.3.2. Incompletely constrained motion.
When the motion between a pair can take place in more than one direction, then the motion is called an incompletely
constrained motion. The change in the direction of impressed force may alter the direction of relative motion between
the pair. A circular bar or shaft in a circular hole, as shown in Fig. 3, is an example of an incompletely constrained
motion as it may either rotate or slide in a hole. These both motions have no relationship with the other.
Fig. 3. Shaft in a circular hole. Fig. 4. Shaft in a foot step bearing.
10.3.3. Successfully constrained motion.
When the motion between the elements, forming a pair, is such that the constrained motion is not completed by itself,
but by some other means, then the motion is said to be successfully constrained motion. Consider a shaft in a foot-step
bearing as shown in Fig.4. The shaft may rotate in a bearing or it may move upwards. This is a case of incompletely

constrained motion. But if the load is placed on the shaft to prevent axial upward movement of the shaft, then the
motion of the pair is said to be successfully constrained motion.
10.4. Kinematic Chain
When the kinematic pairs are coupled in such a way that the last link is joined to the first link to transmit definite
motion (i.e. completely or successfully constrained motion), it is called a kinematic chain. In other words, a kinematic
chain may be defined as a combination of kinematic pairs, joined in such a way that each link forms a part of two pairs
and the relative motion between the links or elements is completely or successfully constrained.
For example, the crankshaft of an engine forms a kinematic pair with the bearings which are fixed in a pair, the
connecting rod with the crank forms a second kinematic pair, the piston with the connecting rod forms a third pair and
the piston with the cylinder forms a fourth pair. The total combination of these links is a kinematic chain.
10.5. Mechanism
When one of the links of a kinematic chain is fixed, the chain is known as mechanism. It may be used for transmitting
or transforming motion. A mechanism with four links is known as simple mechanism, and the mechanism with more
than four links is known as compound mechanism. When a mechanism is required to transmit power or to do some
particular type of work, it then becomes a machine
10.5.1.Inversion of Mechanism
When one of links is fixed in a kinematic chain, it is called a mechanism. So we can obtain as many mechanisms as the
number of links in a kinematic chain by fixing, in turn, different links in a kinematic chain. This method of obtaining
different mechanisms by fixing different links in a kinematic chain, is known as inversion of the mechanism.
10.6. Types of Kinematic Chains
The most important kinematic chains are those which consist of four lower pairs, each pair being a sliding pair or a
turning pair. The following three types of kinematic chains with four lower pairs are important from the subject point of
view :
1. Four bar chain or quadric cyclic chain,
2. Single slider crank chain, and
3. Double slider crank chain.
10.6.1.Four Bar Chain or Quadric Cycle Chain
The simplest and the basic kinematic chain is a four bar chain or quadric cycle chain, as shown in Fig. It consists of four
links, each of them forms a turning pair at A, B, C and D. The four links may be of different lengths.
According to Grashof ’s law for a four bar mechanism, the sum of the shortest and longest link lengths should not be
greater than the sum of the remaining two link lengths if there is to be continuous relative motion between the two links.
A very important consideration in designing a mechanism is to ensure that the input crank makes a complete revolution
relative to the other links. The mechanism in which no link makes a complete revolution will not be useful. In a four bar

chain, one of the links, in particular the shortest link, will make a complete revolution relative to the other three links, if
it satisfies the Grashof ’s law. Such a link is known as crank or driver. Inversions of four bar chain are
1. Beam engine (crank and lever mechanism).
2. Coupling rod of a locomotive (Double crank mechanism).
3. Watt’s indicator mechanism (Double lever mechanism).
10.6.2. Single Slider Crank Chain
A single slider crank chain is a modification of the basic four bar chain. It consist of one sliding pair and three turning
pairs. It is, usually, found in reciprocating steam engine mechanism. This type of mechanism converts rotary motion
into reciprocating motion and vice versa. In a single slider crank chain, as shown in Fig. 5.22, the links 1 and 2, links 2
and 3, and links 3 and 4 form three turning pairs while the links 4 and 1 form a sliding pair. Inversions of single slider
crank chain are
1. Pendulum pump or Bull engine.
2. Oscillating cylinder engine.
3. Rotary internal combustion engine or Gnome engine
4. Crank and slotted lever quick return motion mechanism
5. Whitworth quick return motion mechanism.
10.6.3. Double Slider Crank Chain
A kinematic chain which consists of two turning pairs and two sliding pairs is known as double slider crank chain, as
shown in Fig. We see that the link 2 and link 1 form one turning pair and link 2 and link 3 form the second turning pair.
The link 3 and link 4 form one sliding pair and link 1 and link 4 form the second sliding pair. Inversions of double slider
crank chain are
1. Elliptical trammels
2. Scotch yoke mechanism
3. Oldham’s coupling

1. Welded Joints
1.1 Introduction
A welded joint is a permanent joint which is obtained by the fusion of the edges of the two parts to be
joined together, with or without the application of pressure and a filler material. The heat required for the
fusion of the material may be obtained by burning of gas (in case of gas welding) or by an electric arc (in
case of electric arc welding).
1.2 Advantages and Disadvantages of Welded Joints over Riveted Joints
Following are the advantages and disadvantages of welded joints over riveted joints.
Advantages
1. The welded structures are usually lighter than riveted structures. This is due to the reason, that in
welding, gussets or other connecting components are not used.
2. The welded joints provide maximum efficiency (may be 100%) which is not possible in case of riveted
joints.
3. Alterations and additions can be easily made in the existing structures.
4. As the welded structure is smooth in appearance, therefore it looks pleasing.
5. In welded connections, the tension members are not weakened as in the case of riveted joints.
6. A welded joint has a great strength. Often a welded joint has the strength of the parent metal itself.
7. Sometimes, the members are of such a shape (i.e. circular steel pipes) that they afford difficulty for
riveting. But they can be easily welded.

8. The welding provides very rigid joints. This is in line with the modern trend of providing rigid frames.
9. It is possible to weld any part of a structure at any point. But riveting requires enough clearance.
10. The process of welding takes less time than the riveting.
Disadvantages
1. Since there is an uneven heating and cooling during fabrication, therefore the members may get
distorted or additional stresses may develop.
2. It requires a highly skilled labour and supervision.
3. The capacity of weld structures to damp vibrations is poor
4. Since no provision is kept for expansion and contraction in the frame, therefore there is a possibility of
cracks developing in it.
5. The inspection of welding work is more difficult than riveting work.
1.3 Types of Welded Joints
1.3.1 Lap Joint or Fillet joint
The lap joint or the fillet joint is obtained by overlapping the plates and then welding the edges of the
plates. The cross-section of the fillet is approximately triangular. The fillet joints may be 1. Single transverse
fillet, 2. Double transverse fillet and 3. Parallel fillet joints.

Fig.1. Types of Lap Joints
The fillet joints are shown in Fig.1. A single transverse fillet joint has the disadvantage that the edge of the
plate which is not welded can buckle or warp out of shape.
1.3.2 Butt Joint
The butt joint is obtained by placing the plates edge to edge as shown in Fig.2. In butt welds, the plate
edges do not require bevelling if the thickness of plate is less than 5 mm. On the other hand, if the plate
thickness is 5 mm to 25 mm, the edges should be bevelled to V or U- groove on both sides.
Fig. 2. Types of Butt joints

The other type of welded joints are corner joint, edge joint and T-joint as shown in Fig. 3.
Fig. 3. Other types of Joints
1.4. Basic Weld Symbols

1.4.1 Elements of a Welding Symbol
A welding symbol consists of the following eight elements:
1. Reference line, 2. Arrow,3. Basic weld symbols, 4. Dimensions and other data,5. Supplementary symbols,
6. Finish symbols,7. Tail, and 8. Specification, process or other references.
Standard Location of Elements of a Welding Symbol
The arrow points to the location of weld, the basic symbols with dimensions are located on one or both
sides of reference line. The specification if any is placed in the tail of arrow.
Fig. 1. shows the standard locations of welding symbols represented on drawing.
Fig.1 Standard location of weld symbols.
Some of the examples of welding symbols represented on drawing are shown in the following table.

Representation of welding symbols.
1.5. Strength of Fillet weld
1.5.1. Strength of Transverse Fillet
We have already discussed that the fillet or lap joint is obtained by overlapping the plates and then welding
the edges of the plates. The transverse fillet welds are designed for tensile strength. Let us
and double transverse fillet welds as shown in Fig. 1(
1.5. Strength of Fillet weld
Strength of Transverse Fillet Welded Joints
the edges of the plates. The transverse fillet welds are designed for tensile strength. Let us
and double transverse fillet welds as shown in Fig. 1(a) and (b) respectively.
Fig.1 Transverse fillet welds.
the edges of the plates. The transverse fillet welds are designed for tensile strength. Let us consider a single

The length of each side is known as leg
hypotenuse from the intersection of legs is known as
obtained at the throat , which is given by the product of the throat thickness and length of weld.
Let t = Throat thickness , s = Leg or size of weld= Thickness of plate, and
From Fig., we find that the throat thickness,
Therefore, Minimum area of the weld or throat area,
If σtis the allowable tensile stress for the weld metal, then the tensile strength of the joint for single fillet
weld,
P = Throat area × Allowable tensile stress = 0.707
And tensile strength of the joint for double fillet weld,
P = 2 × 0.707
Note: Since the weld is weaker
is given a reinforcement which
1.5.2 Strength of Parallel Fillet Wel
The parallel fillet welded joints are designed for shear strength. Consider a double parallel
joint as shown in Fig.3 (a). We have already discussed in the previous article,
weld or the throat area,
If τ is the allowable shear stress for the weld metal, then the shear strength of the joint for
fillet weld,
P = Throat are
And shear strength of the joint for double parallel fillet weld,
P = 2
leg or size of the weld(s) and the perpendicular distance of the
ersection of legs is known as throat thickness(t). The minimum area of the weld is
= Leg or size of weld= Thickness of plate, and l = length of w
, we find that the throat thickness, t = s × sin 45° = 0.707 s
Therefore, Minimum area of the weld or throat area,
A = Throat thickness × Length of weld
= t × l = 0.707 s × l
is the allowable tensile stress for the weld metal, then the tensile strength of the joint for single fillet
= Throat area × Allowable tensile stress = 0.707 s × l × σt
And tensile strength of the joint for double fillet weld,
= 2 × 0.707 s × l × σt= 1.414 s × l × σt
Since the weld is weaker than the plate due to slag and blow holes, therefore the weld
is given a reinforcement which may be taken as 10% of the plate thickness.
Strength of Parallel Fillet Welded Joints
are designed for shear strength. Consider a double parallel
). We have already discussed in the previous article, that the minimum area of
A = 0.707 s × l
s for the weld metal, then the shear strength of the joint for
= Throat area × Allowable shear stress = 0.707 s × l × τ
or double parallel fillet weld,
= 2 × 0.707 × s × l × IJ = 1.414 s × l × τ
Fig.3
and the perpendicular distance of the
The minimum area of the weld is
weld,
is the allowable tensile stress for the weld metal, then the tensile strength of the joint for single fillet
than the plate due to slag and blow holes, therefore the weld
are designed for shear strength. Consider a double parallel fillet welded
that the minimum area of
s for the weld metal, then the shear strength of the joint for single parallel

Notes: 1. If there is a combination of single transverse and double parallel fillet welds as
then the strength of the joint is given by the sum of strengths of single
welds. Mathematically,
P = 0.707s
2. In order to allow for starting and stopping of the bead, 12.5 mm should be added to the
weld obtained by the above expression.
3. For reinforced fillet welds, the throat dimension may be taken as 0.85
Problem:
A plate 100 mm wide and 10 mm thick is to be welded to another plate by means of double parallel fillets.
The plates are subjected to a static load of 80 k
the weld does not exceed 55 MPa.
1.6 Strength of Butt Joints
The butt joints are designed for tension or compression. Consider a single V
Fig. 4(a).
tion of single transverse and double parallel fillet welds as shown in Fig. (
ngth of the joint is given by the sum of strengths of single transverse and double
s × l1 × σt+ 1.414 s × l2 × τ Where l1 is normally the width o
and stopping of the bead, 12.5 mm should be added to the
expression.
e throat dimension may be taken as 0.85 t.
The plates are subjected to a static load of 80 kN. Find the length of weld if the permissible shear stress in
The butt joints are designed for tension or compression. Consider a single V-butt joint as shown in
Fig.4. Butt Joints
shown in Fig. (b),
transverse and double parallel fillet
is normally the width of the plate.
and stopping of the bead, 12.5 mm should be added to the length of each
N. Find the length of weld if the permissible shear stress in
butt joint as shown in

In case of butt joint, the length of leg or size of weld is equal to the throat thickness which is equal
to thickness of plates. Therefore, Tensile strength of the butt joint (single-V or square butt joint),
P = t × l × σt
Where l = Length of weld. It is generally equal to the width of plate. And tensile strength for double-V butt
joint as shown in Fig. 4(b) is given by
P = (t1 + t2) l × σt
Where t1 = Throat thickness at the top, and t2 = Throat thickness at the bottom.
It may be noted that size of the weld should be greater than the thickness of the plate, but it may be
less. The following table shows recommended minimum size of the welds.
Problem:
A plate 100 mm wide and 12.5 mm thick is to be welded to another plate by means of parallel fillet
welds. The plates are subjected to a load of 50 kN. Find the length of the weld so that the maximum
stress does not exceed 56 MPa. Consider the joint first under static loading and then under fatigue
loading.

Problem:
A plate 75 mm wide and 12.5 mm thick is joined with
another plate by a single transverse weld and a double
parallel fillet weld as shown in Fig. The maximum tensile
and shear stresses are 70 MPa and 56 MPa respectively.
Find the length of each parallel fillet weld, if the joint is
subjected to both static and fatigue loading.

2. Rivetted Joints
A rivet is a short cylindrical bar with a head and tapered tail. The rivets are used to make permanent
fastening between the plates such as in structural work, ship building, bridges, tanks and boiler shells. The
riveted joints are widely used for joining light metals.
2.1 Methods of Riveting
The function of rivets in a joint is to make a connection that has strength and tightness. The strength is
necessary to prevent failure of the joint. The tightness is necessary in order to contribute to strength
and to prevent leakage as in a boiler or in a ship hull.
When two plates are to be fastened together by a rivet as shown in Fig. (a), the holes in the plates are
punched and reamed or drilled. Punching is the cheapest method and is used for relatively thin plates
and in structural work.
(a) Initial position. (b) Final position.
Fig. Methods of riveting.
When a cold rivet is used, the process is known as cold riveting and when a hot rivet is used, the
process is known as hot riveting. The cold riveting process is used for structural joints while hot riveting is
used to make leak proof joints.The riveting may be done by hammer or by a riveting machine.

2.2 Types of Rivet Heads
Fig. Rivet heads for general purposes (from 12 mm to 48 mm diameter)
2.3. Types of Riveted Joints
Mainly there are two types of riveted joints, 1) Lap joint, 2) Butt joint
1. Lap Joint
A lap joint is that in which one plate overlaps the other and the two plates are then riveted together.
2. Butt Joint
A butt joint is that in which the main plates are kept in alignment butting (i.e. touching) each other and a
cover plate (i.e. strap) is placed either on one side or on both sides of the main plates. The cover plate is
then riveted together with the main plates.

2.3.1 Classification of rivetted joints
2.3.2. Lap joints
(a) Single riveted lap joint. (b) Double riveted lap joint (c) Double riveted lap (Chain riveting).
Joint (Zig-zag riveting).
Fig. Single and double riveted lap joints.
Similarly the joints may be triple riveted or quadruple riveted.
When the rivets in the various rows are opposite to each other, as shown in Fig. (b), then the joint is said to
be chain riveted. On the other hand, if the rivets in the adjacent rows are staggered in such a way that
every rivet is in the middle of the two rivets of the opposite row as shown in Fig. (c), then the joint is said to
be zig-zag riveted.
1. Lap joint
a) Single rivetted joint
b) Double rivetted joint
i. chain type,
ii. Zig-zag type
c) Triple rivetted joint
i. chain type,
ii. Zig- zag type
2) Butt joint.
a) Single strap butt joint
1. Single rivetted joint
2. Double rivetted joint:
i. chain type,
ii. Zig Zag type
b) Double strap butt joint
1. Single rivetted joint
2. Double rivetted joint
i. chain type,
ii. Zig Zag type
3. Triple rivetted joint
i. chain type,
ii. Zig Zag type

(a) Chain riveting.
Fig. 9.7.
2.3.3. Butt joints
Fig. Single riveted double strap butt joint.
(b) Zig-zag riveting.
Fig. 9.7. Triple riveted lap joint.
Single riveted double strap butt joint.

(a) Chain a riveting. (b) Zig-zag riveting
Fig. Double riveted double strap (equal) butt joints.
Fig.1. Triple riveted double strap (unequal) butt joint.
2.4 Important Terms Used in Riveted Joints
The following terms in connection with the riveted joints are important from the subject point of view:
1. Pitch. It is the distance from the centre of one rivet to the centre of the next rivet measured parallel to
the seam as shown in Fig.1 It is usually denoted by p.
2. Back pitch. It is the perpendicular distance between the centre lines of the successive rows as shown in
Fig.1. It is usually denoted by pb.
3. Diagonal pitch. It is the distance between the centers of the rivets in adjacent rows of zigzag riveted
joint as shown in Fig. It is usually denoted by pd.

4. Margin or marginal pitch. It is the distance between the centres of rivet hole to the nearest edge of the
plate as shown in Fig. It is usually denoted by m.
2.5 Failures of a Riveted Joint
A riveted joint may fail in the following ways:
1. Tearing of the plate at an edge. A joint may fail due to tearing of the plate at an edge as shown
in Fig.3. This can be avoided by keeping the margin, m = 1.5d, where d is the diameter of the rivet hole
.
2. Tearing of the plate across a row of rivets. Due to the tensile stresses in the main plates, the
main plate or cover plates may tear off across a row of rivets as shown in Fig.
3. Shearing of the rivets. The plates which are connected by the rivets exert tensile stress on the rivets,
and if the rivets are unable to resist the stress, they are sheared off as shown in Fig.
(a) Shearing off a rivet in a lap joint.
4. Crushing of the plate or rivets. Sometimes, the rivets do not actually shear off under the tensile
stress, but are crushed as shown in Fig. Due to this, the rivet hole becomes of an oval shape and hence
the joint becomes loose. The failure of rivets in such a manner is also known as bearing failure.
Fig. 7. Crushing of a rivet.

2.6. Advantages of a riveted joint
Advantages of a riveted joint compared with a welded joint are as follows:
1. Riveted joints can be used where it is necessary to avoid thethermal after-effects of welding.
2. Riveted joints can be used for metals withPoor weldability like aluminium alloys.
3. When the. joint is made of heterogeneousmaterials, such as the joint between steel
plate and asbestos friction lining, rivetedjoints are preferred
4. Welded joints have poor resistance to vibrations and impact load. A rivetedjoint is ideally suitable in such
situations.
5. Riveted joints are used where thin platesare to be assembled. They are popular,
especially for aircraft structures wherelight structures made of aluminiumalloys are to be fastened.
6. The quality of a riveted joint can beeasily checked while inspection methodsfor welded joint, such a
radiographicinspection of pressure vessel, are costlyand time-consuming-
7. When the riveted joint is dismantled, theconnected components are less damagedcompared with those of
the welded joint.
Disadvantages of a riveted joint comparedwith a welded joint are as follows:
(i) The material cost of a riveted joint is morethan the corresponding material cost ofa welded joint due to high
consumptionof metal.
(ii) The labour cost of riveted joints is morethan that of welded joints. Riveted jointsrequire higher labour input
due to necessityto perform additional operation likelayout and drilling or punching of holes,Besides, the
process of riveting is muchmore complicated and less productive
compared with welding operation.
(ii) The overall cost of riveted joints ismore than that of welded joints due toincreased metal consumption and
higherlabour input. On the other hand, weldingis cheaper compared with riveting.
(iv) Riveted assemblies have more weight thanwelded assemblies due to strap-plates

3. Screwed Fastening
3.1. Thread nomenclature
Major diameter: The major diameter is the diameter of an imaginary cylinder that bounds the
crest of an external thread (d) or the root of an internal thread (D) (Also called as nominal
diameter)
Minor diameter: The minor diameter is the diameter of an imaginary cylinder that bounds the
roots of an external thread (dc) or the crest of an internal thread (Dc)
Pitch diameter: The pitch diameter is the diameter of an imaginary cylinder, the surface of which
would pass through the threads at such points as to make the width of the threads equal to
the width of spaces cut by the surface of the cylinder.
Pitch, p: It is the distance between two corresponding points on adjacent threads measured
parallel to the axis of the thread

Lead: It is the distance that the nut moves parallel to the axis of the screw, when the nut is given
one turn. L = n. p, where n = number of starts of thread, p = pitch of thread
Thread angle: It is the angle between the sides of the thread measured in an axial plane
3.2. Forms of Screw thread
• V-Thread: These are triangular threads with flanks that typically form 60° with each other.
The crests and roots are sharp, but in some cases, as a small flat portion due to limitations
in fabrication.
• American National Thread: Formerly known as the United States Standard Screw Thread,
the American National Thread is a more standardized version of the V-thread which has
specific dimensions to the flatness of the crests and roots of the threads. This form
replaced the V-thread for general use.
• British Whitworth Thread: This was the British counterpart of the American National
Thread.
• Unified Thread: This thread form replaced the American National Thread along with thread
standards from Canada and Britain. This was developed to allow interchangeability of
parts. Unified threads still have the V-shape profile but with rounded or flat crests and
roots. The Unified Thread Standard (UTS) consists of series, namely, Unified Fine (UNF),
Unified Coarse (UNC), Unified Extra Fine (UNEF), and Unified Special (UNS).
• Metric Thread: This thread form was developed to transition from the imperial-based
measurement into the metric system. This was brought by the ISO, displacing the UTS
thread form.
• Square Thread: Square threads are special-purpose threads used for power transmission.
Theoretically, they are the ideal thread for mechanisms and drive applications due to the
perpendicularity of the load-bearing faces or flanks with the axis. However, this form is not
practical due to manufacturing limitations.
• Acme Thread: This thread form is a modification of the square thread. The acme thread is
characterized as having a trapezoidal form with a narrower root than its crest. Acme
threads are stronger and easier to machine than square threads.
• Buttress Thread: In this thread form, one flank is perpendicular or with a slight angle with
the axis while the other has a 45° angle. This thread form is designed to transmit high loads
in one direction.

• Knuckle Thread: Knuckle threads have highly rounded
30°. The rounded profile allows debris to be shifted to not interfere with the meshing of
the threads.
3.3. Advantages of screw joints
A number of advantages offered by threaded joints favours their large
1. Screwed joints are highly reliable in operation
2. They are convenient to assemble and disassemble.
3. For various operating conditions, a wide range of threaded couples can be adopted
4. Screws are relatively cheap to produce due to standardization and highly efficient manufacturing
processes
The main disadvantage of the screwed joints is the stress concentration in the threaded portion which are
vulnerable points under variable load c
3.4. Designation of screw threads
Size of a screw thread is designated by the letter ‘M’ followed by the diameter and pitch, the two being
separated by the sign X. When there is no indication of pitch, course pitch is implied.
Knuckle threads have highly rounded crests and roots with a flank angle of
Advantages of screw joints
A number of advantages offered by threaded joints favours their large-scale applications.
1. Screwed joints are highly reliable in operation
2. They are convenient to assemble and disassemble.
3. For various operating conditions, a wide range of threaded couples can be adopted
of the screwed joints is the stress concentration in the threaded portion which are
vulnerable points under variable load conditions.
Designation of screw threads
separated by the sign X. When there is no indication of pitch, course pitch is implied.
crests and roots with a flank angle of
of the screwed joints is the stress concentration in the threaded portion which are

DME unit 1 notes 2021 rev for diploma mechanical engineering

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