Design of Mechanical
Fasteners/Joints
Dr Krishan Kumar
Assistant Professor
Deptt. of Mechanical Engineering
YMCA University Of Science & Technology Faridabad
Mechanical Joints
A mechanical joint is a segment of a
structure/mechanism which connects two
distinct
parts
for
some
specific
requirement.
Mechanical joints may be of two type;
1. Permanent Type
i. Riveted Joints
ii. Welded Joints
2. Non-Permanent Type
i. Screwed Joints ii. Cotter/Knuckle Joints
Riveted Joints
Failure of Riveted Joints
1. Tearing of Main Plate
i. At edge
Margin, m= 1.5d
ii. Across row of rivets
Failure Area,
Af = (p-d).t
Strength,
P= Failure Area x Stress Responsible for
this Failure
Pt = (p-d).t.σt
2. Shearing of Rivets
Failure Area,
Af = ∏d2/4 (one rivet)
Strength,
P= Failure Area x Stress Responsible for this
Failure
Ps= ∏d2/4 .n .σs
For double shear,
Atot= 1.875x Af
3. Crushing of Rivets/Plate
Failure Area,
Af = d.t (for one rivet)
Strength,
P= Failure Area x Stress Responsible for
this Failure
PC= n.d.t.σc (n= number of rivets per pitch)
Application of Riveted Joints
Steam Boilers
◦ Longitudinal Joints
◦ Circumferential Joints
Structural Bridges
Diamond Riveting
Welded Joints
Failure Area/Shear Plane
Failure Area,
Af= t.l
here,
t= 0.707s
Af= 0.707s.l
Failure of Transverse Fillet
Weld
Strength,
P= Failure Area x Stress Responsible for
this Failure
Pt = 0.707s.l.σt
Pt = 1.414s.l.σt
Failure of Parallel Fillet
Weld
Strength,
Ps = 1.414s.l.σs
P= 0.707s.l1.σt + 1.414s.l2.σs
Screwed Joints
1. Stresses due to initial tightening
2. Stresses due to external forces
3. Stress due to combination of above
Failure of Screwed Joints
1. Stresses due to initial tightening
i. Tensile stress due to stretching,
P= 2840.d
P= 1420.d
(for fluid tight joints)
(for non-fluid tight joints)
ii. Torsional shear stress frictional resistance of
threads
σs = 16T/∏(dc)3
iii. Crushing stress on threads
σc = P/∏.{d2-(dc)2}.n
2. Stresses due to external forces
i. Tensile failure load
ii. Shear failure load
iii. Combined load
(tensile & shear)
,
iii. Stress due to combination of internal and
external stresses
P1 = Initial tension (tightening)
P2 = External load
a = Ratio of elasticity of connected parts to
elasticity of bolt
Cotter Joint
1. Socket & spigot cotter joint
2. Sleeve & cotter joint
3. Gib & cotter joint
Design of Socket & Spigot
Cotter Joint
i.
Tensile Failures
ii. Shear Failures
iii. Crushing Failures
Tensile Failures
Failure of the rods in tension
Failure of spigot in tension across
the slot
Failure of the socket in tension
across the slot
Shear Failures
Failure of cotter in shear
Failure of socket end in
shearing
Failure of spigot end in
shear
Failure of the spigot
collar in shearing
Crushing Failures
Failure of the spigot or cotter
in crushing
Failure of spigot
collar in crushing
Failure of the socket collar in
crushing
Design of Sleeve & Cotter
Joint
i.
Tensile Failures
ii.
Shear Failures
iii. Crushing Failures
Tensile Failures
Failure of rods in Tension
Failure of rod in
tension across the
slot
Failure of sleeve in tension
across the slot
Shear Failures
Failure of cotter in shear
Failure of rod at
slot in shear
Failure of sleeve at slot in shear
Crushing Failures
Failure of the rod or cotter in
crushing
Failure of sleeve or cotter in
crushing
Design of Gib & Cotter Joint
i.
Tensile Failures
ii. Shear Failures
iii. Crushing Failures
Tensile Failures
Failure of square rod in
tension at slot
Failure of strap in tension
at slot
Shear Failures
Failure of the gib and
cotter in shearing
Failure of square
rod in shearing
Failure of the strap
end in shearing
Crushing Failures
Failure of the strap or gib in
crushing
Failure of square rod or gib in
crushing
Design of Knuckle Joint
i.
Tensile Failures
ii. Shear Failures
iii. Crushing Failures
Tensile Failures
Failure of the solid rod in
tension
Failure of
single eye
end in
Tension
Failure of the forked end in
tension
Shear Failures
Failure of the knuckle pin in shear
Failure of the single
eye or rod end in
shearing
Failure of the forked end in
shear
Crushing Failures
Failure of the single eye
end in crushing
Failure of the forked end
in crushing
