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CHAPTER 2
Design of Machine Elements
Subjected to Direct Stresses
MISSAM RAZA Dept. -Mechanical LJ Polytechnic
Two or more rods subjected to tensile and compressive forces
are fastened together
Their axes are not in alignments but meet in a
point
The joint allows a small angular
moment of one rod relative to another
It can be easily connected and disconnectedApplications: Elevator chains, valve rods, etc
Machine elements subjected to direct stresses
Knuckle Joint
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Knuckle Joint
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Knuckle Joint
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Knuckle Joint
• A cotter joints is a flat wedge link piece of steel of rectangular cross section which is inserted through the rods at high angle to their axes .
• It is uniform in thickness but tapering in width , generally on one side only. Usually the taper is 1 in 30. when a special arrangement like a set-screw is provided for keeping the cotter from slackening ,its taper may be as large as 1 in 7.
• The end of the cotter are made narrow to facilitate the hammering for fixing and removing.
• Cotter joints are generally use to fasten rigidly two rods which is subjected to tensile or compressive stress along their axis. This joint is used to connect two circular rods.
Cotter Joint
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Cotter Joint• This joint in not suitable where the
member are subjected under rotation.• Thus they differ from key joints which
are used to fasten shaft and hubs subjected to tensional stress.
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Cotter Joint
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
The enlarged ends of the rods butt against each other with a common sleeve over them
•The rod ends are enlarged to take care of the weakening effect caused by slots
For circular rods
Sleeve and Cotter Joint
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
• Gib and cotter joints are used for rods of square or rectangular cross section.
• The end of one rod fits the end of the other rod which is made in the form of a strap.
• A gib is used along with the cotter to make this joint.
• Gib is likely a cotter but with two gib heads at its ends .
• The thickness of the gib and cotter are same.
Cotter Joint with a gib
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
One bar end is made in the form of a strap
A Gib is used along with the cotter.
Gib is like a cotter but with two gib heads at its ends .
The thickness of the gib and cotter are same
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Gib and Cotter joint for rectangular rods
Gib and Cotter joint for rectangular rods
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
• Non-permanent– Bolted
• Permanent– Bolted– Welded– Bonded
Threaded FastenersTypes of Fastening
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
p pitch in./threadd diameter (major) in.dp pitch diameter in.dr minor diameter in.
L Lead in.
Thread
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
F
F
tt A
F2
224
rp
tdd
A
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Tensile Stress
• jacks for cars• C-clamps• vises• Instron material testing
machines• machine tools (for positioning of
table)
Where have you seen power screws?
Power Screw Application
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
• Square– strongest– no radial load– hard to manufacture
• Acme– 29° included angle– easier to manufacture– common choice for loading
in both directions• Buttress (contrafuerte)
– great strength– only unidirectional loading
Power Screw Types
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
What “simple machine” does a power screw utilize?
dp
L
N
f
P
F
LIFTING
y
x
pd
L
tan
Ld
LdPdT
p
ppSu
2
Load Analysis
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
cc
p
ppcollarSu d
Ld
Ldd
PTTT
u
2
cc
p
pp d
Ld
Ldd
PTd
2
LIFTING
LOWERING
dp
L N
fP
F
y
x
Continue….
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
cc
p
ppcollarSu d
Ld
Ldd
PTTT
u
cos
cos
2
cc
p
pp d
Ld
Ldd
PTd
cos
cos
2
LIFTING
LOWERING
For acme threads
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
oil lubricated= collar w/ bushing=0.15 ± 0.05
collar w/ bearing=0.015 ± 0.005
Friction Co-efficient
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
self-locking – screw cannot turn from load Pback-driving – screw can be turned from load P
costancos ord
L
p
for self-locking:
would square or Acme of same dimensions lock first?
cc
p
pp d
Ld
Ldd
PTd
cos
cos
2
Condition for Self-Locking
MISSAM RAZA Dept. –Mechanical LJ Polytechnic
Ball Screw
MISSAM RAZA Dept. –Mechanical LJ Polytechnic