CHAPTER 2

Design of Machine Elements

Subjected to Direct Stresses

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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

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Knuckle Joint

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Knuckle Joint

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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

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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.

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Cotter Joint

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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

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• 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

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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

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Gib and Cotter joint for rectangular rods

Gib and Cotter joint for rectangular rods

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• Non-permanent– Bolted

• Permanent– Bolted– Welded– Bonded

Threaded FastenersTypes of Fastening

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p pitch in./threadd diameter (major) in.dp pitch diameter in.dr minor diameter in.

L Lead in.

Thread

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F

F

tt A

F2

224

rp

tdd

A

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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

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• 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

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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

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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….

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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

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oil lubricated= collar w/ bushing=0.15 ± 0.05

collar w/ bearing=0.015 ± 0.005

Friction Co-efficient

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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

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Ball Screw

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