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Bolted Joints in tension

Can be disassembled without

destructive methods

Strong enough to resist tensile and

shear loads

The bolt here, has been preloaded to an

initial tensile load Fi

Preload places the bolted membercomponents in compression

Provides better resistance to tensile load

Creates friction force to resist shear load

An external load P and external shear

load Ps are then applied on the joint

Final bolt tension Fb is not affected by

the shear load, PsPortion of a bolted joint

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Analysis of bolted joints

Deflection of a bar in simple tension, = Fl/(AE)

= deflection, F= force, l= length, A = area, E= modulus ofelasticity

Therefore, stiffness constant, k = F/ = AE/l

When an external load P is applied to the bolted connection, the bolt,initially in tension, gets longer

The compression in the connected members gets decreased The increase in deformation of the bolt is,

The decrease in deformation of the connected members is,

If the members are not separated,

bbb kP

m

m

b

b

k

P

k

Pei ..

mmm kP

mb

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Analysis of bolted joints (Contd)

Since P= Pb + Pm , we have,

Therefore the resultant load on the bolt is,

Similarly, the resultant load in the connected members is,

The above equations hold only when some of the initial compression

remains in the connected members.

If the external force is large enough to remove this compression

completely, the members will separate and the entire load will be

carried by the bolt, i.e., Fm = 0

mb

bb

kk

PkP

i

mb

bibb F

kk

PkFPF

i

mb

mm F

kk

PkF

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Analysis of bolted joints (Contd)

b and m are the deformations of the bolt and the members respectively

When an external load is applied, m is reduced by m and b is

increased by the same amount b = m Thus, the load on the bolt increases and the load in the members decreases

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Gasketed Joints

When there are more members included in the grip of the bolt, they

act like compressive springs in series

Total spring rate is given by,

If one of the members is a soft gasket, its stiffness relative to the

other members is usually very small

So, for all practical purposes, deflection of other members can be

neglected

im kkkkk

1......

1111

321

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Gasketed Joints

There are three basic gasket configurations

Unconfined gasket (fig. (a) )

It is subject to the full compressive load between the members

Stiffness of the metal parts within the bolt grip can be considered to

be infinite

Design of the connection is based on the stiffness of the gasket

O ring or confined gasket (fig. (b) )

The members have metal to metal contact (no separation) and the

previous analysis holds

Confined gasket in which the seal is achieved by compressing the gasket

(fig. (c) )

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Gasketed Joints (Contd)

A gasketed joint must be designed to satisfy all the usual requirementsfor preload.

Moreover, the preload must be large enough to achieve the minimum

sealing pressure required for the gasket material. i.e.,

, where Ag is the gasket area and

po is the minimum gasket seal pressure

The resultant clamping load in a bolted connection is given by,

where Pis the external tensile load per bolt.

In a gasketed joint, this clamping load must satisfy the relation,

, where m is called the gasket factor(analogous to afactor of safety), which varies from 2 to 4 for most materials

p is the internal pressure which tends to separate the joint

ogi pAF

mb

bii

mb

mm

kk

kCbygivenisCwhereFCPF

kk

PkF

,)1(

mpAF gi