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Bolt preload

A bolt is often tightened with a preload force far larger than the external load. The preload keeps the parts pressed together so they do not separate, and it means the bolt itself only sees a small part of a varying external load – which protects against fatigue.

F_i \gg F_{ext}

preload should be far larger than the external load

Symbols

$F_i$	preload force	N
$F_{ext}$	external (varying) load on the joint	N

Example

A flange joint with a fluctuating load:

high preload means the bolt experiences a far smaller fluctuation than the load itself.

The correct tightening torque matters – too little causes joint separation, too much can damage the bolt.

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Part of Machine Design: Machine elements.