Why do bolts loosen?

The main reason is insufficient preload, allowing transverse slip of bolt and joint members. Preload, or residual tension, in a tightened bolt means more to assembly strength than actual strength of the fastener itself. In a joint, a bolt torqued to its proper load level can resist a maximum amount of external load without loosening.

Designers can take advantage of this fact to ensure correct bolt loading, and at the same time reduce costs. FACT! : Assembled bolts are tightest when stressed as near as possible to their elastic limit.

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