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.
