Why does a bolt loosen?

From the moment it is pre-loaded, a bolt almost always loosens. Several factors inherent to industrial processes can cause bolts to loosen: changes in temperature, vibration, relaxation of bolts, over-tightening and under-tightening, to name a few. In applications with extreme temperature requirements, such as cryogenic applications, thermal expansion/contraction is a significant cause of bolt loosening.

Well-designed Belleville disc springs protect bolt integrity and ensure that bolt joints remain tight.