The vast majority of threaded fasteners retain their preload because of friction present on the thread and head contact surfaces that resists self-loosening. It was thought that once relative motion occurs between the threaded surfaces and other contact surfaces of the clamped parts (because of a tangential external force being applied to the joint), the bolted connection would become free of friction in a circumferential direction. More recent studies have shown that under conditions of transverse slip there is a small, but measureable, force resisting circumferential movement.

In the above diagram, the red arrow shows the joint surface moving under the nut. When slip occurs at the thread interface as well as the nut face, the frictional resisting forces in the circumferential direction reduce to a very small value. The Jost Effect is the name given to the reduction in the frictional resistance that occurs in a direction different to that inwhich slip is occurring. This effect is used in many applications including the removal of corks from bottles. If the cork is first rotated the force needed to pull the cork from the bottle is significantly reduced. Several machines and applications use indirectly the Jost Effect, for example floor polishing machines, the machine being easier to move when the polishing disk is rotating. It is also the fundamental reason why threaded fasteners experience self-loosening. Frictional resistance is first overcome in the transverse direction by slip occurring on the joint resulting in the frictional resistance in the circumferential direction reducing to a small value. The torque acting on the fastener in the loosening direction (as a result of its preload) that when coupled with the Jost Effect results in self-loosening occurring. This is illustrated in the diagram.

The term is named after the Institute that completed research into this effect, the Jost Institute of Tribotechnology at the University of Central Lancashire in the UK.

More details about the vibration loosening of threaded fasteners