Static friction

Engineering

Stiction refers to the characteristic of start-and-stop–type motion of a mechanical assembly. Consider a mechanical element slowly increasing an external force on an assembly at rest that is designed for the relative rotation or sliding of its parts in contact. The static contact friction between the assembly parts resists movement, causing the spring moments in the assembly to store mechanical energy. Any part of the assembly that can elastically bend, even microscopically, and exert a restoring force contributes a spring moment. Thus the "springs" in an assembly might not be obvious to the eye. The increasing external force finally exceeds the static friction resisting force, and the spring moments, released, impulsively exert their restoring forces on both the moving assembly parts and, by Newton's third law, in reaction on the external forcing element. The assembly parts then impulsively accelerate with respect to each other, though resisted by dynamic contact friction (in this context very much less than the static friction). However, the forcing element cannot accelerate at the same pace, fails to keep up, and loses contact. The external force on the moving assembly momentarily drops to zero for lack of forcing mechanical contact even though the external force element continues its motion. The moving part then decelerates to a stop from the dynamic contact friction. The cycle repeats as the forcing element catches up to contact again. Stick, store spring energy, impulsively release spring energy, accelerate, decelerate, stop, stick. Repeat.