The lost motion or looseness (play) between the faces of meshing gears or threads. In the case of gears, it refers to how much one gear can be moved back and forth without moving the gear into which it is meshed. It may be measured in degrees.
Backlash is required for assembling and making sure that gears run without jamming under operating conditions. However, backlash in the system gives rise to the clonk phenomenon.
Subjects: Mechanical Engineering