In
mechanical engineering,
backlash, sometimes called
lash or
play, is clearance or lost motion in a mechanism caused by gaps between the parts. It can be defined as "the maximum distance or angle through which any part of a
mechanical system may be moved in one direction without applying appreciable force or motion to the next part in mechanical sequence",
p. 1-8 and is a mechanical form of
deadband. An example, in the context of
gears and
gear trains, is the amount of clearance between mated gear teeth. It can be seen when the direction of movement is reversed and the slack or lost motion is taken up before the reversal of motion is complete. Another example is in a
valve train with mechanical
tappets, where a certain range of lash is necessary for the valves to work properly.