Backlash

The amount of free play between mating mechanical components (typically gear teeth) that must be taken up before torque is transmitted from one side to the other. In a gear drive with backlash, commanding a reversal of direction results in no motion until the backlash gap is closed, then sudden motion. Backlash introduces nonlinearity and dead zones that degrade position control accuracy and that state estimators must explicitly account for to avoid predicting motion that is not occurring.

The small amount of free play or slack in a gear train or mechanical linkage, which shows up as position error at the output. Backlash occurring earlier in a multi-stage gear train is amplified by more of the total reduction than backlash occurring later, and preloaded or anti-backlash gearing can reduce the resulting position error below what the raw reduction ratio alone would suggest.

The mechanical dead zone in a gear train caused by clearance between gear teeth. During direction reversal, the motor rotates through the backlash angle without moving the output shaft. Backlash limits achievable position accuracy and can cause oscillation in a closed-loop system attempting to correct errors smaller than the backlash.

The angular dead-band between input and output motion in a gear mesh. When the drive direction reverses, the input rotates through the backlash angle before the output begins to move. At the joint tip, backlash produces a position error equal to the backlash angle (in radians) times the distance from joint to tip.

The amount of output motion that occurs with no corresponding input motion, due to clearance between meshing surfaces in a transmission (gear teeth gaps, belt-to-pulley tooth play, chain pin clearance). Backlash causes a dead zone on direction reversal and leads to position error and chatter in closed-loop control systems.