 Backlash

Backlash is free play in a mechanical coupling that can produce lost motion when the direction of drive reverses. In a lead screw, clearance between the screw thread and nut permits relative movement before the opposite contact surfaces carry the load.^[ref]

Consider an ideal screw with screw lead $L=4\mathrm{mm}$ per revolution and axial backlash $b=0.05\mathrm{mm}$. Suppose one thread flank initially carries the load and a small opposing force holds the carriage still while the screw reverses through the clearance. The angular movement required to take up the gap isDuring that interval, screw rotation need not produce carriage movement. A commanded reversal of $6^{\circ }$ corresponds geometrically to $4(6/360)=0.0667\mathrm{mm}$, but this simple gap model leaves only about $0.0167\mathrm{mm}$ of carriage travel after taking up the backlash.

The example assumes a particular contact state and holding force. If another force keeps the same thread flanks in contact, or moves the carriage across the gap, the observed reversal can differ. Thus backlash must be associated with a load condition, rather than inferred from every mismatch between commanded and measured position.

A useful measurement records screw angle and carriage position while approaching a chosen position from both directions. Direction-dependent results can reveal backlash even when repeated approaches from one direction agree. Separately changing the applied force helps distinguish free play from mechanical compliance: elastic deformation changes with force after contact is established, while clearance permits a gap before that load path engages. Both effects can contribute to a path-dependent relation and hysteresis.