What is Backlash?

Backlash is any non-movement that occurs during axis reversals. Say, for instance, the X axis is commanded to move 1 inch in the positive direction. Immediately after this X movement, the X axis is commanded to move 1 inch in the negative direction. If any backlash exists in the X axis, then it will not immediately start moving in the negative direction, and the resulting distance traveled will not be precisely 1 inch—it will be shortened by the amount of lost motion, backlash.

This lost motion results from the space between the thickness of a gear tooth or (screw) and the width of the space between the teeth in the mating gear (nut).

This space is typically designed to allow for a film of lubricant, binding from heat expansion and eccentricity or manufacturing inaccuracies.

The problem is it causes position errors and vibration in a positioning system.

Backlash is a measurable quantity. The simplest way to measure it in a screw and nut is to push the nut as far as it will go in one direction and then measure how far it moves in the other direction.

This amount of lost motion can be compensated in the machine control (within reasonable limits) by measuring the amount of lost motion and entering that value into dedicated compensation registers in the machine control.
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One way to minimize backlash mechanically is through the use of what is called preload. Preloading, or imposing a load on both sides of the screw thread simultaneously, even while it is not moving, eliminates backlash.

Preload is the amount of force exerted against the rolling elements in a screw without the application of any external forces. Proper preload maximizes bearing life and optimizes machine performance.

See the figure to the right, backlash occurs when the diameter of the rolling element is less than the space within the track (black area in top image). Backlash is eliminated when the diameter of the rolling element is larger than the space within the track (bottom image).

Bearing preload influences accuracy, speed limits, rigidity and other applicable workloads.

For optimum performance and extended life, your Versamill comes complete with large diameter ballscrews with anti-backlash ball nuts with preload.