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Design and Theory

Backlash or Lash or “play”

Backlash is the axial motion between the nut and screw when the load direction is reversed while not turning the nut or screw.  The axial motion comes from the balls shifting from one side of the ball track to the other.  The animation below shows a section view of a nut.  The load applied (red arrow) reverses causing the lash to be visible.  The gaps between balls are exaggerated for this demonstration to make the lash visible.

Backlash Animation, ball gaps exaggerated for visibility

The lash is measured by taking the difference in position between pushing the nut each direction without letting anything rotate.  A nut with 0.002 in. of lash means the nut could move 0.002 inches on the screw without turning the screw.  If the load is always kept in the same direction, backlash will not be seen. In applications where the load reverses and backlash is a problem, the lash can be reduced by selecting better fitting balls or removed by using a preloaded nut.
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