The ball screw drive is an assembly that converts rotary motion to linear motion and vice versa.  The ball screw drive consists of a ball screw and a ball nut with recirculating ball bearings. 

The connection between the screw and the nut is made by ball bearings that roll in the matching forms in the screw and ball nut.  The forces transmitted are distributed over a large number of ball bearings, giving a comparatively low relative load per ball.  With rolling elements the ball screw drive has a very low friction coefficient. 

Ball screw drives typically provide mechanical efficiency of greater than 90% so their higher initial cost is often offset by reduced power requirements.

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Reprint of Thomson Industries, Inc. Article by Jeff G. Johnson