Glossary of Terms

CYCLE - The complete forward and reverse motion of the screw (or nut) in moving the load. One cycle is equivalent to two strokes (one forward and one backward)

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LEAD - Axial distance screw or nut travels in one revolution.

LEAD ERROR - The error in lead length per turn, per inch, per foot or, overall length compared to the basic lead specified

MAJOR DIAMETER - The outside diameter of the ballscrew shaft.

MINOR (ROOT) DIAMETER - Diameter of the screw measured at the bottom of the ball track.

Pitch (Ball Circle) Diameter - The nominal diameter of a theoretical cylinder passing through the centers of the balls when they are in contact with the ballscrew and ball nut tracks.

Pitch - The axial distance between succeeding ball grooves. The distance from a point on a ball track to a corresponding point on the next track, parallel to the ballscrew or ball nut axis.


Design Life Objective - The total number of inches that the ball nut will travel during the desired life of the machine.

Effective Ball Turns - The number of turns of balls contained within the ball nut body.

Load Carrying Balls - The balls in contact with the ball grooves of both the nut and the screw for load carrying purposes.

Return Tube - The tubular member that carries balls from the output to the input end of each ball circuit to complete a recirculating ball path.


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  • Compression Load - A load which would tend to compress or buckle the ballscrew shaft.
  • Dynamic Load Rating - The maximum load which a ball bearing screw assembly can maintain for a prescribed length of travel.
  • Off Center Load - A load tending to cock the ball nut on the screw, reducing the rated life. This must be considered in the selection of the ball bearing screw assembly.
  • Preload - The use of one group of bearing balls set in opposition to another to remove axial lash or backlash and increase ball bearing screw stiffness. All axial freedom is eliminated in preloading.
  • Side Load (Radial) - A load from the side that will reduce the rated life and must be considered in the selection of the ball bearing screw.
  • Static Load - The maximum non- operating load capacity above which brinelling of the ball track occurs.
  • Tension Load - A load which would tend to stretch the ballscrew shaft.
  • Thrust Load - Thrust load is loading parallel to and concentric with the centerline of the screw shaft which acts continuously in one direction. Thrust loading is the proper method of attaching the load to the ball bearing screw assembly.


ballscrew glossary 03


Critical Speed

The speed at which the nut or screw has a tendency to develop severe vibrations. The desired speed should not exceed the safe rate of travel (revolutions per minute) for the unsupported length and type of mounting.

  • Load/Life Rating - The usable life of a ballscrew assembly measured in inches of travel under a specific load.
  • Service Life - The number of years the machine is designed for.
  • Stroke - The axial distance in either direction that the screw or nut travels in moving the load.
  • Ball Circulation:
  • Backlash (End Play) - Axial free motion between the nut and the screw.
  • Circuit - The continuous closed path of recirculating balls in a ball nut.

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