Glossary
of Terms
General Definitions
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).
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 ballnut tracks.
Life ^
Design Life Objective - The total number of inches that the ballnut
will travel during the desired life of the machine.
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.
Effective Ball Turns - The number of turns of balls contained within
the ballnut body.
Load Carrying Balls - The balls in contact with the ball groves of
the nut and the screw for load carrying purposes.
Return Tube - The tubular member that carries the balls from the
output to the input end of each ball circuit to complete a recirculating
ball path.
Loading
^
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 ballnut 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.
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.
