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#269 Ball Screws -6: Glossary on Ball Screw Accuracy

Category : Linear Motion Components
December27, 2016

Terminology related to ball screw accuracy is explained in this table.

Thread grooveThreaded area where a screw shaft faces a nut. A steel ball rotates in this area.
Ball screw leadThe distance that a nut travels in the axial direction for one revolution of the screw shaft.
Screw shaft leadThe distance measured in parallel to the axis of opposing two points on the adjacent thread grooves within a section where an axis of the screw shaft exists.
Specified leadThis is same as the nominal lead in general.
Actual leadActual lead measured for a ball screw.
Cumulative specified leadCumulative lead resulting from rotating a screw for a given number of revolutions based on the specified lead.
Cumulative actual leadAverage trend determined from the lead diagram based on the result of successive measurements or cumulative lead obtained from measurement on a section containing a screw shaft axis.
Actual mean leadA straight line representing the trend of cumulative actual lead.
Lead errorDifference between actual lead and specified lead. The value is positive if the actual lead is larger than the specified lead. Otherwise, the value is negative.
Actual mean lead deviationCumulative actual lead minus cumulative specified lead.
VariationThe maximum value of the cumulative actual lead enclosed by two straight lines drawn parallel to the actual mean lead as defined in the following three items:
a) Variation for the effective travel distance of a nut or the effective thread length of a screw shaft.
b) Variation for any 300 mm interval in the effective thread length of a screw shaft.
c) Variation for one revolution made within the effective thread length of a screw shaft.

[Fig.1] Lead accuracy of precision ball screws (JIS B 1192)

[Fig.2] is an example of inspection data for precision ball screws. The solid line represents measured data.

[Fig.2] Example of inspection data for a precision ball screw

Description of inspection data
  • Cumulative reference of this ball screw (target value) = 0.0 µm --- Setting value
  • The green arrow [1] is the actual mean lead deviation = -15 µm (data readings)
  • The orange arrow [2] is the variation -a) <variation for the effective thread length> = 6.0 µm (data readings)
  • The red arrow [3] is the variation -b) <variation for any 300 mm interval> = 4.0 µm (data readings)