Low Cost Automation Tutorial

#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.

Terms	Explanation
Thread groove	Threaded area where a screw shaft faces a nut. A steel ball rotates in this area.
Ball screw lead	The distance that a nut travels in the axial direction for one revolution of the screw shaft.
Screw shaft lead	The 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 lead	This is same as the nominal lead in general.
Actual lead	Actual lead measured for a ball screw.
Cumulative specified lead	Cumulative lead resulting from rotating a screw for a given number of revolutions based on the specified lead.
Cumulative actual lead	Average 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 lead	A straight line representing the trend of cumulative actual lead.
Lead error	Difference 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 deviation	Cumulative actual lead minus cumulative specified lead.
Variation	The 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)

« PREV