Introduction about ball screws

Major Diameter

(Land Diameter) The outside diameter of the screw thread.

Minor Diameter

(Root Diameter) The diameter of the screw shaft as measured at the bottom of

the ball thread track.  This diameter is used in column load and critical speed

calculations. Minor diameter also is a consideration in support bearing selection.

Ball Pitch Diameter

(Ball Circle Diameter) The theoretical cylinder passing through the center of the balls when they are in contact with the ball screw and ball nut races.

Lead

The axial distance the screw or nut travels in one revolution.

Lead Error

(Accuracy) The difference between the actual distance traveled compared to the theoretical travel based on the lead of the screw. The lead error for a standard screw will not exceed +/-.007” per foot and a premium grade screw will not exceed +/-.003” per foot. Lead error is cumulative based on the actual length of the ballscrew thread. Ref. Class 7-8 ANSI B5.48-1977. Lead charts describing incremental lead deviation offsets can be supplied (upon request). These incremental offsets can be input into motion controllers for lead error compensation.

Matched Leads

(Synchronous Screws) Used when multiple screws are being driven by a single drive in order to keep the screws in sync. Basically the lead errors are matched at the factory in order to minimize misalignments during the stroke. Consult factory for additional information on matched leads.

Pitch

The distance from one thread on the screw to a corresponding point on the next thread parallel to the screw axis. Pitch is equal to the lead on single start screws.

Screw Starts

The number of independent threads on the screw shaft. The lead of the screw is calculated by dividing the threads per inch by the number of starts.