Aerospace Ball Screw

 

Aerospace ball screw is just that a screw which runs on ball bearings. The aerospace ball screw and ball nut have matching grooves, and the ball bearings re-circulate in these races. There is no physical contact between the screw and the nut. As the aerospace ball screw or nut rotates, and the rolling balls reach the trailing end of the nut, they are deflected or guided from this “pitch” contact by means of a return tube and returned to the leading end of the circuit. There, the cycle resumes and the balls recirculate continuously.

Major Diameter (Land Diameter) The outside diameter of the ball screw thread.
Minor Diameter (Root Diameter) The diameter of the ball 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 ball 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 ball screw. The lead error for a standard ball 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 ball screw thread. Ref. Class 7-8 ANSI B5.48-1977. Lead charts describing incremental lead deviation offsets can be found. These incre­mental offsets can be input into motion controllers for lead error compensation.
Matched Leads (Synchronous Screws) Used when multiple ball screws are being driven by a single drive in order to keep the ball 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 ball screw to a corresponding point on the next thread parallel to the ball screw axis. Pitch is equal to the lead on single start screws.
Screw Starts The number of independent threads on the ball screw shaft. The lead of the ball screw is calculated by dividing the threads per inch by the number of starts.
Backlash The axial free motion between the nut and the ball screw. It determines the amount of lost motion between the nut and ball screw on a horizontal application. Backlash on standard nuts range from .005 to .015, depending on the size of the ball screw.
Selective Fit The process of selecting a unique ball size for reducing backlash to as little as .001 inches.
Preloading Method of eliminating backlash in a ball screw assembly. This is accomplished by the use of one group of ball grooves in opposition to another to eliminate back­lash. Preloading increases stiffness (resistance to deflection) and provides for accurate positioning with very little increase in applied torque or load capability.