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#136 Mastering Ball Screws - 6 : Life of Ball Screws

Category : Linear Motion Components
November 9, 2012

Since the ball screws are designed based on rolling steel ball bearings, product service life calculations are backed by a solid theoretical system.

(1) Life of rolling ball bearings

-Here, we consider the ball screw life based on them as being analogous to the general purpose ball bearings. There are following 5 items as representative functionalities of ball bearings.
(1) Little friction
(2) Smooth rotation and quiet
(3) Good accuracies
(4) High load bearing capability
(5) High rigidity
-The life (in a broad sense) can be defined as an ability to retain these functionalities (durability).
-Various defects, damages, and deterioration (accuracy degradation, noise increase, torque increase, and wear, etc.) are what degrade the ability to retain the functionalities.
-Noise and torque increses are often attributed to ball screw systems designs, handling, and installation accuracies.
-However, the Flaking caused by rolling fatigue can occur even with proper designing and installing, and the JIS defines this as Life in a narrow sense.

■Bearing life definition in a narrow sense
The total number of rotations until the bearing race or rolling body will show fatigue induced material damages.

(2) Life of a typical ball screw (Life in a narrow sense)

Other than ball screws with high preloads, the typical ball screw life can be approximately determined by the following calculation formula (07 MISUMI Catalog, P2532)