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#045 Motion Mechanism Design - 15 : Ballscrew Shaft Support Methods

Category : Drive mechanism design
June11, 2010

Ball screw shaft end bearings' structure and design points, and convenient procurement methods are explained here.

(1) Methods to support ballscrew ends

There are three ballscrew mounting methods: Fixed - Fixed, Fixed - Supported, and Fixed - Free. The most common is the Fixed - Supported method. Each method requires design considerations not to cause excessive moment loads and radial loads on the ballscrew shafts.
On the fixed side, the bearing's inner race is fixed to the ballscrew shaft with a slip fit, and the bearing's out race is solidly mounted to a support unit housing to eliminate axial movements. Angular bearings are used for radial and axial loads. On the free side, the ballscrew shaft is not captured in order to account for any heat related shaft expansion. (see [Fig.1])

[Fig.1] Ballscrew shaft end support design

(2) Characteristics of ballscrew end supports

(a) Types of loads

Ballscrew is a machine element that transmits rotational torque to moving objects at high speeds. Since the shaft end bearings will be subjected to both axial and radial loads alternately, appropriate rigidity is required for the bearings and bearing holders.

(b) Differences in shaft end configurations

Ballscrew shaft ends have different configurations (diameter, shape) since the nut unit is assembled from one end after the shaft machining is completed. For that reason, the end bearing types are different making the designing cumbersome.

(3) Selecting the end bearings

For the reasons stated in (2) above, end bearings be different types and the end bearing holders will also be of different designs. In order to eliminate this necessity of designing two different bearing holders, MISUMI provides support units as standardized pairs. (see [Photo 1])

[Photo 1]

(4) Surface treatments of ballscrew end bearings

Ballscrew shaft and end bearings are lubricated and have less chance of rusting, but the bearing holders (support units) may develop rust on the outer surfaces. Rust protection means are recommended for equipment intended for clean room use.