Selecting Support Bearings
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Support bearings must be able to carry not only the thrust produced by the ball screw, but also any additional forces from pre-tensioning the ball screw shaft, plus any side forces generated by belt drives. Ball nuts with many ball circles and/or large balls and shafts with considerable pre-tension may make it difficult to find bearings with sufficient load capacity, especially when the bore (ID) of the bearing has to be no larger than the shaft's root diameter, and the journal shoulder diameter no larger than the ball screw's nominal diameter.
This discussion is only meant to highlight areas of concern. We cannot give detailed recommendations on which bearing to select. Criteria for the selection include:
- Axial dynamic load capacity of the bearing should be approximately equal to the dynamic load capacity of the ball screw, or higher if the screw is pre-tensioned.
- Minimum shoulder diameter for the bearing's inner ring should be no greater than the root diameter of the ball screw (journal shape A), or no greater than the screw's nominal diameter (journal shape B).
- The bearing should be suitable for the same lubrication method (grease/oil) and equipped with the proper seals for that lubricant. Speed ratings must be sufficient with the lubricant selected.
Steinmeyer recommends using INA support bearings. The following table gives examples of typical ballscrew / bearing assemblies. However, it is not possible to cover all combinations in this catalog. Please refer to our engineering service for further information.
This brief overview cannot give a final selection aid to determinate an optimum bearing solution. Radial loads due to drive belt tension or increased axial loads due to pre-tensioning a ballscrew need to be considered, too.
Miniature ball screws are normally supported using either a pair of angular contact bearings on one end, or by a single-row bearing at both ends of the screw, preloaded against one another. It is better to use bearings with increased play here, since this will establish a more desirable contact angle when the bearings are preloaded. Bearings with minimal or no play may generate excessive forces on their balls when preloaded this way, causing potential premature bearing failure and rough motion.