How to Select Supporting Bearings for Ball Screw Assemblies
The ball screw mechanism features distinct characteristics in terms of force bearing and component division of labor: it only provides the axial force required for the linear movement of the load table along the guiding direction through the ball nut, while the table and the radial loads, torque loads it bears are all borne by high-rigidity guiding components. As the core component of the mechanism, the ball nut is usually used in conjunction with guiding components.
Based on the above characteristics, the requirements of the ball screw pair for rolling bearings are both similar to and different from those for the main shaft and transmission shaft. The similarity lies in that the rolling bearings used in both cases need to have sufficient precision and fatigue life; the difference is significant due to the difference in force bearing: the high-precision ball screw mainly bears axial loads, and there is generally no other radial load except for its own weight. Therefore, the supporting rolling bearings for it need to meet high requirements for axial precision and axial rigidity. Meanwhile, the requirements of the feed system for movement flexibility and sensitivity to micro-displacement response determine that the bearing friction torque should be as small as possible; and since the ball screw speed is not high and the high-speed operation time is short, heating is not a major issue.
It should be particularly noted that during the operation of the ball screw kit, the ball nut can only bear the load along the axial direction of the ball screw, and it is necessary to ensure that the axial load passes through the axis of the linear ball screw. Under no circumstances should radial loads or torque loads be directly applied to the nut. Otherwise, radial or torque loads will cause the screw shaft to bend, leading to overload of some balls in the nut, and then resulting in problems such as unsmooth transmission, reduced precision, a sharp shortening of service life, and even poor operation in severe cases. The radial load borne by the ball screw drive mainly comes from its weight.
In terms of design and checking, the ball screw assembly needs to undergo multiple calculations according to specific working conditions: for slender ball screws that bear compressive loads, buckling stability calculation should be performed; for ball screw pairs with high rotating speed and large supporting distance, critical speed checking should be carried out; for ball screw assemblies with high precision requirements, rigidity and moment of inertia checking need to be completed; for CNC machine tools, the moment of inertia needs to be calculated, and for closed-loop control systems, the resonance frequency also needs to be checked.
Considering that the ball screw mainly bears axial force, thrust bearings are mostly used for its support. Under the same size conditions, the axial rigidity of the thrust ball bearing is more than twice that of the angular contact ball bearing and the tapered roller bearing; the rigidity of the thrust roller bearing is about twice that of the thrust ball bearing. If the axial load is small, the angular contact ball bearing can be used instead to reduce the number of bearings.