The resistance caused by lubrication will also increase the friction torque, especially at high speeds. Most ball screws are used at speeds far below 5 m/min. However, modern machine tools require speeds greater than 10 m/min, and some systems use speeds as high as 30 m/min. The type of lubricant that produces the least resistance is determined by the axial rotation speed of the ball screw. Generally speaking, when the speed is below 500 rpm or the moving speed is 3 m/min, grease lubrication is best. At this lower speed, boundary lubrication is mainly used. Rotation speed exceeds 500 rpm, mainly oil body film lubrication, oil is the best lubricant.
When calculating the heat generated by the ball screw itself, the high working load is the biggest potential cause. Generally, the working load is about three times the tightening force of the ball screw. The larger load must be compensated by increasing the size of the ball screw device used or by a larger lubricant cooling capacity. One method of compensating for thermal expansion is to apply a pretension to the ball screw. This is achieved by machining the screw shaft to a negative tolerance size. In this way, the screw torque is slightly shortened, and the ball screw nut is compressed during assembly. The screw expansion device works normally at working temperature. When two ball screw nuts are used together, they are usually separated by washers, and the pre-tightening force is determined by the thickness of the washers. By replacing the solid washer with a butterfly spring, the torque of the ball screw can be reduced. This spring allows axial deformation to reduce compression. Another major source of resistance, the friction between adjacent balls, can be reduced by removing a few balls or replacing some of them with isolated balls.
