Selecting and Sizing Ball Screw Drives

The ball screw drive is an assembly that converts rotary motion to linear motion and vice versa. The ball screw drive consists of a ball screw and a ball nut with recirculating ball bearings (Fig. 1). The connection between the screw and the nut is made by ball bearings that roll in the matching forms in the screw and ball nut. The forces transmitted are distributed over a large number of ball bearings, giving a comparatively low relative load per ball. With rolling elements the ball screw drive has a very low friction coefficient. Ball screw drives typically provide mechanical efficiency of greater than 90%, so their higher initial cost is often offset by reduced power requirements. The features of ball screws in relation to other linear actuation devices are summarized in Figure 2 and the important ball screw specifications are summarized in Figure 3.

Optional Inputs Figure 4 provides an overview of the parameters used in the ball screw selection and sizing process. The optional inputs make it possible for the designer to specify basic preferences for a specific type of ball screw early in the selection process. It’s important to note that specifying optional inputs may unnecessarily restrict component selection. As an example, if you select metric product series you eliminate about half of the possible ball screw alternatives even though it’s possible that an inch series ball screw might work just as well in the application.