Linear motion via ballscrew drives

Linear motion of machine tool axes is often achieved using screws and nuts. In a typical screw-and-ball-nut design, the nut is rigidly attached to the moving structure (table), and the screw is attached directly to the rotor of a servomotor. Via CNC, the rotor rotates the screw. The table cannot rotate because it is supported by the guide ways, so the nut moves along the axis of the screw as the screw rotates, carrying the table along (Figure 1). 

Figure 1. A screw-and-ball-nut drive for a linear axis. Adapted from “Hybrid modeling of machine tool axis drives,” by R. Whalley, M. Ebrahimi, and A.A. Abdul-Ameer, International Journal of Machine Tools and Manufacture, Vol. 45, Issue 14, Nov. 2005, pp. 1560-1576.

To reduce friction between the threads on the screw and nut, balls are placed between them. The screw rotates and the balls roll similar to ball bearings. As the balls roll, they progress from one end of the nut to the other. As the balls reach one end of the nut, they are guided into tubes that carry them back to the other end. 

Sometimes, several nuts are used together so they can be preloaded against each other to eliminate backlash. This kind of nut is called a “recirculating ball nut” (Figure 2 on page 24). A recirculating ball-nut drive is popular for NC machine tools. The friction is so low in the ball nut that it is possible to drive the screw backward. 

In a vertical axis, the weight of the slide can be enough to rotate the screw, allowing the axis to drop if power to the servomotor or CNC is lost. Often a brake, which actuates if power is lost, is included in the design of vertical axes. In a horizontal axis, the cutting force can be enough to cause the screw to rotate unless the servomotor is actively preventing it. Therefore, screw-and-ball-nut designs are generally not suitable for manual machine tools. 

Figure 2. This recirculating ball nut shows the recirculation tubes for the balls on top. Image courtesy NSK America.

The screw-and-ball-nut design is insensitive to cutting forces and to the mass of the workpiece. Such designs typically have high transmission ratios and often incorporate multiple leads (more than one parallel thread). While the servomotor
sees the inertia of the screw directly, the cutting forces and workpiece mass only appear to the servomotor through the square of the transmission ratio. In most cases, the inertia of the screw is the dominant design criteria, and the screw length is the limiting factor. 

A large-diameter screw is stiffer but has more inertia than one with a small diameter. Conversely, a smaller diameter screw has less inertia but is more flexible. A long, flexible section of the screw that is not supported can significantly deflect, or whip. Typical screw-and-ball-nut designs in machine tools are 1" to 2" in diameter and no more than about 4' long. Also, because of the high transmission ratio, the rotary acceleration and rotary velocity of the screw are quite high compared to the linear acceleration and linear velocity of the table.

If more force is required than one screw can provide, multiple parallel screws can be used. Each screw provides force to the nut, and the total of those forces provides the acceleration for the table mass and opposes the cutting forces. However, when multiple screws are used, the motion of the screws must be carefully coordinated or the table can twist and jam. 

Usually, one screw is used as the master, and the motions of the other screws are slaves to it. Every time a screw is added, the force increases. The mass of the table only increases by the mass of the nut and attachment, so the achievable acceleration increases. Therefore, the multiple-screw design is often found in high-speed machine tools. CTE

About the Author: Dr. Scott Smith is a professor and chair of the Department of Mechanical Engineering at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at kssmith@uncc.edu.