Preloading ball nuts: General Industry Coverage

Translating machine tool axes are often driven by screws attached to the rotor of a motor by a coupling, with the motor attached to the fixed frame of the machine. A nut is attached to the moving element, such as a table or carriage, and is fixed so it cannot rotate. When the screw rotates, the nut and the attached moving element translate along the length of the screw.

In this configuration, the screw is called a lead screw. The screw has a large mechanical advantage, meaning its thrust capacity is high. In designs where more turns of the screw are required for a given linear translation, the mechanical advantage and load capacity are higher. Lead screws are stiff because the thrust load is carried on the large contact area between the threads.

Courtesy of All images: S. Smith

Figure 1. A split-nut, spacer-preloaded ball nut.

However, lead screws in this configuration have two significant limitations:

1. The motor has to overcome a significant amount of friction as the screw and nut slide against each other, and the friction generates heat. This heat, in turn, thermally deforms the screw and machine, reducing accuracy.

2. A clearance must exist between the screw threads and nut threads such that only one side contacts at a time. Typically, the position feedback is on the motor, not the translating slide. As a result, when the screw reverses direction, the slide remains stationary as the screw turns and makes contact with the other side of the nut thread. The controller thinks the slide moved, but the slide remained stationary. This is called backlash. A trade-off always exists between machine accuracy (and stiffness) and cost. Some controllers use backlash compensation, so whenever the axis reverses direction, additional motion is commanded to compensate for the measured backlash.

To reduce friction, balls are often introduced between the screw and nut. When the screw rotates, the balls roll, as in a ball bearing. The balls roll through the nut from one end to the other, then are collected into a tube that transports them back to the start of the nut. This configuration is called a recirculating ballscrew. The balls dramatically reduce friction and heat generation, but they also reduce stiffness. The balls act as springs, but they do not exhibit a linear relationship between force and deflection like coil springs do. Rather, because the contact exists only at small points on opposite sides of the balls, a small deflection only requires a small force. As the deflection increases, the force required to cause the deflection quickly increases, and stiffness goes up.

A preload, which would keep the balls in a compressed condition all the time would increase stiffness and eliminate backlash. So how can it be arranged? There are four basic ways: a split nut with a spacer, a split nut with a spring, a variable thread pitch and oversized balls.