Managing hybrid spindle bearings: Drilling Performance

High-speed machine tool spindles often have the rotor of the motor mounted directly on the spindle. Angular-contact ball bearings then support the spindle in the housing. The bearings can support both an axial load (as when drilling) and a radial load (as when milling) on the spindle. Figure 1 schematically shows a spindle mounted with angular-contact ball bearings in the “big X” configuration. It’s called that because the spindle preload makes an X. The bearings play several critical roles, including providing stiffness and geometric accuracy.

The rolling elements, or balls, are carefully chosen to be the same size and very round. The balls are stiff, but not infinitely stiff. If there is no preload on the spindle, the balls do not contact the race or spindle, and the spindle can rattle in the housing.

A spindle without preload is neither accurate nor stiff. If the preload is increased, the balls just make contact—at a point—on both the inner and outer races. In this case, the spindle rotates accurately, but only a small force is required to displace it.

Lightly preloaded bearings are not very stiff. If the preload is increased, then the balls begin to deform, flattening at the contact. The contact between the ball and race changes from a point to a wider contact area, and the deformed balls become stiffer. For this reason, manufacturers typically assemble spindles with a large preload, but the large preload causes other problems.

As the balls roll, ball sections are compressed when in contact with the inner or outer race and then released when not in contact. That stress cycling shortens the fatigue life of the balls and races and generates heat. The heat raises the temperature of the bearing, and can increase the preload (generating more heat), which may cause the spindle to sieze or destroy the lubrication or separator. (The separator is a polymer component in the spindle that prevents the balls from contacting each other.)

Courtesy of All images: S. Smith

Figure 1. A spindle with angular-contact ball bearings in the “big X” configuration.

In addition, the balls begin to slide on the race in high-speed spindles. When the spindle speed is low, the contact on the inner and outer races are on opposite sides of the ball (Figure 2). With low preload, the balls roll between the two contact points. With higher preload, some sliding occurs in the contact area. As spindle speed increases, the centrifugal force on the balls begins to increase.