Managing hybrid spindle bearings

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.

As a result, to keep the forces in balance, the outer contact point moves radially, and the support force changes direction (Figure 3). The balls cannot simultaneously roll on these contact points. Instead, the balls roll on either the inner or outer race, and slide on the other. The sliding generates more heat, shortening bearing life. 

Hybrid spindle bearings can help overcome these problems. Hybrid bearings have steel races and ceramic balls, typically silicon nitride. Hybrid bearings are significantly more expensive than all-steel bearings, but have advantages in high-speed applications. For example, Si3N4 balls are about 50 percent stiffer than steel balls (the modulus of elasticity is 301 GPa vs. 210 GPa for steel). That means the spindle is stiffer, and less heat is generated during stress cycling of the balls. 

Figure 2 (left). Contact forces during low-speed operation.

Figure 3 (right). Contact forces during high-speed operation, when the centrifugal force (red) becomes important and the support force changes direction. 

Centrifugal force is proportional to ball density, to the distance from the spindle axis and to the square of the spindle speed. In high-speed applications, Si3N4 balls are beneficial because they are about 40 percent less dense than steel balls (3.2 g/cm³ vs. 7.9 g/cm³ for steel). Because the balls have less mass, they slide less and generate less heat.

Of course, friction still generates heat, but ceramic balls have a lower coefficient of friction on steel races than steel balls do. Heat is also generated from stress cycling and from the rotor of the electric motor mounted on the spindle shaft. While ceramic balls have a coefficient of thermal expansion about four times less than steel, they are, unfortunately, poor conductors of heat. Typically, the only path for heat to leave the spindle is through the bearings. 

Spindle designers are forced to use expensive measures of controlling preload while allowing thermal expansion of the shaft or providing supplemental cooling to remove the heat, such as chilled oil through the spindle shaft. Hybrid designs are expensive compared to steel designs, but they make accurate high-speed machining possible. 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.