Hollow bearings dampen vibrations

April 18, 2001
Hollow-cylindrical-roller bearings are intended for light load applications, including all types of grinding, machining, and milling spindles.

Hollow cylindrical roller bearings are used in machine-tool spindles and other precision positioning equipment.

Hollow-cylindrical-roller bearings are intended for light load applications, including all types of grinding, machining, and milling spindles. They use hollow cylindrical rollers instead of solid rollers. These provide significant advantages over standard ball bearings, such as reduced vibration, low-radial runout, better radial stiffness, higher operating speeds, and lower operating temperatures.

The hollow rollers in these bearings act like shock absorbers to damp vibrations. Any vibratory load on the bearing, like a load caused by imbalance, is absorbed by roller flexibility. Preloading compresses all rollers to the same radial height, and they rotate as if they were all exactly the same diameter.

Hollow rollers are lighter than solid rollers which reduces centrifugal forces at high operating speeds, lowers contact stresses, and allows higher speeds. Bearing lubricant can also flow through the center of the hollow rollers and dissipate heat more effectively than solid rollers. The resultant cooler running improves machine-tool precision capabilities and permits higher operating speeds. Hollow-cylindricalroller bearings out-perform conventional roller bearings and angular contact bearings by radially preloading the hollow rollers. In contrast, regular roller bearings must have some internal clearance for lubrication, and less than half of the rollers carry the radial load. Angular contact ball bearings can be preloaded to remove all internal clearance, but the ball contact area is small and allows further radial deflection and vibration.

To the casual observer, it would seem that a bearing with flexible hollow rollers would exhibit little radial stiffness. But actual stiffness is several times greater than ball bearings because all the rollers are preloaded between the rings, so all share the load. This configuration doubles effective stiffness over normal radial bearings. Also, contact deflections are taken up and the area of contact on the raceway is larger.

The hollow-roller elements actually change shape to produce a low radial runout. The hollow rollers have a slight oval shape, so when rotated, the radial stiffness of the bearing increases. The hollow rollers resist changing shape rapidly because of hysteresis in bending. Hysteresis is the reluctance of a metal roller to regain its roundness after being forced into a non-round shape. It takes several seconds for a roller to regain its round shape after releasing the pressure.

Radial runout is a critically important measure of bearing precision. The variation in radial distance from the bearing bore surface to the bearing outside diameter as the bearing rotates is a measure of radial runout. Hollow-cylindricalroller bearings have radial runouts down to 0.000050 in. This is because preloaded hollow rollers eliminate all internal radial clearance in the bearing, absorb minute variations in the ring surface finish, and maintain a constant rotating center.

Total bearing runout is composed of two parts, repeatable runout and nonrepeatable runout. Repeatable runout is the difference between high and low indicator readings for one revolution of the bearing, and is often the result of inner-ring concentricity error. This runout can be eliminated by grinding or truing the spindle nose. Hollow-cylindrical-roller bearings have extremely small non-repeatable runout, usually <0.000020 in. Nonrepeatable runout is the difference between high and low indicator readings as measured for one revolution of the complement of rollers. Several shaft revolutions

may be required to obtain the maximum difference.

The absence of internal bearing wear and the heat it would generate permits higher speeds. In contrast, the hydrodynamic nature of conventional rollers always limits speed. Oil in the small gaps between conventional bearing surfaces acts like oil in a shock absorber, and creates viscous shear resistance and reduces speed.

In conventional roller bearings the "at-rest" contact occurs between the bearing surfaces. Bearing surfaces generate an oil film during motion that causes a higher static friction than dynamic friction. The difference between static and dynamic coefficients of friction results in the phenomenon referred to as "stick-slip," and can make it difficult to position conventional rollers accurately in small motion application. Hollow roller bearings have higher speeds than plain-surface bearings, and permit smaller axial steps without "stick-slip."

A hollow roller tends to run straight and true about its rotational axis, which is the axis with highest moment of inertia. Consider a barrel hoop and how well it rolls, correcting itself as it gains speed. To resist skewing, the moment of inertia about the roller's rotational axis should be greater than or equal to the moment of inertia about the skewing axis. Hollow rollers are about half the mass of solid rollers and centrifugal forces are similarly reduced. Therefore, stresses at the outer race-roller contact are considerably reduced.

What's more, hollow-cylindricalroller bearings are microfinished to increase surface contact between rolling elements, minimize fatigue, and extend bearing life. At comparable speeds and loads, microfinished roller bearings last up to 2.5 times longer than their ground counterparts. This also translates into about 30% greater dynamic load capacities.

This information provided by Kaydon Corp., Muskegon, Mich.

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