Lubricating spindle bearings: General Industry Coverage

Rolling-element spindle bearings require lubrication because even the best bearings experience significant sliding contact. The bearings and races are not perfect and deform under preload. As a result, the contacts between the rolling elements and the races are not points, but patches, and some parts of the patches slide. Lubrication reduces the heat generated during sliding and extends bearing life.

When spindle speed is low, such as a few thousand rpm for spindles a few inches in diameter, grease lubrication is sufficient. These bearings can be permanently sealed, or the grease can be replenished as it leaks out. Grease lubricant is quite viscous and will stay in place no matter the spindle orientation. In addition, such spindles usually are belt- or gear-driven, and the motor’s heat source is separated from the spindle.

As spindle speed increases, belts and gears become impractical, and the spindle design is modified so the spindle is the rotor of the motor. The heat generated by the motor escapes only through the bearings, and the bearings are not good conductors of heat. In addition, preload is increased to maintain spindle stiffness, and the bearings themselves become significant heat sources. The resulting temperature in the spindle becomes too high, typically at around 150° C (302° F), for grease lubrication. At this temperature, the grease can liquefy or burn, and oil lubrication is typically used instead.

Oil can be pumped through the bearings, but the sealing strategy becomes important, especially if the spindle is suitable for vertical and horizontal orientations. A liquid flow must not leak out, so a sealing strategy is required. It is easier to seal liquid in a horizontal spindle, because the fluid collects in an area where there is no gap. If the spindle is vertical, the gap between the spindle and housing is difficult to seal. Some spindles change orientation, which changes the oil flow, so sealing is a challenge.

The flowing oil helps remove heat from the spindle and bearings to a certain extent. At low flow rates, the oil provides some cooling, but its capacity to absorb heat is limited. As the flow of oil through the spindle increases, oil’s capacity to absorb heat increases, but the churning of the oil by the bearings eventually begins to generate heat on its own. The lubrication system must dissipate the heat generated by the spindle and by the churning oil. Spindle designers deal with the lubrication and heat problems in three common ways: chilled oil, oil mist and air/oil injection.