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Machinery & Equipment MRO
September 2021
SLIPPAGE OF ROLLING ELEMENTS IN BALL AND ROLLER BEARINGS BY DOUGLAS MARTIN
Normal rolling contact:
With rolling elements, there is always slippage across the contact ellipse. In this picture, the yellow triangles are where there is rolling contact, and all the other areas are sliding contact.
So even in a normally operating bearing, there is slippage, and hence, the importance of a lubricating film to separate the rolling surfaces.
Abnormal contact:
Abnormal contact arises from these cases:
Three points of contact:
With a QJ Bearing (4 Point Angular Contact Ball Bearing), the bearing is designed internally to contact in either of two planes, however, there should only be one plane in contact at any given moment. This allows the bearing to accommodate a thrust load in either direction, saving space compared to a pair of single row angular contact ball bearings or a double row ball bearing. If a radial load is applied to this bearing, it will cause the ball to contact the outer race in two places, while still only in contact
with the inner race in one place. The ball will rotate in the plane with the outer and inner ring contact at 180 degrees apart, and the second contact on the outer race will be sliding. For this reason, these bearings are mostly installed with a relieved housing seat such that the outer ring outer diameter is not in contact with the housing bore, preventing an unintended radial load to be applied to the bearing.
Two-point contact on different planes
With a pair of angular contact bearings, there is one bearing that is taking the thrust load and the other bearing is unloaded or sharing a portion of the radial load. With lower speeds, the unloaded bearing will contact the races in two places, and the ball will spin on a single plane with rolling contact with both races. In higher speed applications, a centripetal force will act on the ball to shiª it in a radial direction. When this occurs, the ball will contact the outer race on a different plane than the inner ring contact and one of these contact points, typically with the outer race, will slide.
Rolling element speed/moment of inertia
In a radially loaded bearing, the rolling element is moving around the bearing and only in full contact with both races in the load zone. While it is in the load zone, the contact with the inner and outer ring provides traction such that the roller is driven and it rotates. When the roller is out of the load zone, it is no longer driven by the race contact. It is pushed by the cage through the unloaded zone. While it is being pushed, the frictional contact with the cage bar will slow the rolling element’s rotational speed. Also, any frictional effects of the grease or oil may also contribute to a slowing of the roller rotation. Depending on the size of the bearing, there may be enough of a distance through the load zone such that the relative speed of the rolling element surface and the race surface becomes significantly different. When the roller then enters the load zone, it quickly accelerates up to the speed of the races. Oªen with more massive roll-
Figure images: SKF.
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n this article, there will be a breakdown of the different forms of “slippage” and what some of the causes are. The term “slippage”, meaning that at some point on the rolling element/race contact there is relative motion. One surface is going a different velocity (speed and/or direction) than the other surface. One important component of a bearing that will make a significant difference in whether that slippage causes damage or leaves no trace is the lubricant, and specifically the generation of an oil film between the two surfaces. This oil film is the buffer between the two differently moving surfaces.
