Ball Excursion and How It Is Prevented
Ball Excursion
12-02-2008

Uniform spacing of the ball set in a rotating bearing is critical to bearing performance and life. After all, every ball bearing—except for full complement designs—includes a cage, which positions the balls at approximately equal intervals around the bearing raceway. In some applications, however, the cage experiences a catastrophic failure by getting torn apart by the ball-set.

Such a catastrophic failure might indicate the presence of a condition known as ball excursion. Ball excursion occurs when the balls fail to move around the bearing at circumferentially equidistant intervals, which applies pressure in a radial direction on the cage’s ball pockets. When the ball-set’s spatial irregularity is excessive, cage-failure often results.

Ball excursion occurs in certain load conditions, which cause a variation in the bearing's contact angle, ball path, and effective pitch radius. Changes to these factors cause the balls to accelerate and decelerate as they move through different segments of the raceway. Because each ball experiences these changes in momentum at different instances, the balls fail to stay within their ideal positions as defined by the cage’s ball pockets.

The conditions that bring about ball excursion are complex and involve many factors, including how load and speed conditions interact with the bearing geometry and cage design. Two possible causes of ball excursion include misalignment and combined thrust and radial loading. Misalignment changes the orientation of the inner raceway in relation to the outer raceway, which forces the ball-set to track off course. Combined loading varies the contact angle through variations in load distribution.

The best way to deal with ball excursion is to determine its likelihood before finalizing the bearing design. If a slight amount of ball excursion is likely, then modifying the shape of the cage's ball pockets can accommodate it. With moderate to severe amounts, it may be necessary to optimize the bearing’s internal geometry. Before adjusting solely for ball excursion, however, it is crucial to analyze how any change affects other performance characteristics.

For help in determining the best approach to dealing with ball excursion, contact one of NHBB’s ball bearing engineering departments at either HiTech or Precision.