Point or Line Contact?

Monday, August 13th, 2012

Rolling elements in bearings make a very small area of contact with the raceways, sometimes smaller than 1 square mm (0.0015 sq. inches.) in ball bearings.  At the point of contact, there is a fundamental difference depending on the bearing type: “point” contact or “line” contact. Ball bearings generally have point contact, while roller bearings have line contact. The difference in operation is critical: in an electric motor with a ball bearing on one end and a roller bearing on the other, you’ll need to relubricate the roller bearing about twice as often as the ball bearing.
One advantage for roller bearings is that by spreading the load over a greater area, they can last longer, or they can be subjected to a greater load over time. This is why “sheave duty” motors often have a cylindrical roller bearing on the drive end. The belts in the pulley add a larger radial load to the roller bearing. The line contact spreads the load across a greater area, reducing the stresses on the bearing steel that could eventually lead to fatigue failure.
The terms “point” and “line” contact are somewhat inaccurate, since the contacts are actually small ellipses, and the area of contact varies depending on the load and other conditions. When you tear down and examine the raceways in damaged bearings, understanding the type of contact will help. The patterns created on the raceways and rolling elements change depending on the type of roller to raceway contact.