Four ball wear and EP (extreme pressure) tests are outlined in ASTM D 2266 and ASTM D 2596, as well as others, and are used for testing different lubricants. In each, three hard steel balls are locked in position while a fourth ball is rotated against the three others. Light loads are applied for the wear test producing a circular wear scar on each ball, and the average wear scar diameter is determined for comparison. The EP test is used to measure high load characteristics of a lubricant and is run until the balls are welded together.
Falex four ball tester
Animation of the four ball configuration (Click on the image to see animation)