An inclined plane test for the breakaway coefficient of rolling friction of rolling element bearings

Abstract An ISO/ASTM standard test has been used for over 20 years for detecting the presence of lubricating films on plastics (usually in web form). The test uses a wire paperclip (on end) as the rider on an inclined plane. A need for a test to measure breakaway friction of rolling element bearings for instrument pivots prompted a study to determine if the “paperclip” test could be adapted to screen bearings for low rolling friction. This paper describes the test modification and the results obtained in intralaboratory tests aimed at making the test an ASTM standard. The test was modified by replacing the paperclip rider with a rolling element bearing with the inner race fixed to an axle on a balanced rider. The inclined plane is raised to imitate rolling of the outer race on the inclined plane. The tangent of this angle is used as the test metric. Seven random ball bearings, some new, some used, were tested by six laboratories in the USA to determine if the test is repeatable enough to be an ASTM standard. The coefficient of variation was quite good within lab, and reasonable between labs. The test seems to be a fast, simple friction test for rolling element and other bearings that should be standardized.