Double-Contact Sliding and Rolling Quasi Bearing Friction Testing Machine and Its Application

In bearings, there are two different types of friction pairs which are the rolling element and groove, the rolling element and cage. The two friction pairs influence each other during bearing operation. Currently, traditional tribological testing methods and instruments typically employ a single contact form to test the friction and wear properties of bearing materials, but they are unable to reflect the friction and wear characteristics of the materials under the real working conditions of the bearing. In response to this problem, A double-contact sliding-rolling quasi bearing friction testing machine was developed. The upper pair (pin-ball) simulates the friction between the rolling elements and the cage, while the lower pair (ball-disk) simulates the friction between the rolling elements and the groove of the inner or outer rings. The ball and the disk are driven by a motor, and the rotational speed is programmable and controllable. A data acquisition system for the testing machine was established, and the operation interface of the system is simple, which can complete the display, processing and saving of the experimental data in real time. After calibration and verification of the testing machine, its static test error does not exceed 5%, and the relative deviation between forward and reverse tests was 2%. Test programs for the variation curves of the friction coefficient with the sliding-rolling ratio (SRR) and the Stribeck curves were programed. Each friction curve was repeated five times. The results indicated that the repeatability standard deviation of the testing machine was not higher than 0.005. To further validate the availability of the testing machine in simulating the friction and wear of bearings, two test cases were conducted. In case one, oil-impregnated porous polyimide (iPPI) pin-steel ball-steel disc double contact was used, and the lower contact was fully flooded in oils with different viscosities. The results showed that as the viscosity of the lubricating oil increased, the friction coefficient also increased as expected due to high viscosity leading to high stirring loss. Case two investigated the blackening observed in actual iPPI bearing retainer after friction. The testing machine successfully replicated the blackening of iPPI retainer, which was commonly found in bearings and difficult to reproduce using traditional friction testing machines. Compared with the single upper contact, the surface of iPPI showed obvious blackening wear in the double contact. When the steel ball in the single upper contact was replaced with a ceramic material, no blackening was observed on the surface of iPPI. This indicated that the wear debris generated by the lower contact was penetrated inside the iPPI, leading to blackening wear, which was totally different from single contact. In this study, it was verified that the double-contact sliding-rolling quasi bearing friction testing machine can effectively simulate the internal friction of bearings and is an effective tool for evaluating the performance of polymer bearing retainers.