Abstract: There are two types of friction pairs in bearings, rolling elements and internal or external channels, rolling elements and retainer, especially in bearings using polymer retainer, the properties of these two types of friction pairs are completely different. The mutual influence cannot be ignored. Oil-impregnated porous polyimide (iPPI) is one of the polymers usually used as bearing retainers. At present, the standard friction testers on the market all use a single contact, which cannot simulate the friction in the bearing well. Aiming at this problem, a double-contact friction testing machine was developed, and the testing machine was calibrated. Calibration include both forward and reverse directions. After calibration, the repeatability accuracy of the friction force of the upper contact was ±0.025 N, and the repeatability of the friction force of the lower contact was ±0.05 N. The rotation speed of the testing machine was controlled by programming, and the Stribeck curve of the friction pair could be quickly measured. The Stribeck curves of metal-metal, metal-iPPI single pair and metal-iPPI-metal double pair were tested, and the results showed that the testing machine had good test accuracy. The blackening process of iPPI bearing retainer materials was studied by this test machine. The friction and wear characteristics of iPPI materials under single and double contacts were compared. The results showed that the developed double-contact friction tester could more realistically simulate the wear and blackening phenomenon on the surface of the iPPI retainer material, while the single-contact friction test was difficult to reproduce this blackening worn surface. The worn surfaces of iPPI under the single-contact and double-contact were analyzed by energy dispersive analysis (EDS) and Raman spectroscopy. It was found that the blackened substances on the surface of iPPI were α-Fe₂O₃ and Fe₃O₄. The blackening mechanism of the worn surface of the iPPI retainer in the bearing was that the friction between the steel ball and the steel ring, the steel ball and the iPPI retainer produced wear debris, and the wear debris was repeatedly sheared into tiny particles during the friction. The tiny particles penetrate into the iPPI material with the extrusion and recovery of the lubricating oil, which resulted in blackening of iPPI surface. Meanwhile, the blackening of iPPI worn surface indicated that the iPPI material had good embeddability. Tiny wear debris could be embedded inside the iPPI material from the micro pores, which was beneficial to the reduction of friction and wear and had a protective effect on the friction pair. The blackening on the surface of iPPI was also an "indicator" of the wear state of the steel friction pair. Therefore, the porous retainer material should not only ensure the storage of enough lubricating oil to lubricate the friction surface and reduce the blackening, but also had a good embeddability, in the case of poor lubrication, the wear debris could be embedded. This was the design principle for porous oil-impregnated retainer materials. From the results, the developed double-contact friction testing machine had high test accuracy and good repeatability and was an effective tool for studying the friction in bearings.