Rolling contact fatigue (RCF) testings of thrust ball bearing under boundary lubrication condition (low lambda condition) are conducted while changing test conditions of load, rotational speed and combination of surface roughnesses for bearing component parts. The RCF test results and measurements of surface roughnesses of before and after the tests show that the raceway surface with larger roughness promotes the surface initiated failure of the other component. In addition, they demonstrate that the behavior of change in surface roughness during rolling contact depends on the test conditions and influences RCF life accordingly. These results suggest that RCF life of rolling bearing used under low lambda conditions should be estimated considering not only analytical relationship between repeated stress in subsurface and surface roughness during RCF, but also experimental database of the running-in behavior depending on RCF conditions.
Tribological performance of polyalkylmethacrylates (PAMAs) with different alkyl groups was studied under the rolling-sliding conditions. First, the coefficient of friction for the oils containing PAMA together with tricresylphosphate (TCP) evaluated with a mini traction machine (MTM) significantly depended upon kinds of PAMA. That is, higher the adsorption ability to the metal surface as well as surface coverage maintaining property of PAMA showed lower coefficient of friction, higher contact resistance and thinner tribofilm derived from TCP. Then, the effect of fatigue life of polymer structure on the rolling bearings was evaluated with a Unisteel fatigue tester for the oils containing PAMA together with the package additive for Automatic Transmission Fluid (ATF). The polymer structure significantly affected the fatigue life and the PAMA (C1Mix) containing methylmethacrylate showed the longest life. A close correlation was observed between the fatigue life and frictional properties evaluated by MTM for the same formulated oils. These results indicate that the polymer having high adsorption ability as well as high coverage maintaining property extend the fatigue life by preventing metal-to-metal contact as well as reducing the ratio of tribofilm showing high friction on the contact.