EFFECTS OF AREA RATIO AND NATURE OF SURFACES ON SCUFFING IN LUBRICATED CONTACTS

Heavily loaded moving machine components encounter severe tribological problems. Typical examples include piston and cylinder contacts in a hydraulic motor. Piston and cylinder bore contacts invariably operate in boundary lubrication regime and the risk for seizure of these contacts is high particularly when lubricated with low-viscosity lubricants. The piston assembly in a radial piston hydraulic motor has conformal (area) contact. In this work, these contacts have been simulated in the laboratory by using a thrust washer test configuration. The influence of area ratio, nature of contacting surfaces, and speed on scuffing has been investigated by using a factorial design of experiments approach. The influence of area ratio on wear at lower speed has also been studied. The results have revealed that the surface power, or mu p nu-value, at scuffing varies by 3-4 times when the area ratio was increased from 8 to 72 per cent. The running-in wear tests have shown that running-in of the upper and lower specimens also varies with the area ratio. During the first 30 s the upper specimen is polished with low area ratio, whereas the lower specimen is polished with high area ratio. The surface temperature was estimated by using finite-element calculation and it was found to be similar prior to scuffing for both 8 and 72 per cent area ratios at 1.7 m/s sliding speed. The comparison of the results with the actual motor tests shows that an area ratio of 24 per cent seems to simulate the piston-cylinder contact better.