Experimental analysis of contact fatigue damage in case hardened gears for off-highway axles

Pitting is one of the main causes of failure in planetary gear sets of axles designed for agricultural vehicles. The high torque and the low wheel speed typical of such machines result in poor lubrication and promote the onset of contact fatigue failure by pitting, which generally occurs earlier in the sun gear than in both the planets and the ring of wheel hub planetary drives. In fact, sun gears are subjected to the highest contact pressure and, at the same time, unfavourable rolling-sliding working conditions. In this paper, six case hardened sun gears damaged by pitting during endurance tests were analysed. The aim of the analysis was to highlight the key aspects of the morphology and the evolution of pitting damage on the case hardened sun gears.