Pitting / Spalling

Pitting is a surface fatigue phenomenon occurring in concentrated rolling and sliding contacts. Particles break out of affected areas, leaving surfaces pockmarked with scattered holes. Progressive macro pitting – spalling – occurs when pits coalesce and form irregular craters, cavities and cracks. Pitting / spalling typically occurs at 60 degrees from the top of the nose whereas sliding wear is concentrated at the top of the nose. This can be explained by considering the Hertzian contact pressure and the film thickness as a function of the cam angle (Figure 7.28). The large particles that pits produce are caught in the oil filter or settle at the bottom of the sump. Inspection of the oil filter or the magnet plug at the bottom of the sump may be adequate to detect the iron particles. Alternatively, the damage of the camshaft lobes can be inspected visually.

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