With the drive to reduce emissions and improve fuel economy, fuel injection pressures have increased. This has increased Hertzian stresses on the roller follower cam system to the point that cam lobe contact fatigue failure has become the “Achilles heel” of diesel engine durability in the 1990s. Contact fatigue failures have occurred on both injector lobes and the exhaust and inlet lobes. This is particularly the case in fleets with frequent engine shut downs and starts, stop-go service and in some line-haul fleets.
This paper describes field service cam failures across several engine types and applications. In our experience supporting fleet customers in cam failure analysis, we found that a combination of ten critical independent variables must be correct in order to prevent cam lobe contact fatigue failures. These variables are each discussed separately.
In bench tests simulating stop-go applications, we found that the silicon nitride roller on a steel pin had low and constant friction under flooded and starved lubrication conditions. In contrast, the bronze pin and steel roller had severe stick-slip with very high friction, which could cause roller skidding on the cam. The silicon nitride roller and steel pin have provided long cam life in many applications.