Sunday, December 27, 2009

Some thoughts on camshaft lobes

From previous articles I've written on inspecting camshaft lobes, one point that possibly wasn't stated clearly is that the typical camshaft lobe failure is not a lubricant failure. Signs of lubricant failure show surface sliding damage such as scuffing, scoring, and metal transfer across the surfaces. Not that this doesn't occur, especially, in later stages of lobe disintegration, but often the primary failure that starts the whole process is not related to lubrication.
If you look at the initial damage on a camshaft lobe what you see is a good engineered surface with irregular shaped potholes. A pothole is a good analogy as both a camshaft with a follower rolling on it and a road surface are both non-conforming surfaces with Hertizian type stresses. Potholes in roads release chunks of asphalt with top road surface in good condition. Same with a camshaft lobe, the surface doesn't look bad in the early stages of failure. It's just that small flakes are missing. There is a simple reason for this, the highest stress is below the surface.
Whenever you have rolling or non-flat surfaces, the highest stress occurs below the surface. This is called hertzian stress. Fatigue cracks start off below the surface, enlarge and eventually a chunk of material is released. The wear scar often clearly shows crack formation and crack growth.
Lubricants are not involved in this process. Not that a lubricant isn't necessary for preventing surface wear, but subsurface fatigue cracks are not affected by lubricants. Failure is determined by the magnitude of the stress and the number of stress cycles. So whenever you add one of those calcium fortified camshaft lubricants to your engine oil, do not expect miracles. Given enough time and hours, all camshaft lobes fail through Hertzian fatigue.

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