Oil coking, the deposition of a hard layer of carbon on a shaft seal’s lip, can result when oil comes in contact with high underlip temperatures. For example, the combination of high shaft speed and high sump temperature can cause excessive underlip temperature, which in turn burns the oil and deposits the layer of carbon on the lip. This carbon crust blocks the pumping ability of the lip, making leakage inevitable. Figure 247 shows what an oil-coked lip looks like.

The only two ways to eliminate oil coking are to reduce the radial load of the lip, thus reducing wear and heat buildup, which in turn may bring the underlip temperature down enough to prevent the coking reaction with the lubricant. If this proves unfeasible, you may need to switch to a different lubricant, possibly a synthetic blend engineered to handle excessively high temperatures.

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