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Most effective sealing is obtained with optimum shaft surface finishes. The sealing efficiency is affected by the direction of the finish tool marks and the spiral lead. Best sealing results are obtained with polished or ground shafts with concentric (no spiral lead) finish marks. If you must use shafts with spiral finish leads, they should lead toward the fluid when the shaft rotates.
No matter the PTFE machining techniques and other processes used in making mechanical parts, they tend to fail due to some reasons. The same thing applies to oil seals. When they are exposed to some factors, they fail. The factors are stated below alongside the solutions.