As noted in Anatomy of a Shaft Seal, shaft seals have a large number of variable features. The ways in which these variables work together to form a successful seal is not simple. On the contrary, a complete understanding of how a shaft seal functions is not arrived at easily. What follows, however, is an overview of the major principles at work in shaft sealing.

Once installed, a typical shaft seal is defined by two sealing surfaces. In order for the seal to perform successfully, both of these surfaces must function properly. The first is a tight static seal formed as a result of contact, or interference, between the seal’s outside diameter (O.D.) and the housing bore. The seal O.D. is designed to be slightly larger than the bore, typically .004" to .008" larger for metal O.D. seals and .006" to .012" larger for rubber-covered O.D.s. (The exact amount of interference depends on the bore size.) This difference between seal size and bore size ensures a tight pressfit that leaves no room for leakage around the O.D. The tightness of this fit also keeps the seal retained in the bore.

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