A seal must always be installed so that the sealing lip is facing the fluid to be sealed. This is because the lip is made so that pressure applied to it from the “wet” side of the seal will tend to increase the pressure the lip applies to the shaft. If the seal is installed backwards, pressure acting on the “wrong” side of the lip will cause it to lift from the shaft, resulting in leakage. On most seals, the correct side is obvious; however, on others it is not.
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It has been a long drive to the lake. The driver carefully backs the trailer down the boat ramp. As the axles hit the water, the hot wheel bearing hubs hit the water simultaneously. As the bearings’ heat is quenched by the lake water on the outside of the hub, the rapidly contracting air and lubricants inside the hub create a vacuum. If this vacuum can’t be held by the seals, the hub will suck in water and contaminants.
While this is an extreme example, this type of contamination can happen to all bearings if the seals are not in good condition. It is clear to see, the most important part of bearings is the seal. If contaminants are able to work their way onto the contact surfaces, or the lubricant is expelled, the bearing will not last long.
Some newer seals are fabricated using hydrogenated nitrile butyl rubber. The manufacturer claims that the material is impervious to attack and degradation by synthetic fluids and additives that can attack conventional nitrile compounds. In addition, the material has a high resistance to abrasives that can become embedded in other compounds, thus causing leakage.
Most seals today are termed “lip seals,” as they have a lip that rides against the outside diameter of a shaft. This “rubber” (nitrile, polyacrylate, silicone, etc.) lip is bonded to a metal shell that fits into the bore of the component to be sealed. A garter spring fits in a groove behind the lip to help the lip maintain contact with the shaft. Sometimes you’ll find a bead of sealer around the outside diameter of the shell to help seal the metal shell to the bore in which the seal is installed. In other cases, the metal shell is completely covered with same material from which the lip itself is made.
Some lip seals include their own integral dust seal, which is a small, additional lip facing the outside of the shell. This small lip has no garter spring. Some bearing seal manufacturers are making seals that can have up to three different lips.