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Grease seals are typically found on automotive wheel-bearing assemblies. The automotive wheel bearing consists of an outer as well as an inner roller bearing along with the grease seals. The roller bearings are packed with grease when they are installed and that grease melts as the wheel spins. The grease seals consist of a soft rubber or nylon ring incorporated into a metal housing which is installed into the axle housing or wheel hub and is positioned close to the axle or spindle. As the grease melts due to the friction of the wheel's spinning, the grease seals prevent the liquid grease from seeping out of the bearing housing.
In the early years of the automobile, grease seals were formed out of leather. The leather grease seals were adequate and successful due in part to the relatively slow speed of the wheels. As the automobile evolved and wheel speeds increased, the leather grease seals proved to be inadequate. An improved version consisting of a rubber seal bonded to a metal body proved to successfully seal the bearings, preventing grease from escaping the bearing assembly. Some vehicles manufactured prior to 1950 used a seal made of rope. The rope http://www.wisegeek.com/back/article.htm?id=128806seals were marginally successful in sealing the bearings.
When installing a new wheel bearing, it is wise for individuals to replace the grease seals as well. The slightest nick or manipulation of the grease seal could result in a faulty seal and grease finding its way out of the bearing. The signs of a faulty seal are streaks of grease running from the center of the wheel to the outer edge of the wheel or tire. In the case of a faulty inner seal, the smell of hot or burning grease when braking is often a tell-tale sign of a problem. This happens when escaping grease is leaked onto the brake rotor and is heated by the friction of applying the brakes.
Replacing a seal is as easy as prying the failed seal out of its location and tapping the new seal into position. It is imperative that the inner lip of the new seal be coated with a light oil or grease prior to being installed onto the axle or spindle. This prevents the seal from becoming over-heated, resulting in a distorted seal. A distorted seal will not fit close to the axle or spindle and will allow grease to escape from the bearing.