Like so many other seals, pinion seals—found in rear wheel drive transmissions where the drive shaft yoke enters the differential housing and so named because they’re near the differential’s pinion gear—are being subjected to increasingly stringent demands. Warranty periods are increasing, as are temperature requirements and the expectations for exclusion of contaminants. Pinion seals (also known as input seals) must also resist being broken down by the increasingly aggressive additives in gear oils. High shaft speeds are common, and so are temperature extremes. The placement of pinion seals means they are constantly inundated by dust, water, and mud. Axial end play is typical. Typical pinion seal placement is shown in Figure 221. In particularly dirty environments, a flinger may also be installed on the yoke to help deflect contaminants.

A couple of different pinion seal configurations are common. As shown in Figure 222, the first is a fluoroelastomer seal with a spring-loaded primary lip and dual dirt lips. The space between the primary lip and the dirt lips is packed with grease as a further barrier to contaminants. The seal is placed in the differential housing, and the yoke connecting the drive shaft to the pinion gear provides the sealing surface.

Contamination reaching the sealing lips can cause wear on both the lips and the yoke surface, so in some cases a unitized pinion seal design is better. As shown in Figure 223, this design features two axial dirt lips, maximizing contaminant exclusion. A thrust bumper is molded on the seal to accurately locate the axial dirt lips. Sometimes a PTFE thrust bumper is utilized to minimize friction between the seal and the flange. This bumper further impedes contamination, as does the flange (which acts as a flinger). The wear sleeve provides the running surface for the seal, thus protecting the yoke.

As shown in Figure 224, a seal to be installed in the differential housing bore may have rubber molded on the I.D. of the wear sleeve to facilitate yoke installation. The exception is high-speed applications; the presence of the rubber may cause unwanted heat build-up, which will, in turn, cause higher underlip temperature and shorter seal life. A seal without rubber on the I.D. should be installed onto the yoke (using a proper installation tool) rather than into the differential housing; the shaft-yoke combination can then be fitted over the pinion shaft.

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