members. Two basic types of oil seals used on vehicles today are synthetic rubber seals and wick seals. Each is discussed below.

SYNTHETIC RUBBER SEALS. The synthetic rubber seal (fig. 3-20) is the most common type of oil seal. It is composed of a metal case used to retain its shape and maintain rigidity. A rubber element is bonded to the case, providing a sealing lip or lips against the rotating shaft. Different types of oil seal designs are shown in figure 3-20. A coil spring, sometimes called a garter spring, is used to hold the rubber element around the shaft with a controlled force. This allows the seal to conform to minor shaft runout. Some synthetic rubber seals fit into bores mounted around the shaft This type is generally a split design and does not require a metal case or garter spring. Figure 3-20 shows the effects of pressure on lip seals. The internal pressure developed during operations forces the sealing lips tighter against the rotating shaft. This type

seal only operates effectively against fluid pressure from one direction. Leather also is used as a lip seal. In this configuration, the inside diameter of the seal is smaller than the shaft As the shaft is installed, the seal bows outward to form a lip seal.

Figure 3-19. - Typical gaskets.

WICK SEALS. The wick seal (fig. 3-21) is made of graphite-impregnated asbestos. Wicking is sometimes used to control oil leakage. This seal conforms to the recess in which it is installed. When using this type of seal, use a knurl finish on the rotating shaft. The oil is contained between the knurls and seal, which rub together. As the shaft rotates, the oil is driven back by the propeller effect of the seal and knurl finish. An oil slinger sometimes is used with wick seals. The oil slinger is a raised washerlike area on the shaft. As oil meets the slinger, it is propelled outward by centrifugal force. A catch trough then is used to collect the oil and return it to the sump.

As you gain experience in the mechanical field, you will be able to recognize the different types of seals and how they work to prevent leaks. Other types of seals are discussed in a later module.

MOVING PARTS OF AN ENGINE

The moving parts of an engine serve an important function - turning heat energy into mechanical energy. They further convert reciprocal motion into rotary

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