As mentioned in chapter 3, the bearings of an engine make up an important group of parts. Bearings serve to support rotating shafts and other moving parts and to transmit loads from one part of the engine to another. Engine bearings consist of two basic types: antifriction bearings and friction bearings. Both types are used in Navy diesel engines.

ANTIFRICTION BEARINGS

Antifriction bearings can be grouped into six general classifications: ball bearings, cylindrical roller bearings, needle bearings, tapered roller bearings, self-aligning roller bearings, and thrust bearings. The use of antifriction bearings is mostly limited to the exterior areas of an engine. You will find them in use in cooling pumps, fuel-injection pumps, governors, starters, flywheel pilot bearings, turbochargers, and blowers.

All antifriction bearings employ a rolling element (rollers, balls, or needles) between the inner and outer rings (races). Either the inner ring or the outer ring will remain stationary. (Refer to fig. 4-24 for cutaway views of two types of antifriction bearings.) Because of the small contact area between the rolling elements and the inner and outer rings and the necessity for the bearing to withstand the high compression stress, the material used for the construction of roller bearings is usually carbonized steel alloy and that used for ball bearings is usually heat-treated chromium-alloy steel.

Figure 4-25.Main journal bearing shells.

Figure 4-24.Cutaway view of a ball bearing and roller bearing.

As an Engineman, you will come into contact with various items of equipment that may require bearing replacement. Bearings that are similar in appearance may not be suitable as replacement bearings. Ball and roller bearings are identified by a numerical code which indicates the bore in millimeters or sixteenths of an inch. The internal fit, or tolerance, and any special characteristics are also coded by number. Letter codes indicate the type of bearing, the outside diameter (OD), the width of the cage, the seal or shield, the modification, and the required lubricant.