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ANTIFRICTIONAL OR ROLLER
AND BALL BEARINGS You have had first-hand acquaintance with ball bearings since you were a child. They are what made your roller skates or bicycle wheels spin freely. If any of the little steel balls came out and were lost, your roller skates screeched and groaned.Antifrictional balls or rollers are made of hard, highly polished steel. Typical bearings consist of two hardened steel rings (called races), the hardened steel balls or rollers, and a separator. The motion occurs between the race surfaces and the rolling elements. There are seven basic types of antifrictional bearings (fig. 11-4):1. Radial ball bearings 2. Cylindrical roller bearings 3. Tapered roller bearings
Figure 11-5.-Ball bearings. A. Radial type; B. Thrust type. 4. Self-aligning roller bearings with a spherical outer raceway 5. Self-aligning roller bearings with a spherical inner raceway 6. Ball thrust bearings 7. Needle roller bearings Roller bearing assemblies are usually easy to disassemble for inspection, cleaning, and replacement of parts. Ball bearings are assembled by the manufacturer and are installed, or replaced, as a unit. Sometimes maintenance publications refer to roller and ball bearings as either trust or radial bearings. The difference between the two depends on the angle of intersection between the direction of the load and the plane of rotation of the bearing. Figure 11-5, A, shows a radial ball bearing assembly. The load shown is pressing outward along the radius of the shaft. Now suppose a strong thrust were to be exerted on the right end of the shaft in an effort to
Figure 11-6.-Radial-thrust roller bearing. move it to the left. You would find that the radial bearing is not designed to support this axial thrust. Even putting a shoulder between the load and the inner race wouldnt support it; instead, the bearings would pop out of their races. Supporting a thrust on the right end of the shaft would require the thrust bearing arrangement of the braces shown in figure 11-5, B. A shoulder under the lower race and another between the load and the upper race would handle any axial load up to the design limit of the bearing. Sometimes bearings are designed to support both thrust and radial loads. This explains the use of the term "radial thrust" bearings. The tapered roller bearing in figure 11-6 is an example of a radial-thrust roller bearing. Antifriction bearings require smaller housings than other bearings of the same load capacity and can operate at higher speeds. SPRINGS Springs are elastic bodies (generally metal) that can be twisted, pulled, or stretched by some force. They can return to their original shape when the force is released. All springs used in naval machinery are made of metalusually steelthough some are made of phosphor bronze, brass, or other alloys. A part that is subject to constant spring thrust or pressure is said to be
Figure 11-7.-Types of springs. spring-loaded. (Some components that appear to be spring-loaded are actually under hydraulic or pneumatic pressure or are moved by weights.) |
 
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