Chapter 11 - Machine elements and basic mechanisms

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CHAPTER 11

MACHINE ELEMENTS AND BASIC MECHANISMS

CHAPTER LEARNING OBJECTIVES

Upon completion of this chapter, you should be able to do the following:

• Describe the machine elements used in naval machinery and equipment.
• Identify the basic machines used in naval machiney and equipment.
• Explain the use of clutches.

Any machine, however simple, consists of one or more basic machine elements or mechanisms. In this chapter we will take a look at some of the more familiar elements and mechanisms used in naval machinery and equipment.

BEARINGS

Friction is the resistance of force between two surfaces. In chapter 7 we saw that two objects rubbing against each other produce friction. If the surfaces are smooth, they produce little friction; if either or both are rough, they produce more friction. To start rolling a loaded hand truck across the deck, you would have to give it a hard tug to overcome the resistance of static friction. To start sliding the same load across the deck, you would have to give it an even harder push. That is because rolling friction is always less than sliding friction. We take advantage of this fact by using rollers or bearings in machines to reduce friction. We use lubricants on bearing surfaces to reduce the friction even further.

A bearing is a support and guide that carries a moving part (or parts) of a machine. It maintains the proper relationship between the moving part or parts and the stationary part. It usually permits only one form of motion, such as rotation. There are two basic types of bearings: sliding (plain bearings), also called friction or guide bearings, and antifrictional (roller and ball bearings).

SLIDING BEARINGS

In sliding (plain) bearings, a film of lubricant separates the moving part from the stationary part. Three types of sliding bearings are commonly used: reciprocal motion bearings, journal bearings, and thrust bearings.

Reciprocal Motion Bearings

Reciprocal motion bearings provide a bearing surface on which an object slides back and forth. They are found on steam reciprocating pumps, in which connecting rods slide on bearing surfaces near their connections to the pistons. We use similar bearings on the connecting rods of large internal-combustion engines and in many mechanisms operated by cams.

Journal Bearings

Journal bearings guide and support revolving shafts. The shaft revolves in a housing fitted with a liner. The inside of the liner, on which the shaft bears, is made of babbitt metal or a similar soft alloy (antifriction metal) to reduce friction. The soft metal is backed by a bronze or copper layer and has a steel back for strength. Sometimes the bearing is made in two halves and is

Figure 11-1.-Babbitt-lined bearing in which steel shaft revolves.

clamped or screwed around the shaft (fig. 11-1). We also call it a laminated sleeve bearing.

Under favorable conditions the friction in journal bearings is remarkably small. However, when the rubbing speed of a journal bearing is very low or extremely high, the friction loss may become excessive. A good example is the railroad car. Railroad cars are now being fitted with roller bearings to eliminate the "hot box" troubles associated with journal bearings.

Heavy-duty bearings have oil circulated around and through them. Some have an additional cooling system that circulates water around the bearing. Although revolving the steel shaft against babbitt metal produces less friction (and less heat and wear) than steel against

Figure 11-3.-Diagrammatic arrangement of a Kingsbury thrust bearing, showing oil film.

steel, keeping the parts cool is still a problem. The same care and lubrication needed to prevent a burned out bearing on your car is needed on all Navy equipment, only more so. Many lives depend on the continued operation of Navy equipment.