Quantcast Chapter 11 Machine Elements and Basic Mechanisms

CHAPTER  11 MACHINE ELEMENTS AND BASIC MECHANISMS CHAPTER  LEARNING  OBJECTIVES Upon completion of this chapter, you should be able to do the following: l Describe the machine elements used in naval machinery and equipment. l Identify the basic machines used in naval machiney and equipment. l 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 11-1


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