CHAPTER 11MACHINE ELEMENTS AND BASIC MECHANISMSCHAPTER LEARNING OBJECTIVESUpon completion of this chapter, you should be able to do the following:lDescribe the machine elements used in naval machinery and equipment.lIdentify the basic machines used in naval machiney and equipment.lExplain the use of clutches.Any machine, however simple, consists of one ormore basic machine elements or mechanisms. In thischapter we will take a look at some of the more familiarelements and mechanisms used in naval machinery andequipment.BEARINGSFriction is the resistance of force between twosurfaces. In chapter 7 we saw that two objects rubbingagainst each other produce friction. If the surfaces aresmooth, they produce little friction; if either or both arerough, they produce more friction. To start rolling aloaded hand truck across the deck, you would have togive it a hard tug to overcome the resistance of staticfriction. To start sliding the same load across the deck,you would have to give it an even harder push. That isbecause rolling friction is always less than slidingfriction. We take advantage of this fact by using rollersor bearings in machines to reduce friction. We uselubricants on bearing surfaces to reduce the friction evenfurther.A bearing is a support and guide that carries amoving part (or parts) of a machine. It maintains theproper relationship between the moving part or parts andthe stationary part. It usually permits only one form ofmotion, such as rotation. There are two basic types ofbearings: sliding (plain bearings), also called friction orguide bearings, and antifrictional (roller and ballbearings).SLIDING BEARINGSIn sliding (plain) bearings, a film of lubricantseparates the moving part from the stationary part. Threetypes of sliding bearings are commonly used: reciprocalmotion bearings, journal bearings, and thrust bearings.Reciprocal Motion BearingsReciprocal motion bearings provide a bearingsurface on which an object slides back and forth. Theyare found on steam reciprocating pumps, in whichconnecting rods slide on bearing surfaces near theirconnections to the pistons. We use similar bearings onthe connecting rods of large internal-combustionengines and in many mechanisms operated by cams.Journal BearingsJournal bearings guide and support revolving shafts.The shaft revolves in a housing fitted with a liner. Theinside of the liner, on which the shaft bears, is made ofbabbitt metal or a similar soft alloy (antifriction metal)to reduce friction. The soft metal is backed by a bronzeor copper layer and has a steel back for strength.Sometimes the bearing is made in two halves and is11-1
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