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
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
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
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 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