Engineering Mechanics

(^38) „„„„„ A Textbook of Engineering Mechanics
3.11.LEVERS
A lever is a rigid bar (straight, curved or bent) and is hinged at one point. It is free to rotate
about the hinged end called fulcrum. The common examples of the use of lever are crow bar, pair of
scissors, fire tongs, etc.
It may be noted that there is a point for effort (called effort arm) and another point for
overcoming resistance or lifting load (called load arm).
3.12. TYPES OF LEVERS
Though the levers are of many types, yet the following are important from the subject point of
view.

1. Simple levers. 2. Compound levers.

3.13. SIMPLE LEVERS

Fig. 3.16. Simple levers.

A lever, which consists of one bar having one fulcrum is known as *simple lever as shown in

Fig. 3.16 (a) and (b).

Let P= Effort applied

W= Weight lifted

a= Length between fulcrum and effort, and

b= Length between fulcrum and weight.

Now taking moments of the effort and load about the fulcrum (F) and equating the same,

P. a =W. b or

Wa

Pb

=

The terms

W

P

and

a

b

are commonly known as †mechanical advantage and leverage. A little

consideration will show that in order to increase the mechanical advantage, either length of the

lever arm (a) is to be increased or length of the load arm (b) is to be reduced.

Note. A simple lever may be straight, curved or even bent.

* The simple levers are classified as

(a) Lever of first order. In this type of lever, the effort and load act on the opposite sides of the

fulcrum.

(b) Lever of second order. In this type of lever, the effort and load act on the same side of the fulcrum.

But effort acting at a greater distance than the load. Or in other words, load is acting between the

fulcrum and the effort.

(c) Lever of third order. In this type of lever, the effort and load act on the same side of the fulcrum.

But load acting at a greater distance than the effort. Or in other words, effort is acting between the

fulcrum and the load.

† This point will be discussed in more details in the chapter ‘Simple Lifiting Machines’.