It is a device, which enables us to lift a heavy load W, by a comparatively small effort P. The following terms are commonly used in lifting machines

1. Mechanical advantage (M.A.).- It is the ratio of load lifted (W) to the effort applied (P).

2. Velocity ratio (V.R.). – It is the ratio of the distance moved by the effort (y) to the distance moved by the load (x).

3. Input of the machine. – It is the workdone on the machine. It is equal to the product of effort and the distance through which it moves (i.e. P x y).

4. Output of the machine. – It is the workdone by the machine. It is equal to the product of load lifted and the distance through which it has been lifted (i.e. W x X).

5. Efficiency of the machine.- It is ratio of output to the input of the machine. Mathematically, efficiency of the machine,

6. Ideal machine. – If the efficiency of the machine is 100 %, i.e. if output is equal to input, then the machine is said to be a perfect or ideal machine.

7. Reversible machine.- If a machine is capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as reversible machine. The condition for a machine to be reversible is that its efficiency should be more than 50 %.

8. Non- reversible or self locking machine.- If a machine is not capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as non-reversible or self locking machine. The condition for a machine to be non-reversible or self locking is that its efficiency should be less than 50 %.

9. Law of the machine.- It is the relationship between the load lifted (W) and the effort applied (P). It is given by the equation,

P = m.W+C
where
m = A constant (called coefficient of friction) which is equal to the slope of the line AB as shown in Fig. 1.23, and
C = Another constant, which represents the machine friction.

10. Maximum mechanical advantage.– The maximum mechanical advantage of a lifting machine is given by
Max. M.A. = 1 / m

11. Maximum efficiency.- The maximum efficiency of a lifting machine is given by