**Lift / Lifting machine – Mechanical advantage, Velocity Ratio, Efficiency, Reversible or non-Reversible machine.**

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

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