Reversible and Irreversible Machine:

Effort is applied in the machine to lift the load. Assume the load has been lifted by a distance 'x'. If effort is eliminate and the load starts falling down, the machine is said to be reversible. If load reside there even if the effort is eliminate the machine is said to be irreversible. In the case of irreversible machine, friction in the machine is adequate to hold the load in case of effort being eliminated.

Work completed by the friction force = Input - Output

                                                     = P_y - W_x

Force irreversible machine

Work finished by friction force > Output of machine

∴          P_y - W_x > W_x

or         P_y > 2 W_x

or         W_x / P_y < 1/2

or         η < 1/2  or 50%

Thus, if efficiency of the machine is less than 50%, it is an irreversible machine.