The mechanical efficiency is the ratio of work done by the machine (Machine output) to the work done on the machine (machine input)

n _{m} = work output / Work input = power / Power input

The efficiency is always less than unity because,

Work output= Work input- Energy losses

n _{m} = Work input- energy losses / Work input

= 1 – Energy losses / Work input

Now, Energy losses> 0

n _{m }< 1

n _{e} = electrical efficiency of a machine

n _{t} = thermal efficiency of a machine

n _{m} = mechanical efficiency of a machine

. . The overall efficiency n_{0} = n_{m} n_{e} n _{t}