The axle is a rod of smaller radius fixed to the centre of the wheel which is of larger radius. This simple machine i.e wheel and axle is a combination of a wheel and an axle where the wheel moves as per the motion of the axle. A wheel and axle make it easy to move heavy things. Vehicles that run on wheels use wheel and axle.

Efficiency: Efficiency of a simple machine is its ability to convert the amount of energy supplied to it into work. Work done by the machine Efficiency = Work done by the machine Energy supplied Efficiency is usually written as a percentage: Efficiency = Workdone by the machine Energy supplied × 100 % Efficiency of any machine cannot be more than 100 % because a machine cannot do more work than the given supply of energy. Generally, efficiency of a machine is always less than 100 % because some energy is lost while operating the machine, such as energy loss due to friction .