Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement. Efficiency is measured as a ratio of the measured performance to the performance of an ideal machine:
Because the power transmission system or mechanism does not generate power, its ideal performance occurs when the output power equals the input power, that is, when there are no losses. Real devices dissipate power through friction, part deformation and wear.
The ideal transmission or mechanism has an efficiency of 100%, because there is no power loss. Real devices will have efficiency less than 100% because rigid and friction-less systems do not exist. The power losses in a transmission or mechanism are eventually dissipated as heat.