In all machines there is more or less friction. The work done by the acting force always exceeds the useful work by the amount that is transformed into heat. The ratio of the useful work to the total work done by the acting force is called the efficiency of the machine.
Fig. 35. - Ball Bearings.
Efficiency = Useful work accomplished / Total work expended
The efficiency of simple levers is very nearly 100% because the friction is so small as to be disregarded. In the inclined plane the friction is greater than in the lever because the two bodies come in contact with a larger surface. The efficiency of a lever is somewhere between 90 and 100%. The efficiency of the commercial block and tackle with several movable pulleys varies from 40 to 60%. In the case of the jack screw there is necessarily a large amount of friction so that its efficiency is often as low as 25%. Gear wheels or chain gears, such as are used in bicycles, are machines of high efficiency, often running as high as 90% or more.