Moving Parts - Mechanical Efficiency and Wear

Mechanical Efficiency and Wear

The amount of moving parts in a machine is a factor in its mechanical efficiency. The greater the number of moving parts, the greater the amount of energy lost to heat by friction between those parts. In a modern automobile engine, for example, roughly 7% of the total power obtained from burning the engine's fuel is lost to friction between the engine's moving parts.

Conversely, the fewer the number of moving parts, the greater the efficiency. Machines with no moving parts at all can be very efficient. An electrical transformer, for example, has no moving parts, and its mechanical efficiency is generally above the 90% mark. (The remaining power losses in a transformer are from other causes, including loss to electrical resistance in the copper windings and hysteresis loss and eddy current loss in the iron core.)

Two means are used for overcoming the efficiency losses caused by friction between moving parts. First, moving parts are lubricated. Second, the moving parts of a machine are designed so that they have a small amount of contact with one another. The latter, in its turn, comprises two approaches. A machine can be reduced in size, thereby quite simply reducing the areas of the moving parts that rub against one another; and the designs of the individual components can be modified, changing their shapes and structures to reduce or avoid contact with one another.

Lubrication also reduces wear, as does the use of suitable materials. As moving parts wear out, this can affect the precision of the machine. Designers thus have to design moving parts with this factor in mind, ensuring that if precision over the lifetime of the machine is paramount, that wear is accounted for and, if possible, minimized. (A simple example of this is the design of a simple single-wheel wheelbarrow. A design where the axle is fixed to the barrow arms and the wheel rotates around it is prone to wear which quickly causes wobble, whereas a rotating axle that is attached to the wheel and that rotates upon bearings in the arms does not start to wobble as the axle wears through the arms.)

The scientific and engineering discipline that deals with the lubrication, friction, and wear of moving parts is tribology, an interdisciplinary field that encompasses materials science, mechanical engineering, chemistry, and mechanics.