Fly-wheel, a large wheel with a heavy rim fitted to steam-engines, or other machinery, in order to equalise the effect of the driving effort. Its action depends on the principle that matter in motion possesses kinetic energy—e.g. a body in having its velocity reduced is capable of doing work. The amount of work it can do depends directly on the mass of the body, and on the difference of the squares of its initial and final velocities, and is numerically equal to . A heavy wheel thus becomes a reservoir of work when set in motion.
There are two principal cases in which the fly-wheel is commonly applied: (1) where the driving effort is intermittent or irregular, while the resistance to be overcome is for the time practically constant; and (2) where the resistance or work to be done is intermittent or irregular. The crank in a foot-lathe is a good example of the first case; the driving effort of the foot is only applied to the treadle on the down stroke, and the crank must rise independently of the effort. A fly-wheel attached to the crank-shaft effects this, the motion it acquires while the foot is acting gives it energy, and in virtue of this it is able to bring the crank up again into the proper position for the foot to act on the treadle. In single-crank engines the fly-wheel carries the crank over the dead centres (see CRANK), and whenever used in engines its function is to keep the speed steady during each revolution of the crank—i.e. to prevent unsteadiness during each turn; this it does by storing up energy during parts of the revolution when the effort is greater than the mean resistance, and giving it up again during those parts of the revolution when the effort falls below the resistance. In the gas-engine, where the effort (explosion) is often only applied during part of every second or third revolution, it does very important work. Its action must be clearly separated from that of the governor, whose function is to determine the mean speed or number of turns the engine shall make per minute; this the fly-wheel cannot in any way do—it can only keep the speed steady during each turn.
The second case is illustrated by a punching-machine. The engine need not be of sufficient power to directly force the punch through the metal, but with the aid of a fly-wheel it easily does it. The machine is so arranged that the actual part of each revolution spent in punching is very small; all the rest of the revolution the fly-wheel is storing up energy, nearly all the effort going in this. Then at the proper moment the work stored up is added to the direct work of the engine, and the punch forced through, the speed of the fly-wheel being proportionately reduced.
The principle of the fly-wheel is sometimes applied in other forms than that of a wheel—e.g. in fly-presses for stamping or coining metals, in which two heavy balls are fixed at the ends of a long lever, which is made to swing round with considerable velocity. The accumulated energy is given up at the moment of impact of the die upon the metal, and a force of great intensity called into play to compress the latter. Fly-wheels are not required in locomotives or marine engines (see STEAM-ENGINE).