FLYWHEEL. The heavy rim of a flywheel stores up energy and is necessary for two main purposes : (1) to prevent irregular running of an engine, (2) to give off a reserve of force with which to overcome sudden resistance. Steam, gas, oil and petrol engines, air-compressors, pumps and some water turbines are concerned with the first-named requirement, and presses, shearing and punching and slotting machines, forging machines, crushers and rolling mills with the second. In engines the pressures transmitted to the crankpins vary, and the flywheel absorbs excess energy, and gives it out again when the pressure is insufficient to meet demands. In explosion types of engines the cyclic variations are very marked, and heavy wheels are necessary to ensure steady running. For single-cylinder internal-combustion engines a specially heavy flywheel is used when driving an electric gen erator.
The mass of a large flywheel constitutes a potential danger, for if the velocity of the rim exceeds the safe tensile strength of the cast-iron, the wheel will burst and scatter death and destruction to a considerable distance. The governing device must therefore be infallible, to prevent over-speeding, or "run ning away." Safety in manufacture is often sought by casting the arms curved instead of straight; as the rim and boss cool in the mould the arms are thus able to accommodate themselves to any pull, nor are they put into a state of stress when all is cold. Large wheels are built up in pieces, for safety and convenience of making and transport; often the rim is cast in several pieces, united with links put in hot, in the recesses, and forming a powerful hold by the act of shrinking. Each arm is separate in the biggest wheels, which are of an enormous weight.
What is termed a high-speed flywheel is made of forged steel instead of cast-iron, and goes on a fast-running shaft to give the same result as a much heavier wheel on a slow-speed shaft.
Large wheels are provided with barring gear, consisting either of a lever which is pushed against teeth in the wheel and so starts up the engine, enables it to be moved round for repairs, etc.; or of a sort of hand winch, steam barring engine or electric motor, which serves for large wheels. A flywheel often serves a dual purpose, when its rim takes a driving belt, or when it is grooved for a set of driving ropes. A clutch also may be incor porated, as in automobile practice or in some stamping presses.