flywheel

A flywheel is a heavy disk or wheel mounted on a rotating shaft, designed to resist changes in rotational speed. Its mass and moment of inertia store kinetic energy; as the machine demands more power than the source can supply, the flywheel releases this energy to maintain consistent speed. Conversely, when power input exceeds demand, the flywheel absorbs the excess and spins faster. This regulating action is mechanical and passive, requiring no active control.

The size and weight of a flywheel depend directly on how much speed variation the application will tolerate. A punch press, for example, demands a surge of energy during the downstroke; a large, heavy flywheel lets the motor run at nearly constant speed while the flywheel gives up energy as the ram descends, then receives it back as the press returns. An automotive engine's flywheel is lighter because internal combustion delivers power in pulses, and the driver tolerates some speed ripple. Industrial flywheels on compressors, pumps, and shear machines typically weigh from several kilograms to hundreds of kilograms, with diameters ranging from 300 mm to over 1 meter.

Material is almost always cast iron or steel; iron is chosen for its high density and established casting practice, which produces flywheels with uniform mass distribution. The rim carries most of the inertia, while the hub and spokes are optimized for strength under centrifugal stress. Rotational speed is limited by the tensile strength of the material; at high speed, centrifugal force can crack a rim or throw blades. Critical speed calculations account for diameter, material density, and maximum safe tangential velocity, typically 30 to 80 meters per second in industrial equipment.

Failure and maintenance

Flywheels fail through rim cracking, web fracture, or gradual imbalance. A cracked rim can catastrophically fail under load, scattering metal fragments; this is why machines with large flywheels are guarded and inspected regularly. Imbalance develops from wear, corrosion pitting, or attachment of foreign material, causing vibration that damages bearings and seals. Balancing is done by drilling or grinding the rim, usually in two or more planes, to bring the center of mass back into the axis of rotation.

The name derives from the wheel's ability to fly past moments of demand without slowing down, a metaphor from the early industrial era. Today the term is standard across mechanical engineering, though modern energy storage for intermittent loads sometimes uses hydraulic accumulators or electrical systems. Nevertheless, mechanical flywheels remain in widespread use because they are simple, durable, and cost-effective for machines where some speed variation is acceptable and where the variability in load is predictable and cyclical.