A flywheel is a battery made of motion. It stores energy in the act of spinning, then gives it back when the load needs it — carrying an engine through the gaps between power strokes, or buffering a grid against sudden demand. The energy it holds is the rotational kinetic energy, ½ I ω². Two levers set it: the moment of inertia (how the mass is spread about the axis) and the speed, which enters as a square — so spinning faster pays off far more than simply adding weight.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: rigid-body rotational dynamics.
The flywheel equations
Energy comes out in joules when mass is in kilograms, radius in metres and ω in radians per second. The shape factor matters: putting the same mass at the rim instead of spreading it through a disk doubles the inertia and therefore the stored energy. But the strongest lever is speed — because ω is squared, going from 600 to 1200 rpm multiplies the energy by four.
Worked example — a press flywheel
Scenario: A solid steel disk, 50 kg and 0.3 m radius, spinning at 600 rpm.
The wheel stores about 4.44 kJ — enough to drive a press through a punch and recover between strokes. The rim runs at 18.8 m/s, well within steel's safe range. To store four times the energy you would not quadruple the mass; you would double the speed to 1200 rpm, raising the rim speed to 37.7 m/s — still safe, and a far lighter solution.
Frequently Asked Questions
E = ½·I·ω², with ω = 2πN/60. A 50 kg, 0.3 m disk at 600 rpm stores ~4,440 J.
I = ½mr² for a solid disk, I = mr² for a thin rim. A rim wheel stores twice the energy of a disk of equal mass and radius.
Energy goes as ω² but only linearly with inertia. Doubling RPM quadruples energy; doubling mass only doubles it.
Rim (hoop) stress rises with rim-speed squared. Steel tops out ~200 m/s; composites go higher. Energy density follows strength-to-density.
Smoothing uneven torque (engines, presses) and fast energy storage (UPS, grid frequency, regen braking).