Hi,
Fly wheel is a heavy wheel coupled to a shaft, so that it maintains a constant speed at the shaft even if there are variations in the input energy due to its high inertia. In other words, the mechanical energy stored in the flywheel is used to even out the fluctuations of speed that would have otherwise come on the shaft in its absence. In a mechanical circuit, it works like a filter capacitor/inductor used in the electronic power supply circuits to filter out ripples.
Regards, Laktronics