flywheel

1. (mechanical engineering) A rotating mass used to maintain the speed of a machine within certain limits while the machine receives or releases energy at a varying rate, or as a form of energy storage.Examples: "They were fitted with flywheels 25 ft. in dia. and weighing 50 tons each. The energy stored in such flywheels is very great, and in this case was of the order of 7,000,000 lb./ft. each."; "As on Nos. 20001-3, the motor and generator armature shafts of the new locomotive each carry a heavy flywheel to provide kinetic energy and help maintain the speed of the motor-generator set during interruptions of supply, as at breaks in the continuity of the conductor rail."