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heavy metal wheel attached to a drive shaft, having most of its weight concentrated at the circumference. Such a wheel resists changes in speed and helps steady the rotation of the shaft where a power source such as a piston engine exerts an uneven torque on the shaft or where the load is intermittent, as in piston pumps or punches. By slowly increasing the speed of a flywheel a small motor can store up energy that, if released in a short time, enables the motor to perform a function for which it is ordinarily too small. The flywheel was developed by James Watt in his work on the steam engine.



a massive round part that is mounted on the driveshaft of a machine to reduce irregularities in its rotation during steady motion. A flywheel is a wheel with a heavy rim connected to a hub by straight spokes or a solid disk. It often also performs the function of a pulley or clutch disk.

The steady motion of most machines is characterized by periodic fluctuations of the angular velocity of the driveshaft. This is caused, on the one hand, by design features of the machine (for example, the presence of a crank gear in the kinematic chain) and, on the other, by the periodic change in the relations between the motive forces and the forces of resistance (for example, during the idle and power strokes). By accumulating kinetic energy during acceleration and yielding it during deceleration, a flywheel reduces the nonuniformity of shaft rotation to a level

permissible for normal operation of a machine. In inertial engines the energy accumulated by the flywheel is used to drive the machine (for example, in a gyrobus). Flywheels are usually cast from gray iron or, for velocities above 30-35 m/sec, from steel.


Artobolevskii, I. I. Teoriia mekhanizmov, 2nd ed. Moscow, 1967.


(mechanical engineering)
A rotating element attached to the shaft of a machine for the maintenance of uniform angular velocity and revolutions per minute. Also known as balance wheel.


A rotating mass used to maintain the speed of a machine between given limits while the machine releases or receives energy at a varying rate. A flywheel is an energy storage device. It stores energy as its speed increases, and gives up energy as the speed decreases. The specifications of the machine usually determine the allowable range of speed and the required energy interchange.

The difficulty of casting stress-free spoked flywheels leads the modern designer to use solid web castings or welded structural steel assemblies. For large, slow-turning flywheels on heavyduty diesel engines or large mechanical presses, cast-spoked flywheels of two-piece design are standard (see illustration). See Energy storage

Typical flywheel structuresenlarge picture
Typical flywheel structures


a heavy wheel that stores kinetic energy and smooths the operation of a reciprocating engine by maintaining a constant speed of rotation over the whole cycle
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