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Flywheel

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Spoked flywheel

Image:Leonardo-Flywheel-screenshot.jpg

For the film, see Flywheel (film).

Flywheel is a heavy rotating disk used as a storage device for kinetic energy. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when an uneven torque is exerted on it by its power source such as a piston-based, (reciprocating) engine, or when the load placed on it is intermittent (such as a piston pump). Flywheels can be used to produce very high power pulses as needed for some experiments, where drawing the power from the public network would produce unacceptable spikes. A small motor can accelerate the flywheel between the pulses. Recently, flywheels have become the subject of extensive research as power storage devices for uses in vehicles; see flywheel energy storage. Flywheel drive is common in low-cost toys.

A momentum wheel is a type of flywheel useful in satellite pointing operations, in which the flywheels are used to point the satellite's instruments in the correct directions without the use of thrusters.

The kinetic energy of a rotating flywheel is

<math>E = \frac{1}{2} I \omega^2</math>

where <math>I</math> is the moment of inertia of the mass about the center of rotation and <math>\omega</math> (omega) is the angular velocity in radian units. A flywheel is more effective when its inertia is larger, as when its mass is located farther from the center of rotation either due to a more massive rim or due to a larger diameter. Note the similarity of the above formula to the kinetic energy formula E = mv²/2, where linear velocity v is comparable to the rotational velocity, and the mass is comparable to the rotational inertia.

In application of flywheels in vehicles, the phenomenon of precession has to be considered. A rotating flywheel responds to any momentum that tends to change the direction of it's axis of rotation by a resulting precession rotation. A vehicle with a vertical-axis flywheel would experience a lateral momentum when passing the top of a hill or the bottom of a valley (roll momentum in response to a pitch change). Two counter-rotating flywheels may be needed to eliminate this effect.

The flywheel has been used since ancient times, the most common traditional example being the potter's wheel. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine, and his contemporary James Pickard used a flywheel combined with a crank to transform reciprocating into rotary motion.