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A 'flywheel' is a 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 a fluctuating 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.

Contents

Physics

Applications

History

See also

References

External Links

Physics

Energy is stored in the rotor as kinetic energy, or more specifically, rotational energy:
:
where
:$omega$ is the angular velocity, and
:$I$ is the moment of inertia of the mass about the center of rotation.

★ The moment of inertia for a solid-cylinder is ,

★ for a thin-walled cylinder is $I\; =\; m\; r^2\; ,$,

★ and for a thick-walled cylinder is .
where ''m'' denotes mass, and ''r'' denotes a radius. More information can be found at list of moments of inertia
The amount of energy that can safely be stored in the rotor depends on the point at which the rotor will warp or shatter. The hoop stress on the rotor is a major consideration in the design of a flywheel energy storage system.
:$sigma\_t\; =$
ho r^2 omega^2
where
:$sigma\_t$ is the tensile stress on the rim of the cylinder
:$$
ho is the density of the cylinder
:$r$ is the radius of the cylinder, and
:$omega$ is the angular velocity of the cylinder.

Applications

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 its 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.

History

The principle of the flywheel is already found in the Neolithic spindle and the potter's wheel.^[1] The flywheel as a general mechanical device for equalizing the speed of rotation is, according to the American medievalist Lynn White, first recorded in the ''De diversibus artibus'' (On various arts) of the German artisan Theophilus Presbyter (ca. 1070-1125) who records applying the device in several of his machines. ^[2]

See also

★ List of energy topics

★ Gyroscope

★ Momentum wheel

★ Regenerative braking

★ Flywheel energy storage

★ Inductor

References

1. Lynn White, Jr., “Theophilus Redivivus”, ''Technology and Culture'', Vol. 5, No. 2. (Spring, 1964), Review, pp. 224-233 (233)
2. Lynn White, Jr., “Medieval Engineering and the Sociology of Knowledge”, ''The Pacific Historical Review'', Vol. 44, No. 1. (Feb., 1975), pp. 1-21 (6)

External Links

★ Flywheel highlight: Hypervideo showing construction and operation of four cylinder internal combustion engine (courtesy of Ford Motor Company)