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Spoked flywheel

Flywheel from stationary engine. Note the castellated rim which was used to rotate the engine to the correct starting position by means of a lever.

A Landini tractor with massive flywheel

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.

Energy is stored in the rotor as kinetic energy, or more specifically, rotational energy:

where

ω 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 ,
• 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.

where

σ_t is the tensile stress on the rim of the cylinder

ρ is the density of the cylinder

r is the radius of the cylinder, and

ω is the angular velocity of the cylinder.

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.

In a more modern application, 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 thruster rockets.

Simple flywheel in motion. Constructed based on drawings by Leonardo da Vinci

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 first decribed in the Kitab al-Filaha of the Andalusian engineer Ibn Bassal (fl. 1038-1075), who applies the device in a chain pump (saqiya) and noria.^[2]

According to the American medievalist Lynn Townsend White, Jr., such a flywheel is also 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.^[1][3]

• List of energy topics
• Gyroscope
• Momentum wheel
• Regenerative braking
• Flywheel energy storage
• Inductor

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

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Flywheels

Video of a simple flywheel in motion Constructed based on drawings by Leonardo da Vinci