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A flywheel is a mechanical device which uses the conservation of
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
to store
rotational energy
Rotational energy or angular kinetic energy is kinetic energy due to the rotation of an object and is part of its total kinetic energy. Looking at rotational energy separately around an object's axis of rotation, the following dependence on the ob ...
; a form of kinetic energy proportional to the product of its
moment of inertia and the square of its
rotational speed
Rotational frequency (also known as rotational speed or rate of rotation) of an object Rotation around a fixed axis, rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second ...
. In particular, assuming the flywheel's moment of inertia is constant (i.e., a flywheel with fixed mass and
second moment of area
The second moment of area, or second area moment, or quadratic moment of area and also known as the area moment of inertia, is a geometrical property of an area which reflects how its points are distributed with regard to an arbitrary axis. The ...
revolving about some fixed axis) then the stored (rotational) energy is directly associated with the square of its rotational speed.
Since a flywheel serves to store mechanical energy for later use, it is natural to consider it as a
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
analogue of an
electrical inductor. Once suitably abstracted, this shared principle of energy storage is described in the generalized concept of an
accumulator. As with other types of accumulators, a flywheel inherently smooths sufficiently small deviations in the power output of a system, thereby effectively playing the role of a
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...
with respect to the mechanical velocity (angular, or otherwise) of the system. More precisely, a flywheel's stored energy will donate a surge in power output upon a drop in power input and will conversely absorb any excess power input (system-generated power) in the form of rotational energy.
Common uses of a flywheel include smoothing a power output in
reciprocating engine
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fea ...
s,
energy storage, delivering energy at higher rates than the source, controlling the orientation of a mechanical system using
gyroscope and
reaction wheel, etc. Flywheels are typically made of steel and rotate on conventional bearings; these are generally limited to a maximum revolution rate of a few thousand RPM.
[; "Flywheels move from steam age technology to Formula 1"; Jon Stewart , 1 July 2012, retrieved 2012-07-03] High energy density flywheels can be made of carbon fiber composites and employ
magnetic bearing
A magnetic bearing is a type of bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with v ...
s, enabling them to revolve at speeds up to 60,000 RPM (1 kHz).
[, "Breakthrough in Ricardo Kinergy ‘second generation’ high-speed flywheel technology"; Press release date: 22 August 2011. retrieved 2012-07-03]
Applications
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Flywheels are often used to provide continuous power output in systems where the energy source is not continuous. For example, a flywheel is used to smooth the fast angular velocity fluctuations of the
crankshaft
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
in a reciprocating engine. In this case, a crankshaft flywheel stores energy when torque is exerted on it by a firing
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
and then returns that energy to the piston to compress a fresh charge of air and fuel. Another example is the
friction motor
A friction motor is a simple mechanism to propel toy cars, trucks, trains, action figures and similar toys. The motor consists of a large flywheel which is connected to the drive wheels of the toy via a very low gear ratio, so that the flywheel re ...
which powers devices such as
toy car
A model car, or toy car, is a miniature representation of an automobile. Other miniature motor vehicles, such as trucks, buses, or even ATVs, etc. are often included in this general category. Because many miniature vehicles were originally aime ...
s. In unstressed and inexpensive cases, to save on cost, the bulk of the mass of the flywheel is toward the rim of the wheel. Pushing the mass away from the axis of rotation heightens
rotational inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular acceler ...
for a given total mass.
A flywheel may also be used to supply intermittent pulses of energy at power levels that exceed the abilities of its energy source. This is achieved by accumulating energy in the flywheel over a period of time, at a rate that is compatible with the energy source, and then releasing energy at a much higher rate over a relatively short time when it is needed. For example, flywheels are used in
power hammer
Power hammers are mechanical forging hammers that use an electrical power source or steam to raise the hammer preparatory to striking, and accelerate it onto the work being hammered. They are also called open die power forging hammers. They ha ...
s and
riveting machines
A riveting machine is used to automatically set (squeeze) rivets in order to join materials together. The riveting machine offers greater consistency, productivity, and lower cost when compared to manual riveting.Parker, Dana T. ''Building Victo ...
.
Flywheels can be used to control direction and oppose unwanted motions. Flywheels in this context have a wide range of applications:
gyroscopes for instrumentation,
ship stability
Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity, centers of buoyancy, the ...
, satellite stabilization (
reaction wheel), keeping a toy spin spinning (
friction motor
A friction motor is a simple mechanism to propel toy cars, trucks, trains, action figures and similar toys. The motor consists of a large flywheel which is connected to the drive wheels of the toy via a very low gear ratio, so that the flywheel re ...
), stabilizing magnetically-levitated objects (
Spin-stabilized magnetic levitation
Spin-stabilized magnetic levitation is a phenomenon of magnetic levitation whereby a spinning magnet or array of magnets (typically as a top) is levitated via magnetic forces above another magnet or array of magnets, and stabilised by gyroscopic e ...
).
Flywheels may also be used as an electric compensator, like a
synchronous compensator, that can either produce or sink reactive power but would not affect the real power. The purposes for that application are to improve the power factor of the system or adjust the grid voltage. Typically, the flywheels used in this field are similar in structure and installation as the synchronous motor (but it is called synchronous compensator or synchronous condenser in this context). There are also some other kinds of compensator using flywheels, like the single phase induction machine. But the basic ideas here are the same, the flywheels are controlled to spin exactly at the frequency which you want to compensate. For a synchronous compensator, you also need to keep the voltage of rotor and stator in phase, which is the same as keeping the magnetic field of rotor and the total magnetic field in phase (in the
rotating frame reference).
History
The principle of the flywheel is found in the
Neolithic
The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several parts ...
spindle
Spindle may refer to:
Textiles and manufacturing
* Spindle (textiles), a straight spike to spin fibers into yarn
* Spindle (tool), a rotating axis of a machine tool
Biology
* Common spindle and other species of shrubs and trees in genus ''Euony ...
and the
potter's wheel
In pottery, a potter's wheel is a machine used in the shaping (known as throwing) of clay into round ceramic ware. The wheel may also be used during the process of trimming excess clay from leather-hard dried ware that is stiff but malleable, a ...
, as well as circular sharpening stones in antiquity.
[Lynn White, Jr., "Theophilus Redivivus", ''Technology and Culture'', Vol. 5, No. 2. (Spring, 1964), Review, pp. 224–233 (233)] In early 11th century
Ibn Bassal
Ibn Bassal ( ar, ابن بصال) was an 11th-century Andalusian Arab botanist and agronomist in Toledo and Seville, Spain who wrote about horticulture and arboriculture. He is best known for his book on agronomy, the ''Dīwān al-filāha'' (An ...
pioneered the used on flywheel in noria and saqiya. The use of the flywheel as a general mechanical device to equalize the speed of rotation is, according to the American medievalist
Lynn White
Lynn Townsend White Jr. (April 29, 1907 – March 30, 1987) was an American historian. He was a professor of medieval history at Princeton from 1933 to 1937, and at Stanford from 1937 to 1943. He was president of Mills College, Oakland, from 1943 ...
, recorded in the ''De diversibus artibus'' (On various arts) of the German artisan
Theophilus Presbyter
Theophilus Presbyter (fl. c. 1070–1125) is the pseudonymous author or compiler of a Latin text containing detailed descriptions of various medieval arts, a text commonly known as the ''Schedula diversarum artium'' ("List of various arts") or ''De ...
(ca. 1070–1125) who records applying the device in several of his machines.
In the
Industrial Revolution
The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
,
James Watt contributed to the development of the flywheel in the
steam engine, and his contemporary
James Pickard
James Pickard was an English inventor. He modified the Newcomen engine in a manner that it could deliver a rotary motion. His solution, which he patented in 1780, involved the combined use of a crank and a flywheel.
James Watt's company Boul ...
used a flywheel combined with a
crank to transform reciprocating motion into rotary motion.
Physics
![Volin](https://upload.wikimedia.org/wikipedia/commons/9/9c/Volin.jpg)
A flywheel is a spinning wheel, or disc, or rotor, rotating around its symmetry axis. Energy is stored as
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
, more specifically
rotational energy
Rotational energy or angular kinetic energy is kinetic energy due to the rotation of an object and is part of its total kinetic energy. Looking at rotational energy separately around an object's axis of rotation, the following dependence on the ob ...
, of the
rotor
Rotor may refer to:
Science and technology
Engineering
* Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
*Helicopter rotor, the rotary wing(s) of a rotorcraft ...
, and can be calculated by
. ω is the
angular velocity, and
is the
moment of inertia of the flywheel about its axis of symmetry. The moment of inertia is a measure of resistance to
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
applied on a spinning object (i.e. the higher the moment of inertia, the slower it will accelerate when a given torque is applied). The
moment of inertia can be known by mass (
) and radius (
). For a solid cylinder it is
, for a thin-walled empty cylinder it is approximately
, and for a thick-walled empty cylinder with constant density it is
.
An electric motor-powered flywheel is common in practice. The output power of the electric motor is approximately equal to the output power of the flywheel. It can be calculated by
, where
is the voltage of
rotor
Rotor may refer to:
Science and technology
Engineering
* Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
*Helicopter rotor, the rotary wing(s) of a rotorcraft ...
winding,
is
stator voltage, and
is the angle between two voltages. Increasing amounts of rotation energy can be stored in the flywheel until the rotor shatters. This happens when the
hoop stress
In mechanics, a cylinder stress is a stress (physics), stress distribution with rotational symmetry; that is, which remains unchanged if the stressed object is rotated about some fixed axis.
Cylinder stress patterns include:
* circumferential str ...
within the rotor exceeds the
ultimate tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
of the rotor material.
Tensile stress
In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...
can be calculated by
, where
is the density of the cylinder,
is the radius of the cylinder, and
is the
angular velocity of the cylinder.
Material selection
Flywheels are made from many different materials; the application determines the choice of material. Small flywheels made of lead are found in children's toys. Cast iron flywheels are used in old steam engines. Flywheels used in car engines are made of cast or nodular iron, steel or aluminum. Flywheels made from high-strength steel or composites have been proposed for use in vehicle energy storage and braking systems.
The efficiency of a flywheel is determined by the maximum amount of energy it can store per unit weight. As the flywheel's rotational speed or angular velocity is increased, the stored energy increases; however, the stresses also increase. If the hoop stress surpass the tensile strength of the material, the flywheel will break apart. Thus, the tensile strength limits the amount of energy that a flywheel can store.
In this context, using lead for a flywheel in a child's toy is not efficient; however, the flywheel velocity never approaches its burst velocity because the limit in this case is the pulling-power of the child. In other applications, such as an automobile, the flywheel operates at a specified angular velocity and is constrained by the space it must fit in, so the goal is to maximize the stored energy per unit volume. The material selection therefore depends on the application.
Energy storage
For a given flywheel design, the kinetic energy is proportional to the ratio of the
hoop stress
In mechanics, a cylinder stress is a stress (physics), stress distribution with rotational symmetry; that is, which remains unchanged if the stressed object is rotated about some fixed axis.
Cylinder stress patterns include:
* circumferential str ...
to the material density and to the mass. The
specific tensile strength of a flywheel can be defined as
. The flywheel material with the highest specific tensile strength will yield the highest energy storage per unit mass. This is one reason why
carbon fiber
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
is a material of interest. For a given design the stored energy is proportional to the hoop stress and the volume.
Design
A
rimmed flywheel has a
rim, a hub, and
spokes
A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface.
The term originally referred to portions of a log that had been riven (split l ...
. Calculation of the flywheel's moment of inertia can be more easily analysed by applying various simplifications. One method is to assume the spokes, shaft and hub have zero moments of inertia, and the flywheel's moment of inertia is from the rim alone. Another is to
lump
Lump may refer to:
* "Lump" (song), a 1995 song by The Presidents of the United States of America
* ''Lump'' (compilation album), a 2000 best-of album by The Presidents of the United States of America
* Lump (dog), a dog who inspired Pablo Pica ...
moments of inertia of spokes, hub and shaft may be estimated as a percentage of the flywheel's moment of inertia, with the majority from the rim, so that
. For example, if the moments of inertia of hub, spokes and shaft are deemed negligible, and the rim's thickness is very small compared to its mean radius (
), the radius of rotation of the rim is equal to its mean radius and thus
.
A
shaftless flywheel eliminates the annulus holes, shaft or hub. It has higher energy density than conventional design but requires a specialized magnetic bearing and control system. The specific energy of a flywheel is determined by
, in which
is the shape factor,
the material's tensile strength and
the density. While a typical flywheel has a shape factor of 0.3, the shaftless flywheel has a shape factor close to 0.6, out of a theoretical limit of about 1.
A
superflywheel consists of a solid core (hub) and multiple thin layers of high-strength flexible materials (such as special steels, carbon fiber composites, glass fiber, or graphene) wound around it. Compared to conventional flywheels, superflywheels can store more energy and are safer to operate. In case of failure, a superflywheel does not explode or burst into large shards like a regular flywheel, but instead splits into layers. The separated layers then slow a superflywheel down by sliding against the inner walls of the enclosure, thus preventing any further destruction. Although the exact value of energy density of a superflywheel would depend on the material used, it could theoretically be as high as 1200 Wh (4.4 MJ) per kg of mass for graphene superflywheels. The first superflywheel was patented in 1964 by the Soviet-Russian scientist
Nurbei Guilia.
See also
*
Dual-mass flywheel
A dual-mass flywheel (DMF or DMFW) is a rotating mechanical device that is used to provide continuous energy (rotational energy) in systems where the energy source is not continuous, the same way as a conventional flywheel acts, but damping any ...
*
Flywheel energy storage
Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a c ...
*
Accumulator (energy) An accumulator is an energy storage device: a device which accepts energy, stores energy, and releases energy as needed. Some accumulators accept energy at a low rate (low power) over a long time interval and deliver the energy at a high rate (high ...
*
Diesel rotary uninterruptible power supply
Most forms of uninterruptible power supply (UPS) can be either powered by battery or flywheel energy. These are ready for immediate use at the instant that the mains electricity fails, but the relatively small and finite amount of stored energy ...
*
List of moments of inertia
Moment of inertia, denoted by , measures the extent to which an object resists rotational acceleration about a particular axis, it is the rotational analogue to mass (which determines an object's resistance to ''linear'' acceleration). The momen ...
*
Clutch
*
Fidget Spinner
A fidget spinner is a toy that consists of a ball bearing in the center of a multi-lobed (typically two or three) flat structure made from metal or plastic designed to spin along its axis with pressure. Fidget spinners became trending toys in 2 ...
*
Flywheel training Flywheel training is a type of strength training where the resistance required for muscle activation is generated by the inertia of a flywheel instead of gravity from weights as in traditional weight training.
In contrast to weight training, flywh ...
References
Further reading
*
*
* https://pserc.wisc.edu/documents/general_information/presentations/presentations_by_pserc_university_members/heydt_synchronous_mach_sep03.pdf
External links
*
Flywheel batterieson ''Interesting Thing of the Day''.
Flywheel-based microgrid stabilisation technology. ABB
ABB Ltd. is a Swedish- Swiss multinational corporation headquartered in Zürich, Switzerland. The company was formed in 1988 when Sweden's Allmänna Svenska Elektriska Aktiebolaget (ASEA) and Switzerland's Brown, Boveri & Cie merged to crea ...
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