A flywheel is a mechanical device with a significant moment of inertia used as a storage device for rotational 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 an intermittent load, such as a piston pump, is placed on it. Flywheels can be used to produce very high power pulses for 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 and power plants; see flywheel energy storage.
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.
You can use those equations to do 'back of the envelope' calculations and find the rotational energy stored in various flywheels. I = kmr2, and k is from List of moments of inertia
|object||k (varies with shape)||mass||diameter||angular velocity||energy stored, J||energy stored|
|bicycle wheel @ 20 km/h||1||1 kg||700 mm||150 rpm||15 J||4 × 10−3 Wh|
|bicycle wheel, double speed (40 km/h)||1||1 kg||700 mm||300 rpm||60 J||16 × 10−3 Wh|
|bicycle wheel, double mass (20 km/h)||1||2 kg||700 mm||150 rpm||30 J||8 × 10−3 Wh|
|Flintstones concrete car wheel (19 km/h)||1/2||245 kg||500 mm||200 rpm||1.68 kJ||0.47 Wh|
|wheel on train @ 60 km/h||1/2||942 kg||1 m||318 rpm||65 kJ||18 Wh|
|giant dump truck wheel @ 30 km/h (18 mph)||1/2||1000 kg||2 m||79 rpm||17 kJ||4.8 Wh|
|small flywheel battery||1/2||100 kg||600 mm||20000 rpm||9.8 MJ||2.7 kWh|
|regenerative braking flywheel for trains||1/2||3000 kg||500 mm||8000 rpm||33 MJ||9.1 kWh|
|electrical power backup flywheel||1/2||600 kg||500 mm||30000 rpm||92 MJ||26 kWh|
|the planet Earth||2/5||5.97 × 1027 g||12,725 km||~1 per day (696 µrpm[nb 1])||2.6 × 1029 J||72 YWh (× 1024 Wh)|
For a given flywheel design, it can be derived from the above equations that the kinetic energy is proportional to the ratio of the hoop stress to the material density.
This parameter could be called the specific tensile strength. The flywheel material with the highest specific tensile strength will yield the highest energy storage. This is one reason why carbon fiber is a material of interest.
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.
In a 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.
The flywheel as a general mechanical device for equalizing the speed of rotation is, according to the American medievalist Lynn White, 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.
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.
For internal combustion engine applications, the flywheel is a heavy wheel mounted on the crankshaft. Its main function is to maintain a fairly constant angular velocity of the crankshaft.
[[File:|thumb|180px|Simple flywheel in motion. Constructed based on drawings by Leonardo da Vinci]]
A flywheel is a heavy disk or wheel that is attached to a rotating shaft. Flywheels are used for storage of kinetic energy. The momentum of the flywheel causes it to not change its rotational speed easily. Because of this, flywheels help to keep the shaft rotating at the same speed. This helps when the torque applied to the shaft changes often. Uneven torque can change the speed of rotation. Because the flywheel resists changes in speed, it decreases the effects of uneven torque. Engines which use pistons to provide power usually have uneven torque and use flywheels to fix this problem.
It takes energy to get a wheel (any wheel) to rotate. If there is little friction (good bearings) then it will keep rotating a long time. When energy is needed, it can be taken from the wheel again. So it is a simple mechanical means of storing energy. The amount of energy stored is a function of the weight and the speed of rotation - making a heavier wheel rotate faster takes more energy. Another factor is the radius (size) because the farther from the axis a part of the wheel is, the more energy it takes to make is rotate. These three factor can be represented by M (mass), (angular velocity) and R (radius). Combining the two equations below gives 2MR2/4. A fly-wheel is not just any wheel, but specifically designed to store energy. So it should be heavy and/or rotate fast. For example, some buses have a fly-wheel that is used for stopping and starting. When the bus stops (eg for a traffic light), the fly-wheel is connected to the wheels, so the rotational energy is transferred to it, so the bus will slow down while the fly-wheel speeds up. Then, when the bus has to start driving again, it is connected again and the energy is transferred back. Of course, you wouldn't want to lug a heavy wheel around on a bus, so it is made of a lighter material that can withstand extremely fast rotation.
The kinetic energy of a rotating flywheel is
Where the moment of inertia of center mass is equal to
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.