FLYWHEEL

A Flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a

significant moment of inertia and thus resist changes in rotational speed. The amount of energy stored in a

flywheel is proportional to the square of its rotational speed. Energy is transferred to a flywheel by

applying torque to it, thereby increasing its rotational speed, and hence its stored energy. Conversely, a

flywheel releases stored energy by applying torque to a mechanical load, thereby decreasing the flywheel's

rotational speed.

Common uses of a flywheel:

Providing continuous energy when the energy source is discontinuous. For example, flywheels are

used in reciprocating engines because the energy source, torque from the engine, is intermittent.

Delivering energy at rates beyond the ability of a continuous energy source. This is achieved by

collecting energy in the flywheel over time and then releasing the energy quickly, at rates that exceed

the abilities of the energy source.

Controlling the orientation of a mechanical system. In such applications, the angular momentum of a

flywheel is purposely transferred to a load when energy is transferred to or from the flywheel.

Flywheels are often used to provide continuous energy in systems where the energy source is not

continuous. In such cases, the flywheel stores energy when torque is applied by the energy source, and it

releases stored energy when the energy source is not applying torque to it. For example, a flywheel is used

to maintain constant angular velocity of the crankshaft in a reciprocating engine. In this case, the flywheel

—which is mounted on the crankshaft—stores energy when torque is exerted on it by a firing piston, and it

releases energy to its mechanical loads when no piston is exerting torque on it. Other examples of this

are friction motors, which use flywheel energy to power devices such as toy cars.

Energy stored in a flywheel

Rotational Kinetic Energy, E = ½ Iω2

where,

I - moment of inertia of the flywheel (ability of an object to resist changes in its rotational

velocity)

ω - rotational velocity (Rad / sec)

The moment of inertia, I = kMr 2 

where,

M - mass of the flywheel

r - radius of flywheel

k - inertial constant.

k depends on the shape of the rotating object. So for solid disk ; I = Mr 2 /2

Stresses in a flywheel rim

A flywheel consists of a rim at which the major portion of the mass or weight of flywheel is concentrated, a

boss or hub for fixing the flywheel on to shaft and a number of arms for supporting the rim on the hub.

The following stresses are induced in the rim.

Tensile stress due to centrifugal force.

Tensile bending stress caused by the restraint of the arms.

1. Tensile stress due to the centrifugal force.

The tensile stress in the rim due to the centrifugal force, assuming that the rim is unstrained by the

arms, is determined in the similar way as the thin cylinder subjected to internal pressure.

ft = ρ.R2.ω2 = ρ.v2 ( v = R.ω )

When ρ is in kg/m3, v is in m/sec, ft will be in N/m2

where ρ = density of the flywheel material

ω = angular speed of the flywheel

R = mean radius of the flywheel

v = linear velocity of the flywheel

2. Tensile bending stress caused by restraint of arms.

The tensile bending stress in the rim due to the restraint of arms is based on the assumption that each

portion of the rim between a pair of arms behaves like a beam fixed at both ends and uniformly loaded,

such that length between fixed ends,

L = π.D/n = 2.π.R / n

where n - number of arms

The max bending moment,

M = w.l2 /12 = b.t.ρ.ω2.R/12(2.π.R/n)

Section modulus, Z = 1/6 (b.t2)

So bending stress fb = M/Z = b.t.ρ.ω2.R/12 (2.π.R/n) *

6 / (b.t2)

Total stress in the rim

f = ft + fb

Stresses in flywheel arms

The following stresses are induced in the arms of the flywheel.

Tensile stresses due to centrifugal force acting on the rim

Bending stress due to the torque transmitted from the rim to the shaft or from the shaft to the rim.

Construction of Flywheel

Flywheels are typically made of steel and rotate on conventional bearings; these are generally

limited to a revolution rate of a few thousand RPM

The flywheel of smaller size( upto 600 mm dia)are casted in one piece. The rim and the hub are

joined together by means of web.

If flywheel is of larger size (upto 2-5 meters diameter), then it is made of arms.

The number of arms depends upon the size of the flywheel and its speed of rotation. But the

flywheels above 2-5 meters are usually casted in two pieces. Such a flywheel is known as “ split

flywheel “.

A split flywheel has the advantage of relieving the shrinkage stresses in the arms due to unequal

rates of cooling of casting.

Modern Flywheel

A flywheel may also be used to supply intermittent pulses of energy at transfer rates that exceed the abilities

of its energy source, or when such pulses would disrupt the energy supply (e.g., public electric network).

This is achieved by accumulating stored energy in the flywheel over a period of time, at a rate that is

compatible with the energy source, and then releasing that energy at a much higher rate over a relatively

short time.

For example, flywheels are used in riveting machines to store energy from the motor and release it during

the riveting operation. The phenomenon of precession has to be considered when using flywheels in

vehicles. 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. This effect is used

in reaction wheels, a type of flywheel employed in satellites in which the flywheel is used to orient the

satellite's instruments without thruster rockets.

Flywheels have also been proposed as a power booster for electric vehicles. Speeds of 100,000 rpm have

been used to achieve very high power densities.Modern high energy flywheels use composite rotors made

with carbon-fibre materials. The rotors have a very high strength-to-density ratio, and rotate at speeds up to

100,000 rpm. in a vacuum chamber to minimize aerodynamic losses.

Benefits in Aerospace

Flywheels are preferred over conventional batteries in many aerospace applications because of the

following benefits:

5 to 10+ times greater specific energy

Lower mass / kW output

Long life. Unaffected by number of charge / discharge cycles

85-95% round trip efficiency

Fewer regulators / controls needed

Greater peak load capability

Reduced maintenance / life cycle costs

Disadvantages

There are safety concerns associated with flywheels due to their high speed rotor and the

possibility of it breaking loose & releasing all of it's energy in an uncontrolled manner.

Its Bulkier, adds more weight to the vehicle

Conclusion

Recent advance in the mechanical properties of composites has regained the interest in

using the inertia of a spinning wheel to store energy.

Carbon-composite flywheel batteries have recently been manufactured and are proving to be viable

in real-world tests on mainstream cars. Additionally, their disposal is more eco-friendly.