Question answers help in understanding

in a better way. Flywheels stores energy

when in excess during the working stroke.

Flywheel supplies back energy in suction,

compression and exhaust strokes.

Question answers help

 in practical applications with

ease. It also help in making improvements.

 What is a flywheel?
It is heavy wheel which stores excess ROTATIONAL energy during power stroke. It releases energy during exhaust, suction and compression strokes. Time duration of storage and release of excess rotational energy is small.

Q. Compare the different types of flywheels.


Comparison of high and low speed flywheels

Sr. No.

High speed flywheels

Low speed flywheels

Speed is 30000 to 60000 RPM
Speed is less than 10000 RPM
Light weight
Heavy weight
Easy commissioning and requires less maintenance
Tedious commissioning and requires heavy maintenance
Easy starting
Tedious starting

Easy shut down

Tedious  shut down

Q. What are advantages of having elliptical

section of the flywheel arm?

This bending moment lies in the plane of rotation of the flywheel. The arm axis also lies in the plane of rotation. Thus the axis should be as large as possible. In a circular section it is equal to the radius. But in case of an elliptical section, the arm major axis can be as large as twice the minor axis. Flywheel arm with elliptical section having major axis twice the minor axis can resist twice the bending moment. It is not obtainable with any other shape of the arm cross section. Thus only elliptical section for the flywheel arm is selected.

Q. What are main considerations for the selection

of material of a flywheel?

It is based on the type of stresses produced. Due to centrifugal force arm comes in tension. There is a tensile stress due bending of arm which is due to torque. There is a reversal of stress due to reverse motion of the flywheel which causes fatigue. Thus the main considerations in the selection of the material are
  • (i) High tensile strength
  • (ii) High fatigue strength

Q. What is fluctuation of speed? What is the coefficient

of fluctuation? What is coefficient of steadiness? What

is mass moment of inertia of the flywheel?


(a) Fluctuation of speed

The difference between maximum and minimum speed during a cycle is
called the fluctuation of speed. Recall the speed of the engine is not the
same during suction, compression, power and exhaust strokes. This
fluctuation of speed can be reduced by having a large mass moment
of inertia ‘I’ of the flywheel.

(b) Coefficient of fluctuation of speed

It is the ratio of maximum speed to mean speed.
Cs = (Nmax —Nmin)/Nmean
Where N is RPM
The range of Cs for various types of machines is given below:

Sr. No.

Type of machine

Coefficient of fluctuation of speed, Cs

Crushing machines
Electrical machines(direct drive)
Electrical machines
Engine with belt transmission
0.05 to 0.03
Machine tools
Gear wheel machines
Hammering machines
Automobile (normal speed)
Automobile (Idling)
Paper, textile and weaving machines
Rolling and mining machines
Spinning mill
0.10 to 0.02
Punching, shearing and power presses
0.10 to 0.15
Flour mill machines
Less value of the coefficient of fluctuation of speed is desirable.

© Coefficient of steadiness

It is equal to the reciprocal of the coefficient of fluctuation of speed. Its symbol is ‘m’.
Coefficient of steadiness =m = 1/Cs

(d) Mass moment of inertia

I = KE / Cs ωmean2
Where ωmean is the mean angular speed
KE = Kinetic energy
Mass moment of inertia of various types of flywheels is    given below:
(i) Disc type of flywheel
I =(1/2) Mr2 = Mk2
where k = r/ 2
Where r is the radius of the disc and k is the radius of gyration
(ii)  Rim type flywheel
I = Mr2  =Mk2       k = rm =mean radius
Where M is the mass of the flywheel
M is 90 % rim mass and 10 % mass of hub and arms


1. What is the meaning of a  governor in a machine?

Maintains a constant speed under changing power requirements over a longer interval of time.  It  controls the supply of driving fuel. Fuel n case of an engine / the supply of steam in case of a steam turbine,

2. What is hunting of a governor?

Hunting is alternate increase and decrease of speed as per load changes on the engine. If load increases, speed decreases. If the load decreases, speed increases. Hunting means governor is highly sensitive and immediately responds to the increase or decrease of load.

3. What is Sticking-failure of a governor?

When the governor does not respond to load changes on the engine, the sticking failure occurs. It may be due to wear of some parts of the governor.

4. What is the controlling force in a governor?

It is the inward pull exerted on each ball. It is equal and opposite to the outward centrifugal force acting on the governor.

5. How to judge the performance of a governor?

Following four parameters determines the quality of performance of a governor:
(i) Sensitiveness: governor responds immediately to load changes. It is ratio of difference in maximum and minimum equilibrium speeds divided by the mean equilibrium speed.
Say maximum speed =100, minimum speed =50
Then sensitiveness = (100 – 50) / 75 = 0.67.
(ii) Stability:  There is only one radius of the balls of the governor. At this radius, it is in equilibrium for entire range of speeds.
(iii) Governor effort: It is the mean force exerted by the governor on the sleeve. This sleeve controls the supply of fuel to the engine in a given range of speed variation. Normally it is 1 % of the range of speed variation.
(iv) Governor power
It is the work done on the sleeve for a given % change in speed.
Mathematically Power = Effort x displacement of sleeve    Flywheel & governor