INTERVIEW SHORT QUESTION ANSWERS-Flywheel

INTERVIEW SHORT QUESTION ANSWERS—Flywheels 

Q. Compare the different types of flywheels.

A.

Comparison of high and low speed flywheels

Sr. No. High speed flywheels Low speed flywheels
1. Speed is 30000 to 60000 RPM Speed is less than 10000 RPM
2. Light weight Heavy weight
3. Easy commissioning and requires less maintenance Tedious commissioning and requires heavy maintenance
4. Easy starting Tedious starting
5. Easy shut down Tedious  shut down

Q. What are advantages of having elliptical section of the flywheel arm?

A.

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?

A.

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.

(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
1. Crushing machines 0.20
2. Electrical machines(direct drive) 0.002
3. Electrical machines 0.003
4. Engine with belt transmission 0.03
5. Pumps 0.05 to 0.03
6. Machine tools 0.03
7. Gear wheel machines 0.02
8. Hammering machines 0.2
9. Automobile (normal speed) 0.1
10. Automobile (Idling) 0.2
11. Paper, textile and weaving machines 0.025
12. Rolling and mining machines 0.025
13. Spinning mill 0.10 to 0.02
14. Punching, shearing and power presses 0.10 to 0.15
15. Flour mill machines 0.02

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