Design of Flywheel

1. Introduction

A flywheel is a rotating mechanical device used to store energy and maintain uniform speed in machines. It absorbs energy when supply exceeds demand and releases it when demand exceeds supply.

Applications:

• IC Engine
• Steam Engine
• Punch Press
• Rolling mills, compressors

2. Functions of a Flywheel

• Reduces fluctuation in speed
• Stores excess energy
• Supplies energy during peak load
• Ensures smooth operation

3. Energy Fluctuation in Machines

In reciprocating machines, torque is not constant → causes speed fluctuation.
Flywheel compensates this by storing/releasing kinetic energy.

4. Turning Moment Diagram

• Graph of torque vs crank angle
• Area between mean torque line and curve gives energy fluctuation

5. Maximum Fluctuation of Energy

$$\Delta E = E_{max} – E_{min}$$

• Represents energy stored/released by flywheel

6. Coefficient of Fluctuation of Energy

$$C_E = \frac{\Delta E}{E_{mean}}$$

7. Coefficient of Fluctuation of Speed

$$C_s = \frac{N_{max} – N_{min}}{N_{mean}}$$Cs​=Nmean​Nmax​−Nmin​​

• Indicates permissible speed variation

8. Energy Stored in Flywheel

$E = \frac{1}{2} I \omega^2$

Where:

• $I$I = mass moment of inertia
• $\omega$ω = angular velocity

Energy Fluctuation Relation

$$\Delta E = I \omega^2 C_s$$

9. Design of Flywheel

(a) Determination of Mass Moment of Inertia

$$I = \frac{\Delta E}{\omega^2 C_s}$$

(b) Rim Design

• Most energy stored in rim
• Assume rim carries ~90% of total energy

(c) Stresses in Flywheel Rim

Centrifugal Stress

$$\sigma_c = \rho v^2$$Where:

• $\rho$ρ = density
• $v$v = peripheral velocity

Bending Stress

• Due to restraint of arms

(d) Design of Arms

• Subjected to bending
• Usually elliptical cross-section

(e) Design of Shaft and Hub

• Shaft designed for torque
• Hub fitted to shaft with key

10. Materials for Flywheel

• Cast Iron (most common)
• Steel (high-speed applications)

Properties required:

• High strength
• Good fatigue resistance
• Good castability

11. Advantages of Flywheel

• Smoothens speed fluctuations
• Improves efficiency
• Reduces vibration

12. Limitations

• Cannot control cyclic speed variations completely
• Adds weight and space
• Initial cost

13. Flywheel vs Governor

Feature	Flywheel	Governor
Function	Controls speed fluctuation	Controls mean speed
Energy	Stores energy	Does not store energy
Application	Cyclic variation	Load variation

14. Applications

• Engines
• Press machines
• Rolling mills
• Power plants

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