Therefore given the supply torque from the prime mover (say an IC Engine, electric motor etc) and the load torque, we can size the flywheel required to ensure that the fluctuation in speed will remain within the desired limits (as prescribed by k_s ). It must be noted that by taking in the above calculation, we would have overestimated the flywheel size somewhat or in other words, the actual fluctuation in speed will be even smaller than demanded. Thus we see that a flywheel acts similar to a reservoir in a fluid line which tries to maintain pressure fluctuations within limits.

A typical flywheel would appear as shown in Fig. 5.7 wherein more material is kept towards the outer radius so as to get maximum moment of inertia. For designing the cross-section of the spokes etc. it is important to realize that the maximum stress would occur towards the root – at any section the internal resisting force developed is the force required to keep the material from flying away. Thus as we go from tip to the root, more and more material tries to fly away at great speed and therefore greater and greater force is required to be developed internally between the sections towards the root.

Fig. 5.7 Typical flywheel

A few issues are left for you to think further about viz.,

• Is there an upper limit on the inertia of the flywheel – obviously the bigger the wheel, the lesser will be the fluctuations in speed?
  
  
• Typically we have a transmission unit between the prime mover and the load, in the form of a speed reducer gear box. Which side of the gear box will you keep the flywheel and why?