Short questions with answers on Design of Flywheel

1.      What is flywheel?

A flywheel is a heavy body rotating about its axis. It acts as a reservoir of energy which is stored in the form of kinetic energy. The extra energy is stored during the idle stroke of the driven machinery and released during the working stroke. Thus flywheel controls the fluctuations of speed during each cycle of the driven machinery.

2.      What are the functions of flywheel in a machine?

The primary function of a flywheel is:

a.       To absorb energy when demand of energy id less than the supply

b.      To give out energy when demand of energy is more than the supply.

3.      What types of stresses are set up in the flywheel rims?

a.       Tensile stress due to the centrifugal force

b.      Tensile bending stress due to restraint of the arms

c.       Shrinkage stresses due to the unequal rate of cooling of casting.

4.      What are the various types of flywheel?

a.       Solid disc type

b.      Rimmed type with either arms or solid web

Solid disc type flywheel is rarely used because they have less capacity of storing energy.

Rimmed type flywheels with arms are preferred because they can store more energy. Small rimmed type flywheels are manufactured with solid web or holes drilled in the web.

5.      Why flywheels are used in punching machines?

Use of flywheel in punching machine is due to the following reasons:

a.       It decreases the variation of speed during each cycle of punching machine.

b.      It decreases the fluctuation of speed due to difference in output and input

c.       It stores energy during idle stroke and releases during working stroke.

6.      Why flywheel is used in IC engines?

In IC engine or stem engine the energy is developed during the power stroke, no energy is developed during suction, compression and exhaust strokes in 4 stroke engine. It helps the crank shaft to run at uniform speed by performing its primary function

7.      What is the difference in the function of governor and a flywheel?

Governor regulates the mean speed of an engine when there are variations in load by changing the supply of working fluid. Flywheel does not maintain a constant speed. It reduces the fluctuations.

8.      Coefficient of fluctuation of speed is ------------ of maximum fluctuation of speed and the mean speed

Ratio

9.      Due to centrifugal forces acting on the rim, the flywheel arms will be subjected to ------------- stresses

Tensile

10.  Why flywheel arm are usually elliptical?

This shape helps in more section modulus for the dame weight. This results in more strength than circular section

11.  Under what consideration the shaft for a flywheel is designed?

It is designed under shear stresses produced due to the combined action of torsion and bending moment.

12.   In a flywheel, the major axis of the elliptical section of the arm is the plane of rotation. Write done the reason for this arrangement.

The arms may have to carry the full torque load due to high inertia of the flywheel when the energy input to its shaft is cut off. The arm may be assumed as a cantilever fixed at the hub and carrying the load at the rim end. This bending moment lies in the plane of rotation of the flywheel. Therefore, the major axis of the arms must be parallel to the tangential force F acting on the flywheel.

Bending moment on the arm, M = F(R-d_h/2)

                                    F = t/(nR)

Where n = number of arms

Section modulus of arm, Z = (π/32 )b₁a₁²

Bending stress σ_b = M/Z

13.   On what basis the material of flywheel is selected?

a.       High tensile strength

b.      High fatigue strength

c.       Low shrinkage

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

The flywheel arms are made of elliptical with major axis twice the minor axis. The major axis lies in the plane of rotation and provides double the resistance against bending moment.

15.   Difference between flywheel and governor

The function of a governor in engine is entirely different from that of a flywheel. It regulates the mean speed of an engine when there are variations in the load, e.g. when the load on the engine increases, it becomes necessary to increase the supply of working fluid. On the other hand, when the load decreases, less working fluid is required. The governor automatically controls the supply of working fluid to the engine with the varying load condition and keeps the mean speed within certain limits.

As discussed above, the flywheel does not maintain a constant speed; it simply reduces the fluctuation of speed. In other words, a flywheel controls the speed variations caused by the fluctuation of the engine turning moment during each cycle of operation. It does not control the speed variations caused by the varying load.

16.  Define the following terms

Coefficient of Fluctuation of Speed, coefficient of steadiness, fluctuation of energy, maximum fluctuation of energy, Coefficient of Fluctuation of Energy

Coefficient of fluctuation of speed: The difference between the maximum and minimum speeds during a cycle is called the maximum fluctuation of speed. The ratio of the maximum fluctuation of speed to the mean speed is called coefficient of fluctuation of speed.

Coefficient of steadiness: The reciprocal of coefficient of fluctuation of speed is known as coefficient of steadiness and it is denoted by m.

Fluctuation of energy, maximum fluctuation of energy: 
The fluctuation of energy may be determined by the turning moment diagram for one complete cycle of operation. Consider a turning moment diagram for a single cylinder double acting steam engine as shown in Fig.  The vertical ordinate represents the turning moment and the horizontal ordinate (abscissa) represents the crank angle.

 

A little consideration will show that the turning moment is zero when the crank angle is zero. It rises to a maximum value when crank angle reaches 90º and it is again zero when crank angle is 180º. This is shown by the curve abc in Fig. and it represents the turning moment diagram for outstroke. The curve cde is the turning moment diagram for instroke and is somewhat similar to the curve abc. Since the work done is the product of the turning moment and the angle turned, therefore the area of the turning moment diagram represents the work done per revolution. In actual practice, the engine is assumed to work against the mean resisting torque, as shown by a horizontal line AF. The height of the ordinate aA represents the mean height of the turning moment diagram. Since it is assumed that the work done by the turning moment per revolution is equal to the work done against the mean resisting torque, therefore the area of the rectangle aA Fe is proportional to the work done against the mean resisting torque. We see in Fig.  that the mean resisting torque line AF cuts the turning moment diagram at points B, C, D and E. When the crank moves from ‘a’ to ‘p’ the work done by the engine is equal to the area aBp, whereas the energy required is represented by the area aABp. In other words, the engine has done less work (equal to the area aAB) than the requirement. This amount of energy is taken fromthe flywheel and hence the speed of the flywheel decreases. Now the crank moves from p to q, the work done by the engine is equal to the area pBbCq, whereas the requirement of energy is represented by the area pBCq. Therefore the engine has done more work than the requirement. This excess work (equal to the area BbC) is stored in the flywheel and hence the speed of the flywheel increases while the crank moves from p to q.

Similarly when the crank moves from q to r, more work is taken from the engine than is developed. This loss of work is represented by the area CcD. To supply this loss, the flywheel gives up some of its energy and thus the speed decreases while the crank moves from q to r. As the crank moves from r to s, excess energy is again developed given by the area DdE and the speed again increases. As the piston moves from s to e, again there is a loss of work and the speed decreases. The variations of energy above and below the mean resisting torque line are called fluctuation of energy. The areas BbC, CcD, DdE etc. represent fluctuations of energy. A little consideration will show that the engine has a maximum speed either at q or at s. This is due to the fact that the flywheel absorbs energy while the crank moves from p to q and from r to s. On the other hand, the engine has a minimum speed either at p or at r. The reason is that the flywheel gives out some of its energy when the crank moves from a to p and from q to r. The difference between the maximum and the minimum energies is known as maximum fluctuation of energy 

Coefficient of Fluctuation of Energy 

It is defined as the ratio of the maximum fluctuation of energy to the work done per cycle. It is usually denoted by CE