1. FLYWHEEL
(PSG Design Data Book p: )

2. The principle of flywheel in found before the many centuries ago ---- in the potter’s wheel
A potter’s wheel is a mechanism with a rotating turntable on top where the clay is shaped.
A shaft from the turntable to the pedal has a flywheel attached. As the person operating the potter’s wheel presses down and releases the pedal, the flywheel keeps the turntable moving at a constant speed.
The operator could even stop pedaling and the turntable would keep moving due to the energy stored in the flywheel.

4. Flywheels-Function need and Operation
A flywheel is a mechanical energy storage/delivery device that stores energy in the form of kinetic energy.
If the source of the driving torque or load torque is fluctuating in nature, then a flywheel is usually called for.
The main function of a fly wheel is to smoothen out variations in the speed of a shaft caused by torque fluctuations.
Internal combustion engines with one or two cylinders are a typical example. Piston compressors, punch presses, riveting machine, rock crushers etc. are the other systems that have fly wheel.
In case of steam engines, internal combustion engines, reciprocating compressors and pumps, the energy is developed during one stroke and the engine is to run for the whole cycle on the energy produced during this one ,stroke.
Flywheel absorbs mechanical energy by increasing its angular velocity and delivers the stored energy by decreasing its velocity

5. Governor Vs Flywheel

6. This results in speed variation.
Two following types of cases where reciprocating engine mechanism is used :
-Variable torque is supplied where demand is a constant torque
Ex: An internal combustion engine or a steam engine which is used as a prime mover to drive generators, centrifugal pumps, etc.
(b) -Demand is variable torque whereas constant torque is supplied.
Ex: A punching machine which is driven by a prime mover like electric motor.
In both these cases there is mismatch between the supply and demand.
This results in speed variation.
In case of generators, speed variation results in change in frequency and variation in voltage.
On the other hand, punching machine requires energy at small interval only when punching is done.
To supply such large energy at the time of punching, motor of high power shall be required and also large variation in speed.
To smoothen these variations in torque, flywheel is used which works as a energy storage. This results in usage of low power motor in punching machine.

7. Types of Fly Wheels Disc type : Up to 600 mm diameter - One piece
2. Rim & Arm type : Diameters (600 to 2500 mm)
Hub is connected with 4,6 or 8 arms
3. Split Fly wheel: Larger size diameter above 2500mm
Fly wheel is spitted in 2 ,3 or more no. of components.

8. The turning moment diagram (also known as crank-effort diagram)
It is the graphical representation of the turning moment or crank-effort for various positions of the crank. It is plotted on Cartesian coordinates, in which the turning moment is taken as the ordinate and crank angle as abscissa.
Let, Fp = Piston effort, r = Radius of crank,
n = Ratio of the connecting rod length and radius of crank, and
Ѳ = Angle turned by the crank from inner dead centre.

9. Turning Moment Diagram
for a Single Cylinder Double Acting Steam Engine
The turning moment T is zero, when the crank angle Ѳ is zero. It is maximum when the crank angle is 90 and again zero when the angle is 180.
This is shown by the curve abc 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.

10. 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.
1. When the turning moment is positive (i.e. when the engine torque is
more than the mean resisting torque) as shown between points B and C (or D and E) in Fig. the crankshaft accelerates and the work is done by the
steam.
2. When the turning moment is negative (i.e. when the engine torque is
less than the mean resisting Torque) as shown between points C and D in
Fig., the crankshaft retards and the work is done on the steam.
3. If, T=Torque on crankshaft at any instant & Tmean = Mean resisting torque.
Then accelerating torque on the rotating parts of the engine = T - T mean
4. If ( T - T mean) is positive flywheel accelerates and if (T- Tmean) is
negative, then the flywheel retards.

11. Turning moment diagram for a four stroke internal combustion engine
Only one cycle is powered directly by the combustion of gases in 4-S engine. The movement of the piston in the other three steps is powered by the flywheel. As the piston is pushed down by the combustion, its linear kinetic energy is transferred into rotational kinetic energy in the flywheel. Because the flywheel has such a high moment of inertia, it has a high amount of kinetic energy. This kinetic energy is in turn transferred to the piston in the other three phases in order to move it down, up, down in the cylinder. Were it not for the flywheel, the piston would be pushed down in the cylinder once, and would have no way to return to the top.

12. TURNING MOMENT DIAGRAM TOR TRIPLE CYLINDER COMPOUND STEAM ENGINE
Three cylinders double acting compound steam engine. the diagram for each engine is drawn separately and After summation of all three, the resultant diagram is drawn.
Here generally first cylinder is of high pressure second is of Medium and third is low pressure.
The arrangement of cylinders are done in such a way that there is minimum possible variation in turning moment. That’s why cranks arranger on the angle of 120 degree.

13. A large flywheel is needed to smooth the output from a single cylinder engine as excessive vibration can cause problems with parts resonating and fatiguing and nuts and bolts working loose. Single cylinder low speed diesels need particularly heavy flywheels because they have large heavy pistons and connecting rods which run at low speeds; traditional designs are "open flywheel"
In a 4 cylinder engine about 40% of the energy of the cycle is temporarily stored. However, not all of this energy goes into flywheel.
In a 6 cylinder engine the proportion of the energy which must be absorbed & returned by the moving parts amounts to about 20%.
The greater the No. of cylinders the smaller the flywheel capacity required per unit of piston displacement, because the overlap of power strokes is greater & besides other rotating parts of the engine have greater inertia.

14. Turning Moment Diagram
In a combustion engine the crankshaft would deliver a fluctuating torque to its load.
Inserting a flywheel at the crankshaft reduce the fluctuations.
Flywheel absorb energy when T >Tt
and
deliver back into the system such excess energy when T <Tt .
Figure illustrates the above concepts, also showing that over one cycle of a repeated event the excess (1) energies and the deficient (2) energies are equal.
The greatest crank speed change tends to occur across a single large positive loop, as illustrated in Fig.
Flywheel does not maintain a constant speed, it simply reduces the fluctuation of speed

15. There are two stages to the design of a flywheel.
Design Approach
There are two stages to the design of a flywheel.
The amount of energy required for the desired degree of smoothening must be found and the (mass) moment of inertia needed to absorb that energy determined.
Then flywheel geometry must be defined that caters the required moment of inertia in a reasonably sized package and is safe against failure at the designed speeds of operation.
The moment of inertia is the measure of resistance to torque applied on a spinning object (i.e. the higher the Moment of Inertia, the slower it will spin after being applied a given torque).

16. Energy Stored in a Flywheel
A flywheel is shown in Fig We have already discussed that when a flywheel absorbs energy its speed increases and when it gives up energy its speed decreases.
Let I be the mass moment of inertia of the flywheel.
Neglecting mass moment of inertia of the other rotating parts which is negligible in comparison to mass moment of inertia of the flywheel.

23. Stresses in a Flywheel Rim
A flywheel consists of a rim at which the major portion of the mass or weight of flywheel is concentrated (85-90%), a boss or hub for fixing the flywheel on to the shaft and a number of arms for supporting the rim on the hub.
The following types of stresses are induced in the rim of a flywheel:
Tensile stress due to centrifugal force
2. Tensile bending stress caused by the restraint of the arms

24. [PSG (p.7.120)]

25. w = width of the key

31. (PSG D.B.p-5.16)

32. Unless a flywheel is used, the speed of the crank shaft will be very high when resisting torque is very small and substantial decrease in speed shall take place when punching operation is done.
If flywheel is provided, the excess energy shall be absorbed in the flywheel and it will be available when punching operation is being done where energy is deficient. It will result in reduction of the power of motor required if a suitable flywheel is used.

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