flywheel

flywheel

Flywheel A flywheel is an inertial energy-storage device. It absorbs mechanical energy and serves as a reservoir, storing energy during the period when the supply of energy is more than the requirement and releases it during the period when the requirement of energy is more than the supply. Flywheels-Function need and Operation The main function of a fly wheel is to smoothen out variations in the speed of a shaft caused by torque fluctuations. If the source of the driving torque or load torque is fluctuating in nature, then a flywheel is usually called for. Many machines have load patterns that cause the torque time function to vary over the cycle. Internal combustion engines with one or two cylinders are a typical example. Piston compressors, punch presses, rock crushers etc. are the other systems that have fly wheel. Flywheel absorbs mechanical energy by increasing its angular velocity and delivers the stored energy by decreasing its velocity
Design Approach There are two stages to the design of a flywheel. First, 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. Design Parameters Flywheel inertia (size) needed directly depends upon the acceptable changes in the speed. Speed fluctuation The change in the shaft speed during a cycle is called the speed fluctuation and is equal to ωmax- ωmin Fl max min = ω − ω We can normalize this to a dimensionless ratio by dividing it by the average or nominal shaft speed (ωave) . max min Cf ω − ω = ω Where ωavg is nominal angular velocity Co-efficient of speed fluctuation The above ratio is termed as coefficient of speed fluctuation Cf and it is defined as max min Cf ω − ω = ω Machine Design II Prof. K.Gopinath & Prof. M.M.Mayuram Indian Institute of Technology Madras Where ω is nominal angular velocity, and ωave the average or mean shaft speed desired. This coefficient is a design parameter to be chosen by the designer. The smaller this chosen value, the larger the flywheel have to be and more the cost and weight to be added to the system. However the smaller this value more smoother the operation of the device It is typically set to a value between 0.01 to 0.05 for precision machinery and as high as 0.20 for applications like crusher hammering machinery. Design Equation The kinetic energy Ek in a rotating system = ( ) 1 2 I 2 ω Hence the change in kinetic energy of a system can be given as, 1 2 2 E I K m max min 2 ⎛ ⎞ = ω⎜ ⎟ −ω ⎝ ⎠ E E K 2 = − E1 ( max m ) in avg 2 ω + ω ω = ( )( ) 1 E I 2 C K s avg f avg 2 2 E E C I 2 1 f Ek I s 2 Cf avg = ω ω − = ω = ω Thus the mass moment of inertia Im needed in the entire rotating system in order to obtain selected coefficient of speed fluctuation is determined using the relation