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23.7 FLYWHEEL Flywheels store kinetic energy as a mechanical battery and are used to smooth the variations in shaft speed that are caused by loads or power sources that vary in a cyclic fashion. Overall performance of the flywheel depends on a sufficient moment of inertia, matching the power source to the load, and resulting performance requirements. One of the main considerations in flywheel design is balancing. By design, flywheels are devices with large inertia and they must, therefore, be balanced to remove eccentric loading and reduce the loading on bearings and other components. The spinning of a flywheel creates stress at the inner hub connection which can lead to fracture Estimating the failure rate of a flywheel must therefore consider the flywheel velocity. Rotating parts such as a flywheel can be simplified to a rotating ring to determine stress levels. 23.7.1 Flywheel Failure Modes Flywheels develop large stresses at their inter hub connection due to dynamic forces caused by spinning. These stresses can lead to failure. Table 23-7 includes some failure modes to consider when evaluating flywheel reliability. Table 23-7. Typical Failure Modes for a Flywheel FAILURE MODE FAILURE CAUSE FAILURE EFFECT Flywheel loosened from shaft Broken shaft/wheel connection Uncontrolled release of energy Flywheel fracture Centrifugal forces Complete loss of output energy System vibration Unbalanced flywheel Shaft/bearing damage 23.7.2 Flywheel Failure Rate The failure rate of a flywheel depends on a design balance between rotational speed, material density and tensile strength. Flywheel performance depends on an optimum energy-to-mass ratio and the flywheel must therefore spin at the maximum possible speed since kinetic energy increases only linearly with mass but increases as the square of rotational speed. However, a rapidly rotating object is subject to Miscellaneous Parts 23-10 Revision B

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