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