A flywheel is a rotating wheel optimized to store energy and/or resist rotational speed changes. Flywheels are often used to smooth the torque in a piston engine.
The energy stored in the rotating member can be computed by the following formula which assumes the mass of the spokes and hub are negligible.
- Let VelocityFtPerSec be the velocity of the mean radius of the rim.
- Let AccelGravityFtPerSec2 = 32.16
- Let RimLbs be the weight of the flywheel rim
- EnergyFtLbs = RimLbs * (VelocityFtPerSec ^ 2) / (2 * AccelGravityFtPerSec2)
The amount of energy that can safely be stored depends on the point at which the rotor will warp or shatter which in turn depends on the material. Materials with the highest specific tensile strength will yield the highest energy storage.
The theoretical limit on rim velocity, which is independent of radius, when considering only hoop tension stress is:
- VelocityFeetPerSec = SQRT (10 * RimMaterialTensileStressLbsPerSqInch)
One also has to consider the bending stress of the rim between spokes. This stress can be minimized by increasing the number of spokes, by using a disk structure or by increasing the rim’s radial thickness.
Safety factors must also be included to account for a variety of other factors such as material imperfections, impact stresses, etc. For example, a safety factor of 10 to 13 is commonly used for the tensile strength of a cast-iron flywheel.
Designing flywheels for vehicles requires consideration of precession and the resulting resistance to change of the axis of rotation. For example, a flywheel having a horizontal rim would encounter a lateral force when driving over a hill or into a valley.
Also see Flywheel Dynamic Balancing.
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