TABLE OF CONTENTS Flywheels store energy. Small ones the sort found in children's toys are made of lead. Old steam engines have flywheels; they are made of cast iron. Cars have them too (though you cannot see them) to smooth power-transmission. More recently flywheels have been proposed for power storage and regenerative braking systems for vehicles; a few have been built, some of high-strength steel, some of composites. Lead, cast iron, steel, composites there is a strange diversity here. What is the best choice of material for a flywheel?
An efficient flywheel stores as much energy per unit weight as possible. As the flywheel is spun up, increasing its angular velocity, ?, it stores more energy. The limit is set by failure caused by centrifugal loading: if the centrifugal stress exceeds the tensile strength (or fatigue strength), the flywheel flies apart. One constraint, clearly, is that this should not occur.
The flywheel of a child's toy is not efficient in this sense. Its velocity is limited by the pulling-power of the child, and never remotely approaches the burst velocity. In this case, and for the flywheel of an automobile engine we wish to maximize the energy stored per unit volume at a constant (specified) angular velocity. There is also a constraint on the outer radius, R, of the flywheel so that it will fit into a confined space.
The answer therefore depends on the application. The strategy for optimizing...
