What Is a Flywheel?

Flywheels have been used since the Industrial Revolution in machines and vehicles requiring the need for potential energy to be stored. If not functioning properly, cars would be unable to move, and manufacturing equipment could suddenly come to a halt. In this article, we will discuss how flywheels work, and under what conditions they may fail.

Flywheels work by converting and storing rotational energy to be used later as mechanical energy. When torque is applied to a flywheel, it begins to rotate and increase its momentum, which is then stored as energy. In order for this process to work, the flywheel will employ gyroscopic motion to the object creating torque. This also means that flywheels are limited in their mass, as too much rotational inertia could make it difficult for the vehicle or machine to maneuver.

Based on their operating principles, flywheels can be used to modulate energy and inertia in several ways. The first and most common use for a flywheel is to release stored energy when required, mostly in situations where the associated machine must exert more energy than it can produce. It can also even out the motion of an attached crankshaft by regulating the torque fluctuation, all the while acting as a counterbalance. Finally, flywheels can be used to reduce lag time in high-power demand situations, such as a car's engine starting from its rest state.

Flywheels are manufactured in a rugged manner due to the high demands placed on them during operation. The wheel's material is chosen based on its specific tensile strength, which is a ratio of kilojoules of energy density in kilograms. For applications not requiring much tensile strength, lead alloys and cast iron are typically the first choices. These materials are much cheaper than other options, but have limited uses. In the 100-300 kJ/kg range, metals such as beryllium, steel, aluminum, and titanium are used, each varying in performance and cost. These metals account for the majority of flywheels in production and use. For higher performance applications, composites such as CFRP and GRFP are chosen. Ceramics can endure more compressive forces, withstanding up to 2000 kJ/kg, but they are brittle and prone to failure when exposed to tensile forces.

Another important choice manufacturers make when designing flywheels is the type of bearings used. Magnetic bearings are optimal for most applications since they are less prone to friction loss and can be placed inside a vacuum to operate more efficiently. Traditional, non-magnetic bearings are also used sparingly in lower energy applications. Regardless of the material, it is important to periodically check the bearings for overheating or failure.

A failed or overheated flywheel will result in a sequelae of obvious symptoms. If in operation, the most noticeable sign will be a burning smell generated from the heat being dissipated because of increased friction. Another telltale sign of a bad flywheel is a dramatic increase in vibration. In a car, this vibration will propagate through the entire vehicle, including the floor and frame. Finally, if a machine with a flywheel suddenly stops working or is unable to change gears in the case of a car, there is likely a failure.

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