The free wheel units are fitted so that the main rotor can not drive the engine (torque reversal) in the event of main rotor RPM over-running the engine This occurs during flaring the main rotors and when the engines fail or have been throttled back for practice Autorotation during any critical situation.
There are two main types, the Roller and Sprag type.Some types can be manually selected for ground running without causing any rotation of the rotor systems.
This unit consists of an inner drive from the engine on which is mounted a cam ring and an outer drive to the rotor. Circumferential interposed between the two are caged rollers which act as the driving medium.
When the transmission drive from the engine rotates, the rollers ride up the slopes of the cams and are jammed between the transmission drive shaft and the rotor drive forming a positive coupling between them.
Whenever the rotor overruns the engine the rotor outer drive of the free wheel unit is rotating faster than the inner engine drive thus releasing the rollers from their wedging action and making the rotor side of the transmission independent of the engine side.
will rotate without the engine. The same would happen if the engine stopped.