What is the purpose of a freewheel fitted to a helicopter?
Refer to figures.
The freewheel unit is an essential part of the transmission or powertrain, functioning as an overrunning clutch. Its primary role is to enable the helicopter blades to continue rotating even if the engine drive shaft comes to a stop.
In other words, it automatically disengages the main rotor from the engine when the engine's rotational speed falls below the equivalent rotational speed of the rotor and prevents the rotor from driving the engine.
These freewheel units are typically filled with oil and receive most of their lubrication from the main rotor gearbox.
Two common designs include:
The sprag clutch. This tends to be used at higher speeds. It has inner and outer races with an annular space between them, supported by ball bearings, but filled with sprags. Theoretically, the inner race is driven by the engine, but on a helicopter, the outer race typically does the work. On one design, the sprags are figure-of-eight shaped lumps of metal (tumblers, with springs) which don’t quite fit into the gap between the sections (they are slightly longer), so they are pushed up, or wedged, by the springs against the outer race, forcing it to move. If the drive is taken away, the outer race overruns past the sprags.
The ramp and roller is used for lower-speed applications. It has ramps on the inner race, up which rollers move until they are stopped by the outer race, forcing it to move. When the engine drive is taken-away, the rollers move down the ramps and allow the outer race to move freely. A snatch engagement of a freewheel unit may shear the engine drive shaft.
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