From RC Helicopter Wiki
The main gear is the large gearwheel (sometimes a gear chain of two gearwheels) that transfer power from the motor to the main rotor and tail rotor. Typically, the main gear is connected directly to the main mast via a one-way bearing (sometimes called the freewheeling unit), which allows the main gear to stop rotating while allowing the main rotor to continue rotating during autorotations.
The main gear is a reduction gear, reducing the high engine RPMs down to the much lower rotor RPM. The motor delivers power to the main gear via a pinion on its output shaft, and the ratio of the tooth count on the gear defines the reduction ratio:
- Engine RPM / Rotor RPM = Main Gear Teeth / Pinion Gear Teeth
Particularly for electric motors, it is useful to be able to change the pinion to match the motor to the helicopter.
The coarseness of the teeth on the gear and pinion are defined by its module, and it is essential that you use a pinion of the correct module to avoid damaging the main gear. While most main gears use straight teeth, a few use helical gears, which promise higher efficiency and lower noise, at the price of much smaller pinion choice.
It is necessary for there to be a tiny amount of space between the pinion and the main gear; without this proper gear mesh, the drive train will be noisy and inefficient.
The tail drive gear is often mounted coaxially with the main gear, directly or indirectly to the main mast.
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