Learn About Helicopter gearboxes

Many helicopters have multiple engines.

The transmission systems needed for three-engined helicopters are immensely complex. Each of the three engines drive though a series of reduction gears to the third-stage collector gear. The collector gear drives the rotor at 210 rpm through a sun and planet gear. The tail rotor shaft is driven off the collector gear at 3,312 rpm. Accessory gearboxes are also driven off collector gears, however when the rotor is stationary it is possible to drive the accessory gearbox by the APU or from No. 1 engine by pilot selection. Accessory gearboxes drive hydraulic pumps and AC generators. The third hydraulic pump is driven directly off the main gearbox.

The main gearbox lubrication system comprises two independent lubrication circuits, each with its own oil pump filter and cooler.

The EH 101 Merlin main transmission system (GKN Westland Helicopters) is typical.

The EH 101 Merlin main gearbox and engine installation comprises three engines, the APU to the rear of the main gearbox and just above the tail rotor drive shaft. The accessory gearbox is located on the front of the main gearbox and the main rotor drive rises vertically from the main gearbox.

Due to the obvious importance of helicopter transmission systems, a considerable degree of monitoring is in-built to detect failures at an incipient stage. Typical parameters which are monitored are pressures and temperatures, bearing temperatures, wear, and in some cases accelerations. The role of the health and usage monitoring systems HUMS on board helicopters is assuming paramount importance.

Aircraft gearboxes

Gearboxes for many other applications are also commonplace on aircraft generally, for example wing flaps, landing gear gearboxes, etc.

As well as being overhauled and/or repaired all associated drive shafts have to be dynamically balanced.

Semi-independent flap and slat systems raise and lower using hydraulic motors, drive units, torque tubes, and screw jack-type actuators. Pressure from the combined hydraulic system powers the flap drive motor and gearbox assembly.

If the combined hydraulic system fails, a hydraulic brake locks the hydraulic motor, and an emergency   electric   motor   provides   continued operation. Flap drive gearboxes, through a series of torque tubes and offset gearboxes, drive all eight flap actuators. The flap actuators drive the carriage and attaching flaps out and down.