Tail gearbox (TGB)

Application description

The tail gearbox, mounted at the top of the tail pylon, supports and drives the rotary rudder. It reduces shaft speed (typical gear ratio around 3:1) and changes the direction of drive by 90°. It also enables pitch changes of the tail rotor blades through the flight control system. Lubrication is by means of an oil splash/mist method that is also monitored by a chip detector. In addition to normal chip detector function, it detects overheating of the tail gearbox. The gearbox is splash-lubricated and air-cooled.

SKF products and typical solutions

Shaft systems are typically supported by a cylindrical roller bearing coupled with a pair of angular contact ball bearings, usually preloaded. An actuator is connected to the internal shaft, which then supports a system called the “spider”, regulating the collective blade pitch. A specific bearing is mounted to support this shaft, which is subject to the rotor aerodynamic thrust.

Tail rotor actuator bearing

If a paired angular contact ball bearing set is used, the bearing set is typically preloaded in order to avoid skidding on the unloaded row, since the axial load is dominant in one direction.

Lessons learned / Best practices

Tail gearbox comprises many bearing types, therefore it is recommended to apply the general guidelines given for each product line. Specifically for the tail rotor actuator position, a well consolidated design does not exist. Although the configuration is often represented by a paired angular contact ball bearing arrangement, some bearing parameters (i.e. bearing preload) vary considerably across applications.

For this type of bearing, collaboration with the customer on working conditions has to be even stronger, with special focus on fittings, operating temperatures and lubrication conditions. These three parameters can have a large impact on the preloaded bearing behaviour. Bearing and gearbox performance will more than likely be compromised if the input technical specification data are wrong or not precise enough. For that reason it is very important for SKF to get involved in the qualification tests and incremental bearing investigations, in order to validate the working conditions with the customer.
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