Section 11
Air Turbine Gearbox- Repair and reconditioning.
DC and AC power supplies are maintained by a Rotol ADE 383 air driven gearbox.
The gearbox is of two-part construction. Comprising
1) Gearbox assembly
2) Air Turbine exducer / nozzle housing assembly.
The oil filled gearbox is driven by the air turbine, providing rotary movement for the DC generator and AC alternator.200v, 400hz
Hot air is directed from the main engine NRV’s (non return valves), through a pre-cooler to the Air turbine assembly duct. Air is directed over a collection of small governor vanes, which in turn directly control the airflow to the main turbine exducer wheel, translating rotary motion at 16,000 rpm, to the main gearbox. The gearbox is both lubricated and cooled by circulatory oil. The main draw back for the ATGB is the air turbine exducer wheel and NVA. The turbine wheel and NVA housings are constructed from mild steel, and as such are not tolerant to moisture ingress, this results in corrosion of the turbine wheel and NVA guides.
Due to the location of the turbine exhaust duct, water ingress is guaranteed, when the aircraft is stored outside, unless the exhaust duct is plugged. This results in the bottom half of the turbine wheel assembly filling up with water and corroding. This has occurred on ZF579. The ATGB therefore requires re-conditioning. 4 spare gearboxes and turbines have been acquired to construct 2 serviceable units.
Minor damage to blades can be repaired, however the whole blade assembly will require re-balancing to inhibit main gearbox shaft and bearing vibration during operation. Approximately 12 governor vanes in the existing ATGB’s will require new bearings and taper pins, the severity of corrosion will be revealed upon inspection of the assembly. In order to gain access to the vanes, the nozzle vain housing will need to be split in to two. To achieve this, the Exhaust housing, Exducer, pre-cooler and front cover of the ATGB will need to be removed, to separate the entire nozzle vein housing from the ATGB. Special extraction tools will be manufactured in order to remove the various components without damage.
Lower left corner of picture: Gold casing is Air Turbine Housing, including Governor nozzle vein housing.
Centre section of picture: Black with secondary turbine on top is, Gearbox and pre-cooler
Top half of picture: Two circular modules attached to gearbox are, Dc generator and AC alternator.
Below: Same ATGB from a different angle
Above: Air turbine within turbine housing, before removal. Corrosion is clear to see, plus minor damage to one blade leading edge.
Below. Taurus Housing removed, to reveal governor vein assembly. Corrosion seen
Turbine, removed with special extraction tool. Coated in phosphorous acid, reaction with corrosion can be seen.
Below: After several hours immersed in Phosphorous acid and two rounds of sandblasting, the air turbine is ready for re-balancing before refitting to main gearbox.
17/12/03
1) Pre- cooler unbolted and removed
2) ATGB front casing removed, along with one main shaft-bearing race. (8 bearings)
3) Pre-cooler bearing housing removed, to allow main shaft removal.
4) Main shaft bearing housing, removed from nozzle vain housing, to allow nozzle vein housing to be separated.
5) Nozzle vein housing separated, revealing 12 veins badly corroded. Bearings to be replaced with 12 good bearings from spare housing, taper pins to be manufactured to original material specification and profiled to fit, oversize may be required.
