Accessory drives

Accessory drives:



INTRODUCTION
1. Accessory units provide the power for aircraft hydraulic, pneumatic and electrical systems in  addition to providing various pumps and control systems for efficient engine operation. The high level  of dependence upon these units requires an extremely reliable drive system.taken from a rotating engine shaft, via an internal  mount for the accessories and distributes the starter may also be fitted to provide an input torque   by-pass engine takes between 400 and 500
2. The drive for the accessory units is typically gearbox, to an external gearbox which provides a appropriate geared drive to each accessory unit. A to the engine. An accessory drive system on a high  horsepower from the engine.

Fig. 7-1 Mechanical arrangement of accessory drives.

Gear train drive ,Gear train drive ,External gearbox,Auxiliary gearbox

Gear train drive :
13. When space permits, the drive may be taken to the external gearbox via a gear train (fig. 7-1). This involves the use of spur gears, sometimes incorpo-rating a centrifugal breather (Part 8). However, it is rare to find this type of drive system in current use.
Intermediate gearbox
14. Intermediate gearboxes are employed when it is not possible to directly align the radial driveshaft with
the external gearbox. To overcome this problem an intermediate gearbox is mounted on the high pressure compressor case and re-directs the drive, through bevel gears, to the external gearbox. An example of this layout is shown in fig. 7-1.

Gearbox sealing,Materials,An external gearbox with auxiliary gearbox drive

Gearbox sealing

Fig. 7-5 An external gearbox with auxiliary gearbox drive.

26. Sealing of the accessory drive system is primarily concerned with preventing oil loss. The
internal gearbox has labyrinth seals where the static casing mates with the rotating compressor shaft. For
some o! the accessories mounted on the external gearbox, an air blown pressurized labyrinth seal is employed. This prevents oil from the gearbox entering the accessory unit and also prevents con-tamination of the gearbox, and hence engine, in the event of an accessory failure. The use of an air blown
seal results in a gearbox pressure of about 3 lbs. per sq. in. above atmospheric pressure. To supplement a
labyrinth seal, an 'oil thrower ring' may be used. This involves the leakage oil running down the driving
shaft and being flung outwards by a flange on the rotating shaft. The oil is then collected and returned
to the gearbox.