Wednesday, July 11, 2012

Accessory Gearbox

The rear section of piston engine is usually is cast construction, and the material may be either aluminium alloy, which is used most widely, or magnesium, which has been used to some extent.  On some engines, it is cast in one piece and provided with means for mounting the accessories, such as magnetos, carburettors, and fuel, oil, and vacuum pumps, and starter, generator, etc., in the various locations required to facilitate accessibility.  Other adaptations cast magnesium cover plate on which the accessory mounts are arranged.

Accessory drive shafts are mounted in suitable bronze bushings located in the diffuser and rear sections.  These shafts extend into the rear section and are fitted with suitable gears from which power takeoffs or drive arrangements are carried out to the accessory mounting pads.  In this manner the various gear ratios can be arranged to give the proper drive speed to magneto, pump, and other accessories to obtain correct timing or functioning.
In some cases there is a duplication of drives, such as the tachometer drive, to connect instruments located at separate stations.
The accessory section provides a mounting place for the carburettor, or master control, fuel injection pumps, engine-driven fuel pump, tachometer generator, synchronising generator for the engine analyser, oil filter, and oil pressure relief valve.

Gear trains, containing both spur- and bevel-type gears, are used in the different types of engines for driving engine components and accessories.  Spur-type gears are generally used to drive the heavier loaded accessories or those requiring the least play or backlash in the gear train.  Bevel gears permit angular location of short stub shafts leading to the various accessory mounting pads.