External & Auxiliary gearbox

External Gearbox

The external gearbox contains the drives for the accessories, the drive from the starter and provides a mounting face for each accessory unit. Provision is also made for hand turning the engine, via the gearbox, for maintenance purposes. Fig. 7-4 shows the accessory units that are typically found on an external gearbox.

The overall layout of an external gearbox is dictated by a number of factors. To reduce drag whilst the aircraft is flying it is important to present a low frontal area to the airflow. Therefore the gearbox is 'wrapped' around the engine and may look, from the front, similar to a banana in shape. For maintenance purposes the gearbox is generally located on the underside of the engine to allow ground crew to gain access. However, helicopter installation design usually requires the gearbox to be located on the top of the engine for ease of access.

The starter/driven gearshaft (fig. 7-4) roughly divides the external gearbox into two sections. One section provides the drive for the accessories which require low power whilst the other drives the high power accessories. This allows the small and large gears to be grouped together independently and is an efficient method of distributing the drive for the minimum weight.

If any accessory unit fails, and is prevented from rotating, it could cause further failure in the external gearbox by shearing the teeth of the gear train. To prevent secondary failure occurring a weak section is machined into the driveshafts, known as a 'shear-neck', which is designed to fail and thus protect the other drives. This feature is not included for primary engine accessory units, such as the oil pumps, because these units are vital to the running of the engine and any failure would necessitate immediate shutdown of the engine.

Since the starter provides the highest torque that the drive system encounters, it is the basis of design. The starter is usually positioned to give the shortest drive line to the engine core. This eliminates the necessity of strengthening the entire gear train which would increase the gearbox weight. However, when an auxiliary gearbox is fitted (para, 21) the starter is moved along the gear train to allow the heavily loaded auxiliary gearbox drive to pass through the external gearbox. This requires the spur gears between the starter and starter/driven gearshaft to have a larger face width to carry the load applied by the starter (fig. 7-5).

When a drive is taken from two compressor shafts, as discussed in para. 7, two separate gearboxes are required. These are mounted either side of the compressor case and are generally known as the 'low speed' and 'high speed' external gearboxes.

Auxiliary gearbox

An auxiliary gearbox is a convenient method of providing additional accessory drives when the configuration of an engine and airframe does not allow enough space to mount all of the accessory units on a single external gearbox.

A drive is taken from the external gearbox (fig. 7-5) to power the auxiliary gearbox which distributes the appropriate gear ratio drive to the accessories in the same manner as the external gearbox.