Radial driveshaft

The purpose of a radial driveshaft is to transmit the drive from the internal gearbox to an accessory unit or the external gearbox. It also serves to transmit the high torque from the starter to rotate the high-pressure system for engine starting purposes. The driveshaft may be direct drive or via an intermediate gearbox (para. 14).

To minimize the effect of the driveshaft passing through the compressor duct and disrupting the airflow, it is housed within the compressor support structure. On by-pass engines, the driveshaft is either housed in the outlet guide vanes or in a hollow streamlined radial fairing across the low pressure compressor duct.

To reduce airflow disruption it is desirable to have the smallest driveshaft diameter as possible. The smaller the diameter, the faster the shaft must rotate to provide the same power. However, this raises the internal stress and gives greater dynamic problems which result in vibration. A long radial driveshaft usually requires a roller bearing situated halfway along its length to give smooth running. This allows a rotational speed of approximately 25,000 r.p.m. to be achieved with a shaft diameter of less than 1.5 inch without encountering serious vibration problems.

Direct drive

In some early engines, a radial driveshaft was used to drive each, or in some instances a pair, of accessory units. Although this allowed each accessory unit to be located in any desirable location around the engine and decreased the power transmitted through individual gears, it necessitated a large internal gearbox. Additionally, numerous radial driveshafts had to be incorporated within the design. This led to an excessive amount of time required for disassembly and assembly of the engine for maintenance purposes.

In some instances the direct drive method may be used in conjunction with the external gearbox system when it is impractical to take a drive from a particular area of the engine to the external gearbox. For example, fig. 7-1 shows a turbo-propeller engine which requires accessories specific to the propeller reduction drive, but has the external gearbox located away from this area to receive the drive from the compressor shaft.

Gear train drive

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 incorporating a centrifugal breather (Part 8). However, it is rare to find this type of drive system in current use.

Intermediate gearbox

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.