6. To minimize unwanted movement between the compressor shaft bevel gear and radial driveshaft
7. To spread the load of driving accessory units, some engines take a second drive from the slower rotating low pressure shaft to a second external gearbox (fig. 7-1). This also has the advantage of locating the accessory units in two groups, thus overcoming the possibility of limited external space on the engine. When this method is used, an attempt is made to group the accessory units specific to the engine onto the high pressure system, since that is the first shaft to rotate, and the aircraft accessory units are driven by the low pressure system.A typical internal gearbox showing how both drives are taken is shown in fig.7-3
Radial driveshaft :
8. 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).
9. 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
10. 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
drive shaft 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.