Description

Four-Wheel Drive Systems

OPERATION
The power transfer unit is driven directly from the transmission differential via a hollow shaft, through which the RH (right-hand) half shaft passes. The LH (left-hand) half shaft is driven directly from the transmission differential. The unit transfers drive from transmission differential to the drive shaft and the rear differential. No drive from the power transfer unit is passed to the front half shafts.
Drive from the transmission differential drives the crown wheel drive gear, which in turn rotates the pinion gear. Drive is passed from the pinion gear to the drive shaft and to the rear differential via the drive flange attached to the pinion gear shaft.

DESCRIPTION









Power Transfer Unit (Petrol version shown)









The crown wheel drive gear is mounted longitudinally across the unit. The gear is a low-offset hypoid bevel gear configured for minimal power loss across the speed range. The LH (left-hand) end of the gear has splines which mate with corresponding splines in the transmission differential output sleeve. The drive gear is hollow which allows for the fitment of the RH (right-hand) half shaft. The half shaft is located through the hollow drive gear and mates with splines in the transmission differential. The half shaft is driven by the transmission differential and receives no drive from the power transfer unit.
The crown wheel is supported in the casing on opposing taper roller bearings which are pressed into the casing and sealed with an O ring seal. The housing is fitted with a triple edge oil seal on the outer end of the housing which directs dirt and moisture away from the area of the drive gear. Another seal is fitted which prevents the ingress of dirt and moisture between the drive gear and RH (right-hand) half shaft.
The pinion gear is locked in the housing at 90 degrees to the crown wheel drive gear. The pinion gear is supported in the casing on opposing taper roller bearings. The pinion gear is retained in the housing with a pre-load nut. The outer end of the pinion gear is splined and mates with the drive shaft output flange. The flange is secured with a flanged bolt which is screwed into the pinion gear.
A collapsible spacer is located between the outer taper roller bearing and a shoulder on the pinion gear. The collapsible spacer holds the bearing in alignment and also collapses under pressure applied the pre-load nut. This allows the pre-load nut to be tightened to a predetermined torque, which collapses the spacer, setting the correct bearing pre-load and therefore the correct meshing of the pinion gear and the crown wheel gear teeth.