Land Rover Discovery: Power Transfer Unit (PTU)

1. Breather and tube
2. Drive flange
3. Hydraulic pressure line from AWD valve block (P4)
4. Hydraulic pressure line from AWD valve block (P3)
5. Input shaft
6. Left cover
7. Casing
8. Engine mounting bracket
9. Right cover
10. Crown wheel shaft
11. Level plug
12. Drain plug
13. Hydraulic connection (P3)
14. Hydraulic connection (P4)

Power Transfer Unit (PTU) Sectional View

1. Left cover
2. Input shaft
3. Vee seal
4. Oil seal
5. Bearing
6. Synchroniser sleeve
7. Synchroniser hub
8. Taper roller bearing
9. Casing
10. Crown wheel drive gear
11. Right cover
12. Taper roller bearing
13. Triple lip oil seal
14. Crown wheel shaft
15. Pinion gear
16. Taper roller bearing
17. Collapsible spacer
18. Taper roller bearing
19. Drive flange
20. Pre-load nut
21. Oil seal
22. Pinion shaft housing
23. Synchroniser selector fork
24. Needle roller bearing
25. Oil seal
26. Circlip

The PTU compromises a main casing, left and right covers and a pinion shaft housing. Within the casing the main components are a crown wheel drive gear and shaft, a pinion drive gear and shaft, an input shaft, a triple cone synchroniser and a hydraulic piston.

The PTU on the Active Driveline is similar in construction to the PTU used on vehicles without active driveline. The active driveline PTU has a hydraulically operated synchroniser which allows the drive from the transmission differential to be fully disconnected from the crown wheel drive shaft, allowing the vehicle to be operated in FWD only when conditions allow. This reduces frictional losses improving performance and economy.

The input shaft is connected to the transmission differential right side output shaft. Splines on the input shaft mate with splines on the transmission differential output shaft. The input shaft is supported on a ball bearing in the left cover and a needle roller bearing between the input shaft and the crown wheel shaft. The input shaft contains the engagement ring which is part of the triple cone synchroniser assembly. Weicon Anti-Seize grease is used on the input spline between transmission and the PTU to lubricate the splines.

Power Transfer Unit Internal Components

1. Vee seal
2. Oil seal
3. Ball bearing
4. Input shaft engagement ring
5. Synchroniser sleeve
6. Crown wheel drive gear
7. Pinion shaft drive gear
8. Pinion shaft
9. O ring seal
10. Piston
11. Circlip
12. Detent
13. Synchroniser selector fork
14. Input shaft

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 left end of the crown wheel shaft has splines which mate with corresponding splines in the synchroniser sleeve. The crown wheel shaft is hollow which allows for the fitment of the right halfshaft. The halfshaft is located through the crown wheel shaft and the input shaft and mates with splines in the transmission differential. The halfshaft is driven by the transmission differential and receives no drive from the PTU.

The crown wheel shaft is supported in the casing on opposing taper roller bearings which are pressed into the casing and sealed with an O ring seal.

The casing is fitted with a triple edge oil seal on the outer end of the right casing 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 right halfshaft.

The pinion drive gear is locked in the casing at 90 degrees to the crown wheel drive gear. The pinion drive gear is supported in the casing on opposing taper roller bearings. The pinion gear is retained in the casing with a pre-load nut. The outer end of the pinion gear is splined and mates with the drive flange. The drive flange is secured on the pinion gear by the pre-load nut.

The pinion drive gear is locked in the casing at 90 degrees to the crown wheel drive gear. The pinion drive gear is supported in the casing on opposing taper roller bearings. The pinion gear is retained in the casing with a pre-load nut. The outer end of the pinion gear is splined and mates with the drive flange. The drive flange is secured on the pinion gear by the pre-load nut.

A collapsible spacer is located between the outer taper roller bearing and a shoulder on the pinion shaft. The collapsible spacer holds the taper roller bearing in alignment and also collapses under pressure applied by the preload nut. This allows the pre-load nut to be tightened to a predetermined torque, which collapses the spacer, setting the correct taper roller bearing pre-load and therefore the correct meshing of the teeth of the pinion drive gear and the crown wheel drive gear.

Triple Cone Synchroniser

1. Hub
2. Inner balk ring
3. Selector sleeve
4. Bearing
5. Crown wheel shaft
6. Crown wheel drive gear
7. Pinion drive gear
8. Pinion shaft
9. Seal
10. Piston
11. Circlip
12. Circlip
13. Detent
14. Selector fork
15. Detent spring and ball (3 off)
16. Detent housing (3 off)
17. Blocker ring
18. Inner cone
19. Outer balk ring
20. Needle bearing
21. Intermediate cone
22. Oil seal
23. Input shaft
24. Ball bearing
25. Circlip

The triple cone synchroniser has three frictional interfaces. The intermediate cone has friction material on its inner and outer surfaces, the inner cone has friction material on its inner surface. These co-axial surfaces are supplemented by the inner and outer balk rings.

The synchronisers work as brakes. They brake or accelerate the components to align the rotational speed of the input shaft and the crown wheel shaft to allow the synchroniser to engage.

The synchronisation process starts with the selector sleeve which is moved by the selector fork towards the engagement ring on the input shaft . While there is a speed difference between the selector hub and the engagement ring, the selector sleeve is blocked by the blocker ring. The synchroniser cones create a friction torque which aligns the rotational speed of the crown wheel shaft to speed of the input shaft. When the speeds of both shafts are synchronised the selector sleeve can be moved further and engages into the engagement ring on the input shaft, providing a positive drive to the crown wheel shaft.

The synchroniser is operated by a double acting piston located in a bore in the PTU casing. Hydraulic pressure lines from the RDU valve block supply hydraulic pressure to the piston. The pressure lines are connected to drillings in the PTU casing marked P3 and P4. P3 and P4 correspond to the markings on the AWD valve block. Connection P3 supplies pressure to engage the synchroniser and P4 supplies pressure to disengage the synchroniser.

The piston is sealed in the bore with two seals and retained with a circlip. An extension on the piston locates in the selector fork and is retained with a circlip. A spring loaded detent ball is fitted in the casing. The spring loaded ball positively retains the selector fork in the required position, until hydraulic pressure is applied to move it in the opposite direction.

Four-wheel Drive Systems - Vehicles With- Active Driveline/ Description and OperationRear Drive Unit (RDU)

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