Differential of the GPA20 rear axle

The crown wheel and pinion, driven by the gearbox output shaft, drives the rear axle. The helical pinion is supported on either side of its teeth by two tapered roller bearings fitted in opposition. Lubrication of the bearing (30) (37) is ensured by a flow of transmission oil from the PTO top cover plate located at the rear of the tractor. The bearing cones are force fitted to the pinion and the rear cup, smeared with Loctite 603, is also force fitted in the rear axle housing. The front cup is free in its bore to allow for shimming of the pinion.
The ring gear is fixed to the differential unit by rivets. The differential assembly turns on two tapered roller bearings supported by two lateral flanges centred by rings and screwed to the rear axle housing. The differential assembly comprises two half-units holding two planet gears for models up to 75 hp and four planet gears for models whose power output exceeds 75 hp. The unit also holds two sun gears. The pinion is fitted in the rear axle housing. The crown wheel and pinion is adjusted using shims fitted between the centre housing and the bearing cup (37). The pre-loading of the tapered roller bearings (31) (33) and (30) (37) is obtained by shim(s) placed between the hand brake plate and the centre housing.
The pre-load shimming of the differential assembly is carried out by deflectors (13) of different thicknesses fitted behind the left-hand bearing cup (14). If needed, an additional shim (49) can be fitted between flange (9) and the deflector.

Differential lock
The left-hand flange contains the differential lock mechanism. The system consists of a piston and a mobile dog clutch splined with the left-hand trumpet housing input sun gear. The mobile dog clutch is moved by the piston which is supplied by the 17 bar hydraulic system via a solenoid valve fitted on the right-hand cover plate. The piston moves and pushes against the mobile dog clutch, compressing the spring. The teeth of the mobile dog clutch engage with a fixed dog clutch that is attached to the differential unit. In this position, the right and left-hand trumpet housing input sun gears turn at the same speed. When the pressure is released, the mobile dog clutch is driven back by the spring.

Parts list
(1) "O" ring (2) Stud (3) "O" ring (4) Circlip (5) Piston (6) Spring (7) Mobile dog clutch (8) "O" ring (9) Left-hand flange (10) Differential lock hydraulic assembly (11) Friction washer (12) Finger (13) Deflector (14) Bearing cup (15) Screw (16) Fixed dog clutch (17) Bearing cone (18) Washer
(19) Sun gear (20) Planet gears (21) Washer (22) Spider (23) Ring gear (24) Rivets (25) Right-hand flange (26) Screw (27) Bearing cup (28) Bearing cone (29) Unit (30) Bearing cone (31) Bearing cone (32) Screw
(33) Bearing cup (34) Pinion (35) Thrust plate (36) Shim(s) (37) Bearing cup (38) Shim(s) (39) Union (40) Differential lock supply pipe (41) Lubrication plug (42) Clip nut (differential support) (44) Washer
(45) Locating ring (differential support) (46) Stud (47) Pipe (48) Union (49) Shim(s) (50) Shim(s) (51) Nut (securing the lubrication pipe) (52) Lubricating union (53) Spool valve tank return pipe (54) Clip nut (55) Clip nut (differential support) (56) Screw

Removing the left-hand flange and differential lock assembly

It is possible to carry out maintenance on the differential lock hydraulic assembly (10) by removing only the left-hand trumpet housing.

Remove the left-hand trumpet housing.
Disconnect the supply pipe (2), unscrew the union (39) and take out the pipe (40). If access to the union (39) is difficult on tractors fitted with 4-speed economy PTO, remove the control (1)Remove the brake piston.

Special points
Purpose of the studs (2) and (46).
- Stud (2):
• this helps to attach the left-hand flange (9).
• it holds the lubrication pipe (47) for the pinion (34).

To reach the nut (3), remove the auxiliary spool valve support and work through the resulting opening. If this operation has to be carried out on a tractor fitted with 4-speed economy PTO, it is necessary to also remove the hitch hook support and the PTO top cover plate located at the rear of the tractor, the double gear and the control fork.
- Stud (46):
• this attaches the right-hand flange (25).
• it supports the return pipe (53) of the hydraulic spool valves.
The two studs (2) and (46) are fitted in the housing with a locking product (Loctite 242 or equivalent). The nuts on either side are tightened like the other screws holding the flanges, to a torque of 85 - 100 Nm.

Remove the flange (9), the locating ring (45), and if necessary, the friction washer (11), the mobile dog clutch (7) and the spring (6).

Disassembling and reassembling the differential lock assembly

Take out cup (14), deflector (13) and shim (49) (if fitted). Mark the deflector fitting direction and the location of the shim (49). Remove the circlip (4).

Take out the piston (5) using a jet of compressed air.
Remove the "O" rings (3) (8) (1). Unscrew the finger (12) (if necessary).

Check the components and replace those that are defective.
Clean finger (12), smear it with Loctite 542, then Install it and tighten it on flange (9).
Smear the "O" rings (3) (8) with miscible grease ("Amber Technical" or equivalent), and fit them correctly in the bottom of their respective grooves.

Using a plastic hammer, insert the piston (5) into the flange (9) while respecting the location of the finger (12).
Install the circlip (4), the shim (49) (if necessary), and the deflector (13), respecting its fitting direction, and the cup (14). The shim (49) (if used) must be fitted between the flange (9) and the deflector (13).

Reinstall the left-hand flange and the differential lock assembly

Install the "O" ring (1).
Assemble the washer (11), the mobile dog clutch (7) and the spring (6).
Reinstall the assembly in the left-hand flange (9).

Flange (9) fitting method. 

Screw two guide studs "G" (L = 60 mm approx.) onto the centre housing. Check for the presence of the ring (45)

Position the flange assembly (9) turning it so that the oil passage is facing downwards. Install the washer (44). Tighten alternately and uniformly the screw (56) and nut (42) to a torque of 85 - 100 Nm.
Install the pipe (40) and the union (39).

Sealing test
If maintenance has been carried out on the piston (5), seals (3) (8) or flange (9), it is necessary to check the sealing of the assembly.
Install a pressure gauge that has been previously tested for tightness to the supply union (39).

Supply the system with compressed air at approximately 5 bar to correctly place the piston and the "O" rings in the left-hand flange (9). Reduce the pressure to 0.3 bar and carry out the sealing test.
Close the valve. For approximately one minute, no drop in pressure should be observed on the pressure gauge.
Disconnect the pressure gauge and connect the supply pipe (2). If removed, reconnect the 4-speed economy PTO control and set the control.
Reinstall the brake piston and the left-hand trumpet housing.
Start the engine. Check the sealing of the supply pipe (2) and the correct operation of the differential lock.

Removing the differential assembly

Disconnect the tractor between the gearbox and the rear axle (the cab remains attached to the gearbox)Remove the wheels.
Separate the left and right-hand trumpet housings from the centre housing.
Remove the linkage cover plate.

Remove the hitch hook support. Remove the PTO top cover plate, the driving gear and the intermediate shaft. Take off the pipe (40).

- the brake pistons. Carefully fit a sling to the differential assembly using a clamp.
Before working on the screws (56) and the nut (42). Loosen the screws (56) and the nut (42) alternately to gradually release the spring (6).
- the flange (9), the spring (6), the dog clutch (7) and the washer (11).
- the screws (26).
- the flange (25).
Take out the differential assembly from the centre housing.

Disassembling the differential assembly and the ring gear

Disassembling the differential assembly
Place the differential assembly on a workbench.
Extract the cones (17) (28) and cup (27), recover shims (38).
Mark the two half-units (29) with two joining paint lines. Remove the screws (15). Remove the fixed dog clutch (16). Separate the two half-units (29).

Remove the washers (18), the sun gears (19), the planet gears (20), the washers (21) and the spider (22). If the ring gear is replaced, the pinion must also be replaced. These components both bear the same number. They must be fitted as pairs. The ring gear and unit are originally assembled using rivets. During repairs, these rivets are replaced by screws and nuts.

Disassembling the ring gear 
Remove the ring gear from the unit.
Centre punch each rivet (24) (on the toothed side of the ring gear).
Using a 5 mm drill bit, drill the rivets to a depth of 10 mm.
Carry out a second drilling operation with a 12 mm drill to the same depth.
Drive out the rivets using a suitable pin punch.

Removing and disassembling the pinion

- the right and left-hand hydraulic cover plate assembly.

- the circlip (1).
- the gear (2) (if fitted).
- the hand brake mechanism assembly.
- the screws (32), the thrust plate (35) and the shims (36).
- the cup (33) and the pinion complete with cones (30) and (31).
- the lubrication pipe (47). Remove and discard the plug (41), extract the cup (37) and recover the shims (50).

Disassembling the pinion
Extract the cones (30) (31).

Reassembling the ring gear and the differential assembly

Reassembling the ring gear
Check that the ring gear and pinion bear the same number.
Clean the mating faces of the new ring gear (23) and the unit (29) and the nuts and screws referenced in the spare parts catalogue.

Smear the first threads of the screws (1) with Loctite 270 and place them in the ring gear and unit. Tighten and lock the nuts (2) to a torque of 150 - 160 Nm.

Reassembling the differential assembly
Check the components and replace those that are defective.
Reassemble the differential assembly. 
Smear the screws (15) with Loctite 270 then tighten to a torque of 85 - 130 Nm. The clearance "J1" between the planet gears and the sun gears must be between 0.08 and 0.30 mm.
Insert the cones (17) (28) respectively in the bottom of the shoulder of the fixed dog clutch (16) and the right-hand flange (25), using a press and a suitable fixture. Place shim(s) (38) in the half-unit (29) to pre-adjust the backlash and then Install the cup (27).