GBA20 gearbox- Output shaft



The bearings (28) (30) separated by a spacer (80) rotate the driving gear (29) on the output shaft. The bearings (79) (81) operate as axial stops. A single cone synchroniser allows range shifting between gears (29) (Hare) and (36) (Tortoise). Preloading, or the clearance between the cones (39) (40) and their respective cups (48) (37) is adjusted according to the thickness of the circlip (49).

The output shaft transmits movement from the different ratios to the rear axle transfer shaft. It is fitted on the lower transmission line at the rear of the unit. It is supported at the front by a needle roller bearing fitted in the layshaft bore and at the rear by two opposing tapered roller bearings. The shaft (44) supports:
- the driving gear (29) fitted idle onto the combined bearings (28) (30);
- the idle-fitted gear (36) and ring (35) assembly;
- the Hare/Tortoise synchroniser (31), integral rotation by splines.

To ensure correct operation of the assembly depending on the load applied to the transmission, several adjustments are required:
Shimming J3: via the circlip (49), this shimming cancels out the clearance between the cones (39) (40) and their respective cup.
Shimming J4: the shim(s) (38) positioned between the shoulder of the shaft (44) and the cone (39) provide axial clearance for the gear (29).
Shimming J5: the shim(s) (42) fitted between the cone (40) and the shim (43) cancel out the axial clearance of the cones (39) (40) on the shaft (44). The GBA20 gearbox output shaft without creeper gears may be replaced by two other shafts relating to the following options respectively: creeper unit or super creeper unit.

1

Parts list (Fig.1)
(21) Synchroniser cone (Hare) (22) Synchroniser cone (Tortoise) (24) Needle roller bearing (27) Thrust washer (28) Needle roller bearing (29) 3rd driving gear (Hare) (30) Needle roller bearing (31) Hare / Tortoise single cone synchronizer (32) Thrust washer (35) Ring (36) Tortoise gear
(37) Bearing cup (38) Shim(s) (39) Bearing cone (40) Bearing cone (41) Screw (42) Shim(s) (43) Shim (44) Output shaft (45) Circlip (46) Stop plate (47) 3rd synchroniser cone (48) Bearing cup (49) Circlip
(79) Needle roller bearing (80) Spacer (81) Needle roller bearing

2

Disconnect the tractor between the gearbox and the rear axle.
Remove the selector cover plate.
If necessary, remove the screw.

Disassembling and reassembling the selector rail and the forks

The tractor is disconnected. Consequently, selector rail removal is made easier by this separation which is to be made from the rear side of the gearbox housing. Apart from this point, the procedure remains
unchanged.
If the tractor is fitted with a creeper unit, remove the pin, the counter-nut, the adjustable lock, and the fork with the sleeve. If the tractor is fitted with a super-creeper unit, the fork and its lock are removed as described above, except for the absence of the pin.
Remove the selector rail and the forks.
Clean and check all components. Replace those that are defective.
Install the selector rail and the forks.
Depending on the equipment, refit the sleeve, fork, adjustable lock and pin. To adjust the fork.

Disassembling, reassembling and shimming the shaft

If the tractor is fitted with a creeper unit or super-creeper unit, remove the unit. The bearing cones and cups are to be paired if reused.
Remove the screws (41).
Remove the stop plate (46).
Remove the bearing cup (48).
Remove the circlip (45).
Remove the shim (43) and the shims (42).
Remove the bearing cones (40) (39).
Remove the shims (38) and the cup (37).
Remove the circlip (49).
Pull and remove the shaft (44) backwards while holding the gear assembly.
Through the selector cover plate opening, take out:
- the synchroniser (31) and its rings;
- the Hare / Tortoise synchroniser cones (21) and (22);
- the gear assembly (29) and needle roller bearings (28) (30);
- the cone (47) and the 3rd synchroniser ring;
- the two needle roller bearings (79) (81);
- the thrust washers (27) (32). The synchroniser cones and rings are to be paired if reused.

If necessary, remove the Tortoise gear (36). If work is necessary on this gear, first remove the input unit in order to lift the gearbox intermediate shaft to take out the gear concerned through the selector cover plate opening.
Remove the needle roller bearing (24), while marking the assembly direction.
Check and clean all components. Replace those that are defective.
Position the needle roller bearing (24) in the bore of the layshaft "R", taking care to position the smaller diameter of the flexible ring towards the 3rd driving gear (Hare) (29) (Fig.3).

3

Install the shaft in the housing without the gears. Install a temporary circlip (49), minimum thickness. Position the cup (37).

Preparation for J3 shimming (Fig.4)

4

Slide cones (39) (40) and hold them against the shoulder of shaft (44) by applying pressure by hand. Fill space Y between the rear of cone (40) and the groove of circlip (45) with shims (42) and shim (43) which will be placed against circlip (45). Take a new circlip. The circlip must be held closed in the groove under a slight pressure.
Install the cup (48) and the stop plate (46). Tighten screws (41) to a torque of 25 - 35 Nm. For gearboxes fitted with a creeper unit, use the unit plate to replace the stop plate (46). Tighten the screws to a torque of 33.8 - 51.5 Nm.

J3 shimming (Fig.5)

5

Put the dial gauge feeler pin against the end of the shaft (44).
Push the shaft, while turning it alternately from right to left to correctly "seat" the cones in the cups.
Set the dial gauge to zero.
If the axial clearance is greater than 0.05 mm, set a definitive thickness of the circlip (49), to obtain J3 = -0.05 to +0.05 mm.
Remove the circlip (45).
Remove the shim (43) and the shims (42).
Remove the screws (41) and the stop plate (46).
Remove:
- the cup (48);
- the bearing cones (40) (39), pairing them with their cups;
- the cup (37);
- the temporary circlip (49) fitted;
- the shaft (44).

Assembling the gears and the shaft
On the workbench, pre-assemble the 3rd driving gear (29) with the synchroniser cones (21) and the 3rd gear (47) fitted with their respective synchronizer ring. Fit the bearings (79) (81) and washers (27)(32).
If removed, position the Tortoise gear (36) with its synchroniser ring (22) into the housing. If the ring (35) of the Tortoise gear (36) is damaged, replace the gear assembly (the ring is rebored after fitting).
Install the input unit.
In the housing, continue to fit:
- the 3rd driving gear (29) fitted with the needle roller bearings (28) (30) (recess "R" facing towards the 3rd-4th synchroniser (Fig.6));

6

- the synchroniser (31). The needle roller bearings (28) (30) are force-fitted and are separated by a spacer (80).
Install the output shaft (44) while holding the gear assembly.

J4 shimming (Fig.7)

7

Install the circlip (49) defined. Slide the cones and cups paired.
Install the stop plate (46). Tighten screws (41) to a torque of 25 - 35 Nm. For gearboxes fitted with a creeper unit, use the unit plate to replace the stop plate (46). Tighten the screws to a torque of 33.8 - 51.5 Nm.
Put the dial gauge feeler pin against the end of the shaft (44).
Push the shaft, while turning it alternately from right to left to correctly "seat" the cones in the cups.
Set the dial gauge to zero.
Depending on the value read on the dial gauge, determine the thickness of shims Z (38) to obtain: J4 = 0.20 to 0.40 mm (Fig.7)
Remove the screws (41) and the stop plate (46).
Remove the cup (48).
Remove cones (40) (39).
Slide the required thickness of shims Z (38), and cones (39) (40) onto the shaft.

J5 shimming (Fig.8)

8

Carry out shimming to obtain the clearance J5 = 0.
Using the thickness of shims Y (42) (43), determine the thickness of shims Z (38) to obtain the clearance J5 = Y - Z.
Install the cup (48) and the stop plate (46). Tighten screws (41) to a torque of 25 - 35 Nm.
Slide the thickness of shims.
Position the shim (43) on the circlip groove side. Position the circlip (45): it must be held closed in the groove under slight pressure. Check that it is correctly positioned.
Install the creeper or super creeper gearbox (depending on option).
Manually check the rotation of the shaft and its gears.
Install the forks and the selector rail.
Install the selector cover plate, by first checking for the presence of the screw.

Reconnect the tractor between the gearbox and the rear axle.
Check:
- the tightness of the systems;
- correct operation of electrical circuits.
Carry out a road test of the controls.
Check the tightness of the mating faces (selector cover plate, gearbox on rear axle)