GBA20 gearbox with mechanical reverse shuttle - Shuttle

The mechanical reverse shuttle transmits drive from the Speedshift to the input gear (1) of the gearbox.
The mechanical reverse shuttle is located in the rear housing of the input unit, behind the Speedshift. It comprises:
- two helical gears (forward operation (5) and reverse operation (23)) fitted on the needle roller bearings (6) and (20);
- a double cone synchroniser (18) whose hub (2) is splined to the secondary shaft;
- a secondary shaft (19) fitted on two ball bearings and supported by the two bearings of the reverse shuttle unit;
- a selector rail/synchroniser control fork assembly;
- a double layshaft gear (15) constantly meshed with the reverse gear (23) and the input gear (1) of the gearbox.

Parts list 
(1) Shim(s) (2) Synchroniser hub (3) Sliding coupler (4) Cone (brake) (5) Forward driving gear (6) Needle roller bearing (7) Ring (8) Friction washer (9) Ball bearing (10) Set screw (11) Spacer (12) Friction washers (13) Reverse gear pin (14) Needle roller bearings (15) Double reverse gear (17) Locking device (18) Synchroniser assembly (19) Secondary shaft (20) Needle roller bearing (21) Rings (22) Coupling flanges (23) Reverse driving gear (24) Ball bearing (25) Circlip (26) Housing (35) Friction washer
(39) Cones (brake)

Forward travel kinematics
The movement of the synchroniser sliding coupler (3) to the rear joins the gear (5) with the shaft (19). The gear (1) is splined to the layshaft. The constant meshing of gears (5) and (1) allows drive to be transmitted to the gearbox.

Reverse travel kinematics
The movement of the synchroniser sliding coupler (3) to the front joins the gear (23) firmly with the shaft (19). Drive is transmitted via the gear (23), the teeth of the layshaft gear (15) and the gear (1). Consequently, the drive transmitted to the layshaft of the gearbox is reversed.
If the input unit is removed to replace the complete unit or just the housing (26), it is necessary to shim the intermediate shaft of the gearbox again after removing the input unit (see chapter 5).

Disconnect the tractor between the engine and the gearbox. Drain the gearbox and the rear axle.
Remove the input unit. Remove the locking device, the selector rail and the shuttle fork. Remove the Speedshift.

Disassembling and reassembling the reverse shuttle

Disassembling the layshaft gear (15) for reverse travel
Take out the set screw (10).
Take out the pin (13).
Remove the layshaft gear (15), the washers (12), the needle roller bearings (14) and the spacer (11).

Disassembling the reverse shuttle
Remove the circlip (25).
Remove the shim(s) (1).
Drive the secondary shaft out of the gear/synchronizer assembly.
Take out the gear/synchroniser assembly (18) from the housing. Remove the washer (8), marking its positioning.

Place the gear/synchroniser assembly on a workbench.
- the gear (5)
- the ring (7)
- the needle roller bearing (6)
- the cones (brake) (4) and (39)
- the coupling flanges (22)
- the gear (23)
- the needle roller bearing (20)
If required, extract the ball bearing (24) from the secondary shaft (19). Remove the washer (35), marking its positioning.

Reassembling the reverse shuttle
Clean and check all components. Replace those that are defective.
Check that the ports and channels of the secondary shaft (19), the pin (13) and housing (26) are not blocked.
Lubricate the secondary shaft, the ring (7) and needle roller bearings (6) and (20).
If removed, place the washer (35) on the secondary shaft (19). Fit the ball bearing (24) against the washer (35), using a press and a makeshift sleeve (internal Ø = 47 mm; L = 60 mm). The sleeve is used to push against the internal cage of the ball bearing.
- the gear (5)
- the ring (7)
- the needle roller bearing (6)
- the cones (brake) (4) and (39)
- the coupling flanges (22)
- the gear (23)
- the needle roller bearing (20)

Install the ball bearing (9) in the housing.
Install the gear (5) (23)/synchroniser (18) assembly and the washer (8) in the housing.
Turn the lubricating grooves of the washer (8) towards the gear (5).
Position the washer centrally (8).
Insert the secondary shaft (19) into the gear/synchroniser (18) assembly through the bore in the front of the gearbox.
Turn the shaft from right to left and vice versa to partially engage the splines of the secondary shaft in those of the synchroniser.
With the help of an operator, install the fitted unit on a table press.
Position a makeshift sleeve (internal Ø = 44 mm; L = 32 mm) under the internal ring of the ball bearing (9).
Position the sleeve on the internal ring of the ball bearing (24).
Gradually fit the secondary shaft (19) securely using a press until resistance is felt. Simultaneously check the gears rotate smoothly. It is recommended to use a press and sleeves to fit the secondary shaft under the correct conditions, and to avoid exposing the ball bearings to excessive force when fitting.
Install the shim(s) (1) removed during disassembly, and the circlip.
If it is necessary to shim the secondary shaft (19).
Check the axial clearance and rotation of the gears (5) and (23). Manually check the rotation of the shaft (19).

Reassembling the layshaft gear (15) for reverse travel
Slide the needle roller bearings (14) separated by the spacer (11) into the layshaft gear (15).
Place the assembled layshaft gear in the housing (26). Slide a washer (12) onto each of its faces.
Install the pin (13).
Lightly smear the thread of the set screw (10) with Loctite 242 or equivalent. Tighten to a torque of 28 - 43 Nm.
Manually check the axial clearance and backlash of the reverse layshaft gear.

Shimming the secondary shaft

Secure the unit in a vice fitted with protected jaws.

Manually check for clearance on the secondary shaft (19) in the housing (26). If there is no clearance, temporarily remove one or several shims (1).

Position the dial gauge feeler pin on the end of the shaft (19).
Pull hard on the secondary shaft (19) to correctly position the ball bearing (24) against the circlip (25).
Set the dial gauge needle to zero.
Pushing to bring the ball bearing (9) up against the shoulder of the housing (26).
Depending on the clearance measured, select a new thickness of shim(s) (1) to obtain a clearance of J1 = 0.05 to 0.20 mm. If possible, shim the secondary shaft close to the minimum tolerance value.
Remove the circlip (25).

Position the definitive shim(s) (1) on the bearing (24).
Install the circlip (25). Check it is correctly fitted at the base of the groove.
- the Speedshift
- the shuttle fork, the selector rail and its locking device.
Install the input unit.
Top up the oil level in the housings. Check it using the gauge at the rear of the centre housing.
Reconnect the tractor between the engine and the gearbox.
Carry out a road test on all controls.
Check the tightness of the seals and hydraulic unions.