Single and Double cone synchronizer GBA20



Single cone synchronisers

On GBA20 gearboxes with mechanical reverse shuttle, the 1st - 2nd and 3rd - 4th gear synchronisers and the Hare / Tortoise synchroniser are single cone type.


Parts list (Fig.1)
(1) Sliding coupler (2) Cone (brake) (3) Coupling flange (4) Ball bearing (5) Pressure elements (6) Spring

Locked position 
When the sliding coupler (1) moves towards the gear to be locked, it presses against the cone (2) which in turn presses against the male cone of the coupling flange (3) by means of the ball bearings (4) and pressure elements (5). When the synchronisation is set, the sliding coupler (1) can mesh and silently lock with the teeth of the coupling flange (3).

Neutral position 
The sliding coupler (1) is in the middle position. The balls (4) are pushed into the V groove of the sliding
coupler by the pressure springs (6). The gears can turn freely on the shaft. In this neutral position, the sliding coupler is locked by three balls held in place by the pressure springs.

Overhaul
If the single cone synchronisers (11), (23) and (31) have been disassembled, check the wear to the cone (2) as follows:
Stack the coupling flange (3) and the cone (2) on the relevant gear.
Correctly position the cone (2) on the male cone of the coupling flange, turning it one way then the other several times and applying manual pressure.
Using a set of shims, measure dimension X at three equidistant points.

2

Calculate the average of the three values and proceed as described below, depending on the result obtained. On a new synchroniser, dimension X must be 1.5 mm maximum.
After operation, if X is less than or equal to 0.80 mm:
- replace the cone (2)
- check the measurement of X again, using the same process.
If dimension X is still incorrect, also replace the coupling flange (3) or, if necessary, the entire synchroniser.

Double cone synchroniser


On GBA20 gearboxes, the mechanical reverse shuttle synchroniser is double cone type.


Parts list (Fig.3)
(1) Sliding coupler (2) Cone (brake) (3) Coupling flange (4) Ball bearing (5) Pressure elements (6) Spring (7) Ring (8) Cone (brake)

Advantages and operation
The double cone synchroniser has the following advantages: improved reliability and increased resistance to transmission loads. The operating principle of the double cone synchronizer is similar to that of the single cone synchroniser. The positions (locked and neutral) are obtained in the same way.

Overhaul
If the double cone synchroniser of the mechanical reverse shuttle has been disassembled, check the
cones (2) and (8) for wear as follows:
Stack the coupling flange (3), cones (2) and (8) and ring (7).
Correctly position the ring (7) on the cones (2) and (8), turning it one way then the other several times
and applying manual pressure.
Using a set of shims, measure dimension X (Fig.4) at three equidistant points.
Calculate the average of the three values and proceed as described below, depending on the result obtained. On a new synchroniser, dimension X must be 1.6 mm minimum.
After operation, if X is less than or equal to 0.60 - 0.80 mm:
- replace the cones (2) and (8)
- check the measurement of X again, using the same process.
If dimension X is still incorrect, also replace the ring (7) or, if necessary, the entire synchroniser.

4

Slow speed range (Tortoise)
Gear engagement is obtained by moving one of the synchroniser sliding couplers (11) or (23) to join, in rotation, the layshaft (69) with one of the freely-mounted gears (1st, 2nd, 3rd, 4th). No matter what ratio is
selected, the drive is transmitted to the intermediate shaft (54). The output shaft (44) is driven by the teeth machined on the intermediate shaft, which is constantly meshed to the idle-mounted gear (36). The low range (Tortoise) is obtained by moving the synchroniser sliding coupler (31) backwards.

High speed range (Hare)
The high speed range is obtained by moving the synchroniser sliding coupler (31) forwards, meshing the driving gear (29) and the output shaft. Consequently, in 3rd gear Hare, the intermediate shaft (54) is passive. The other gears are obtained by moving the synchroniser sliding couplers (11) or (23) as with the Tortoise range. Drive is transmitted to the output shaft (44) by driven gears (29) and (55).

Input unit and shuttle GBA20

The GBA20 input unit with mechanical reverse shuttle consists of an interchangeable module. It is located at the front of the main gearbox. It is made of two separate parts: the mechanical reverse shuttle and the Speedshift. The input unit receives drive from the engine clutch and transmits it to the transmission via a driving gear (forward operation) or a layshaft gear (reverse operation). These gears are located in the rear compartment of the input unit.

Removing and refitting the input Unit

Disconnect the tractor between the engine and the gearbox.
Mark the PTO shaft position and remove it from the gearbox.
Separate the hydraulic mechanism from the input unit spacer.
Drain the transmission.
Remove the selector cover plate.
Remove the selector from the mechanical reverse shuttle.

Attach tool to the front cover plate of the input unit. To attach the tool to the front cover plate of the input unit correctly, use locally obtained screws that are long enough
and of suitable strength.
Take out the M10 screws. Do not take out the M8 screws to avoid removing certain parts from the input unit.
Remove the gearbox input unit and the handling bar.
After removing the input unit, separate the tool from the front cover plate if necessary.
Discard the "O" rings (2) and (3).
Recover the cup and the dowel (4).


 Refitting the input unit
Clean the mating faces of the gearbox and input unit.
Check that the cup is present.
Install the gearbox and input unit with new "O" rings (2) and (3). Position the dowel (4).
Screw diametrically opposed guide studs "G" into the gearbox. The guide studs "G" assist insertion of the unit into the gearbox. Their use is advisable but not mandatory.


With the help of an operator, insert the input unit into the gearbox and align the gearbox intermediate shaft with the input unit: slightly raise the intermediate shaft through the selector cover plate to ensure alignment.
Turn the input shaft of the unit.
Engage the unit in the dowel. Place the input unit on the gearbox bearing face.

Fit and tighten the M10 screws to a torque of 50 - 70 Nm.
After refitting the input unit, remove the and the handling bar from the front cover plate.
Assemble the spacer and hydraulic mechanism of the engine clutch on the front cover plate of the input unit.
Install and adjust the selector on the mechanical reverse shuttle.
Install the selector cover plate.
Install the PTO shaft.


Reconnect the tractor between the engine and the gearbox.
Bleed the hydraulic mechanism of the engine clutch.
Carry out a road test of the mechanical reverse shuttle and the Speedshift (low range and high range).