GBA20 gearbox - Power Shuttle

The housing of the GBA20 gearbox with Power Shuttle consists of the following main components, in the
order given:
- the Power Shuttle,
- the Speedshift input unit,
- the gearbox input gear train,
- the main gearbox,
- the creeper or super creeper gearbox (depending on option).

Power Shuttle
The Power Shuttle consists of two multidisc wet clutches (one for forward operation and one for reverse operation). These clutches are located at the front of the input unit, in front of the Speedshift unit. The Power Shuttle is controlled by two proportional solenoid valves operated by a lever located to the left under the steering wheel and managed by the tractor electronic system.

Speedshift input unit
The Speedshift input unit consists of a hydraulic mechanism and an epicyclic gear train. It is located to the rear of the Power Shuttle. It allows two gearbox input ratios to be obtained via a hydraulic control unit and a solenoid valve.

Gearbox input gear train
The gearbox input gear train consists of two meshed gears:
- a driving gear located at the rear of the input unit;
- a driven gear attached to the gearbox layshaft.
The input gear train transmits drive from the engine to the gearbox.

Main gearbox
The main gearbox consists of four basic gears and two speed ranges (Hare and Tortoise), giving a total of eight ratios for each direction of travel. This transmission covers all usage requirements.

Creeper gearbox 
The creeper gearbox is fitted to the gearbox output shaft. It allows slow travel. It operates at a ratio of 4/1.
It is controlled mechanically and should only be engaged when the gearbox is in Tortoise range.

Super creeper gearbox 
The super creeper gearbox is also fitted to the gearbox output shaft. It consists of two epicyclic gear trains which allow a reduction ratio of 14/1, and therefore allows very slow travel. Like the creeper gearbox, it is controlled mechanically.

Parts list 
(3) Screw (4) Ring (5) Secondary shaft (6) Output sun gear (7) Input sun gear (8) Planet gear pins (9) Planet carrier unit (10) Friction washer (12) Double planet gears (13) Unit (14) Spacers (15) Hub (16) Needle roller bearings (17) Friction washers (18) Spring washer (Belleville) (19) Ring restrictor
(20) Thrust plate (21) Brake disc (22) "O" ring (23) "O" ring (24) Screw (25) Hydraulic cover plate (26) "O" ring (27) Front cover plate (28) Clutch piston (29) "O" ring (30) Intermediate plates (31) Rings (32) Needle roller bearings (33) Ring carrier (34) Screw (36) Primary shaft (37) Circlip (38) Tab washer (39) Washer (40) Ball bearing (41) Screw (42) Discs (43) Spring washers (48) Brake piston (50) Planet carrier assembly (30 kph version) (51) Screw (52) Cover plate (53) Shim(s) (54) Bearing cup (56) Driving gear
(57) Bearing cup (58) Bearing cone

Parts list 
(1) Bearing cone (2) Bearing cup (3) Shim(s) (6) Splined washers (7) 1st driving gear (8) Ring (11) 1st - 2nd double cone synchronizer (14) 2nd driving gear (16) Bearing cup (17) Bearing cone (18) 4th driving gear (23) 3rd - 4th double cone synchronizer (24) Needle roller bearing (26) Circlip
(27) Washer (28) Needle roller bearings (29) 3rd driving gear (Hare) (31) Hare / Tortoise double cone synchronizer (32) 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
(48) Bearing cup (49) Circlip (50) Deflector (51) Shim(s) (52) Bearing cup (53) Bearing cone (54) Intermediate shaft (55) 3rd driven gear (56) 4th driven gear (57) Spacer (58) Shim(s) (59) 2nd driven gear (60) 1st driven gear (61) Circlip (62) Needle roller bearing (63) Bearing cone (64) Bearing cup
(65) Gearbox housing (67) Input gear (68) Nut (69) Layshaft (70) Snap ring (71) Lubrication pipe (72) Spring (77) Washer with flat sections (78) Tab washer (79) Needle roller bearing (80) Spacer (81) Needle roller bearing (82) Spacer

Power shuttle GBA20 gearbox construction

The GBA20 gearbox with Power Shuttle consists of three gear trains fitted to the:
- layshaft;
- intermediate shaft (or main shaft),
- output shaft.
It allows eight basic synchronised gear ratios:
- four gear ratios: 1st, 2nd, 3rd and 4th;
- two range ratios: Hare and Tortoise.
Its main characteristic is its assembly of three double cone synchronisers. The main gearbox consists of four synchronised gears. A gear idle-mounted to the output shaft and controlled by a synchroniser allows the four initial gears to be doubled to obtain the eight basic gears. The layshaft and the intermediate shaft are supported by tapered roller bearings. The output shaft is fitted on two tapered roller bearings and one needle roller bearing. The layshaft bearing cups are fitted up against the housing. All gears have helical teeth and are constantly meshed. To ensure optimum gearbox reliability, the bearings are fitted:
- with preload for the layshaft,
- with clearance for the intermediate shaft.
The output shaft may be set either with a slight clearance or a slight preload.

The lubricating oil of the lower shaftline is supplied from the tractor hydraulic system via a 1.5 bar valve. It passes through a channel in the layshaft and output shaft. Radial bores direct the oil to the gears, bearings, rings and synchronisers. The splines of the Hare/Tortoise synchroniser hub are lubricated via a radial bore on the output shaft. The oil flowing inside the intermediate shaft lubricates the tapered roller bearings and needle roller bearing (62). These bearings are fitted respectively at the ends of the shaft.

Drive is transferred from the main gearbox by the driven gear (67) which is permanently meshed with the driving gear for the input gear train (Power Shuttle). The driven gear (67) is splined to the layshaft (69). The two synchroniser hubs (11) (23) are splined to the layshaft (69). The 2nd driving gear (14) is freely mounted on a ring (8). The 1st (7) and 4th (18) driving gears are freely mounted directly on the shaft. The 3rd driving gear (29) is fitted on needle roller bearings (28). It is also fitted with a needle roller thrust bearing on each of its faces to absorb any axial pressure. The hub of the Hare / Tortoise synchroniser (31) is secured to the output shaft (44). The driven gears (55), (56), (59) and (60) drive the intermediate shaft (54) by means of splines. The rear teeth of this shaft are constantly meshed with the Tortoise gear (36) idle-mounted on the output shaft. On GBA20 gearboxes with Power Shuttle, the 1st - 2nd and 3rd - 4th gear synchronisers and the Hare / Tortoise synchroniser are double cone type. The double cone synchroniser has the following advantages: improved reliability and increased resistance to transmission loads.

Synchronisers (double cone)

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

Locked position 
When the sliding coupler (1) moves towards the gear to be locked, it is pressed against the ring (7) by the
ball bearings (4) and pressure elements (5). The ring (7) transmits the pressure received to the cones (2) and (8) to set a synchronisation speed. 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 ball bearings (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.

If the synchronisers (11), (23) and (31) are disassembled, check the wear to the cones (2) and (8) in the following manner:
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 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.

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).