Mechanical reverse shuttle GBA20



The housing of the GBA20 gearbox with mechanical reverse shuttle consists of the following main components, in the order given:
- the engine clutch hydraulic mechanism,
- the Speedshift input unit,
- the mechanical reverse shuttle,
- the main gearbox,
- the creeper or super creeper gearbox (depending on option).

Engine clutch hydraulic mechanism
The engine clutch hydraulic mechanism comprises two slave cylinders, a guide and a thrust ball bearing.
They are fitted to the front of the main gearbox input unit.

Speedshift input unit
The Speedshift input unit consists of a hydraulic mechanism and an epicyclic gear train. It is located at the front of the mechanical reverse shuttle. It allows two gearbox input ratios to be obtained via a solenoid valve.

Mechanical reverse shuttle
The purpose of the mechanical reverse shuttle is to transmit drive from the Speedshift to the layshaft fitted at the front of the main gearbox. The mechanical reverse shuttle is located in the rear
housing of the input unit, behind the Speedshift.

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 (optional)
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 (optional)
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.

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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) Seal (7 mm thick) (11) Seal (10 mm thick) (12) Double planet gears (13) Unit
(14) Spacer (15) Hub (16) Needle roller bearings (17) Friction washers (18) Spring washer (Belleville) (19) Rivets (20) Thrust plate (21) Brake disc (22) "O" ring (23) "O" ring (24) Seal (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 bearing (33) Ring carrier (34) Threaded plug Ø 8 (35) Threaded plug Ø 10 (36) Primary shaft (37) Circlip (38) Tab washer (39) Washer (40) Ball bearing (42) Discs (43) Spring washers (46) Seal (47) Seal (48) Brake piston (50) Planet carrier assembly (30 kph version) (51) Circlip (52) Shim(s) (53) Ball bearing (54) Washer (55) Reverse driving gear (56) Needle roller bearing (57) Double cone synchronizer (58) Needle roller bearing (59) Friction washer (60) Ball bearing (61) Ring(62) Forward driving gear (63) M8 screw (64) M10 screw (65) Needle roller bearing (66) Seal (67) Washers (68) Pin (69) Screw (70) Spacer (71) Needle roller bearings (72) Reverse layshaft gear

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Parts list
(1) Bearing cone (2) Bearing cup (3) Shim(s) (6) Splined washers (7) 1st driving gear (8) Ring (11) 1st - 2nd single cone synchronizer (14) 2nd driving gear (16) Bearing cup (17) Bearing cone (18) 4th driving gear (23) 3rd - 4th single cone synchronizer (24) Needle roller bearing (26) Circlip (27) Washer
(28) Needle roller bearings (29) 3rd driving gear (Hare) (31) Hare / Tortoise single cone synchroniser (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

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Construction
The GBA20 gearbox with mechanical reverse 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 single 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.

Lubrication
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.

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Description (Fig.3)
Drive is transmitted from the main unit by the driven gear (67) which is permanently meshed with the driving gear (62) when in forward position (Fig.4) or the layshaft gear (72) when in reverse position (Fig.5). 



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.