GBA25 gearbox - Powershift module


The Powershift module is fitted on GBA25 Dyna-4 gearbox. It is fitted at the gearbox input and is driven directly by the engine. This module provides the GBA25 Dyna-4 transmission with four ratios for shifting under load. These four ratios are provided by two epicyclic gear trains, operating with two brakes and two clutches. The ratios can be controlled with the Power control lever (to the left under the steering wheel) or with the right-hand armrest lever.


Ratios are shifted in "Speed matching" mode, where the most suitable ratio for shifting under load is automatically selected when changing range. The brakes and clutches are controlled by proportional solenoid valves (N and L). They are fitted to a hydraulic unit to the front right of the gearbox that operates on the tractor 20 bar circuit.

Principles of operation

The Powershift module is made up of two epicyclic gear trains:
- The front epicyclic gear train, located on the gearbox input. This comprises a sun gear (36), a planet carrier (25), three planetary gears (48) and a ring gear (65). It is controlled by a brake and a clutch. Its ratio is 1.23/1 (speed multiplied) or 1/1 (train locked).
- The rear epicyclic gear train, located on the gearbox output. This comprises a sun gear (49), a planet carrier (30), three planetary gears (50) and a ring gear (29). It is controlled by a brake and a clutch. Its ratio is 1/1.5 (speed reduced) or 1/1 (train locked).

Front epicyclic gear train
The input shaft (10) of the module is linked to the planet carrier (25) of the primary epicyclic gear train. The sun gear (36) is either attached to the planet carrier (25) (clutch M engaged, epicyclic gear train locked) or to the gearbox housing (brake L engaged, input speed multiplied). Drive is transmitted by ring gear (65) to ring gear (29) of the rear epicyclic gear train.

Rear epicyclic gear train
Input drive is transmitted by the ring gear (29). The sun gear (49) is either attached to the planet carrier (30) (clutch O engaged, epicyclic gear train locked) or to the gearbox housing (brake N engaged, input speed reduced). The output shaft (47) is attached to the planet carrier (30), and is linked to the Power Shuttle.

Lubrication
The Powershift module is lubricated under pressure by a network of internal channels. Oil enters the upper main shaft of the gearbox, then travels through radial bores to the elements to lubricate. The oil flows from the tractor hydraulic circuit.


Parts list 
(1) Housing (2) Cover (3) Clutch M housing (4) Belleville washer (5) Discs (6) Thrust plates (7) Clutch M cover (8) Discs (9) Thrust plate (10) Input shaft (11) Lip seal (12) Ball bearing (13) Friction washer (14) Friction washer (15) Screws (16) Clutch M/brake L junction plate (17) Brake L piston (18) Powershift module front housing (19) "O" ring (20) "O" ring (21) "O" ring
(22) Spring (23) "O" ring (24) "O" ring (25) Front planet carrier (26) Cover (27) Ring gear carrier (28) Rear housing (29) Rear ring gear (30) Rear planet carrier (31) Snap ring (32) Clutch O cover (33) Discs (34) Thrust plates (35) Clutch O/brake N junction plate (36) Sun gear (37) Snap ring (38) Needle bearing (39) Pin (40) Plug (41) Friction washers (42) Snap ring (43) Bronze spacer
(44) Ball bearing (45) Spacer (46) Circlip (47) Output shaft (48) Planetary gear (49) Sun gear (50) Planetary gear (51) Needle bearing (52) Pin (53) Plug (54) Friction washers (55) "O" ring (56) "O" ring (57) Brake N piston (58) Spring (59) Snap ring (60) Ball bearing (61) Clutch hub
(62) Thrust plates (63) Discs (64) Screws (65) Front ring gear (66) Clutch O housing (67) Belleville washer (68) Belleville washer (69) Friction washer (70) Spacer (71) Bearing (72) Spring washer (73) Needle bearing (74) "O" ring (75) "O" ring (76) "O" ring (77) Washer

Kinematics


Parts list 
(Av) Forward clutch
(Ar) Reverse clutch
(L) Sun gear brake, front epicyclic gear train of Powershift module
(M) Sun gear clutch, front epicyclic gear train of Powershift module
(N) Sun gear brake, rear epicyclic gear train of Powershift module
(O) Sun gear clutch, rear epicyclic gear train of Powershift module

Logic of the ratios (see table)


The four ratios are obtained by combining the two epicyclic gear trains. The front epicyclic gear train multiplies the input speed to 1.23/1, while the rear epicyclic gear train reduces the speed to 1/1.5. Each epicyclic gear train may be engaged (multiplication or reduction) or locked (ratio 1/1). The two epicyclic gear trains are used in combination to obtain the four ratios A, B, C and D.

Clutch/brake operation 


The brakes and clutches operate as a 1 brake/1 clutch set. The brakes (F) mesh an element to the housing (fixed), and the clutches (E) mesh two elements in rotation. The clutches/brakes are controlled by a single piston (P). A junction plate (L) allows the clutch to be released when the brake is engaged, and vice versa. When there is no pressure on the piston (position A), the Belleville washer (R) engages the clutch (E). When the piston (P) is actuated (position B), it engages the brake (F) and pushes back the Belleville washer (R) via the plate (L).

Ratio A (1/1.5) 


Brake piston "L" is not pressurised, clutch "M" is engaged and locks the front epicyclic gear train. Brake piston "N" is pressurised. The sun gear of the rear epicyclic gear train is held in relation to the gear box housing, enabling the rear epicyclic gear train to reduce the input speed.

Ratio B (1/1.22) 


The two brake pistons L and N are pressurised. The sun gears are held in relation to the gearbox housing. The two epicyclic gear trains are engaged. The front epicyclic gear train increases the input speed to 1.23/1 and the rear epicyclic gear train reduces the speed to 1/1.5, giving an overall ratio of 1/1.22.

Ratio C (1/1) 


None of the pistons is pressurised, clutches "M" and "O" are engaged by their respective Belleville washers. Both epicyclic gear trains are locked and the input speed is identical to the output speed.

Ratio D (1.23/1) 


Brake piston "L" is pressurised, it holds the sun gear of the front epicyclic gear train in relation to the housing. The front epicyclic gear train multiplies the input speed to 1.23/1. Brake piston N is not pressurised. Clutch "O" locks the rear epicyclic gear train; its ratio is 1/1.