GBA25 gearbox - Construction and description



The housing of the Dyna-4 GBA25 gearbox consists of the following main components, in the order given:
- (1) the Powershift module, providing four ratios for shifting under load (A, B, C and D)
- (2) the Power Shuttle (3) the main gearbox comprising four robotically controlled mechanical ratios (4) the creeper or super creeper gearbox (if fitted). 

Powershift module
This module provides four ratios for shifting under load (A, B, C and D). The Powershift module comprises two epicyclic gear trains, two brakes and two multidisc clutches. The brakes are controlled hydraulically by two proportional solenoid valves with an ON/OFF control, and the clutches are engaged by Belleville washers and disengaged by plates connected to the brakes located in the same module.

Power Shuttle
The Power Shuttle is fitted in the centre of the unit. It links the ratios for shifting under load and the gearbox. The Power Shuttle comprises two multidisc wet clutches controlled by two proportional solenoid valves with PWM control. A mainshaft ensures reverse position.

Robotic gearbox
The gearbox has four ranges. The ratios are selected by two hydraulically-controlled synchronisers.
Gearbox input drive is supplied by the Powershift module. The four gearbox ranges are automated, so all ratios can be engaged or disengaged automatically by the tractor electronic management system.

Creeper and super-creeper unit
A unit fitted on the secondary shaft, to the rear of the gearbox, provides very slow travel speeds. This unit comprises one or two final drives providing a reduction of 4/1 (creeper speeds) or 14/1 (super creeper speeds). These ratios are engaged mechanically.

Lubrication
Lubrication is provided by the tractor hydraulic circuit. Oil arrives at two points located on the left-hand side of the gearbox housing. Oil flows to the bearings and clutches to be lubricated via internal channels in the gearbox shafts.


Parts list 
(A) Powershift module (B) Power Shuttle module (C) Reverse driving gear (D) Mechanical unit module providing four robotically controlled ranges (E) Single cone synchronizer (F) Double cone synchroniser
(G) PTO shaft (H) Gearbox output shaft (Av) Forward clutch (Ar) Reverse clutch (L) Brake (M) Clutch (N) Brake (O) Clutch


Parts list 
(1) Gearbox housing (2) Compartment (3) Shims (4) Primary shaft (5) Bearing cup (6) Bearing cone (7) Double pinion (8) Circlip (9) Spacer (10) Shims (11) Bearing cone (12) Bearing cup (13) Bearing cup (14) Bearing cone (15) 3rd and 4th single cone synchronizer (16) Spacer (17) Circlip (18) Circlip
(19) Spacer (20) Needle bearing (21) Needle bearing (22) Spacer (23) Circlip (24) Circlip (25) Ring (26) 4th driving gear (27) Bearing cup (28) Bearing cone (29) Plug (30) "O" ring (31) Power take-off shaft (32) Secondary shaft (33) Bearing cone (34) Bearing cup (35) 4th driven gear (36) Spacer
(37) 4th driven gear (38) Bearing cup (39) Bearing cone (40) Lubrication pipe (41) Spring (42) "O" ring (43) Shims (44) Spacer (45) Circlip (46) Washer (47) Shims (48) Bearing cup (49) Bearing cone (50) Bearing cone (51) Bearing cup (52) 2nd gear pinion (53) 1st gear intermediate pinion
(54) 1st and 2nd double cone synchronizer (55) Needle bearing


Parts list 
(56) Powershift module housing (57) Spring (58) Brake "L" disc/thrust plate assembly (59) Clutch "M" disc/thrust plate assembly (60) Clutch "M" bell housing (61) Ball bearing (62) Front epicyclic gear train primary shaft (63) Lip seal (64) Friction washers (65) Belleville washer
(66) Cover (67) Clutch "M" cover (68) Planet carrier (69) Brake "L" piston (70) Ring gear (71) Ring-gear carrier (72) Sun gear (73) Planetary gear (74) Ring (75) Ball bearing (76) Circlip (77) Spacer (78) Pin (79) Needle bearing (80) Planet carrier (81) Ring gear (82) Pin (83) Needle bearing (84) Sun gear (85) Snap ring (86) Clutch "O" cover (104) Brake "N" piston
(105) Spring (106) Brake "N" disc/thrust plate assembly (107) Clutch "O" disc/thrust plate assembly (108) Belleville washers (109) Secondary shaft (110) Ball bearing (111) Clutch "O" bell housing (112) PowerShuttle assembly (113) Driven transfer gear (114) Reverse mainshaft
(115) Driving transfer gear (116) Bearing cup (117) Bearing cup (118) Bearing cone (119) Bearing cone (120) Cover (121) Hub (122) Ball bearing


The Dyna-4GBA25 gearbox provides four basic synchronised ratios and four ratios for shifting under load. The 16 ratios can be engaged in forward and reverse position without using the clutch pedal.
The unit housing consists of two sections:
- front housing: fitted just behind the engine, it supports the shifting under load section of the transmission (Powershift module) and the Power Shuttle.
- rear housing: fitted just behind the front housing, it supports the main gearbox and the robotically controlled section of the gearbox.

Construction of the main gearbox
The gearbox is completely crossed by the PTO shaft (31) which links the engine flywheel to the PTO clutch in the rear axle. This shaft turns at the centre of a series of shafts forming the upper shaftline.
In the rear housing (1), a vertical compartment (2) supports the front bearings (5)(6) and (38)(39) of the primary (4) and secondary (32) units. It also contains the lubricating channels. The housings are bolted together and act as an oil tank for the tractor hydraulic circuit. The main gearbox comprises four synchronised ratios with two synchronisers (15) and (54). Four gear trains are distributed on the two shafts (4) and (32). Each ratio is obtained by engaging only one synchronizer at a time.
The first ratio uses three gear trains, while the three other ratios each use one gear train. The primary shaft (4) is supported by two taper roller bearings (5)(6) and (27)(28). Adjustment is ensured by shims (3) fitted under the bearing cup (5). The secondary shaft (32) is also supported by two taper roller bearings (38)(39) and (34)(33). Adjustment is ensured by shims (43) fitted under the bearing cup
(38).

Primary shaft (4)
On the primary shaft (4), the 1st and 3rd ratio pinions are integral and form a double pinion (7) that turns idle on the shaft. This assembly is supported by two taper roller bearings (11)(12) and (13)(14) fitted in an X shape, and two needle bearings (20) and (21). The position of the bearings (11)(12) and (13)(14) is adjusted by shims (10). The 2nd ratio pinion is machined in the primary shaft (4), and the 4th ratio pinion (26) turns idle and is supported by a lubricated ring (25).

Secondary shaft (32)
On the secondary shaft, the 2nd ratio pinion (52) turns idle. It is supported by two taper roller bearings (48) (49) and (50)(51) fitted in an X shape, and a needle bearing (55). The bearings (48)(49) and (50)(51) are adjusted by shims (47). The 2nd ratio pinion (52) acts as a rotating pin for the 1st ratio pinion (53), which turns without a bearing. The friction surface is lubricated by internal channels. The double cone synchroniser (54) is fitted on the 2nd ratio pinion and is meshed with the pinion (53) or (37). The 3rd (37) and 4th (35) ratio pinions are meshed by the splines on the secondary shaft (32).

Construction of the Powershift module 
The Powershift module on the Dyna-4 GBA25 transmission is located behind the engine flywheel. Each module is fitted in a cast-iron housing screwed into the gearbox housing. It includes two epicyclic gear trains with three planetary gears.

Front epicyclic gear train
The front epicyclic gear train functions with the clutch (M) and the brake (L). The primary shaft (62) is linked to the planet carrier (68). It links the PTO shaft and engine flywheel. The clutch discs (59) are splined to the primary shaft (62). The thrust plates of the clutch (59) are integral with the clutch cover (60) and sun gear (72). The brake discs (58) are connected to the clutch bell housing (60) and the sun gear (72) via the cover (67). The front epicyclic gear train ring gear (70) drives the rear epicyclic gear train ring gear (81) via the ring-gear carrier (71).

Rear epicyclic gear train
The rear epicyclic gear train functions with the clutch (O) and the brake (N). The clutch discs (107) are splined to the output shaft (109) and the planet-carrier (80). The thrust plates of the clutch (107) are integral with the clutch bell housing (111) and sun gear (84). The sun gear is held to the clutch bell housing (86) by the snap ring (85). The brake discs (105) are connected to the clutch bell housing (111) and sun gear (84). Drive enters via the ring gear (81), driven by the front epicyclic gear train ring gear (70). Drive exits via the planet-carrier (80) towards the Power Shuttle.

Reverse position
The reverse position is ensured by a mainshaft comprised of a shaft (114) and two pinions (113) and (115). It transmits drive from the Power Shuttle (112) to the 2nd ratio pinion (52) on the secondary shaft (32) of the main unit. The pinions (113) and (115) are linked to the shaft by splines. The pinion (115) is fitted on bearings (116)(118) and (117)(119) with preload shimming at the bearing cup (116).

Clutch and brake operation
The clutches/brakes that select the Powershift module ratios are each controlled by a piston (P). When a brake is engaged, the related clutch is disengaged, and vice versa. The control pistons (P) act both on the brake discs (F) and on an intermediate plate (I). When the piston (P) is pressurised, the plate (I) pushes back the Belleville washer(s) (B) of the clutch (E). When the pressure is released, the Belleville washer (B) pushes back the piston (P) via the intermediate plate (I) and tightens the clutch discs (E).