GBA25 gearbox – Power Shuttle

The Power Shuttle, also called "ISC", is a system consisting of two electro hydraulically controlled multidisc clutches. It enables the direction to be reversed under load. It is fitted to all tractors with a Dyna-4 GBA25 gearbox. The reverse shuttle is fitted in the centre of the gearbox. It links the gearbox Power shift module to the gearbox mechanical section. The Power Shuttle is used as the main clutch of the transmission. The Power Shuttle design is simple. It comprises two multidisc clutches, one for forward travel and one for reverse travel. The reverse shuttle transmits the drive from the gearbox Power shift module to the gearbox mechanical section. Forward drive is transmitted directly to the mechanical gearbox primary shaft. For reverse travel, a gear train and a mainshaft transmit drive to the second ratio pinion on the secondary shaft of the mechanical gearbox.

A common lever under the steering wheel to the left reverses the direction of travel and shifts the Power shift ratios A, B, C and D. It is also fitted with a position for manual declutching of the transmission.
Depending on the position selected by the operator, the lever controls two proportional solenoid valves via an Autotronic 5 calculator fitted in the cab floor. The clutch pedal or maneuvering pedal also controls the clutch via sensors and the calculator. It is useful when carrying out precise maneuvers or attaching implements. The two solenoid valves are fitted on a hydraulic unit located at the front right of the gearbox. This unit also includes the control solenoid valves of the Powershift ratios. It is fed by the tractor hydraulic circuit with at a pressure of 20 bar. A speed sensor fitted to the right-hand side of the gearbox informs the calculator of the rotational output speed of the Power Shuttle. The measurement is taken at the second ratio pinion of the secondary shaft.

Parts list 
(1) Power Shuttle primary shaft (2) Snap ring (3) Bearing (4) Rings (5) Bearing (6) Reverse driving gear (7) Bearing (8) Circlip (9) Snap ring (10) Collar (11) Discs (12) Thrust plates (13) Washer (14) Reverse shuttle unit (15) Thrust plates (16) Discs (17) Belleville washers (18) Circlip (19) Snap ring
(20) Forward output hub (21) Gearbox housing (22) Mechanical gearbox primary shaft (23) Power take-off shaft (24) Lubricating oil deflector (25) Snap ring (26) Powershift module cover (27) Snap ring (28) Reverse clutch cover (29) Circlip (30) Forward clutch piston (31) Belleville washers
(32) Reverse clutch piston (33) Seal (34) Seal (35) Seal (36) Seal (37) Washer (38) Reverse driving gear (39) Bearing (40) Forward clutch cover (41) "O" ring (42) Sealing ring (43) Ring (44) Snap ring (45) Ring (46) Gearbox internal compartment (47) Mechanical gearbox secondary shaft (48) Second range driven gear (49) Reverse transfer gear (50) Bearing (51) Mainshaft (52) Shims (53) Bearing cup (54) Bearing cone (55) Bearing cup (56) Bearing cone (57) Bearing (58) Circlip (59) Flange (60) Needle bearing (61) Circlip

Construction GBA25 gearbox

The Power Shuttle unit comprises two identical clutches fitted opposite one another. The Power Shuttle consists mainly of a gearbox (14) fitted to the output shaft (1) of the Power shift module. The shaft (1) is supported on the Power shift module side by a ball bearing (3) held on the bearing (57) by the flange (59). On the mechanical gearbox side, the shaft (1) is supported by a ball bearing (39) fitted on the primary shaft of the mechanical gearbox (22). The forward position output hub (20) is splined to the shaft (22). The reverse pinion (6) turns on two ball bearings (5) and (7). Each clutch mechanism comprises a piston (30) and (32), a series of discs (16) (11) and thrust plates (15) (12). The tightness of the pistons is ensured by seals with square sections (35) (36) and (33) (34). Each piston is repositioned by a series of Belleville washers (17) and (31). A thrust washer (13) or (37) is positioned between the piston (32) or (30) and the Belleville washers (31) or (17). The Belleville washers (31) and (17) are held on the shaft (1) by the rings (10) and (45) and snap rings (9) and (44) respectively. The reverse transfer shaft (51) is supported at the front by a ball bearing (50) fitted tightly in the cover (26) of the Powershift module. On the mechanical gearbox side, the pinion (38) splined to the shaft (51) turns on two taper roller bearings. This part of the mechanism\ is described in the "Mechanical gearbox" section. The pinion (49) is splined to the shaft (51). A circlip (58) holds the pinion (49) in position on the shaft (51).


To engage the forward position, the piston is supplied by the 20 bar hydraulic circuit of the tractor. Oil flows through the channel (B) in the shaft (1) and moves the piston (30). The discs (16) and thrust plates (15) are compressed between the piston and cover (40). Input drive is transmitted via the unit (14) to the thrust plates (15) and discs (16). The splined hub (20) directly drives the gearbox primary shaft (22).

The clutch mechanism is identical to the forward position. The reverse pinion (6) drives the mechanical gearbox second gear pinion (48) via the transfer shaft (51); the pinion turns idle on the secondary shaft (47). The direction of travel is reversed by a series of reverse driving pinions (6), (49) and (38).
The forward and reverse position ratios are identical.

Pressure is supplied to the pistons (30) and (32) by internal channels (A) and (B) located in the shaft (1).
Two solenoid valves located on the gearbox right-hand cover control the clutches. The internal channels are located in the bearing (57) and in the shaft (1). Three rings (4) provide the seal between the bearing (57) and shaft (1). The channels are blocked at the end of the shaft (1) by plugs.

A flow of oil from the tractor hydraulic circuit is sent to the bearing (57) on the left-hand side of the gearbox. The oil flow is channelled to the centre of the upper shaftline. Radial bores distribute the oil to the various elements. The channels are blocked at the end of the shaft (1) by plugs. The seal ring (42), "O" ring (41) and ring (43) prevent the oil from leaking out where the shafts (1) and (22) join.

Power Shuttle control
Reversing the direction of travel is controlled electro-hydraulically by an Autotronic 5 calculator used to control the transmission. The calculator controls two proportional solenoid valves that control the Power
Shuttle clutches. To control the gradual reverse shifting or engagement of a clutch, the calculator receives information concerning engine rotational speed, ground speed and Power Shuttle output
rotational speed.