GBA25 gearbox - Kinematics and Synchronizers


Kinematics of the GBA25 gearbox  


Parts list 
(Av) Forward clutch
(Ar) Reverse clutch
(L) Planetary gear brake, Power shift module front epicyclic gear train
(M) Planetary gear clutch, Power shift module front epicyclic gear train
(N) Planetary gear brake, Power shift module rear epicyclic gear train
(O) Planetary gear clutch, Power shift module rear epicyclic gear train

Power shift module kinematics 

Ratio A (1/1.5)
The front epicyclic gear train piston (69) is not pressurised; the clutch (M) is tightened by the Belleville washers. The sun gear and the planet-carrier are attached, thereby creating a mechanical lock; the front epicyclic gear train is thus "locked" and its ratio is 1/1. The rear epicyclic gear train piston (104) is pressurised. The brake (N) is tightened and holds the sun gear in relation to the housing. The rear epicyclic gear train turns and reduces the input speed.

Ratio B (1/1.22)
The front epicyclic gear train piston (69) is pressurised; the clutch (M) is loosened. The brake (L) is tightened and holds the sun gear (72) in relation to the housing. The front epicyclic gear train turns and multiplies the input speed. The rear epicyclic gear train is in the same position as at ratio A.

Ratio C (1/1)
The two epicyclic gear train pistons (69)(104) are not pressurised. The two clutches (M) and (O) are tightened by their respective Belleville washers. The sun gear is attached to the planet-carrier in each epicyclic gear train, thereby creating a mechanical lock. The two epicyclic gear trains are "locked", the module input speed is the same at its output and the ratio is 1/1.

Ratio D (1/0.813)
The front epicyclic gear train piston (69) is pressurised; the clutch (M) is loosened. The brake (L) is tightened and holds the sun gear (72) in relation to the housing. The front epicyclic gear train turns and multiplies the input speed. The second epicyclic gear train is "locked" as at ratio C; its ratio is 1/1.

Kinematics of the Power Shuttle 
When the transmission is in neutral, the Power Shuttle clutches are not pressurised. In forward position, the "Av" clutch is engaged, and directly transmits drive from the Power shift to the primary shaft of the gearbox. In reverse position, the "Ar" clutch is engaged, and transmits drive to a mainshaft then to a 2nd ratio pinion on the secondary shaft of the main gearbox.

Kinematics of the main gearbox



Range 1

The reduction is carried out on three gear trains. The first gear train drives the second train via the 1st gear synchronisers, then the third train.




Range 2

The reduction is carried out on the first gear train. The 2nd gear synchronisers secure the 2nd ratio pinion to the secondary shaft.




Range 3

The reduction is carried out on the third gear train. The primary shaft is secured to the 3rd ratio pinion by the 3rd gear synchronisers.




Range 4

The reduction is carried out on the fourth gear train. The primary shaft is secured to the 4th ratio pinion by the 4th gear synchronisers.



Synchronisers of the GBA25 gearbox  

Single cone synchronisers (Ranges 3 and 4)



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


Locked position 
When the sliding coupler (1) moves towards the gear to be locked, it presses against the cone (2) which in turn presses against the male cone of the coupling flange (3) via ball bearings (4) and pressure elements (5). When the synchronisation is set, the sliding coupler (1) can mesh and silently lock with the teeth of the coupling flange (3).


Neutral position 
Sliding coupler (1) is in the middle position. The balls (4) are pushed into the V groove of the sliding coupler by pressure springs (6). In this neutral position, the sliding coupler is locked by three balls held in place by the pressure springs.


Check
If the single cone synchronisers (11), (23) and (31) have been disassembled, check the wear to the cone (2) as follows:
Stack the coupling flange (3) and cone (2) on the relevant pinion.
Correctly position the cone (2) on the male cone of the coupling flange, revolving several times and applying manual pressure.
Using a set of thickness 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 1.5 mm maximum. After operation, if X is less than or equal to 0.80 mm:
- replace the cone (2);
- check the measurement of X again, using the same process. If dimension X remains incorrect, also replace the coupling flange (3) or, if necessary, the complete synchroniser.

Double cone synchronisers (Ranges 1 and 2)



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



Advantages and operation
The double cone synchroniser has the following advantages: improved reliability and increased resistance to transmission loads. The operating principle of the double cone synchronizer is similar to that of the single cone synchroniser. The positions (locked and neutral) are obtained in the same way.


Check
When disassembling the double cone synchroniser of the mechanical reverse shuttle, check the cones (2) and (8) for wear as follows:
Stack the coupling flange (3), cones (2) and (8) and ring (7).

Correctly position the ring (7) on the cones (2) and (8), revolving them alternately each several turns and applying pressure manually.

Using a set of thickness 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 remains incorrect, also replace the ring (7) or, if necessary, the complete synchroniser.

Main gearbox robotic control


Description 
The synchronisers are controlled conventionally by forks (1) and (2) and selector rails (3) and (4). The selector rails are controlled hydraulically. The selector rails are fitted on double-acting pistons (5) and (6). On the selector rail (4) that controls the 1st and 2nd gear synchroniser, the piston (6) is damped by a spring (7) to smoothen shifting between ranges 1 and 2. The selector rails are adjusted by tightening or loosening the adjustable stops (8) and (9). Once adjusted, the stops are held in rotation by screws (10)
and (11). A safety pin (12) prevents the selector rails from being engaged simultaneously. When a selector rail is engaged, the pin locks the other one at the groove (13).