Robotic mechanical GBA25 gearbox - Disassembly and reassembly the primary and secondary shafts


Disassembling the primary shaft
The bearing cones (2) and (50) are fitted tight on the shaft.


Position the shaft on a work surface with a suitable supporting fixture to hold the shaft in a vertical position.


Take off the ring (53) at the end of the shaft (rear axle end).
Install a shouldered washer at the end of the shaft (1) in order to Install an extractor. Using the extractor, pull the pinion (48) and remove the bearing (50) and the pinion (48)Recover the lubricating bush (49).


Remove the synchroniser assembly (60). Remove the circlip (42).
Take off the circlip (18) to remove the double pinion (26). The bearings (37) and (38) stay in the double pinion (26). The bearing cones (28) and (29) stay on the shaft (1). Recover the washer (19) and the shims (20).


Remove the bearing cup (30) from the pinion (26). Check whether or not there is a shim behind the cup.
Remove the snap ring (40). Take off the needle bearings (38) and (37) with the spacers (36) and (39). Remove the snap ring (35) if required.
Remove the circlip (34), washer (33) and bearing cones (28) and (29).
If necessary, remove the bearing cone (2).

Reassembling the primary shaft
Pair up the cones (28)(29) and cups (27)(30).
Install the bearing cones (28) and (29) on the shaft (1). Next install the washer (33) and circlip (34). It is assembled as an X. The cones are placed back to back. Install the cup (30) in the double pinion (26).


Check the presence of the snap ring (35) in the double pinion.
Install the spacer (36), greased needle bearings (37) and (38), and spacer (39) in the pinion (26)Install the snap ring (40).
Slide the shaft (1) into the pinion (26).
Install the washer (19) and circlip (18). Do not insert shims to measure the clearance during this operation.

Shimming the pinion (26)
In order to position the bearings correctly, push and pull the pinion (26) while turning it.
Place a dial gauge feeler pin on the surface of the pinion (26).
Measure the clearance A at three points. Calculate the average of the three readings. Remove the circlip (18) and the washer (19).


Between the cup (27) and washer (19) insert the thickness of shims required to obtain clearance A: A = -0.05 to 0.05 mm Shim to the maximum tolerance (low preload).

Shimming the synchroniser overlap
Install the thickness of shims.
Install the circlip (42) holding the synchroniser in position (60).
Using a depth gauge, measure distance B between the mating face of the synchroniser (60) on the circlip (42) and the mating face of the pinion (26).


Ensure distance B does not exceed 16.33 mm:
- If B<16.33: the dimensions are correct.
- If B>16.33: take out one 0.40 mm shim (20) used to shim the pinion (26), and place it between the pinion (26) and the cup (30). There are no intermediate shim sizes; use either the 0.40 mm shim or no shim.


Once the shimming is complete, refit the synchronizer (60). Make a paint mark to correctly align the lubricating ports on the synchroniser hub and the shaft (1). Reinstall the bush (49) and pinion (48).
Using a makeshift tool, fit the bearing cones (2) and (50) on the shaft (1).

Disassembling the secondary shaft
Take care not to pull the pinion (25) or the synchroniser will come apart. The bearing cone (9) is freely mounted on the shaft (1).
Remove the bearing cone (9) and the spacer (14).
Install the shaft vertically on the pinion (25). Pull the shaft (13) upwards with the pinions (43) and (45), taking care not to disassemble the synchronizer (69)Recover the needle bearing (32).
Extract the snap ring (68). Remove the synchronizer (69) and pinion (31).
Turn over the pinion (25). Take off the circlip (15), washer (16) and shims (17)Remove and pair up the bearings (21)(22) and (23)(24).
Using an extractor, pull the pinion (45) and remove the bearing (46).
Remove the spacer (44) and pinion (43).

Reassembling the secondary shaft
On the shaft (13), Install:
- pinion (43)
- spacer (44)
- pinion (45).
Install the bearing cone (46) using a makeshift tool.
In the pinion (25), fit the roller bearings (21)(22) and (23)(24).
Install the washer (16) then the circlip (15). Do not fit shims (17).
Using a set of shims, measure the clearance C between the circlip (15) and the washer (16).
Between the bearing (21) and washer (16) insert the thickness of shims (17) required to obtain clearance C: C = -0.05 to 0.05 mm Shim to the maximum tolerance (low preload).


On the pinion (25), reinstall the pinion (31), its inner surface lightly greased.
Reassemble the synchroniser (69) on the splines of the pinion (25). Make a paint mark to correctly align the lubricating ports on the synchroniser hub and the pinion (25). Install the snap ring (68).
Install the pre-greased bearing (32).
Position the pinion (25) on a support, with the synchroniser turned upwards.


Thread on the shaft (13) assembled during, paying special attention when inserting it through the needle bearing (32).
Install the spacer (14) and bearing cone (9) on the shaft (13). Ensure the spacer shoulder is positioned the right way.



 Shimming the primary and secondary shafts in the housing

Shimming the primary shaft and reverse transfer pinion
Check the presence of the bearing cups (51), (47) and (118) in the gearbox housing (5).
In the gearbox housing (5), position the primary shaft (1) assembled during operations. At the same time, Install the reverse transfer pinion (116).
In the cover (6), Install the bearing cups (3) and (114) (assembled without shims) with grease to keep them in the cover (6).
Reinstall the cover (6) on the housing (5). Install and tighten the cover screws (A).


Place a dial gauge feeler pin at the end of the primary shaft (1). In order to correctly seat the bearings, pull and push hard on the shaft while turning it. To pull the shaft, use a lever positioned underneath the gearbox.
Measure the clearance with a dial gauge. Calculate the thickness of shims required to obtain preload P1: P1 = 0.05 to 0.10 mm. If possible, shim to obtain maximum tolerance.

Example if the clearance measured is 0.73:
0.73 + 0.05 = 0.78
0.73 + 0.10 = 0.83
The thickness of shims required will be between 0.78 and 0.83. Get as close as possible to 0.83.

Place the dial gauge feeler pin on the reverse transfer pinion.
Using a makeshift bent rod, pull the pinion to correctly seat the bearings.
Measure the clearance with a dial gauge. Calculate the thickness of shims required to obtain preload P2: P2 = 0.05 to 0.10 mm. If possible, shim to obtain maximum tolerance.


Remove the screws (A) and the cover (6).
Take out the primary shaft and reverse transfer pinion.

Shimming the secondary shaft
Check the presence of the bearing cup (47) in the gearbox housing (5).
In the gearbox housing (5), position the secondary shaft (13) assembled during operations.
In the cover (6), Install the bearing cups (8) (assembled without shims) with grease to keep them in the cover (6).
Reinstall the cover (6) on the housing (5). Install and tighten the cover screws (A).


Place a dial gauge feeler pin at the end of the secondary shaft (13). In order to correctly seat the bearings, pull and push hard on the shaft while turning it. To pull the shaft, use a lever positioned underneath the gearbox.

Measure the clearance with a dial gauge. Calculate the thickness of shims required to obtain preload P3: P3 = 0.05 to 0.15 mm. If possible, shim to obtain maximum tolerance. Reinstall the shafts in the housing and fit the cover with the calculated shim thicknesses.