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Gearbox main shaft question 19 Jun 2021 15:28 #222559

  • jon_d
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I've been working on the gearbox in the Bedford and have a question regarding play in the main shaft.

Long story - short. The gearbox is an Isuzu MLD6Q. (six speed overdrive - steel casing). The rear main and counter shaft bearings were rotating in the casing. The gap was minimal, (ie a 0.02mm feeler wouldn't fit). So I've removed the bearings and then glued with bearing retainer.

When every thing was reassembled, and the tail shaft flange attached. (the nut was tightened to 580 ft-lbs as per the WSM) I have noticed a detectable forward/backwards ( F-B ) play in the main shaft. The movement must be limited by the F-B travel in the roller bearings.

I don't know whether the movement was there prior to disassembly. I didn't detect it prior. However, it may not have been noticeable as the bearing was effectively a slip fit in the case and it was moving about anyway. (Now it's fixed - aka interference fit)

My question is; in gearboxes, is there normally a detectable (say 1--20 thou) F-B movement in the main shaft?..... I'm trying to determine if I haven't pressed (hammer and drift) the inner bearing race down on the shaft enough and hence, there is a gap in things along the shaft.

I'm thinking/hoping that now the outer race is glued/fixed, I'm just seeing the actual F-B movement in the race rollers and the hard stop as they rest up against the inner/outer race walls.

The work was done under the bus. Any more work might be limited to trying to push the inner rear race forward a whisker and then reassembling.

Thanks, any insights would be really helpful.

The bearing looks like this.

The inner race is at at the bottom and slides in - not easy to see the step in the race. The second ring is a thrust washer.



Jon
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Gearbox main shaft question 19 Jun 2021 18:20 #222562

  • overnite
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To tighten a bearing in a worn housing, before you use bearing loctite or similar, do several centre punch marks in the bearing housing to tighten it up.

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Gearbox main shaft question 19 Jun 2021 19:47 #222570

  • Blackduck59
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Roller bearings are not designed to carry thrust so I would guess a little float is normal.

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Gearbox main shaft question 19 Jun 2021 20:06 #222571

  • JOHN.K.
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Ive never seen a truck gearbox with a roller at the back of the mainshaft.....every on I ever worked on had a big ball bearing ,or even two like a roadranger ,to take the endload of the tailshaft moving ........However ,a well oiled roller can take considerable thrust,and a lot more load than a ball bearing.

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Gearbox main shaft question 19 Jun 2021 21:13 #222572

  • jon_d
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Thanks all,

I rang an old mate in SA and in QLD and both just said it's all ok with the end float. No problems. Pity the WSM didn't mention it.
In hindsight, I reckon I didn't feel it previously because the bearing was slipping in the case.

Overnite; Yes, centre punched all around the inside of the casing.

Hopefully all will be good, as it's going across across the Nullarbor in a few months. Thanks again. :-)

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Gearbox main shaft question 19 Jun 2021 22:19 #222575

  • JOHN.K.
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Outer race turning in the case is not a problem.......inner race turning on shaft will overheat the bearing......so you are good to go..

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