Diff bearings

[JCD]

My Montecarlo gearbox is on the workbench at the moment and I have a couple of questions about it.

The differential taper bearings look tired and I want to replace them. I have two new SKF bearings 32009X.
The procedure for bedding in the bearings is described in the factory shop manual and its associated technical data book. The Haynes Beta manual describes the same in slightly other words.
This procedure looks cumbersome and awkward to me.
I’m somewhat surprised by the way the diff bearing is designed. The procedure for bedding in the bearings requires the cover to be torqued before pressing in the gearbox side outer race.
This means it should be pushed sideward while it is already clamped, which looks odd to me.
Is there a more pragmatic approach for bedding in these bearings?

The bearing outer races are clamped between the diff cover and the gearbox casing.
I don't see anything else preventing them from turning.
Mine did though. There is a slight marking of the outer race seat in the gearbox casing and the diff cover showing both bearings have turned around, luckily without real damage.
I have measured the bearing outer races and the bore in the diff housing when the cover was mounted and torqued.
Bearings outer races are measured 74.99 mm, not 75. Is this a measurement error or are they going to expand under influence of the pretension - for the time being I’ll leave that for what it is.
In the plane parallel to the cover joint the bore is larger than the bearing, there is  0 to 0.03 mm play (with respect to 75 mm), probably due to the slight misalignment between the cover and the gearbox housing.
In the plane perpendicular to the cover joint I find an interference fit of 0.02 – 0.03 mm.
How can I best avoid bearing rotation?

[Nigel]

Hi J,
You should have found a shim on the engine side, directly under the cover.
This shim thickness determines your pre-load.
The outer races are 'clamped' in place by the covers when torqued.

The Haynes manual is quite good on this except for one critical detail.
When measuring your pre-load, you need to place a piece of suitable tube
between the hub and the inner shaft, in order to lock the planet gears.

Take a look at http://www.betaboyz.myzen.co.uk/forum/index.php?topic=4277.140  which is
where I did the pre-tension on my VX box. It appears crude, but it really works.

Edit: To add, as per Eric's advice a while back, if you're just replacing the bearings, with
all other things the same, pre-load should not be necessary. But it's always a good thing to
do if there's any doubt. And assuming you've found the shim.

Nigel

[JCD]

Hi Nigel,

Thanks for the info. Nice thread and nice car!

I've already done the shimming of the end float.
I found output shaft splines have some visible wear as well but not really to the extent that it worries me.
The ring nuts holding the flanges in place were just hand tight while they should be torqued at 16 kgm. They were correctly crimped and don't seem to be reused ones.
So this might be a recurring problem, and worth checking every time driveshafts are taken off.

I understand the diff should be locked in order to carry out the rolling torque measurement.
If I'm lucky it will be possible to re-use the shim for the adjusting of the bearing preload, like you mention - as per Eric's advice a while back.
You also mention the outer races are 'clamped' in place by the covers when torqued. I don't think that is impossible, but to me it looks improbable.
I think the outer races are clamped between the gearbox and the diff cover. That's why one needs that much force to bed them in; at least according to my logic.
To find out the required shim for the preload I would be tempted to mount the diff and tighten the cover with much lower torque to ease  the bedding in.
After that, all can be mounted and torqued to specs.
Do you think it might make sense to do it that way?
The rolling torque measurement will be the final check anyway.

Cheers,

Jean-Claude

[Nigel]

Hi Jean-Claude,

I did this work a while back and actually worked on 2 different gearboxes.
Some of what I've already stated is not necessarily correct.

The first time was when a gearbox was fully built up, and in that state
pre-load cannot be carried out due to the pinion being in place.
At this time, I simply changed the bearings, used the existing shim, and bolted it all
together. Back in the car and all was good.

I then, a few months later, acquired 2 VX gearboxes.
It was during the build of one of them, that I did the pre-load adjustment, and as I had mixed
parts from both boxes,[spline wear!] this was essential. At this time, the casing
was minus both gear shafts during the adjustment.

Sorry for my confusion/memory lapses.

I may well be wrong about the 'clamped' bit, but in practice the outer races don't spin
in action.

Nigel

[JCD]

Hi Nigel,

That was you 666th post. A sign on the wall? 

I think it is possible to carry out bearing preload measurement in a built up gearbox when the spur gear is left out, but I won't go that far.
I'll mount everything and use my intuition to interpret the observations.
As you already quoted Eric's advice - when just replacing the bearings, with all other things the same, pre-load should not be necessary.
That was also my experience for the one time I came across a similar situation in the past.

Outer races should not spin at all. If preload and rotating torque look and feel good I might use a drop of Loctite to fix them in place.

I'll post my findings here.
That might take a while. Assembly of the gearbox is foreseen this month.
The engine is away for overhaul so road test will not be before summer I guess.

Jean-Claude

[JCD]

Nigel,
The idea of assembling the diff without the spur gear is nonsense.
I’m glad you noticed without nailing me to the pillory
Bearing outer shells and flange ring nuts are secured with Loctite 270.
A KM-5 socket was not a perfect fit but happily did the job. Together with the fixture holding the flange I had a guide preventing the socket to tilt.
https://drive.google.com/file/d/1LicNr93dWd-_Mn-DQpExGw22TIV1eNZH/view?usp=sharing


https://drive.google.com/file/d/1lcacWJAgNdxC_q38rkDUD01v2l4aoPUf/view?usp=sharing

I couldn't find a better way to share the pictures sorry.

Now patiently waiting for the engine to come back from the workshop.
I’ll update when there will be news.