10 replies to this topic

[colinu]

Folks,

I'm doing a little work to my gearbox, and needed to remove the layshaft. When I refitted it I thought it wise to check the shimming. The 3rd motion retainer has stumped me. It wasn't shimmed to start with, and when I fit it the gap I'm measuring is tapered... goes from 12 thou to 1.5 thou along the top edge (see pic). Can someone tell me...

a) is this normal... and if not how do I correct it?
b) at which point should I take the measurements?
c) what size shim(s) do I need (and what part number... e.g. a couple of 2A3714's)?


Cheers,
Colin.

[MRA]

How tight are the 3 retainer bolts ?

Also thow out the tab washers and use loctite 648...... much better than any tab washer )

Edited by mra-minis.co.uk, 24 October 2010 - 03:03 PM.

[colinu]

They are "pinch tight" at present (i.e. not fully torqued yet, but that won't make a difference).

[colinu]

OK, I need to buy a sub-20lb/ft torque wrench before doing the job properly (I’m mislaid mine in my last house move!), but I tried nipping them up a bit tighter… hand tight then approx 10th of a turn tighter (can’t be far off the required 13lb/ft)… it didn’t make any difference to the "tapper" of the measurements.
Hopefully I’ll have a new torque wrench by this evening and try tightening them to 13lb/ft… but if that doesn’t work does anyone have suggestions (e.g. shim to the average of the tapper, or do I need to remove the 3rd motion shaft and investigate)?

Cheers,
Colin.

[colinu]

OK, torqued the bolts up to 13 lb/ft last night... didn't make any difference. Suggestions???

[Ethel]

Check the thickness of the retainer is uniform and it's flat; that the bolt holes are at least flush to the faces (not proud). The torque isn't really the issue, it's getting all three bolts the same distance into the holes so the retainer is parallel to the casing. It would be easier to measure under the bolts, put a 6 thou blade in the gap, next to the bolt, tighten, then slack off each bolt so you can just get the blade out. Then tighten all the bolts by angle until one hits the required torque and you can tighten the other 2 to the same angle.

[colinu]

Hmmm, I’m still having trouble with this… not the just the physical aspect of the job, but also understanding exactly how/what it is that’s being torque for pre-load.

Before moving on, let’s check I’ve got the whole concept of this pre-loading exercise correct…

We want to give the bearing a very small amount of pre-load by making sure no more than 6thou of “side-loading pressure” is applied. We do that by hand tightening the retainer/bolts (i.e. so the retainer lip is just touching the bearing carrier), measuring the gap between casing and retainer, then if required choose a shim that would take the gap to 6thou or less… then tighten it all up to 13lb/ft. CORRECT????

I think I follow Ethel’s reasoning of how to shim… getting the retainer parallel to the casing, tighten each bolt until one is at the correct torque, then adjust the other two by angle (not torque) hence keeping the retainer parallel to the casing (or rather I’d argue the bearing carrier).

However, here’s my problem (or just plain dumb confusion!)…

The gap next to each bolt is negligible (say 1thou) when hand tight, but at the corner where there’s no bolt the gap is 12thou. If I follow Ethel’s suggestion then I’d need to “pre-gap” using a 15thou feeler, the first bolt to torque to 13lb/ft will be with the retained dead flush against the casting (in fact all bolts would do this!)… and I still have a 12thou gap at the top-left side of the retainer. Hmmm, have I confused myself? I still don’t know if that helps me determine what shim to use (or in fact if I need a shim at all… the previous owner/builder hadn’t shimmed, hence may have faced the same dilemma)?

I’ve checked the retainer and it’s flat and of even thickness to within 2thou (at both the main body and the ‘lip’ that presses/pre-loads the bearing on 3rd motion shaft). What I’m starting to think is either:
a) The casting (beneath the retainer) is not flush/true
b) The bearing isn’t sitting true within the casting
c) Or a combination of both

All of the above is difficult to determine without removing the 3rd motion shaft… which I rather not have to do.

Or (a possible brainwave just stuck me)… do I just put a piece of 11thou shim under the offending “12thou corner” (thus making sure the retainer is parallel with the bearing) then torque it all up to 13lb/ft… and pray that bit of shim doesn’t dislodge itself sometime in future?

Hmmm. Suggestions? Or have I totally got the concept of this pre-loading exercise all wrong?

[Ethel]

I'm betting there's a coffee mug on your desk...

Stick a coaster on top of it. In this demonstration your desk is playing the part of the gearbox casing, the mug is the bearing and the coaster the retainer

Push down on one corner of the coaster and the other corners will flip up so the coaster is no longer flat on top of the mug and level with the table. It's a simple as that, just the "flipping" retainer is doing it so minutely it's not obvious.

It will never be perfect, since they took the 4th bolt away, but it obviously didn't bother the open side of the retainer either. The tightest bit will be under the bolts so avoid overloading in those areas.

Preload just takes up the slack.

Oh as well, it couldn't be an A+ gear set in a pre A plus box could it? Check the lay shaft locking plate thingy isn't sitting proud.

[Guess-Works.com]

simple way...

Torque up the bolts as you have then loosen them off, this means the bearing is in as far as it will go... then using you HAND push the retainer in and measure the clearance along the top. You then add shims to fit..

< 5 thou, none required
5 - 6 thou, use 5
6 - 8, use 7
8 - 10, use 9
10 - 12, use 11
12 - 14, use 13
14 - 15, use 15

Over that, change the retainer or there's something wrong in the installation of the double roller..

You are not installing the bearing with a pre-load you are just fitting it...

Edited by Guess-Works.com, 29 October 2010 - 05:18 AM.

[colinu]

Ethel, Guess-works,

OK, the penny has dropped. As the gaps at each bolted corner are <5thou (approx 1thou) I'm going to assume no shim is required (and ignore the fact that I have a 12thou gap at the unbolted corner of the retainer). Hopefully all will be well

The only other question I have is... with the box on the bench (and diff removed) how easy should it be to select gears by hand (e.g. thin screwdriver through the hole in the selector rod and turn/push/pull into gears). I've never tried that before on the bench so don't know what to expect... this box feels "stiff" to me and I have to punch it into each gear (1st/2nd being the most difficult). Is that expected, or do I need to strip this box down further? When in the car the box was a little akward into 1st gear, but I'd assume that was due to clutch problems and a knackered (destroyed!) thrust washer on the crank.

Cheers,
Colin.

[Ethel]

It can be done, it will obviously feel a lot stiffer without the leverage.