Lotus Elan

Dummy Layshaft

PostPost by: seniorchristo » Thu Feb 15, 2018 3:03 am

The Workshop Manual refers to using a "Dummy Layshaft" to disassemble the gearbox. Can this be a regular layshaft or perhaps a wooden dowel of similar diameter?
Thanks,
Chris :)
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PostPost by: 2cams70 » Thu Feb 15, 2018 3:37 am

If you have an old worn layshaft you can cut it down so it's the same length as the laygear or use a piece of dowel. You can get away without a dummy layshaft if you stick the needle rollers in carefully with grease and are very careful to keep things in line as you push in the layshaft.
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PostPost by: seniorchristo » Thu Feb 15, 2018 3:43 am

Thanks 2cams
So the purpose of the Dummy shaft is to keep the needle bearings in position? :)
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PostPost by: 2cams70 » Thu Feb 15, 2018 4:13 am

Yes - that's what it is for.
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PostPost by: promotor » Thu Feb 15, 2018 8:44 am

To disassemble the gearbox you don't absolutely need a dummy shaft and unless you know which is the correct length and diameter then you may as well just push the original shaft out and let the rollers just collapse inside the cluster gear.

However you should make the correct length one for re-assembly. If using an original shaft you need to shorten it but you also need to take some of the outside diameter down to allow it to pass through the casing locating points as they are a press fit in both ends, with the end closest to the tailshaft the tightest of the two points due to increased diameter on the layshaft at the point. A wooden dowel is good as well - just make sure there is no sawdust or particles likely to drop into the needle rollers.
You will want a dummy shaft of some kind on re-assembly as attempting to assemble the gearbox only to find the rollers have moved/dropped will annoy you immensely!

The shaft needs to be exactly as long as the cluster gear (but not even a fraction longer) to allow the cluster gear to drop between the thrust bearings without the shaft "hanging up" on them and thus not dropping in to the bottom of the gearbox properly.
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PostPost by: Chancer » Thu Feb 15, 2018 10:51 am

promotor wrote: If using an original shaft you need to shorten it but you also need to take some of the outside diameter down to allow it to pass through the casing locating points as they are a press fit in both ends, with the end closest to the tailshaft the tightest of the two points due to increased diameter on the layshaft at the point. A wooden dowel is good as well - just make sure there is no sawdust or particles likely to drop into the needle rollers.
You will want a dummy shaft of some kind on re-assembly as attempting to assemble the gearbox only to find the rollers have moved/dropped will annoy you immensely!

The shaft needs to be exactly as long as the cluster gear (but not even a fraction longer) to allow the cluster gear to drop between the thrust bearings without the shaft "hanging up" on them and thus not dropping in to the bottom of the gearbox properly.


Absolutely, I have a cut down layshaft but it took me hours of grinding to reduce the diameter to a clearance fit over its whole length, for a one off I would turn down a piece of broom handle or dowel.

Back in the day I did a lot of gearbox rebuilds and the steel shaft does not get damaged rattling around in my toolbox, it shares its space with a gearbox first motion shaft used for clutch aligning.
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PostPost by: Certified Lotus » Thu Feb 15, 2018 11:17 am

Chris, I didn’t use a dummy layshaft when I rebuilt my trans. Just use firm grease to install the needle bearings. They did not move when I installed the shaft.
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PostPost by: 2cams70 » Thu Feb 15, 2018 11:19 am

It was almost twenty years ago when I last rebuilt one of these boxes but from memory once the laygear with rollers was placed in the bottom of the box and mainshaft installed I turned the box upside down so the laygear dropped into mesh with the mainshaft gears. Slight adjustments to the position of the laygear were possible by inserting a large screwdriver through the case and into the bore of the laygear and lowering or lifting it slightly as appropriate to get things centralised. Once centralised the layshaft was able to be carefully passed through. The key thing is to STOP should pushing the layshaft become difficult. If that's the case it's likely some of the needle rollers have moved out of place during the process. Do not force it. You don't need a dummy layshaft for gearbox disassembly.
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PostPost by: seniorchristo » Thu Feb 15, 2018 1:01 pm

Very helpful information! Thanks all. :)
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PostPost by: Chancer » Thu Feb 15, 2018 3:08 pm

Yes it can be done without, thats how most of us learned the hard way what happens when a roller or two falls out of position :(

I could blame youthfull impatience, inexperience and knowitallness back then, at our ages there is no excuse for not taking heed of good advice.

Drop the layshaft assembly with dummy layshaft into the box with a couple of loose loops of string or wire, tywraps etc around it, that way you can lift it into engagement with one hand whilst looking down the layshaft axis to ensure the alignment then introduce the proper layshaft, you normally need to lift the front of the laygear a tad more for it to push home into the bore at the front of the gearbox casing.

Amazing that I can recall all these détails from years back but not where i put my keys 30 or a screwdriver seconds ago :D
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PostPost by: englishmaninwales » Thu Feb 15, 2018 4:25 pm

Yes, I found it possible using grease as said, but there is a reasonable chance you’ll be frustrated....
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PostPost by: Chancer » Thu Feb 15, 2018 5:21 pm

When I have been given gearboxes to rebuild that sounded like a box of spanners the first step is to drain the oïl and fish out all the shrapnel and débris, usually I can identify the remains of a few layshaft needle rollers as being what started the box to eat itself and I know that the bodger before did not have a dummy layshaft to use.

With the rarity of these boxes and their parts its just not worth taking a risk IMO.
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PostPost by: promotor » Thu Feb 15, 2018 6:06 pm

Chancer wrote:
Absolutely, I have a cut down layshaft but it took me hours of grinding to reduce the diameter to a clearance fit over its whole length, for a one off I would turn down a piece of broom handle or dowel.

Back in the day I did a lot of gearbox rebuilds and the steel shaft does not get damaged rattling around in my toolbox, it shares its space with a gearbox first motion shaft used for clutch aligning.


I've got 3 dummy layshafts and sometimes that isn't enough depending on how many gearboxes I need to have in pieces at any one time! Also have to have a shorter one for Anglia 997 gearboxes in case anyone is daft enough to want one of those rebuilt! :)

The dummy shaft also makes it easier loading the ends of the cluster gear with the needles - no need to stick them to the cluster with grease. Just pack a little in after they've been loaded to give a little extra lubrication for first use in case the oil level is too low or not circulating well enough.
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PostPost by: Craven » Thu Feb 15, 2018 8:51 pm

Not only does the dummy shaft need to be the correct diameter but it’s length is critical as well.
Not mention so far are the needle trust washers located both ends of the sets of needle rollers, these washer are fairly thin and the outside ones ( each end ) can be pushed out of position as the close fitting laygear is slid into position.
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PostPost by: Elan45 » Fri Feb 16, 2018 2:04 am

Hi Chriis,

I've made a dummy shaft from hardware store 3/4" bar stock. The exact diameter isn't that critical, you just have to wiggle a bit and slide it in carefully. You must keep the new layshaft and the dummy in constant contact when you are sliding through the bearings. And be careful that the new shaft must go into the box from the rear; the layshaft is slightly larger on the back end.

A positive for dummy layshaft use is you don't need to use grease to hold the needles in place. I was told many years ago that the grease used in assembly like holding the needles in place breaks down in later operation and causes the gearbox oil to foam. I have no proof, but I decided back then I didn't need to take a chance of that occurring, so I always use a dummy.

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