Burman 4th Gear Selector Meltdown

[Matty]

Thanks tatty500 - I will check when I rebuild mine if there is enough thread left and perhaps "win" a mm or two by using a thinner spacer on the output shaft or some sutch. I don't think the chain alignment should cause any problems unless it catches on the gearbox case.

Hello Bob - I attach an inverted picture of the numbers cast onto my old welded up gearbox casing which is still on the bike. This is a 1952 bike and the gearbox is as fitted at the factory.

Matty

 

[Matty]

Hi Vic - This is just to tie up the loose ends of this saga for a while as far as I am concerned.

Have picked up your "broken" parts from Mayland via tatty500. They are very interesting and obviously the tolerances of the depth of the sliding 4th gear dog into the output gear were very dodgy from new. Yours had only a mm or so engagement even though there was not very much wear on the selector pins, etc. The 4th gear dogs were worn very tapered however for a mm or so due I suspect to both cause and the effect of insufficient depth of engagement.

When I rebuild my gearbox with a replacement casing, I will look at the tolerances in my box compared with your old bits. Then maybe, as suggested by "tatty500" after also examining Vic Youle's damaged gears, will, if necessary try to move the output gear in a mm or two with a spacer washer, after grinding a taper on the gear teeth or removing some metal from the selector casting to win some clearance. This would hopefully increase the depth of engagement at the expense of slight drive chain misalignment, but could perhaps also cause a problem with the length of thread left for the nut to hold on the drive chain sprocket - will investigate.


Will keep you all informed when/if I get a result.

Matty

 

[Matty]

Hi All

Just an update with my latest findings now that I have the "new" gearbox casing to replace the old damaged one I have been using since 1957, but which now has a loose, leaky output bearing in the casing.
With this "new" casing I have been playing around with the damaged parts - layshaft with gears, output gear, sliding dog gears and selector shaft with damaged and melted 3/4th gear selector fork, ex Vic Yuole which were sent to me by "tatty500". Have fitted a new output bearing in the "new" casing and it seems to me that the 4th gear dogs engage by about 3.3mm with all the bits pulled in the worst direction - providing that there is no end float on the selector shaft.

I can not of course measure the end float in my gearbox yet because the middle part of the casing is still on my bike.

However when I do take my box out and rebuild it, I think I may have to do two things to ensure better 4th gear engagement:-

Firstly I aim to put a 1mm spacer between the output gear and its bearing - this spacer would be 1 and a half inches internal diameter and about 2" outside diameter. Perhaps another may be needed between the chain drive sprocket and the output gear to re-align the chain, if there is enough thread for the large nut? This would provide another 1mm of dog engagement and the output gear would not quite touch the selector shaft, other gears etc.by 1mm or so!!

Secondly I would try to eliminate any end float in the selector shaft by putting suitable hardened shims behind the roller bearing end of the shaft.

Do these two ideas seem OK please?

Am at present looking for the shims and spacers so that I have all the bits when I start on my gearbox rebuild in a few weeks time.

Matty

 

[nkt267]

Perhaps another may be needed between the chain drive sprocket and the output gear to re-align the chain,

If I remember correctly, isn't the sprocket alignment due to the spacer that fits between the outside of the bearing and the sprocket. If I am right then moving the output gear inboard by any amount should not affect the chain alignment..John
Just nipped down the garage and checked and there is a spacer between the sprocket and output bearing.

 

[tatty500]

Matty,
I’m glad to hear that there seems to be at least one combination of parts that give this much engagement. Perhaps Vic and I have been unlucky.
I have just two point to help prepare you.
First, the OD of the 1.5inch shim needs to be about 46mm to fit inside the casting’s bore on the inside of the output bearing (……but this won’t change the chain-line because the bearing hasn’t been moved)
Second, the selector shaft OD is smaller than the bush OD so there will be a thin unworn rim on the bush face that will need removing……or a new bush. I wizzed about a millimetre off the face on mine and made the shim thicker.
My improvement measures are now twofold.
1. A 1mm or 1.2mm big shim as you are proposing.
2. A new selector with mods.
This entailed enlarging the peg hole in the nice new perfect selector to 3/8inch, as eccentric as possible, and then using a 3/8 peg with original size offset end to engage in the groove. This has given me 33thou ie 0.8mm shift of the fork.
(I’ll have to shim 3[SUP]rd[/SUP] over to reduce the loss of engagement at that end).
So with these changes I should get 2mm more giving a total of 3.4 or more…….not far from your starting point!
Best wish

 

[Matty]

Don't really know yet why the dog engagement with Vic's old parts in my "new" casing seems to be better - other than the selector end float which I can not measure yet - though it did not seem to be excessive in your case.

Must have had a senior moment about the chain sprocket alignment, because it obviously uses the bearing as a reference and not a step on the output gear !!!

Hope I don't need to make an eccetric selector pin for my box - will see when I do the rebuild.

Will also deal with the wear on the selector shaft bush, which I gather from "clevtrev" is made of mild steel and usually wears a bit to create end float on the selector shaft.

Interested to hear how Vic Youle's and tatty500s mods. work out.

All the best

Matty

 

[Matty]

Just done a double check on the engagement of 4th gear dogs using Vic's damaged parts in the "new" casing and I have confirmed that with all the slop on the selectors in the worst direction, his 4th gears engage by 3.5mm which is a little beyond the worn, tapered bits of the dogs and should be OK.

This is with the selector shaft "bottomed" at the drive gear end. So if there was little or no end float on Vic's selector shaft, there should not have been a problem.

However maybe the selector shaft bush face is worn on my "new" casing and hence the shaft further in than on the casings you have tested, so giving me more engagement depth.

Maybe the flange on this bush was a bit thicker in your boxes, which would result in poorer 4th gear engagement.

However I'm sure the various fixes which have been discussed to these tolerance/wear problems should get us Burman gearboxes which will hopefully last another 60 years or so !!!

Matty

 

[vibrac]

So nice to see a useful thread carried to its conclusion over 2 odd months
What is it they say? "without repitition,or deviation"

 

[Matty]

Thanks "vibrac"

In danger of repeating myself !!!

I think we now have solutions to fix the gearboxes, but "clevtrev" says by email, that he checked the drive end selector bush in the casing before he sent it to me and it is still unworn. So wear on the bush would not seem to be the reason why Vic Youle's damaged bits seem OK in the "new" casing.
So why "tatty500's" measurements are different from mine is still a bit of a mystery - he seems to have better facilities than I have (also I am an electronic/electrical Engineer and I suspect he is a Mechanical one) and I trust his figures. I do not believe he is mistaken in his diagnosis which is consistent with the way the failure happened, but I do not believe it would have happened with the measurements I have taken with largely the same parts in the "new" casing.

Hopefully all will become clear when I strip and rebuild my box - and in any case I think we now have a set of solutions which will cover most problems.

Thanks to All for your help

Matty

 

[Matty]

Hi All

Have now removed my Burman Box and found the following :-

The output bearing housing had been modified to take a sleeve so that a "standard " bearing could be used and the flange on the inside was broken off, but with enough left to stop the sleeve going right through. This I already knew, but the sleeve had become loose in the gearbox casing and the bearing had since worked loose in the sleeve, allowing the output gear to move in if it wanted to. I'm surprised there were not nasty noises as the output gear touched the selector fork casting or a gear on the layshaft !!

This could however have resulted in better meshing of 4th gear, so the marks (about 4mm) of the meshing of the gears (see attached photo) may be false ones.

Even though I have had the bike since 1956, I did not do this mod - so the box must have had a serious problem before this.

I have tried my gears in the "new" casing and get nearly 4mm of meshing into 4th gear with all tolerences in the worst direction - but get very much the same figures with Vic Youle's old parts !!

I had about 0.6 mm of backlash on my selector shaft which I have got to nearly zero by putting a shim behind the steel bearing which the rollers go into.

So far therefore I can not account for the small meshing of Vic and tatty500's gearboxes unless two things have happened.

1. There is a lot of backlash in their selector shafts - which with their measurements does not seem to be the case.

2. Or their output gear has moved outwards for some reason.

I shall put a 1mm shim behind the output gear and have checked layshaft end float (about 10 thou) .

With new bearings etc I am fairly confident my gearbox will be OK, but some puzzles still remain on the cause of Vic and Tatty's problems as far as I am concerned.

Will come back onto the forum if I find anything else.

Matty