Author Topic: 3 speed gearbox on my 2 3/4hp model W  (Read 390 times)

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Offline Lat Fuller

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3 speed gearbox on my 2 3/4hp model W
« on: 14 Oct 2017 at 01:53 »
I recently did the Australian National Veteran Run in Nuriootpa, South Australia.   It was a fantastic event covering nearly 600km of quiet, good roads, and my '17 Douglas did the whole run with few problems.
However, I had noticed before I even started that it seemed to jerk every now and then in first gear when it was under heavy load.   But this got worse, and on the last day I had no first gear at all, despite adjusting linkages etc.
When I got home, I stripped the gearbox.   There was nothing significantly wrong with the gears, although the engagement dogs on first and third gear were both a little worn, (which can be expected).   Those who know these gearboxes will know that the selectors move under quite a high spring pressure, and need to be held in gear by the gear linkage, so this has to be correctly set up. I replaced bearings, changed the dogs around  etc. but could not find any obvious reason for a gear to jump out.   Upon reassembling it, it was better, but still jerking in and out of first.   But there is no gear noise (ie grating of gears).
I have checked everything on the rest of the drive system, tightened the drive belt and clutch, but the problem persists.
Does anyone have any ideas ?   Suggestions would be appreciated.

Offline Hutch

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #1 on: 14 Oct 2017 at 06:13 »
Hi Lat,

The two largest gears (pinions) - one on the main shaft and one on the secondary shaft are prevented from rotating on their respective shafts by two diametrically opposed keys (two for each gear). Maybe the first gear drive path, which is through these larger gears has failed due to some sheared keys?  The large pinion on the primary chain side of the gearbox is also used by the second gear drive path so is probably ok, therefore the keys on  large gear (aka first gear pinion in the Handbook of the Douglas Motorcycle 2 3/4HP) on the main shaft on the belt drive side would be worth looking at?

-Ian

Offline Lat Fuller

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #2 on: 18 Oct 2017 at 23:02 »
Hello Ian
Thanks for that info - it much appreciated.   
I have however stripped the gearbox (again!!) but no problem on the keyways.   I have increased the pressure on the first gear selector by adjusting the selector shaft but that does not solve the problem.
The "jerking"  is still there.   I am not sure where to go from here.
selector "dogs" are quite badly worn, but I am not sure what to do here.   I thought of dressing them up a bit with a grinder, but have not yet had the courage to try that !
I am still not convinced that this is what is causing the jerking, as there is no gear grinding or crashing happening when it jerks.
Best regards Lat

Offline ian scott

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #3 on: 19 Oct 2017 at 03:01 »
Could it be caused by clutch or belt slip?

Offline Hutch

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #4 on: 19 Oct 2017 at 04:15 »
Hi Lat,

If the keys are ok then i cannot think of any other internal gearbox issue, other than the disengaging-under-load dogs that could be causing your first gear symptoms, assuming that the selectors are operating and set up correctly. I have a 3 speed gearbox with a damaged key and keyway on the first gear pinion so thought that might have been the cause of your issue. I will have a look at a dismantled 3 speed on the weekend and see if i can work out anything else but it appears that all your accumulated wear in the dogs and selectors has got to a point where it won't stay in first??



-Ian

Offline eddie

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #5 on: 19 Oct 2017 at 07:29 »
Hi Lat,
          I am not familiar with the 3 speed 'box, but could the problem be worn or incorrect thickness spacers/thrust washers that are allowing the gears to float to the point that 2 gears are trying to engage at the same time? The fact that it gets worse as the gearbox warms up points to something loosening up and floating.

  Regards,
                Eddie.

Offline graeme

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #6 on: 19 Oct 2017 at 22:34 »
Hi Lat
I had as issue with the top gear in the 3 speed box of my 3 12/hp jerking a lot under load, and found like you have that the dogs were worn. I got them undercut by a machinist and that fixed the issue completely.
Regards,
Graeme

Offline Lat Fuller

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Re: 3 speed gearbox on my 2 3/4hp model W
« Reply #7 on: 12 Nov 2017 at 02:17 »
Thanks Graeme.   I am sure you are right - yes they are worn.   I will take the gearbox out when the biking season is over and get them refaced. Thanks for your reply    Lat