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Author Topic: Poole DMU split gears - my solution  (Read 5488 times)

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Offline DesertHound

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Poole DMU split gears - my solution
« on: December 21, 2014, 10:26:14 AM »
Good morning all

Thought I would share with you something I have been working on since yesterday. As we know, the press-fit 25 and 16 tooth gears on the armature and drive shafts of the DMU (full chassis version) are notorious for splitting and, to my knowledge, the said gears are no longer available (I'm not sure about NGS, but since the shop is closed, I know of nowhere else who has them for sale).

These 25 and 16 tooth gears are the same as the ones you will find in your bogie towers for various Poole built diesels, except for one quite important difference. They have a smaller diameter hole in the middle. This is because they are designed to be "press-fitted" onto the armature / drive shaft for a tight fit. You will immediately see that a 25 or 16 tooth gear designed for the bogie towers will not fit on the armature / drive shafts, since the whole in the middle is too big.

So, no we have a problem. With these press-fit gears having split (I'd hazard a guess at saying on nearly all DMU's) and with no replacements available, what is the solution? Having thought about this for a while, and knowing that the bogie tower 25 and 16 tooth gears were otherwise identical, I focussed my efforts on using these somehow.

After examining my guinea pig DMU I was pleased to find that the 25 tooth (larger) gears were fine, still snugly press-fitted on to their shafts. It was the two 16 tooth gears, attached to the armature, which had split. These were duly removed.

Now, the solution I came up with, even to me, sounded as though it wouldn't work. However, I had nothing to lose and so thought it must be worth a try. My solution was to take two 16 tooth (normal) gears used in the bogie towers and, due to their larger diameter hole, epoxy them on to the armature shaft.

My reservations as to this not being successful were as follows:

1) Longitudinal alignment - will the glued on 16 tooth gears align longitudinally with the 25 tooth gears?

2) How would I get the 16 tooth gear concentrically placed around the shaft (would rotation give a wonky gear)?

3) Would the epoxy hold?

Below you can see a picture of the two gears in question. These are the ones at each end of the armature.

(Sorry, but you are not allowed to access the gallery)

So, on we move to actually attaching the gears. I used two-part epoxy (Araldite) and the plan was to let it set overnight for a secure bond. the first attempt resulted in slightly too much epoxy on the shaft and this migrated to the inner white bearing. Absolutely no good, so required removing, wiping clean and starting again on that end. Second attempt, with less epoxy, was successful. I glued both gears at the same time, but it might be better to do one one night, followed by the other the next. The reason for this is that if you do need to make adjustments at one end, it is very tricky not to disturb the other end, which might otherwise be fine.

So, with epoxy applied, it was time to let it set. Whilst this was happening the rest of the chassis and mechanism had the full Rolls Royce treatment - complete strip down, clean / wash etc. Indeed it didn't really need it, since the chassis was clean and looked quite fresh. I bought this particular DMU some years ago second hand, but I suspect it was sold on after the gears split, which was most probably not long after purchase, hence I doubt it has seen much running. The brushes and clean condition of the pick-ups would also suggest this.

So, with all the chassis parts nice and clean, and the epoxy setting, there was nothing else to do until the morning. A quick check every few hours to see that the shaft was freely rotating in the bearings was all that was really needed. I didn't want to wake up in the morning to find that the bearings had bonded to the shaft - that most definitely would be the end, or at least return to square one, of this experiment.

So, this morning I dropped the armature back into the chassis and checked that it aligned with the 25 tooth gears. Hey presto! Also, twiddling the pinions with my thumbs, I was happy that there was free rotation. Still, I wasn't assured of success and I still didn't know what would happen when the chassis underframe was reattached and power was supplied. With great anticipation I proceeded. To my joy the armature turned sweetly, freely at low speed and without any evidence of gears binding, biting, or any other symptom that I thought might be likely.

It was drawing 120mA without the bogies attached, which to me is more than acceptable and what I would expect from comparable large diesel chassis (class 37/47). One thing I did notice, however, and I cannot be certain that this is related to the gear change or not, is that slight slop in one of the long pinion drive shafts (let's call it that) meant that the end of the shaft could pissibly make contact with the chassis end. The picture at the end is of the "good end" but it shows that there is a small hole at the end of the chassis block (it's actually for the screw holding on the metal bogie retainers) but it also doubles up as giving clearance for the shaft ends.

(Sorry, but you are not allowed to access the gallery)

Given this recess at the ends, I was happy that the shaft end was clear of the chassis. I oiled very lightly the shaft end anyway, just in case there was the slightest contact (although this would have ultimately meant a strip down and reassessment).

So, forward it was with the reattachment of the bogies. I still didn't know for sure if this whole thing was going to work. Perhaps the load of the bogie gears would somehow fail the project. With the bogie gears reattached, power was applied. Hey presto! She purred sweetly, almost as if new. She was now drawing between 120mA - 130mA, barely any change from bogieless, which suggested to me that the bogies were not introducing any undue load. Even I would have expected a jump of 20mA or so with bogie reattached (certainly I would on other diesels, but perhaps the gearing design on these chassis helps).

Anyway, popped on the track, and after an intial stutter, she was running smoothly. She's now being run in on the rolling road and I'm listening for any odd sounds, feeling for heat, and looking for any wobbles. So far so good!

The most important point to note, actually two, are these;

1) The 16 tooth gears had to be glued flush with the end of the armature shaft for alignment with the 25 tooth gears

2) Do not use too much glue so as it migrates to the bearings. Carry out periodic checks as the glue sets, and if you notice it has migrated, strip and wipe clean. I would say if it is going to migrate, it is going to happen early in the setting process, whilst the glue is less viscous.

Now, I know this isn't a conventional fix, and it may have its detractors. However, with the lack of availability of press-fit gears (perhaps forevermore) I needed to come up with a plan which would enable me to run these DMU's again. The adverse effects, as far as I can see, are so far none. It might just be that I was "lucky" with my alignment - I won't know until I try this on a second DMU.

The point is, as certain spares become scare, those few of us who still enjoy tinkering with Poole models will be faced with more and more such challenges. The enjoyment is coming up with a solution. So far, in this case, I think I have found one. I shall continue testing over the next month and report back. I think only then can we really start to comment on the success, or otherwise, of this project.

Oh, and for the purists, motorbike oil was used  :smiley-laughing:

Cheers and I hope this is of help to some of you.

Dan
« Last Edit: December 21, 2014, 03:07:32 PM by DesertHound, Reason: Original post quoted 12 tooth gears, when it should have been 25 tooth gears. »
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Online Bealman

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Re: Poole DMU split gears - my solution
« Reply #1 on: December 21, 2014, 10:32:34 AM »
You, sir, are a genius.

I wish you and yours all the best for Christmas!


All the best,

George
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Offline Newportnobby

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Re: Poole DMU split gears - my solution
« Reply #2 on: December 21, 2014, 10:52:16 AM »
Thanks for that, Dan :thumbsup:
I fear a mass of old Poole DMU power cars will be heading to you, though :goggleeyes:

Offline bees

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Re: Poole DMU split gears - my solution
« Reply #3 on: December 21, 2014, 11:13:46 AM »
Hi Dan,

Obvious and brilliant, thanks for sharing, i might be having a little tinker over the next few days!

Thanks
Steven

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Offline PostModN66

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Re: Poole DMU split gears - my solution
« Reply #4 on: December 21, 2014, 11:33:19 AM »
slight slop in one of the long pinion drive shafts (let's call it that) meant that the end of the shaft could pissibly make contact with the chassis end.


That is the worst type of contact!!!  :confused1:

Looks like a successful mod; well done Dan.   For anyone considering this there is also another method which was invented by Dr Al and pinched by me...

http://www.ngaugeforum.co.uk/SMFN/index.php?topic=16759.msg166177#msg166177

Somewhat more drastic but quite dooable,

Cheers  Jon  :)
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Offline Dr Al

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Re: Poole DMU split gears - my solution
« Reply #5 on: December 21, 2014, 12:11:17 PM »
Thought I would share with you something I have been working on since yesterday. As we know, the press-fit 12 and 16 tooth gears on the armature and drive shafts of the DMU

12 tooth? There are no 12 tooth gears in the chassis - they are 16 tooth on the armature and 25 tooth on the worm shafts....

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

“We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces.” – Dr. Carl Sagan

Offline Dr Al

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Re: Poole DMU split gears - my solution
« Reply #6 on: December 21, 2014, 12:17:45 PM »
Looks like a successful mod; well done Dan.   For anyone considering this there is also another method which was invented by Dr Al and pinched by me...

http://www.ngaugeforum.co.uk/SMFN/index.php?topic=16759.msg166177#msg166177


Thanks - but not really invented my me - was done by Richard Deas a long time before that (he remotored and Poole Farish model in this way, including 20s, 47s etc). I just used a different motor which is the only original thing about mine.

I didn't do very many of these - only a couple as an experiment. The running was comparable to an un-split-geared original model, but the noise was spectacularly cut down which made them seem much much smoother. Now that gears are unobtainable from BR Lines (Lytchett Manor models had told me they have some in production, but that was a fair bit back and I haven't heard any more), this is a viable alternative. It's not too drastic as only needs a small piece cut from the chassis, a motor (~£10) and a couple of bits of drive spring.

Personally though, I don't do this now as I've sold on all my old Poole versions and replaced with the vastly superior new-tool model.

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

“We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces.” – Dr. Carl Sagan

Offline Dr Al

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Re: Poole DMU split gears - my solution
« Reply #7 on: December 21, 2014, 12:24:00 PM »
One thing I did notice, however, and I cannot be certain that this is related to the gear change or not, is that slight slop in one of the long pinion drive shafts (let's call it that) meant that the end of the shaft could pissibly make contact with the chassis end. The picture at the end is of the "good end" but it shows that there is a small hole at the end of the chassis block (it's actually for the screw holding on the metal bogie retainers) but it also doubles up as giving clearance for the shaft ends.

There is little friction produced by this (shaft diameter is small where it contacts the inner chassis, so frictional torque is next to nothing), and it's normal.

Simple fix would be to centre up by sliding the worm further onto the shaft. If it bothers you try that, but I doubt it'll actually result in an improvement in performance - not sure about the noise though, might reduce that....?

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

“We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces.” – Dr. Carl Sagan

Offline Dr Al

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Re: Poole DMU split gears - my solution
« Reply #8 on: December 21, 2014, 12:30:16 PM »
For completeness, another solution (and one I use now if I need to repair these) is to open up the central hole in the gear and use a small piece of heatshrink tubing to create a boss for the gear to fit on. Again, not original, but very successful.

This has the advantage of:
- being slightly flexible, so any shaft diameter expansion with heat and running is partly absorbed by the boss rather than the gear (so less stress in the gear)
- no glue is used meaning gears can be more easily adjusted to allow perfect commutator centering, and aren't permanently bonded to the shaft
- you can use thicker profile gears too, to further reduce any future possibility of failure.

Hopefully the easiest solution (replacement gears from Lytchett) comes to fruition soon though. I suspect too that BR Lines do still have stocks if you get them to do the work though - I think they are being retained for their own repairs, so another option if you don't want to tinker.

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

“We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces.” – Dr. Carl Sagan

Offline DesertHound

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Re: Poole DMU split gears - my solution
« Reply #9 on: December 21, 2014, 01:12:41 PM »
Thought I would share with you something I have been working on since yesterday. As we know, the press-fit 12 and 16 tooth gears on the armature and drive shafts of the DMU

12 tooth? There are no 12 tooth gears in the chassis - they are 16 tooth on the armature and 25 tooth on the worm shafts....

Cheers,
Alan

Duly noted - thanks. I meant to say 25 tooth gears. I'm not home at present, will amend the opening post when back.

Cheers
Visit www.thefarishshed.com for all things Poole Farish and have the confidence to look under the bonnet of your locos!

Offline DesertHound

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Re: Poole DMU split gears - my solution
« Reply #10 on: December 21, 2014, 03:08:18 PM »
Opening post reference to 12 tooth gears now amended to read 25 tooth gears.

Thanks
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Offline DesertHound

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Re: Poole DMU split gears - my solution
« Reply #11 on: December 23, 2014, 01:09:58 AM »
You, sir, are a genius.

I wish you and yours all the best for Christmas!


All the best,

George

Thank you George. Same to you and yours! I trust you'll be celebrating 7hrs ahead if me, and 11 hrs ahead of the rest of the community  ;D
Visit www.thefarishshed.com for all things Poole Farish and have the confidence to look under the bonnet of your locos!

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Re: Poole DMU split gears - my solution
« Reply #12 on: December 23, 2014, 01:59:04 AM »
Yep, Santa calls at NZ first then Australia.  ;D
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Offline DesertHound

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Re: Poole DMU split gears - my solution
« Reply #13 on: December 23, 2014, 09:11:06 AM »
For completeness of the thread, I did speak with Bob at BR Lines and he does have a mod to repair the DMU's.

Just thought I'd mention that, as it might be useful to some as reference. I recognise that my method might not appeal to all and Bob does provide an alternative for those who don't wish to get "sticky fingers".

Dan
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Offline Dr Al

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Re: Poole DMU split gears - my solution
« Reply #14 on: December 23, 2014, 11:46:08 AM »
For completeness of the thread, I did speak with Bob at BR Lines and he does have a mod to repair the DMU's.

What is the mod?

Cheers,
Alan
Quote from: Roy L S
If Dr Al is online he may be able to provide a more comprehensive answer.

“We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces.” – Dr. Carl Sagan

 

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