N Class running badly - why?

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muggins
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N Class running badly - why?

Postby muggins » Tue Dec 01, 2020 3:41 pm

I've got myself a new and unused Bachmann 32-163 N Class which has apparently been sitting quietly in somebody's collection for the last 17 years, and it has me puzzled. Upon trying it out, I found that it was happy enough (considering) running backwards, but it ran forwards like it has at least one flat tyre. Suspecting that the first thing to check after the obvious quartering and motion-fouling was hardened grease, I opened it up and found that the grease is perfectly OK.

Everything else also seemed to be in order, except that I spotted what looks an awful lot like a defective tooth on the last small nylon pinion in the drive i.e. the one that mates with the large (final drive) one which is attached to the axle. Nothing else seemed to be amiss, so I put it all back together and tested. It's now worse than it was!

I've done a video of it running in its present state which is here https://youtu.be/CazRai13siI. If you'd care to take a squint at that with the volume turned up full so you can best appreciate the sound effects and then tell me what you recklon the problem is, I'd be very grateful.

Bigmet
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Re: N Class running badly - why?

Postby Bigmet » Tue Dec 01, 2020 4:21 pm

When running forward I counted 26 clicks in nearly 9 revolutions of the driving wheels - so probably there would have been 27 clicks if a full 9 revs were observed - a 3:1 ratio. That's not easy to generate on the outside rods as the quartering generates 2 (or 4) per revolution.

I would suggest it's gear tooth count time. If the smaller gear with the tooth damage you observed has a third the teeth of the axle gear count, then it's almost certainly that damaged tooth you have seen causing the row. (The gear trains 'throw over' because the worm applies a lateral force which changes in direction on reversing, thus more noise one way than the other.) If the tooth count ratio isn't 3:1, then it's think again time...

The good news is that the mechanism runs. So if you confirm the fault location, then correcting the deformed tooth cleanly slicing off only the damaged piece that is protruding from the tooth profile - look at it under 10x magnification with a good light source while attempting this - may do the trick.

muggins
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Re: N Class running badly - why?

Postby muggins » Tue Dec 01, 2020 6:07 pm

Cheers Bigmet. I made it about 3:1 too, so I reckon the finger points ...

What odds on Bachmann still having any spares for a 13-year-old chassis? I might even phone 'em tomorrow and try my luck. Quite how user-replaceable that pair of gears might be is of course another matter, but I've always favoured the principle that if human hands put it together, more of the same should be able to take it apart and then repeat the process ...

Bigmet
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Re: N Class running badly - why?

Postby Bigmet » Tue Dec 01, 2020 9:01 pm

Worth trying Bachmann, consistently helpful.

The gear trains have always been easy enough to disassemble in my experience to date, though I haven't tackled this particular model to prove it applies to this one... You are quite right, these are just assembled kits.

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Mountain
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Re: N Class running badly - why?

Postby Mountain » Wed Dec 02, 2020 2:13 am

Just a thought. Were those some of the locos that were effected with mazak rot issues?
That noise is loud. Louder then I expect a clicking gear tooth to sound so it is odd. I have heard loud sounds like that when the wheel quartering is out but usually the loco will display this in both directions and also it will be travelling in noticeable jumps for the noise to be that loud.
But I have once heard a noticeable sound like that in just the one direction on a loco that had its Walschaerts valve gear rubbing, in that one rod was colliding with another rod which on one direction the rods would slide past each other, but in the other direction they would catch, so check the locos rods. Try and look from above or below as the wheels are moving as looking from the side and it was not noticeable. You may need to turn the loco upsidedown and apply power to the wheels to visually see this, to check to see if any of the wheel rods are colliding with each other.

Bigmet
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Re: N Class running badly - why?

Postby Bigmet » Wed Dec 02, 2020 8:48 am

Mountain wrote:Just a thought. Were those some of the locos that were effected with mazak rot issues? ...

Only the first production run, made well before this particular model, and the affected part was the loco footplate which visibly deforms before fracturing. That can safely be ignored in this case, which is clearly a mechanism problem.

Mountain wrote:...That noise is loud. Louder then I expect a clicking gear tooth to sound so it is odd. I have heard loud sounds like that when the wheel quartering is out but usually the loco will display this in both directions and also it will be travelling in noticeable jumps for the noise to be that loud. But I have once heard a noticeable sound like that in just the one direction on a loco that had its Walschaerts valve gear rubbing, in that one rod was colliding with another rod which on one direction the rods would slide past each other, but in the other direction they would catch, so check the locos rods ...


It is loud, and louder than I would expect a rod catch to be. If a rod were catching and making that much noise I would expect to see movement disturbance too, and I don't see any. (Also a rod catch is typically once per wheel revolution, second possibility twice, because of the quartering. Three evenly spaced per wheel revolution as seen here is very unlikely from the rods.)

If it isn't the gears, then something else will have to be looked for.

Seen an effect like this from a pick up wiper out of position, and thus catching on wheel back features such as depressions in the casting. Similarly detail parts on the chassis fouling on the wheel backs. But these tend to act in both directions, and here the effect is on forward movement only: for which there is a good reason in the gear train, because of the effect of the worm slightly moving the gear train laterally to alternate positions between forward and reverse.

muggins
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Re: N Class running badly - why?

Postby muggins » Wed Dec 02, 2020 8:52 am

Job currently on hold pending email reply from Bachmann. I'll report back in due course ...

muggins
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Re: N Class running badly - why?

Postby muggins » Wed Dec 02, 2020 9:37 am

No sooner said than done. All being well (which I have every confidence that it will be), one new motor cradle with gear will shortly be on its way to me from Barwell, and this for just six quid! I'm well impressed :)

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Mountain
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Re: N Class running badly - why?

Postby Mountain » Wed Dec 02, 2020 11:25 am

Wow. Bachmann are good.

muggins
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Re: N Class running badly - why?

Postby muggins » Sat Dec 05, 2020 2:18 pm

By way of an update, the new motor cradle and gear arrived lunchtime today and I've just spent an enjoyable (and very interesting) hour installing it. My lovely brand new 17-year-old N Class is now behaving itself very nicely indeed trundling round the tracks on its running-in turn.

Having cleared the grease off the gears that I replaced I can now see that 90% of one tooth on the smallest gear is indeed missing. In fact, drawing on my injection moulding experience, I can say with some certainty that it was never all there in the first place. It shows all the characteristic signs of overheated crud in the cavity, so a pound to a penny says that the dud gear was off one of the first couple of shots after a stoppage. Just one of those things ...

Bigmet
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Re: N Class running badly - why?

Postby Bigmet » Sat Dec 05, 2020 4:22 pm

Brisk service as well as the very reasonable price from Bachmann. Good to read that a satisfactory result has been obtained.

I shall know in future where to apply should any duff moulded mechanism parts need attribution of cause of failure ... I managed to duck out of learning about injection moulding, by instead being assigned to clear up a 'little local difficulty', caused by a marketing group having contracted to deliver to a very major client a product which fouled a law of physics.


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