Backwoods Miniatures D&HR C Class Pacific - Electrical Short Problem

[Phil Court]

I have been building a Backwoods Miniatures D&HR C Class Pacific that I purchased from the OO9 Societies Sales many years ago. It took some time to summon up the courage and I started 12 months ago. Taking my time and planning each stage in advance with a good recovery period between stages. All went well and a few changes to improve the model (Easy disassembly, miniature plug &sockets for power feed, power pickups in the tender and a CLAG drive shaft www.clag.org.uk/silicone-tube.html ) were made.
Eventually prior to  painting I was ready to start running the motor in having checked and tested smooth running at every stage of the construction. All went well for several hours of the big "run in" and then disaster struck.

Fortunately the loco had been built so some level of disassembly of the main components smoke box, boiler, firebox, cab and chassis could all be easily separated from each other for painting and maintenance.

The motor suddenly started operating intermittently. After investigation and isolating any potential causes I discovered the problem. Motor functioned ok and the coupling rods and valve gear were freely moving. The problem was a partial short between the live wheel and chassis. This was finally traced to a 7ohm short between one axel and metal wheel. It was getting dark at the time and I noticed (and heard) arcing on the axel & wheel. The area was cleaned with a small brush and IPA then given a delicate probe with a fine pin. The short was reduced to 50ohms. Reapplying power did not "blow" the short away and there was arcing with smoke and short reverted to 7ohms.

To replace the wheel/axel will be a b****r of a task as the drive wheels are soldered/glued into the frame and surrounded by the coupling rods.
This now leaves me in a difficult position on how to proceed so I have a number of questions that if any member can provide any answer I would be great full.

The wheel details are
Wheel Diameter    9mm
Flange Diameter   10.6mm
Thickness      2.6mm
Flange Thickness     0.6mm
Axel Diameter      2mm
Axel Length      18.5mm (flat ends)
Wheels insulated on one side of axel
Pinion Gear (20T) offset by aprox. 1mm from non insulated wheel
Shape is plain dimpled


Is shorting of the insulation on a wheel/axel a common problem (ie breakdown of plastic)?

Does anyone know the make/type of wheel used in the Backwoods kit?

Who if anyone sources the wheels or wheel and axel (they were supplied as a pair of wheels and pinion gear preassembled on the axel in the kit)?

Any other advice or suggestions on how to resolve the issue would be appreciated.

[JulienW]

I'm having a little difficulty grasping the concept of a short - the passing of current where it isn't needed - expressed in units I associate with electrical resistance so I apologise if I've got the wrong end of the stick.

If by 'live wheel' you mean one that is solid metal and electrically bonded to the axle, then it will also be live to the chassis though you may get a small amount of arcing between the axle and its bearing in the frame as that's where the current should be flowing to get to the motor, and if the bearing is too sloppy a fit then you'll be getting an air gap and arcing.

Alternatively if by 'live wheel' you mean the ones that need pick ups because they have a plastic insulating bush at the middle then they should not be live to the axle. It's possible that in soldering the cranks onto the axle (if that's how you'd fixed them) the bush has been softened by the heat and is now allowing the wheel and axle to make a circuit, but I'd have thought that such a deformation would make the wheel wobbly to the point where the thing wouldn't have run at all, let alone for 'several hours'. It's possible the bush may have degraded but the materials used are generally chosen to be stable, and again I would expect to see mechanical issues (wheel wobble) before electrical ones.

I wonder if the issue is not the wheel arcing direct to the axle but to the frame, by rubbing on the inside of the frame. This could be fixed by slipping a thin plastic washer between the wheel face and the frame, but suggests the wheel has shifted on the axle to check the back to back measurements and maybe used a bit of Loctite 603 or other gear retaining compound carefully applied between the wheel and axle (from the back of the wheel) to keep it in place.

The other thought is that a pick up that has shifted slightly and is somehow touching both the wheel and the chassis. Again this could be directly or via the copperclad you may have used as a buss bar for the pickups - this could be something as insignificant as a whisker of copper from cutting/gapping the material or maybe not enough clearance between the live copper cladding and any screws used to hold it in place.

If it really is that the insulator bush had broken down in some way (personally unless it has been heated I think it unlikely) then I'd have thought running some adhesive in to fill the gaps and restore the bush's insulating properties (superglue? epoxy?) would be better than using an electrical cleaner like Isopropyl alcohol, as I'd have thought that would only improve current flow where you don't want it.

Hopefully that might give you a few pointers, but it's odd that from something that ran fine you now have issues, so the key is to think about what might have changed...

Finally it's worth mentioning to check that not only have the wheels on the loco been assembled correctly (with insulation bushes on the same side) but also those on the tender. This is especially the case if using the 'American' system where the loco is live to one side and the tender is live to the other, as I've acquired more than one US N-scale loco as a non runner where it turned out this was because one of the bogies on the tender had gotten turned around.  But that usually manifests as a dead short, not a partial/intermittent one.

[Matthew Mawson]

On the subject of the wheels as far as I know they are Sharman wheel which are now managed by Phoenix although it seems not all are still produced due to ageing tooling:
https://www.phoenix-paints.co.uk/products/4mm-products/sharman-wheels

[B.Meldon]

Railzip, or using graphite on the track and/or wheel contacts can cause this type of issue.

You have said you have cleaned the wheels and this helped.
Perhaps cleaning them again using a de-greaser of some type worked in with a brush may help?

[Graeme Dunbar]

7 ohms seems quite low for build up but I guess it could be over a large area.

Probably barking up the wrong tree entirely, but it is what should be an insulated part that is going short and not what should be a conducting part that has started to go open circuit?

It was the cleaning and improvement followed by reversion to lower resistance that got me thinking. If contamination has been washed away it is unlikely to build up again so quickly. Perhaps there was a conducting whisker that got displaced during cleaning and it reseated itself to form a lower resistance again?

Also, things like dry solder joints can be most unpredictable.

[Phil Court]

PROBLEM SOLVED
Thanks for all your suggestions all of which helped me consider optional courses of action to pursue.

I decided that if I had to replace the wheel then a strip down of the chassis would be a major operation.

The wheel photo shows the place where the short was between the axel and wheel after I had eased the wheel inwards (this is an insulated wheel)
Arcing and smoke originated from here when the loco was on the powered track.

Firstly I gave it all another good clean with a degreaser then with isopropyl  alcohol (IPA)
Retesting gave the same results as before.

If I had to strip it down then the wheels would have to be moved on the shaft so there was nothing to be lost moving the wheels in situe. 

Having eased the wheel inwards I noticed a small speck on the shaft.
(This was only possible to see with an Optvisor x2.5 and Optiloupe -an excellent piece of kit)
At first I thought it was corrosion from the arcing or just a swarf blob.
The lump was small and hard. About 2mm long (see photo)

But on investigation it was solid and now no partial short was measured now between axle and wheel.

So at some point in time after starting the running in swarf or fash must have been dislodged then jammed between wheel and axel that was not removed by cleaning.
All your comments were gratefully received and prompted me to re investigate in more depth.

The lesson has been learnt that keeping the parts clean and free from unwanted particles at all times is a must. Also when running in give the movement a deep clean regular intervals to wash away the crud.

Thankyou all for your help

[Graeme Dunbar]

It's good to hear you have it sorted. It would be a shame to have to have done major surgery to fix it.

I wonder it it could have been a splash of solder that got dislodged and fell into the gap?