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Insulation of crankpins


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Hello

 

I have decided to turn my own locomotive wheels for my current construction project. I have never done this before and I have taken advice from several people including Mark Wood and Raymond Walley who have both been extremely helpful. In order to insulate the driving wheels from the chassis it is necessary to insert tufnol into the axle holes, drill out to leave 1/2mm of tufnol into which the axle is pressed. With both sides insulated like this, I anticipated no further problems. However, all the driving wheels will be live  on both sides which means that the crankpins will be live. It follows that once the motion is fitted, there will be a short circuit from the crankpins on one side, through the coupling and connecting rods to the cylinder, through the frame and out through the opposite side wheels.

 

I'm building a 7mm Class 4MT which is not a 'conventional' locomotive in that it doesn't have a chassis with running plate on which the boiler and cab sits. It is no easy matter to separate parts of the build so that the loco is electrically sound. Maybe I could insulate the cylinders from the frames, but there is also the motion brackets.....maybe I need to insulate those too somehow.

 

It would appear therefore, that the only solution is to insulate the crankpins. This is relatively simple to do....Mark Wood suggests a top hat bush.......perhaps an epoxy glue would do it. Despite this, it just sort of complicates matters somewhat as I want to be sure that even if I do this, I maintain electrical integrity, especially as I will have an expensive decoder installed too!

 

I can't find any information on insulation crankpins so it would be very useful to hear from anyone with experience of this (potential!) problem.

 

Thanks

 

Ray

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It's common enough in 7mm, Alan Harris wheels usually had insulated crankpins - the wheels had a tufnol plug for the crankpin on one side. Drill the crankpin holes oversize, press in a tufnol bush and press in the crankpin, if the bush isn't top hat shaped add a fibre washer before fitting the rods.

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It's what Hornby Dublo did when they changed the design of their outside cylinder 3 rail locos to suit 2 rail in the 1950s.   They used top hat bushes into which they pushed the crankpins and return cranks.  One side pressed into the mazak wheel and the other into a small top hat bush.  Both return cranks were splined, it was OK on the insulated side but its a one hit only pressed into mazak so insulated bushes both sides is probably preferable.  I have insulated the insulated side cylinders on some Hornby Dublo 3 to 2 rail conversions so I can just insulate the axles, but its a fiddle  Actually insulating all the valve gear from the wheels makes a lot of sense.   Keeping the rods away from the wheel tyres can be a problem, probably less so in 0 gauge.

Having pickups both sides helps a loco run central down the track but make sure you use a substantial pickup as the small springs used by Airfix in their 14XX plunger pick ups which are otherwise ideal tend to shrivel up and die if shorted out.  I have bits of 5 of these things and can't even make up 1 set of 6 good pickups as almost all the springs have failed.

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15 hours ago, Michael Edge said:

It's common enough in 7mm, Alan Harris wheels usually had insulated crankpins - the wheels had a tufnol plug for the crankpin on one side. Drill the crankpin holes oversize, press in a tufnol bush and press in the crankpin, if the bush isn't top hat shaped add a fibre washer before fitting the rods.

 

Michael

 

Thanks very much. This is what I thought it would have to be..... and I had realised that a fibre washer would be needed if the bushes are to be simple parallel inserts. I want to keep the coupling rods as close to the wheel as is practically possible as even in this 7mm kit, clearances are tight. I will need to experiment to determine how thin the base of the top hat can be cut without tearing  or breaking it off, or what sort of minimum thickness of washers are available.

 

Tufnol seems to be the preferred material for this but do you (or anyone else out there) have any experience of using Delrin for such purposes?

 

Ray

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15 hours ago, DavidCBroad said:

It's what Hornby Dublo did when they changed the design of their outside cylinder 3 rail locos to suit 2 rail in the 1950s.   They used top hat bushes into which they pushed the crankpins and return cranks.  One side pressed into the mazak wheel and the other into a small top hat bush.  Both return cranks were splined, it was OK on the insulated side but its a one hit only pressed into mazak so insulated bushes both sides is probably preferable.  I have insulated the insulated side cylinders on some Hornby Dublo 3 to 2 rail conversions so I can just insulate the axles, but its a fiddle  Actually insulating all the valve gear from the wheels makes a lot of sense.   Keeping the rods away from the wheel tyres can be a problem, probably less so in 0 gauge.

Having pickups both sides helps a loco run central down the track but make sure you use a substantial pickup as the small springs used by Airfix in their 14XX plunger pick ups which are otherwise ideal tend to shrivel up and die if shorted out.  I have bits of 5 of these things and can't even make up 1 set of 6 good pickups as almost all the springs have failed.

 

David

 

Thanks for your comments.

 

Your experience with the pick up springs is interesting. I'm not sure how or why your  springs have burned out as the path of a short circuit would, I assume, travel through the wheels, motion and frames as that would be the path of least resistance. Any short through the pick up springs would have to travel through the motor/DCC decoder too wouldn't it? In your case, the springs seem to have acted in the same way as fuse wire does....which is rather worrying as you would hope that the safety devices on controllers would be designed clear a short circuit fault much quicker than such a 'fuse'.  (Although MCBs in domestic installations are generally much slower than fuses when subject to short circuit currents).

 

My current thought is to use Slaters pick ups. They too have small springs which I shall stretch, then cut in half. This provides enough pressure on the plunger to ensure good contact, but doesn't add too much friction. Hopefully, I get things insulated properly and I avoid the pitfalls you have experienced.

 

Ray

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4 hours ago, Hull Paragon said:

 

David

 

Thanks for your comments.

 

Your experience with the pick up springs is interesting. I'm not sure how or why your  springs have burned out as the path of a short circuit would, I assume, travel through the wheels, motion and frames as that would be the path of least resistance. Any short through the pick up springs would have to travel through the motor/DCC decoder too wouldn't it? In your case, the springs seem to have acted in the same way as fuse wire does....which is rather worrying as you would hope that the safety devices on controllers would be designed clear a short circuit fault much quicker than such a 'fuse'.  (Although MCBs in domestic installations are generally much slower than fuses when subject to short circuit currents).

 

My current thought is to use Slaters pick ups. They too have small springs which I shall stretch, then cut in half. This provides enough pressure on the plunger to ensure good contact, but doesn't add too much friction. Hopefully, I get things insulated properly and I avoid the pitfalls you have experienced.

 

Ray

Hi Ray.

The damage occurs when you run into the frog end of a wrongly set live frog point.  the short is left front wheel, to frame, to left rear wheel across the insulated joiner. It only needs to happen once to take the temper out of the springs.  Modern overload cut outs with Polyswitches will not usually cut out quickly enough to prevent this damage, and if they do they reset and try again and again. the good Hornby Dublo went click at 1 amp buzzed and lit a red light end of but those old cut outs are far too expensive to be used these days.

Edited by DavidCBroad
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17 hours ago, Happy Hippo said:

Why not use Slaters wheels, which have plastic centres and steel rims?

 

If you must have cast iron wheels, then have you considered rim insulation instead?

 

 

 

HH

 

I don't like them! I had several problems with the plastic centres moving, and with concentricity.......and I think metal wheels look better.

 

Ray

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17 hours ago, DavidCBroad said:

Hi Ray.

The damage occurs when you run into the frog end of a wrongly set live frog point.  the short is left front wheel, to frame, to left rear wheel across the insulated joiner. It only needs to happen once to take the temper out of the springs.  Modern overload cut outs with Polyswitches will not usually cut out quickly enough to prevent this damage, and if they do they reset and try again and again. the good Hornby Dublo went click at 1 amp buzzed and lit a red light end of but those old cut outs are far too expensive to be used these days.

 

David

 

Thanks again. I hadn't considered the short at that point!

 

Ray

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19 minutes ago, Hull Paragon said:

 

HH

 

I don't like them! I had several problems with the plastic centres moving, and with concentricity.......and I think metal wheels look better.

 

Ray

That's a fair point.

 

Any reason why you have dismissed rim insulation, which is probably an easier machining operation on the lathe than having to make and fit  very small insulated bushes for the crankpins?

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1 hour ago, Happy Hippo said:

That's a fair point.

 

Any reason why you have dismissed rim insulation, which is probably an easier machining operation on the lathe than having to make and fit  very small insulated bushes for the crankpins?

Unless its a matter of pride in making everything yourself there are a wide variety of top hat axle bushes available from RTR stock which would only mean turning the crankpin to the size of the RTR axle and drilling the wheel to the same size as the RTR wheel.  I would suggest that the old Hornby Dublo / Wrenn 00 loco axle bush is a nice size for an 0 gauge crankpin.  The current Bachmann and Hornby may also be good while UK sourced Triang / Hornby will I suggest be be too big.  I would suggest a splined H/D Wrenn 00 driving wheel axle  could make a good 0 gauge driving axle crankpin if you machined the return crank to fit it and shortened the axle to suit and used the H/D Wrenn bush..

Edited by DavidCBroad
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3 hours ago, Hull Paragon said:

 

HH

 

I don't like them! I had several problems with the plastic centres moving, and with concentricity.......and I think metal wheels look better.

 

Ray

How long ago was that? Current Slater's production is always concentric and it's hard to imagine the rigid plastic centre moving much. A long time ago there was a very bad batch of axles which caused all sorts of problems with Slater's wheels but long since dealt with.

I do agree that cast iron wheels are better in many respects though, personally I insulate them by cutting and aralditing the spokes, in my experience insulating bushes at the wheel centre are far more likely to cause trouble.

Cutting the rim to insulate is possible but far from an easy lathe operation.

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22 hours ago, Happy Hippo said:

That's a fair point.

 

Any reason why you have dismissed rim insulation, which is probably an easier machining operation on the lathe than having to make and fit  very small insulated bushes for the crankpins?

 

I haven't dismissed anything. I am loathe to rim insulate though because a slight error could result in too small a surface for the pick-ups.....and I am unsure of the strength of epoxy if/when I turned a groove on the second side. You have to remove all the metal to reach the epoxy and without jigs to take the stress there could be problems. On top of that, I don't have an adequte tool.

 

Turning a bush takes 5 minutes.

 

Ray

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21 hours ago, DavidCBroad said:

Unless its a matter of pride in making everything yourself there are a wide variety of top hat axle bushes available from RTR stock which would only mean turning the crankpin to the size of the RTR axle and drilling the wheel to the same size as the RTR wheel.  I would suggest that the old Hornby Dublo / Wrenn 00 loco axle bush is a nice size for an 0 gauge crankpin.  The current Bachmann and Hornby may also be good while UK sourced Triang / Hornby will I suggest be be too big.  I would suggest a splined H/D Wrenn 00 driving wheel axle  could make a good 0 gauge driving axle crankpin if you machined the return crank to fit it and shortened the axle to suit and used the H/D Wrenn bush..

 

David

 

Many thanks for all of this....of which I was not aware!

 

Ray

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19 hours ago, Michael Edge said:

How long ago was that? Current Slater's production is always concentric and it's hard to imagine the rigid plastic centre moving much. A long time ago there was a very bad batch of axles which caused all sorts of problems with Slater's wheels but long since dealt with.

I do agree that cast iron wheels are better in many respects though, personally I insulate them by cutting and aralditing the spokes, in my experience insulating bushes at the wheel centre are far more likely to cause trouble.

Cutting the rim to insulate is possible but far from an easy lathe operation.

 

Michael

 

It is an interesting point when challenged on one's beliefs or prejudices. Despite Slaters status in the hobby, I have written them off because of past experience. The fact that they may have improved does not lessen my (irrational?) bias against them. I suppose it's a bit like a car...once you've had a problematic Ford, you never have another?

 

I should add that I have had concentricity problems with Walsall wheels too but because they are metal, I was highly confident that skimming them on a lathe would be safe. The bending force on plastic spokes when machining at the tyre surface would probably be excessive if the tool grabbed or bit into the tyre. Again, I know that with very tiny cuts (and a perfectly sharpened tool) the exercise would probably be safe......but I can't shake the prejudicial thoughts no matter how easy it might make my life.

 

I have commented above on the question of rim cutting. I assume that JPL have jigs to do it to prevent damage or distortion.........maybe they use a miller with a fine cutter. Whatever, it is unlikely that I will insulate at the rim.

 

Ray

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3 hours ago, Hull Paragon said:

 

Michael

 

It is an interesting point when challenged on one's beliefs or prejudices. Despite Slaters status in the hobby, I have written them off because of past experience. The fact that they may have improved does not lessen my (irrational?) bias against them. I suppose it's a bit like a car...once you've had a problematic Ford, you never have another?

 

I should add that I have had concentricity problems with Walsall wheels too but because they are metal, I was highly confident that skimming them on a lathe would be safe. The bending force on plastic spokes when machining at the tyre surface would probably be excessive if the tool grabbed or bit into the tyre. Again, I know that with very tiny cuts (and a perfectly sharpened tool) the exercise would probably be safe......but I can't shake the prejudicial thoughts no matter how easy it might make my life.

 

I have commented above on the question of rim cutting. I assume that JPL have jigs to do it to prevent damage or distortion.........maybe they use a miller with a fine cutter. Whatever, it is unlikely that I will insulate at the rim.

 

Ray

 

How about buying a set of Slaters Wheels, assemble each one on an axle in turn and then hold the axle in a lathe chuck (collet chuck if you have one).  Revolve by hand whilst at the same time measure any run-out using a dial gauge.  Should put your mind at rest; if they're not up to scratch then return to vendor for a refund.  Be realistic though - expecting less than half a thou at the tyre may be asking at bit much....  :)

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I've used hundreds of Slater's wheels with no problems for many years, I've also re-machined them to S7 profile (with a profile tool) with no difficulty. The axle problem was more of a fault in the machining of the square ends resulting in quartering difficulties but as I said earlier it's been fixed many years ago.

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20 hours ago, polybear said:

 

How about buying a set of Slaters Wheels, assemble each one on an axle in turn and then hold the axle in a lathe chuck (collet chuck if you have one).  Revolve by hand whilst at the same time measure any run-out using a dial gauge.  Should put your mind at rest; if they're not up to scratch then return to vendor for a refund.  Be realistic though - expecting less than half a thou at the tyre may be asking at bit much....  :)

 

I could do that but I am now committed to using MW wheels. It's been an interesting dialogue and all comments have validity, but in the end my original query was about insulating crankpins and having enjoyed all the comments, I would prefer to drift back to the subject.

 

Thanks

 

Ray

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14 hours ago, Michael Edge said:

I've used hundreds of Slater's wheels with no problems for many years, I've also re-machined them to S7 profile (with a profile tool) with no difficulty. The axle problem was more of a fault in the machining of the square ends resulting in quartering difficulties but as I said earlier it's been fixed many years ago.

 

Thanks again Michael.

 

Regardless, I am not using Slaters again.

 

Ray

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Having said I want to get back to the crankpins, I realised that I have some pictures of a J71 that I built fitted with diferent wheels. In order they are Slaters, Walsall and Original. It's a personal choice as to which you prefer.

 

One final comment on the choice of wheels, regardless of quality issues and/or personal bias, Slaters don't do a V Rim profile driver......which is quite apparent on Ivatt locos.

 

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Ray

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  • 3 months later...
On 28/02/2019 at 12:37, Michael Edge said:

How long ago was that? Current Slater's production is always concentric and it's hard to imagine the rigid plastic centre moving much. A long time ago there was a very bad batch of axles which caused all sorts of problems with Slater's wheels but long since dealt with.

I do agree that cast iron wheels are better in many respects though, personally I insulate them by cutting and aralditing the spokes, in my experience insulating bushes at the wheel centre are far more likely to cause trouble.

Cutting the rim to insulate is possible but far from an easy lathe operation.

 

Michael

 

After some considerable thought, I've concluded that cutting and aralditing the spokes will be my preferred solution.

 

If this post reaches you would you please tell me what your method is......tools used; width of slit in spoke (I reckon 0.5mm will be about right); type of araldite etc?

 

Thanks

 

Ray

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I cut through half of the spokes with a piercing saw - size 0 (less than .5mm - about .35 or so), clean off any raggy bits which may have got in the gap and fill with ordinary (slow setting) araldite. Put the wheels on a steel plate in the oven (about gas mark 2) for an hour or so. When they have cooled down I file the araldite smooth before cutting the other half of the spokes and repeat the process. To save time with the piercing saw I usually (depending on the number of spokes) cut them in blocks of three or so, cleaning up half way through helps me to see which ones I have already done. I also test them electrically after cutting the second set of spokes and at the end of the process, this is done with the aid of my test track power supply which has a very slow acting cut out. Connect the power to the rim and the centre with crocodile clips and switch on, if there is a small short (usually from iron dust in the araldite) it burns out quite spectacularly. Sometimes a bit of filler is needed where the araldite has shrunk or moved in the gap but this method is extremely strong, I've never had a mechanical failure in more than 40 years of doing this.

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Have tried some of the foregoing methods, but now will not do anything other than turn and fit separate tyres.  Tyre insulation has mostly been thin rings turned from PVC (more easily turned to exacting dimensions than Delrin), and secured with super glue. 

Two locos have ordinary notepaper for rim insulation (0.004" or 0.1mm thickness), soaked in Araldite , with the tyres wrung on. 

Another set used a variation of Michael's method above, but by cutting around the thickness of the rim instead of the spokes, filling one half length at a time with warmed up Araldite - to make it run and fill the groove fully.  These wheels were from my own nickle-silver lost wax castings, which had a small slot cast in the rim for starting the piercing saw blade.

(Slaters wheels have the plastic centres moulded into the rims, which are grooved internally - so don't see how those tyres could come loose.) 

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  • 4 weeks later...
On 04/06/2019 at 09:28, brianCAD said:

Have tried some of the foregoing methods, but now will not do anything other than turn and fit separate tyres.  Tyre insulation has mostly been thin rings turned from PVC (more easily turned to exacting dimensions than Delrin), and secured with super glue. 

Two locos have ordinary notepaper for rim insulation (0.004" or 0.1mm thickness), soaked in Araldite , with the tyres wrung on. 

Another set used a variation of Michael's method above, but by cutting around the thickness of the rim instead of the spokes, filling one half length at a time with warmed up Araldite - to make it run and fill the groove fully.  These wheels were from my own nickle-silver lost wax castings, which had a small slot cast in the rim for starting the piercing saw blade.

(Slaters wheels have the plastic centres moulded into the rims, which are grooved internally - so don't see how those tyres could come loose.) 

 

Thanks Brian....a very interesting post.

 

Ray

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