Scratchbuild Loco Challenge - PSMT No 4

[Stubby47]

The Patiala State Monorail Trainways - Locomotive No 4

 

As part of my new layout, I need to scratch build a loco.

 

Photo copied from the main layout thread, courtesy of Eric (Burgundy)

 

The main challenge is what scale to build it...

 

The Loco

 

The loco's wheel base is 3'11", which in 7mm scale is only 27mm long.

 

In 1:35 scale it's 34.1mm and in 1:16th scale it's 74.6mm

 

For comparison, the wheel base of a 4mm 0-6-0 is 62mm

 

So, I'm probably leaning towards 1:35 scale.  There are lots of plastic kits available in this scale, mostly for military modellers, but there will be some items that are useful.

 

 

 

From the drawing, using the known sizes of wheel base and balance wheel diameter, it can be deduced that the boiler has an outside diameter of (at 1:35 scale) 20.6mm.

 

A 4mm boiler of 3'9" is 32.7mm in diameter and a 2mm boiler of 5'6" is 11.3mm in diameter. So neither of these are suitable, meaning I'll probably need to scratch built the boiler as well.

 

The cylinders are 5 1/2" x 14", which equate to 4mm x 10.2mm - hopefully I can find something suitable in the spares boxes at Halifax at the weekend.

 

 

 

Making the loco move...

 

As I see it, the main challenge of sending 12v dv (or 16v ac) along a single rail is, well, there will be a short circuit.

 

So I have a cunning plan...

 

I will split the single rail into sections, each being fed via a SPDT reed switch, biased to negative. A strip of magnets on the loco will trip the reed switches under and ahead of the loco to positive, which will be collected via the loco wheels. Meanwhile, the coach wheels will be on the sections of track fed via the reed switches which are no longer affected by the magnets, so will be negative.

 

 

Lots of other ideas have been thought of and suggested (via the main layout thread - thanks again !!), but, for the time being, these have been placed on hold.

 

 

Next steps...

 

I need to find suitable wheels, double flanged, with a diameter of 14.5mm. The outside cranks, valve gear, sliders, cylinders, etc., can all be found or fabricated to suit.

The body will be mainly plasticard, with whatever bits 'n' bobs are needed to represent the various attachments.

 

I'm hoping to make the coach from strip wood, though the chassis may well come from a cannibalised 4mm wagon.

 

As ever, your comments, suggestions and out-right guffaws are warmly welcomed !! 

 

Stu

 

 

 

[Northroader]

Just looking at the side elevation, could you simplify things by cheating a little,? (Or a lot). Have motor and gears driving a transverse axle with the road wheel on one end and an oversize rail wheel at t'other. (Not necessarily double flanged) both same diameter, which would necessitate road surface same level as rail surface, which shouldn't be a problem. The leading and trailing wheels would need to be spaced out slightly further to clear the oversize centre wheel, and would need double flanges, but with the both centre wheels driving, the loco. shouldn't crab as badly as the prototype must have done. The oscillating bits are just attached to the outer wheels, everything is hidden behind outside frames and low down under the loco overhang, so it's barely visible anyway. Then it's just getting a centre of gravity sorted and having one idler wheel mounted in a rigid frame with main axle mount, and the other one lightly sprung so's not to take adhesive weight from the drivers. The problem then is hiding the motor and gear drive in the sooperstructure. Simples. Sits down, thinks carefully, and presses post.

[Northroader]

Thinking a bit more about this today, I worked out what a person standing by the loco would look like, from the given dimensions, to get a better idea of size. . . . My! It is small, a Welsh slate quarry engine would run it close. I usually view things with 7mm to the foot spectacles, but having dabbled with On16.5, you'd have a hell of a game doing this at that scale, you really do need the 1:35 scale you propose, at least. Sooo..... Back to psmt (I'm a bit worried if I google it, I might end up in a medical directory!) the trains that are still very small, so you'll need a small layout or it will get swamped, roundy - roundy on a door, if not shorter, view from the front, central divider with semi flat buildings, colourful bazaar scene, I think you're on a winner, aren't you?

[Stubby47]

I like the idea of a little train in a larger landscape.  6' x 4' with a paddy field, small creek, bridge, cutting, rustic bungalow with chickens and an old lorry of some sort, rusting in some weeds.

[uax6]

20mm plastic conduit for a boiler? Why not go for the largest scale possible, that way it will be much easier to model it.

Also why not have the path that the balance wheel sits on as the other conductor rail? Much better than switching the rail sections. I'm sure you can find some metal traction engine wheels somewhere. The path can be copperclad board....

 

Andy G

[Huw Griffiths]

This sounds like a very "interesting" project - interesting in the sense of being offbeat, with loads of technical challenges - interesting in the sense that one of my maths teachers in school used to say, of those challenges being right **#@**!!**s to solve. Saying that though, I hope you are able to solve the challenges - best of luck and all that … .

 

Turning to these challenges, when I started reading your initial post on this topic, I wondered if you might have been planning to use onboard battery power - much like the Faller Car System - but without the need for an underground guidancd wire.

 

I then started to think that something like this might be likely to run along a continuous loop - well, a loco reversing and running round its train would probably be awkward, to say the least - which made me wonder if the loco even needs to be powered, or merely fitted with a magnet and propelled by another magnet on a rope (in a similar manner to a certain pushbike, which created a buzz at Warley a few years back).

 

However, if this model really does need to be rail powered, the only other option I can think of would involve some sort of split running rail - most likely both sides of the rail being small section rails (N gauge?), possibly with inner flanges removed or thinned, with a grey plastic filler section between them. It would be awkward to do - but probably nowhere near as awkward as certain other arrangements could turn out to be.

 

Incidentally, a similar concept to your moving contact idea was tried a number of years back on some electric tram systems - if I recall correctly, I think metal studs were embedded in the road surface and energized as trams passed over them. I don't think any places stuck with it for very long - which makes me wonder if they might have had difficulty in making it work reliably. Obviously, I could be mistaken (I probably am) … .

 

Whatever the score, I wish you every success in building this model and making it work.

 

 

All the best,

 

Huw.

[Stubby47]

Thanks Huw for all the ideas.

 

In the main layout thread ( link below) quite a few of these are discussed.

 

Whilst they are all sound ideas, for the time being I am setting them to one side as I want to ( probably foolishly) try the sectioned track method.

 

I've drawn up a rough diagram of the coach behind the loco, to try to gauge the length of each track section. This will then be compared with a possible track layout ( I.e. length of the single loop) so I know how many reed switches are needed.

 

I'm also thinkng about making up a test rig, using OO set track and suitably altered rolling stock, to prove / disprove the theory.

 

Yes it is a challenge, but if it works it could end the OO/ EM/ P4 gauge wars...

[Horsetan]

....if it works it could end the OO/ EM/ P4 gauge wars...

I think you'd still have arguments about what the correct flange depth should be - I presume the driven wheels have double flanges.

[Stubby47]

I think you'd still have arguments about what the correct flange depth should be - I presume the driven wheels have double flanges.

 

Yes, the outer two do, the middle one is flangless.

 

I'm really hoping to find some suitable wheels at Halifax this weekend....

[Stubby47]

Having worked out the probably distance between the front coach wheel and the last loco wheel, (approx 75mm), this then gives me the length of each section of track.

 

Using a nominal circuit of 370mm radius (15"), this give a circumference of 2,324mm, which is 31 x 75mm sections.  Not too bad.

 

 

 

 

[peter220950]

Two things,

 

any idea what the wheel diameter of the 'drivers' is (roughly)?

 

secondly might it be worth reversing your magnetic polarity scheme to put the row of magnets under the coach and just change polarity for the coach wheel pick-up's,as it would be easier to hide the row of magnets and /or have a height adjusting mech for them. I'm thinking it would be more difficult to hide amongst all of the loco valvegear and stuff?

 

Peter

[Stubby47]

Peter,

Good thinking !!

 

The drivers are 1'8", or 14.5mm in 1:35 scale.

 

Stu

[Stubby47]

Re-evaluating the reed switch drawing above, in light of Peter's suggestion, I realise the magnet needs to also cover the 3rd left switch.

 

Meanwhile, I'm trying to visualise how setting the coach wheels to pick up the switched polarity would work.

[peter220950]

There looks to be plenty of room in front of the front wheel on the coach to activate the next sevtion switch in front, so I wouldn't have thought it to be a problem, as regerds pick up, either the coach, or just the coach chassis is insulated from the loco, as with American pick-up in tender loco's. There's not really any difference I can see between this and your earlier scheme, the rear wheels were always going to pick up from one polarity of feed?

 

I'd missed the fact that you were now up to 1.35th, this makes more sense as there's a bit more room and size to the components, and room to add weight for adhesion.-  It may be a risk too far but if you only picked up from the rear carriage wheel the track joint spacing could be greater, but I suspect it's a bit too much of a risk, unless there's a way of concealing an additional pick up under the coach.

 

I think the driver size is about the same as an 00 wagon wheel, so whether two wheels turned down a bit thinner and bolted face to face might work I'm not sure. I suspect one of the experiments will need to be to see if the flanges need to be deeper to keep the thing on the track. All interesting stuff, you will have to publish your 'standards' for the new scale/gauge!

 

Peter

[Stubby47]

First batch of reed switches ordered - £8.41 for 10  http://www.ebay.co.uk/itm/like/141745187091?clk_rvr_id=904088060508&item=141745187091&lgeo=1&vectorid=229508&rmvSB=true

 

Plus, having looked at the reed switch diagram at the bottom of post #10, I now know I need to fit the switch as close to the end of the section as possible.  This is for two reasons : 

1 - it will mean the section just ahead of the loco wheels will be set positive before the wheels arrive

2- the section between loco wheels and coach wheels will be turned 'off' as soon as possible after the loco wheels have left the section

 

The magnet will need to extend behind the rear loco wheel to a distance where the wheel has cleared the associated section.

 

[Stubby47]

Quick question ; 

 

Can anyone tell me the width of the top of Code 100 rail, please?

 

I can possibly source some brass sheave wheels, but the groove is only 1.5 mm - will they fit ?

 

Thanks all.

[uax6]

My Mk1 eyeball reads the width on a ruler as just under a 1mm, approx 3/4 a mm. looks like you will be ok. Whats the shape of the inside of the pulley? My guess it may well be a conical shape. Hopefully this won't affect running?

 

Andy G

[peter220950]

Assuming you are still using code 100, now you've upscaled to 1/35th, it should be fine, Peco code 100 is .040" at the head, which is just around 1mm.

 

http://fast-tracks.net/forums/viewtopic.php?f=4&t=840

 

If you are going up to code 124 it might be a bit tight, as I've just measured it at 1.56mm.

 

Peter

[Stubby47]

Thanks for the quick replies - these are the items I'm after  : http://www.model-dockyard.com/acatalog/mantua-Rigging_Block_Sheaves.html- Code 32640 14mm dia

[Rusty.J]

Just visited the NRM yesterday. Been to Delhi for a week. All the best with this.

[Stubby47]

Just visited the NRM yesterday. Been to Delhi for a week. All the best with this.

Thanks.

If you have any photos they would be very much appreciated.

[Stubby47]

A set of 10 sheave wheels arrived in the post today.

They have a square profile groove, which I have yet to try on a piece of rail, but it looks ok.

After some sage advice, the next task is to purchase a piercing saw and some brass sheet, with which to make the chassis.

A mock verion will be made in plasticard first, to assess what is required and what the minimum track radius can be.

[Stubby47]

The rolling stock wheels (as I shall now call the sheave wheels) have been compared to Code 75 track and the flanges are too long, resulting in them rolling on the chairs.

 

However, the flanges don't extend beyond the bottom edge of the Code 75 rail, so I'm hoping that the Code 100 will be a better height to use.  I also don't want the rail to appear to be too light weight on a 1:35 scale scene, so the heavier Code 100 will not be out of place.

 

Width wise, the groove between the flanges is not too great to be sloppy, so I think in a rigid chassis the wheels will work.

 

 

Further thoughts on the user of Reed Switches.

It occurred to me that the only reason the magnetic strip needed to extend behind the wheels was to ensure the current piece of track the wheels were on was not switched back to the negative polarity.

 

If, however, I use two sets of simple (& cheaper) on/off normally open reed switches, one set each side of the rail, then I could switch the polarity from off to positive for the loco wheels and off to negative for the coach wheels.  The magnets would only need to be long enough to switch the sections under and in front of the respective wheels sets, as the section behind (between the sets of wheels) could be allowed to be switched off.

 

I need to experiment a bit more with lengths of section, etc, but this could be a viable alternative.  Plus, a deeper rail profile will allow more scope to bury the reed switches without appearing as if the rail is laid in a groove.

[peter220950]

I would have thought Code 75 has got to be too small at around 2.5" for the real rail, even code 100 is only 3.5" but given the wheels you have it's a good compromise. With regard to slop, if the curves are a problem, depending on which wheel(s) you drive the trailing one could be left to move a little from side to side, or you could follow prototype practice and leave the centre wheel flangeless, it's probably a matter of suck it and see with the development plastic card mule.

 

The reed switch ideas throw up all sorts of possibilities the more you think about it. -  Not sure if your proposal means shorter sections to get the unpowered 'gap' between loco and coach pick-up, and trying to switch one section from positive to negative at the same time seems a bit ambitious, particularly with curved track, where radii are different each side of the loco. With regards to hiding them, the switches can always be let into the board surface to reduce their impact on scenery,.

 

Although the mark2 system is cheaper, it doubles the wiring and risk of failure to trigger, but it's certainly worthy a try, I think I would be going down a route of operating relays from on/off reed switches, to change polarity when activated. At around £4 for a board of 8 relays it's a bit more expensive than your proposal, but less wiring. - Good luck with the experiments, I'm finding it very hard not to join in, but it's your project and I've got too many other diversions on the go, perhaps when you've established 'Proto-Stubby' there might be a few 'guest' engines built by followers of the new gauge! It's very unusual to find a completely new topic that hasn't been done before, this is fascinating.

 

Peter

[Stubby47]

Guest locos..... wow, that would be something !