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SRBF/ SRBP/ PTFE/ Wood for Boiler Mountings?

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Nigel Graham 217/03/2020 23:44:49
586 forum posts

I need to mount a copper boiler in a steel chassis, for a 4"-scale steam-wagon I am building from archive photos, so no detail drawings. I have to design it as I go along... 3 steps in full forward gear, 2 to 5 back..


The boiler is of an unusual pattern, externally a T-piece on its side. It has three plate lugs on the outer firebox to sit on chassis parts, and the barrel rests directly in the smoke-box and via spacers, on a cross-member.

My thought is to fit supporting / insulating pads of Synthetic Resin-Bonded Fabric (SRBF) or Paper (SRBP), e.g. from the Tufnol range, PTFE or indeed hardwood, between the lugs and shell, and the chassis components.

Further, to use similar material for the cladding crinolines, possibly as blocks screwed to bands of thin brass strip rather than using a lot of material for trepanned rings.


Working temperature:

The standard steam-tables show 338ºF / 170º C, at 100psi gauge, slightly above the designed 90psi.

[Fingers crossed that those degree signs don't bring up stupid face symbols!]


Any particular pros, cons, problems etc, of these materials for this, please?. Would timber need some suitable preservative, given that the manufacturers of wood-preservatives tend not to consider such high operating temperatures?

Jeff Dayman18/03/2020 08:47:36
1793 forum posts
45 photos

I think if you keep the area of metal to metal contact at a minimum you could make all metal boiler mounts as on the original. To minimise conduction the contact surfaces could be relieved with ribs or pockets. If any of the brackets are exposed to flame or burning cinders / ash near fire door I would say metal is a must in these areas.

If you feel you must use something non metallic and there is no exposure to flame, maybe printed circuit board material with the copper removed , laminated with epoxy to thickness required, would be a candidate. It is very strong, an excellent heat insulator, commonly available, and relatively cheap. Tempered glass blocks might also work. Both these materials would be overkill in my opinion though, and not as durable long term against vibration and shock from road running as metal would be.

SillyOldDuffer18/03/2020 09:35:15
5638 forum posts
1159 photos

The problem with SRBF/SRBP & friends, is their limited resistance to high temperatures. It would have to be looked up, but the first Tufnol specification I found quotes 120°C max continuous, and 130°C max peak.

PFTE should be OK - good up to at least 260°C. It's pricey!

Wood might manage 170°C especially in an infrequently used engined. Preservatives are designed to stop biological attacks - maybe wood in a hot oily engine wouldn't attract inserts, fungi, bacteria or woodpeckers!

There's an excellent series of bubble charts showing the strength/max operating temperature ranges of just about everything on the Cambridge Uni's Materials Website.

I like Jeff's suggestion of using metal pocketed or bored to minimise conduction. Metals vary considerably in their ability to conduct heat. Avoid Silver, Copper, Aluminium and other good conductors. Brasses are all over the place, some being good, others bad - also avoid. Worst conductors are the expensive alloys like Constantin, Inconel, Monel & Nickel-Silver. High Carbon steels conduct heat 3 times less well than mild-steel, but the obvious cheap choice is a Stainless Steel.

A stainless pillar could be sat on a glass, ceramic or mica base for a little extra insulation, but I'd worry about breakages - a loco boiler needs to be fairly robust on the track and being lifted.

Stainless steel doesn't solve the cladding problem though! Off-hand I can't think of an effective insulator that could be wrapped thinly enough around a model boiler without spoiling the appearance. A few inches of glass-fibre wrapped in Aluminium foil would do a fair job thermally, but it's plug-ugly! I'd guess far more heat is lost from the boiler shell than through its supports.


duncan webster18/03/2020 12:36:21
2546 forum posts
49 photos

Compared with the amount of heat transferred from the fire to the boiler, loss by convection from the outer shell is small. It is reduced considerably by the steel sheet cladding, it keeps the draughts off and traps a blanket of air round the shell. Whether to put insulation between the shell and the cladding is a moot point. It is the trapped air which does the insulating, all that rockwool etc does is stop the air moving around (presents convection).

I've seen people squashing 2" insulation down to less than 1" on the basis of the more the merrier, but not a good idea. If you can get it thin enough, Rockwool will stand >200C, Balsa wood is good, and I reckon that corrugated cardboard would work. There is no stress on the lagging, and paper doesn't burn until it reaches 451F.

Finally, why does anyone use brass for cladding, It's expensive and difficult to paint

Nigel Graham 221/03/2020 22:35:24
586 forum posts

Thank you gentlemen!

Regarding what was done on the original... I have no idea! The only surviving documents are assorted publicity photographs and few dimensions from the time. My guess is that the boiler was simply bolted down to steel brackets on the chassis - in fact the boiler itself is of partially bolted construction.

I am replicating to one-third size a steam-wagon, not locomotive, which has a lot of details hidden. This can be both blessing and curse. It allows me to use non-prototypical practices or structural arrangements as and where appropriate; but being hidden means the advertising photos do not reveal what was really there!

The boiler I am using is copper, by Western Steam.

I ought really have given as salient details, that the outer firebox is a vertical cylinder with has 3 copper L-brackets silver-soldered to it, one at the back (w.r.t. to the vehicle) and one each side, at its half-height. The relatively short barrel will sit without fastenings in a ring fitted in the back of the smoke-box, as locomotive practice, with much of its weight taken by a cross-member via a reasonably substantial cladding crinoline.

My thought is to run two parallels to the chassis rails for the firebox side brackets, whilst the rear bracket sits on a plate already riveted to a hefty cross-member that will also be one support for the inverted-vertical, mid-mounted engine.

The firebox brackets will need slight expansion freedom whilst keeping the boiler in place, gained by restraining bars and the insulating pads. My query relates to:

1) the pads between the brackets' undersides and the chassis components - they are thin (5mm) and will be busy, as heat-blocks, expansion-bearings and some cushioning of copper from steel;

2) the crinolines. I am allowing for insulation up to half-inch thick. It would bring the outwards appearance a bit closer to scale if I also lag the boiler top; and as I have allowed for a super-heater not on the original, deep cladding will disguise and lag most of the pipework. (The prototype fed the cylinders from a globe-valve serving as regulator, directly on the boiler, via an exposed loop of pipe. In cold weather especially, the poor engine must have been gasping on hot, wet fluff!)

As it happens, after posting that and making another slight change to " design ", I discovered I have a small block of PTFE (I think - white, glossy, slightly soapy feel) and am using this for the bracket pads, but I still need decide what goes under the cladding. I will test very small samples for heat-resistance by " frying " in cooking-oil.


Dave - Information new to me! I'd naturally assumed most metals are of fairly similar heat-conducting properties, though I knew copper is better than steel. Stainless-steel being a relatively poor conductor surprises me, given its use in saucepans, though perhaps the lower conductivity is far outweighed by other qualities, and anyway a conventional hob loses a lot of heat into the open air. However the mechanical duties of this boiler's mounting are the more important factor.

Jeff - I like the idea of old PCB material. Some are of SRBF so avoid that as you advise, but many are of fibre-glass. I do have some off-cuts of hardwood too, that would be amenable to trimming down. The pads are well away from flames, and won't be seen on the finished assembly, down in dark recesses.

Duncan - good points about how insulation works, and the cost of brass for cladding. I intend sheet-steel, having shared successful experience with it on club-built locomotives. (True to prototype, too!) I'd be reluctant to use cardboard though, since there is always a slight risk of water finding it. You could say that of wood, too, hence my asking about preservatives.

John Paton 122/03/2020 00:38:50
268 forum posts
17 photos

PTFE gives off nasty fumes when burnt - so avoid use within the firebox but I suggest would make good bushes elsewhere if you have it. Teak or Iroko strips for cladding? (pretty good thermally and don't need painting)

Nigel Graham 223/03/2020 00:28:34
586 forum posts

Thank you John.

Yes, I know the danger of overheating PTFE. Other plastics can emit toxic fumes too, if burnt.

However, these parts are outside the boiler, and a long way from either the fire-hole or grate.

The first requirement is three pads that act as insulation, cushioning and expansion-bearers between the copper shell's mounting-lugs and the chassis. By chance, after submitting the question, I discovered I had an oddment of PTFE plate, and made those pads from it.

For the rest, summarising what I've been advised here, I think my best approach will be hardwood for supporting the lagging and outer cladding.

Some of my source photos suggest the original vehicles having no boiler cladding, but they may have been photographed incomplete to show some of the details. If they were clad, most likely it was with sheet-metal over some insulation material, unknown to me, though possibly wood.

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