References for steam passage design

[Andy StopfordParticipant @andystopford50521]

I’m working on the design for a 1″ scale ploughing engine, and need to work out suitable dimensions for things like diameters of steam pipes, valve port cross sections and the like. Does anyone know of any source of information about this, whether book or online?

[noel shelleyParticipant @noelshelley55608]

Investigate the building of 1″minnie ! I have drawings for the Bassett Lowke 1.5″ that may help and there are books, Building the Allchin, by W.J.Hughes, Introducing Model Traction Engine Construction by John Haining, and The Building and Running of Steam Traction Engine and Roller Models by H.R.Plastow. If your model IS scale then the prototype will dictate some of what you need. Cylinder portways may be cast in. remember that in altering the scale or size, dimentions change by the square. Good Luck Noel.

[JasonBModerator @jasonb]

I can think of only one commercially available model that has anywhere near scale passages cast in. The majority will be a solid cylinder block with drilled or otherwise formed passages. So as Noel says you would probably be better off looking at other designs of similar sized traction engine models.

Is it a single or compound ploughing engine you are doing? If a compound then you are unlikely to be able to get the pressures needed for compounding and at 1″ I’d say you are probably better off running it as a double high.

Most traction and ploughing engines unlike locos don’t have steam pipes, the underside of the cylinder block acts like a dome and the cylinder is cored or drilled to allow the steam up to the regulator chest usually via a jacket around the cylinder.

[Andy StopfordParticipant @andystopford50521]

I’m going for a model of the Fowler Z7S – I’d originally thought of scaling down the John Haining design to 1 1/2″, but being realistic, even that is too big for my limited facilities, so 1″ it has to be (which of course is pretty hefty compared with the Minnie I’m building at the moment).

I’ll be making my own castings, so probably nothing very complicated in the way of cast in internal details, all passages drilled etc.

I hadn’t really thought of making it a 2 cylinder simple – in a way I’d like to be as true to prototype as possible, but since inevitably I’ll have to make concessions to practicality that’s worth looking at. What sort of pressure is regarded as attainable for this sort of size boiler (it works out at 76mm diameter and about 320mm long) by the way?

[JasonBModerator @jasonb]

You won’t get much more than 100psi out of a copper boiler, you want 150plus to see the effects of compounding.

Haining’s design is quite a long way from prototype so you may want to do a lot more research if you want it true. In practice the compound angles of the original port faces will be a real challenge, I’ve only seen them done true a couple of times and that was on larger scale engines.

Not sure if it warrants castings at 1″ you could get better detail cutting from solid or fabricating

[Andy StopfordParticipant @andystopford50521]

For the Z7S he has the port faces flat (presumably to make it look more like the piston-valved original). In fact, for quite a lot of detail, I’ve been using photos of a class AA7 I took at the Tinker’s park rally last year, so I’m more into getting the general idea of the thing rather than super accuracy. In the original magazine articles, Haining mentions a Z7 in a collection/museum about 5 miles from me. Annoyingly it seems to have closed a few years ago.

I agree, for a lot of things, castings are more trouble than they’re worth; the cylinder block is the most obvious thing which I’d look to cast, just to avoid having to buy a large, expensive, and difficult to source bronze section. The chimney base and some parts of the cable drum come to mind too.

[noel shelleyParticipant @noelshelley55608]

The block is bronze ? Noel.

[JasonBModerator @jasonb]

Probably more cost effective to cut the cylinder from a lump of Cast Iron. Cable drum would solder up from mostly laser cut steel

[LukerParticipant @luker]

You’ll find a few references for passage design but I’ve found they are empirical guesses. For my designs I use a non-dimensional analysis to balance the engine. You need to start with the boiler and work from there. I ended up writing an algorithm to iterate for the best solution within each prototypes general dimensions.  For cylinder steam passages my starting point is 22m/s for steam inlets and 1.5-2 times that for the exhaust (this is where you need unobstructed flow). The steam port design is important. Martin Evans’ book is in the ballpark but I’ve made one of his freelance engines and managed to double the efficiency with a few minor tweaks.

I’ve just finished a design for a steam traction engine, some of the castings done! The gearing changes things a little if you want an efficient design.

Home casting is easy! Once you get the hang of it you’ll have no problem casting anything. Do a little reading on feeding and coring before casting things like the cylinders.

 

[JasonBModerator @jasonb]

This is a good archived site, although it covers building a couple of BB1 engines the methods apply to others, I’ve linked to the page showing the beginning of the winding drum but the rest are good.

If you are on Traction Talk take a look at Doug’s TE1 build, this is his cylinder carved from solid, fabricated governor bracket.

[Anonymous]

For my 4″ scale engines I scaled the steam port dimensions from full size, as best as I could. In full size a velocity of ~6000ft/min is often stated as suitable for passageway design. There is a tradeoff, but turbulent flow is usually better than laminar flow as the pressure drops are lower. For turbulent flow the Reynolds number needs to be around 10^4 or higher.

From a practical point of view on models it is difficult to get turbulent flow. Since areas decrease as a square law and volumes decrease as cube law the mass flows are in a model as disproportionally small. Just scale the pipes, and passages, from full size. The flows will be slower than full size and possibly laminar, but that won’t be an issue in a model.

There is a lot of informtion on steam flows on the Spirax-Sarco website.

Andrew

[Andy StopfordParticipant @andystopford50521]

Thanks for your thoughts, chaps.

I haven’t decided on what material for the cylinders yet – the actual design may determine what’s most suitable anyway.

When I was thinking of 1 1/2″ scale, I gave some consideration to a fabricated construction, like the welded frames used for big diesels, though silver soldered. The nice thing about that might be freedom to arrange steam passages which would otherwise need complicated coring. I’m not sure it wouldn’t be too fiddly in 1″, but it’s another possibility.

I forgot the hornbrackets as something which would be appropriate for casting:

I don’t think I’d fancy machining something like that from solid on a non-CNC mill.

Thanks for those figures, Luker and Andrew, I’ll see how that looks when I open up the CAD file this evening (and have a look at what Messrs. Spirax and Sparco have to say).

The BB1 build that Jason’s linked to looks interesting as well (once I’d persuaded Three to let me see it – a possible Tea Room Topic there). I applied to Traction Talk a while ago, but have heard nothing so far from the High Priests of TT.

Does anyone have views on suitability of piston valves for this sort of size, by the way? The cylinder diameters, as scaled from the Haining design, are about 21mm and 29mm, though there’s room for bigger.

 

[JasonBModerator @jasonb]

The horn brackets are another contender for silver soldering, Build it up with more material on the parts to be machined and treat the fabrication as a casting doing the final work after soldering. If you can stick bits of wood together to make a pattern then silversoldering bits of metal together is not a lot different and you don’t have to worry about getting the metal to flow into thin section castings.

I’ve an almost complete set of Superba castings but am unlikely to make the engine now. However Even when I was likely to have built it not many of those castings were going to get used and now I have the CNC even less would be used. The Hornbrackets would have been fabricated.

A bronze cylinder would have the advantage that it is easily silver soldered be it a full fabrication or just the ability to add bits that would not be so easy with Cast Iron. There is a very nice R3 Fowler on TT with built up cylinder, another Dutch guy. Similar cylinder in here for a “Big Lion”

[Andy StopfordParticipant @andystopford50521]

Yes, that’s just the sort of thing I had in mind for a fabricated cylinder. I haven’t got time to look now, but it looks like there are some other unconventional techniques described in that .pdf.

I was thinking more in terms of 3D printing patterns – though that means doing battle with a 3D printer. Still, I’m told they’re less contrary now than the early one I had, and disposed of.

[JasonBModerator @jasonb]

 

 

[Andy StopfordParticipant @andystopford50521]

Very nice

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