From Firth Came Forth… Compounding & Valve Gear?

[duncan webster 1Participant @duncanwebster1]

I used to work for a company that was renowned for changing the design of a plant half way through build. The official acronym was reputed to stand for Build Now, Flowsheet Later. It appears Nigel is well down this route. It’s always a good idea to freeze the design before you start cutting metal, otherwise you have major rework and scrapped components to look forward to. Something I always did when in paid employment, I wish I was that disciplined in my home shop, it’s always tempting to get on with it.

It might be possible to fit a liner to the existing LP cylinder, this would preserve the look of a compound

[JasonBModerator @jasonb]

May be useful for people to look at these photos of the full size which I think is a replica, you can see the boiler shape and also the questionable regulator.

Is there any really bad reason not to run HP steam to both existing size cylinders?

[Martin Johnson 1Participant @martinjohnson1]

Thanks for the photos, Jason.  I knew roughly what the Hindley looked like, but not to the detail of where the regulator was (is?).  A wheel valve was quite common practice for lorries and underlines two things:

1. A much slower pace of traffic and life in general back then.
2. Far greater reliance on driving on the reverser than is common practice among the steam fraternity now.

Nigel,

Having thought further about your design problem, a point not considered so far is that the boiler has superheater flues if I understand you correctly.  If you leave these empty you have a low resistance short cut for the flue gases to take in preference to the tubes.  That being so, you would drop the boiler performance from what it should be or would be if it were full of firetubes only.

I remain convinced the better way would be to have the pipework going to and from superheaters.  With a bit of luck they could be tucked under the boiler lagging – or a bulge thereof, somewhere deep in the fuel bunkers never to be seen by a living soul.  I won’t tell if you don’t let on.  Another ruse to consider is having the regulator valve AFTER the superheater as was done on Sentinel waggons.  That arrangement makes for easier and safer driving as you don’t have a long run of pipe full of live steam even after the regulator is shut.  It is also possible to get a slug of water carry over into the superheater, which flashes to steam and tends to make things go faster just when you want them to stop!  Shutting the regulator in such cases might give the driver something to do, but does nothing to kill the speed.  So, could you have a dummy regulator up top as shown in Jason’s link, with an extended spindle going to the real valve somewhere below footplate level?  I think that might help with pipe run length as well.

The offer still stands for assistance with the heat transfer sums if you want to prove it one way or t’uther.

Martin

[Nigel Graham 2Participant @nigelgraham2]

Thankyou Jason, for finding those photos.

Yes, it is a replica, built I think in the 1990s by Richard ‘Turbo’ Vincent, to commission. I saw it shortly after he started, when it was a bare chassis upside down on trestles, with the boiler standing nearby.

That camera angle looking up at the driving position may exaggerate the height of the regulator above the boiler. On the original photo I have it was bolted to a flanged elbow much more compact than that.

The picture shows what you thought might be the regulator but I think more likely the starting-valve, is remarkably badly positioned for anything, and quite how the driver was meant to operate both valves and steer is anyone’s guess.

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However, you have given me an idea. Let’s look at photos I took this evening:

Driving a miniature steam-lorry means you have to sit on its platform, with your feet through a big hole in the floor, or on footrests sticking out from the side. I concocted a rough idea of the 12″ -scale driver’s view, by sitting on a footstool on planks across the chassis, and placed a brick and box to show very roughly the engine’s location.

The steam take-off is that large bronze disc eventually to take 4 studs, with a central blanking-plug, hydraulic test for the use of.

Now, suppose I fit the globe-valve for fidelity but with a simple-expansion engine turn the simpling-valve into the regulator itself? Though moving across rather than fore-and-aft it would certainly be quick to use, and easier to reach from an ungainly driving position.

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There is a reason for not using the cylinders I have already made: a lot of re-work. Yes, the ex-LP one could be given a proportionately earlier cut-off than the HP one, or an inlet restrictor, for fairly equal power.

However when I came to start making the valve-chests I discovered some of the studs ran into the passages. Also, the top and bottom of the block are to have full-area cover-plates with the circular cylinder top covers fitted to them, but the shape and disposition of the steam-passages give serious sealing difficulties.

…

 

Turning to Martin’s points…

Thank you for that about empty flues taking too much of the hot gases. I’d not thought of that. They could be blanked off or fitted with restrictors.

I’ve looked more carefully and realised there is in fact far more room in there than I thought. The chimney choke is in the roof of the smoke-box.

Putting the pipes to and from the superheater under the lagging is exactly what I had in mind – I think I had already said so. The smoke-box is considerably larger than the shell, allowing specially-made bulkhead unions through the adaptor flange. There is one for the blower pipe too, though I later re-routed that to a side-entry elbow. I can revert it to the original through-flange fitting.

I don’t want them in the bunker for three reasons, apart from appearance: heat-loss / lagging difficulties, vulnerable to damage, and also because I am making the major assemblies like the superstructure to be readily removable for servicing.

The two bunkers are joined together and held to the chassis by just a few screws through their floors, tapped into the chassis rail flanges.

The smoke-box is not fastened to the boiler but apart from the plumbing, is held by just four screws and nuts into two brackets: remove them and slide it forwards. A cradle behind it supports the front-heavy boiler on a simple wooden Vee-block.

Similar principles apply to the footplates and seats, platform and canopy. (The last, similar to that on a traction-engine, seems to have been an optional extra in 1908!)

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If I arrange things as above, the difference between prototypical and model pipe-work might not be too noticeable, and the driving regulator would be disguised as the starting-valve.

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Thank you for your calculations offer: I may yet take you up on them.

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I took these photographs this evening so now we know what we are up against:

Approximate driver’s-eye views with and without the engine’s location very roughly approximated, and the darker grey areas showing the model seat positions. The steam take-off is the bronze disc, yet to be fitted with the correct flange-joint studs. Its central detail is a blanking-plug for hydraulic test. Temporary steering worm gear, and the steering-wheel is under-size and due for replacing. The pale oval thing is my knee as I perched on the vehicle!

 

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General view of the near-side. The original through-flange blower-pipe union is just visible behind the pipe. The superheater in and out connections are similarly placed on the off-side. Those extra holes in the chassis are now surplus to requirements, reflecting changes as I went along. The injector water-valve is an old motor-cycle type petrol-tank cock. The ash-pan is visible with one of its two suspension-rods. It proved extremely difficult to make a dropping grate and pan assembly for this, and my efforts merely created a horrible confection of steelwork.

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Smoke-box. Flues approx. 11/16″ bore (how do I fit two pipes and their spear in that?). fire-tubes approx 3/8″ bore. 44 of ’em if I counted correctly. The original blower union is visible, blanked with a stainless-steel screw and nut. The wiggly pipe lying underneath is the engine exhaust-pipe disconnected so I could make the boiler front support. The axle-beam, made to represent the vague photographs, consists of two lengths of folded 14swg steel channel welded back-to-back and dressed to resemble a single I-beam.

 

 

 

 

 

 

 

[Martin Johnson 1]

On Martin Johnson 1 Said:

…but have to differ with him about flow regime. Within model steam pipes, you typically have turbulent flow (Reynolds number in the order of several thousand).

My comments were based partly on the Spirax Sarco book on steam system design. Having re-read it I think I was a little out on my numbers. They say Reynolds numbers from 100 to 2300 are mostly likely to be laminar flow, from 2300 to 10000 transitional and above 10000 most likely turbulent. Of course all these numbers are not written in stone!

I can’t remember what the exact numbers were but when I designed the passageways for my Burrell SCC I got a Reynolds number in the low thousands. So probably transitional flow. To get the 5000ft/min steam velocities recommended by the older text books would have meant very small passageways. So I compromised on lower velocities, and lower Reynolds numbers. I guess this is the scale effect working, volume going down as a cube law.

In reality I am not sure it will make much difference to my engines. I’ll be happy if they actually run.

Andrew

[duncan webster 1Participant @duncanwebster1]

I think losses in steam ports are dominated by entry, right angle bends and exit losses, and that the length compared to cross section is too short to establish either laminar or full turbulent conditions. As you rightly point out, piston speed in (most) models is so low that if made to scale the pressure drop is very low

[Nigel Graham 2Participant @nigelgraham2]

Gentlemen,

I very much appreciate your advice and you are perfectly welcome to discuss Thermodynamics and Fluid Dynamics on my thread, but please remember I have not had your university education. I do not have an Honours Degree in Mechanical Engineering – I don’t even have a GCE O-Level in Physics!

I have no idea what a Reynolds Number is, nor do I know what gases get up to as they rush through wriggly pipes and valves. I will leave that to you who find such things simple.

All I want is a vehicle that might actually move its own weight plus mine across a field, but I could never have learnt mathematics to anywhere near the level involved in designing such a thing from theory.

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Regarding piston speed in a model, I assume it is faster in a steam road vehicle than a railway locomotive of comparable scale, due to the gearing, but easier to work out.

IF my engine reaches its prototypical maximum of 400rpm the pistons would move 400 X 2 X 2 = 1600 inches / minute; approx 133 ft / min, slightly over 2 ft /s.

Mean speed, neglecting all their very complicated accelerating and decelerating.

I think.

….

I managed to spend a few hours on the project today, modifying the very wriggly half-inch copper tube that one year (if I live long enough) might convey exhaust steam from engine to blast-pipe. It has to wind its way round the boiler from the level of the valve-chest down to the underside of the smoke-box, without getting in the way of lagging yet to be made, and various pipes and fittings.

Also tried designing the top cylinder covers, as their layout will affect such cylinder details as the passages, chest studs, etc. That was unexpectedly hard because I want the engine to at least look as if a compound, but enlarging the former HP cylinder by 1/4″ creates geometrical mayhem that stretched my Alibre-driving ability to the limit.

(The HP cylinder bore is increased from 1.25″ , the LP decreased from 2″ , so both end up at 1.5 X 2″ .)

I might yet revisit modifying the original compound block, but that won’t be at all easy, with recesses and holes in all the wrong places.

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The rear axle chain-drive sprocket needs replacing, and various other details on the axle completing. I investigated modifying the sprocket’s mounting on the differential to give a bit more room at the other end of the chain, for the 2-shaft transmission and the engine’s nearside main bearing.

That’s already complicated by finding the crankshaft is too short for two sliding pinions, so I’ve to shorten it further and fit a coupling to a separate gear shaft. Plus I’ve also to find room for the feed-pump and its drive-gear I have not yet designed, but the region around the engine space and boiler is already filling with metalwork.

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Oh, and just to add to the fun I have lost the water inlet pipe for the injector so will have to make a new one!

If it’s not one thing it’s another.

 

[Nigel Graham 2]

On Nigel Graham 2 Said:

I do not have an Honours Degree in Mechanical Engineering…

Neither do I, my degree is in electronics, aka semiconductor physics.

Andrew

[Nigel Graham 2Participant @nigelgraham2]

Well, I was being a bit tongue-in-cheek, but my point was that the discussion has risen academically far beyond anything that was ever possible for me.

It is interesting, I read it as far as I can, you are all welcome to continue it; but the mysteries of flow conditions and their Numbers cannot help me design a model steam-engine.

I have to use books like Martin Evans’ on miniature locomotive construction to assess the proportions of ports and so on; and drawings such as Luker’s in ME are also helpful because I know his engines work.

I only wish I could understand such physics, be it steam in pipes or electrons in silicates; but it is mostly advanced mathematics, and mathematics beyond about GCSE level is impossible for me.

[Nigel Graham 2]

On Nigel Graham 2 Said:

Well, I was being a bit tongue-in-cheek, but my point was that the discussion has risen academically far beyond anything that was ever possible for me.

It is interesting, I read it as far as I can, you are all welcome to continue it; but the mysteries of flow conditions and their Numbers cannot help me design a model steam-engine.

…

Time for an rethink then!   The discussion got into cube laws, thermodynamics, and Reynolds Numbers as a consequence of the fundamentally difficult questions Nigel is asking.    He is in deep water, and I diagnose insufficient learning for the challenge in hand.   As projects go, I put Nigel’s endeavour into the very difficult class.

The easiest way to engineer is to copy what already works.  It’s because someone else slogged through most of the agonizingly hard work needed to get results.   Many of the machines we see today look simple enough, but we forget it took decades to get them right.

Design of anything remotely complicated is, I think, by far the most difficult aspect of engineering, especially when done from first principles, as Nigel is attempting.

Rapid progress requires a good understanding of materials and the relevant physical laws.  That understanding can be acquired ‘on the job’, but gaining it takes forever and is highly error prone.  Far better to swot Newton’s Laws up – he was a genius and I’m not!

Swotting isn’t to everyone’s taste.  The alternative is to learn from existing designs.   Not just by building an example, but studying and understanding how it works, then looking for ways to improve it.   The proportions, layout and materials were all chosen for good reasons, not guesswork.

LBSC’s locomotives are firmly based on full-scale practice developed professionally before he was born by thousands of talented engineers.  Later designs aren’t massively better than LBSC’s efforts,  I suggest because the laws of physics cannot be bypassed.  Easy enough to get a model wet-steam loco to work, but then much harder to make it efficient.  For example hard to design a superheater that doesn’t behave in the real-world like a wire-threading super-cooler.

This puts beginner designers into a very difficult place.  Not understanding what works practically or the theoretical limits creates massive fog and multiple disappointments.

Professional engineers approach design by nailing down the requirement and specification first.    Although the idiot customer usually gets the requirement wrong,  the engineer can only go so far to clean it up, so he does his best with the spec.   It gets particular attention, making sure that it doesn’t contain anything that’s theoretically impossible like Perpetual Motion, or high-risk, including bleeding edge.   Then during development, the design is continually reviewed and tested, generally seeking simplifications.  On a large project the process becomes incredibly elaborate, and engineers still get it wrong sometimes!

Nigel’s approach feels like a slogging match to me, perhaps a death-march. So I recommend a reset – build a similar steam tractor from an existing plan; study it’s arrangements carefully; and then copy as much as possible into the next attempt.   I can’t think of a better way,  anyone else?

Dave

 

 

[Martin Johnson 1Participant @martinjohnson1]

I am away from home, so can’t write a lengthy epistle but:

Nigel’s original query concerned whether the losses associated with some rather long pipe runs would negate superheating.  Sadly, friction losses and heat exchange depend very much on Reynolds number and a whole bunch of other flute music.  As before, I have it all coded up in a spreadsheet if required, otherwise ’tis guesswork.

Promise not to mention Reynolds again (oops), but will check in with Andrew as one of us may have made an error and I am worried it might be me!

Have looked at Nigel’s photos and I reckon that is excellent and well worth persevering.

The question of compound Vs twin simple is a bit borderline.  If bits are already made, stick with what you have.  If it doesn’t work out, you can overcome by sleeving down the LP and maybe change gear ratios to suit.

It will make a refreshing contrast to endless Fodens on the rally fields.

Good luck,

Martin

[Nigel Graham 2Participant @nigelgraham2]

“Swotting” is more than not to my taste. I don’t mind it if I can cope with the subject and want the results, and I have a small library of “full size” as well as model-engineering text-books. But you can’t swot subjects you genuinely can’t learn, like advanced mathematics.

I have some basic but key information for my project: wheel diameters, cylinder sizes and the patent specification (un-dimensioned) for the boiler, plus old photographs from which to gain at least fair proportions. This will never be a rivet-counting replica of something that has detail differences from one photograph to the next anyway, but I never claimed it will be.

Even that full-size replica probably depends on a lot of assumptions but had the benefit of being built by a professional restoration engineer – who did admit to me that the lack of original works drawings and surviving vehicles is a big obstacle. He did say he thinks its front wheels might be original Hindley ones – I forget what he said was their source!

So with no original Hindley drawings, no original lorry to examine,  and no mathematical ability, I have to fall back on adapting proven work such as in Evans’ text-book and published designs of comparable size.

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Building a known vehicle first?

I have been battling with this project for a very long time, through a major false start, two house moves, periods of lost enthusiasm and even periods of clinical depression in which I came near to scrapping the poor thing. It survived partly by my thinking I owed it to too many people to give up – one of whom passed away four years ago, leaving parts of a pair of 4″-scale Fowler ploughing-engines that was his equivalently ambitious project.

If I stop now and build some known miniature, I might finish that but almost certainly not the Hindley. Or if I do I will probably never be able to drive it around: I am nearly 72 now.

When I started no-one else had even heard of the name I met as a photograph and brief note in a history-magazine article. I was going to build a Foden ‘C’-type as they are plentiful and large-scale drawings for them became published; but everyone has a Foden ‘C’ and this was both unique and made in Dorset.

[duncan webster 1Participant @duncanwebster1]

Full size locos were designed (if that is the word) for 6 revs/sec (bridge stress committee). That’s 360 rpm, not far from Nigel’s 400. If you use ports typical for similar size loco cylinders you won’t go far wrong, in fact I think they will be bigger than they need to be. I don’t see any problem with studs breaking into ports, as long as you have enough full thread. Even if the stud had to pass right through the port, it won’t completely block it by a long way.

Binning it and starting again would be an over reaction. As a twin HP it will work on saturated steam, it would just work better on superheat, easier to keep a sufficient supply of steam. It might be interesting to work out the Ewins A factor, which relates grate area to steam demand. Not very well, but better than nothing. Don’t take any notice of the other factors, I believe they are flawed.

EDIT, who said both cylinders of a HP twin have to be the same size? One at 1.25 and one at 1.625 has nearly the same swept volume as 2 off 1.5. It might sound a bit odd, but probably not as you have a long wiggly exhaust.

[JasonBModerator @jasonb]

Well I did ask a few posts back if there was any major problem running the two existing 1.25″ and 2″ bores both on HP. Apart from increased steam volume but you could use the reverser to play with cut off.

Or if you wanted to keep the look of the cylinder ctr lines what about taking the HP out by 1/8″ to 1.375 and sleeving the LP down to 1.875″ or 1.75″?

 

[Nigel Graham 2Participant @nigelgraham2]

Thank you!

If I modify the exiting block I think it would be safer to leave the existing LP cylinder as it is and either sleeve the LP one down or arrange some sort of restrictor, small ports or early cut-off.

These two photos might show some of the problems.

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The passages are drilled parallel to the surfaces as that was the easiest and safest way I had at the time, but this puts them close to the surface and gives a very narrow sealing area for the flat end-plates that hold the circular covers themselves.

One option is to cut the ports faces right down to the passage floors, then machine sloping passages that meet the cylinders within the covers, and fit a new port-face to each with a ramp to create a new passage roofs.

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Those connecting-rods at 4.5″ centres are probably too short and I forget why I did that, given that I envisaged copying K.N. Harris’ Stephenson’s Link Motion designed for the ‘Maid of Kent’, needing connecting rods around 7″ centres. The little brass thing peeping out of the big-end is a grub-screw tapped through the turning-centre hole, as an anti-rotation pin for the half-brasses. An oil-hole is drilled down through the flank of the big-end, both for manual oiling and hopefully to collect oil trickling down the rod, at least when the engine is stopped.

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The cross-head castings happened to have been a pair in the waifs-and-strays tray on M.J. Engineering’s sales-stand! They were probably intended for a 7-1/4″ g loco. Those semicircular notches are oil-scrapers that divert “surplus” oil from the vertical guide-bars into holes drilled through to the cavity, to lubricate the small-end. The gudgeon-pin is a commercial item: one of those ground-shank socket-head screws intended as pivot-pins.

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I cut the crankshaft from solid, from a piece of old quarry tramway wagon axle! I did create proper drawings for this, one for the item itself, the other to guide how to make it from the “supplied” material. What I have not done is set it up between Vee-blocks to test for parallelism etc. Nor considered how it would drive the transmission-gears. I envisage possibly ball-bearing outer main bearings, but they can be plain bushes. The centre bearing will have to be split of course, leaded-bronze or cast-iron lined.

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The cast-iron eccentric sheaves are retained in the mild-steel straps by circlip-like sheet-steel rings (I may replace with brass). This allowed turning the bearing surface right across in a simple sliding cut. A gap in the ring allows access to the grub-screw. I thought I’d need prevent the ring from rotating but realised it would not matter if it does.

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I spent several hours on the project this afternoon and evening, towards completing the boiler ancilliaries. My aim is to complete the steam-making department first to a satisfactory point, then concentrate on the engine and transmission, though it’s not to a set schedule and I might well move from one area to another.

 

 

[duncan webster 1Participant @duncanwebster1]

The condensation problem means working at very short cut off isn’t attractive. An alternative, which I don’t really like, would be to leave the bores as they are, admit steam to both cylinders for starting, but shut off the small one for running. You’d then need to have an arrangement to connect both ends of the small one together to reduce pumping posses, and it would make it tricky to drive, I did say I didn’t like the idea!

[Andrew JohnstonParticipant @andrewjohnston13878]

Nigel seems to have a rather odd view of engineering, as opposed to modelling.

For sure an engineering degree involves a fair amount of mathematics. If I was being cynical I’d say that was at least partly because it is easier to create exam questions around it rather than messy real world engineering. In my working life I have never used the vector calculus I was taught at university. Although, for electronics at least, a working knowledge of complex numbers (*) is essential.

I have done a lot of calculations during the design of my engines, taking note of what has gone before and referencing older textbooks. In order to do the calculations it is necessary to have a conceptual understanding, which is fundamental to being an engineer, but it does not require advanced mathematics. All my calculations have involved nothing more than secondary school arithmetic.

Andrew

* – I don’t think complex numbers are complex, I prefer to see them as 2-dimensional numbers, being part of a larger picture including 4-dimensional numbers (quaternions, used in tensors and 3-D computer graphics) and 8-dimensional numbers (Cayley numbers, which have no practical engineering application, as far as I know).

[Martin Johnson 1Participant @martinjohnson1]

A lot of road vehicles had relatively short con rods – its a space problem not shared by rail vehicles.

If you want to convert to double high engine, sleeve the HP otherwise the two cylinders will drain the boiler too quickly.  Remember a compound only takes one cylinder’s worth every rev.  Models boilers do tend to make more steam at small scale, but wont perform miracles.

With any ME project you set your own parameters and Nigel seems to have a good idea of what he wants to achieve.  The other thing needed is commitment, which requires that the project inspires the builder sufficiently to stick at it.  Building somenthing else would not satisfy the “sticking at it” test and would just result in another unfinished project sold to a dealer.

The rest of us can offer opinions and even talk about thermodynamics.  However bottom line is it’s your baby.  I would certainly love to see it in steam whether it be compound, simple, saturated or superheated.

Martin

[Andrew Johnston]

On Andrew Johnston Said:

Nigel seems to have a rather odd view of engineering, as opposed to modelling.

For sure an engineering degree involves a fair amount of mathematics. If I was being cynical I’d say that was at least partly because it is easier to create exam questions around it rather than messy real world engineering. In my working life I have never used the vector calculus I was taught at university. Although, for electronics at least, a working knowledge of complex numbers (*) is essential.

I have done a lot of calculations during the design of my engines, taking note of what has gone before and referencing older textbooks. In order to do the calculations it is necessary to have a conceptual understanding, which is fundamental to being an engineer, but it does not require advanced mathematics. All my calculations have involved nothing more than secondary school arithmetic.

Andrew

* – I don’t think complex numbers are complex, I prefer to see them as 2-dimensional numbers, being part of a larger picture including 4-dimensional numbers (quaternions, used in tensors and 3-D computer graphics) and 8-dimensional numbers (Cayley numbers, which have no practical engineering application, as far as I know).

We once had a long lecture about Laplace Transforms. At the end the lecturer turned to the audience (he’d had his back to us until then) and asked ‘any questions?’ a brave soul at the front just asked ‘why’. If the topic had been introduced as ‘here’s an engineering problem, and this is how you solve it’ we might have been more interested, but the lecturer was a mathematician, so practical applications were foreign to him. I’m with Andrew, for 95% of engineering sums you don’t need advanced mathematics, perhaps a little beyond what you get at school  but not the hieroglyphics we were force fed. Suffice to say I’ve never transformed a Laplace in my subsequent career.

[Nigel Graham 2Participant @nigelgraham2]

Andrew –

Your approach, using reference-books, seems much the same as mine. In fact I bought the ancient books I have, not just to understand how to design a steam-engine but also to find typical, general engineering trade practices of the time so I could make a fair representation of an Edwardian machine.

As for complex numbers, you might not see them as living up to their name, but they are beyond me even before you wrap them up in mystical alchemy like tensors in umpteen-dimensions! I know complex numbers have something to do with a.c. electrical theory, but I wonder how much of the really rarified maths that exists has any practical use at all.

 

A friend once told me, ” Maths is all squiggles and I doesn’t do squiggles!”

I think I commented that as an anaesthetics nurse turned Senior Medical Lecturer she deals with squidgies, and being squeamish I don’t “do” squidgies.

A squiggles series invented by George Green, printed round a museum souvenir-shop mug I found at work:

ʃdxdydzUδV  +  ʃdσU dV/dw  –  4 πU’’  =  ʃdxdydzVδU  +  ʃdσVU/dw  –  4 πV’ …..

Yes, those are integral signs and the letter ‘pi’. No I have no idea what it means, nor if it’s the slightest use to anyone!

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Martin –

You are right! Yes, I’d love to complete it and successfully.

I can use long con-rods on this engine because the unit is vertical and about the same total height as the firebox. The various texts I have suggest centres over 3 times the stroke, though certain classes of steam engine did have less than that.

Modifying the cylinders already made will not be easy, and it may be best to keep the 1.25″ bore as it is and sleeve the other down to match, or nearly so. It might even be worth completing it as a compound first to see if that actually works, then altering it if that proves necessary as does seem likely.

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Duncan –

The basic shop-floor maths and arithmetic is not too hard, but once you start delving into machine theory to be able to design things properly you need be very good at Higher Mathematics.

I have inherited some of my Dad’s engineering text-books, and collected a few others. Much of the mathematics in these, especially dealing with heat-engines, and stresses and strains, is formidable indeed.

Even the familiar [ PLAN/33000 ] for estimating the Indicated Horse-Power of a steam-engine cylinder invokes a hyperbolic relationship, though I think there are fairly direct, arithmetical ways to work out the mean effective pressure without horrendous calculus.

I also worked at lab-floor level in a company combining high-level physics and engineering.

It was via this that I took the ordinary GCSE Maths course as a refresher, hence met the weird Wonderland of Matrices (Lewis Carroll developed them, in his day-job). The scientist who tried unsuccessfully to help me understand their basics said they are part of the computer programme she used to analyse and predict very complicated vibrations in surfaces and solids. Other areas of the work made numbers complex and transformed Laplaces.

This was all Engineering in that it was all towards designing and testing physical objects doing physical things in the real world.

 

[Martin Johnson 1]

On Martin Johnson 1 Said:

…

With any ME project you set your own parameters and Nigel seems to have a good idea of what he wants to achieve.  The other thing needed is commitment, which requires that the project inspires the builder sufficiently to stick at it.  Building somenthing else would not satisfy the “sticking at it” test …

Ideas and commitment aren’t the whole story.  You also need a workshop and be able to use the tools!  And, if as Nigel is doing, you are developing from scratch rather than building from an existing design, then you also need design skills.

Unfortunately, unless the object is very simple, designing from first principles as Nigel is doing is seriously difficult.   It’s because an assembly like a Steam Tractor contains a multitude of interrelated parts that have to fit together in a practical way, be strong enough, and they may have to move without hitting anything else in the structure.

It’s design skills that Nigel is missing.   One way of obtaining them is by slogging through the textbooks and taking a degree course, eek.    Another way is to learn from others work.   I suggested learning whilst building from an existing plan by thinking hard about how and why the designer decided each dimension, relationship and material.  It’s one way of breaking in.  Design is a different skill-set, and I infer from Nigel’s questions that he’s not got enough background yet to tackle major design problems.  That’s why I recommend building something else as a learning project.   The purpose is to get inside the designers mind, not the build itself.   I learned a lot from studying Stewart Hart’s PottyMill in relation to the full-size engines that inspired him.

Nigel is tackling a difficult project and it feels as if every move he makes is a leap into the dark.  Good fun if you enjoy pottering and learning by doing, but agonisingly slow and frustrating when time is limited.   I suggest his current approach is over-ambitious, and the cure is to develop better design skills by studying what others have done.   Once a good start has been made, the rest of the design tends to fall into place, like eating an elephant a mouthful at a time.   Don’t slash the thinking stage just because its hard work!

As a general design principle it’s good practice to defer detailed decisions for as long as possible.   So, if one decides to build a steam loco from the wheels and chassis up,  don’t veer off into superheaters!   Conversely, if building from the engine down, don’t get distracted by the steering geometry.   Design is an incremental process, part of which is bringing the whole together, which might require wholesale changes to previous work, itself a challenge.    Above a certain level of complexity, design becomes desperately difficult.  It mustn’t be under-estimated.

Dave

 

[Nigel Graham 2Participant @nigelgraham2]

I am not designing fully from cold.

I have helped build steam and petrol/hydraulic locomotives as club projects, partly from published drawings partly freelance.

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If I may be excused a little immodesty it was me who cracked a problem in each of the two steam-locomotives that that had confounded their main builders for a long time. They were far better model-engineers than me: excellent craftsmen at following drawings, and they understood the conventional loco valve-gears (Hackworth baffled them!)

They were stymied for months on one, by a mistake too simple to have occurred to experts: someone forgot exhaust-elbow gaskets need an ‘ole for the steam through ’em. That was revealed when I found and cured a tiny hole caused by slight over-drilling. Even before then I had successfully sealed a leaking silver-soldered joint in one of the fabricated cylinders, by soft-soldering a brass shim to the port’s leaking floor. The other project was delayed even longer by a curious drawing error: the slide-valves were to a “thou” of their own drawing but the builders had not spotted that the motion GA on another sheet, showed their correct length, with much shorter laps. I found this only by dogged re-measuring, and we proved it with a transparent dummy valve, made from acrylic.

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So I’m not a total novice and I am not building my wagon completely from nothing.

I have only old advertising photographs and some leading dimensions as original material; but I use a combination of “full-size” engineering text-books, model-engineering books, and studying published miniature locomotive and traction-engine designs.

Unfortunately I could not draw everything in sufficient detail all at the start. I had to start screwing bits of steel together to determine the next steps. Chassis, wheels and steering first, then the bunkers, footplate and water-tank. Some parts need refining and finishing, the springs need heat-treating (if I can find someone to do that.)

I also made the woodwork, suggesting progress when displayed. The entire superstructure is easily removable for servicing and storage without having to untangle a spaghetti of pipes and small parts.

From there the project drifted slowly from paper-first to trial-and-error metalwork. I measure what exists so I can “design” the next parts; and design them to fit, or modify / replace existing work, as appropriate.

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Worse, in a fit of CAD over-optimism I dismantled my drawing-board beyond repair. So unless I can re-assemble the board to sufficiently useable form, I cannot create proper drawings of more than single, simple parts.

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I was not being “distracted” by asking about steering. I had completed the axle and links to the photographs and using an NTET reprint of a steam-vehicle operators’ handbook for the principles. I asked here, only how many turns of the steering-wheel would be typical, to help me replace the temporary worm-gear. It was not me who was distracted, but respondents who started discussing steering-geometry, which was not what I’d asked! The turning-circle is rather large but the layout follows the fully-frontal photographs.

[Nigel Graham 2]

On Nigel Graham 2 Said:

I am not designing fully from cold. …

Yet Nigel’s first post ended with:

Given I have to design (ish), re-design, make, re-make… this thing all the way through….. Any thoughts on the above, please, chaps?

To me the questions Nigel asks suggest it is the design/redesign element of this project that is causing most of the grief.   What Nigel already knows is fine, and I’m sure he could build a steam-tractor if someone else provided all the ‘words and music’.   Here though, effectively a new model, Nigel is often exploring virgin territory, having to come up with design solutions himself.

I suggest it’s what Nigel doesn’t know about his build that’s the problem.   I feel filling the gaps requires design skills, not workshop savvy with a smidgen of previous experience.   How best to get the skills Nigel needs to make progress?

Dave

 

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Well said, Dave

I think we’re getting into ‘Donald Rumsfeld’ territory:

https://en.wikipedia.org/wiki/There_are_unknown_unknowns

is well-worth contemplating.

MichaelG.

[Martin Johnson 1Participant @martinjohnson1]

I really find SOD & Michael G’s approach rather a put down.  I am also concerned about earlier disparaging remarks about concurrent design and build.

I was a professional engineer, Fellow of the I Mech. E and have been in engineering design all my professional life.  While some design is pretty high level stuff, much of it is simple; I don’t see any reason why lack of design knowledge should be a hindrance to a model engineer.  As a group, ME’s rise to conquer 3D cad (I can’t), 3D printing (I can’t), CNC (I can’t), electronics (I can’t), clockmaking (I haven’t), etc. etc.  So figuring out how to adapt a historic design of steam lorry is really not such a mountain.

The OP has a part finished very viable project that will be unique when (not if!) it is complete.  Steam lorry building is a peculiar art and there are many problems with finding routes for pipework and controls around the basic blocks of engine, boiler, chassis and transmission.  Especially when one adds in the need to house a very over-scale driver!  I should know as I have spent many hours on 2D cad or laying on my back looking at a part finished lorry figuring how in heaven’s name I can get pipe X from A to B while missing all the junk in the way during my own build which was a concurrent design and build project forced by the fact is was a big project and I am not immortal.  The OP is absolutely right that concurrent design and build is just about the only way to proceed on such a project.  It does work, but some scrappage is inevitable, but a worthwhile price to pay to cut down start to finish time.

I look forward to hearing more of Nigel’s progress and chipping in with comments if asked.

Martin

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