Don Ashton’s valve gear for “Speedy”

[JAParticipant @ja]

I am now cutting metal for my “Speedy”. I am building it to LBSC’s book with small modifications such as using two axle driven water pumps and Don Ashton’s valve gear.

I have studied his valve gear using my own spreadsheets, various simulators and reading (and rereading) his articles. Essentially his three modifications are

1. Redesigning the valve porting to give the correct lap
2. Lengthening the lifting arm by ¼”, which means moving the expansion link forward and a longer gear frame, to give good valve performance in both forward and reverse
3. Recommending better support for the reversing (weigh) shaft to reduce deflection of the reversing and lifting arms.

The first two are relatively easy but the last is left to the builder.

LBSC’s design, and, I think, every one else, mounts the reversing shaft in through bushes on the frame plates. The shaft diameter is 5/16” and the distances between the bushes 4” and lifting arms 8 1/8”. I am about to calculate the loads on and, thus, deflections of the lifting arms at the extreme reversing lever settings.

I suspect that no one has done the last modification but if someone has, please let me know. Any details would be useful.

JA

[WearyParticipant @weary]

Strongly recommend that you post your enquiry on Model Engineering Clearing House forum where there is one active Speedy builder, and a couple of active members with experience of this locomotive and its’ valve-gear.

Regards,

Phil

 

[Speedy Builder5Participant @speedybuilder5]

It will be interesting to see how much you calculate the deflection to be. I have incorporated the first 2 of Don’s modifications and my Speedy seems to run well in FWD and REV.

[Nigel Graham 2Participant @nigelgraham2]

Has anyone ever really tried to calculate stresses and strains in miniature-locomotive valve-gear?

I think the well-tried designs are all fine, but from experience with helping build and look after the club’s doubled-size Juliet (from 3.5 to 7.25 inch gauge) and studying the drawings for the Maid of Kent, the weakness in LBSC’s valve-gears was not of component strength but of poor support.

He sometimes suspended key areas of inside motion work from just one side, and guided some parts by swinging links instead of linear slides, so as the bushes wore much of the eccentric travel went into lost motion.

So are Don Ashton’s modifications to beef up the parts, or to fit them with double-sided suspensions as they should have, of course?

There are many Speedies speeding around club tracks, IMLECS an ‘all,  so I would be surprised if you need worry about the strength of motion-work parts.

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I recall this being aired in the magazine a long time ago, with the observation that if the mechanical parts are made to scale (e.g. from original builder’s drawings) they are quite a bit over-strong. That is because their safe working loads are controlled largely by cross-sectional area so reduce by a square law, while the masses and loads reduce by a cube law.

The force on the piston-rod will reduce by not only by the scale of the piston area, but further by the ratio of the miniature steam-pressures over full-size in the cylinder – and that is likely to be lower than a direct boiler-pressure comparison.

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So while it would be an interesting exercise, unless Don Young’s design really does economise on steel I doubt any stress and strain calculations will be really useful.

[duncan webster 1Participant @duncanwebster1]

Speaking from complete ignorance, can you support the weighshaft from a modified gear frame? I wish you luck calculating the forces involved, not an easy undertaking

[Nigel Graham 2Participant @nigelgraham2]

Is “gear frame” applicable here? it’s a definite term for part of the Baker valve gear used on American locomotives.

I don’t know Don Ashton’s modifications so I don’t know if the one I am looking at on Station Road Steam is to LBSC or Ashton, but it has outside motion with Walschaerts’ gear.

From your dimensions above the overhang of the weighshaft is two inches each side so any bending effect on that is on the exposed piece of shaft between the frame journals and the inner faces of the lifting-arms.

I can’t really think the forces on it are sufficient to deflect it, although it seems a bit thin for a shaft under a cantilevered load. On any such mechanism the journals should really extend out to cover that exposed section, partly for support and partly to maintain axial alignment. Either that or a stiffened, cantilevered bracket extends out from the main frame plate, over the top of the moving bits, to support the outer end of the shaft; and that looks the case on this locomotive.

I’m puzzled why the lifting-arms might deflect. They are under tension, not sideways load against their smaller cross-dimension.

I wonder if the real problem is that LBSC’s mounting of the weighshaft was insufficient, leading to rapid journal wear that would spoil the valve-events and give the illusion the parts are bending.

“Curly” Lawrence did sometimes skimp on motion-work journals and suspensions, needlessly because the better design would give little extra work and metal for a better quality machine. As he built so many engines (if he did complete all his own designs), perhaps he did not use any sufficiently for enough wear to show their weaknesses. I don’t know if he did drive them anywhere other than on his track at home or perhaps at occasional exhibitions.

[Nigel Graham 2]

On Nigel Graham 2 Said:

……..

I can’t really think the forces on it are sufficient to deflect it, although it seems a bit thin for a shaft under a cantilevered load…

 

Any force, no matter how small, will cause a deflection, the question is how much. Without knowing the forces involved or acceptable deflection we have to rely on past practice. This can give things which are stiffer/stronger than they need to be, a lot better than the alternative

 

 

[JAParticipant @ja]

Many thanks for the replies.

They sort of suggest that the mounting of the reversing/lifting arms has been ignored.

The loading on the lifting arms is cyclic and 90degree out of phase with each other. This would give a torsional load with the right-hand side lift arm being held rigid by the reversing rod. With LBSC’s overhang and bearing clearances the left-hand arm would rattle around the place. Moving the bearings to the gear frame (LBSC’s nomenclature) would reduce this by giving a far stiffer assembly but not cure it. I also think that increasing the reversing shaft diameter would be sensible. It may be necessary to stiffen the mounting of the gear frames.

I will return to the design of the reversing gear when not cutting metal (when the workshop is too cold or hot or when I am recovering from encounters with the medical profession). I also intend to post the question on the Model Engineering Clearing House forum and look the 15xx and 7 ¼” locomotive arrangements.

I do wonder what manufacturers of full-size locomotives did. Obviously, there was a lot of drawing and making of wooden models. It is quite possible that once a good working Walschaerts gear was arrived at it was used on all future engines.

Hopefully there will be more about this during the winter.

JA

[Nigel Graham 2Participant @nigelgraham2]

Is there any torsional load on anything?

All the moving parts are suspended from pin-joints so the force transmitted to the weighshaft is very small.

What might be loaded? Let’s look at diagrams of the gear (in Martin Evans’ book, for me), and see what is moving where and by what.

The return-crank and crank-pin are under cyclic bending stresses.

The eccentric rod is a relatively long, slender beam alternately in compression and tension, with some lateral force in the vertical plane as it swings across the centre-line, hence is typically stiffened by fluting it.

The expansion-link swings on a bearing on the frame, so that bearing is under a bending force unless supported on both sides. Its journal pin is under alternating shears.

The radius-rod is another beam under tension and compression, and it it lifted up and down the expansion-link by a short link pinned to the lifting-arm. There will be some force transmitted through the latter to the weigh-shaft, but not much because the lifting-link is swinging to and fro. The largest torsional force might even be more from the mass of the suspended steelwork than power-stresses. If so that is negligible on a miniature locomotive.

The valve-spindle is obviously under compression and tension, and there is a low sideways thrust on its own cross-head.

The combination-lever is “simply” a collection of swinging links with some load on the anchor on the crosshead.

By all means try and calculate the forces in these but to be honest I think you are worrying too much. The design, including Don Ashton’s improvements, has been around for a long time and is well-proven. Because the parts are made to scale they tend to have factors of safety well above their full-size equivalents even if we make ours from “ordinary” mild-steel, not high-tensile alloys.

…..

What did the original builders do? Yes they probably did use Wood-Aided-Engineering and accumulated experience but certainly in the 20C engineering design was becoming highly scientific, including stress and strain calculations, and materials testing.

How much Walschaerts gear can be standardised across locomotive classes I would not like to say because each design had its own cylinder dimensions controlling port sizes and valve-events, but they could probably use common parts across ranges of similar classes.

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