question regarding scale thickness valve gear

[Nova]

So brassmasters appears to do scale thickness coupling/connecting rods, but to my understanding it's simply the coupling/connecting rods and return crank and not the components for valve gear as seen in the diagram below. of course I may be mistaken in this regard:
http://msts.steam4me.net/tutorials/imag ... rts_01.jpg

are there any companies that do scale thickness valve gear to accompany those components?

additionally is it not practical to have the brassmasters detailing parts & connecting rods + scale thickness valve gear fitted on an A3 that I intend to run on a minimum of 4th radius curves (the local club has helixes on the club layout with 4th radius inside curves). meaning would there be issues with clearance or the rods binding?

[Hatfield Shed]

The pressed metal valve gear components on RTR models: return crank, eccentric rod, radius rod, combination lever. etc. are typically fairly close to scale thickness (albeit of incorrect form in some respects) while the coupling and connecting rods are typically underscale, not thick enough.

It is well to be aware that putting scale cross section coupling and connecting rods on a RTR model is not straightforward on three counts.

On an outside cylinder loco, the crankpin on the driven wheel will probably be too short to accept scale thickness bushes of the coupling rod and connecting rod.

The greater coupling rod thickness means the supplied screws used as coupling rod crankpins may not have a long enough unthreaded shoulder, and will thus either bind the coupling rod to the wheelface if tightened, or rapidly unscrew if left slack enough for motion.

Then if these components are replaced to enable the scale thickness rods to be mounted, the lateral displacements of the axles necessary for the loco on well below scale radius curvature, combined with the coupling rods and thus crankpin heads projecting further from the wheelface, will probably lead to fouling of the connecting rod on a crankpin head, immediately halting the loco, and often with some damage. (There is constraint on width, only about 8mm total from wheelface to furthest outboard rod face - that of the eccentric rod - if the model is not to look seriously overwidth and probably start clonking platform copings...)

I would want to look very closely at the clearances of a pacific fitted up this way to operate on 4th radius. Hopefully among the club members there will be some mechanism experience to provide guidance on what is practical on the 4th radius helix?

[Nova]

On an outside cylinder loco, the crankpin on the driven wheel will probably be too short to accept scale thickness bushes of the coupling rod and connecting rod.

The greater coupling rod thickness means the supplied screws used as coupling rod crankpins may not have a long enough unthreaded shoulder, and will thus either bind the coupling rod to the wheelface if tightened, or rapidly unscrew if left slack enough for motion.

thank you, I'll definitely keep that in mind.

though IIRC the center drivers do have a sizable crank pin so that may well be left as is (see below linked picture)
http://www.petersspares.com/images/_lib ... 437000.jpg

I may also see if the existing crank threads can be removed and markits/romford/etc. equivalents OR spares used for the center driving wheels can be fitted instead

[drmditch]

Is it worth pointing out that materials do not 'scale'.?

Steel, brass, water, and oils retain their same basic structures and molecular/ crystalline sizes whether you are using a large or small amount. This is well known by people who make working ship models! Although the working loads of locomotive models (at least in 4mm scale) do not represent those of the originals, some parts, for example coupling rods, do exercise comparable functions.

Since we expect models to encounter vastly different curvatures, accelerations, and even handling ( I don't think a real A3 ever had problems with fingers accidentally squeezing it's valve gear), then moving parts have to be engineered for the conditions they actually do encounter.

I am full of admiration for people who make complex live steam models, but I suspect that even they would not attempt to make boiler plates of scale thickness!

[Nova]

Is it worth pointing out that materials do not 'scale'.?

I realise this, I was referring to valve gear with physical dimensions that are a scale 4mm:foot compared to the real thing

[1H was 2E]

It's not just the materials themselves that do not scale; for instance mechanical strength is related to cross sectional area. The con. rod will be 1/76th of the height and 1/76th of the thickness meaning the model con. rod has (1/76)2 of the csa of the prototype (=1/5776th). Mass (weight) which produces inertia/momentum is proportional to volume; to take the con rod example further it's length is also 1/76th of the prototype so the model con rod has (1/76)3 - 1/488,976th) of the volume, and therefor mass and inertia of the prototype. This explains why moving parts are intrinsically more jerky on a model than the prototype but the con rod of a 1/76 model has the strength (=area) to mass (=vol) ratio (i.e.. ability to withstand impact for instance) is 76 times higher than the real thing.
It's not as simple as every physical property being just 1/76th of the prototype (because a 1/76th model of a 76 ton loco would weigh 1 ton) but different properties scale different. That of course is why a model can stop so much more smartly than the prototype but, if it doesn't stop and hits something, generally little damage is done.
Edit to say 2,3 after the brackets mean squared, cubed resp - didn't come out quite as I expected.

[Nova]

It's not just the materials themselves that do not scale; for instance mechanical strength is related to cross sectional area. The con. rod will be 1/76th of the height and 1/76th of the thickness meaning the model con. rod has (1/76)2 of the csa of the prototype (=1/5776th). Mass (weight) which produces inertia/momentum is proportional to volume; to take the con rod example further it's length is also 1/76th of the prototype so the model con rod has (1/76)3 - 1/488,976th) of the volume, and therefor mass and inertia of the prototype. This explains why moving parts are intrinsically more jerky on a model than the prototype but the con rod of a 1/76 model has the strength (=area) to mass (=vol) ratio (i.e.. ability to withstand impact for instance) is 76 times higher than the real thing.
It's not as simple as every physical property being just 1/76th of the prototype (because a 1/76th model of a 76 ton loco would weigh 1 ton) but different properties scale different. That of course is why a model can stop so much more smartly than the prototype but, if it doesn't stop and hits something, generally little damage is done.
Edit to say 2,3 after the brackets mean squared, cubed resp - didn't come out quite as I expected.

that is not what I was referring to at all.

I was referring to the side rods being a scale size, so that it looks like a 1/76ths scale version of the real thing, rather than a thin sliver of stamped metal.

I am fully capable of comprehending that physical properties do not scale, as are most people here I'm sure. and to be honest you're adding needlessly to the conversation after the original question had been answered, which was that whilst the connecting rods and side rods are too thin, the valve gear is a reasonable scale representation of the real thing.

[wlkr]

Dave Bradwell produces excellent etched chassis and locomotive kits for ER 4mm. scale with scale components for the motion. Brassmasters will when the former Martin Finney ex. ER kits are reintroduced. Remember, for almost dead scale 4mm outside motion with the above named products you are looking at very fine forked joints and .05 max. brass wire motion pins. .03 looks better! And don't forget the oiled "Blue Rizlas" when soldering the pins up!