John, a picture may help, have a read of this to see how to add one to a post, just a screen shot of the basic concept should do.

I think I get the idea of what you are suggesting but at the moment the valve and piston will move in unison but you want the valve to lead the crank by approx 120deg to get the timing right.

Also are those sizes right, they sound big for Gauge O

Edited By JasonB on 09/02/2021 13:07:00

This is the picture that came to my mind from the description. Green pendulum pivoted at the top has it's other end moved 190mm by piston rod. The valve rod being nearer the pivot will not move as far.

Figures are probably what John has used to model the layout on paper.

There is also no real need for an elaborate linkage system to get the valve above the cylinder, just mount a traditional eccentric on outside of wheel and then have the crank pin go right through that. Then simply angle the piston valve hole so it aims at the ctr line of the axle then all you need is a simple clevis to join valve rod to eccentric rod.

If you were really clever then have a slot curved slot in the eccentric where the crank pin comes through and it could be made into a slip eccentric so engine will run either way

Edited By JasonB on 09/02/2021 14:39:54

The most difficult part of building the loco in question will be making the cylinder and piston. In comparison a single eccentric or slip eccentric valve gear is an easy job and will work well.

You could consider using an oscillating cylinder engine with a geared drive but this is not really much easier. Oscillating cylinders are not quite as simple as they look, and have to be accurately made to work properly – perhaps more so than the slide valve equivalent.

I would suggest that if a good "alternative" valve gear for small models existed, then it would have emerged already in the 100 or so years that we have been playing with steam engines.

No reason why not, though perhaps a bit fiddly in 0 gauge and of course only suitable for outside cylinders. It depends how well it fits in with the overall design. It is sometimes overlooked that valve gear is three-dimensional, and has to fit in the space available.

Good to see that people are building live-steam locos from scratch in these smaller scales.

To be honest though, unless you intend the engine to run only ever forwards, any gear more elaborate than a slip-eccentric will need about as many separate parts as any other; but is not straightforward to design..

As Redsetter points out, an oscillating cylinder is not the answer it may seem, and also needs a fair of work to disguise the fact if outside the frames – though some owners of commercially-built oscillator models have done that successfully.

My own society has quite a number of members who own Gauge 0 and 45mm -gauge locos, admittedly mostly commercially built, and the club facilities include a raised track for them. Although I am not such an owner I have noticed that none of their engines normally needs a helping hand to start, but they are all two-cylinder machines and seem to be double-acting, many with a simplified Walschaert's Gear radio-operated along with the regulator. (I don't think they can be notched-up, nor need it.)

So I am a bit puzzled by Norman Dewhurst's observation, and whilst I'm sure he was a good craftsman and his point about crank-axles is fair, I suggest his engines need the Maradonna touch because they rely on a single cylinder – not vice-versa!

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Just to add my 2p worth, have a look at the 16mm soc site. there are plans on there for a quarry hunslet (wild rose) which is slide valve slip eccentric. I used the dimensions to make a slip crank version with outside valve chests like you want. It took a few goes to get it to work but it did and it is a lot easier than making eccentrics and rocking lever to bring the motion outside . Not much metal is wasted if you get it wrong either. It doesn't look too bad visually either. I am sure that it will work for piston valves. Sadly I do not have pictures and the loco was made for a friend so not in my posession anymore. I do have a video of it running somewhere. Don't reinvent the wheel!

Hi John

It might be worth drawing your valve gear at different positions of the crank and comparing the valve openings to the Dockstader simulations. Several of them use an oscillating lever similar to yours, but derive their action from the vertical movement of the crank which is automatically 90 degrees out of phase with the piston.

Brian G