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Piston Valves - much ado about nothing?

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La Cathedrale23/02/2021 14:56:53
6 forum posts

I want to build a loco with piston valves instead of slide valves, but I see them much more rarely than slide valves.

Other than the challenges of turning the bobbin and the associated faff of another cylinder to bore, more piston rings to make, etc. - is it intrinsicly more difficult? It just seems like a bit more long winded, rather than actually problematic?

If not, and I am being overly sure of myself - is there a suggested resource for me to get my teeth into?

Many thanks,

Andrew Tinsley23/02/2021 15:18:30
1283 forum posts

Simple answer is that slide valves do not need very high precision to stay steam tight, Piston valves on the other hand do need high precision to prevent steam leakage. Piston valves are forced onto their seats by steam pressure. Piston valves rely on excellent fits to prevent leakage.

Andrew.

Tomfilery23/02/2021 15:37:44
128 forum posts
4 photos

Hi,

I suggest you look up John "Bogstandard"'s Paddleduck engine for a place to start.

You didn't say how big a loco you are thinking of building and John's is a small marine engine for driving a small paddle steamer. It was aimed at beginners, so provides a lot of good constructional detail and advice.

Regards Tom

La Cathedrale23/02/2021 15:59:25
6 forum posts

Hi Andrew,

Isn't this the 'question' with cylnders also (i.e. requiring precision to prevent steam leakage) ? i.e. we need to do it (i.e. precise machining) twice rather than once if we use a piston valve?

Hi Tom,

I was looking at either an A1 or a B1 - much like the question of piston valves, I'm also very interested to know how much of a factor a tapered vs parallel boiler is. Basically, is it something that just takes longer with more prep, or is it a cliff edge brick wall of difficulty that will require a bought-in boiler/etc.

La Cathedrale23/02/2021 15:59:57
6 forum posts

ps. I wil review the paddle steamer engine right now

noel shelley23/02/2021 16:47:35
349 forum posts
9 photos

If you have wet steam, a slide valve will lift and prevent a hydraulic lock and blown cylinder covers with luck, a piston valve will not. Cylinder drains are vital with piston valves. Noel

Nigel Bennett23/02/2021 16:53:32
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386 forum posts
11 photos

Many piston valves have been made as solid bobbins, with or without "oil grooves". (The theory is that leaking steam passes into the first groove, expands, and then leaks into the next groove at a much lower pressure. By the time it reaches the fourth groove it's got fed up, and the valve has now reversed direction and steam is on the other side now anyway...)

To maintain steam-tightness with such things requires that the bobbins are turned to such a dimension that at working temperature steam does not leak through to any significant extent. To that end, people have used O rings and (more usually) cast iron rings to provide a seal. It is not easy to provide the necessary porting in a piston valve bore so that the O ring isn't chewed up the first time it moves.

Material choice for the bobbins, liners and/or cylinder block is also a minefield due to dissimilar expansion rates. Non-metallic piston valve bobbins have been made from solid Fluorosint, or cunningly-designed composite arrangements whereby the sealing element can be expanded into tight contact by opposing tapers. I tried solid PEEK in my last loco but I ended up making solid stainless ones in the end. They still leak a bit more than I would like.

It only needs a small amount of detritus (ash, swarf, grit or whatever) to chew lumps out of a piston valve bobbin and/or the liner and start it leaking. Restoring such damage inevitably means renewing the bobbins, whereas with a slide valve you may get away with a quick rub of the damaged faces on an oilstone or whatever to restore the sealing surfaces.

LBSC always maintained that piston valves were easy - read up about it in for example the Speedy book...

La Cathedrale23/02/2021 20:00:06
6 forum posts

@nigel - Mr. Crispin on YT (rolls royce fitter?) uses cast iron pistons/spindle valves with stainless steel shafts, inside cast iron lined cylinder blocks - materials-wise, that seems fairly straight forward? That said, as far as I know despite being an excellent engineer this is his first loco build.

Paul Lousick23/02/2021 21:42:54
1688 forum posts
624 photos

If you have wet steam, a slide valve will lift and prevent a hydraulic lock and blown cylinder covers with luck, a piston valve will not. Cylinder drains are vital with piston valves.

I used to drive an Aveling and Porter steam roller which was fitted with piston valves. The action of the throttle lever was much easier and smoother to operate than those with slide valves. It was also fitted with spring loaded, automatic drain cocks on the cylinder to prevent hydraulic lockup.

Paul

Andrew Tinsley23/02/2021 21:45:31
1283 forum posts

Piston valves are smaller diameter than the pistons themselves, thus are more difficult to make and remain steam tight. I have made both piston valves and slide valves and I know which are the easiest to make! By all means use piston valves if that is your fancy, but be prepared for more servicing to keep them tight.

Andrew.

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