Piston Valves

I'm trying to design and build my first model steam engine. Having thought over the possible drawbacks of slide valves I thought it might be worth trying to build a piston valve but I can't find much information about designing and building piston valves (all the books concentrate on slide valves there doesn't seem to be much information online about general design of piston valves.).

I'm not sure how to make the cylinder in which the piston valve would operate - it would probably be too small to bore and I'm not sure if reaming would give a good enough surface. At the moment drilling and reaming seems the best option.

I'm also not sure how to seal the piston valve - whether to use an o-ring or cut oil groves in the piston.

Maybe there better valve designs that I've overlooked.

Any advice gratefully received.

Will

My first attempt was a crude piston valve engine. I later built a more sophisticated one.

While it is possible to do this succesfully with little experience you will find that a slide valve is much easier as the mating faces are flat so it is easy to get a good seal. Getting piston valves right requires machining to a higher standard to get the same level of performance.

Oil grooves or o-rings will work, but o-rings can be cut to pieces by the sharp port edges that give good crisp vave events.

But you won't go far wrong with teh link that Bog's has suggested.

Joey

Will, just follow Bogs, he won't put you wrong, and even if you'r like me, and wander off on your own, the stuff in Bogs posts will show you how to do the bits . Ian S C

Thank you very much - I've started reading Bog's blog and it's wonderful

What level of accuracy do I need to be able to achieve for the valves to work correctly? (I'm using an old borrowed lathe with a 3 jaw chuck - no 4 jaw - so I'm not sure whether it will be capable of the required accuracy.) Paxman's literature about the Paxman-Lentz engines said "Our tools and men are quite accustomed to work to one two-thousandth part of an inch." so I'm guessing that that's the level of accuracy needed for piston valves as well - I could be completely wrong though.

http://www.paxmanhistory.org.uk/paxlentz.htm#brochure

My plan was to do a trial-run of one valve, check it with micrometer callipers then check its function with compressed air. Is this appropriate or can anyone suggest a better way of determining whether or not the lathe I'm using is up to the job?

Thanks again for all your help!

Will

Hi John,

Thank you very much - I didn't know you could get ground rod stock. Very excited now about the opportunities it opens out.

Is it suitable for making piston rods as well? (Pistons even?)

Am I right in thinking that ordinary ground stock is OK and that there's no need for the 'mirror finish' sort?

Could you suggest some suppliers for ground rod stock?.

A little steam leakage is fine (I sometimes suffer a little steam leakage myself). My plan is for the valve block to be easily detachable from the cylinder and replaceable so that once the engine is made I can experiment with different sort of packing for the piston steam gland and try making a second version of the valve block with o-rings if necessary.

I thought about an oscillating engine but decided to go for one with separate valves - I was fascinated by mill engines when I was a child and wanted for years to build something like a miniature version of one of them. I also like the flexibility of being able to trim the valve timing. (If I'm crazy enough, in future I might even think about one of those complicated regulators that can adjust valve timing - that's unlikely though.)

Will

Sorry - I forgot to ask - what type of steel is best for the pistons of the piston valves and what is best for the valve blocks? Would Silver Steel (BS-1407) be suitable for the pistons or would this be too difficult to machine?

I'm guessing that the piston and valve block should be made from dissimilar metals to reduce wear - because of the precision of the valve do we need to think about differences in the coefficients of thermal expansion of the two metals or is that insignificant?

Sorry for another question and sorry if my guesswork is way wide of the mark.

I'v built a few small, and very small (3 mm bore x 6 mm stroke), wobblers, and I used stainless steel for pistons, and most have brass cylinders, the wee one has bronze. The first engine I attempted was a Elbow engine that I found in a Popular Mechanics mag, not too good, i won't work, it has plain steel pistons in bronze cylinder blocks, as it was for running on air the pistons seem OK. Over the last 20yrs or so, I'v learned a little about different metals, and how they work together. Ian S C

I made my 9f with cast iron cylinders and stainless steel piston valves.

No ground rod as I turned them to fit the reamed and then lapped bores and the fit is good to my standard.

Choose your type of S/steel carefully as some will machine well and another will make junk of the item and the tool.

The main cylinder piston is bronze and with a groove for packing with woven square section PTFE when I can find it.

The glands I use rolled up PTFE tape.

Clive

Hi John,

Thank you very much for the advice to avoid silver steel - that's probably saved me a lot of heartache.

Is mild steel also too rust-prone for use with steam?

I've found the stainless steel road on RS.

Do you have any general advice about selecting metals for valve and engine building? (I don't have a large enough junk collection so I'll need to buy some of the metal.)

Hi Clive,

Thank you very much for the advice about packing. Any more info about the woven square section PTFE would be great! Is the PTFE tape for the glands just the ordinary Boss Blue or Boss white type PTFE tape used for plumbing?

What did you use to lap the valves? Course and fine valve grinding paste or basso - or something else?

Will

Will before you make your decision to go ahead have a look at the design of ports and pistons used by the Hydraulics people. Here 'O' rings are used a-pleanty and last for many hundreds of thousands of operations. the ports however consist of rows of small holes each row is close to its neighbours and are staggered. There was a 'letter to the editor' in ME some years ago to which our good editor added a complimentary comment about learning from others.

Rdgs

Dick

Will, look through REEVES2000 website and they have a good selection of packing materiel for glands and pistons.

Clive

Hi Dick,

Thanks - I thought about looking at the hydraulics community for machining cylinders but I didn't think to look there for valves - I'll see what I find out.

Hi Clive, Thanks very much ffor metnionging Reves - i didn't know about them - I noticed that they've got nitrile o-rings - wonder if they'd be suitable for glands?

Will

Hi John,

Thank you very much - again you've saved me a lot of confusion and heartache.

Thanks for your advice about graphite string and viton o-rings. (I used to have an enormous tub of graphite flakes when I was a child which were a source of constant fascination because of their appearance, texture and electrical and mechanical properties.)

Thanks for your advice about avoiding nitrile and silicone - sounds like it's handy stuff in the chemistry lab but no good for steam (I wish they'd mention this in the catalogues!).

Is there any particular type of stainless steel, brass or bronze that I should use to give a good balance between corrosion resistance and ease of machining? I know there are BS specifications for different stainless steels, brasses and bronzes but I've no idea which one to use. (Here people often talk about 'chrome steel' and say it's got a reputation for being indestructible but difficult to machine.)

Thanks for the photos of the engines - I don't think I've ever seen such beautiful oscillating engines!

Will

The two main types of Stainless steel used by the likes of us are 303 and 316 they both machine OK but you need higher speeds and fine cuts.I usually allow 1/100 of a mm for each 10mm of diameter for clearance and it seems to work OK.I bore/ream and then turn to fit the bore as its a ready made gauge.Beware of burrs on s/steel, dress off with a stone.

Clive

Apologies for my silence - work's been heavier than usual and there's been a delay getting carbide tips through customs.

Hi Clive, Thank you for the SAE 303 and 316 steel grades - now that I've got these I've looked up the chemical compositions of the steels and their properties and have a number that I can hopefully use in dealing with suppliers Thank you also for your advice about clearance and higher cutting speeds - what sort of cutting speed and feed rate should I aim for for these steels? (At the moment I don't have any cooling.)

Re. Dressing off with a stone, should I use the fine diamond stone I use for honing HSS or should I use a different sort of stone?

Hi John, Unfortunately, at the moment my steel supply comes as random lumps of unidentified scrap - so when I find a piece that works well there's unlikely to be another piece the same in the box I'll try to build up experience to tell more quickly whether a given lump is likely to work or not. Let me know if you can find any sort of identification number or classification for the steel that gave good results for you.

Will

I would try speeds at about 150% faster for the stainless steel, its always works for me.

Cooling, I use WD40 or 1 to20 cutting oil.

I use various tooling HSS and Carbide and pick which one gives the best finish.

I dont 'Dress' the finished article with stones but use Buff sticks.

Be careful with Stainless steel as there is sometimes a hard layer just under the surface of the bar and avoid end pieces that have been cut off with an abrasive disc cutter as it leaves a hard bit at the edge.

Clive

Edited By Clive Hartland on 29/05/2012 19:26:33