# Piston Valve Vs Slide Valve



## Dom

Good Morning all,
                       I'm in the finishing stage of my first steam engine, a Stuart S50 scratch built from stock material (with the exception of the bed and flywheel) and am now planning my next project, a twin victoria.

Being a larger model i want the Twin Victoria to be capable of "doing work" most likely driving a water pump although i'm open to suggestions.

My question is; are there any benefit's to be found in replacing the slide valve with a piston valve? Also, could i get away with simply redesigning the steam chest or would i have to redesign the entire cylinder valve face, steam chest and associated linkages?
Ill be machining the entire block from stock material so a complete redesign isn't the end of the world if it will result in a better performing engine.

Your thoughts appreciated,
Dom


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## kvom

I know it's possible to make a steam chest with piston valves having the same appearance as the slide valve and no change to the cylinder.  Some locomotives in the past had their slide valves changed to piston in this manner.  In full size equipment there is an advantage to piston valves, but in a model I'd probably stick to slide valves as they are both easier to fabricate and easier to time.  YMMV.


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## Ken I

Slide valves are forgiving of condensate (if you are running on steam) and if the piston hydraulic locks it will simply lift the slide valve - not so with piston types.

Ken


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## GWRdriver

If it was me I would stay with slide valves.  The old saying is, "Slide valves wear in, piston valves wear out," and although both have their advantages the slide valve has more advantages (or less disadvantages) for small, low pressure, engines than piston valves.

Another consideration is this, piston valves are "inside admission" meaning that the steam supply is admitted to a central cavity on the valve spool - the cavity on the spool IS the steam chest .   Slide valves on the other hand are "outside admission," meaning the steam is admitted outside the valve body, into the steam chest proper.    The potential problem is that steam passages for piston valves are not exactly the same as those for slide valves, so if your cylinders are cored or drilled for slide valves they may not be able to be reworked for piston valves without a great deal of time and effort.    The question would be why make that work for yourself, when functionally there would be virtually nothing to gain?

There is such a thing as "faux" piston valves in some small engines, mostly the manufactured engines, but also in some kit designs.  I don't know much about these valves, which are usually a round rod or bar with milled flats which cover and uncover steam passages as the valve moves.  This valve type doesn't appear to produce much power.


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## cncjunior

It has been awhile since I have read up on the valve configuration.  What GWRdriver said about the inside verses outside steam admission is a good point.  The other point that goes with that is the valve gearing needs changing depending on inside or outside admission.  If you are going to change to piston valve you may want to study valve gearing.

Daniel


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## cfellows

The main advantage of a piston valve is that you can reverse the engine by simply reversing the air flow.

The biggest disadvante in my experience is getting the piston valves to fit without an air leak.  I've never been able to make a piston valve engine that didn't have that annoying hiss of air (or steam) leaking.

Chuck


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## Runner

I can't comment on the performance benefits if any of using piston valves over slide valves. However, piston valve engines are easier to make. You don't have the problem of drilling long steam inlet passages at a carefully set up angle or do you need to fabricate the valve chest and cover with often numerous fixing holes required to affix it to the cylinder. Because the valve rod is only exposed to exhaust steam and not HP inlet steam there is no need for a gland on the valve rod. Very careful turning of the piston valve cylinder and piston valve is required. But that's all done in the lathe.

Brian


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## robcas631

Dom have you incorporated a Walschaerts valve gearing system into your first design?


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## robcas631

Anyone have a rough design of a slide valve?


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## robcas631

I used a piston valve on my first steam engine. It worked well! I'm sure it will withstand full steam pressure. However I am thinking about a slide valve to save brass and increase efficiency


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## Philjoe5

Here's a typical slide valve design:







Phil


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## Tin Falcon

Looks like the slide valve wins here. 

IMHO if you are making a stuart  engine or a copy of a stuart engine stick to the original classic design.  And like others said if you are running on steam they are more forgiving  on condensate of course you can install drain cocks to deal with that issue as well. from what I remember seeing most full size steam engines have either slide valves or corliss valves. 

It is good to have a variety of designs in models so you may want to build a piston valve engine at some point as well. 


robcast look at the elers engines designs many of his engines have the classic  slide valve in several orientations. the prints  show the pistion and all the valve assembly parts. 
Tin


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## Dom

Thankyou for all the advice, 
i guess ill stick with the slide valves on this build and move onto piston valves and possibly Walschaerts gearing next time. Could anybody suggest a nice model to build? I want to stick to stationary or vertical engines but experiment with piston valves and Walschaerts gearing. I know these would normally only be found on loco's but for me this is about the learning curve.

Regards Dom


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## kvom

The "paddleducks launch engine" that BogStandard designed and published here and on other sites uses piston valves, and is relatively easy to build.  Bogs did a great writeup explaining everything as you go along.  There are several build threads on this site.  I  built mine in 1.5x.  Plans are metric, so for imperial it's just a matter of multiplying dimensions.


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## Tin Falcon

A simple but nice horizontal engine that uses a piston valve is the PM Research model 3 IIRC the kit is about $77  cast iron and bronze. Rudy Kouhoupt also had a couple piston designs that were basic. 
Tin


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## Septic

I agree totally that slide valves at smaller scale are much more forgiving, cheaper and easier to build, but the challenge of producing an efficient piston valve delivers a nice sense of achievement when it's right....


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## robcas631

I have made three engines that work rather well utilizing the cylinder valve steam chest system.  When I obtain a proper mill I will attempt a slide valve. Being busy I will upload pictures of current projects, I have not had time to do so.


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## Ryker Carruthers

GWRdriver said:


> Another consideration is this, piston valves are "inside admission" meaning that the steam supply is admitted to a central cavity on the valve spool - the cavity on the spool IS the steam chest .   Slide valves on the other hand are "outside admission," meaning the steam is admitted outside the valve body, into the steam chest proper.    The potential problem is that steam passages for piston valves are not exactly the same as those for slide valves, so if your cylinders are cored or drilled for slide valves they may not be able to be reworked for piston valves without a great deal of time and effort.



Not always, Alot of piston valves were made for outside admission so you still have the steam chest. Traction engines from the factory were all made for slide valves but companies sold branchhouses kits for piston valves. The steam chest cover is changed usually, Makes it extend out for more room. and the valve and cage. They use the same valve face, The cage is held against the seat by two bolts from the steam chest. In all cases there is much less wear on the valve gear, on a traction engine take 4x9  (about the size of the valve) times 150 or 175 for boiler pressuer... 5400 lbs plus. On a piston valve there is darn near nothing. 
Ryker


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## kvom

For a stationary engine a Stephenson linkage is easier than Walschaerts and works just as well.  A major reason for locomotives abandoning the Stephenson linkage was that the mechanism is between the frames under the boiler, so maintenance access was difficult.


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## bikerbob

Philjoe5 said:


> Here's a typical slide valve design:
> 
> 
> 
> 
> 
> 
> Phil



Good day Phil
 If plans for the engine above are still available; could you scan and email a copy to me (bikerbobatmagmadotca)
Thanks
Bikerbob


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## Blogwitch

Dom,

There are two main differences between piston valve and slide valve.

A piston valve is no more efficient than a normal oscillating engine, using the same principles for steam use and reversible by just swapping over the steam exhaust/inlet lines by use of a normal fwd/rev/speed valve. But a slide valve does look nicer than an oscillator because of the eccentric system rather than wobbling cylinders.
If I was going to use an engine in say a model boat, I would choose an oscillator any day, less messing about and super reliable, less things to go wrong. Just requiring one servo to control fwd/rev and speed. 

A slide valve is usually more steam efficient, especially if more than one cylinder and expansion cylinders are used, the down side is that it requires mechanical means to swap the eccentric timing to make it run in reverse.
If you want it to power something remotely, it would require two servos, one to control the reversing linkage and another to control the speed.

John


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## Charles Lamont

The OP was three and a half years ago so it does not matter much, but John, I am puzzled by your claims here. You say a slide valve is more efficient than a piston valve. Why?


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## Blogwitch

Norman,

We are talking about MODEL engines here, whereas in full size the opposite should be the case.

In full sized, slide valves (and oscillators) came first, and stood the test of time, but as boiler pressures got higher slide valves became inefficient and wore away quite quickly, so a few of the old loco engines got refitted with piston valves and outside operating systems (Walchaerts) whereas new engines got built with piston valves from the start. 
They can do this in full sized as it is fairly easy to put piston rings onto the piston valve to prevent blow-by, whereas in model size, it boils down to high precision machining to do the same thing (you can't use rubber 'o' rings to seal them as the sharp edges cut them up, so invariably, in model size, after a very short while, piston valves start to blow steam out of the top and bottom because of the wear between the piston and it's sleeve (if it isn't leaking steam already because of not a tight enough fit between the piston and sleeve) and it doesn't really help by fitting a stuffing gland at either end of the piston rod, you get internal leakage then between top and bottom ports, which causes all sorts of problems at times.

You can also bring into the equation on a slide valve engine that of steam injection timing, where, as the engine reaches higher speeds, a smaller dose of steam can be injected into the cylinder at a later timing and still have the same speed and power output. This would be used on model locos rather than on an engine designed for a model boat as there would be an operator on board of a loco to do it rather than on a model boat where the operator wouldn't be in intimate contact with the model and so couldn't control it correctly.

In the model fraternity, we are blessed with not having to resort to high pressures to get our steam engines to work, so you will find that in our sizes, what I stated before holds perfectly true, you will get more efficient running from a slide valve than you will from a piston valve, purely because in the sizes we are talking about, the slide valve will always have less steam blow-by loss.

I didn't want to say anything on his post about the 'Borkum' build as it wouldn't be right to criticize it there, but that has the most inefficient piston slide valve engine available, and if you were to ask him to show the model on the water after say an hour running it in, you wouldn't be able to see the boat for the steam coming out of either end of the piston valves. I know, I have thrown many of those engines away for people and fitted an easy to operate wobbler instead. 
I designed the 'Paddleducks' engine, not for use in anything, although it could be used if it was made to very tight tolerances and kept that way, but as an engine to give experience in making one. 
If it wasn't to be seen, in fact, even if it could, I, as before, would recommend a wobbler every day of the week over any others.

John


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## Charles Lamont

Blogwitch said:


> Norman,
> You can also bring into the equation on a slide valve engine that of steam injection timing, where, as the engine reaches higher speeds, a smaller dose of steam can be injected into the cylinder at a later timing and still have the same speed and power output.
> John


I don't know wkat you mean by a later timing, but I think you are talking about using lap on a slide valve and eccentric 'advance' to provide an earlier cut-off and consequently a degree of expansive working. This is generally done to some extent even in engines without reversing gear (but is, of course, impossible with the simple form of model oscillating engine). And it also applies to piston valves, which are exactly the same as slide valves in this respect. If a non-expansive engine is altered to work expansively, the power output, at a the same pressure and speed, would be less. However, because the engine is working more efficiently, it can give more shaft power (for example by running faster) for the same *boiler* output.


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## goldstar31

Blogwitch said:


> Norman,
> 
> 
> 
> John


 
Well, not this one! I positively hate 'steam' and I think that my last incursion was trimming and stoking a small but full size thingy which was about the same size as a sea going tug. Before that, my great uncle had been buckled and arthritic from driving the Royals on the old LMS line out of Carlisle and a family who had been 'in steam' since Timothy Hackworth who presumably employed my great or was it great great grandfather at Shildon?  Somewhere 'Blackadder' Rowan appears as well. Whatever but we all drifted away.

For such, probably the best is dear old Don, writing as D.L.Ashton and his epic Walshaert's Gear and Stephenson's Gear stuff. Last heard of was building a fullsize steam loco as a world authority on tug boats and the saxophone.

I jest not- he used to write and dedicate music to my late wife! The last real involvement  really was something called 'acid corrosion' in engines.

Heady stuff- indeed.:wall:


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## cfellows

Hey, John, good to see you out and about.  I have to agree with you, it's near impossible to get piston valves to seal and even if successful, they will wear and start leaking.

Oscillator engines might not look as sexy or sophisticated, but as prime mover model engines go, they get the job done with far less work, they take up less room in your model, and they are quite reliable.

On the other hand, as a running scale model, double acting, reciprocating steam engines with slide valves are among the most beautiful and fascinating to watch.

Chuck


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## Blogwitch

Hi Chuck,

Still trying to get over my driving holiday, 2,200 miles of bad roads on the way back (including lots of goat tracks in Romania and Bulgaria) over 5 days really got to me, about a week later. Ended up laying on my stomach for nearly a week, and still a bit out of it now. Time to think about retiring from doing things like that, but on the other hand, I am off on another one, just a short one this time, to southern Germany, early September.

Really glad to hear that you are almost in the clear, you had me very worried for a while.

With regards to oscillators, I still have people from the model boat fraternity contacting me to see if I am going to make any more batches, the last batch I made (qty 6 upright and 2 horizontal) a few years ago turned out perfect and I haven't had a single complaint about them, they seem to be lasting forever. Fully stainless ballraced with all stainless rods, shafts and fasteners plus brass or bronze almost everything else.







I do agree with you about slide valve engines and their control gear, they look so majestic in operation.

I have a very rare set of laser cut control gear and main plates for a 'Steamech' engine. 

[ame]https://www.youtube.com/watch?v=Fc3qxbap1qY[/ame]






and a very full casting set for 'Borderer' by John Bertinat, in either slide valve or piston valve configuration, but as things with me are going, they will never be made.

John


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## larrydoucet1946

Could you tell me where you got your casting for this engine?


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## Blogwitch

The two at the top are pure barstock, a modified design of a French engine to make it easier to produce and they run much better as well.

The video is a Steamech engine, again made from bar stock, so no castings, but at a certain point in time a few years ago, the laser cut linkages shown in the last picture were made available for a very short length of time (definitely no longer available).

John


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## Anatol

Blogwitch said:


> With regards to oscillators, I still have people from the model boat fraternity contacting me to see if I am going to make any more batches, the last batch I made (qty 6 upright and 2 horizontal) a few years ago turned out perfect and I haven't had a single complaint about them, they seem to be lasting forever. Fully stainless ballraced with all stainless rods, shafts and fasteners plus brass or bronze almost everything else.
> 
> 
> 
> 
> 
> 
> I do agree with you about slide valve engines and their control gear, they look so majestic in operation.
> 
> I have a very rare set of laser cut control gear and main plates for a 'Steamech' engine.
> 
> https://www.youtube.com/watch?v=Fc3qxbap1qY
> 
> 
> 
> 
> 
> 
> and a very full casting set for 'Borderer' by John Bertinat, in either slide valve or piston valve configuration, but as things with me are going, they will never be made.
> 
> John



Hi John
I know this is an old thread but hopefully you'll see this note. Speaking as someone who has do quite a lot of research hang thinking but no actual making yet - 

1.Wobblers. I'm attracted to idea due to the minimizing of parts, but I wonder about two issues. A. port timing. How optimal can you get it?  B. Seal. How do you make the best seal on those plates that slide over each other without overly increasing friction from the bolt use to force them together?

2. Piston valves. Interesting conversation. At what size do you think piston valves become viable? Are there any new materials which might be applicable? (self lubricating composites with minimal thermal expansion...?

3. Paddleduck. Might just have to be my first build


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## Blogwitch

Anatol,

To cover your questions in the same order.

Wobblers - they can be got to very efficient engines. In fact, some of the early steam ships had oscillating engines, with something like 7 foot bores or even larger, so they weren't just used for models.
Having made piston valve and slide valve engines as well, I always liked oscillators purely because of their simplicity, and if made well, are just as efficient as piston valve engines. My motto is that what isn't there, can't go wrong.
The more precise you can get the port timing, the more efficient and well the engine will run, if you can get them spot on, you can get a great engine. You can easily get rid of a lot of friction between the port faces by machining away most of it (just about 0.005" deep), leaving where just the port faces sit plus a little more will remove most of the friction. In the two pictures below, you will see an engine that was sent to me to get running, the ports were in the wrong position, the second one shows the holes repositioned perfectly and the friction area cut back between the two sets of ports.
One thing you most do, after the engine is run in, is to slacken off the spring loading until just before the faces start to lift off each other and start to lose steam. It pays to have two steam control valves, one to control the max amount of steam out of the boiler and the other to control the amount of steam sent to the engine, the second one should be locked up solid to prevent the engine getting too much steam and lifting the port faces. The first control valve can be used to shut steam off from the boiler so that you can top up your lubricator without resetting the steam eventually going to the engine.

Piston valves - in the sizes we make them we are limited to what we can do to eliminate steam leakage, plus sometimes you have to get very close tolerances onto the spool to allow them to work, so as such, not much you can do as sharp edges tend to chop up any seals you try to introduce. The correct materials can make the engine last a lot longer, do not try to use silver steel as the spools because of it's good fit and finish, it will rust up in no time and ruin everything you have done. I tend to use ground stainless in either a brass or bronze valve block. If you can't get stainless, I have seen used either bronze or brass for the spool to fairly good effect to reduce steam leakage.
BTW, both wobblers and piston valves can be controlled by just two channels on a model boat whereas a slide valve requires a third channel to operate fwds/reverse.

The 'Paddleducks' was made on the fly, making it out of bits from my scrap box and designed as it went along from the junk I had, but even so, it will give you a good understanding about how things are made and held while machining to get to a finished product, and if you do build it, you will be a lot further along in you education than you were before.
Although a little large for what it is (because of it being a slow revving long stroke) in say a 4 to 5ft boat and a reliable boiler, it should do well. Plus if you have a little spare time you can change it from the basic first picture to the fully blinged second one.

I hope this helps a little

John


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## Anatol

John
Thankyou, I really appreciate your reply. 

"I always liked oscillators purely because of their simplicity, and if made well, are just as efficient as piston valve engines."
I agree entirely about the wobbler having less to go wrong. Your wobbler with the gear output is similar to my thinking, though I'd use a belt or chain. Are plans available, or do you recommend a particular design (some of the 'famous' designers - Elmer, ET Westbury, etc had wobblers.)

My general design goal is to find ways of reducing precision and moving parts, so I'm looking at wobblers, uniflow exhaust, and rotary valves, and overhung crank geometries to avoid making crankshafts. I currently think a double acting two cylinder V on overhung cranks might be a good way to go, with eccentric operated piston valve intake and uniflow exhaust. But I'm looking at all kinds of layouts. - triradial is nice. I've thought about a single acting 4 cylinder boxer too.  

"the ports were in the wrong position"
so what is the right position? Can you say more about port geometry on the wobbler.  As I understand it, the intake opens before TDC, and the exhaust is only open around BDC. So there seems to be lots of negatives - inlet too early, incomplete exhausting. Do you think these concerns are significant? I can see making  the exhaust port elliptical for longer exhaust, what else can you do to improve timing? 


"I tend to use ground stainless in either a brass or bronze valve block".
noted. so you're saying that with enough precision and the right materials, piston valve are viable even at small scale?  What about delrin an that family of plastics?I'm thinking a 'self lubricating' composite with minimal thermal expansion would be a good thing, but don't know how they hold up in steam. 

"The 'Paddleducks' was made on the fly,"
I know, and as I recall you used hollow rotating valves, which interests me, , but I need to review the files.
more later, gotta go.


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## pipemakermike

I have produced all the drawings for a 2ft narrow gauge loco that is now running on the Lynton to Barnstaple railway in Devon.  This loco is a recreation of an 1898 Baldwin for which there are no drawings but a lot of photos.  The design was significantly modernised to improve the power and drivability of the loco and part of that was to replace the Baldwin slide valve design with a piston valve design that fitted within the external shape of the original slide valve.
You will find pictures of the redesigned valve here
you will need to scroll down a bit.

Mike


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## Bentwings

Dom said:


> Good Morning all,
> I'm in the finishing stage of my first steam engine, a Stuart S50 scratch built from stock material (with the exception of the bed and flywheel) and am now planning my next project, a twin victoria.
> 
> Being a larger model i want the Twin Victoria to be capable of "doing work" most likely driving a water pump although i'm open to suggestions.
> 
> My question is; are there any benefit's to be found in replacing the slide valve with a piston valve? Also, could i get away with simply redesigning the steam chest or would i have to redesign the entire cylinder valve face, steam chest and associated linkages?
> Ill be machining the entire block from stock material so a complete redesign isn't the end of the world if it will result in a better performing engine.
> 
> Your thoughts appreciated,
> Dom


my chilertern quad has piston valves otherwise then a lot of very small parts it’s going together ok the instruction are a bit sketchy but after reading them o near memory ad picking up inferences like valve timing instructions say look at picture for eccentric positioning or timing.
picturs are worth a lotof words but noting where the Allen wrench is positioned and an “ about angle” it’s going ok . As for advantages vs a slide valve I think wear is the biggest issue the slide piston valve just uncovers ports . As long as there is not excessive piston wear I don’t think timing will vary much over the life of the engines. The way mine is set up there will be direct lubrication whether steam or compressed air. Brass is the usual material but bronze might be better in the long run as steam can get corrosive and might get after he zinc in brass I’m not really up to date on that. My cylinders have a piston slid valve block attached to the cylinder block it uses an eccentric to operant the piston port valve there ar a bunch of parts involved the timing is set much like automotive timing go too far and he engine runs backwards I already have a reversing valve . My guess is someone spent a lot of time coming up with the timing event much like automotive cam timing . There just is no spec and not an easy way to find top dead center or the reverse . I thought of a micro degree wheel but I don’t have a way to even print one let alone attach it . You could experiment with port timing I guess but there really is not an easy way to judge performance short of a prony brake or some kind of dyno. That’s too far down the road for now. 
byron


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