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Old 06-08-2016, 06:48 PM   #21
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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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Old 06-08-2016, 10:20 PM   #22
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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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Old 06-09-2016, 02:31 AM   #23
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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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Old 06-09-2016, 07:24 PM   #24
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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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Old 06-13-2016, 07:50 AM   #25
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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.
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Old 06-14-2016, 03:00 PM   #26
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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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Old 06-14-2016, 05:09 PM   #27
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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.




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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Old 03-10-2017, 07:13 PM   #28
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Could you tell me where you got your casting for this engine?
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Old 03-10-2017, 07:34 PM   #29
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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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