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Balancing the steam pressure on the slide valve

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Joe26/08/2009 13:56:37
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Gudday all,
I am building a 5" Simplex and I would like to balance the pressure of the steam within the steam chest so the pressure forcing the slide vale is greatly reduced or eliminated. How can I do this? I have heard of it being done but I dont know the design of the parts or how efficiently it works. Can someone please help me. I have almost completed the chassis and running gear. I have only to make the expansion links and their coupling rods, join the steam pipe up and it will run on compressed air.
many thanks for any help.
Joe (Australia)
John Baguley28/08/2009 18:56:15
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517 forum posts
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Hi Joe,
 
I'm presently working on an 0-6-2 tank loco based on Simplex castings and my intention is to fit balanced slide valves to reduce the load on the valve gear.
 
The principle behind balancing the valve is to isolate part of the upper surface of the valve from the steam pressure in the steam chest so reducing the force pressing the valve onto the port face. If you do a search for balanced slide valve on the net you'll find some info which may be useful.
 
There are various ways by which this can be done in 'our' sizes but will probably require the valve design altering and also changing the drive from the slot and nut to a buckle type drive on the outside of the valve.
 
One simple way is to turn a short spigot on top of the valve which holds an O ring between the top of the valve and the underside of the steam chest cover. This seals the area inside the O ring from the steam chest pressure. I am not sure how long the O ring will last used like this as they are not really designed to slide. The underside of the steam chest cover would need to have a very smooth finish. This method has been used though and described in Model Engineer a couple of times.
 
Another way is to fit a 'balance' piston into a bore in the top of the valve which presses and seals against the steam chest cover, again isolating the part of the valve containing the piston. Another version of this has the balance piston mounted in the steam chest cover and the piston has a flat plate that presses on the top of the valve. If you have already made the valve, this option might be possible as the plate could press on the four corners of the valve left after machining the slots for the drive nut and the valve spindle?
 
Both of the above methods need a small 'vent' hole so that any steam that does get past the O ring or balance piston is vented to atmosphere or exhaust otherwise the pressure will build up on the top of the valve and render the balancing ineffective.

I've done a bit of research through articles in Model Engineer etc. and it seems to be the practice to make the area of the balance piston (or O ring) about 0.9 times the area of the exhaust cavity in the valve.
 
I'm just designing my set up at the moment but haven't finalised my ideas yet so I can't say for definite how I will do mine.  However, I do intend to have the exhaust exiting through the top of the valve and a balance piston and then through the top of the steam chest cover. (This is the method Don Young used for his 5" gauge Hunslet). This gives a freer exhaust as the steam does not have to exit via the usual tortuous route through the exhaust port in the port face. If you like, I'll post some more details when I've finally sorted the design.
 
John
 
 

nick feast29/10/2009 07:59:10
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75 forum posts
7 photos
Morning Gents,
Is there any advantage in doing this extra work? If the valve is restrained from lifting it won't clear water in the cylinders so cylinder drains will definitely be needed and possibly cylinder relief valves in case of priming on the move.
You need the down force to seal the face and it will take many many miles of running before the valve wears as long as it is lubricated.
Full size engines managed with unbalanced valves ok as well in the past. Best of luck!
 
Nick
 
John Baguley29/10/2009 10:10:19
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517 forum posts
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Hi Nick,
 
I think it will have an effect on reducing wear although how significant that will be remains to be seen! I'm doing it more as an experiment (as a lot of my 'mods' are) to see what the effect will actually be. I want this loco to be as maintenance free and long lived as possible as it's going to be used for passenger hauling at the club on a regular basis.
 
I've more or less finalised the design and am going for a balance piston on top of the valve. This will be hollow and allow the exhaust to escape through the top of the steam chest rather than the rather tortuous route via the normal exhaust port. The diameter of the balance piston is such that the force holding the valve onto the port face will only be 90% of what it would normally be, but still leaving enough force on the valve to hold it on the port face.
 
The valve can still lift if necessary to allow any condensate to clear as the balance piston slides in a bore in the valve.
 
Much of this loco will be 'experimental' . The boiler has been redesigned to the late Jim Ewins formulae with a much bigger firebox and more fire tubes. I am now looking at designing a Lempor exhaust system for it as one will fit nicely using the original chimney height so it won't look out of place. All good fun!
 
John
Ian S C29/10/2009 10:31:49
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7468 forum posts
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HI guys I'm no steam man,but definitly an experimenter,I'd tend to try one cylinder balanced,and the other unbalanced to compare the wear rate,could be interesting.IAN S C
nick feast30/10/2009 07:55:49
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75 forum posts
7 photos
Hi John,
 
Experimenting is great fun if it works out! My experience with running the Q1 for a couple of seasons passenger hauling is that the biggest problem seems to be grit. With a closed regulator there is a chance grit will be drawn down the blastpipe and with inside cylinders it doesn't have far to go to cause trouble. A snifting valve is the answer but thats something else to go wrong!
Good lubrication is important, obviously. Next Q1 I'm building will have CI cylider and bronze valve, will be interesting to compare against bronze on bronze.
 
Nick
 

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