Author Topic: Steam chest/D valve questions  (Read 1696 times)

Offline crueby

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Steam chest/D valve questions
« on: March 12, 2017, 05:22:41 PM »
On my Lombard build, I am about to draw up the engine parts in 3D, and have come across some features in the pictures of the steam chest that I have not seen before - maybe due to just seeing simplified models, maybe that this is a later engine that I am used to?

Anyway, two items:
1) around the surface of the valve chest, where the valve ports are on the outside of the engine block, there is a recessed groove at the ends beyond where the d-valve slides.

2) On top of the d-valve, they put in a set of spring loaded wipers that push against the top cover. Not sure why they would bother. I tried searching out this one, did find a diagram of a simaler one that even had a vent to the outside in the gap that the wipers created, but it did not say the purpose of all that.

Here is a photo of both features:

and a view of just the surface the valve slides on:

The wipers are not to let the steam out through an upper exhaust port in the cover, in this pic you can see the cover is solid on the near cylinder, and you can see the exhaust port in the far cylinder (with a rag in it):

Any info on why they did it like this? Must be a good reason to go to the effort.

Thanks!
Chris

Offline Jasonb

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Re: Steam chest/D valve questions
« Reply #1 on: March 12, 2017, 05:41:32 PM »
The valve usually just slides passed the recess which stops the valve wearing a ridge in the port face. You also find teh equivalent of that groove running around all four sides of the portface as it also made machining easier particularly when teh valve chest was an integral part of teh cylinder eg you did not have to plane the surface right into a corner.

J

Edit

This is a good example of the port faces being raised from the surrounding area.

« Last Edit: March 12, 2017, 05:50:53 PM by Jasonb »

Offline crueby

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Re: Steam chest/D valve questions
« Reply #2 on: March 12, 2017, 05:46:28 PM »
The valve usually just slides passed the recess which stops the valve wearing a ridge in the port face. You also find teh equivalent of that groove running around all four sides of the portface as it also made machining easier particularly when teh valve chest was an integral part of teh cylinder eg you did not have to plane the surface right into a corner.

J

That makes sense on the recess in the face, thanks!

Any ideas on the wipers on top of the d valve?

Offline Jasonb

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Re: Steam chest/D valve questions
« Reply #3 on: March 12, 2017, 05:54:31 PM »
Wipers may just be there to help keep the valve pushed down against the port face you do see various spring arrangements to do this. Given that the engine was likely to be on rough ground they may have thought it helpful if the valve puck did not bounce off on evey bump in the trail.

J

Offline crueby

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Re: Steam chest/D valve questions
« Reply #4 on: March 12, 2017, 08:00:10 PM »
Wipers may just be there to help keep the valve pushed down against the port face you do see various spring arrangements to do this. Given that the engine was likely to be on rough ground they may have thought it helpful if the valve puck did not bounce off on evey bump in the trail.

J
Thats likely - I've never seen this on diagrams of train engines, though did they do something like that too? Amazing how many variations there are on engines over the years....

Offline paul gough

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Re: Steam chest/D valve questions
« Reply #5 on: March 12, 2017, 10:10:31 PM »
Chris, on your labelled picture of the valve it appears as if you can see through it to the port face. Is it 'hollow'? To me the 'wipers' look like  sealing strips similar to those found on 'balanced' slide valves, relatively common in locomotive practice. Jason mentions they may be to keep the valve against the port face but I would have thought the steam pressure would have been sufficient for this? Paul Gough.

Offline crueby

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Re: Steam chest/D valve questions
« Reply #6 on: March 12, 2017, 10:28:17 PM »
Chris, on your labelled picture of the valve it appears as if you can see through it to the port face. Is it 'hollow'? To me the 'wipers' look like  sealing strips similar to those found on 'balanced' slide valves, relatively common in locomotive practice. Jason mentions they may be to keep the valve against the port face but I would have thought the steam pressure would have been sufficient for this? Paul Gough.

You are right Paul, I found some other drawings they did during the restoration, and the slide valve is a hollow vertical box, it slides on the valve face, and has spring loaded wipers that seal against the cover, and it connects through the body of the valve vertically.

Why? What is the advantage of that style? I haven't heard of a 'balanced' slide valve, going to do some searches on that one....

EDIT:
Okay, to follow up after some searching on 'balanced slide valve', now it makes sense. With larger D valves in larger engines, the force exerted by the large surface area of the top of the d valve gets quite large, causing more friction and more wear on the valve plate and valve. With the balanced valve, the top of the slide valve is instead riding against the top cover, and there is little surface area to push down on the valve face. That way, the force down is constant and determined by the springs under the wipers. Hence, less wear, and less force needed to move the valves. Clever.

Thanks for the pointer on the name Paul!! Always more fun to know why something was designed a certain way, not just blindly copying it.

 :cheers:
« Last Edit: March 12, 2017, 10:35:57 PM by crueby »

Offline crueby

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Re: Steam chest/D valve questions
« Reply #7 on: March 12, 2017, 10:38:41 PM »
The valve usually just slides passed the recess which stops the valve wearing a ridge in the port face. You also find teh equivalent of that groove running around all four sides of the portface as it also made machining easier particularly when teh valve chest was an integral part of teh cylinder eg you did not have to plane the surface right into a corner.

J

Edit

This is a good example of the port faces being raised from the surrounding area.



Great picture! Looks like a compound engine that someone cut in half? Or was that the core used to make the casting mold? Very cool!

Offline crueby

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Re: Steam chest/D valve questions
« Reply #8 on: March 12, 2017, 10:41:00 PM »
Actually just noticed something else - in the first picture of the original post (reproduced here), you can see the steam inlet pipe coming in on the upper right corner of the steam chest.


Offline paul gough

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Re: Steam chest/D valve questions
« Reply #9 on: March 13, 2017, 12:24:51 AM »
Chris, Glad you got something out of my observation. This sort of 'hollow' valve reduces the load on the valve as with a normal balanced valve but with the hollow one's seal 'slips', 'strips' or 'rings' whatever you like to call them it is critical they seal properly. Obviously, if they don't you will get steam from the steam chest going down the ports or straight up the stack wasting it. I would advise you consider abandoning this valve type for your model and fit a solid one to get away from the sealing issues, I doubt the load on the valves in your size will impede successful operation. However it you are wanting to follow prototypical design, then maybe appropriate grades of Teflon, Rulon or even the white kitchen cutting board material will prove successful if they are made to fit exactly and loaded evenly. Looking at the 'grooves' on the port face I suspect that the valve just overruns the inner edge thus preventing a wear ridge forming on an otherwise flat surface, they may also assist in providing wells for lubricating oil to collect (with condensate) and as the valve edge goes over the well it might pick up some oil. Such small but obscure things were often thought of back in 'the good old days'. Regards, Paul.

Offline crueby

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Re: Steam chest/D valve questions
« Reply #10 on: March 13, 2017, 12:44:24 AM »
Good points Paul. At the small size on the model, the open top valve body is not really practical. The slots in the valve face should be doable though.
Like with the differential once its case is on, won't be visible, but we'll know!


Thanks guys!

Offline zeeprogrammer

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Re: Steam chest/D valve questions
« Reply #11 on: March 13, 2017, 12:57:06 AM »
I can't help you Chris.
But then, that's not my job.
I'm the distractor.
How am I doing?
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Offline crueby

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Re: Steam chest/D valve questions
« Reply #12 on: March 13, 2017, 01:27:55 AM »
I can't help you Chris.
But then, that's not my job.
I'm the distractor.
How am I doing?


Huh? What? Did you say something?


 :ROFL:


 :cheers: