Bilgram's valve diagram a graphical method to design a steam valve

[MJM460]

Hi Dan, A very interesting thread.  I like that Bilgram diagram.  I have not seen it before, but it seems more straight forward than the others I am aware of.

 You have clearly put a lot of time into the study of valve gears.  Thanks for taking the time to write it up.

Interesting that Sabina had inside admission at the HP end and outside admission at the LP end.  I suggest that the difference comes from consideration of packing leakage in each case.  Putting the lower pressure makes sense at the HP end as already mentioned, and also helps with valve balance as the LP on the rod area leaves a lower unbalanced force.

At the LP end of a condensing engine, the issue is air leakage in through the packing due to the condenser vacuum.  This air all has to be removed by the air pump, and reduces the vacuum due to its effect on the condensing temperature.  So similar consideration, gland leakage, leads to a different solution in each case.

MJM460

[Dan Rowe]

You have clearly put a lot of time into the study of valve gears.  Thanks for taking the time to write it up.

At the LP end of a condensing engine, the issue is air leakage in through the packing due to the condenser vacuum. 

Hi MJM, thanks for stopping by, and yes I have spent a fair amount of time studying valve gears. As Chris mentioned without a teacher or even answers in the back of the book, it is very difficult to master a subject. I am still learning new things mainly because I am taking the time to write this up and have knowledgeable folks read it and ask questions or point out errors in my understanding of the subject.

The only reversing valve gear that I have spent much time with is Stephenson valve gear. I need to do a refresher on that subject to finalize the model designs for two Shay engines. I will be explaining the graphical method to correct for the angularity of the rods by finding the optimum location for the suspension point of the link. This is covered in Don Ashton's book but it uses an Excel spreadsheet with an add on called solver. I never worked out how to get the answer with the spreadsheet but I did a Shay design for a fellow Shay fan and he got in touch with Don Ashton to work the problem with the spreadsheet. I got the very same EXACT answer. A graphical solution using cad is just as accurate as a spreadsheet and it might not be as simple as a plug and chug solution, but I like the ability to visualize the math with a drawing.

Now that you mention the vacuum on the LP cylinder I realize that this must be the main reason that many of the marine engines have D slide valves for the LP cylinder. Now if it has a piston valve I know to check for outside admission.

Cheers Dan

[Dan Rowe]

Here is the piston valve with the D slide valve. I used P. 214 in Audels book for the procedure and the numbers from the Bilgram diagram. I originally thought that the steam lap for the piston valve should be on the inside because it is inside admission, but this did not work at all on the drafting board. After a lot of tries and fails I switched the steam lap to the outside just like the D slide valve and that worked, so much for logic. Here are both valves in the mid-travel position.



The port opening and the port width were set equal and the bridge on the D slide valve is also equal to the port width. I also set the middle port for the piston valve equal to port width so both ports on the piston valve are the same size. Here are both valves at full travel.



Looking at the drawings both valves have the same size path at extreme travel.

The main difference in the valves is one should be traveling up while the other one is traveling down. Both valves are symmetrical to the centerline so if we turn one over just simply think it is going the other direction. I think the piston valve works fine for both outside admission and inside admission, so no need to draw both valves for the Sabino, which has an outside admission valve for the HP cylinder and an outside admission valve for the LP cylinder.

The final part of this discussion will be about the merits of using over travel or under travel.

Cheers Dan

[crueby]

Hi Dan,
One issue I see with the way you drew the piston valve side. For inside admission, the port H is the steam inlet, the outside ends of the piston are the exhaust. As drawn, the piston valve is so long that the exhaust is not being opened till after the port at the inlet end is opened, which would mean that the exhaust end of the cylinder would not be opened till well after the piston in the cylinder was on its way down, causing back pressure and possibly stalling the engine. The cause is the fact that you moved the steam lap to the outside - that is where the exhaust is on inside admission so you actually introduced exhaust lap. If you compare the piston valve side of the drawings to the slide valve side, the distance from A to F on the slide valve is the steam lap - the distance from B to F on the piston valve should be that same distance.

Oh, and the steam inlet passage for the piston valve version, H in your drawing, is usually larger, since you want free flow in to the center area of the piston valve. Not that important for these drawings, since that passage is never covered. Of more import is the diameter of the ends of the valve vs the diameter of the center post, since that controls the volume that the steam can flow from the inlet to the port to the cylinder. Typically the ends are several times the diameter of the post.

Chris

[steamer]

Inside on the HP is simple    it keeps high pressure steam away from the valve gland.   In marine compounds/ triples  the LP is usually outside admission to keep vacuum away from the glands     low pressure wet steam lubricates packing very well.

[Dan Rowe]

Hi Chris, yes I see your point. I drew it the other way the first time and something did not work right that way. I forget what it was as my brain went to meltdown about an hour ago. I will put the outside lap back on the outside tomorrow and take a fresh look then.

This was the first piston valve I have drawn with a Bilgram diagram, it would have been nice to get it correct on the first try, but no joy 

Yes, Dave, it should have been simple I am fairly sure I made an obvious goof swapping the lap, I can not remember why I did not like it the other way a new drawing tomorrow and hopefully, I will spot the issue or it will just work.

Cheers Dan

[Dan Rowe]

As drawn, the piston valve is so long

Yes, Chris, it would have been simple the first time if I could have simply made the valve shorter, but I have to stick with the valve length you gave me.

Well, that should work for the LP cylinder, so I get a second try on the HP tomorrow.

Cheers Dan

[crueby]

As drawn, the piston valve is so long

Yes, Chris, it would have been simple the first time if I could have simply made the valve shorter, but I have to stick with the valve length you gave me.

Well, that should work for the LP cylinder, so I get a second try on the HP tomorrow.

Cheers Dan

Hmmm, I phrased that badly - what I meant was the length of the valve end (green arrows) in relation to the port it is covering (blue arrows):


You are adding more lap than I had drawn, which is the point of the whole diagrams, I get that, its just that it should have been added on the inside of the valve rather than the outside. Sorry for the confusion, I should have drawn a picture, then even I would have followed what I was trying to say!   

[Dan Rowe]

Hi Chris, no you did not really confuse me I was already there and then some.

I worked it out and it is a simple change to get the HP from the LP I drew. Move the ports away from each other to make the small lap on the outside like it should be.

If you change the size of the pistons on the valve you are not using the numbers on the Bilgram diagram so the cutoff would change.

I will have the drawing tomorrow but the numbers on the Bilgram will work if you just have to drill some holes in metal.

Note to self:....put the distance between the ports on the diagram so Chris can cut metal soon.

Cheers Dan

[crueby]

Great that it's going to come together. I'm still a ways from cutting ports and cylinders, need to do the crossheads, after that may be time to start roughing out the two cylinder blocks and thier mounting plates, lots to do there. The reverse gear was going to be next instead, but I think it may be easier to be able to work from the valve rids down so the parts have something to hang from. The curves in the follower arms and the shapes of the clevises are complex, so being able to have both ends to fit to would help. Bottom line, will be a couple weeks at least till the ports need to be finalized on paper, probably at least a month given summer travel season is coming!

[Dan Rowe]

Hi Chris,
This thread has brought a bunch of facts about piston valves to my attention so it has been fun learning new stuff from books that were published 90 to well over one hundred years ago. I will have time to write the conclusion of this study soon to reinforce the new concepts that were brought out by this open to the world discussion.

I have to draw the double barn door I am installing on the master bath. One of the instructions was to cut a 7mm by 22mm grove in the bottom of the door with a CHAINSAW


WoW, that is the worst idea I have ever seen in an instruction manual and the most dangerous. It makes me not trust anything in the instructions. That is why it is on my drafting board. Doing things twice is a real drag.

Cheers Dan

[crueby]

Barn doors on the master bath...  Cows or horses need to get in there?  Guess things really ARE different in Texas... 

[Dan Rowe]

Yes Chris, things are indeed different in Texas. The State Capital building in Austin only about 40 miles from here has two large doors big enough for a horse and rider and it has been tested for a fact. As in you and the horse you rode in with.

I was thinking more like wheelchair access as the overhead is not large enough for the horses.

Cheers Dan

[crueby]

Yes Chris, things are indeed different in Texas. The State Capital building in Austin only about 40 miles from here has two large doors big enough for a horse and rider and it has been tested for a fact. As in you and the horse you rode in with.

I was thinking more like wheelchair access as the overhead is not large enough for the horses.

Cheers Dan

Wow, they make wheelchairs for horses down there.... Who'da thunk it!   

[Dan Rowe]

Hi Chris, I just posted this on the Sabino thread and it really belongs with this thread:

The main record for any Shay has the angle of advance and the valve travel on the first page. This not only gives key dimensions for the eccentric casting it is enough to make a Bilgram diagram with only one more fact that the lead is 1/16".

I have worked a bunch of Bilgram diagrams for Shay engines. Here is the secret the lead is not really 1/16" but only fairly close. The valve travel and the angle of advance, and the lap distance are whole numbers with fractions. To make that come out exactly the lead is the fudge factor.

Now time for the pop quiz.
Someone give me a number between 200 and 2999.
I will take that shop number and list the angle of advance and valve travel.

Then I will make a Bilgram diagram to show the cutoff only knowing that the lead is approximately 1/16".

Cheers Dan