Welcome! Log In Create A New Profile Recent Messages

Advanced

Simple slide valve design for model steam engine.

Posted by vandallas 
Simple slide valve design for model steam engine.
May 18, 2011 08:25AM
Hey guys,

I'm making a simple radial 4 cylinder model steam engine for a CAD/CAM project at school, I wanted to use maybe some simple slide valves. Can anyone recommend a certain design?

Thanks a lot.
Re: Simple slide valve design for model steam engine.
May 19, 2011 02:50AM
You have to decide first if you want single acting cylinders or double acting.
Most radial steam engines I have seen are single acting.
Hear is a link to a kit you can get.
Myers models [www.myersengines.com]
Re: Simple slide valve design for model steam engine.
May 19, 2011 01:40PM
Rolly,

Just a single acting, uniflow cylinder. Nothing too fancy.

Another thing I was wondering about is how is thermal expansion accounted for in steam engines? I've read about piston rings being used but obviously they aren't the ones from a IC. Are steam piston rings expensive?

Thanks!

Van
Re: Simple slide valve design for model steam engine.
May 19, 2011 02:41PM
Van it all depends on how large you build your model. I seem to remember the smallest ring I saw was ½”. Most guys I know for small pistons make two-piece pistons and make their own rings. I’ve made plenty of rings but never under one inch. The largest was seven inches. The last model I built was of the Derr boiler and a small chuck about eight years ago.
I haven’t made models for a long time.
You should get a copy of Model Stationary and Marine Steam Engines by K.N.Harris.
Google it. Also check some of the model suppliers. [www.pmresearchinc.com]

Rolly


Re: Simple slide valve design for model steam engine.
May 19, 2011 03:00PM
Rolly

I never thought I'd say this, but that is an adorable chuck.

What material would be best for piston rings?

I'll definitely check that book out, thanks for the link.

I also have another question, while I have you: in a steamchest, how is the valve rod sealed to prevent steam escaping? Is it just machined to a really high tolerance? or is there a special jacket used.

Van.
Re: Simple slide valve design for model steam engine.
May 19, 2011 03:30PM
Van I remembered I had some small parts in a draw. Hear is a small ring I found 0.611.
I don’t remember where I got it.
Old style engines use packing glands. Today you could use vitron O rings. I use ground stainless for models, on large engines I use ground Monel. When I make piston rings I like to use meehanite.
Rolly


Re: Simple slide valve design for model steam engine.
May 20, 2011 06:49AM
Van,
Just my 2 cents, but would model gas airplane engines have rings that might be useable for your project? I haven't fooled with model planes in over 30 years so I'm not sure how they seal the pistons on them now.
Good luck with your project. I'm looking forward to how it works out.

SteveW
Re: Simple slide valve design for model steam engine.
May 25, 2011 02:58PM
Hey guys,

Here's a few pics of what I have so far. I was hoping to get some comments on the valve design. I tried to make it as simple and robust as possible. The eccentrics are in a radial design with a master rod. The plan is to be able to change cut off by altering the stationary eccentric's position.

What kind of metal should I use for the pistons and cylinder housing?

Of course any and all design suggestions are more than welcome.

Thanks a lot for the feedback.

Van


Re: Simple slide valve design for model steam engine.
May 26, 2011 12:40AM
The plan is to be able to change cut off by altering the stationary eccentric's position.
Van

You do not change eccentric position to change cutoff. The eccentric is keyed to the shaft. The valve face is designed to the valve travel and the cutoff is a design factor of the angle of the eccentric position.
The eccentric angle is with the piston at TDC and the eccentric at 90+ 20 to as far as 90+ 45 degrease in the direction of rotation. 90+45 is about the shortest cutoff.

Your design shows open rods, which is the best for increasing lead as you link up.

Audels power plant and engineers guide 1945 starting on page 989 is one of the best books for slide valve design. On page 1019, to 1024 it describes the Bilgram diagram. This is how you lay out and design a slide valve. Way to much to decribe hear.

There are engines where the eccentric can change the offset or valve travel by sliding in a slot so as to have no valve travel, but not the angle. These were used a governors and the engine did not use the link bars.
Rolly



Edited 1 time(s). Last edit at 05/26/2011 12:42AM by Rolly.
Re: Simple slide valve design for model steam engine.
May 26, 2011 12:43AM
Hi Van,

The valve gear should work, I know for a fact I've seen it at least twice in period steam engine books circa 1880-1910, though not in conjunction with a radial engine. That's unique.

The applications for the valve gear were limited, but depending on what you want to do with the engine that may not matter since apparently it is for demonstrating CNC work and not engine design.

Hard to comment about materials without knowing what you want out of it. Since it's a project for school maybe the main concern is easy to machine and just has to run on air for a bit. On the other hand, maybe you want to run it hard over a long period of time... If it's going to sit on a desk or shelf and mostly look pretty, that may be a different matter again. I've built some stuff both for personal and industrial use that was made out of real crud because it was expected to be used once and tossed out, so ease of fabrication and cost were more important than strength and wear. Kinda your call.

Ken



Edited 1 time(s). Last edit at 05/26/2011 01:46AM by frustrated.
Re: Simple slide valve design for model steam engine.
May 26, 2011 01:46AM
Hi Van,

Stanley & my valves for comparison. B139 is 80% scaled 740 Stanley. Corrected for foundry tolerances.

The classic steam engine approach is "cast iron on cast iron" for pistons, cylinders, slide valves, and slide valve platforms/seats.

"Cast iron on cast iron" gives excellent results.

Counterflow/"contraflow" slide valves, way to go!

Peter



Edited 1 time(s). Last edit at 05/26/2011 02:01AM by Peter Brow.


Re: Simple slide valve design for model steam engine.
May 26, 2011 01:55AM
Hi Van,

Slots in these valves allow valves to move toward valve seat/platform, relative to valve stems, to adjust for wear, compensate for misalignment, and also to lift for steam [not hydraulic] pressure relief.

David Nergaard uses carbon steel drill rod for valve stems and piston rods. See mcmaster.com and other suppliers.

The squares on my graph paper shop drawings are 1/8". [0.125"}. Valve platform drawing not dimensioned, but dimensions can be derived by counting squares.

Peter



Edited 3 time(s). Last edit at 05/26/2011 02:45AM by Peter Brow.
Re: Simple slide valve design for model steam engine.
May 26, 2011 01:19PM
Your design shows open rods, which is the best for increasing lead as you link up.
Rolly
Van
I looked at your drawing again and I was wrong you have crossed rods even through your only using one side. That is not what you want. Crossed rods decreases lead as you hookup.


Re: Simple slide valve design for model steam engine.
May 26, 2011 02:45PM
Rolly,

So I can't have one of the rods be stationary?

I've been trying to play with the geometry but can't find a solution. Are there any simple principles someone can relay so I can get a grasp as to what I am aiming for?

Van
Re: Simple slide valve design for model steam engine.
May 26, 2011 03:05PM
Van
Yes you can have one rod stationary. Your only using a single acting cylinder. It’s the relationship of your eccentric that’s wrong.
Re: Simple slide valve design for model steam engine.
May 26, 2011 04:21PM
Rolly,

So the stationary eccentric isn't concentric?
Re: Simple slide valve design for model steam engine.
May 26, 2011 11:55PM
Van
You stated in your opining topic you wanted to use a simple slide valve.
A slide valve is an outside admission valve requiring an eccentric.
As I stated before if it is outside admission the angle when the piston is at TDC is between 90+20 to 90+45 degrease in the direction of rotation.
90+45 being about the shortest cutoff with this type of valve.
Your drawing as I understand it shows a single acting piston requiring only one end of a conventional slide valve with a link bar. The only reason for that type of bar is to hookup or increase cutoff as the engine is running. To do this the rods need to be uncrossed or as it is referred to as opened. Not crossed, crossed rods decrease the lead or open the valve after TDC. With only one eccentric the engine is non-reversible so you’re only using one end of the link bar. Its not necessary in this design, just design the valve with the cutoff you want.

If I am miss understanding your drawing please say so or give us more information.
Rolly



Edited 1 time(s). Last edit at 05/27/2011 12:06AM by Rolly.
Re: Simple slide valve design for model steam engine.
May 27, 2011 02:43AM
Van
Hear are two drawings maybe this will help.
Rolly


Re: Simple slide valve design for model steam engine.
May 27, 2011 11:50PM
Hi Van,

If it is acceptable to eliminate variable cutoff and reversing in a school project, then one eccentric with 4 eccentric rods on it [one per cylinder], with your engine layout, would make a nice, simple, good-running fixed-cutoff non-reversible engine. Just set the eccentric/valve travel for the desired cutoff. Then you have a "will definitely run well" engine design.

If variable cutoff and reversing are desired, then a 2-cylinder double-acting engine with standard Stephenson link motion valve gear starts looking a lot simpler.

Peter



Edited 2 time(s). Last edit at 05/28/2011 11:06AM by Peter Brow.
Re: Simple slide valve design for model steam engine.
May 28, 2011 07:48AM
Hey Peter,

I'd like to be able to variate cut off while still maintaining the radial uniflow design. Reversing is not a priority and I'd rather not even have it be reversing.

Van
Re: Simple slide valve design for model steam engine.
May 28, 2011 10:57AM
Hi Van,

Sounds like a plan! BTW, you have a single-acting "contraflow" engine there, not a "unaflow" engine. Unaflows have exhaust ports in the cylinder walls.

Peter
Re: Simple slide valve design for model steam engine.
May 30, 2011 12:28PM
Peter,

Yeah I know, I rolled them back to simplify what I was putting up here because I wanted the focus to be on the slide valve.

I still haven't been able to get it to work. Right now I'm trying to get all the paperwork done so I can focus on design.

Van
Re: Simple slide valve design for model steam engine.
May 30, 2011 04:06PM
Van,

Here are some links to two great books on the subject of valve gear as applied to steam locomotives:

[books.google.com]

[books.google.com]

They are really worth checking out. Being able to draw and understand a Zeuner or bilgram diagram will also help a great deal. They show very clearly that with a shorter cutoff(inlet closed) admission begins earlier(inlet open), the steam is released earlier(exhaust open) and the compression begins earlier(exhaust closed). For a valve to be optomized for a short cutoff the laps must be right and to be able to do that the valve action must be understood.

Good luck with the project.

Caleb Ramsby
Re: Simple slide valve design for model steam engine.
May 30, 2011 06:24PM
The programs available at the following link allow you to compare the different style / types, and change parameters, etc.

Valve gear simulations


Re: Simple slide valve design for model steam engine.
May 31, 2011 03:22PM
Thanks for the links Caleb,

Once I have some spare cash after paying for school, food and transportation I will definitely pick those up winking smiley

Ahhhh the joys of student life.

The 20 degree advancement, won't that open the inlet port before the piston has finished returning to TDC?

Also,

Is it feasible to make everything out of cast iron? I have the resources to print off most parts in acrylic and then use a lost wax method for casting.

I am going for a function over form type of build, I would really like to reuse this engine later in a project.

So basically, my requirements are: 4 cylinder, radial, variable cutoff, very reliable.

Van



Edited 1 time(s). Last edit at 05/31/2011 03:29PM by vandallas.
Re: Simple slide valve design for model steam engine.
May 31, 2011 04:17PM
Van
The criteria for a good design of an outside admission D valve is to have the valve open before TDC. Using an eccentric operated valve the more, lead which is the term used for the angle the valve opens before TDC the more the valve will be opened just after TDC. And the sooner it will close. Designing the valve itself that will give you lead is lap, the term used for how much the face of the valve over runs the port opening.
In geometry of a piston moving to the end of its stroke there is a point depending on the length of the bore or stroke of the cylinder that there is very little movement of the piston to the top of the cylinder. It’s the feet per second that the piston slows down to almost no movement in the rotation of the crank. This is measured in an angle before TDC and many engine designers would have the lap / lead 10 to 12 degrees before TDC. Two things you want for a double acting cylinder is to have the lap at each end of the cylinder and the compression at each end to be the same.
The eccentric strap increases this angle of lead all the way to the center of the strap that the valve is in the dead spot and the engine will not run as the port is not open to admit steam. On the other hand it can also decrees the angle if the rods are crossed making the valve open well after TDC and closing close to the end of the stroke.
You can get Audels power plant engineers guide used at Abebooks.com used for $11.50

[www.abebooks.com]
Re: Simple slide valve design for model steam engine.
May 31, 2011 07:44PM
Hey Van,

The two links I gave are to books that were past their copyright and scanned by the people at Google. They are free to read fully online or to download as a PDF file. If you follow the link, in the upper right there should be a down arrow with PDF beside it, just click the PDF and it should begin a download of the file.

However, you are in Canada I believe and I know that some of the books that are down loadable for free here in the States are not able to be downloaded from some other countries.

Caleb Ramsby
Re: Simple slide valve design for model steam engine.
June 16, 2011 04:40AM
Thanks for the links Caleb.

I'm wondering about machining the pistons. Will I need O rings? I have access to cylindrical grinders so it shouldn't be hard to get to within 1/10 of a thou if needed.

I also would like help with my calculations if possible.

I used the P*L*A*N/33000 method.

So I have:

600 revolutions at 60kpm
200 psi (so 100 as mean)
1.87 piston travel length * 4 (because 4 pistons)
1.77 Piston diameter

I got roughly 16 Horsepower. Do my figures seem right to anyone?

Thanks a lot!

Van
Re: Simple slide valve design for model steam engine.
June 16, 2011 06:07AM
Van, What is your major? Engineering?
Re: Simple slide valve design for model steam engine.
June 16, 2011 06:34AM
Robotics actually, but I'm doing this for a CADCAM course.
Sorry, only registered users may post in this forum.

Click here to login

All files from this thread

File Name File Size   Posted by Date  
Derr boiler.jpg 36.4 KB open | download Rolly 05/19/2011 Read message
P1010001.JPG 67.1 KB open | download Rolly 05/19/2011 Read message
P5190049.JPG 178.3 KB open | download Rolly 05/19/2011 Read message
1.jpg 58.6 KB open | download vandallas 05/25/2011 Read message
2.jpg 23.6 KB open | download vandallas 05/25/2011 Read message
top.jpg 33.9 KB open | download vandallas 05/25/2011 Read message
angle.jpg 57.2 KB open | download vandallas 05/25/2011 Read message
valve.jpg 79.9 KB open | download vandallas 05/25/2011 Read message
stanleyvalvecasting.jpg 134.3 KB open | download Peter Brow 05/26/2011 Read message
stanleyvalvemachining.jpg 146.9 KB open | download Peter Brow 05/26/2011 Read message
stanley740valvestem.jpg 138.7 KB open | download Peter Brow 05/26/2011 Read message
b139valve.jpg 305.8 KB open | download Peter Brow 05/26/2011 Read message
b139vplatform.jpg.jpg 152.7 KB open | download Peter Brow 05/26/2011 Read message
open rods.jpg 17.8 KB open | download Rolly 05/26/2011 Read message
eccentric settings-1.jpg 21.1 KB open | download Rolly 05/27/2011 Read message
Drawing3.jpg 35.1 KB open | download Rolly 05/27/2011 Read message
ValveGearSimulation.JPG 197 KB open | download Scott Finegan 05/30/2011 Read message
radial 4.zip 6.7 MB open | download vandallas 06/22/2011 Read message
radal4.2.3.png 55.6 KB open | download vandallas 06/22/2011 Read message
radal4.2.1.png 128.8 KB open | download vandallas 06/22/2011 Read message
radal4.2.png 53.5 KB open | download vandallas 06/22/2011 Read message
radal4.2.4.png 162.7 KB open | download vandallas 06/22/2011 Read message
radal4.2.5.png 81.7 KB open | download vandallas 06/22/2011 Read message
radal4.2.6.png 126.2 KB open | download vandallas 06/22/2011 Read message
radal4.2.7 bottom dead center.png 75.3 KB open | download vandallas 06/22/2011 Read message
radal4.2.7 bottom dead center view 2.png 154.2 KB open | download vandallas 06/22/2011 Read message
P6300075.JPG 96.2 KB open | download Rolly 06/30/2011 Read message
P6300077.JPG 87.4 KB open | download Rolly 06/30/2011 Read message
Valve packing sleave.jpg 77.2 KB open | download Rolly 06/30/2011 Read message