Model Engine Maker
Help! => Specific Engine Help => Topic started by: Cp489 on February 05, 2020, 04:53:15 AM
-
Hey guys,
I have been trying to design a replica of James Watt Steam engine, only this replica will be using compressed air. I am having trouble thinking of how I can get a hold of a 4-way butterfly valve that will turn on and off through an attached push-rod with levers on the main beam. I am wondering if I can get this part machined or does anyone suggest buying one? I have tried to look online for a valve that would work but they all seem to not fit my design. Attached are two pictures for clarity. The size of this valve is small. Right now it is modeled at 1 cubic inch with four 0.5 inch tubes connecting to it.
I am new to this so all help is appreciated. Thanks!
-Chris
-
I would say you will need to make it yourself, the spherical hole in the middle of the block should prove interesting to make!
You may well find it easier to make one with a cylindrical barrel that is drilled for the steam(air) passages as that will be a lot easier to make and seal, another option may be to mill flat into the cylinder rather than drilling.
-
I have tried to look online for a valve that would work but they all seem to not fit my design. Attached are two pictures for clarity. The size of this valve is small.
Think about a valve design something like an oscillating engine reverser's ports (attached)
A 25mm square is about the normal size for one of these valves.
Jo
-
The ancient 4 way plug valve would do the job as well.
https://en.wikipedia.org/wiki/Four-way_valve :cheers:
-
8) that is more likely what was fitted.
Jo
-
I am sure that I have read that Watts first engine had a boy to flip the valve at each end of the stroke. What a horrible job! A great incentive to invent an automatic valve. Your diagram is the first I can remember that so clearly shows how it would operate. I am sure there are many around, I just haven’t seen them.
I suspect you will have to have the valve machined, as most commercial valves are intended for manual operation and have sealing as the priority, rather than highly cyclic operation. Of your engine runs at even 50 rpm, that valve will operate at 100 times per minute. That is highly cyclic operation by any definition. Even though you engine will probably not operate for many hours at a time.
I suggest a cylindrical plug would be quite suitable. Even if you could find the machinery to manufacture a sphere to the appropriate tolerances to achieve the required sealing, I am sure that such machinery would not have been available to Watt, so would not have been true to the type. Alternatively, the type suggested by Jo that has flat sealing faces, well proven in oscillating engines, may be easier to manufacture. Two other posts while I have been typing are in a similar vein.
First think about the steam passages you need. If you are more familiar with electrical switches, it would be a double pole, double throw.
The attached sketch shows the basic requirements. You can see the basic idea. At the top of the stroke, the mechanism will trip the plug anticlockwise, while at the bottom of the stroke, it will be flipped clockwise. About 90 degrees in each case. Then define the body around the plug.
The other not so obvious requirement is that you want to isolate the steam/air supply from the exhaust when the valve plug is half way through its movement. Again the electrical analogy is “break before make”. And obviously the plug should fit well enough to minimise/prevent leakage from supply to exhaust around the outside of the plug. This simply requires careful proportioning of the passages in the plug in relation to the inlet and outlet ports.
I have also made a sketch of a longitudinal section showing a shaft seal to minimise leakage to the atmosphere. It is not pressure balanced, but has only one leakage path to be sealed. Commercial plug valves often use a tapered plug for better sealing, but might be prone to jamming in this operation. I suggest a cylindrical plug, carefully lapped to a reamed bore in the body would be possible, as leakage at least is limited by the low pressures involved. However other forum members will be able to suggest other options.
I am looking forward to following your progress so please keep posting. Just a minor point, may I suggest that you continue by just replying to your own earlier post to keep the whole project in one thread. Much easier to follow that way.
MJM460
-
They simple valve used on "rocking valve" engines is what I was thinking of and very similar to what CNR posted in it's simplest form. Must get round to making my one of these.
-
The Trevithick model dredging engine has a similar valve (I think) plans are online.
Mike.
-
Trevithick used a tapered 4-way cock.
-
Thanks for the help and suggestions so far. I will see what I can do to make it possible and easy to machine.
I'll update when I find a solution.
-
In a simple form just a spindle in a block with the "butterfly" formed by milling a slot on opposite sides. You will need to play about with hole sizzes in th eblock and thickness of the butterfly to get the timing right and stop the incoming air going directly to exhaust.
Also a good example to your students to think about how and if a part can be made given available tooling and a budget when they are designing it.
-
Infact the butterfly can be done if the 4 holes are sized so that their edges touch as if they were mitred then the butterfly shaped as an ellipse with minor diameter equal to the hole size and major diameter 1.404 x hole size and edges rounded to the same elliptical form
-
The butterfly approach might be difficult to seal. Keeping tiny sharp sealing edges engaged to the butterfly may prove difficult. A plug valve with large bearing/sealing areas around the ports, close fitted by lapping, with thick lubricant applied, will likely seal better. (and did, on ancient engines by Watt, Trevithick, etc) Just food for thought.
Boyle's law (p1v1=p2v2) is a good one to remember when designing seals for shafts and valves. :cheers:
-
Jasonb,
I will look at that design and weigh at the differences. Thanks for modeling that. It's very easy to understand. Here is another valve I made, it would need to be redesigned so that the diameters would be closer in length. I think this is what several people were saying. I am not 100% sure though.
-
That would work and very similar to the first option I posted, just holes instead of a milled away area.
-
Perfected the control valve. At least in my opinion. Here is some pictures of the two valve designs, thanks to your help guys.
I added pins that would help keep the lever in place. A tac-weld or two will be needed inside the valve block in order to keep the valve in the correct position.
What do you all think? Any other things I can improve on this?
Thanks again
-
What are you planning to tack weld? I have to say that any sort of welding AFTER machining a close-fitting system seems fraught with danger - but maybe that's just my ham-fisted welding abilities!
-
Hi Cp489 Are you planning to lap in the plug valve in the body to achieve sealing? If so I would not do any welding afterward, or the parts will distort and probably leak. If the part on the back of the plug is a cap to retain it, why couldn't it be attached with a couple of screws to the plug body? Two screws because they won't loosen with valve rotation like a single screw at centre of cap might.
If you use spring plungers to act as detents for the operating lever, be sure to make the mount hole a through hole so you can adjust the ball end's position against the lever easily. you will likely need to. There will be a VERY narrow window of position where the lever just holds vs lever can not be moved.
If the spring plunger was mounted in the body and acted against shallow drill dimples in the plug diameter at the front or rear away from the ports, the action would be a lot smoother and more likely to stay adjusted as the plug to body fit is likely the highest precision fit in the whole assembly.
Have a look at the link below for some info on the design of spring plungers and angled dimples and holes. Vlier make very good spring plungers.
https://www.vlier.com/product_index/sld/sel_06_diam.html
-
Awake,
I am planning to make something that will stop the valve from going past certain points. What would stop the valve from going past the dotted line?
Would the air inlet not allow for the valve to go past this point? May be a dumb question/idea.
-
cnr6400,
Yes, lapping would be an option and is what is mind now. The cap is sealing to the valve, not the valve body. I agree with you and achieving a better seal by screwing it to the valve body itself. I will change that. I will also try to find a different alternative than a tack-weld . Unless the air flow itself will keep it in the desired position.
I understand the fine line between too much and not enough when it comes to the ball sticking out of the valve block. I will research this and see visit the hyperlink you posted. Thank you so much.
I honesty didn't even know they were called "spring plungers". At least I'm learning!
-
Hi Cp489, In plug valves, the seal is made only on the cylindrical parts with the ports cut in them. The end flange on the plug and the back end cap do not need to seal if the plug valve fit is good, and well greased.
You could also control the angle of movement by relieving part of the plug flange outside diameter or the rear cap outside diameter and putting a simple peg in the body to allow the plug valve and cap to move until either end of the relief cut hits the peg. This would save you buying spring plungers and spending time adjusting them. Jpeg is attached showing this idea.
This idea will not provide any force to hold the lever / valve at a particular angle though, if the plug friction is not sufficient to hold it, or the operating rod has two pins around the lever so it can't move. (or a curve fork set as many early engines had). Having the operating rod fully control the valve operating lever position rather than using detents or peg / cut on valve will allow adjusting valve events more precisely, giving better engine timing and higher efficiency.
There are usually many ways to do any mechanical job. :cheers:
-
I suspect there will always be enough friction to prevent the plug from travelling further than intended. The steam pressure and exhaust pressure are always creating a pressure difference in the same direction, thus holding the plug against one side of the housing. The cylinder end connections to the valve reverse twice each revolution of the engine so probably tend to bump the plug up and down a bit, depending on how much clearance there is, which might be beneficial in preventing the plug sticking. A nicely lapped in plug will not have much clearance.
If you do put motion limiting pegs or detents of any kind, it will be important to not over constrain the motion, as the little trip levers which operate the valve have a specific geometry and stroke. It will be worth laying out and investigating their motion before worrying about limiting the plug motion.
I suspect that Watt would have started simple, and only added complication where it was proven necessary.
MJM460