Discussion in 'A Work In Progress' started by Jmccrack, Jul 20, 2018.
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In fact it's not good idea to use 180 degree when it's the same bore and stroke in both cylinders due the steam engine can't self start. Also a fault from designer of steam engine.
Never seen the rounded edge inside the valve for exhaustion in the drawings. All edges around the valve that are in contact with the cylinder ports must not be rounded as to the timing shall be correct, even if the valve / cylinder must once again be grinded due to wear when the valve / cylinder is not longer tight against leakage. With rounded edges, timing is changed when the valve/cylinder is worn down and grinded at new again.
I would not use a threaded hole in the valve for the valve rod. The same design as Stuart does with the valve / valve rod as the valve is movable in a direction to hold the valve on the cylinder by the vapor pressure and keep tight without leakage. See photo.
Hey guys check out the media part of this forum.
Here she is running (sort of). I can’t get any speed. It runs very free but still takes 30 psi to run.
I can’t figure out how to post a video here but there is one in the media part.
Hey guys it’s me again. I can almost get this puppy to run (see media). I have to much play in the slide Valve linkage and if you look at the Grab Cad drw. The linkage is secured to the shaft by a small setscrew. There are to links and a setscrew in each one. After a while the set screws come loose and then the timing is off. This is a weak spot in this design. I am trying to come up with a different design from the eccentric to the slide valve. Because of the position of the steam chest in relationship to the eccentric this could be tricky. The alternative is to come up with some better way to secure each link to the shaft.
It’s ingesting to watch it run. There seems to be a knock on each stroke. And some times if I run just one side I can get the piston to move back and forth just 1/2 a stroke.
The new valves look good. However, it sounds as though there is a constant large blow to exhaust. How are the pistons sealed and how close a fit are they? You don't seem to have much oil in the valve chests.
The piston seals are nice and tight. And you are right I forgot to oil the steam chest.
Those valves don't look to be moving far enough, with a 3/32" offset to the eccentric you should be getting 3/16" movement but the valve looks to be moving less than 1/8" judged by the thickness of the valve nut against the valve chest inlet.
The arm that goes to the eccentric rod does not need to be slotted, so if you do alter them bear that in mind. All the arms should be able to be set without needing to run the engine so set them up, mark where the grub screw touches the shaft when in the right position and mill/file a flat for the screw to bear against. Make sure you set both sides to the same angle, ideally I would have the arms at 180degrees which would mean drilling another hole in the eccentric rod. This could be where you are loosing movement.
Does sound to be blowing somewhere, make sure the valves are free enough to settle onto the port face, if the tapped hole in the nut is a little off that is enough to tilt a tight nut and stop it sealing.
I was thinking the same thing. I took the unit apart the other day to check the offset on the eccentric. It’s is .093”. But you are right when I measure the actual throw on the slide valve I am only seeing .065” the difference is in loose connections. Also I put pressure in the steam chest and tried to move the slide valve by hand and it was very hard to slide back and forth. But that was because I had NO lubrication in it. Back at it today. There is no question when you have situations like this one sure learns a lot
A technical digression, if I may. With a simple eccentric, there are errors in the valve events caused by connecting rod angularity. These can be largely, but not completely, corrected by making the valve slightly asymetrical; the lap and exhaust cavity edges are made slightly different at each end. The introduction of a rocker allows for a further correction to be introduced. By arranging the arms of the rocker at something not quite 180°, it is then possible to have admission, cut-off, release, and compression all at exactly the same percentage piston stroke at both ends of the cylinder. The valve modifications I understand, but I have to confess that getting to grips with the geometry of this rocker modification is way down my to-do list.
I was thinking the same thing. I took the unit apart the other day to check the offset on the eccentric. It’s is .093”. But you are right when I measure the actual throw on the slide valve I am only seeing .065” the difference is in loose connections. Also I put pressure in the steam chest and tried to move the slide valve by hand and it was very hard to slide back and forth. But that was because I had NO lubrication in it. Back at it today. There is no question when you have situations like this one sure learns a lot.
I am going to put a dial indicator on the slide valve itself. I will report back what the actual movement is.
Just had a quick look at the renderings and think I have spotted what the problem may be. They show the arms that connect to the eccentric rods sticking up not down as you have them.
Moving them upwards will solve two issues
1. They will be in line with the arm that goes to the valve rod so no loss of movement due to the angular alignment you have at the moment
2. I said the eccentric rods would need shortening to get things in a straight line but as they need to reach a further distance with the arms at the top the length should now be OK.
Does this help.....
Valve timing is 90° off crank throw.
Obviously both eccentrics must be 90° in the same direction from their respective crank throws.
And the crank throws for a 2 cylinder should be 90° apart not 180° which makes it a "Twingle"
Also if the "throw" on your "D" valve is too great you might also be admitting pressure during the exhaust cycle whilst also blocking off the exhaust port.
The valve stroke has to be correct as does the cavity size in the "D" valve.
Hey guys thanks for all the help. I have tried everything anybody has suggested and still no joy. As you can see I can get to run but not well. I have no play in the valve string. I put a dial on the slide valve itself and I should have .196 total travel and I have .131. As Jasonb sayes I will switch the eccentric lever to point up and see what happens. I am losing movement somewhere. Ken I and Jansonb thank you for your drawings. What you have drawn is exactly what is happening only over a .130” travel not .198” as it should be. As I see it the short stroke works but does not allow the cylinder to produce its full power. Does this make sense?
Here is an Interesting question. If the slide valve is positioned properly but the stroke is 30% short how would the cylinder react?
In an engine with reversing gear you can 'notch up', to shorten the cut-off point and so make better use of the expansion of the steam. The reversing gear alters the timing, but with conventional slide valves that inevityably results in a reduced stroke of the valve. Your valves are opening the ports far enough for perfectly satisfactory running (if properly timed), especially with no load. I think we need to start look elsewhere. Some of the things you have said so far lead me to wonder about friction.
I am not sure how to explain it but when the engine runs now it has a hard knock in it. As if part way thru the stroke air is getting to the other side of the piston. I have a lot of lube in the unit now and if I turn it over by hand there is no tight spots.
Just so everyone knows. This the unit running just after I built it. See the media section
Wouldn’t you know that you pick the drawing I did not use. All the drawings and grab cad show the eccentric linkage up. The one I used shows the eccentric linkage down. I took the steam chest apart flipped the slide valve upside down and then hooked everything up with the cover off. When I rotate the fly wheel by hand my total travel is .084” it should be .198”. Moving the lever to the upright position make no difference still .084” travel.
I am wondering. I have .098” offset. I wonder if this is enough.
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