36x60x54 Twin Tandem Mill Engine
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I lost the source on these photos (not mine).
The links would appear to be two-piece, bolted together, with a forked connection on the side ?
So the slot in the link is 90 degrees to the typical position ? Am I seeing that correctly ?
.
The links would appear to be two-piece, bolted together, with a forked connection on the side ?
So the slot in the link is 90 degrees to the typical position ? Am I seeing that correctly ?
.
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In practical terms as this is not going to be an exact copy I think it would be easiest to base the valve gear on a known model, just scaling it to suit your engine.
The curved expansion link is the most common and not too hard to machine with a rotary table so finding suitable examples would be easy and save getting bogged down in designing from scratch as you have enough on your plate with this engine already.
Something like this would work, all the parts were quite straightforward to make. The two "D" shaped bosses are what the eccentric rods connect to, lever bottom left goes to the control rod that is moved to change position.
The curved expansion link is the most common and not too hard to machine with a rotary table so finding suitable examples would be easy and save getting bogged down in designing from scratch as you have enough on your plate with this engine already.
Something like this would work, all the parts were quite straightforward to make. The two "D" shaped bosses are what the eccentric rods connect to, lever bottom left goes to the control rod that is moved to change position.
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I'm not sure you have your links from the bottom bell crank in the right place.
It is usual to raise and lower the expansion link. As it is raised or lowered you then get a "straight line" from the eccentric and it's rod to the horizontal rod that goes to the die block and then onto the valve. If you do it as your sketch suggests then the double link will be angled.
If the double links are to be raised and lowered then I would have expected to see them lifted nearer to the right hand end, this combined with a straight slot rather than curved is Allen valve gear not Stephensons
It is usual to raise and lower the expansion link. As it is raised or lowered you then get a "straight line" from the eccentric and it's rod to the horizontal rod that goes to the die block and then onto the valve. If you do it as your sketch suggests then the double link will be angled.
If the double links are to be raised and lowered then I would have expected to see them lifted nearer to the right hand end, this combined with a straight slot rather than curved is Allen valve gear not Stephensons
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The most common arrangement of a Link Motion, with the expansion link moving up and down is indeed known as Stephenson, although it was invented by his employee Howe.
If the expansion link moves on a fixed rocker, and a radius rod to the valve rod moves up and down the link, then the link is curved the other way, towards the cylinder, and it is called Gooch.
If the expansion link and radius rod both move, in opposite directions, and the expansion link slot is straight, then it is called Allan.
If the expansion link moves on a fixed rocker, and a radius rod to the valve rod moves up and down the link, then the link is curved the other way, towards the cylinder, and it is called Gooch.
If the expansion link and radius rod both move, in opposite directions, and the expansion link slot is straight, then it is called Allan.
Hello All, It has been a minute, a lot of other projects and things happening. I spent a ton of time studying the drawings to at least figure out the "essence" of what the valve gear looked like on this engine. I have a pretty clear image of this system now. Unfortunately, the bed drawings do not show bolt patterns for the fixtures attached to it, so I am designing how to hold everything where it should be.
As a whole, I think the reversing gear is very similar to a set of plans for the A THWATTES AND CARBUTT TYPE REVERSING ROLLING MILL STEAM ENGINE, drawn by J.A.M. DE WAAL PAPAKURA.
The "lay shaft" is an idler shaft driven with a 1:1 ratio off the crank. This engine uses the Stephenson link but is a bit odd as it uses a separate reversing rod for the HP and LP valves on each side of the engine.
The reversing linkages are moved up/down via a steam piston on the outside of the engine. All 4 rods are moved via one common shaft under the engine. The counterweights for those reversing rods are also under the engine.
I spend a ton of time on the links. Since each link needs to hold two reversing rods, I was pretty confused on how to feasibly build this. I ended up referring to Kozo's Pennsylvania A3 Switcher book and completely based these links off of his design. These are a built up "sandwich" that would be silver soldered together.
Up next is continuing coming up with support structures for the links and the valve stem linkages. There is a center casting between the two beds, right behind the LP cylinder, that hold all of the linkages and shafting for the valves.
As a whole, I think the reversing gear is very similar to a set of plans for the A THWATTES AND CARBUTT TYPE REVERSING ROLLING MILL STEAM ENGINE, drawn by J.A.M. DE WAAL PAPAKURA.
The "lay shaft" is an idler shaft driven with a 1:1 ratio off the crank. This engine uses the Stephenson link but is a bit odd as it uses a separate reversing rod for the HP and LP valves on each side of the engine.
The reversing linkages are moved up/down via a steam piston on the outside of the engine. All 4 rods are moved via one common shaft under the engine. The counterweights for those reversing rods are also under the engine.
I spend a ton of time on the links. Since each link needs to hold two reversing rods, I was pretty confused on how to feasibly build this. I ended up referring to Kozo's Pennsylvania A3 Switcher book and completely based these links off of his design. These are a built up "sandwich" that would be silver soldered together.
Up next is continuing coming up with support structures for the links and the valve stem linkages. There is a center casting between the two beds, right behind the LP cylinder, that hold all of the linkages and shafting for the valves.
The Stephenson valve gear uses a forward and reverse eccentric to get the motion of the link. The eccentric lobes would be set 180 deg apart such that the large part of the lobes are opposite each other. However in real life, there is an advanced angle that would bring the lobes away from the 180 deg.
I am not too familiar with the Stephenson valve gear enough to know yet. Do you all have any good literature on the theory?
The reason I am going this deep is because I do not know what the dimension is between the linkage pivot point axis to the eccentric rotational axis.
Thanks!
I am not too familiar with the Stephenson valve gear enough to know yet. Do you all have any good literature on the theory?
The reason I am going this deep is because I do not know what the dimension is between the linkage pivot point axis to the eccentric rotational axis.
Thanks!
Richard Hed
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I'm looking at your drawing, but I don't understand what it is doing. If you could show a couple other angles, maybe I could undersstand what is going on.
A good rule of thumb when setting the timing on a steam engine is that the high spot of the eccentric leads the crank by 90deg plus 30 deg so 120degrees in total.
If you apply that to your two eccentrics then one will be 120deg ahead when the crank is turned in one direction and 120deg when turned in the other. From that your two eccentrics high spots will be 120 deg apart rather than 180.
Richard, it's standard Stephensons valve gear, only difference is the shaft the eccentrics are on is not the crankshaft, just a lay shaft driven by 1:1 gearing.
If you apply that to your two eccentrics then one will be 120deg ahead when the crank is turned in one direction and 120deg when turned in the other. From that your two eccentrics high spots will be 120 deg apart rather than 180.
Richard, it's standard Stephensons valve gear, only difference is the shaft the eccentrics are on is not the crankshaft, just a lay shaft driven by 1:1 gearing.
If it were standard Stephenson's then the expansion links would be the other way round - concave towards the eccentrics
JasonB,
I ended up missing a diagram showing the angle between the eccentric lobes. You are spot on, 30 degrees!
Richard,
Here is a better diagram of the layshaft to the actual spool valves. The arms the reversing piston attaches to seems a little odd, but refer to the second image. It is a shaft with 5 arms, 4 arms to move the reversing rod, and 1 arm to attach to the piston. It gets the reversing piston out to the side of the engine.
Third image is looking at the back of the engine showing a casting that connects the two beds. This is located just behind the LP cylinder. You can see the two connection points to the reverse linkage and why there must be two for each side of this engine.
I ended up missing a diagram showing the angle between the eccentric lobes. You are spot on, 30 degrees!
Richard,
Here is a better diagram of the layshaft to the actual spool valves. The arms the reversing piston attaches to seems a little odd, but refer to the second image. It is a shaft with 5 arms, 4 arms to move the reversing rod, and 1 arm to attach to the piston. It gets the reversing piston out to the side of the engine.
Third image is looking at the back of the engine showing a casting that connects the two beds. This is located just behind the LP cylinder. You can see the two connection points to the reverse linkage and why there must be two for each side of this engine.
