A four-cylinder single-acting steam engine, with the cylinders mounted radially at 90-degree intervals. All pistons are connected to one eccentric wheel, at 90-degree intervals on its circumference. Valves are connected to another eccentric, also at 90-degree intervals. This design provides two desirable features:
This prototype engine will have a 7/16” bore and 3/4”
stroke, with sleeve bearings on the crankshaft (2) and the crank wheel (1).
If this engine works as I hope it will, I’ll build a second one with
1” bore and stroke, double acting, with a reversing gear and throttle.
It will have ball bearings and (perhaps) o-ring piston rings. If the prototype
proves the design, I’ll be willing to invest the time, effort, and
money to produce an elegant machine.
This writeup will grow as the project continues, to be used as a history
of the project and notes for use on future machining tasks.
Dave Goodfellow
September, 2000
| Four cylinders set at 90-degree intervals, each using the same central eccentric. The eccentric has 4 tie points for the piston rods, set at 90-degree intervals. | |
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Plumbing to be embedded in the frame? Possible, but may be more difficult than it is worth. |
 These are strictly basic ideas; probably the actual design will differ greatly as I find out what works and what doesn't. |
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Eccentric Disks -The one on the bottom is 1-1/8” diameter x 5/16” brass with a 1/4” reamed hole in the center for the crankshaft and a 3/16” reamed hole for the eccentric. The one on the top mounts on the eccentric hole and is 1” diameter x 3/8” thick brass. It is grooved 1/8” in the center beyond the 4 connecting rod holes. |
How it Mounts -A 3/8” hole is drilled through the bearing block, with a 1/4” bronze sleeve bearing at each end. The eccentric disks are shown how they eventually will be mounted. |
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| The base is 1/2” aluminum, 5” x 8”. The frame is 3/8” aluminum, 1-3/4” x 8” (2 pieces). The cylinders are 7/8” square by 1-3/4” brass, reamed 7/16”. The frame is dadoed to make the X. |
Cutting the Dado -The Grizzly mini-mill with a 3/8” end mill cutting a 3/16” x 1-3/4” dado in one of the two legs of the frame. The two dados gave me a press fit, with the two pieces flush to each other. |
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Top Front View-The 4 cylinders, bearing block, eccentric, drive shaft and flywheel. |
The dados are a perfect fit; the marks are from a marking pen. The lower legs of the frame are embedded 3/16” deep into channels in the base; each leg is anchored in place with 3 4-40x1/2” bolts.
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These pictures show the parts “in the rough” - useable but not finished. Once I have the engine finished and running, I’ll go back and “pretty up” the parts. For example, the bearing block will be smoothed and contoured on the lathe- perhaps rounded in the middle to a slender column.
Probably I’ll fill the scars on the base and frame with aluminum putty, and paint this scrap aluminum black. The brass parts will be polished bright and sprayed with clear Krylon to protect the finish.
Parts still to make at this stage are -
Aluminum pistons (4)
Steel wrist pins (4)
Brass piston rods (4)
Brass steam chests (4)
Brass valves (4)
Brass valve connecting rods (4)
Brass valve eccentric (1)
Brass steam inlet port (1)
Steel eccentric pins (4)
And plumbing.
That’s 30+ parts to go. Lots of work to do before this baby’s up and running! I hope to finish in about a month of evenings and weekends. That would be shortly before Halloween, 2000.
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Pistons, 1st Stage -Lathe work is done on these four pistons. They’re turned to a slip fit in their cylinders and parted to 3/4”. I’ve always had a problem with parting, but have finally gotten the hang of it. Smooth and easy. Next step is to move them to the mill, where I’ll drill and slot them for the wrist pin. |
Finding the Edge -The first step is locating the hole for the wrist pin. I used an edge finder and then dialed in to the center of the piston. In this operation I used my Sherline mill, as it’s more convenient for these small parts than my Grizzly. |
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Drilling for the Wrist Pin -Here the 1/8” hole is being drilled through for the wrist pin. The next step will be slotting the piston for the piston rod. I’ll do that while the piston is still set in the vise. That way the slot will be perpendicular to the hole - that would be a good thing!. |
Slotting the piston -The slot is cut at right angles to the wrist pin hole, so that the piston rod will rotate on the wrist pin within the slot. It took two passes with my slotting saw to open the slot wide enough for the piston rod to move easily. |
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Each piston required two operations on the mill - drilling and slotting - with the same setup. I finished each piston before working on the next, to guarantee that the holes and slots were perpendicular to each other. |
Major Screw-up -Not everything goes according to plan. While turning the first piston, I tried to fit it into a cylinder. A nice snug fit - perfect! Then I tried to pull it out. Perhaps temperatures equalized. Whatever the cause, the piston was stuck fast. Even after I gave up on saving the cylinder, I could not drive out the piston. |
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Making the wrist pins and piston rods, after which I’ll be able to assemble the pistons, cylinders, rods and eccentric. That will allow me to locate the bearing block accurately in relation to the pistons, and bolt it down. Hopefully, I’ll have that done this weekend (9/23-24, 2000).
After That comes the steam chests, valves, valve rods and valve eccentric. That should take a couple more weekends, and leave only the plumbing to finish. Maybe my Halloween completion schedule is realistic after all!
| The concept of the single eccentric wheel controlling four piston rods was based on the assumption that it would ride on its axle evenly, as shown at right. On that assumption I made the piston rods of equal length. The assumption was basically wrong, as that would require each cylinder/piston/piston rod and linkage to be equally balanced. |  And even then, torque in operation would throw it off. That's so obvious to me now, but I had to build it and try it to understand. |
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When I assembled this mess and turned the flywheel, the eccentric immediately rotated clockwise until the piston rods bound against the adjacent wrist pins, as shown at left. This caused two pistons to bottom out and the other two to fall out! |
Undaunted, I made two piston rods 3/8" shorter, and the other two 3/8" longer. Same problem, only worse. This time the eccentric rotated counter-clockwise, doubling the problem. That's when I realized the concept of an eccentric wheel was basically flawed.
Disappointment got in the way of clear thinking for the rest of the day. But tonight I awoke from a sound sleep, wide-eyed at the simplicity of the solution. So now it's 2:30 a.m. and I'm getting this down in writing so I won't lose it.
The solution? Trash the eccentric wheel (sob) and link the piston rods directly to the eccentric axle the wheel used to ride on. With this arrangement, only one of the rods can be a straight line from piston to eccentric. The other three will have to be bent slightly to allow them to link side-by-side to the shaft. I'll make them tomorrow, and post pictures.
The same arrangement will have to be made for the valve rods.
October 1, 2000 It worked! I drilled and tapped a 2-56" hole in the end of the shaft to put a washer to hold the piston rods on. By that time it was too late at night to make a proper part, so I "kluged up" an unsightly mess to try it out. Today I'll make a proper part. |
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Nothing goes as fast as expected. It's 8:00 p.m. and I haven't yet made the screw/washer for holding the piston rods to the eccentric. Nor have I finished the four valve chests. They're all cut to length with the valve cylinders drilled/reamed. But only one is finished. Here's what I have so far:
| And here's the bottom of the valve chest. The channel passes steam from the inlet port (the hole in the top in the photo above) to a hole near the end of the cylinder. The valve gates incoming steam to the cylinder and spent steam out the end of the steam chest. |  |
Probably I won't get back to this until next Saturday. Got to make a living
| Saturday, 10/7/00 All four valve chests are done and mounted. They took about an hour apiece, plus an hour to assemble them on the engine. Note also that the hole in the middle of the frame has been enlarged. I needed that hole to run a screwdriver through to attach the piston-rod retainer. |
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Saturday, 10/21/00 Hindsight is 20/20! When I blow it, it's big time! I originally chose a 1/4" drive shaft, for the super-intelligent reason that I had a flywheel in my junk box that was already drilled for 1/4". The project needed only the valves, linkages and eccentric (plus plumbing) when I discovered that the 1/4" drive shaft required too big an eccentric to allow the 3/8" travel I needed for the valves. I had to discard the drive shaft, the flywheel, the bearing block, the crank wheel and the oversize eccentric, and build these parts again.This time I used a cast bronze flywheel I bought at N.A.M.E.S. last April, and contoured the bearing block to add a little class to the engine. Here's the new bearing block and flywheel. The bearing block is reamed through at 3/16". I may put in bearings later if need be, but things turn smoothly right now. (Later, the shaft froze. I put in bronze bearings. That fixed it.) |
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And here's the engine as it stands today. Today I'll turn the valves and (maybe) build the linkages. When those two things are done, all that's left to do will be the plumbing. I have a couple of ideas on that ...  | |
December 10, 2000 It runs! I had to test it. I just couldn't wait. So I made some nipples for the valve chests and for the manifold, hooked them up with plastic tubing, and punched the air to it. It started nicely at 10psi, with no dead spots. Started every time, for about 20 tries. Then I throttled back to about 5psi, and it ran smoothly, dead slow. Pushed it up to 40psi and it ran like a banshee. More air probably would have blown the plastic hoses. |
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At present, the valve rods are attached to the valves with paper clips. Those will be replaced with brass brads. The manifold will be attached to the top of the bearing block (where the red spot is) and the plastic tubing will be replaced with copper. I still have to contour the cylinders with a belt sander. The upper left one is partially done. I'll paint the aluminum black, to contrast with the brass. I started this project in September, and thought I'd finish by Halloween. I might have it done by Christmas. Stay tuned. December 23, 2000 Nope. It won't be done by Christmas. Just had a brainstorm regarding the manifold. Instead of mounting one on top of the bearing block, I'll drill the block itself so that it will be the manifold. Less clutter. | |
January 1, 2001. |
Here she is, all done and ready to run. I scrapped the idea of painting the aluminum, after looking at another project I painted. Maybe someday, when I learn how to do a decent job of it ... Lots of mistakes in this, but it runs fine. Hopefully, one day I'll look back and see this as the crude attempt that it is. But for now, I'm kind of tickled with it. |
| Click here to see a test run. (700k) | |
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