"Scotch Twins"

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Two-Cylinder Scotch Yoke Steam Engine

After a year's retirement doing other things -- exploring the American and Canadian San Juan Islands in a 19-foot outboard boat with a canvas cabin -- I got the itch to build again. Actually, the cost of gasoline has put me on the beach for a while. So I built this steam engine, out of odd pieces of brass, aluminum and steel I had lying around. I was determined to use only material I had on hand. You may have a different mix of metals. Use what you have, keeping in mind only that you don't want an aluminum piston in an aluminum cylinder, as in operation they may want to weld together. I would make one or the other out of brass. The same is true for other areas of sliding contact.

This single-acting engine has two opposing cylinders with a single "Scotch Yoke" type piston rod. This makes it easy to tune, for it needs only one eccentric to adjust both cylinders. When one cylinder is tuned properly, they both are. Also, I find a Scotch Yoke in operation fascinating to watch.

A link to plans in .pdf is at the end of this discussion.

Here's the finished engine. Eventually, I will replace the plastic tubing with 1/4" copper, both for looks and for running under steam. (Plastic tubing does not do well with steam.) The engine runs fine on compressed air, but I like to use steam at times.

There are two plugged holes in the valve cylinders, the result of an experiment on steam intake location. If it had worked, the engine would have required less tubing.

The Parts

Base

I built the base out of 1/2" aluminum plate, 4-1/4" x 7". Quarter-inch plate would have worked just fine, but 1/2" plate is what I had. The slot is a recess for the flywheel, and the three holes (countersunk on the bottom) are for mounting the frame and bearing block.

Frame

The frame is made from 1/2" aluminum bar, 1-1/2" x 7". The 1-1/8" dia. hole in the middle is made with a boring tool in the mill. All other holes are drilled.

I made this part early in construction, so I'd have something to mount the other parts on as I made them. I drilled mounting holes when needed, as other parts were ready to be mounted.

Bearing Block

The bearing block is milled from a piece of 1-1/2" x 1-1/2" x 1-1/4" aluminum. Two 3/16" bronze bearings are installed and sealed in place with Loktite. The two mounting holes at the top are drilled through. The two at the bottom are drilled to a depth of 3/8". All four holes are tapped 4-40. I then set the part aside to await the crank and crankshaft.

Crank and Crankshaft

I turned the crank from a piece of 1-1/2" aluminum, drilled and reamed the center hole 3/16" and drilled the offset hole for a 4-40 tap. Offset is 3/8" from center. I cut the piece off at a length of 1/2", to match the thickness of the frame, then drilled a hole through to the center and tapped it 6-32 for a setscrew.

The crankshaft is a piece of 3/16" steel rod, with flats milled at each end to accept the setscrews of the crank and the flywheel. The picture shows the crankshaft, crank and crank pin assembled.

Piston Cylinders

The two cylinders are made from 3/4" x 3/4" aluminum, each cut to 2", drilled and reamed 1/2" by 1-1/2" deep. The extra half inch provides space for mounting screws. Do not drill the ports in the cylinders at this time. Wait until you have mounted both the valve cylinders and the piston cylinders on the frame. Then you can drill straight through the valve cylinder into the frame. Remove the valve cylinder and continue the hole in the frame through and into the piston cylinder. This way, you can be sure of precise alignment through the three pieces of each side. (My 1/16" drill is not long enough to drill all three pieces in one pass.)

Valve Cylinders

I made the valve cylinders from 1-1/4" x 1-1/4" aluminum, each cut to 2" to match the length of the piston cylinders. They didn't need to be this long, but I think the whole engine looks better with all cylinders the same length.

The hole for the port is 1/16", but countersunk 1/4" deep to accept a 10-32 tap.

Eccentric

I've been told that eccentrics should be made out of steel because it's more resistant to wear than (for example) aluminum. I have made many steam engines with this type of eccentric in contact with brass linkages, and the brass shows no sign of wear on these engines. So I made this one out of aluminum. I'll be surprised if it shows wear any time soon.

Scotch Yoke

The Scotch Yoke was made from 1/16" brass, taken from a kick plate for a sliding door. I bought the kick plate some time ago at Home Depot for $10, and pieces of it have gone into many projects since.

Valve Linkage

The two valve linkages were made from the same kick plate described above. The half-inch hole is cut for a sliding fit over the eccentric shown above. Note that center-to-center hole spacing must be identical for these two pieces, so that the one eccentric can tune both cylinders.

Valve

I made the two identical valves from 3/16" brass rod. The groove (near the center of the valve) was made with a cutoff tool in the lathe. Other operations (the flat, the slit, and the hole) were done on the mill. The hole is tapped 2-56 and a 2-56 x 1/4" screw joins the valve to the linkage.

Manifold

The manifold is made from 1/2" x 1/2" x 2" brass. The steam passages are drilled 1/8" diameter, and tapped 10-32 x 1/4" deep. I settled some time ago for 10-32 threads on all nipples for my steam engines. This allowed me to make a carload of nipples in one session, so I would always have them available for new projects.

Nipple

This engine uses five nipples. That number will be reduced if and when I redo the plumbing with copper tubing. The nipple is 1/4" x 1" brass round, with a 1/8" diameter hole drilled through the center, and 1/4" of it threaded 10-32.

Crank Pin

I made the crank pin from 1/4" x 3/4" steel round. The body, a nominal 1/8" diameter, is actually turned "just a skosh" under 1/8", to provide a sliding fit in the slot of the Scotch Yoke. The threaded portion is 1/4" of 4-40.

Flywheel

The flywheel started life as a 3" diameter aluminum disk, 1/2" thick. I first drilled and reamed the center hole 3/16" on the lathe, then transferred the piece to the mill to drill the hole patterns. Back to the lathe, mounted between centers so I could true the circumference the full width of the piece. To hold the piece tightly, I knurled a portion of sacrificial 3/16" steel rod, and pressed the piece onto it. That worked great, and was easy to remove after the work was done.

Assembly

Assembly needs to be done in the sequence shown, for some parts hide the mounting screws for others.

Step One

Mount the bearing block and one of the piston cylinders as shown. Momentarily insert the crankshaft with crank, to ensure that the crank is centered in the hole in the frame and the shaft turns freely. Remove the crankshaft/crank.

Step Two

Reinsert the crankshaft/crank assembly, adding the eccentric and the two valve linkages as shown. Do not tighten the setscrew on the eccentric at this time.

Step Three

Insert the Scotch Yoke/piston assembly into one cylinder. Keep the Scotch Yoke vertical, to prevent the wrist pin from falling out of the exposed piston.

Step Four

Insert the free piston into the other cylinder, and attach that cylinder to the frame. Insert the crank pin, and tighten it securely. Now install the two valves in their cylinders and mount these cylinders on the frame. Ensure that the valves are oriented so that their flats are facing the frame.

Step Five

Install the manifold on the top of the frame, and install the nipples as shown.

Step Six

Install the flywheel, and tighten its setscrew to the flat on the crankshaft.

Step Seven

Mount the engine on its base. This had to be done after installing the flywheel, for there is no room to install the flywheel after the engine is mounted on its base. Add the tubing as shown, and you're done!

Tuning the Engine

Tuning this engine is simpler than tuning other multi-cylinder engines, because the opposing sides of the engine are symmetrical -- mirror images, so that the one eccentric is common to both cylinders.

Remove the nipple from one of the valve cylinders, so that you can observe the valve inside.

Rotate the flywheel in the direction of intended operation until the piston whose valve you are observing is at bottom dead center -- that is, fully into the cylinder.

While holding the flywheel in that position, rotate the eccentric until the valve groove in the valve is just coming into view, traveling toward the eccentric. Tighten the eccentric and replace the nipple.

To test that cylinder, remove the tubing from the other cylinder so that it can free-wheel. Then apply air directly to the cylinder you just tuned, through its nipple. Spin the flywheel in the intended direction of rotation. It should run nicely. If not, adjust as needed.

Once you have one cylinder tuned, verify operation of the other one by running it alone. If you made the parts identical on each side, the two cylinders will run equally strong.

Replace the tubing, apply air to the manifold, and enjoy!

Detailed plans are at Scotch Plans