Double Wobbler

(January, 2004)

The double wobbler shown at the right has a 1" bore and 3/4" stroke. When I started the design, I wasn't all that sure that my compressor could put out enough volume to keep it running. I needn't have worried. It ran like a champ from the moment I put the air to it.

The basic idea was to mount two cylinders on a single frame. I had intended to mount them on the inside of the frame rather than the outside as shown, but that would have required a crankshaft with two throws -- a lot of silver soldering.

My heat source for silver soldering has died and I'm still trying to decide what to replace it with, so for this project I decided to put the flywheel in the center and mount the cylinders as shown. This allows a straight crankshaft, and no soldering.

I would have liked to have made the frame as a single piece, but my endmills are not long enough to mill a deep enough pocket. So I made it as a four-piece assembly, bolted together,

Note that the ramblings below describe the way I built this engine, based on tools and materials available to me. There are many ways to do it.

Frame

The frame consists of two pieces of 1/2" x 1-1/2" aluminum, 4" long. The hole at the top is for the crankshaft, and is sized to accept 3/16" bushings. The hole in the center (pivot hole) is for mounting the cylinder. It is drilled slightly oversize to allow the 4-40 screws that hold the cylinder to the frame to turn freely with the cylinder. The two small holes just below that are the ports; not seen are the holes in the side of the frame that go to the ports. The air nipples screw into these. The picture on the right shows the two spacers that hold the frame in place.

The frame was done in several stages, because I needed to build up most of the engine before I could locate the ports.

A better mathematition than I am probably could have located them in

the planning stage and done all theholes at once. Their location is given on the plans, however, so a builder following those plans could do it all at once.

The distance between the pivot hole and the ports is 3/8", half of the piston throw. I used a compass set for 3/8" and anchored at the pivot hole to scribe an arc on which the ports would lie. I located the position of the ports on that arc after temporarily mounting the cylinders, pistons and crank.

The spacer is made from 1/4" brass rod, threaded 10-32. The nuts are made from 1/2" brass rod, 1/4" thick. This was my chance to try out my new knurling tool, and I was impressed with the results. The unthreaded portion is 1" long. The dimension itself is not critical, but the need for the two pieces to be identical is; if they are not identical, the frame will come together twisted, and the crankshaft will bind.

The spacers alone do not provide enough rigidity for the frame, but addition of the manifold eliminates this problem,

Two manifolds are required so you can select in which direction you want the engine to run. Apply steam (or air) to one manifold and the engine runs clockwise; apply it to the other manifold and the engine runs counter-clockwise.

At left is the manifold, made of 1/4" aluminum, 1" x 1-3/4". The picture at the left is the "outside" face. The hole in the center is threaded 10-32 for the nipple. It is drilled 1/8" deep to meet a 1/8" tunnel drilled lengthwise down the center of the piece. I drilled the tunnel just short of through the piece, and then plugged the open end with an aluminum putty. The point is to plug it to prevent air or steam leakage; how you do it is up to you.

The picture at the right shows the "inboard" face. The pockets at the top and bottom were made with a 5/16" end mill. They are 1/8" deep to meet the tunnel described above.

The result is one air intake hole on one side and two "output" holes on the other, which gate air equally to the two major frame pieces.

Cylinders and Pistons (2 each)

I made the cylinders first, out of 1-1/4" x 1-1/4" x 2" aluminum. The cylinder is 1/8" off center from one face, centered from the other. This allows enough "meat" on one face to drill and tap a pivot hole.

The cylinder is bored 1" diameter to a depth of 1-3/4". This depth allows me to use a 3/4" long piston. (A short piston, for example 1/4", would tend to twist and bind in the cylinder, from the side forces exerted in the conversion from reciprocating to rotary movement.)

I then turned the brass pistons to a sliding fit in the cylinders. The two cylinder bores are "purt near but not plumb" identical, so each piston is turned to fit its cylinder. A builder with a better eye and measuring equipment could make them interchangeable. The pistons are center drilled and tapped 6-32 for the piston rod.

The piston rod is made from 1/4" brass rod, threaded 6-32 for 0.656" on one end, and a 1/16" hole drilled at the other for attachment to its eccentric.

Cranks (2 each)

The cranks are made from 1" brass round, 1/4" thick. These are center drilled and reamed 3/16", with a 1/16" hole drilled through at 3/8" off center. This hole accepts a press-fit 1/16" x 1/2" steel pin, protruding 1/4" from the surface of the wheel. The offset of 3" establishes the 3/4" stroke of the piston. Opposite the eccentric pin is a setscrew hole drilled through to the center and tapped 2-56. I countersank this hole 1/4" deep to avoid having to tap it all the way; the deeper you tap, the easier it is to break a tap. (Don't ask me how I know.)

Crankshaft

The crankshaft is made from 3/16" steel rod, 2-3/4" long. Aside from cutting it to length, the only operations done on this piece are the three flats to accept setscrews from the two cranks and the flywheel. I had other stuff set up on the mill, so I used a file to make these flats. The flats at the two ends are 180 degrees apart. This makes the pistons 180 degrees out of phase, giving the engine some self-start capability, and smoother running than if they were in phase.

Nipples (2)

I made the nipples to fit my testing setup, which consists of a tire-pump hose off my compressor. Depending on your setup, you may want something different. Mine are made from 1/4" brass round, 1" long, drilled through with a 5/32" drill and tapped 10-32 for 1/8". I made a couple dozen of these a year or so ago, so I did not have to make them special for this project.

Flywheel

The flywheel is made from 3" aluminum round, 3/4" thick. I mounted the piece in the 3-jaw chuck and drilled a center hole. Then I removed it from the chuck, ran a steel rod through the center hole and mounted the piece between centers. After turning the piece to make its circumference concentric with the center hole, I faced both sides as shown, making the hub slightly thicker than the outside rim. This allowed clearance for the flywheel mounted inside the frame.

At this point I removed the flywheel, and with a compass scribed a circle midway between the hub and the inside of the rim, and located 8 holes equidistant along that circle, centerpunched and drilled the holes as shown, then drilled and tapped 2-56 for a setscrew to anchor the flywheel to the crankshaft.

Assembly

Assembling this engine is a little bit tricky, for what you do on one side affects what you can do on the other. This sequence worked for me:

1. Mount the cylinders to the pivot holes, using 4-40 x 3/4" screws, spring-loaded. I used the springs from a couple of defunct ball-point pens.

2. Assemble the frame, using the spacers and the manifolds.

3. Insert the 3/16" bushings for the crankshaft. Leave these loose in the holes for now.

4. Insert the crankshaft and ensure that it turns freely. Mine did not at first; the holes the bushings were in were slightly out of alignment. I had to slightly enlarge one of the holes to allow its bushing to find its proper alignment.

5. Remove the crankshaft and reinsert it, with the flywheel between the uprights of the frame.

6. Drop a piston into its cylinder, and attach a crank to it. Move the crankshaft so its end is flush with the bushing, and slip the crank over it. Push the crankshaft back so its end is flush with the outside face of the crank, and tighten the setscrew onto the flat of the crankshaft.

7. Drop the other piston into its cylinder and attach its crank. Move the crankshaft back as far as it will go and slip the crank onto it. Move the crankshaft back so that its end is flush with the outside face of the crank, and tighten the setscrew onto the flat of the crankshift.

8. Center the flywheel in the frame and tighten its setscrew against the flat on the crankshaft.

9. Turn the flywheel to ensure that everything moves smoothly. There will be some resistance and scraping sound as the cylinders rub against the frame, but there should be no binding.

CAUTION:

In Step 10, make sure that no Loktite leaches to the inside of the bushings,
as this will freeze the crankshaft in place and you'll have a difficult time freeing it.

10. If all is well, disassemble the top part of the engine. Remove the crankshaft and wet the outside of the bushings with Locktite. Replace the crankshaft to ensure alignment and let it set a few minutes for the Loktite to cure.

Note: I elected to powder coat the aluminum parts, and so I did this before performing Step 10 because I did not want to powder coat the bushings.

This completes assembly of the engine. Put a drop or so of light oil (I used the transmission fluid I use to oil the ways of my lathe and mill) on the edge of the piston and turn the engine over by hand to let it work in. Then punch the air to it and watch it go. Mine went like a banshee at 40psi, and I could reduce the pressure to run it dead slow.

Plans

"As-built" plans have been completed and are available in .pdf format. The link below accesses these files.

Plans