Hand-Held, Lung Powered "Steam" Engine

I needed a "quicky" project for a birthday present to a 9-year-old friend of mine. Something I could do over a weekend, out of my scrap box. I came up with a hand-held, lung-powered "steam" engine. The engine is a single-acting wobbler with a 1/2" bore and stroke. I made it hand-held by building the engine at one end of a 7" piece of aluminum, leaving 5" as a handle. I made the ports oversize (1/8" diameter) and used a super-weak spring to reduce friction between the frame and the cylinder. This, I thought, might make it easier to run on lung power.

This would be a good first project for a beginner. I used a Micro-Mark 7x14 lathe and a Taig mill, but it could as easily be done on Sherline equipment. In fact, if you went to a rectangular block for the cylinder you wouldn't need a mill at all! You do, however, need a means of drilling accurate holes.

The result was pretty much what I'd hoped for. It runs (barely) on lung power and super-fast on 20psi from a compressor -- much to my surprise, for I thought the weak spring would make the cylinder "float" at 20psi, allowing air to escape through a gap between the cylinder and the frame. Probably some does, but not enough to prevent high-speed operation. Probably it will run better on lung power once it's broken in. Meantime, my friend can run it off a can of Endust or an inflated tire.

Here are some notes on construction, followed by a link to plans in .pdf:

Frame

The frame is made from a 7" piece of 1/2" x 1-1/2" aluminum. The hole at the far right is for the crankshaft. To the left of that is the pivot hole for the cylinder, and to the left of that the two ports. The ports were located after center-punching the pivot hole, but before drilling it. The distance between the pivot hole and the ports was to be 5/8", so I used a compass to scribe an arc with a 5/8" radius. The two ports would lie somewhere on that arc.

The pivot hole is drilled through 1/8", and countersunk on the backside 5/16" by 1/4" deep, so the head of the 4-40 pivot screw will not protrude beyond the frame.

The hole in the side of the frame is 1/8", and meets the port nearest to it. A similar hole is on the far side. These two holes are countersunk 1/4" deep and threaded 10-32 for the air intake nipples.

Cylinder

I made the cylinder from a piece of 1" aluminum round. On a tip from Mert Baker (7x10Omnilathe group) I offset the bore by placing a 1/8" spacer between the workpiece and one jaw of my 3-jaw chuck. This was simpler than putting on the 4-jaw chuck and offsetting the workpiece in it.

After boring and reaming to 1/2", I milled the side farthest from the bore to a depth that left 1/8" between the flat face and the edge of the bore. This gave me the flat face required for a wobbler.

Note the two holes in the flat face. Both go through to the bore. The one closest to the open hole is the pivot, and is tapped 4-40 to just short of the bore. This allows the pivot screw to jam, so it won't work out in operation. The other hole is the port.

Piston

The piston is made from 1/2" brass round. The piece I had fit the bore precisely -- no turning required for that portion. The remainder was turned to 5/32", with a 1/16" hole at the end to fit the pin in the crank.

Crank/Crankshaft

The crank is 3/4" brass round. with a 3/16" hole in the center for the crank, and a 1/16" hole offset 1/4" from center for the crank pin. Both of these holes are drilled for press fit, and on assembly are secured with Loktite. The crankshaft is 3/16" brass round, with a flat milled in one end to accept a setscrew from the flywheel. The flat allows the setscrew to hold the flywheel with only moderate pressure, and any burr the setscrew may make in the flywheel will not interfere with removal/reinstallation.

Air Intake Nipple

The air intake nipple is 1/4" brass round, 1" in length and drilled 3/32"" through. 1/8" is turned down and threaded 10-32 to insert in the frame. This hole is the limiting factor for air intake. It should have been 1/8" diameter to match the ports, but that would have required a larger diameter workpiece and a size up from 10-32. I had neither the stock nor the tooling to make that happen. If I'd not been able to make it work on lung power I'd have bought a 12-28 tap and die and a suitable piece of brass, and reworked the nipple and the holes in the frame.

The 1/4" outside diameter is just right for a snug fit on my compressor's air hose. This nipple fits nicely to the plastic tubing on a can of Endust (a compresserd air can used to blow dust off electronics). This allows operation from "canned" air.

Flywheel

You can use almost anything for a flywheel, as long as it's heavy enough to smooth engine operation. I used the one shown, which I had built previously. Its construction is discussed at --

http://www.davegoodfellow.com/flywheel.html

I built it for a 1/4" crankshaft, and so had to put a sleeve in it to bring it down to 3/16" as used in this engine.

Note the setscrew in the hub, at about 7 o'clock in the picture. It's 2-56 x 1/4", and screws down onto the flat on the crankshaft.

This was a good weekend project, made especially rewarding when it ran beautifully with the first test. I hope my friend is having as much fun running it as I had making it.

Plans

Detailed plans of major components are at Plans. You'll need Acrobat Reader to access them.