A model of theincredible elbow engineBy JEREMY HARRIS AND WILLARD SLAY

Build this simplified power-maker and you'll be able to ask friends, "Would youlike to see a six-cylinder engine that

has only five moving parts?"

SLEEVE SLIPPEDOVER PISTONFOR LEVERAGE

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WITH AN ENGINE as novel as this one, youdon't ask, "What's it for?"—it's for motivatingyou to say "Wow!" after you've hooked it upto an air or steam line and observed what itcan do. It's worth a "wow," too: This is a six-cylinder engine that has only five moving parts!

The pistons are paired into three elbows thatbridge two cylinder barrels. In order to movewhen they're activated, the pistons have to turnboth barrels; thus back-and-forth motion is con-verted into rotary motion—without gears. Switchthe pressure line (steam or compressed air) tothe exhaust port and the whole thing spins in re-verse. Either way, there's more elbow actionthan you'd get from a line of seiioritas in amaraca contest.

Valving is accomplished without adding amoving part. There are two crescent-shaped slotsin the plates that the cylinder blocks ride against.These slots are spaced so that pistons on an up-stroke lie over one of them, while pistons on adownstroke lie over the other. Both ends of eachpiston pair are actuated simultaneously, so thepower-pulse pattern is like a three-cylinder en-gine. The action is based on a type of constant-

The drilling fixture is just a socket forchucking the cylinder in the lathe. Thealuminum one, inset left, has already beenused. Before usage there is only thetapped middle hole and an index mark

These sequence shots showhow two completed cylinders(mounted at right angles) arespun by the in-out action ofthe elbow pistons, which exerta push-pull force

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engine models

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velocity joint known as "Hobson's coupling."But the way the pistons dance is what engineerscall "simple harmonic motion."

Although a milling attachment is useful forcutting these valve crescents, you can make theentire engine on a lathe alone, working from theover-size plans on pages 920 and 921. For thosevalve ports, for example, you'd just drill a seriesof overlapping holes along a scribed arc, usinga bottoming drill. (The exact shape and depthof the ports isn't critical, as long as the cylinderbarrel covers and seals them.)

Use bronze bar stock for the cylinders. Squareboth ends of each barrel accurately and finish-turn the outside diameter. Make two identicalblanks, and postpone turning down a shoulderon one until later. Drill and ream a 1/4-in. centerhole in each barrel; then (with use of an indexinghead if possible) scribe three axial lines 120 deg.apart on the side of each barrel, running about1/4 in. up from one face.

You'll need the simple fixture shown at theupper right to help space the cylinder bores.Chuck a piece of plate or bar stock which has aface area larger than the cylinder barrels. Ma-chine a 1/16-in.-deep recess to nest—snugly—thebase of the barrel. Next, drill and tap a centerhole for a 1/4-28 bolt. In the recess, make a center-punch mark on a 5/16-in. radius and scribe a singleradial line at any one of the corners, as shownin the inset.

fixture turns freely but snuglyMount this fixture in the lathe so the punch

mark is centered; check the recessed face for truerunning, then use a 1/4-28 Allen-head screw toclamp one of the barrel blanks to the fixture,making sure it turns freely but snugly in therecess. Rotate the barrel until one of its indexmarks is in exact alignment with the radial markon the fixture, then tighten the bolt and drill thefirst hole, using a center drill to start. If a reameris available for running the last few thousandthson the diameter of the cylinder holes, this willgive maximum straightness and smoothness. Atany rate, work to a close slip fit with the 1/4-in.stock used for the pistons. When the first hole iscompleted, simply loosen the clamping bolt androtate to the next scribed line on the cylinder.Make the second cylinder barrel the same way,then turn down its o.d., as seen on page 920in the drawing, to provide a press fit in the fly-wheel web.

The piston elbows are made from 1/4-in. drillrod. This material is recommended because of its

accurately ground finish and precise diameter.Three pieces are cut to length, faced and neckeddown in the center to form the elbow of the 90-deg. bend. Keep that section cherry red whilebending, and hold the sleeve you've slipped overthe undamped end firmly against the face of thejig to prevent misalignment by twisting.

There are several ways to make a flywheel, butthe two-piece construction shown is probablyeasiest. The rim is 3-1/2-in. steel pipe, with its i.d.reduced enough to form a press-fit shoulder forthe aluminum web. The inside bore of the web,on the other hand, provides a press fit for theo.d. of the cylinder barrel. Once this three-partassembly is complete, test it for squareness. If itwobbles, lightly tap the web and rim until the unitruns true.

Make the base from 3/8-in. steel plate. Besidesthe tapped holes for attaching the port blockand flywheel support, the base contains drilledpassages for intake and exhaust of both cylinderbarrels. These passages are identical 1/8-in. holeswhich run straight in from the back edge. Fourshort vertical holes intersect them, and matewith passages in the port block and flywheelsupport. Use thin paper gaskets under these partsto avoid leakage.

find the best arrangementWhen assembling the engine, switch the pistons

and bores in various combinations to find thebest free-running arrangement. The engine shouldturn smoothly, with very little drag, once thepivot screws are tightened. End play of the bar-rels should be reduced until they just turn freely;this play can be adjusted either by shimmingunder the screw heads, or by machining thescrew shoulders. Don't be discouraged if a littletuning is needed to make the parts move freely.The action of the mechanism is inherently verysmooth, so it's worth some extra time to take fulladvantage of this characteristic.

Either of the two ports may be used as the in-take. Press lengths of 3/16-in. o.d. tubing into thecounterbored ports, for attaching the pressureline of your choice. The right load for your en-gine must be determined by experiment. Themodel shown runs best when compressed air isregulated to between 20 and 40 p.s.i., and thepistons are kept lubricated with SAE No. 10motor oil. After you have lubricated, of course,the engine will kick up an oil spray, so it will bewise to do your engine-tuning in the workshop.And wear clothing that you won't mind gettingsoiled.

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