Wobbler Design Guide

Western Kentucky University

Mechanical Engineering Program Fall 2006

What is a Wobbler Steam engine? A Wobbler is a valveless oscillating steam engine. The engine has no connecting rod and wrist pin on the piston. Rather, the cylinder pivots around a fixed point and a crank disk helps convert the linear motion of the piston to rotation of the flywheel. As the piston moves in and out of the cylinder, the cylinder must pivot to accommodate the motion of the piston. The crankshaft is straight, and simply functions to support the flywheel and crank disk. What type of Wobbler are we reviewing today? Students at WKU call this style a “Lucy” steam engine, named by Edward G. Warren. He wrote a book, “Home Made Steam Engines Volume 1: The Wobblers”, published in 1998 by Camelback Books of St. Cloud, MN. Although it seems to be out of print today, it is an excellent resource for building these small engines. Plans for his Lucy engine can still be found floating around on the web, and many hobbyists build Wobblers in their home shops. How long have Wobblers been a part of the WKU Freshman Experience? Freshmen ME students at WKU have been building Wobblers since Fall 2001. To date, nearly 230 have been made by freshmen. However, they don’t assemble a kit. They design, draw, machine, assemble, test, and demonstrate their own unique creations. Some students build variations of engine designs found on the web, such as the Lucy, but they still do all their own work. Will we have to do any machining to build our Wobbler? Students use a small milling machine, a bench lathe, a drill press, and a horizontal bandsaw to fabricate their parts. They also use layout equipment to dimension their parts and tap all threaded holes with hand taps. A small arbor press is used to press a bushing in for the shaft of the engine.

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What are the pieces of the Wobbler? Wobbler engines consist of only seven major components: Flywheel Base Plate Crankdisk Valve Plate (with pin) (with bushing) Cylinder Piston Crankshaft 10-32 x 1”

SHCS There is also a small parts kit:

Hose barb 10-32 x 2” SHCS Flat Washer

6-32 Set Screws

Compression Spring

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How do these parts fit together?

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How does the Wobbler work? Air enters through the hose barb, passing into the cylinder through the moving air inlet hole. The air pressure forces the piston out. This movement causes the crank disk to turn, and thus the cylinder begins to pivot or oscillate.

Air inlet hole with hose barb

Cylinder pivot

bolt with spring The cylinder continues to oscillate around its pivot as the cylinder reaches top dead center. The spring on the pivot bolt holds the cylinder tight to the valve plate, keeping the pressure in the cylinder.

Note moving hole in cylinder

Air exit hole

with no barb The cylinder oscillates to its other extreme position, allowing air to escape from the cylinder through the other valve plate hole. Note that the crank disk is spinning, causing the flywheel to spin. The flywheel momentum carries the cylinder past bottom dead center on its way to receive another charge of pressurized air. If the hose barb had been installed on the upper hole in the valve plate, the engine would run in the opposite direction.

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How do I assemble the pieces? We know it’s hard, but please read each step completely before acting. 1. Use the 5/32” hex key wrench to fasten the base plate to the valve plate, using two 10-32 x 1” SHCS. Make sure the counter bores are positioned to accept the heads of the bolts, they go on the bottom. Make sure the holes are lined up and the pieces are square before tightening the bolts. 2. Use the 1/16” hex key to install the 6-32 set screws in the crank disk and flywheel. Don’t drive them completely to the bore, just make them flush with the surface.

3. Use the ¼” combination wrench to tighten the hose barb into the lower hole on the valve plate. Do not overtighten the threads, they are soft and will strip. NOTE: Be sure the two air holes are on the right and the bushed crankshaft hole is on the left before inserting the barb.

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4. Insert the crankshaft into the center of the crank disk, making sure the crankshaft is flush with the face and extends out the side opposite the crank pin. Use the 1/16” hex key to tighten the set screw firmly on the shaft, without stripping the threads. Insert the crankshaft through the valve plate as shown, passing the crankshaft carefully through the bronze bushing in the valve plate. Note: Be sure the crank disk is on the opposite side of the hose barb on the valve plate. 5. Put the flywheel on the other end of the crankshaft, making sure the flywheel is pressed tightly against the valve plate. Note: Leave a small gap, about 1/16” (the thickness of the 1/16” hex key wrench for the set screws) between the crank disk and the valve plate. Carefully tighten the 6-32 set screw firmly on the shaft with the 1/16” hex key wrench, being careful not to strip the threads.

1/16” gap here

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6. Slide the piston down into the cylinder, making sure it moves smoothly in the bore. Align the hole in the piston vertically as shown, parallel to the two holes in the cylinder. 7. Slip the hole in the piston over the pin on the crank disk and position the cylinder with the two holes towards the valve plate.

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8. Put the washer on the 10-32 x 2” SHCS pivot bolt. Slide the spring up the pivot bolt as shown. Note: Be careful, the ends of the spring may be sharp. 9. Now the tricky part: Using two hands, hold the cylinder as shown and carefully thread the 10-32 x 2” pivot bolt into the cylinder. You will have to press the SHCS through the valve plate, since the spring must be compressed. Note: Be careful not to strip or cross-thread the shallow threads in the cylinder.

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10. Tighten the pivot bolt using the 5/32” hex key wrench, being careful not to strip the shallow threads on the cylinder.

Finished!

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