Introduction: Model Steam Engine
This model is powered by an electric motor, and demonstrates the working parts of a double acting piston type steam engine. Most of the parts are 3D printed except for the steam chest, which was made from flat brass stock and brass tubing. I think brass shows up better than plastic for small parts. One of the more interesting features is the Stephenson valve gear, which can reverse the rotation of the steam engine by changing the steam valve timing. This is accomplished through the use of a slotted link that can be raised or lowered to vary the timing- a pin in the center of the slot is connected to drive the valve gear.
Step 1: Materials
3 ea. 625Z bearings for flywheel and connecting rod
21 ea. #4 x 1/2" pan head Phillips stainless steel sheet metal screws
1 ea. 10-24 x 2-1/4" flat machine screw and nut for flywheel axle
4 ea. 8-32 x 1-1/4 pan head machine screws an nuts
3 ea. 0-80 screws and nuts
1 ea. 3/16" dia brass tube
1 ea. 5/32 dia brass tube
1 ea. 1/4" wide brass flat stock 3/32 thick
1 ea. 3/4" wide brass flat stock 3/32 thick
1 ea. 1" wide brass flat stock 3/32 thick
1 ea. 1" dia x 0.14" wall acrylic tube
1 ea. 8-32 x 1" pan head bolt with nut
1 ea. spring 1/2" long to fit above bolt
1 ea. DC 6V 37GB 100RPM gear box electric motor 500mA 37MM
3 ea. 2" USS flat washers (OD 4-1/2" ID 2-1/8") for flywheel
1 ea. bicycle tube for flywheel belt
1 ea. bicycle patch glue
Step 2: Construction
Use a 3D printer to print out the plastic pieces. Assemble the 3 ea. 4-1/2" o.d. heavy steel washers onto the plastic flywheel hub, using epoxy to glue the washers together and to the plastic hub. Install the 10-32 axle into the hub, then secure with a nut. Place a bit of heat shrink tube over the axle then slide the 625z bearing on; bearing will be against the nut. Install a bearing spacer after the first bearing is installed, then a second bit of heat shrink and the second bearing and secure with another nut. Install the bearing assembly into the flywheel stand with 4 ea. #8 nuts and bolts to hold the bearing retainer halves of the flywheel stand together. Place the stand on a 6" x 12" x 1/2" board and secure to the board with 4 ea. #4 pan head screws. Install the third 625z bearing into the connecting rod big end and use bolts and nuts to secure in the connecting rod cap to the connecting rod. Install a 10-32 nut into the plastic hub hex opposite the connecting rod, and install a 10-32 pan head screw through the connecting rod bearing, and through the hub to the nut and tighten securely. Install the cam and cam pushrods onto the flywheel axle. The cams greatest throw should be at 90 degrees to the crankshaft. Secure with a washer and nut.
Cut a 2- 3/4" section of the acrylic tube for the clear cylinder. Push the 3D printed cylinder heads on the tube. Fit a 7- 3/4" brass rod through one of the screw tops of the cylinder head then install the piston on the rod. Screw the top onto the cylinder head. Brass rod should fit through the other screw top; insure that the rod can move freely. If necessary, enlarge the hole in the caps so the rod can move freely. Flatten 1/4" at the end of the brass rod and drill a hole for a 0-80 screw. Secure the brass rod to the connecting rod end with the 0-80 nut and bolt. Position the 3D printed cylinder stand on the board so that the full range of motion of the piston can be seen and that there is no binding when the cylinder assembly is on the cylinder stand. Secure to the board with #4 pan head screws. Use double sided tape to attach the cylinder assembly to the stand.
Attach the slotted link 3D printed part to the cam pushrods using 2 nuts and bolts. Attach the slotted link lifter arm and lifter bellcrank to the wood base, and attach to the slotted link. Install a bolt, spring, and nut to the bellcrank base to hold the bellcrank / slotted link in a set position.
Make the brass steam chest outer perimeter out of 3/32" thick x 3/4" wide flat brass stock. Bend the 4 corners so that the length is 1- 5/8" and width is 1". Solder or glue the ends together to form the rectangle. The flat bottom (valve plate) should be cut to 1- 5/8" by 1". The D valve should be made next, basically it is just a rectangular sliding cup moved across the valve plate by a rod which is connected to the slotted link pin. The D valve is 1/4" brass stock bent into a rectangle 13/16" x 5/8" capped with a similar sized flat plate. Solder or glue together. Sand or file flat on the bottom so it can slide smoothly over the valve plate. Glue the 3D printed cross piece to the top of the D valve. Drill or hand punch 2 ea. 3/16" dia holes centered on the steam chest ends so that a 5/32" rod can fit completely through 3/16" brass guide tubes (and through the center of the cross). A cross pin is fitted into the 5/32" rod at the cross so that the D valve moves when the rod is moved. The cross may need to be filed a bit so the fit is loose; steam pressure in the surrounding steam chest (in an actual engine) would force the D valve against the valve plate, so additional force is unnecessary. Drill and file the valve plate to have 3 rectangular holes, one central rectangle 1/4" x 3/8" (for exhaust) and 2 side rectangles 1/8" x 3/8" which, when uncovered, would allow steam from the steam chest to enter the one of cylinder heads while the other port would allow lower pressure steam from the other side of the piston to pass inside the D valve to the exhaust port. Leave 3/16" between the central and side ports. Deburr the edges of the holes so the D valve can slide smoothly over those ports. Glue the steam chest to the steam chest stand and mount to the board. Attach 2 ea. brass tubes to the cylinder and 2 ea. to the steam chest stand and connect with plastic tubing. Cut a clear acrylic top for the steam chest if desired.
Cut a 2- 7/8" piece of rectangular brass stock for the pushing and pulling the valve, solder in a piece of 5/16" x 5/8" flat stock to the rectangular stock with a 3/32" pin to fit the slotted link at the end of the flat stock. Position the 3D printed slide stand and attach to the wood base. Attach a 1/4" wide x 3/4" long link between the rectangular brass and the steam chest valve rod and secure with 0-80 nuts and bolts.
Attach the driving gearmotor to the base using the 3D printed stand. Wrap a few turns of old bicycle innertube around the motor to reduce vibration before sliding the motor into the stand. Make a driving rubber band out of a section of old bicycle innertube. It should be made 5/8" wide x 16" long. Wash the innertube strip to remove the talc coating, and sand 1- 3/4" of each end. Glue together with bicycle patch glue and hold together in a clamp or vise to cure.
A DPDT switch with center off can be used to control the motor direction and demonstrate steam engine operation via moving the sliding link. Alternately, the sliding link bellcrank can control the direction and speed of the motor (via electronic control) for more realistic forward/reverse action.
The electronic control uses an IR reflective sensor and a microcontroller to determine the position of the bellcrank lever. From that information, the correct forward/ reverse direction and speed is determined and controls the motor via an H Bridge.
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