The R-66 is a continuously rotating pneumatic rotary engine. It is not a stepper motor, so no complicated valve manifold is required to get it running. One pressurized pneumatic line is sufficient. Video: https://youtu.be/Nayswdk_PZ0
Multiple versions are available for the pneumatic connections (4 mm tube or M5), piston type (conventional or monolithic) and front rotary plate (solid shaft or hex bit). Seal cutting tool can be customized with thing:3020690.
Enjoy creating and running this motor! If you have any questions, just ask.
Sept 15, 2018: Added version 2 of BackPlate to allow better fixation of the crankshaft screw, Crank_Rear to allow insertion of a M3 lock nut and Rotor_Front_Acrylic_3mm to allow internal inspection while running.
Support is only needed for the front rotor plate, on build plate only.
Cut 3mm brass rods in lengths 2x 31 mm and 2x 14 mm (other rigid materials should also be fine). File down the edges to make insertion easier. Enlarge holes in the rod preparation block with a 3 mm drill.
Hammer the two 31 mm rods into the long holes of the rod preparation block. Drill through one short hole with a 3 mm drill, as straight as you can. Take care because the drill may bite into the metal due to the cut-out shape. After drilling the first hole, hammer one short rod in that hole and then drill through the other hole.
Take the metal rods out. We need only one, the other one was used to balance the drilling action. The rod should have two aligned circular cut-outs as pictured.
Enlarge the holes in the front and rear cranks with a 3 mm drill, as straight as you can.
Assemble crankshaft and hammer one short rod through the alignment hole. The crankshaft should be reasonably straight and parallel as pictured. Place a M3 nut inside the rear crank (right).
Update Sept 15, 2018: The new version (v2) of the rear crank now allow insertion of a lock nut instead of a normal nut, to avoid the crankshaft screw getting loose.
Prepare piston rods by enlarging large holes to 3.1 mm (or 3 mm plus a bit of creativity if you don't have a 3.1 mm drill) and small holes to 2 mm. Grind surfaces so that the five piston rods can be assembled in line and move without too much friction. Optionally, apply grease (vaseline or equivalent) to reduce friction.
Assemble piston rods on crankshaft (I use order A-C-E-B-D to better balance forces) and hammer the other 14mm pin through the crank.
Grind cylinder top surface. Ensure that each piston can slide smoothly inside the cylinder. Grind piston surfaces if necessary, but not more than absolutely necessary. A wobbling piston makes sealing more difficult, especially if the seals are fixed to the pistons with adhesive.
Cut five seals out of 1 mm thick silicone rubber using the cutting tool.
After cutting, the dimensions should be approximately 24.3 mm x 14.2 mm.
The seal dimensions can also be checked by placing it between opposite walls of the cylinder. A quick test cylinder setup can be constructed by clamping the cylinder walls between two flat plates. The seal should be able to touch both opposite walls without excessive deformations or falling out by gravity.
The question is whether to attach seals to the piston heads or not with some adhesive. This both has advantages and disadvantages in sealing and its effectiveness depends on the characteristics of the particular motor. The wide aspect ratio (12:7) of the cylinder cross-sectional area makes sealing relatively difficult. So I recommend to use double-sided tape to fixate the seal's smaller face to the piston head. Make sure that the seal is precisely centered, the test cylinder setup can be used here.
Connect pistons to piston rods using M2 x 14mm screws and nuts in the correct orientation, as pictured. Ensure that the pistons can easily rotate around the piston rods, grind appropriate surfaces if necessary.
Apply blue silicone on the top/bottom surfaces of the straight cylinder walls. Insert M2 x 30 mm bolts in the front rotor plate through the cylinder walls. Two front rotor plate styles are available: one has a solid axle with flat side, the other has a slot for hex bits.
Apply grease such as vaseline to reduce friction. Place crankshaft assembly with all pistons and seals in motor. Ensure that crank can rotate, do not worry too much about the combined friction of all moving parts and seals.
Carefully grind top surface (five-hole ring) of rear rotor plate. It needs to be smooth and flat in order to make the rotary valve work well. Position rear cover and tighten all 30 screws.
Two versions of the backplate are available: one with M5 pneumatic connection and one for 4 mm tubes. The M5 version needs to be tapped with a flat tap drill, a makeshift one is easily made by making a cut in a M5 bolt using a saw or dremel.
Apply lubricant (vaseline etc) on rotary valve contact surface. Position crankshaft in backplate and insert M3 x 20 mm bolt through the hole. Mount pneumatic fittings (if used).
Update Sept 15, 2018: The new version (v2) of the backplate now allows insertion of an additional M2 x 10mm screw and nut to lock the crankshaft screw.
Screw frame to backplate using M3 x 12 mm (or longer) bolts and nuts. The motor is now ready!
Adjust the crankshaft screw while motor is running to get the ideal tension for the rotary valve such that motor performance (torque, speed or power) is optimal.
A different piston system is also available. This one uses monolithic pistons without separate rods. This design uses fewer moving parts and while it does actually work, it is more prone to leakage because the contact angle with the curved cylinder walls is variable.
The cylinder walls are curved to compensate for the nonlinear piston movement. In theory the seals should be able to completely seal off each cylinder in any position.
It is possible to pressurize individual cylinders using the special cylinder fitting plates. Both M5 and 4 mm variants are available. This is useful to study the behaviour of the piston and seal of an individual cylinder at different pressures and motor phases.
The functioning of the rotary valve can be investigated with this setup. When pressure is applied, air should flow through two or three air holes. By covering them with your fingers, leakages in the valve system and inconsistent valve airflow can be detected.
Update Sept 15, 2018: a design for laser-cutting a transparent front rotor plate is available. This one is made of 3 mm acrylic, with 1 mm engraving for the screw heads. This is very useful in investigating sealing issues while the motor is running.
The curved-cylinder engine type should run fine with decent torque and speed, although the airflow may be a bit high. If you are able to match the straight cylinder type's performance, please let it know in the comments!