The R-64 is a powerful pneumatic rotational stepper motor. It uses three pistons which engage on a gear with 8 or 16 teeth. The torque is huge thanks to the 24 mm x 24 mm piston surface area. A M5 threaded bottom plate is available for use with convenient push-in fittings.
I also designed several two-cylinder rotational (and linear) motors as described in this paper, but these are patented and therefore not so easy to share here. In contrast, the three-cylinder, single-acting motor designs only make use of prior existing technology and are no problem to share. The R-64 presented here is my best attempt so far to circumvent my own patent and maximize torque within the given dimensions. Good luck in building it!
July 18, 2018: Changed axle tolerance to 0.2 mm.
The pistons need support due to the floating teeth.
The motor can also be printed on ancient low-end printers such as the good old Da Vinci 1.0 in ABS (20% infill).
Additional materials and tools
Silicone rubber sheet, 1mm thick
4mm (outer diameter) pneumatic tubes, e.g. polyurethane tubes or Lego pneumatic tubes
M3 x 30mm and M3 x 16mm bolts, and M3 nuts. Longer bolts are fine.
(Recommended) Sealant such as blue silicone
(Optional) Lubricant such as vaseline (petroleum jelly)
(Optional) Something to make 3D printed parts airtight. Impregnating agent "waterdichtmaken.nl" works fine. (Not needed when printing at 100% infill on a good printer, or if small leaks are acceptable.)
Stanley knife with 9mm blade size
(Recommended) 6x push-in fittings M5 to 4mm tubes, I use these.
If not using push-in fittings: a drill with right size (3.8 mm) for tightly fitting 4mm tubes
Pressurized air source (compressor, ...)
5/2-way valves, or use https://www.thingiverse.com/thing:2932081
Overview of all parts. Not shown are blue silicone and vaseline. Make sure that pistons can slide inside the housing, grind if necessary. The seals are cut using the cutting tool, see my earlier designs for specific instructions.
Grind top surface, also the lowered one in the center. Apply blue silicone on top and bottom surfaces.
Put the triangle in place and push seven M3 x 16mm screws (or longer) through its holes.
Apply vaseline all over pistons and seals. Insert as shown. The larger surface of each seal must face the air chamber, the smaller surface of each seal contacts the piston.
Assemble bottom cover and tighten the seven screws. Two versions are available: one with M5 tapped holes designed for push-in fittings, and one with conventional holes designed for inserting 4mm tubes.
Apply vaseline on gear and put it in place. Besides the pictured 8-teeth gear set, a 16-teeth set is also available for download.
Assemble top cover and tighten fifteen M3 x 30mm screws. A square top cover with four mounting holes is also available for download.
The six air vents are printed with M5 threads. Use a M5 tap with flat end to get the thread dimensions right. A makeshift tap can be created by machining a M5 bolt using a saw or dremel tool.
Assemble the six push-in fittings.
The motor is ready! Connect tubes to some suitable valves, and lauch!