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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.

Notes:

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

Instructions: see PDF