The R-25B is a powerful pneumatic rotational stepper motor. It measures only 25 mm, step size is 10° and output torque is quite high for its size (order of 0.1 Nm). Pressure range depends on material strength, typically 0.1-0.4 MPa (1-4 bar).

The R-25B was designed for printing on high-resolution Polyjet printers, such as the Stratasys Objet260. It might work on FDM printers too (especially with small nozzle sizes) but this has never been tested.

The top is a multi-material part consisting of a rigid body and a rubber component for sealing. Vero is used for rigid and Agilus (or Tango) for rubber components. If you do not have a multi-material printer then just print everything in rigid material.

The seals are to be laser-cut from 0.5 or 1 mm thick silicone rubber (Shore 60A). The dimensions of the seal may need to be adjusted for an optimal fit.

Use petroleum jelly (e.g. Vaseline) to lubricate the axle, pistons and seals. This grease also protects the parts from accidentally gluing together. The top can then be glued (Loctite 406+770) on the housing. Suggested method is to first put the parts together and then apply glue all around the model along the slit. Operate the motor initially at low pressure to "break" accidentally-glued components free if necessary, then let it fully dry before applying high pressure to avoid squeezing out the liquid glue.

Tube sockets are 3 mm in diameter. Festo sells matching tubes. I also have a housing for 2 mm tubes (contact me).

The original R-25 motor has been described in this paper. Note that the paper describes an slightly older version: more teeth (13 instead of 9) and smaller step size (6.9° instead of 10°). The original teeth shape was found to be too weak and would break under high stress, version R-25B is much more robust.