The R-38 is the third rotary engine developed after the R-66 and R-52. The R-38 measures (38 x 38 x 48) mm³, making it the smallest rotary engine in this series so far. It is also the fastest and better suited to the airflow of 4 mm pneumatic tubes than the larger models.
The crankshaft is all-metal, the crank radius is 3 mm so the piston stroke is 6 mm. The piston piston surface area is (16 x 12) mm² = 1.92 cm² which yields a piston force of 38 N at 0.2 MPa (2 bar) pressure. Combined, this gives a theoretical torque of 110 Nmm (at 0.2 MPa).
Filament sponsored by Fillamentum. All PLA, in following colors: Everybody's Magenta (rotor), Pearl Night Blue (housing), Gold Happens (bearing cage).
List of materials
Brass rods, with diameters 1 mm, 2 mm and 3 mm
M2 screws, length 10, 16 and 20 mm
1 mm thick silicone rubber
M5 pneumatic sockets (optional)
4 mm tubing
Saw for cutting brass rods
Drills in sizes 1 mm, 2 mm and 2.1 mm
Cut the 2 mm brass rod in lengths 45 mm and 60 mm. Drill the central hole in the wire drill block to 2.1 mm. If you don't have size 2.1 then you can use 2 mm as well, just be sure that you can perform the next steps. Also drill the six screw holes in the wire drill block to 2 mm, and do the same for the fourteen screw holes in the solder block. Insert the 60 mm rod in the wire drill block. Align one end of the rod to the marker. Clamp the rod using the three caps and six M2 x 16 screws with nuts.
Drill two holes with a 1 mm (or 1.1 mm) sized drill bit right through the 2 mm rod inside the drill block. You can use either the horizontal or vertical hole pair. I myself find the horizontal holes easier to drill through. Take the rod out and repeat with the other rod. The holes should be parallel and spaced 20 mm.
Cut the 1 mm brass rod in two pieces of length 40 mm and round the ends. Insert these pieces through the small holes of the 2 mm rods. Align the four rods with the matching grooves in the solder block as pictured, and screw both halves together. Make sure that the rods are exactly centered in the grooves.
Solder the intersecting brass wires to each other like pictured. Make sure that the solder flows all around each rod for a strong attachment.
Bend the long end of the 2 mm rod upwards as pictured.
Unscrew and separate the two halves. This is not trivial as the high temperature may have fused the parts together. There are several slots to put a small screwdriver into, just work all around the parts until the halves are free.
Cut off the 1 mm rods and put some marks, at 11, 14, 31 and 34 mm as measured from the right (the short end of the shaft).
Flatten the rightmost connection between the rods to allow it to slide through the piston. Cut the 2 mm rods along the marker lines using a dremel or other suitable tool. Remove excess material using a small file until you get the finished crankshaft as pictured.
Close-up of the crankshaft.
Cut four silicone rubber seals (thickness 1 mm) with the seal cutter.
In this picture a 16.3 x 12.3 mm cutting block is used. If the seals are too large or small then other sizes can be generated with the customizer: https://www.thingiverse.com/thing:3020690
A difference of 0.1 mm can already give a huge improvement in the friction/leakage tradeoff, so make sure that the seals are consistent and match the space in the cylinders well.
Put double-sided adhesive on the faces of each piston to attach the seals to. While the use of adhesive is often not needed in stepper motors, it appears to be quite useful in rotary engines like this.
Mount seals on the sides of the pistons. Each seal has a large and a small face. The small face must be adjacent to the piston, the larger faces point outwards (otherwise the seal cannot keep the pressurized air inside the chamber). Apply petroleum jelly (vaseline grease) all around the pistons and seals.
Prepare roller bearing by cutting eleven pieces of length 5 mm from a diameter 3 mm metal rod. It is also perfectly fine to use 3-D printed rollers, but be sure to remove 3-D printing artifacts on the rolling surfaces.
Enlarge screw holes with a 2 mm drill in parts like these. The center holes should be slightly bigger, use a 2.1 mm dril (or 2 mm with some creativity) to allow smooth rotation of the crankshaft. Screw holes in certain other parts need to provide some lock to the screws, so do not drill all of them yet.
Make the top face of this part as smooth and flat as possible.
Apply blue silicone on both sides of the rotor housing. At this moment the countdown clock starts ticking: you must finish the whole motor before the blue silicone hardens out (usually within a hour or two), in order to allow the motor to push away excess blue silicone inside the cylinders. So be prepared to perfom all remaining steps, possibly after a "dry run" (without silicone) if this is your first pneumatic engine.
Assemble rotor as shown.
Fasten together using M2 x 20 screws and nuts. The rotor is now ready.
Insert crankshaft in backplate and mount backplate and frame together using M2 x 10 mm screws and nuts (in the slots). If you are using the M5 variant: screw in two M5 x 4 mm pneumatic sockets. There is also a 4 mm lock socket variant available.
Place cage and eleven rollers on the rotor.
Mount top plate and tighten with M2 x 10 mm bolts and nuts. Do not over-tighten, the rotor must be able to rotate freely. The bolts must be secured to prevent from getting loose due to vibrations. The screw holes in the frame (if not drilled out) should provide enough locking friction, alternatively you can use locktite.
Bend crankshaft upwards, trim excess length if desired.
The motor is now ready and can be tested. If excessive air is leaking from the valve (between backplate and rotor) then tighten the front screws a little bit. You can optimize for maximum speed (low friction), maximum torque (low leakage) or maximum output power