I'm doing a lot of magnetic bead DNA purification protocols, and they frequently have a step that requires rotating the tube at a low speed for 5-10 minutes. Until now, I'd been taping the tubes to the rotator shaft of the forgotten hyb oven in the hood of the lab next door. Then I realized I had a set of spare parts laying around that could do the job for me!
This lab rotator is designed for light duty around the same DC gear motor and PWM board I used in my low-cost remix of the Adafruit Crickit lab shaker. It uses a USB charger as a power source, and two ball bearings to keep the motor shaft captive. I designed it with a modular, magnetically-attached tube holder so that I could swap it out for different tube sizes in the future. There are spaces for eight magnets, but in practice a pair of 1/8" x 3/8" magnets or four 1/16" x 3/8" magnets per side are more than sufficient.
The version the I built uses a 1:48 geared motor, but the 1:90 geared version would be more appropriate for this application.
- I printed the motor shaft with 4 perimeters, as all of the weight of the rotator assembly will be transmitted through that point.
- The stand / bearing housing should be printed with the legs sticking up in the air, so that the bearing contain is laid down parallel to the print bed
- The only piece that needs support is the motor shaft, which has a small pocket to accept a little hexagonal nut.
- I printed the tube holder in PETG, as the more flexible plastic should help the flexible tube holders last longer. The rest should be fine in PLA, PETG, or ABS.
Assemble the motor shaft
The hexagonal nut has two sides with very slight tapers. The shorter/wider side snaps into the corresponding pocket on the shaft, and serves to index the tube holder to the shaft. I put a dab of super glue on the nut before snapping it in to the shaft.
The magnets should just snap into the holes on the shaft. You can also dab some super glue to keep them in there if you want.
Press in the bearings
I press fit the bearings into the outer bearing housing, starting the first bearing with my fingers and then turning it upside down and pressing it in with the bench top. I then pushed it a bit farther with my fingers and then put the second bearing on top of it the same way.
Then, the motor shaft could be carefully pressed in to make a rotating assembly. You might find that the shaft is a little loose or a little tight depending on your printer. You can either adjust the tolerances using the Fusion360 file, or either sand the shaft down a little or wrap it in some teflon tape to make it fit.
Assemble the electronics
Cut off all but about 2 inches of the cord from the panel mount USB connector. Remove all of the external rubber cable guard, the insulation, and the green and white wires. Strip a few mm of insulation from the red and black wires and connect them to the Power + and Power - terminals on the PWM board, respectively. Then connect the red and black wires from the DC gear motor to the Motor + and Motor - terminals on the board.
Mount the USB connector to the enclosure shell with the included M3 screws. Pull the plastic knob off the PWM board, remove the nut, and push the metal potentiometer shaft through the corresponding hole in the enclosure. Tighten the nut back on to retain the potentiometer shaft in the housing. Two holes in the edge of the circuit board should line up with holes in the outside of the enclosure. You can affix the board using short M3 screws.
Attach the motor to the flat housing piece and the stand / bearing housing with 2 30mm M3 screws. Line the nylon shaft of the gear motor up with the corresponding notch in the 3D printed shaft, and push M3 nuts into the lower hexagonal spaces in the bearing housing. Push the 30mm M3 screws through the holes in the gear motor, through the holes in the triangular housing panel, through the holes in the bearing housing, and tighten into the retained M3 nuts.
Assemble the rest of the housing
There are four more spots for captive nuts in the bearing housing -- two slots on the bottom of the legs, and two hexagonal spaces near the top. You can now click the housing 'shell' onto the triangular panel, and push 45mm M3 screws through all three pieces and tighten them against the captive nuts.
Now, drop the tube holder over the hexagonal plastic nut on the center of the motor shaft, and press magnets into the spots on the tube holder opposite the magnets on the shaft. Make sure you get the polarity right!