Needed to test a small motor under load, for a mini-winch like application. So I made this holder & drum device that loads the motor via weights hanging on a rope / string.
The whole thing was designed, printed, and the motor tested, in a day. Pretty simple stuff. Results below.
The CAD model can be modified relatively easily for other small motors with a diameter in the common 35-60mm diameter range.
The Fusion360 CAD model (messy), STEP and 3MF files are on Github and MakerWorld
Happy torque maxxing to all !
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Motor & Test
Motor used: 6-22v 200W DC Planet Gear Motor 90kg.cm (9Nm / 65ft.lb) High Torque Mini Metal Geared Motor ($30) 
Its shaft has a D shape with an 8mm diameter, making it easy to securely mount any custom gear to it.
This motor is pretty much the same as used in 18V cordless drills, and comes with 36:1, 151:1 or 216:1 reduction ratios. Specs:
The test went off without a hitch.
This motor started struggling with a 22.5lbs (10kg) load, resulting in a max load torque of ~4Nm (3ft.lb). That's far from the advertised 9Nm. But the shaft wasn't symmetrically loaded (unlike in a planetary gear train for instance), its end wasn't supported by a bearing, 9Nm might be only at 22V, and it is possible that my lithium battery wasn't holding 18V under load. Or the Amazon no-name-supplier is Pollyanna and the specs are 'optimistic'...
Note that in my test the motor was only seeing half the attached 45lbs weight seen in the top picture as a pulley was used. Specifically, a Petzl Micro Traxion pulley that provides one-way-rope-locking. This prevents the weight from dropping when the motor is powered off, thus keeping feet safe. Alternatively, much cheaper ratchet pulleys would do too:
Of course it can also be used without a pulley, and directly pull the load up instead. Just mind your feet 😅
Calculating the torque
Since the drum has a 7cm (2.75") diameter, the formula is: W(kg) x 9.81 x 7 / 2 / 100. W being the testing weight attached to the rope. This said, as the rope coils around the drum during the test, my average coil diameter looked more like 8cm, so I used this simplified formula:
- in N.m : W(kg) x 0.4
- in ft.lb : W(lbs) x 0.3
The max test load for this not-specifically-reinforced 3D printed design is probably around 12Nm / 30kg (8.8ft.lb / 65lbs).
But, realistically, the shaft of the motor under test will likely give
before that as those aren't usually designed to be heavily loaded in
shear without a supporting bearing on their end.
CAD
The upper carabiner hole was X positioned in line with the middle of the drum, and Y aligned to the back of the drum. This ensures, when spinning the shaft clockwise, that the string the holder hangs from, and the string getting coiled, remain mostly coincident under load. Worked well: despite being free-hanging the system remained stable, and the motor horizontal, during the tests.
The
motor mounting screw holes in my model correspond to that motor's gearbox, but they
can easily be changed in CAD.
The drum diameter is 7cm
(2.76"). That can also be easily modified in the model.
The screw used to hold the drum on the shaft sticks out by about 10mm as it is also used to hook the end of the load string onto the drum. It is simply a 1.75" #10 wood screw (45mm, ~4.7mm OD). The screw hole in the CAD model is ribbed to provide plenty of material to bite into, and avoid cracking.
Printing
Settings
- PLA, 0.4mm nozzle, 0.24 layer height
- 4 walls, 6 top, 6 bottom (overkill for the tested weight, 3/4/4 would work too)
- 20% Adaptive Cubic infill
- Tree Auto support, 35° Threshold Angle, On Build Plate Only (should have split the drum into 2 halves, with interlocking fingers, to avoid supports and for a better finish. Pero soy flojo...)
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