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. It's not exactly a dynamometer since it doesn't log Watts / speed / torque, but is a convenient way to manually test various loads.
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: DC90 6-22v 200W DC Planet Metal Gear Motor 90kg.cm (9Nm / 6.6ft.lb) High Torque ($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.
The 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, and it is possible that my Makita 18V LTX 3AH lithium battery wasn't holding 18V under load. Or maybe the Amazon no-name-supplier is Pollyanna and the specs are 'optimistic'...
Also, the No Load speed was ~300rpm at 19.5V. Pretty spot on with the spec.
Note that in my test the motor was only experiencing half of the attached 45lbs weight seen in the top picture as a pulley was used. Specifically, a Petzl Micro Traxion 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 in Newton.meter is: W(kg) x 9.81 x 7 / 2 / 100. W being the testing weight. 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 0.135
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 & Assembly
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 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.
Assembly
- mount motor on holder
- slide drum onto shaft
- secure drum with 1.75" screw (or shorter if drum fits tight enough)
- make a short loop at the end of the load string, hook it around the drum screw's head
- hang holder up
- attach weight to load string
- go brrrrrrrrrrrrrrr
The
motor mounting bolt 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 holding 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). Note that if the drum fits tightly onto the shaft this securing screw is optional and can be replaced by a shorter one that just holds the string's end.
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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