If you're interested in finding the stall torque of a motor, you need:
1) A very good power supply capable of sustaining a set voltage while stalling the motor(hard to do with cells).
2) A method of measuring force(digital scale)
3) A lever of a known length
4) A motor



First a note about torque. Torque is an angular force, which has 2 basic components, which are distance and force(T = d*f). Distance being the span(in centimeters) from the motor shaft to the end of the lever applying force on the measurement point. The force being the actual force, in grams, applied to the measuring point by the motor, through the lever.

My lever is an aluminum rod I had machined, with a perfect 15cm dimension from the 2mm hole drilled in the end, to the other end, and a set screw to tighten it on the motor.


My measurement point was a simple digital scale, prop'd in a way so that the measurment point was right at the end of the shaft. When you set it up, you have to then tare out the weight of the mechanism, so that the only force being monitored is that of the motor pushing down on the scale, through the lever.



A thing to note about this test and power supplies....you have to have a power supply that will sustain a set voltage while stalling the motor, which is a little tricky. Most power supplies, when stalling the motor, will give all the current they can, and then have to drop in voltage. This being said, if you set the voltage too high, the power supply will have a hard time sustaining it while loaded with a stalled motor. If the voltage droops, then your data will be off when you try to compile a load line and efficiency curves. The voltage must sustain to be accurate. A remedy for this, in an effort to maintain accuracy, is to set your voltage lower, so that the power supply will not bottom out in supplying current, and thus sustain the set voltage. This is just an FYI for those of you that might be quick to ask why I set the voltage to 3V, instead of 7.2V, while doing this test. The picture below shows what happens when I set the voltage to 7.2V and stalled the motor.



See how the voltage dropped, and the current is max'd at 10.15A? That means my power supply is max'd out, and had to drop in voltage. So, in all my motor comparisons from here on out, I'm using 3V. Don't worry, it won't effect the comparisons of the motors, as long as I use 3V for every motor. Yes this means that I can't include brushless motors, as I doubt there are any that run on such a low voltage, nor handle being stalled while measuring stall torque....but anyway. I just had to get that out about the voltage thing.....Here's what a sustained voltage of 3V delivers to the stalled motor in terms of current.



So at 3V, the stalled motor is drawing 7.22A. And at 3V, a stalled SPEED 400 will deliver force reading of.....



22.1 grams. So we plug our known variables of torque into our equation. T = d*f = 15cm*22.1g = 331.5 gram centimeters of torque, which isn't too shabby. Take a look around at data sheets for motors at http://www.mabuchi.com and you'll see what I mean.

So that's a method of measuring stall torque, which is one point in the motor's load line. The other point is no-load speed, which I need an opto-isolator setup to measure.....to be continued.........

There were a couple more pics in that post, but I couldn't include them......damn the man.