# VEX 393 Motor Question

Oh ok, so if a 393 motor has a stall torque of 1.67 Nm, then I should use 10g - 100 kg masses, as the greatest mass I’m lifting, 100 g, will take 1 Newton to lift 1 meter, which is less than the stall torque of 1.67 Nm. But if I were to use the 100 g - 1 kg masses, then the winch wouldn’t be able to lift anything greater than 1.67 Nm. The 200 g mass would take 2 N to lift 1 m, the 300 g mass 3 N to lift 1 m, and so on.

Thank you so much, man. You quite literally saved my science fair project from being a disaster. I think it’s really starting to come together now.

I think you might still be misunderstanding torque. It’s not a matter of how high it can lift it, it’s a matter of the radius of the pulley (so, wheel or whatever) attached to the motor.

Imagine in this picture that instead of two ropes, there’s just the one going down, and the wheel is hooked to the 393 motor. The distance you’re measuring is the distance between that center dot and the outside of the circle (technically a little bit inside, it’s the distance between the center of the circle and where the rope is coiled around)

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I have prepared a high quality diagram.

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I already built the winch I’m using for a project for my engineering class a couple of months back. This is what the pulley looks like:

The pulley itself is just a shaft. Let’s say, for convenience sake, the radius is 0.1 meters. So, using Newtons acting on a 100 g hanging mass due to gravitational force times the radius of the pulley, I get 0.1 Nm.

So I should use greater masses, depending on the width of the shaft, to maximize the spread of data points and for me to see a visual difference depending on the weight of the mass I’m using.

Am I on the right track here, or is there something else I’m missing?

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Do you have any way of making an actual pulley? The radius of that pulley is not 0.1 meters, it seems to be more like 0.003m. So you’re going to have a hard time nearing stall torque that way.

EDIT: not sure I already said this, the stall torque on these motors is 1.67 Nm, you’re gonna want like… 1.3 ish to get a good range while still being sure you’re not going to stall it.

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Good point. Something to consider. I know VEX makes pulleys but they’re still way too small for my liking. I could look into constructing one on my own. I could make the radius large enough so I wouldn’t have to worry about getting too many weights.

You also bring up a good point on stall torque. I’ll try my best to vary the data points as far as possible.

Just to throw one more thing into your experiment: you already know that power (watts) is Volts x Amps. But with your pulley setup and a stopwatch, you could also measure power by how fast your motor is able to lift the weight over a given distance. You could then compare your electically measured power by your mechanically calculated power. I’ll leave the formula to you…it should be in your physics book, you’ll end up with a very small fraction of a horsepower.

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You can just gear it down to decrease how much input force is needed to stall it. An 84t driving a 12t gear would decrease the torque required to stall by a factor of 7

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Also from what I see in the picture you sent, there’s a ton of friction in that system which will cause inaccuracies in the data. Plates aren’t very sturdy vex parts- try c channels and you’ll have a more rigid and precise setup. It’ll make a big difference compared to the bending that will take place with those plates

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Hello,
It’s been a while since we’ve talked, but I just figured I’d let you know that I won the science fair!
I placed 1st in physics as well as 1st overall in the 9-12 grade age bracket.
I won late February, but I wasn’t able to get my hands on the certificate until just yesterday.
I just wanted to give you one last “thank you,” as my project probably wouldn’t have been able to win anything without your guidance.
I’m moving on to the next level of the science fair, which has over 700 competitors throughout the tri state area. It’ll take place the 12th of this month.

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In case anybody wanted to take a closer look at my project, this was everything I had submitted:
Abstract: James Lee - Final Science Fair Project Part 3_ Abstract.pdf (29.9 KB)
PowerPoint Presentation: Science Fair (Final).pdf (342.5 KB)