# How do I measure torque from a shaft?

I’m using a planetary gear set and I need to figure out the torque of the output. I couldn’t find anything that would measure the torque of a shaft (mostly the torque and efficiency of cortex motors). The only way I could think of how to do it for now is timing how long it takes to reach maximum speed with an encoder to get the acceleration and then convert that into torque. Any ideas are welcome also please post threads that I might have missed that have a working idea.
I don’t know if this helps but its a Ravigneaux gear set.

you could probably use a pulley with weights and keep adding weight until it starts to stall. then you can do math to figure out how much torque it takes to lift the heaviest weight is could do.

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Torque varies based on a few things. What you probably want to find is the stall torque, which you can measure by applying a torque in the opposite direction to it until it is unable to turn.

Directly measuring torque is a challenge. When I worked for La-Z-Boy, I used something like this https://www.omega.com/en-us/sensors-and-sensing-equipment/load-and-force/torque-sensors/p/TQ103 to measure the torque for operating the chair handle. You might be able to work something out with a less expensive “beam” type torque wrench, like this one: https://www.amazon.com/Performance-Tool-M195-Torque-Wrench/dp/B00SNICDJC/ The potential issue you may have with using a wrench, however, is that your torque will always be the torque at the point the motor stalls. Only an in-line transducer like the one from Omega would let you measure torque while the machine is running. You could also build your own stall-torque measure device by attaching a beam of known length to the end of your shaft and measuring the force at the end of the beam with a small gram or ounce scale. Torque is just the length of the shaft tiimes the measured force on the scale.

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Okay I’m going to do that. Also I’m assuming that I should still find acceleration speed but I should calculate torque with what Kmmohn said.
Also does anyone know the stall torque of V5 motors?

Edit:

In this topic callen said something about stall torque being less important

So with this knowledge I might just calculate torque with the acceleration and not use stall torque as my main way of calculating it. I’m assuming that the first link kmmohn sent would be the most accurate way I could calculate torque and I don’t want to spend all my money on that.

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you can find it on the VEX website. I believe for the 100 RPM (red cartridge) it’s 2.1 Nm

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He’s looking for the experimental values, not just the theoretical ones, I’m assuming.

As was said earlier you can test this with a pulley, or, you can use an encoder and measure the rpm at the shaft, and then use the formula P=ωτ to find the torque. The power value is listed on vex’s website, I don’t recall what the exact number is at the moment. ω is angular velocity, and while rpm is one commonly used unit to measure this, I would be sure to double check your conversions before simply plugging in rpm. Worst case scenario if you can’t math it out just google the conversions. I’m sure there’s some online calculator out there for this kind of thing

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Here’s an example test rig you could set up. Torque would be the length of the beam (from the center of the motor shaft to the point where it is pressing on the scale) multiplied by the scale readout.

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τ=FSin(θ). It’s multiplied, not divided

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Right…thanks for catching that. (I’ve corrected my posts). Long day, and I forgot all my arithmetic when I took Calculus, and I haven’t used Calculus for 30 years, so I’ve forgotten that too

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No worries : )

Yea I’m looking for the experimental values because my gear set is semi inefficient and also thanks kmmohn for the CAD picture I was confused on how the setup would be. Also is that example for how to do stall torque or for torque (sorry I’m not good at words) I’m assuming stall torque because your measuring for how much weight it can push until it cant push anymore.

Right, the set-up shown would be stall torque…or it might be the torque where your shaft starts twisting…be careful if you’re using a low strength shaft in a V5 motor!

I’m assuming that stall torque would be more helpful in pushing battles but torque would be more helpful when accelerating. But they both would go up with a gear ratio that increases torque.

this product looks a little out of the typical vex team’s price range lol

nice try though, mr. force_measurement_sp

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yes but this helpful fellow made an account just to post this super useful tool, the least we could do is buy a few dozen. if it’s 3 and a half thousand dollars it must be good!

Naw, we go in as a group buy and get one, less than a dollar per team Now if just shipped in less than 6 weeks…

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500 4” flex wheels would be a better investment ngl

yes, but how are you going to measure torque on those flex wheels hmmm?

use them as weights. when the string attaching them to the gears stalls u can calculate the torque