Foo - What would be the best way to measure necessary torque

Okay I have a shaft that needs to be rotated. I want to use an appropriately sized motor to rotate said shaft. Torque necessary will not be a lot, it will be under 2 Nm.
What would be a fairly accurate way to measure the torque necessary? The point of this is for me to avoid buying a motor larger than I need (therefor adding cost), so the method to measure torque needs to be inexpensive as well. Any suggestions?

Will the loads be constant? Attach a pulley to the shaft of a known diameter. Hang weights off the pulley and find the minimum weight that barely spins the shaft. Calculate torque based upon weight and radius of pulley. Add a certain percentage to deal with stiction, friction, heat, expansion, bearing wear & tear, etc. I'd use double the minimum torque just to be on the safe side.

The loads will be constant. I know I can do a setup like you suggest, but I was kind of hoping to avoid all that due to laziness. Do light duty torque wrenches exist that might trip at a torque as low as 2Nm?

Will the loads be constant? Attach a pulley to the shaft of a known diameter. Hang weights off the pulley and find the minimum weight that barely spins the shaft. Calculate torque based upon weight and radius of pulley. Add a certain percentage to deal with stiction, friction, heat, expansion, bearing wear & tear, etc. I'd use double the minimum torque just to be on the safe side.
+1

A nice exercise from ME UG days. :D

The loads will be constant. I know I can do a setup like you suggest, but I was kind of hoping to avoid all that due to laziness. Do light duty torque wrenches exist that might trip at a torque as low as 2Nm?

They do exist, I know that for sure because a friend of mine has one.

I'll call him and ask where he got it.

PC... You'll probably pay as much for the torque wrench as you are the entire contraption you're building.

Spent a couple of bucks, buy a pully, add weight and do the math. For someone as bright and industrious as you I'd think you'd welcome the practical application of your knowlege. :)

I guess I could. I think it may be easier to just buy a bigger motor, or go by what other people have done. I don't want to make the setup for just one test.

I phoned him and it turns out it's his bro's, and he's in Calgary right now.

I guess I could. I think it may be easier to just buy a bigger motor, or go by what other people have done. I don't want to make the setup for just one test.
Bigger motor = $$$$ Test = $

Plus, it's a life experirence.

BTW ~ How's that oragami rat coming along?

You just need to rotate a damn shaft?
Technically near 0 nm torque would rotate any sized shaft. It's acceleration and overcoming static moment of inertia from friction you have to worry about. Like danno says, use a cheap pulley. You don't even need a cheap pulley, just clamp a bar coming out of the shaft and start attaching weights to it until it moves. That's the necessary force to overcome static friction. It will be your bare minimum required to get it moving. How fast you want it to come to whatever rpm's you want, well, figure it out, you know how to do math right?

BTW: 2nm is about 18lbin (x9). Plenty of cheap torque wrenches on mcmaster to do the job.

Okay I have a shaft that needs to be rotated. I want to use an appropriately sized motor to rotate said shaft. Torque necessary will not be a lot, it will be under 2 Nm.
What would be a fairly accurate way to measure the torque necessary? The point of this is for me to avoid buying a motor larger than I need (therefor adding cost), so the method to measure torque needs to be inexpensive as well. Any suggestions?
Wind a string on the shaft and add weight til it starts unrolling. That will tell you the minimum torque in ft-lbs or whatever standard you use to overcome static inertia and rotate the shaft. Now as to whether that torque will be enough to actually handle any work load is a different story.:D

Check you work, Tom... You're forgetting a step

Check you work, Tom... You're forgetting a step
Well, I'm operating under the assumption that the shaft is mounted on bearings already.....

I used ft-lbs but the standard is whatever he chooses to apply. What am I missing?

A mounted shaft, add weight till it unrolls, the mass will indicate the amount of torque necessary to overcome the static inertia quite neatly and simply through the initiation of rotation!!

The calculations!

Weight/Radius of the shaft, or is that the other way around? It's all about the calculations. PC dosen't want to do therm. :)

Another method might be to determine the brake HP of the motor and apply a calculation
Torque = 5,252 x hp/Rpm.

the rpm is not a fixed variable, so don't treat it as a constant

Wooo :).
The reason I was wanting to do this was because I was afraid that the current step motors I have are not up to the job, due to a lack of torque. So I just got my ballscrews mounted and decided to give it a whirl. Got my step motors mounted on hte X and Y, and it works with plenty of leftover torque! I will take a picture soon