How to measure torque

[JFettig]

Hey guys, Im getting some motors, The torque is unknown but at least 300oz/in
4.2v 4.7A nema 34

Is there a way I can measure the torque? like mount the motor, attach a rod of known length and push up on that rod with a scale and do the math?

Thanks,
Jon

[Chagrin]

If you put a 2" diameter spool (1" radius) on the shaft and use it as a sort of "winch" to lift weights, however many ounces you can lift is the definition of the oz/in rating. I.e., 300 oz/in is the ability to lift 300 ounces at a distance of 1" from the turning point.

Your method would also work but you need a rod of "zero" mass in order to measure accurately (well, just as you need a spool of "zero" mass) and the measurement is only accurate at the point in time where the scale forms a perfect right angle with the rod.

[JFettig]

Thanx for the information. I appreciate it.

At what voltage should these measurements be taken?
Is it fair to look at other motor specs and compare them? I cannot find any information on these motors other than wahts there, the dimentions too, but not torque.

Thanks
Jon

[ESjaavik]

Originally Posted by JFettig Thanx for the information. I appreciate it. At what voltage should these measurements be taken? Is it fair to look at other motor specs and compare them? I cannot find any information on these motors other than wahts there, the dimentions too, but not torque. Thanks Jon

Current, not voltage. The torque is proportional to current. Use the current from the label or specification sheet.

[HomeCNC]

Most specs on steppers are for HOLDING torque, not lifting torque.

[arvidb]

So then attach a bucket to the string hanging from the spool, and add weights to it until the motor drops it.

The weight of the spool is of no importance, and if you use a rod, connect it to the rotor in the middle (so equally long parts of the rod sticks out in opposite directions), and its weight will not matter. Or you can attach it in the end, and add half its weight to the final bucket weight before calculating torque.

Also, torque is the product of force and distance. That is, oz*in, not oz/in. So when you have your weight, you multiply that with the length of the rod (or radius of the spool) to get the torque.

Arvid

[Chagrin]

I would think that the moment of inertia of the spool would create some discrepancy in the measurement, so the weight does matter. I'm also not sure how you figure that "half" the weight of an end-attached rod should be added to the bucket weight, but you might know something I don't. Back in college the physics classes all seemed harder than that

Suffice to say I think you should give the bucket a push to get it moving and see if the motor can sustain the motion to determine if it's capable of the tested torque -- static friction could cause havoc in this measurement.

[arvidb]

Well, this is mostly guessing, but: the inertia would only make the bucket accelerate slower. It would not increase or decrease the weight needed to set the bucket off. This is of course if we start with a stopped motor, to test the holding torque, as Jeff suggested (and to which I agree).

The end-connected rod: the mass centre of the rod is in its middle (assuming it's a uniform rod that doesn't get thicker in one end, for example). So the torque exerted by the rod equals its weight times half its length (assuming now that it's horizontal). Since you multiply the final weight with the full length of the rod, you must add only half its weight to the final weight - this is the same as first calculating the torque produced by the bucket, and add that to the torque of the rod. With M = total torque, Fb = force-bucket, Fr = force-rod, Lr = length-rod:

M = Fr*Lr/2 + Fb*Lr = (Fr/2 + Fb)*Lr

I agree it would be a good idea to give the bucket a push - but we want to see if the motor is able to stop the motion, not sustain it (it's the HOLDING torque we are after). Still, any moment of intertia of the spool would only delay the braking a bit, and not affect the motor's ability to actually hold the bucket or not.

Arvid

[quickerz]

The only way to calculate the moment accuratly would be to mesure the static hold moment. To measure the moment when the motor is turning would require some sort of brake to stall the motor to it`s breaking point. This is quite overkill for steppers. But the static moment could be calculated quite easy by the "bucket" metod. Tourqe=F*L. So if the motor shaft is 4.75mm in D, that makes the radie is 0.00237meters. The F would be the mass you can put into the bucket including the buckets own mass in kilograms before the motor slips. The total weight would then be the mass * 9.81 (earths gravity).

Motor hold tourqe=(mass*9.81)*0.00237m. The hold tourqe is then exspressed in Newton Meters.

Jon

[JFettig]

Thanx for the info guys, Im looking for a close, but not extremely acurate measurement. I think I will try the string and bucket thing. Ill have to find some weights of some sort.

Instead of using a spool, Ill use a rod with 3 holes, 1" apart, just have nothing in the one hole sticking out the side and the string hanging off the other.

Thanx
Jon

[3t3d]

Originally Posted by JFettig Thanx for the info guys, Im looking for a close, but not extremely acurate measurement. I think I will try the string and bucket thing. Ill have to find some weights of some sort. Instead of using a spool, Ill use a rod with 3 holes, 1" apart, just have nothing in the one hole sticking out the side and the string hanging off the other. Thanx Jon

Use the disk, with the string wrapped loosely around the disk. You
can spin the motor at whatever speed you are interested in, and then
when you pull the string tight, it will load the motor. Use a windlass
arangement, rather than a winch.
This will allow you to find the torque of the motor at various speeds,
rather than only the static torque. At any speed of interest, get the
motor up to speed, then see if it can lift the current load. If it can
move that load, increase the load, untill you find the torque capability
of the motor for the speed in question.
Untill you apply tension to the string, there is no loading on the motor.
Without tension, there is no friction. You only need to apply enough
tension to develop friction, not enough to affect the torque measured.

Pete