Measuring torque on a motor with a leadscrew with a torque wrench

A prony Brake and kitchen scales can do the job. I saw a good description a while back but can't find it now. A basic setup is found here:

but you will have to ignore all the over unity free energy garbage. A test engineer for a local stepper motor manufacturer confessed that they used to use a bit of cord wrapped around the output shaft and some scales.

Potentially more accurate but a bugger to set up would be an electrically regulated load. Essentially a suitable electric motor with the terminals connected to a load to vary the current and, therefore, the torque.

In any event, you are going to want a bag of good experimental technique to get decent results.

Pete Harrison

If your motor can be unbolted, just mount it on the torque wrench and start 'er up! The torque on the motor cannot easily be measured when it's bolted down, however.

It is possible to characterize the motor and use the back-EMF on the windings (voltmeter measurement, basically), but it might almost be as easy to measure the power input and the rotation rate and divide. You'll have to account for waste power, too, of course.

What is wrong with using the stepper motor's manufacture specs?

Paul

Stall torque is easy. Just attach an arm to the shaft and use a spring scale to measure the force exerted at the end of the arm. If the arm is one foot long, and the force is one pound, the torque is one foot-pound.

But steppers, unlike DC motors, don't have a useful stall torque. DC motors produce max torque at stall, so for DC motors you find the stall torque and figure you can use 50-75% of that. The key number for a stepper is the load at which it starts missing steps. Stalled, it will just vibrate, not produce a steady torque. What you actually need to measure stepper torque is a smooth load like a water brake dyno, which is simply a paddle turning in a viscous fluid.

Lacking that, put a big pulley or spool on the shaft and have the stepper wind up a string or rope with weights on the end. Increase the weight until you get a stall. Figure on no more than

50% of that value as useful output.

John Nagle

--OBTW something I've learned about using steppers on a leadscrew: the stepper motor *likes* to have a certain amount of drag on it; i.e. ballscrews are nice for servo motors but not worth the money if you're using a stepper. I've found that my stepper-driven X-axis (on a Bridgeport mill) works more reliably if I ever so slightly engage the brake lever on the front of the table.

Hmm ... is this the rotating X-axis leadscrew? Perhaps it is the mass of that long rotating screw and the impulses from the stepper combining to cause the leadscrew to wind up a bit, and unwind for a few cycles. This may be one of the reasons that the Bridgeport BOSS-3 and later CNC machines (at least through BOSS-6) which used steppers also redesigned the setup so the leadscrew does not rotate, but instead is rigidly attached to the right-hand end of the table, and the *nut* rotates within a pair of opposed tapered roller bearings. (Another reason for this is to avoid the leadscrew whipping during a fast move.)

Enjoy, DoN.

--Interesting! Hadn't thought of moving the nut; doesn't sound trivial tho.. Yes, there's the mass problem, probably a screw torsion problem and then there's the issue of synch points or whatever one would call them. That is to say the stepper doesn't have a totally smooth speed range: there are points when the harmonics seem to interfere with the stepping process and that's where trouble seems to occur most often. The solution for my system is to adjust the speed ever so slightly one way or the other; not ideal but it's cheaper than a redesign of the whole thing.

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It sounds to me like resonance -- probably the twist goes down the leadscrew hits the end, and reflects back. Your problems are happening when the time required for the round-trip matches fairly closely the time from step to step.

It might be worth trying a flywheel-shaped weight at about where the leadscrew joins the stepper to see what effect that has. Is this direct coupled, or through timing belts?

Enjoy, DoN.

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From the first days of the enactment of this measure, and all the way through until 1877, the inspectors of brothels had standing orders to enter any native house that they suspected of containing any women of loose character, and arrest its inmates in accordance with the following plan: The inspector would secure an accomplice, called an informer, or often more than one. The accomplice would enter a native house plentifully supplied with marked money out of the Secret Service Fund. This accomplice was often a friend or relative of the family he called upon. He would often offer them a feast and drinks, and send to a near-by restaurant and procure them at Government expense. After feasting

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--Neat idea! I've got the motor directly attached with a honkin' big helical coupling that I snagged many moons ago (I helped build the machine the manufacturer needed to make 'em back in the early '80s). I suppose there's room enough to put a hunk of iron somewhere; will give it a look.