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

Hello,
I am determining the technique used to measure torque on a linear motion system, using a stepper motor with an ACME lead screw attached
to it. I learned from one source that one could use a torque wrench (preferably a digital one). Does anybody know the exact steps needed to use one of these wrenches to measure torque? I know a fish scale could be used also, but I need precise values. If there is any other way other than these two, I would like to hear.
Thanks Mike
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eljainc wrote:

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:
http://jnaudin.free.fr/html/nwmechts.htm
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
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Peter Harrison wrote:

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
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    --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.
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    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.
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    --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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    --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.
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smuggled into brothels, some licensed and others unlicensed, or otherwise dedicated to immoral purposes. But the enormous extent to which slavery in this Colony has grown up has called into existence a greatly increasing traffic, especially in women and children. The number of Chinamen in this Colony has increased and is increasing rapidly, whilst their great increase in wealth has fostered licentious habits, notably in buying women for purposes sanctioned neither by the laws nor customs on the mainland. I hold in my hand a placard in Chinese, torn down from the wall of the Central School, Cough Street steps, in this city. The translation appears at length in the Hong Kong _Daily Press of_ August 15th, 1879. The purport of that translation is shortly that the advertiser, one Cheong, has lost a purchased slave girl named Tai Ho, aged 13 years. After a full description of the girl a reward is offered in these terms:--'If there is in either of the four quarters any worthy man who knows where she is gone to, and will send a letter, he will be rewarded with four full weight dollars, and the person detaining the slave will be rewarded with fifteen full weight dollars.' These words are subsequently added:--'This is firm, and the words will not be eaten.' I recently spoke in reprobation of slavery from this Bench, and in consequence of my remarks a gentleman who tore down this placard gave it to the editor of the _Daily Press_, and in a letter in that paper he stated that such placards are common, and that he had torn do
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under the Stars and Stripes; and when you can think of nothing else to do, you can always go aside and cry to the Judge of all the earth to "execute righteousness and judgment for all that are oppressed," as He has promised to do, if we but call upon Him.
Now read on with a heart full of courage, not caring for the haunting pain that will be left when you lay the book aside. What others have had to suffer, you can at least endure to hear about, in order to put a check upon like suffering in the future, and in our own land, too. A country bathed in blood as ours has once been has met already its terrible judgment for not throttling the monster, Slavery, in its infancy, before it cost so much blood and treasure. We will be wiser another time, and refuse to trifle with such great wrongs. We cannot brave the Omnipotent wrath in a second judgment for the same offense, lest He say to us: "Ye have not hearkened unto Me, in proclaiming liberty, everyone to his brother, and every man to his neighbor; behold, I proclaim a liberty unto you, saith the Lord, to the sword and to the pestilence and to the famine."
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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Chinatown early, that our coming may not be signaled by those on the streets at a later hour. If the alarm is given, every slave den will be doubly bolted and barred; and perhaps little Seen Fah, whom we wish to save, will be spirited away beyond reach of help." Well did the questioner know the terrible truth of these words. A sympathetic shade of sorrow and anxiety crossed her bright face. She, too, was a rescued girl and had not forgotten the dark, mysterious ways of Chinatown. The Superintendent rose to answer the summons of a small electric bell. Two trusted detectives had arrived. After a short conference, the rescuing party set forth on its strange mission. One who had eagerly thought and planned for the success of the undertaking felt her heart throbbing between hope and fear, but was reassured when a slender hand slipped into hers and a sweet, encouraging voice whispered: "I have faith to believe God will give us the girl." Faith triumphed that day. Through two of C
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"The zeal of inspectors of brothels and informers had been stimulated by occasional solid rewards from the Bench, and the numerous prosecutions commenced seldom failed to end in conviction and substantial punishment."
Ten years after the Ordinance of 1857 had been in operation, the Registrar General, C.C. Smith, wrote:
"There is another matter connected with the brothels, licensed and unlicensed, in Hong Kong, which almost daily assumes a graver aspect. I refer to what is no less than the trafficking in human flesh between the brothel-keepers and the vagabonds of the Colony. Women are bought and sold in nearly every brothel in the place. They are induced by specious pretexts to come to Hong Kong, and then, after they are admitted into the brothels, such a system of espionage is kept over them, and so frightened do they get, as to prevent any application to the police. They have no relatives, no friends to assist them, and their life is such that, unless goaded into unusual excitement by a long course of ill-treatment, they sink down under the style of life they are forced to adopt, and
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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.
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What is wrong with using the stepper motor's manufacture specs?
Paul
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