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.
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,
In any event, you are going to want a bag of good experimental technique
to get decent results.
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.
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.
--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.)
--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.
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
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
Ten years after the Ord "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
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?
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
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
--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.