# Servo torque calculation

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I'm contemplating an application for a servo. How do I figure out how much torque I'll need? The application is for twisting tempered steel wire into a twisted knot. You've probably seen a knot-type wire wheel brush. I do this with hydraulics now but I know it's WAY overkill. Then I need to figure out if I should use a gear box or a timing belt.

Hydraulics are persnikety, the oil has to be just the right operating temp.

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Measure the torque required, by means fair or foul.

For me this would mean making some sort of fixture (or using an old machine, if you have one) to do the job close to how you do it now, then use an indicating torque wrench or a torque watch (possibly home-brewed) to measure what it takes to make it happen.

Then multiply by 2, and use that as a minimum. Depending on how much speed you want, you very likely want to gear things down considerably -- electric motors are usually at their best when they're going as fast (or faster) than the manufacturer recommends. You really only want to direct- drive things if it's going pretty close to the motor's maximum speed already, or if you have a need for light, well-controlled torque.

(You should already know this, but if you don't -- take the efficiency of the gearbox into account. Nothing's more embarrassing than leaving out the loss of a component in your calculations, and coming up short in the end).

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Why not get a torque meter and measure the torque? You can make a crude one with an arm and a weight or fish scale. My understanding is the torque will be about the same at any speed to perform this kind of task. Most likely a gearbox with a relatively low ratio will be needed. Possibly belt reduction can do it, also.

Jon

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Right. That's how a Prony Brake dynamometer works. I've made several of them for measuring torque on small electric motors.

I've used a DC motor, running as a generator, into a big variable resistor as a brake. That's how robotics hobbyists often do it -- there's a whole book written about those things.

The first one I made, back in the '60s, was for the purpose of measuring torque on small racing outboard motors. For that, I used the classic Prony Brake setup, right on the prop shaft, with a flywheel and a piece of automobile brake lining material running on the outside of it, with a lever to adjust the braking. It was a little rough, pulsing with the cylinder firings, but it did work. And the spray from the cooing water outlet didn't make things any easier...

Using a motor/generator with a resistive load works much better. You don't need to measure the output of the generator; its only purpose is to provide a load, adjusted until the spring arm doesn't move and the measured spring rate is stable. You need to arrange some stops for the arm so you don't wreck your spring scale or break your knuckles. d8-)

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Ha! Here's a really nice homemade Prony Brake, a much nicer version of the one I made in high school to measure outboard motor torque:

If you google Prony Brake you'll also see some that use DC motors as generator/brakes. The simplest brakes are just two spring scales, a leather belt, and a small flywheel:

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Possibly a dumb question, not seeing the machine, but why is a servo required at all? Twisting wire knots would seem to be a "rotate x times" thing that would be simple limit switches, not requiring any sort of servo setup.

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Perhaps he means servo _motor_? Tom?

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The bit about the current hydraulics and temperature sensitivity imply proportional hydraulic servo valves. Without seeing the machine I'm having a hard time picturing why such would be needed for what would seem to be a simple "twist it until you hit the limit switch" type of operation.

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Unless he wants flexibility in the number of turns to make, in which case some sort of digital control would make things easier overall.

But we're both just guessing...

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Tom, if I were doing it, I'd build a simple test apparatus to twist the knot, and apply a torque wrench to do the job. I think you'll find that the twisting torque increases significantly with the amount of twist, and that it rises asymptotically to vertical as you approach the maximum twist the wire bundle can withstand while still "chucked" at both ends.

A torque wrench would give you close approximations of both the starting and ending torques, and give you an idea of what portion of the duty cycle is occupied by what torques. From that information, you can deduce whether to get a motor/drive train that covers the whole envelope within it's 'normal' operating torque, or whether you can over-torque it at a certain duty cycle without harm.

LLoyd

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I suspect it doesn't increase much because the metal is being strained beyond its yield point where the stress-strain curve flattens out. If it was in the linear region it wouldn't stay twisted.

I just went down to the sekrit laboratory and twisted some 1/16" soft iron wire with safety wire twisting pliers. The wire started to twist at ~25 Lbs pull, rising slowly to 50 as the coils wound tight.

jsw

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The twisting nozel has to realign itself perfectly with the disk at the start of each cycle and varying the speed in different parts of the twist would have a great advantage. I assumed a servo motor would be the way to go and there would have to be two more servos, one for indexing the disk and one for the pull-back. A stepper could work for indexing.

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I've played with twisting in a jig and starting the twist is the hardest then it gets easy.

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Tom Gardner fired this volley in news:frudnYmRecYrNWbTnZ2dnUVZ snipped-for-privacy@giganews.com:

OK... I was wrong. DON'T use a torque wrench to measure it!

LLoyd

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Sure sounds a like a good servo application to me. Its been too many years but I used to work with a company, Parker Compumotor, for such applications. Their application engineers were the best. A few phone calls and we'd have the right equipment selected. Later, at machine startup, they'd all but write the machine code to run their motors. I'd at least ask if they still provide such excellent service.

Karl

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