Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: Overloaded on April 04, 2008, 11:25:25 PM
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Hello folks,
Some stepper specs. only give "Holding" torque, no curve.
Is there a general sort of formula to determine the expected low speed to high speed torque curve ?
Just roughly........"Rule of Thumb" sort of thing.
Thanks,
RC
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As a rule of thumb I think you can say dynamic is generally about 70% of holding for low speed - or at least that's what I've read - not sure why!!!.
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That's good to keep in mind.
Looks like some motor ads are somewhat deceiving by bragging on holding and not supplying running info.
Thanks Stirling,
RC
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Holding torque is easy to spec as it is pretty much independent of the type of drive used. In steppers 'running' torque is inversely proportional to speed. The faster you step the less torque you have. Then you have to consider the type of power supply and driver that you have. Modern stepper drivers use a power supply 2-3 times what the stepper is rated for and control the current flowing through the stepper coils. This allows for the current/torque to build up faster providing more torque while stepping.
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Hello Rich,
You solidify my point exactly.
When a relative newcomer to hobby CNC, or CNC in general looks for stepper motors, the main piece of bait in the ad is the torque and they mostly refer to holding torque which can be a bit misleading. As you stated, there are other more important aspects to consider.
Seems like many of us greenhorns only learn these things after the fact. Some mfgrs. go to great length to describe their systems while some hobby dealers supply very little info. I guess maybe they assume we/I know already.
It has become clear to me, thanks to you guys, that there is much more to consider when building an optimal set-up.
Thanks RICH,
RC
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I knew I should have been more careful of how I phrased my answer. ;D
Of course I fully agree that the only reliable way to predict a motor's possible performance is to use the torque curves. (discounting previous experience of any given motor). I also agree that a motor's torque can be to an extent, maintained over higher speeds by increasing the voltage. This we all know. But of course it cannot actually be increased by doing this.
So at the risk of digging my hole even deeper - I'll re-phrase my original answer: As a rule of thumb - the MAXIMUM dynamic torque of a motor will generally be in the order of around 70% of its holding torque.
Now I'm going to run for cover ;D
Ian
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I'll supply cover fire for ya, that's the "general" sort of answer I was looking for.
Thanks Ian,
RC
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I always thought that the reason for using stepper motors was
1. To move by a number of steps to another position
2. When at that position - HOLD IT THERE
hence the holding torque is an important figure, if not the most important torque for this type of motor.
I would also think that it is an easier figure to both calculate, and verify by experiment, since it is stationary.
When we complain of loosing steps, we all assume that we loose them whilst moving - is it possible we are loosing them whilst the motor is stationary???
I think that a 70% figure sounds reasonable.
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Hello Jim,
For a rack and pinion, linear toothed belt or a high helix lead or ballscrew, holding torque is definitely important. A heavy uncountered mill Z axis as well.
But for acme leadscrews, allthread and the like, (hobby stuff), running torque is all that really matters. Once in position, there is (virtually) no torque required to hold it there. Just like manual lathes and mills.
Even with the ballscrews on my lathe, there is far less torque needed to hold than to feed a heavy cut.
Thanks,
RC
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Yes, and many of the small steppers found in office equipment for example have a very light load to move but holding is most important.
But for a converted lathe with a leadscrews, holding is not so important. They stay put by themselves anyway. Sure, the spindle is another matter especially with live tooling.
Good stuff,
thanks.
RC
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In the flavour of this thread is there also a rule of thumb for maximum RPM expected out of motor or even maximum RPM where torque is still 90% of the 70% if you know what I mean :)
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So you buy one of those high torque motors but can't get a torque to speed curve from the seller.
What can you "simply" do to generate a curve that would be "somewhat" indicative of preformance?
Now a stepper mounted in a fixture and it's shaft turned over a range of speeds ( verticaly in a vise and driven by a mill / drill press chuck) you could measure the voltage generated out of the wiring configuration and plot the speed to the generated voltage at say 24 volt output. Now I'm brain dead.............
RICH
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Rich,
Not exactly what you wanted but have a look at this posting :-
http://www.machsupport.com/forum/index.php/topic,4764.0.html
Also, a helpful reply from Mariss Freimanis (The Gecko man) on the Yahoo group showing what you can achieve with steppers in this thread :-
http://finance.groups.yahoo.com/group/geckodrive/message/13626
Ian
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Thanks Ian,
Torque gages and wrenches come in handy and have used them to check holding torque and more importantly to see outcome of table adjustments and required torque to move a table. Need to think more about that other post.
The cloud over my head envisioned some formulas and general guidelines into an excell spread sheet.....and magic
.....there's a "ball park graph" for an unknown stepper!
RICH