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Author Topic: "Holding" vs "Running" torque  (Read 9563 times)

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"Holding" vs "Running" torque
« on: April 04, 2008, 11:25:25 PM »
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

Offline stirling

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Re: "Holding" vs "Running" torque
« Reply #1 on: April 05, 2008, 06:11:09 AM »
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!!!.
« Last Edit: April 05, 2008, 06:32:35 AM by stirling »
Re: "Holding" vs "Running" torque
« Reply #2 on: April 05, 2008, 08:43:32 AM »
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

Offline Jeff_Birt

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Re: "Holding" vs "Running" torque
« Reply #3 on: April 05, 2008, 10:50:12 AM »
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.
Happy machining , Jeff Birt
 
Re: "Holding" vs "Running" torque
« Reply #4 on: April 05, 2008, 10:47:35 PM »
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
« Last Edit: April 06, 2008, 12:38:54 AM by Overloaded »

Offline stirling

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Re: "Holding" vs "Running" torque
« Reply #5 on: April 06, 2008, 06:32:58 AM »
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
Re: "Holding" vs "Running" torque
« Reply #6 on: April 06, 2008, 10:34:56 AM »
I'll supply cover fire for ya, that's the "general"  sort of answer I was looking for.
Thanks Ian,
RC

Offline jimpinder

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Re: "Holding" vs "Running" torque
« Reply #7 on: April 06, 2008, 12:36:06 PM »
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.
Not me driving the engine - I'm better looking.
Re: "Holding" vs "Running" torque
« Reply #8 on: April 06, 2008, 03:54:56 PM »
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
« Last Edit: April 06, 2008, 04:59:10 PM by Overloaded »
Re: "Holding" vs "Running" torque
« Reply #9 on: April 07, 2008, 12:29:32 AM »
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