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Author Topic: question on torque for motors?  (Read 642 times)

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Offline vipulan12Topic starter

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question on torque for motors?
« on: July 06, 2013, 03:12:31 PM »
why is the torque for motors rated in Kg/Cm or Oz/in instead of Newtons/meter?
is their a difference in these types of torque?

Offline Pogertt

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Re: question on torque for motors?
« Reply #1 on: July 06, 2013, 04:55:14 PM »
I think Kg/Cm is used for 50 Hz metric motors and Oz/ In is used for 60 Hz NEMA motors in North America.


Or is it the other way around?



Actually, any of the three mentioned scales can  be used.
Just a different way of measuring. 

Similar to how Celsius and  Fahrenheit both refer to temperature.
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Offline jwatte

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Re: question on torque for motors?
« Reply #2 on: July 07, 2013, 10:51:43 AM »
Why do we measure temperature in Centigrade rather than Kelvin? Why do we measure short distances in centimeters instead of meters?

The answer is: Practicality. For most smaller motors, the units of kgcm or ozin have more "handy" numbers than Nm. (Note that it's not Newtons per meter -- it's Newtons times meter!)
These numbers all convert using a simple conversion constant, so it doesn't really matter which one it's measured with.
kgcm is used in most of the world, as it's based on SI units. ozin is used in the US because the US is too stubborn for its own good.

Offline jkerns

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Re: question on torque for motors?
« Reply #3 on: July 08, 2013, 09:28:11 AM »
Because people tend to ignore the fundamental difference between force and mass - particularly in metric systems where g has a numeric value = 1 (Kg/N)/(M/sec^2)

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Offline jwatte

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Re: question on torque for motors?
« Reply #4 on: July 08, 2013, 10:37:33 AM »
I think your units are wrong, if by "g" you mean the gravitational acceleration constant, not the unit of mass.

The "typical" value of g is 9.81 m/s^2 (although it varies a little bit over the planets' surface,) which means that 1 kg of mass exerts 9.81 N of force at sea level on a "typical" place on Earth.

The actual mapping between position and g (and thus between position and N/kg) is typically defined as a complex equation in a 360 term spherical harmonic. There are also higher-order definitions up to many tens of thousands of terms if you need really good calculations.

Same thing with ounces/pounds: "pounds of thrust/force" is equal to the gravitational force on a pound of mass at some "typical" sea level spot on Earth.

So, with the imperial units being defined as exact scales on metric units, and assuming you can use a standard value for g (the gravitational constant,) then you can convert between any of the "convenience" units (kgcm, ozin) and the "actual" unit of Nm.

Offline jkerns

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Re: question on torque for motors?
« Reply #5 on: July 08, 2013, 12:42:58 PM »
I think your units are wrong, if by "g" you mean the gravitational acceleration constant, not the unit of mass.



No.

g as in F = (M*A)/g 

without the g which has units of (mass/force) * (distance / time^2) the units in Newtons equation don't balance.

The numeric value of g depends on the unit system and not the acceleration of gravity.

For example, in the pounds mass / pouds force system that used to be common for engineering applications in the U.S. g = 32.2 lbm/lbf * ft./sec^2 which matched the acceleration of gravity. But for other systems such as pounds force / slugs or the pounds mass / poundel  system g has a numeric value of 1 and, of course, does not match the acceleration of gravity. 

Similarly with the MKS version of the metric system when force is in units of Newtons, g = 1 which does not match the acceleration due to gravity as you note. (strictly speaking Newtons have the units of Kg m / sec^2  which solves the unit conversion problem and eliminates the need for g - but for most of the other systems of units that I have used, you need to keep track of g)

But, back to the original question, specifying torque in units of mass*distance (e.g. Kg cm) makes no sense at all because torque is a force times a distance.

Now, it is possible that instead of using Kg as a unit of mass (which it is) , the specifications are referring to the kilogram-force (= 9.8 Newton) thing (which is not part of the International System of Units (SI) http://en.wikipedia.org/wiki/International_System_of_Units), but  then it should be written as Kgf not Kg so we could use the specification without having to guess at what they may or may not have intended.
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Offline jwatte

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Re: question on torque for motors?
« Reply #6 on: July 08, 2013, 03:19:31 PM »
Quote
so we could use the specification without having to guess at what they may or may not have intended

While you are technically correct (which is the best kind of correct,) I, and many others, make this assumption daily when seeing torques listed in kgcm, and no robot or space shuttle has yet blown up because of that. So it's likely a very safe assumption ;-)


Offline Pogertt

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Re: question on torque for motors?
« Reply #7 on: July 08, 2013, 03:35:46 PM »
@jwatte............While you are technically correct (which is the best kind of correct,)

And how does  "technically correct" compare to:

The truth, the whole truth, and nothing but the truth.

and

The plain unvarnished truth?
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Offline jkerns

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Re: question on torque for motors?
« Reply #8 on: July 09, 2013, 07:13:35 AM »
Quote
so we could use the specification without having to guess at what they may or may not have intended


While you are technically correct (which is the best kind of correct,) I, and many others, make this assumption daily when seeing torques listed in kgcm, and no robot or space shuttle has yet blown up because of that. So it's likely a very safe assumption ;-)


Orbiter and lander: $327.6 million
Spacecraft development: $193.1 million
Launch: $91.7 million
Mission operations: $42.8 million
Having your spacecraft disappear behind Mars never to be heard from again because someone made a mistake with the units: Priceless.

http://en.wikipedia.org/wiki/Mars_Climate_Orbiter


 ;)
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

 


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