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I was looking up the motor parameters for some stepper motor where they listed the torque of the motor at different current/voltage but the torque they listed was in kg/cm.

How is kg/cm even a remotely acceptable unit for torque?

How do I calculate the torque in Nm from kg/cm?

Clarity note: Its not kgcm which represents [0.098 kilogram force = 1 Nm.]

Website where this happens.

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  • $\begingroup$ kg/cm AKA kg-cm AKA kgcm is still better than ozin that many websites use, even if oz is technically a measure of weight rather than mass. It drives me nuts too. They should all switch to N*m, which is an easily recognizable standard and different by almost exactly an order of 10. $\endgroup$ – user3685427 Oct 20 '17 at 20:39
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It is most likely kgf-cm, as kg/cm is not a valid unit for torque.

Apparently writing kg/cm when kgf-cm is meant is a common mistake made on datasheets.

According to this conversion of kgf-cm to Nm, 28 kgf-cm of torque is 2.7 Nm, and 150 kgf-cm of torque is 14.7 Nm.

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  • $\begingroup$ Still does not clear the confusion. If its kg-cm then its absurdly low. If its kg/cm then why is it in those units. The calculations do not match. $\endgroup$ – Naresh Mar 29 '13 at 3:04
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    $\begingroup$ It's a typo. See the answer here: electronics.stackexchange.com/questions/2749/… $\endgroup$ – Ian Mar 29 '13 at 21:14
  • $\begingroup$ It would be even better if they used N-m. "kgf" may be a unit of force but it's going to get confused with actual kg which it is not, and at that point you might as well be using pounds and furlongs and firkins. Pardon the crankiness. High school physics teacher effectively drilled the difference between force and mass into my head, and it gave me an appreciation for N and kg, vs. pounds and pounds, and it just grinds my gears to see "kg-cm" used for torque. It's not a measure of torque. $\endgroup$ – greggo Dec 29 '16 at 5:57
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Y[Nm]=X[kg-cm]*9.81[m/s^2]/100[cm]

1kg multiplyed with gravitational constant will give you 10N and normalizing the cm to m by multiplying it with 0.01.

Basically, unit in their table is wrong. It is not kg/cm but rather kg-cm. Kg/cm would mean normalized linear pressure, which makes no sense for torque.

Also, what you have mentioned that 0.02 Nm is small torque. It is but not for 0.1A and 0.4V. If you take a look at torque for 1.5A, it is 0.18Nm for 9W of power.

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  • $\begingroup$ Can this be verified? Because then the so called 'high' torque stepper motor would be rated at 0.222x10x0.01 = 0.0222Nm which is absurdly low. $\endgroup$ – Naresh Mar 28 '13 at 6:45
  • $\begingroup$ i agree, something is fishy here .. i will check this and edit my answer $\endgroup$ – Gossamer Mar 28 '13 at 7:32
  • $\begingroup$ If its kg-cm then its absurdly low again.Most DC motors on the website have ~28-150 kg-cm torque. If your formula holds then it would still have below acceptable levels of torque, and how do they justify with kg/cm as a unit for torque? They use this unit only for the stepper motors and not for the DC motors which is what makes me suspicious. $\endgroup$ – Naresh Mar 28 '13 at 8:58
  • $\begingroup$ What would be the physical meaning of kg/cm as torque. Torque is defined as $T[Nm] = r[m] x F[N]$. I dont know. Maybe someone else have an idea ... $\endgroup$ – Gossamer Mar 28 '13 at 11:46
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The motor they listed apparently has 1.548 kg·cm of torque at 4.8 V. (As Ian pointed out, on this website -- like too many other websites -- "kg·cm" is often misspelled as "Kg/cm").

It is apparently a NEMA 17 motor. From the photo, it looks about half as long as a typical NEMA 17 motor, so I'm not surprised it has about half the torque of a typical NEMA 17 motor.

A torque of 1.548 kg·cm is more than adequate for many robots -- 1.4 kg·cm of torque is adequate for axis motors on a RepRap.

They use this unit only for the stepper motors and not for the DC motors

Huh? Every motor Several motors listed on that site, both stepper and DC motor ( a b ), are rated using the "Kg/cm" unit, which in every case is a misspelling of "kg·cm".

0.222 ... its absurdly low again.Most DC motors on the website

The "0.222 kg·cm" applies when driven at 0.4 V. That's a small fraction of its rated voltage, which is 6 V where the motor gives 1.869 kg·cm.

As far as I can tell, all the DC motors on that website include a gear box, which multiplies the torque.

It is unfair to compare motors by comparing only the torque at the output of a torque-multiplying gear box of one motor driven at its full rated voltage, to the torque of some other motor without a gearbox and when driven at a small fraction of its rated voltage.

On the other hand, I have to laugh when this website calls this a "High torque" motor when it is almost the lowest-torque NEMA 17 motor I've ever seen. It reminds me of the "Schnell-Bahn" (literally "fast train"), which are the slowest trains still operating in Germany.

EDIT: I shouldn't have said "every motor"; most of the motors are rated in "Kg-cm" or "kg-cm", which is close enough to "kg·cm".

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  • $\begingroup$ 60 RPM, 12V, 45 kg-cm stall torque Side Shaft 37mm Diameter High Performance DC Gear Motor. Suitable for small robots / automation systems. As an example. So, they use Kg/cm for stepper motors and Kg-cm for DC motors. $\endgroup$ – Naresh Apr 2 '13 at 4:20
  • $\begingroup$ 1.869 kg-cm is pretty low torque. For example, if I need my motor to pull 1kg load up an incline of 45 degrees, it would require torque equal to around 4 kg-cm torque. Assuming wheel size = 6 cm radius. $\endgroup$ – Naresh Apr 2 '13 at 4:26
  • $\begingroup$ Gearbox would increase torque at the cost of rpm, causing my robot to move significantly more slowly. $\endgroup$ – Naresh Apr 2 '13 at 4:28

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