Why is are some motor torques listed in Kg*cm instead of a force*distance? - Page 1
Forum section: electronics / beginners

author: cbc02009
author_rank: Regular Contributor
timestamp: September 18, 2018, 10:34:25 pm
content: Hello,I'm trying to size a motor for a project I'm working on, and I'm really confused. I have a flat square panel that I'm rotating, with the length and width parallel to the ground. The center of rotation is through the center of the panel, and perpendicular to the ground.I've calculated the angular acceleration I need in radians/(s^2), and the moment of inertia of the panel in Kg*cm^2. So the torque I calculated has the correct unit of Newton meters.But most of the motors I've found for sale list their rated torque in Kg*cm, which is not the same units as Newton meters. Do they actually mean kgf*cm? Or am I missing something?Any help would be greatly appreciated.these are my numbers in case they're helpful:These are the dimensions of the panel:H = 3.5cmW = 62cmL = 66.5cmmass = 10.2kgI got a moment of inertia of 7.033 Kg*cm^2 using I = (1/12)*mass*(L^2+W^2)The acceleration I want is .314 radians/sec^2The torque I calculated was therefore .11 Newton meters

author: Rerouter
author_rank: Super Contributor
timestamp: September 18, 2018, 10:56:06 pm
content: 1kg*cm would mean able to lift a 1kg mass on a spindle with a 1cm radii. So for all intents it is N*m (i know 9.81... not 10 for acceleration)

author: langwadt
author_rank: Super Contributor
timestamp: September 18, 2018, 11:20:46 pm
content: Quote from: Rerouter on September 18, 2018, 10:56:06 pm~0.1 N*m

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 18, 2018, 11:28:42 pm
content: Quote from: Rerouter on September 18, 2018, 10:56:06 pmI'll give you your 2 percent rounding error from 9.81 to 10, but you can't have the factor of 10 error in converting cm to meters.A newton is a kilogram-meter/second squared. In earths gravity you get close to 10 meters/second squared. So a kilogram weight on earth produces 10 newtons of force. At one centimeter (0.01 meter) you get 0.1 Newton-meter.There are two advantages to the metric system. One is the decimal relationships, which is nice but not compelling. The other is complete self consistency. No need to differentiate between pounds force and pounds mass or other dodges like poundals or slugs. This is the real advantage of the metric system, and is compelling. When people use bastard units like kg-cm they are throwing away the real fundamental benefit of metric.

author: Rerouter
author_rank: Super Contributor
timestamp: September 18, 2018, 11:51:05 pm
content: Yep sorry, got my head flipped between centi and deci, I've seen that on motor specs before.

author: Brumby
author_rank: Unknown
timestamp: September 19, 2018, 01:55:17 am
content: I just wish everyone would stick to the same units. Comparison between products becomes a pain otherwise.Ideally N.m - prefixed as necessary (kN.m, mN.m)

author: glarsson
author_rank: Frequent Contributor
timestamp: September 19, 2018, 05:47:48 am
content: No. Use Nm. We do not garnish SI units with a sprinkle of dots.

author: IanB
author_rank: Super Contributor
timestamp: September 19, 2018, 06:27:17 am
content: Quote from: cbc02009 on September 18, 2018, 10:34:25 pmYes, they mean kgfcm. It is common to find kgfand lbfused in everyday engineering, mainly because they are more accessible quantities. Actual force units like Newtons, poundals or dynes are slightly obscure to the average person.

author: helius
author_rank: Super Contributor
timestamp: September 19, 2018, 06:56:48 am
content: Quote from: glarsson on September 19, 2018, 05:47:48 amAnd this is really unfortunate, because the torque is the cross-product of the moment-arm and force vectors (not a product involving a dot). It is not equivalent at all to Joules, despite appearances.Better would be \$ \lVert\mathbf{N \times m}\rVert \$, but good luck getting companies to be consistent.

author: KaneTW
author_rank: Frequent Contributor
timestamp: September 19, 2018, 07:32:53 am
content: A cross product is still a product for the purpose of assigning a unit, not something special.And of course it's not equivalent to Joules, that's why it's called Nm.

author: helius
author_rank: Super Contributor
timestamp: September 19, 2018, 07:51:39 am
content: Joules are Nm by the definition of work.

author: Brumby
author_rank: Unknown
timestamp: September 19, 2018, 08:15:03 am
content: Quote from: glarsson on September 19, 2018, 05:47:48 amMea culpa.(I just used the dots for clarity.)

author: TimFox
author_rank: Super Contributor
timestamp: September 19, 2018, 02:02:46 pm
content: My mnemonic for converting force values is to remember that a kilogram mass weighs 9.8 Newtons or 2.2 pounds av.

author: babysitter
author_rank: Frequent Contributor
timestamp: September 19, 2018, 02:37:36 pm
content: Its great to live where it is done right. =)

author: den
author_rank: Contributor
timestamp: September 19, 2018, 06:18:44 pm
content: Since the unit conversion question already has been answered, I'll just point out that you calculations are somewhere wrong. The moment of inertia should be 1000 times as much (it's a big and heavy plate actually) and the resulting torque is twice as much: 0.22 Nm.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 20, 2018, 02:01:15 am
content: Quote from: babysitter on September 19, 2018, 02:37:36 pmThe first place I encountered gm-cm was in Schwabia. Maybe your part of Germany is different.

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2018, 08:37:35 pm
content: Quote from: cbc02009 on September 18, 2018, 10:34:25 pmThey mean kgf just because it is more practical in most applications.In a way it is the opposite of how Imperial units are used in the same situation where everything is in pounds force and the Imperial unit for mass, slugs, is ignored.