Flywheel Inertia sanity check 4

[KirbyWan]

Howdy all,

So I'm designing a test stand for an electric motor. It wants a flywheel to resist the motor acceleration. Here is the flywheel the CMM (component maintenance manaual) wants:

Inertia flywheel, 0.9 Nm2 (2.15 lbf ft2), 0 to 10000 RPM.

Now, I'm confused, because I thought the units of inertia were Kg*m^2. That is Kilograms meters squared (MLL), not N*m^2 or Newton meters squared (MLLL/SS).

Based on the conversion, I expect they meant a flywheel of .092 Kg*M^2 which would be equivalent to .2 * (3.28^2) lb-ft^2.

But I've never designed a flywheel before and thought I might have missed a lesson a couple decades ago.

Can anyone give me a sanity check? Did they just put the wrong units calculating the weight instead of using the mass?

Thanks all,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

I think they've been lazy (or confused) and used weight rather than mass. I'd divide by g (9.817 = 10 … near enough) … which is near enough to your calc !
not sure I get your imperial conversion, i'd divide by 32 ft/sec2.

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

The Imperial conversion is reasonably close; I get 2.18 lb-ft^2

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[rb1957]

so the original number (2.15) is correct ?

unless we say that 0.9Nm2 is equivalent to 0.09 kgm2 and these are equivalent to 2.15 lbf*ft2 ?

but is the imperial moment of inertia in slug*ft2 ? (which'd be 2.15/32 = 0.06, or something like) ?

I guess the important thing is to be careful with your units.

another day in paradise, or is paradise one day closer ?

 

[3DDave]

The true beauty of "slug" as a unit is that it's not got any common usage for weight. I see plenty of scales that measure weight in kg and pounds. The rarity is such that no one goes to the store to buy 1/6th of a slug of flour.

 

[rb1957]

'cause you buy flour and sugar by weight (and not mass)

the happy coincidence on the metric system is that a mass of 1 kg is equal to the weight of 1kgf (or 10N).

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

sorry, should have clarified it as lbm-ft^2, where lbm = 0.4536 kg; Mathcad's default lb unit is that conversion.

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[3DDave]

That's the point, there's no kilogram-force unit in the SI system, but people use it that way. No one uses a slug like that.

 

[KirbyWan]

Yeah, the mass vs weight issue we will be dealing with as long as we remain a single planet society since it doesn't really matter in the grocery store which you mean since they are connected by a constant.

I'm going to assume they meant pound mass, because even as an engineer I have never used slugs except in Physics 101 and now here. Confusing pound mass and pound force was probably their original mistake which is why they converted it Newtons instead of Kilograms. Though I will send them a question which might get them to correct their CMM in the next revision.

Thanks all!

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[PNachtwey]

I do all my basic calculations in MKS units. I find imperial units to be a pain and prone to error. In the past I have used slinches which are slugs defined in terms of inches instead of feet.
Like IRstuff, I use Mathcad to keep my units straight of consistent when not dealing in the MKS world.
It is too bad the American people are too stupid to adopt the metric systems.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[electricpete]

For rotary inertia, it might help to talk about the symbols...

Most people would be familiar with polar mass moment of inertia J (or Ip), whose units would be mass times distance squared.

Some standards refer to something called WK^2. W stands for weight and K stands for radius of gyration. From straightfoward interpretation of the symbols you can guess the units are force times distance squared. So the two are related by: WK^2 = J * g where g is the acceleration of gravity on earth's surface. WK^2 may be what op was referring to.

[to add to the confusion, some people use the symbol WK^2 along with the units of J... in which case why not call it J or MK^2 instead of WK^2].


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(2B)+(2B)' ?

 

[KirbyWan]

electricpete,

Thanks for that connection to the radius of gyration. I'm looking at the wikipedia page for that and moment of inertia and see how to convert one to the other. But why take the radius of gyration and multiply by a weight? The inertia of a flywheel is an intrinsic property and doesn't depend on the gravity well it happens to be in. If it's just that, then my plan to design a flywheel is in line with the value they intended.

Would you know of some other reason to use weight rather than mass? That is other than historical reasons, which I understand even when they don't make logical sense.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[electricpete]

Would you know of some other reason to use weight rather than mass? That is other than historical reasons...

Nope, there's no logical reason to use weight imo.

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(2B)+(2B)' ?