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# Calculation of CG from asserted weight

Pioneer

Is there a way to calculate centroid from the input asserted weight of assemblies. We have only the weights but not the densities.

17 REPLIES 17

# Re: Calculation of CG from asserted weight

Siemens Phenom

All you need are the weights.  The densities of the material used when modeling a parts plays no direct role in computing the CG except where the weight of a model needs to be computed.  But if the weight has been asserted, then the density plays no role whatsoever.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA

# Re: Calculation of CG from asserted weight

Pioneer

John,

I have tried with simple assembly of 2 components. It would have helped me if weight assertions updated the densities in the models.

Even I tried without assertions and whenever densities are varied, I am getting different c.g. values.

What could be the solution when we did not have densities before hand to capture correct centroid?

# Re: Calculation of CG from asserted weight

Gears Esteemed Contributor

I'm wondering...If you assert the weight, don't you also have to assert the CG location?

As the part could be an assembly of materials (e.g. a motor - you have iron, copper, insulation, etc.) which vary in space, so the actual CG is not necessarily the same as if it were a "solid body of one density" CG?

Ken Akerboom Sr CAx Systems Engr, Moog, Inc.
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled

# Re: Calculation of CG from asserted weight

Pioneer

Exactly. I interprete by your reply that, I need to assert c.g. value too for each component.

Coming to my question about centroid, in general terms, will it vary with density?

As you said an assembly like motor will have heterogeneous parts. I would know only the weights, not individual densities. Any help in finding out c.g. of system will be appreciated.

# Re: Calculation of CG from asserted weight

Gears Esteemed Contributor

If you have a heterogeneous assembly, you have to accurately model  the assembly (including all the component parts' densities) to get the true CG.

If it is a purchased part, the supplier SHOULD be able  to get you the "true" CG location/moments of inertia/etc.

Ken Akerboom Sr CAx Systems Engr, Moog, Inc.
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled

# Re: Calculation of CG from asserted weight

Siemens Phenom

As I stated before, th especific 'density' of the material plays NO direct role in where the CG of a model is located.  Only the weight of the model, or in the case of an assembly, the weights of the Components, will have an impact of the CG.  The only possible exception to this would be if a model was created using a non-homogeneous material, which would be difficult to even do in NX.  Therefore, when you assert the weight of a part model, that is technically all that you need to do.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA

# Re: Calculation of CG from asserted weight

Gears Esteemed Contributor

John -

I disagree.

Imaging I model a cube.  CG is at the geometrric center.

Now say the cube is really 1/2 steel and 1/2 styrene.

You could assert an average density to get the weight correct (or assert a weight & get an "average density").

But the true CG would NOT be at the center - it would be somwhere in the steel half.  Without modeling the actual components & their densities & distribution, you don't know where the actual CG is.

Now I will admit my example is somewhat an extreme case, for many assemblies the actual CG would probably be close to the "asserted" CG based on geometry alone.  But not always.

Ken Akerboom Sr CAx Systems Engr, Moog, Inc.
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled

# Re: Calculation of CG from asserted weight

Siemens Phenom

Did you miss where I stated:

"The only possible exception to this would be if a model was created using a non-homogeneous material, which would be difficult to even do in NX."

What you've described, a SINGLE block where it was half one material and half another, is what's referred to as a "non-homogeneous material".  Now if you had TWO blocks next to each other, one of one material and the other a difference material, now everything would work as expected.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA

# Re: Calculation of CG from asserted weight

Gears Esteemed Contributor

I guess I'm making an un-stated assumption :-)

If you are asserting a weight, then it is because you aren't accurately modeling an assembly (From what I see, this is typically due to it being a part purchased from a vendor, that is really an assembly, but we treat it as "one" part - e.g. a motor or an LVDT or ....)

Ken Akerboom Sr CAx Systems Engr, Moog, Inc.
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled