Centroid same as Center of Gravity? 1

[AlexDring]

I have a 3D model in Autocad Mechanical 6.0 and I need to find the center of gravity. It is an assembly drawing, meaning different solids used in one drawing that I created. If I unioned all the parts to make one solid, could I find by using the mass properties? Is the centroid the same as the center of gravity? I have other seats in CAD software, if someone knows a to figure it. The seats include: Mechanical Desktop 5.0, Autocad 2002, Solid Works 2001+, Solid Edge, TurboCad 9.0 Pro and CoCreate's One Space Designer 11.0 and 11.6

Any help would be nice. Thanks.

 

[Binary]

For a a 3-dimensional mass, the term centroid is used synonymously with center of mass (which is collquially referred to as "center of gravity&quot.

Using ACAD, you can create a composite solid and determine its mass properties which will include the x,y,z coordinates of its center of mass.

You can also determine the center of mass of an assembly in Solid Edge. I'd have to think SW and MDT permit it as well.

Have you tried it an are viewing the answers as suspect?

 

[AlexDring]

I did, and used the centroid. I put some geometery in and it just doesnt look right, since I know that one end of it is a lot heavier then the other. It looked like the point wasnt offset enough to compensate.

 

[Binary]

Perhaps you could create a composite assembly using shapes whose centroids are easily determined (spheres, cubes, retangular solids...), run the centroid, and then verify the centroid locations. This would at least satisfy you that the algorithm is working.

Is your composite assy something whose centroid you could numerically estimate, at least in one plane?

Another thing: In "vanilla" ACAD, all bodies have the same density so different materials all get treated the same. The could give seriously erroneous results.

I just used ACAD 2k2 and it correctly determined the centroid of 2 cubes spread 4" apart and also when I added a 3rd cube stacked on top of one of the originals. I did not need to "union" them into a single solid.

Be sure to realize that the centroid coordinates will be with respect to whereever the origin happens to be set.

 

[cowski]

If you have parts made from different materials, I don't think it is a good idea to join them all together before calculating the cg. The software will need to know the density of each part and if you join them all together you only have one part (now everything is uniform density). It would be better to determine each part's cg then using that info, calculate the assembly cg (some sort of macro would be ideal here).

 

[AlexDring]

There all the same material, steel which is .284 in denisty if I am not mistaken. I am looking into where my axis are right now.

 

[FredGarvin]

A few of things to be careful of:

1. Where your CG is referenced to, i.e. coordinate system reference. I place the UCS somewhere that is a good reference and then make sure you reference the UCS instead of the WCS.

2. Make sure all of your parts have the material properties specified. I can't remember what MD does, but it assigns some default material properties if you don't.

3. Make sure you select all parts that make up the assembly when you start the mass properties.

I just did a quick and dirty assembly of 3 parts and did the mass properties to find the CG and it worked perfectly.

 

[AlexDring]

I didnt have to worry about different material denisties, it was all steel. I cant remember where Autocad keeps that stuff, I only saw it once. I typed all when it asked to pick the solids, and gave me a location. I put the assembly into other programs using the asic model and others validated the centroid even ones that you easily assign material props.

 

[shabba]

In MDT6.0, go to Assembly-Analysis-Mass Properties

Pick your assembly(ies),

Use the dialogue box to set your units, set your co-ordinate system to either UCS or WCS, and set all your component materials and properties.

Go to the results tab and press Calculate.

This should give you everything you need to know. Also you can switch back to the setup tab and change materials etc and re-calculate straightaway.

Hope this helps...

 

[shabba]

Oh, you're using Autocad mechanical not Desktop. You could load it into your mdt5 seat, convert the solids into 'parts' (amnew,p,select solid) then try the above, can't remember if this feature is in 5.0 though...

In any case, for an assy of components of the same density, the centroid given by Acad is the same as the CoG.

 

[AlexDring]

Thanks Shabba, I verified the results in just about all the programs that I have. I noticed in MDT it displayed the center of gravity, but didnt check it because I didnt have time to convert the solids into parts. My skills in MDT are not exactly the quickest, so I used other programs I am quicker at.