Assembly Center of Gravity and Weight

[afrsited]

I was wondering how I could find the center of gravity in an asm. I know in a part file, you create a CSYS with relations to PRT_CSYS_DEF, but when I do it in an asm, it doesn't work. Also, how can I get the weight of a part and/or asm? These facts would be very useful to have on hand for the upcoming FRC build season.

[sm]

Analysis, Mass Properties. The process is the same for parts and assemblies. To get a proper weight, you must assign the proper material file to your components or enter a proper value for the component's density. If you want to keep the CG on screen, there is a feature option in the Mass Props result box.

[ptc-6828354]

Hey Scott, we found that the weights in the CAD program is 14 percent off from the actual weight and the weight on Andy Mark. Is this normal?

Lexi Team 4931

[ptc-6828354]

Here's what I found. CAD has the 2015 Chasis light by about 17%. Below are my results:

Name	Andy Mark weight	CAD weight (oz)	Actual (oz)	Material
churro 24.25 inches	5.173	4.32	5.135	Aluminum 6061
hex bolt 4 inches	2.24	1.55	2.241	Metal Steel low Carbon
end sheet	36.48	31.584	36.4	Aluminum 5052
hex hub	1.408	1.136	1.4	Aluminum 6063
outer sheet	21.92	17.856	21.6	Aluminum 5052
Name	Andy Mark weight	CAD weight (oz)	Actual (oz)	Material
churro 24.25 inches	5.173	84%	99%	Aluminum 6061
hex bolt 4 inches	2.24	69%	100%	Metal Steel low Carbon
end sheet	36.48	87%	100%	Aluminum 5052
hex hub	1.408	81%	99%	Aluminum 6063
outer sheet	21.92	81%	99%	Aluminum 5052

[dschenken]

What's it say the densities are for those materials?

Aluminum is approx .1 lb/in^3 and Steel is approx .3 lb/in^3.

[rjohnson-4]

I've been working with Lexi on this and all the densities in Creo look correct for each of the materials (using the "configure for schools" standards).

So even with the right densities, things seem to be coming up a little light. Has anyone else experienced this? We're wanting to use CAD for a weight and CG calcs, so this is important.

Thanks,

Rande J

Mentor FRC4931

[dschenken]

Create a 1 inch cube and assign a material and see what it reports. If it doesn't match the density then there is a bug in the program. If it does match then the models probably don't match the real parts.

[sm]

Are you including the wheels and gearboxes? I did not assign exact densities or material properties to items that did not have OOTB materials. Is the published weght you have for the complete chassis and have you measured the weight of all the items? Creo is typically spot on with weight as long as all the correct densities or mass properties are assigned.

[rjohnson-4 · Accepted solution]

We found the problem. We were dealing with the AndyMark "square" chassis in CAD while weighing the original chassis parts as shipped ("long" & "wide"). Once we adjusted the lengths to match, the weights matched well enough.

Still need to look into why the bolts and churros were different - they should have been right.

But for now, thanks for your help chasing the rabbit...