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Assembly Center of Gravity and Weight

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Assembly Center of Gravity and Weight

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.

1 ACCEPTED SOLUTION

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Re: Assembly Center of Gravity and Weight

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...

View solution in original post

8 REPLIES 8
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Re: Assembly Center of Gravity and Weight

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.

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Re: Assembly Center of Gravity and Weight

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

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Re: Assembly Center of Gravity and Weight

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

NameAndy Mark weightCAD weight (oz)Actual (oz)Material
churro 24.25 inches5.1734.325.135Aluminum 6061
hex bolt 4 inches2.241.552.241Metal Steel low Carbon
end sheet36.4831.58436.4Aluminum 5052
hex hub1.4081.1361.4Aluminum 6063
outer sheet21.9217.85621.6Aluminum 5052
NameAndy Mark weightCAD weight (oz)Actual (oz)Material
churro 24.25 inches5.17384%99%Aluminum 6061
hex bolt 4 inches2.2469%100%Metal Steel low Carbon
end sheet36.4887%100%Aluminum 5052
hex hub1.40881%99%Aluminum 6063
outer sheet21.9281%99%Aluminum 5052
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Re: Assembly Center of Gravity and Weight

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.

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Re: Assembly Center of Gravity and Weight

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

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Re: Assembly Center of Gravity and Weight

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.

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Re: Assembly Center of Gravity and Weight

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.

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Re: Assembly Center of Gravity and Weight

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...

View solution in original post

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