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ptc-52502211-Visitor
1-Visitor
December 10, 2013
Question

Sub-assemblies registering as 0 mass in top assembly?

  • December 10, 2013
  • 4 replies
  • 9147 views

I'm trying to run a dynamic analysis using a servo motor and just gravity for a landing gear, but everytime I try I get an error message stating parts have zero mass. I know it's the sub-assemblies that are causing this because when they are removed, the dynamic analysis works just fine. Unfortunately the subassemblies are a requirement for the project. Material files are assigned to every part, all part files and assembly files are in the same units and the sub-assembly files give mass measuremtns for the parts contained in the subassembly. It's only when I go to the top assembly that the analysis fails, and mass analysis of the top assembly says the two sub-assemblies have unknown material and masses. I have no idea how to fix this and would appreciate any and all help!

4 replies

D
dschenken1-Visitor
1-Visitor
December 10, 2013

Based on general debugging (not so much with dynamics package) it is referring to bodies, not parts or assemblies. I know the analysis package puts items together as bodies, so look at the mechanism explorer to see if the bodies make sense.

P
ptc-52502211-VisitorAuthor
1-Visitor
December 10, 2013

I'm really new to this and taking an intro course so could you explain a little more in depth if possible? thank you!

C
ChrisKaswer16-Pearl
16-Pearl
December 10, 2013

You will never get this error if each and every part has a density. By therory, only individual "bodies" need to have a mass, as David mentions. I have found that I never have issue if I am as formal as possible with MDO rules.

P
ptc-52502211-VisitorAuthor
1-Visitor
December 10, 2013

Is there any extra step I'm missing somewhere? All the parts are assigned materials in the part files and all the material files contain the density for each respective material.

C
ChrisKaswer16-Pearl
16-Pearl
December 10, 2013

This may be one of those issues that is not possible to help diagnose unless we have your set of parts and assemblies. Do you have a "skeleton" part that you connect others to in a sub-assembly? If so, you'd likely need to give a mass to this "ground", or skeleton part of the sub-assembly.

3
346gnu13-Aquamarine
13-Aquamarine
December 18, 2013

Hi,

Does the attached help?

It works with assmblies with subassemblies for us.

True skeleton parts should be ignored. by mechanisms. 'homemade' skeletons will need to be made part of ground or attached in some other way.mechanism+mass+props.jpg

You will have to pedantically work your way through every component whether its this way or o set up mass properties before going to mechanisms.

Regards

V
vzak12-Amethyst
12-Amethyst
December 18, 2013

As already mentioned by people here, the problem is in that you have subassemblies that are bodies by themselves - hence have zero mass. Have you had few parts in these subassemblies belong to same body as subassembly this will give mass to the body and resolve the problem.

But given you need such subassemblies by design structure, try this : after entering Mechanism go to Body Tree / find this Body that only has such subassembly / select it and RMB > Exclude Fom Mechanism. Check if analysis run now.

Regards

- Vlad

J
jhansson1-Visitor
1-Visitor
June 12, 2017

Hi,

I have the same problem as stated above since I have a subassembly with parts of which none of them can be rigidly attached to the ground.

The Exclude from mechanism solution seems very conevient but when I RMB click a body in the mechanism tree I can only choose Info. Is it possible to find it in some other way? I am using Creo 2.0.

Kind Regards

-Jimmie

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