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Mechanism Dynamics and Subassemblies


Mechanism Dynamics and Subassemblies


I'm having a problem with the Student Edition of Pro Engineer Wildfire 4.0 and Mechanism analysis with the dynamic analysis option on. If I have an arbitrarily simple subassembly (containing one part that is just a simple cube) within a top level assembly containing only that subassembly, I get the error message:

"The mass of one or more bodies in your mechanism is zero. Dynamic analyses require nonzero mass for all bodies. Please review mass properties using Info, Mechanism, Details."

The problem doesn't occur if the top level assembly contains only the nested part and no subassembly. The problem doesn't occur if only the subassembly is being analyzed with no top level assembly. Nested subassemblies seem to be the problem. Can anyone help me with this?A small zip file containing the part and assembly files is attached. It can also be found here:


Any help would be appreciated. Thanks.



Hello Ted,

Pro/Engineer automaticly creates other body (body 2) when you use kinematical connection for costraining of subassembly. In your case body 1 is cube and body 2 is datum geometries of subassemly (datum planes, points, curve, axis ...) these geometries are nonvolume and has zero mass. Solution of your problem is in creating of "new part" with any DOF (degrees of freedom) in subassembly. The cube should by reconnected to the "new part" which will be ground body (frame) in subassembly. Consequently for definition of kinematical constrains betwen subassembly and top assembly you will have to used geometry of the "new part" of subassembly. The new part will be body 2 with nonzero mass in top assembly and dynamic analysis should be successful.

Good luck,

Slavo M.


I was getting the same error because the subassembly was not fully constrained with mechanism joints. Once I constrained the components inside the subassembly, the error got solved.

Thanks, Slavo. You lead me to the solution.I was way too primitive in my understanding of the mechanism model tree and constraints as they relate to mechanisms. I just needed to rigidly constrain (coord system) at least one (in this case only) part in the subassembly to the subassembly itself. Now I know whya part would go sailing off under the influence of gravity, while its parent assembly datumsjust sat there. Page 7 of this link helped:


Your solution(as I understand it) of putting an arbitrary partin the top level assembly (which defines the ground body) and rigidly attaching it (as I read your advice) may beappropriate for other situations though. Thanks!

I had the same problem with Creo 3.0.

SOLUTION FOR ME> Check the connections of each sub assembly, if at least the first part of those sub assemblies is not set to DEFAULT, then you got that error. So SET THAT THING TO DEFAULT! And then connect the remain parts.

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