Inertia relief

[mikechy]

If in my model I have mass elements and want to use an inertia relief option, do I need to create gravity or centrifugal forces to analyse effect of mass forces, or 'inertia relief' will do it for me? If last one is correct, then what will be a direction of mass forces (in other words, what will be a direction of accelerations)?

Thanks in advance,
Fred

 

[alwaysdrunk]

Fred


I only use inertia relief when analyzing a part that is from a mechanism assembly that means it's a moving part. when you pull out a part with mechanism loads to mechanica, inertia relief will constrain your part , so you do not need to add displacement constarints.I don't know if were on the same boat but i think that is what inertia relief is doing

 

[Kaz Z06]

Inertia relief offers automatic balancing/constraining of residual loads present in a Mechanica model to eliminate rigid body motion. This option should be used when small residual loading is present, for example in models where loading is transferred from Mechanisms/MDO. Create inertial loading only if it is present in your problem. But, be aware that this option should only be used when you know there are only small body loads present as a net result.

The help topic on this option is very complete and well defined - please consult the internal documentation in the future, it almost always provides the best answer.
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[mikechy]

First of all, 'alwaysdrunk' and 'Kaz Z06', thanks for replies.
My question was about some static structure, not the mechanism. Consider, that we want to analyze frame of machine. There are a lot other devices assembled with such frame: engine, fuel tank, transmission, and so on. Weights and CG of these devices are given. We know external forces from maneuvers of the machine. We want to run fatigue life optimization of frame design. The size of the model is huge already. One of ways to decrease of the size is in using masses of all of additional devices instead of real geometry. And because that we would like to see improvement of each design iteration (not absolute value), use of inertia relief could be (time) saving solution.
But there are problems in such approach.
First, with inertia relief option we can

 

[alwaysdrunk]

I think what you've found in proe documentation is not the whole thing. here it is.hope it will give you ideas

Inertia Relief
Use the Inertia Relief option on the Static Analysis Definition dialog box to analyze an unconstrained model. You can use this option only for a linear static analysis. Using this option, Mechanica analyzes your model as if it were floating freely in space, without any constraints, but with the loads applied.
Note: You can also use the Inertia Relief option to analyze an underconstrained model. However, if the model already has some constraint sets, then Mechanica ignores these constraints during the analysis.
During an analysis with the Inertia Relief option selected, Mechanica internally creates a new Cartesian coordinate system (UCS) and defines a constraint set containing three-point constraints with respect to the newly created UCS. Mechanica also automatically applies body loads that balance your applied loads.
The three-point constraints affect the displacement solution but not the stress solution.
An analysis with inertia relief should always run fine, as long as there is non-zero stiffness for all the six degrees of freedom between two bodies.
Before you run an analysis with inertia relief, Mechanica asks you whether you want error detection to be performed. If you do, Mechanica checks for various modeling conditions including the presence of multiple bodies in the model. If Mechanica encounters more than one body, then it displays a message indicating the number of separate or disjoint bodies that it finds. You can use this information to determine whether parts you thought were connected are truly connected. If you see an unexpected number of bodies, you may want to cancel the analysis or study and correct the assembly.
For an analysis with Inertia relief ensure that the model does not have more than one disjoint body. If multiple disjoint bodies exist in your model, then the analysis fails with an underconstrained error. To run an inertia relief analysis with multiple disjoint bodies, ensure that all the disjoint bodies are connected in such a way that there is no relative motion between the bodies. If the connections are such that relative motion exists between any two bodies in the model, then the inertia relief analysis fails with an underconstrained error. For example, if two bodies are connected by bolts, then the inertia relief analysis fails if the bolts have no stiffness for some degrees of freedom.
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