sequence of rigid body modes and signifiance
sequence of rigid body modes and signifiance
(OP)
Hello All,
I have started learning frequency or modal analysis using FEA. I have one basic question.
Will the sequence of first six rigid modes during frequency or modal analysis will always remain same? Also, what is the significance of these rigid body modes?
Please bear with me for this basic question
I have started learning frequency or modal analysis using FEA. I have one basic question.
Will the sequence of first six rigid modes during frequency or modal analysis will always remain same? Also, what is the significance of these rigid body modes?
Please bear with me for this basic question





RE: sequence of rigid body modes and signifiance
Hope this answers your question.
236804
RE: sequence of rigid body modes and signifiance
You may even not see these rbm at all, if the FE program you use filter them out automatically.
The significance of the rbm in a modal analysis is null, because they are only the trivial solution of the eigenproblem where all eigenvalues are zero.
The sequence will remain the same for the simple reason that they all have a frequency of 0 Hz and thus they will be ordered not by frequency but by degree of freedom (UX, UY, UZ, ROTX, ROTY, ROTZ).
Regards
RE: sequence of rigid body modes and signifiance
The significance of an RBM depends entirely on your modeling intent. If you have a static object with no moving parts, an RBM will tell you that you haven't properly constrained your model. If you have a rotating shaft on a piece of machinery and your FEM calculates 1 RBM, you may have some confidence that the model is performing as expected.
I don't think most FE programs necessarily filter RBM, but they do allow you to idenify how many you expect. They will also usually identify excessive deflections and that your model isn't tied down (constrained) sufficiently.
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
RE: sequence of rigid body modes and signifiance
For instance, when we mount an engine into the car, the frequencies of the bounce pitch and roll modes are fundamental to the performance of the system.
Note that just because they are rigid body modes they are not necessarily zero frequency - it is well worth differentiating between the two phenomena.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: sequence of rigid body modes and signifiance
I'm not familiar with vehicle dynamics, but what you are describing in "bounce pitch and roll" doesn't sound like what I think of as a classic "rigid body mode". It sounds more like the motion of a rigid body on an elastic foundation...a single degree or multidegree of freedom system?
I think of a "rigid body mode" as the result of zero restraining stiffness, so the body is able to deflect infinitely. Since, for a single DOF system, omega = SQRT(k/m) and k (stiffness) is zero, omega would be zero. Am I missing something from vibrations?
I'd have to think about what this means in a multi DOF system with a forcing function. Guess I've been away from the books too long!
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
RE: sequence of rigid body modes and signifiance
I think you have muddied the waters just a bit ! In your "funny old world of vehicle dynamics" the term rigid body mode must mean something different to what the rest of the world thinks it means (as defined by Garland)!
RE: sequence of rigid body modes and signifiance
RE: sequence of rigid body modes and signifiance
Can you please explain, if possible, somewhat about "the frequencies of the bounce pitch and roll modes" so that we can have better understanding about topic,
RE: sequence of rigid body modes and signifiance
If you analyse an engine then the first 6 modes are rigid body modes, in which the engine itself is rigid, but it moves in various ways on the engine mounts, typically at frequencies between 6 and 35 Hz.
At a frequency of (say) 80 Hz we get the first flexural mode of the engine, where it bends in its own right.
So in our terminology a RBM is merely a non-flexural mode whereas a zero frequency mode is an unconstrained motion.
The same terminology is used with smaller components on the engine, so the alternator, for example, will have 6 rigid body modes where its mounting brackets are flexing, but the alternator itself is just a mass, and then at say 1000- 1500Hz it will have its first flexural mode, where the casing starts to flex.
Typically the alternator's rbms are important for engine noise, and the flexural modes are responsible for whines and so on.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: sequence of rigid body modes and signifiance
Greg, it seems to me that in automotive (which is definitely not my field) you use a very specific terminology which is not adherent with the common definition of "rigid body motion". It's not a problem, it's just a matter of terminology as you say. When you say that the first freqs of an engine are "rbm", you don't want to say that the eigenvector is null for a particular dof, rather that "the engine moves as an infinitely-rigid body upon an elastic foundation", as Garland said.
Garland, I fully agree with your remarks. I also used a terminology which could sound unclear: the "significance" of a rbm in a modal analysis is null in that sense that it is not a vibration; nevertheless, the information given by a rbm CAN be extremely significant! your definition of rbm, by the way, exactly matches mine in mathematical terms: when you solve the eigenproblem, one trivial solution of the system is to have null eigenvector (so that anything multiplied by zero equals to zero by definition!!!), but of course this is not a vibrational shape.
Btw: a solver of Cosmos for example automatically recognizes RBMs and doesn't display them, so for a free/free beam the first shown eigenvalue (and eigenform) is in reality the "seventh" (the first 6 being RBMs for the fundamental 6 DOFs of the system). On the countrary, Ansys treats the RBMs as "real" eigenvectors.
Regards
RE: sequence of rigid body modes and signifiance
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: sequence of rigid body modes and signifiance
Greg, this is interesting and, I think, not really surprising: if we consider the whole car "turning around" the wheel axis, then this is a RBM in the common definition of this term: ROTZ (being "Z" the wheel axis) is the DOF, the eigenform is a constant unit for any point of the car (or the entire car mass value if we normalize to mass, or also a constant zero because this makes no difference, 'cause the value of the eigenvector only states the initial position w.r.t. the axes), and the frequency is zero (i.e. it's a constant movement, not a "true" vibration). It's the same thing I encounter in my world made of turbine rotors, where the first torsional eigenform is the null one around the machine axis (same thing also pointed out by Garland).
Am I correctly interpreting all this?
Regards
RE: sequence of rigid body modes and signifiance