How to Model a Revolute Joint Correctly with Nastran
Altair EmployeeHi,
I'm trying to model a revolute joint as part of a 1D element actuator mechanism in NASTRAN. I have created the simple model in MotionView and it works, however I need to understand how to model this correctly and that the kinematics work for a static analysis when I introduce the rest of the structure and stops into the system.
My problem is that my RBE2 is elongating - I need it to stay the same length so that the actuator deforms correctly. Can someone check my .bdf input and see if I've made any errors with the DOFs or missed something simple? It is really very simple - only 4 GRIDS!
For reference, I've attached the MotionView output of how the elements should displace.
Thanks in advanced,
Matt
SOL 101 CEND VECTOR(PLOT,SORT1,REAL)=ALL DISPLACEMENT = ALL ELFORCE = ALL GPFORCE = ALL SPCFORCES = ALL $ SUBCASE 1 LABEL= Lift SPC = 1 DEFORM = 2 ANALYSIS = STATIC BEGIN BULK PARAM,POST,-1 GRID 1 0.0 0.0 0.0 GRID 2 345.656 0.0 -15.423 GRID 3 287.795 0.0 -36.262 GRID 4 287.795 0.0 -36.262 RBE2 2 2 123456 4 RJOINT 3 4 3 12346 CROD 1 1 1 4 $ PROD 1 1314.159315707.96 MAT1 172000.0 0.3 $ $ SPC 1 1 12346 0.0 SPC 1 2 12346 0.0 DEFORM 2 1 109.71 $ ENDDATA
Answers
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Hi
DOFs settings were fine I think.
But your RJOINT is not working because its end is free ! (F10 check 1D>free node)
RBE2 and CROD are connecting to grid 4, while one must connect to grid 3
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Hey, I noticed this problem after I posted - but it still doesn't work.
For example, the deflections of Node 3 are never on the circle produced by the radius of the RBE2 element - I've tried with DEFORM cards, Forces applied along the CROD element on Node 3, non-linear analyses, modal analyses...
I'm not sure if I am missing something stupid or if there is a nuance with the maths within the NASTRAN code.
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/emoticons/default_rolleyes.gif' title=':rolleyes:' /> nastran code is ok, i think.
Could you show result? (Result values in f06, do not show results by hyperview)
Anyway i tested with optistruct, it is ok
You don't need RJOINT because CROD has no bending dofs
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I ran the following bdf:
SOL 101 CEND DISPLACEMENT = ALL ELFORCE = ALL GPFORCE = ALL SPCFORCES = ALL $ SUBCASE 1 LABEL= Lift SPC = 1 DEFORM = 2 ANALYSIS = STATIC BEGIN BULK PARAM,POST,-1 GRID 1 0.0 0.0 0.0 GRID 2 345.656 0.0 -15.423 GRID 3 287.795 0.0 -36.262 RBE2 2 2 12346 3 CROD 1 1 1 3 $ PROD 1 1314.159315707.96 MAT1 172000.0 0.3 $ $ SPC 1 1 12346 0.0 SPC 1 2 12346 0.0 DEFORM 2 1 109.71 $ ENDDATAThis gives the following displacements from the .f06:
POINT ID. TYPE T1 T2 T3 R1 R2 R3 1 G 0.0 0.0 0.0 0.0 0.0 0.0 2 G 0.0 0.0 0.0 0.0 -3.931020E+00 0.0 3 G 8.191853E+01 0.0 -2.274528E+02 0.0 0.0 0.0Note that the length of the RBE2 is [(345.656 - 287.795)^2 + (36.262-15.423)^2]^0.5 = 61.499
Therefore, the equation of the line that Node 3 can move is x^2 + y^2 = 61.499^2 relative to the origin of the circle which is Node 2.
In the reference system where Node 2 is the origin, the original position of Node 3 is 57.861, 0.0, -20.839. Double checking it lies on the circle: (57.681^2+20.839^2)^0.5 = 61.499
When we calculate the new position of Node 3, we get 57.861+81.19853 = 133.06 and -20.839 - 227.4528 = -248.292. Obviously we can already see that the model has not behaved as expected since the X and Z positions shouldn't be able to be more than +/- 61.499. I have also tried the model with forces applied instead of DEFORM cards for similar results.
Just to prove with the equation of the circle: (133.06^2 + 227.839^2)^0.5 = 263.85 (i.e. not 61.499).
When you say it's OK, do you mean the model ran? Because my model runs, but the results are incorrect based on the structure I am trying to model.
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Are you sure your calculation is correct?
I think you need to be more careful !
Please calculate it again.
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