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MattM

How to Model a Revolute Joint Correctly with Nastran

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Hi,

 

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

 

Actuator Test.h3d

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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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: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
$
ENDDATA

This 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            0.0

Note 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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