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Modeling of Bolts

ron.kunz
Dear all,
I have a big assembly and want to model bolts in the complete assembly. What is the best and accurate way to model this.
Cbar analysis is accurate enough to analyse the bolt forces. Boltstress is of secundair interest.
Are there examples for these bolts?
In addition bar orientation for this CBAR property, how can this best be modeled?
 
Thanks in advance
  • 0
    On to bolts end A and end B create spider using RBE2 ,can use utility >> FEM-General>> Rbe2 spider
    Center grids of the two spiders can be connected to cbar. Cbar x-axis is from GA to GB hence ensure Y and Z axis are always perpendicular to x axis of cbar
  • 0
    Thanks Vivek,
    -This method I also know from Hypermesh/Nastran combination.
    With Patran my predecessor has patched this hole and let Patran create a center node. So no RBE2 are created. Is there a difference in output results with this kind of modeling and the way you describe with RBE2?
    -In addition:
    Does the direction of the Y and Z matter (so negative or positive value) ?
    -In addition:
    If one has 3 plates connected to eachother how is the procedure?
    -In addition:
    For Hypermesh one CBAR is created between two nodes. In Patran I notice 2 elements between the different connecting points. If one wants to read out the boltforces what is the best approach?
    Thanks.
  • 0
    Hi Ron,
     
    If you have already center grid, then can create directly rbe2, no need to use the utility. This utility helps in creating center grid automatically.
     
    For circular bolt direction of the Y and Z does not matter, as it is symmetric at each quadrant.
     
    For 3 plates create three spiders, one for each plate. Connect them with cbar(1st cba = node 1 to node 2; 2dn cbar = node 2 to node 3)
     
    For bolt forces in the case control use : FORCE = ALL
     
    Thanks,
    Vivek
  • 0
    Thanks Vivek,
    -Regarding: "For bolt forces in the case control use : FORCE = ALL"
    In output request this is the Element forces?
    How can one show these results in Patran after solving? And especially filter the CBAR results? Besides showing, one can get also a list of forces?
     
    -Attached I have a CBAR in a current model I have
    CBAR
    So the CBAR created seems to consist of 2 elements (upper part and bottom part). This is an old model. Is this correct? Or should only 1 element been created?
     
    -An additional question regaring creating CBARS. If one select the two center nodes of a CBAR, does Patran recognise this as the x-axis? The axial direction is always the x-axis? Or if the boltaxis is not in the x-axis direction one should create a local axis system?
     
    Thanks
     
  • 0
    Ron, there are many many ways to model bolts...the best "best and most accurate" way depends on the results you need. I have seen all of the methods describe above (and more) for modeling fasteners. For example, you mentioned bolt force - what about the moment? Also, how important is interaction of the bolt with the local structure? Another thing you didn't mention is the need to model a bolt preload. All of these influence the choice of best method.
  • 0
    In the results Patran gives BAR forces as option to look at the bar forces. You can also create group of those bars and plot them
  • 0
    Thanks Ken,
    Indeed that is true what you are saying. Usually we read out the axial and shear forces and we compare these with manual calculations accoring to norms/guidelines.
    Can you also have a look at my last questions (with the picture attached)?
    Thanks.
  • 0
    Yes, FORCE = ALL in case control outputs Element Forces to print file (f06) and plot file (XDB, or OP2 or HDF5). Additional details in Quick Reference Guide.
    Bar force orientation depends on the local element coordinate system and offsets. The x axis is always node 1-2 (including offsets), so the x force is the axial force (and torque is x moment).
     
    Vivek explained how to view in Patran.
     
    Regarding your model (using 2 bars vs 1 bar), either can be "correct". The better question is "is it realistic for my analysis?", which is model and load dependent. With 2 bars, you will get a deformation at the midspan. Note that Bar/Beam elements are designed for bending, and shear response should be checked.
  • 0
    You may also want to look into CFAST and CWELD elements.
  • 0
    Thanks all!
    @ Ken: I understand the x-axis is the axial force. However I don`t know if Patran does generate the x-axis by itself by selecting the 2 points for the bar. Or that I have to specify a local axis system and add this to the cbar.
     
    @Sanjay
    Could you explain more? CFAST and CWELD are recommended for what purpose?
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