Learning Forum

[Hex_screw]

I face a problem to solve a model I have been working with in the last few days. The model is :

• a thin-wall cylinder (modeled as a solid, because there are different thicknesses involved and it did not behave proporly with shell)
• a first profile welded to the cylinder
• a second profile screwed to the first profile, but also supported by horizontal push screws on the cylinder skin

To keep the model a reasonable size, I used a quarter cylinder, and I decided to model all screws with beam elements.

I have a first version of the model with no pretension in the screws. This model is solving fine.

My issue is when applying pretension in the screws. The solution fails to solve at the first iteration of the pretension step, with error message :

The value of UX at node 314638 is 1.922248161E+14.  It is greater than 
 the current limit of 1000000 (which can be reset on the NCNV command). 
  This generally indicates rigid body motion as a result of an          
 unconstrained model.  Verify that your model is properly constrained.

The node number refers to a number above the node count of the model. Thanks to this forum I diggedd into the ds.dat file and found that the node was involved in a bonded contact between a push screw and the cylinder skin. I have played all day with this ssue and can't find a way to make this contact working properly.

The contact formulation is MPC, with a pinball radius big enough to capture some nodes. But I am not sure the issue is there as in my first model I did not pick a pinball value and the contact was working fine (I can see in the results the beam push screw being compressed). I meshed the cylinder a bit finer to help but I can't afford to mesh too fine,

The initial status of the contact is closed. I tried solving with sub-steps, I tried lowering my pretension force. I tried a beam formulation instead of MPC... Not much idea left....

[Hex_screw]

In addition here is picture of the MPC constraints on these contacts in my working model (the one without bolt pretension) :

[Hex_screw]

Okay I found the issue. My beam elements acting as push screws were meshed with only one element. Now it's solving fine. ?