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I am currently working on a project using MSC Nastran and Patran trying to model only radial stiffness between two hollow cylinders. I have been able to idealize a connection which has both radial and axial stiffness in a cylindrical coordinate system using CBUSH elements. However when only radial stiffness is required I am encountering a fatal error due to excessive pivot ratios. I am wondering if it is possible to model radial stiffness only and if so how?

Jack Boggs
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  • If you have two separate parts which are only connected in the radial direction, you need to restrain the axial motion of both of them. You may also need to restrain any possible rigid body rotations of both components, but your post didn't give enough detail to know if that will be a problem. Note, you also need to restrain the axial motions in "real life," otherwise the structure can fall apart! – alephzero Nov 22 '18 at 12:50
  • Thank you for your reply. The connection which I am trying to idealize only posseses radial stiffness, i.e if body 1 moves axially then body 2 will move axially. The same for the theta degree of freedom. Is this possible to achieve within the software or for radial stiffness does axial stiffness also have to exist. I don't believe that rotations are a concern as I have idealized a connection which posseses axial and radial stiffness successfully. Thanks again, if more detail is required I can communicate this. – Jack Boggs Nov 22 '18 at 13:22
  • If the two bodies move together axially, that sounds like a rigid connection. You could model that with an RBE2 element, connecting just the axial degree of freedom at the dependent node. Use the same two nodes as for the CBUSH element - it probably doesn't matter which is dependent and which is independent in the RBE2. – alephzero Nov 22 '18 at 22:21

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