I'm attempting to model bearings on a cylindrical shaft so that I can vary the stiffness in order to study the effect of the bearings on shaft deflection. I was hoping to do this with spring elements, but I need the element to be compressive only and the spring elements (CELAS) are both tension and compression elements. Gap elements seem like they would work, but I'm having difficulty with the inputs. Is there a simple method to model this in NX Nastran?
By the way. It would be very unusual to model a shaft bearing as compressive only, since it would typically act in all directions unless the shaft were just resting on the bearing on one side. A bearing gap is more common, and can also be modeled with a PELAST (or PBUSHT). For dynamic solutions you can also model nonlinear springs using NOLIN1 inputs.
Paul Blelloch, Ph.D.
Director, Aerospace Analysis
ATA Engineering, Inc.
11995 El Camino Real
San Diego, CA 92130
Very interesting the question: 1st suggestion is to use CGAP elements, but also agree with Paul to take a look to CBUSH elements, in my opinion the CBUSH elements are the big unknow element of the NX NASTRAN library: the only spring element I use is the CBUSH, it has all the advantages of a beam element with the ability to dial in spring values.
Here you are some advantages of using the CBUSH element over CELASi elements:
• The generalized spring-damper element CBUSH is a structural scalar element connecting two noncoincident grid points, or two coincident grid points, or one grid point with an associated PBUSH entry. This combination is valid for any structural solution sequence.
• if you use CELASi elements and the geometry isn’t aligned properly, internal constraints may be induced.
• The CBUSH element contains all the features of the CELASi elements plus it avoids the internal constraint problem.
• You can also use the PBUSHT entry to define load-displacement dependency in SOL 106.
Other factors to take in mind:
• If the element type used to mesh the shaft is cbeam/cbar, then CBUSH is perfect to characterize the stiffness bearings.
• Using 2-node CGAP elements is perfect to manage node-to-node contact compression-only problems, but you need to have experience using CGAP elements in order to avoid singular matrix.
• The analysis type is critical as well: CBUSH is valid for all solution types, but forgot CGAP for dynamic response analysis (it behaves as a spring).