Using the NX Nastran Cbush Spring Element

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1 1/ 9 Springs versus CBUSH There is no versus, the only spring element that I use is the CBUSH. Why? Take a look at the note below written by a well respected Nastran architect (Ted Rose). For example, if you use the standard dnx Nastran spring element between non coincident id nodes, you have the potential to suck force out of your model; that is, your OLOAD won t match your SPCFORCE in the F06 file and your model could be out of whack. I made this mistake once and was schooled by senior analyst at Navistar when she noticed that t my model had some strange stresses. In short, the model was garbage. The correction was to simply swap out the CELAS for CBUSH elements. The CBUSH element does account for any offset between GRID points, so if you use this, it is acceptable to have offset GRID points. If you use CELASi elements (regular NX Nastran springs), you are "adding terms directly into the stiffness matrix" i" and there is no offset between the GRID points. That is the reason for the recommendation in the Common Q&A that the GRID points be coincident. Most pre processors allow spring elements between noncoincident points. In this case, if the spring does not act along the line connecting the points, you have a "spring to ground". The spring will not allow rigid body rotation.

2 2/ 9 CBUSH Usage: Femap has several good little sections on how to create the CBUSH element rved 1 All Rights Reser

3 3/ 9 CBUSH Usage: Femap has several good little sections on how to create the CBUSH element rved 1 All Rights Reser

4 4/ 9 CBUSH Usage: The NX Nastran Element Library Section 6.2 gives you the mathematical background that you will need: rved 1 All Rights Reser

5 5/ 9 CBUSH Example with Solid Elements: A hexed sphere is suspended by four CBUSH elements. The constraints create the minimal set for stability. The CBUSH should be treated like a delicate beam element. Each node has 6 DOF. When attached to a solid element with 3 DOF, the rotational DOFneed to be constrained or the model will be unstable. At the free end of the CBUSH element, the rotational DOF are fixed (456) and the vertical DOF (2).

6 6/ 9 CBUSH Example with Solid Elements: The CBUSH element is selected as a Spring/Damper. Then, and this is the step that most people miss, is that one needs to change the Formulation. When you press the Formulation button, you have anotherscreen where you can change the Nastran type to 1. CBUSH

7 7/ 9 CBUSH Example with Solid Elements: CBUSH requires an orientation vector for creation (like a beam element). However, the spring constants can be setup in the CBUSH coordinate system or that of the Orientation CSys. For this example, the latter approach is used. For stability, small values are used to keep the rotational DOF stable. This is necessary when the 6 DOF CBUSH is attached to a 3 DOF element.

8 8/ 9 CBUSH Example with Solid Elements: Constraining the CBUSH is the biggest challenge to its use. To get the constraints right you must use the constraint set and the CBUSH card together. When the CBUSH is attached to solid elements, DOF 123 are automatically constrained but not DOF 456. To create a full constraint set for the CBUSH, we need to constrain all 6 DOF adequately. The translational (123) are covered by the solid element but rotational (456) are problematic. One solution is to fix one end (456) and then use small rotational values in the CBUSH formulation.

9 9/ 9 CBUSH Example with Plate Elements: The difference with attached to 6 DOF elements is that the CBUSH has something to anchor to for all 6 DOF (123456). To demonstrate this, the model will run with only the vertical DOF constrained. The models used in this example are availablewith this note at the website.