1 / 9Using the NX Nastran Cbush Spring Element
Springs versus CBUSHSprings 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
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example, if you use the standard NX Nastran spring element between non‐coincident 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
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1–All Rights Reser noticed that my model had some strange stresses. In short, the model was garbage.
The correction was to simply swap out the CELAS for CBUSH elements.
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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 th tiff t i " d th i ff t b t th GRID i t Th t i th f th d tithe stiffness matrix" 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 / 9Using the NX Nastran Cbush Spring Element
CBUSH Usage:CBUSH Usage:Femap has several good little sections on how to create the CBUSH element
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3 / 9Using the NX Nastran Cbush Spring Element
CBUSH Usage:CBUSH Usage:Femap has several good little sections on how to create the CBUSH element
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4 / 9Using the NX Nastran Cbush Spring Element
CBUSH Usage:CBUSH Usage:The NX Nastran Element Library Section 6.2 gives you the mathematical background that you will need:
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5 / 9Using the NX Nastran Cbush Spring Element
CBUSH Example with Solid Elements:CBUSH Example with Solid Elements:
A hexed sphere is suspended by four CBUSH elements The constraints
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CBUSH elements. The constraints create the minimal set for stability.
The CBUSH should be treated like a
1–All Rights Reser delicate beam element. Each node
has 6 DOF. When attached to a solid element with 3 DOF, the rotational DOF need to be constrained or the
201 DOF need 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).
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CBUSH Example with Solid Elements:CBUSH Example with Solid Elements:
The CBUSH element is selected as a Spring/Damper
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a Spring/Damper.
Then, and this is the step that most people miss, is that one
1–All Rights Reser needs to change the Formulation.
When you press the Formulation button you have another screen
201 button, you have another screen
where you can change the Nastran type to 1. CBUSH
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CBUSH Example with Solid Elements:CBUSH Example with Solid Elements:
CBUSH requires an orientation vector for creation (like a beam element)
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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
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used.
For stability small values are used to
201 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.
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CBUSH Example with Solid Elements:CBUSH Example with Solid Elements:
Constraining the CBUSH is the biggest challenge to its use To get the
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challenge to its use. To get the constraints right you must use the constraint set and the CBUSH card together.
1–All Rights Reser 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
201 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.the CBUSH formulation.
9 / 9Using the NX Nastran Cbush Spring Element
CBUSH Example with Plate Elements:CBUSH Example with Plate Elements:
The difference with attached to 6 DOF elements is that the CBUSH has
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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
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The models used in this example are available with this note at the website
201 available with this note at the website.