Suppose that I have a long shaft modeled with 3D elements. There are two bearings at the end of each shaft and would like to see the effects of bearing stiffnes on shaft vibration modes.
What can be the best modeling approach for ball bearings in Simcenter, CBUSH elements ?
Looking forward to you valuable comments.
Using springs (i.e. CBUSH) for bearings is a widely used approach. If you aren't modeling the structure beyond the bearings, then you could use node to ground CBUSH elements. These are 1 node elements that connect to your structure, then to ground as if it is a 2 node element with one of the nodes fixed.
This is a study we could do in Simcenter Motion using flexible bodies. We can mount the shaft on bushings that you can define either a linear or varible stiffness and damping. We can then spin the shaft at a range of RPM's and study the shaft vibrations at each RPM or bearing stiffness.
As you say it may be easier to solve it in Motion, however we have to do it in using FEM.
I agree with the recommendation to use CBUSH. Since CBUSH is node-to-node (or node-to-ground) you would need to create connect nodes on your 3D shaft model. I would recommend that you use RBE3 element to connect a group of nodes on your shaft to a centerline node (that is not part of a shaft element). Then you can connect CBUSH to the RBE3 node.
Moment stiffness is often the most important to get right. Generally the translational stiffness of bearings is high relative to the shaft stiffness lateral stiffness. In that case it almost behaves rigidly. But bearings also transmit moment stiffness by virtue of having a length. This stiffness is often on the order of shaft bending stiffness so will have a signficant effect on the response. You may be able to find moment stiffnesses and use those as part of the CBUSH. Or you can create several CBUSH elements that span the lenght of the bearing and give them traslational stiffness only. This will also result in bending stiffness.
If you have no idea what to use for bushing stiffness values, you could make a detailed break-out model of your shaft and the bearings modeled explicilty with 3D elements. make contact connnections between the shaft and bearings. Apply a vertical (or lateral) load to the shaft representing the weight that goes into the bearing. This will establish your reference condition. Then apply another load that may be twice the load of the first. You can use the delta load and delta displacement to compute a stiffness for the CBUSH. You can do similar to get a moment stiffness.
Thank you for your explanation.
What about for the other side of the bearing (supoort structure or housing)? Should we also create an independent node and connect with RBE3s to the support structure, then connect the two nodes with CBUSH ?
Do you have any sample models to show the usage of CBUSH as bearings ?
We just made a sample model using the technique mentioned by Mark. You can see the effects of the bearing stiffness on the modal frequencies. See the attachmnet.