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Found 15 results

  1. what is the dof for CBUSH elements? If i want to create a pin joint ain which i want to constraint all dof except one rotation..what dof i must select while creating zero length CBUSH element among 1-6 dof??
  2. Hello, I have few questions regarding spring in optistruct. 1. How can I load a Cbush o cbush 1D? I know how to stablish the spring K but not a load. 2. Can I choose the direction of the load? Thank you!
  3. Hello, i need some help because i cant find any solution to my problem. I've a Project where i have to work with dynamic stiffness (pbusht element) of a structure and want to see the Eigenvalues and normal modes of my structure. To get into the theory i set up a simple model: A CBUSH element with a pbush property and with a 1D Mass (m=1kg) on top and a BC on the bottom. I specified in pbusht property with elastic stiffness of K=100 (N/mm) and a loss factor of GE=0.3 Then i did an normal mode analysis and i know that the "Damping is neglected" (see link (1)) and i have just my K=100. The solution of my normal mode analysis is a normal mode frequency of omega = 50,33 Hz. This is correct, because the formula for free, undamped oscillations is f = sqrt(k/m)/2PI = sqrt(100 N/mm / 0,001 tonnen) / 2PI = 50,33 Hz (i use N, mm, tons as units) Now i want to include the damping, so i use the complex eigenvalue analysis. In theory i learned that for free, undamped oscillations with the damping factor D<1 the cycle frequency omega_d = omega * sqrt(1-D²). Because of the relation that 2*D = tan(phi) = GE i can fill in the my loss factor of GE=0.3. Then i got my omega_d = 49.76 Hz. This is also logically, because for D<1 the oscillations of damped systems are always slower than the ones of the undamped system. ...so omega_d < omega is okay. Now i did the complex eigenvalue analysis (just defined a load collector EIGC card and connected it to the CMETHOD of the complex eigenvalue load step (of course the SPC and EIGRL card to the SPC and Method(Struct) too like i did for the normal modes). The result of the simulation is a frequency of 50,88 Hz and a loss factor of 0,2935. Now i'm confused - first: why is my loss factor not 0,3? Yes, its just a little difference but its a really simple model - in the normal modes analysis the eigenfreqency was also exact to 5 decimal places. But okay, maybe some numeric accuracy. But i definitely do not understand the frequency of 50,88 Hz. Where is the mistake? Its higher than before whats definitely wrong in theory and not my calculated value. Anyone got some ideas? I already thank you Regards Rene (1) https://altairhyperworks.com/hwhelp/Altair/2019/help/os/topics/solvers/os/analysis_normal_modes_r.htm?zoom_highlightsub=eigra (2) https://altairhyperworks.com/hwhelp/Altair/2019/help/os/topics/solvers/os/analysis_complex_eigenvalue_r.htm CBUSH.hm modal_complex.fem modal_complex.h3d modal_normal.fem modal_normal.h3d
  4. I have a spider of CBARs that I want to change into CBUSHs using Hypermesh. If I use the "config edit" panel it creates 1 noded CBUSHs, since CBUSHs are created under the "masses" panel. How can I make this change? Thank you. Phil Brown
  5. I'm modeling a representative volume element that it's basicaly made by a cylinder inserted into a cube. I need to make a spring contact between this two parts, changing the spring stiffness i want to analyse the perfect and the imperfect contact. Anybody has a tip to build this model with spring elements (CBUSH) only in the common nodes between the cylinder and the rest of the cube? Those spring elements need to be at a cylindric system. I have something like this:
  6. Hello all, I am trying to load a femur model according to a local coordinate system. When loading in the non-desired direction for testing (Z-dir) the model node moves predictably. But when I try to load in the desired direction against the springs (X-dir), the model does some crazy twisting and weird movement. Would love some help resolving this issue. I have a suspicion it may have to do with the springs... making them very compliant still didn't solve my issue though. .fem files attached 6_25_Altair_Question.zip
  7. I don't see how to select an orientation coordinate system while creating a CBUSH element on the springs panel. When using coincident nodes the CBUSH must be oriented by a coordinate system. Can someone tell me the way to set the CBUSH orientation CID from within HyperMesh? My model will only work by editing the input (*fem file) manually.
  8. Hello everybody, is it possible to create pre-loaded CBUSH elements in Optistruct? I would like to model with spring elements braced components (tet-meshed) with a defined spring stiffness. The spring elements are attached by RBE3 to the component's surfaces (RBE3-spider -- CBUSH -- RBE3-spider). My goal is to model pre-loaded human ligaments restraining the hip. I'd be glad to get any advice on this topic. Best regards Jo
  9. I am having trouble figuring out why there are strange reaction forces in my model. The rightmost node allows for rotation (vector out/in screen) and the bottom left node pushes up at a set displacement. The rightmost node should not have so much force in the x-direction. Any help in figuring out why this is happening is much appreciated. Femur_Model_7_1.hm
  10. Hello, I have a group of springs from which I want to extract the total force in a simulation. Is there a quick and easy way to do this? I'd really appreciate your help
  11. The Optistruct manual says that when defining a CBUSH element with coincident GA and GB one must specify a CID. (Optistruct manual, CBUSH page, Comment 5) I do not see a way of specifying a CID in HM. Editing the element has the fields for X1, X2, X3, but the there is no field 9 for the CID. How can assigning a CID to a CBUSH be done? Thank you. Phil
  12. In my model, I am attempting to use RBE3 elements as supports to avoid artificial rigidity imposed by RBE2s. Because a support cannot be attached directly to an RBE3 element, I am trying to create a zero length CBUSH element with high stiffness, as suggested in this help document. However, since the CBUSH element is a spring that requires two nodes to be specified, I don't understand how to achieve zero length without selecting the same node for the ends (which gives the error "The independent and the dependent nodes are the same"). This post indicates that it is possible to create coincident nodes, but when I attempted to create a temp node at the same location as the center of my RBE3 (Geometry -> nodes -> x,y,z), it fails to create a distinct node that I can select in the spring creation panel. My questions are: Is it possible to create coincident nodes? If so, how is this done? If not, how do I create a zero length CBUSH element?
  13. Hello , Are there any options in HM to visualize zero length CBUSH elements . Thanks Girish
  14. Hi, I have a large MSC Nastran model and when I try to show cbush loads on Hyperview as Vectors It does not show them whereas It does as contour loads. Do you know how can fix it? Thanks. Regards.
  15. I am trying to squeeze a femur model with two sets of stiff springs top and bottom, and allow it to rotate with the other (spring BCs come from a research paper). Instead, the model bends where the mesh meets the springs that should remain mostly straight. However, I have tried adjusting all DOF stiffnesses in the PBUSH card without luck. All springs have been assigned to the local coordinate system (shown in right on photos) using CBUSH card. It seems to me I need to stiffen K2 or K6 (y-tension, z-moment), but neither of these solves the issue. Attached graphs show "TY" stresses. Femur_Model_6_26.fem
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