10 Tutorial: The Slider Crank Mechanism
(With Flexible Con-Rod)
Preprocessing Using A Flexible Body
Usually in this simulation, the con-rod is modeled as flexible body. So we will convert the rigid conrod into a flexible body. Assume that conrod geometry is available, it is already meshed in HyperMesh and the input deck (.fem) was exported.
To preprocess the flexbody we make use of the FlexPrep Tool
Step 1: Generating/Building The FlexBody (Using The FlexPrep Tool)
To launch FlexPrep, click on the FlexTools menu near the top of the HyperWorks window. See the figure below.
The FlexBodyPrep dialog, shown below, will appear.
Figure 34: Flex Body Prep GUI – Bulk Data File
Clicking Create RBE2 Spiders opens HyperMesh with a three-step-approach to generate rigid body elements to connect a flexible body.
Figure 35: Creation Of RBE-Spiders – Part 1
The conrod bushing nodes shall be used as dependent nodes of the RBE2.
Similarly another RBE-spider can be created for the second bushing. After the modification is complete, the flex body can be saved and closed by clicking button Step 3. See figure below.
Figure 37: Save And Close RBE-Spiders
The h3d graphic file of the flexible body is created. This is then loaded into the FlexPrep dialog. See figure below.
The field Specify Interface Node List shows the two independent nodes of the RBE-spiders. Then the performance of stress recovery and strain recovery should be selected. The other fields have to be left default and the flex body preparation shall be finished by clicking OK.
Step 2: Adapting Body Properties
To convert the rigid conrod body to be flexible, choose it in the model tree. In the Properties menu check the select box Deformable and specify the Flex h3d-file as the Graphic file.
Figure 40: Body Properties Adaption – Graphic File
After opening the Graphic File the selected one is also taken over at H3D file, which is the default and shall be used for this tutorial. Then the dialog Nodes... shall be opened. This dialog box will help finding the node attachment points for the part connectors (in case of the tutorial the joints). Click the Find All button on the Nodes dialog to find nodes on the flexible body which are located the closest to the interface points on the crank mechanism model.
Hint:
Sometimes there is a little offset between the flexible body interface node and its corresponding interface point in the model. When one clicks the Align button, MotionView moves the connection point in the model (and subsequently the markers that define the connection) to the node location on the flexible body. This affects other entities that reference this point. If there is a non-zero offset, MotionSolve will insert a dummy body between the flexible body and the nearest connection point.
Figure 41: Nodes Dialog
The dialog Modes... allows the user to choose the modes which shall be included in the simulation (see image below). For this tutorial the default-settings should be used.
This window which is prompted after the “Modes” option was activated shows which modes are active during the simulation, and allows to select different options to define the modal damping during the MotionSolve run. Although the dialog box has the option to turn on or off individual modes, it is strongly advised to never turn off any of the flexible body modes other than the rigid body modes. The first six modes are rigid body modes, and are always turned off by default since the rigid body motion is already computed by MotionSolve and thus these modes are not necessary. The modes dialog box is shown below.
Figure 42: Modes Dialog
Step 3: Set The Simulation Parameters
As in the rigid model before, the same Simulation Parameters shall be used. Afterwards, the solver input deck has to be saved.
Step 4: Check The Model
Before running the solver, again, the model check should be used to prove the model for possible errors.
Figure 44: Check Model
Step 5: Execute The Solver Run
The solver can be started in order to calculate the MBD-simulation with the flexible conrod.
Step 6: Post-Processing
In an exemplary manner to the stress, it should be shown how the results of the simulation can be displayed by using flexible bodies. As of missed out pressure forces within the cylinder and the very massively designed conrod, the stresses are, as expected, very low.
Conclusion
This tutorial demonstrates the use of MotionView/MotionSolve to solve a simple multi-body model. MotionView is the modeling environment. It allows you to build models graphically. You can even add flexible bodies in your system model. MotionSolve is the solver. Given a model description, it formulates the equations of motion and solves these using numerical methods.
The results generated by MotionSolve may be used to understand and improve the behavior of the system.
List Of References
[1] Rill, Georg; S., T.: Grundlagen und Methodik der Mehrkörpersimulation. Vieweg+Teubner, 2010. ISBN 978-3-8348-0888-2. [2] www.altairuniversity.com. Altair Engineering 2013.
[3] Kecskeméthy, Andrés: Mehrkörpersimulation. Essener Unikate, 2007. [4] MotionView Introduction. Altair Engineering.
[5] Schaeffer, Thomas: Mehrkörpersimulation (MKS): Einsatzmöglichkeiten, Entwicklungstrends und neue Möglichkeiten der Kooperation. Info-Workshop 4, 2009.
[6] Schmalzl, Jürgen: Simulation des dynamischen Verhaltens von Flurförderzeugen in der Lagertechnik. Dissertation, 2006. [7] Grote, K.-H.; Feldhusen, J.: Dubbel – Taschenbuch für den Maschinenbau, 23. Auflage. Springer, 2011. ISBN 978-3- 642-17305-9.
