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Kinematic Elements Enable Mechanical Event Simulation for CAD Solid AssembliesAlgor Staff In the past, mechanical engineers have been limited to performing static stress analyses on individual parts of CAD solid assemblies due to both FEA software and hardware limitations. Now, with the addition of Algor’s kinematic elements to Mechanical Event Simulation software, engineers can simultaneously determine motion and stresses for the full assembly of interconnected parts with practical run times. Algor’s Mechanical Event Simulation software combines the capabilities to replicate motion (kinematics), dynamic loading (kinetics) and flexing (stresses) of an assembly of interconnected components (mechanisms) during a virtual "event." This What-You-See-Is-What-You-Get environment lets engineers examine the behavior of the entire mechanical system -- bending, twisting, stretching, squashing and buckling-- without physically building a prototype. In addition, recent improvements in desktop computer hard drive space and computer speed let the average engineer run and study comprehensive Mechanical Event Simulation scenarios in a practical amount of time using a desktop computer. The key is to use Algor’s proprietary kinematic elements. Kinematic elements can drastically reduce analysis run times for Mechanical Event Simulations of full assemblies that contain rigid components because these rigid elements behave like regular, flexible finite elements, but do not produce stresses. Engineers can insert kinematic elements in areas of the assembly where dynamic effects are essential but for which stresses are of secondary importance. This saves time because the engineer can focus the analysis on the part of the mechanism being optimized. For example, the coupler of this drive train model (see Figure 1) was analyzed using flexible finite elements while the rest of the model was specified as kinematic elements to dramatically speed up the processing time. This approach is the most typical application for using kinematic elements in Mechanical Event Simulations.
Figure 1. Algor has compared the speed of kinematic elements to flexible elements and test results are impressive. This 37,659-element drive train model created from a CAD assembly completed one timestep in 24 hours using flexible elements. The same model that used a combination of 7,622 flexible and 30,037 kinematic elements completed one timestep in only 8.9 minutes -- 161 times faster -- on the same Pentium II 266 MHz computer. (Processing times depend on many factors that vary with each model and event.) Performing a Mechanical Event Simulation on full CAD solid assembly parts using kinematic elements involves three simple steps:
To generate an FEA solid mesh, engineers first must transfer the CAD solid assembly to Algor. Algor software capabilities can be launched from within popular CAD solid modelers such as SolidWorks, Pro/ENGINEER and Mechanical Desktop, when they are installed on the same computer as the Algor software. (If they are not, engineers can always import CAD universal files such as Parasolid or IGES directly into Algor.)
Figure 2. This fastener assembly was created using Pro/ENGINEER. All of Algor’s design and analysis capabilities can be accessed from within popular CAD solid modelers, such as Pro/ENGINEER, SolidWorks or Mechanical Desktop, when they are installed on the same computer. A simple "Algor:Algor Mesh" selection in the CAD solid modeler seamlessly exchanges memory and launches the CAD assembly into Algor’s CAD Solid Model Interface. An engineer created this automotive fastener assembly (see Figure 2) using Pro/ENGINEER and then chose the simple Pro/ENGINEER menu selection "Algor:Algor Mesh" to launch Algor’s CAD Solid Model Interface. This interface enables engineers to view their entire model prior to FEA surface meshing to determine if the design contains model defects, such as surface holes or self-intersecting surfaces. For the fastener assembly model, the engineer then created an FEA surface mesh with an easy-to-use sliding mesh control (see Figure 3), which enables quick adjustments to be made to the initial FEA surface mesh density if necessary.
Figure 3. Algor’s CAD Solid Model Interface enables engineers to view an entire assembly prior to surface meshing to determine if the design contains model defects, such as surface holes or self-intersection surfaces. An easy-to-use sliding mesh control enables engineers to quickly adjust the initial FEA surface mesh density. Once the initial surface mesh is created, engineers can activate Superdraw III, Algor's single user interface and precision finite element model-building tool, with a single mouse click. From this interface, engineers can access automatic or manual surface mesh refinement options, automatic mesh engines for high quality FEA solid meshes using bricks, tetrahedron or a combination of both, and Algor's full breadth of analysis options. In our fastener assembly example, the engineer used a combination of advanced meshing techniques in Superdraw III, working directly on the FEA model surface, and automatic mesh refinement options available with Algor’s Merlin Meshing Technology. An advanced surface matching feature ensures that adjacent surfaces align correctly. Then, the engineer used Hexagen, an automatic solid mesh engine, to create a hybrid solid mesh based on the surface mesh. Algor’s solid meshing technology works from the optimized surface mesh inward to ensure that the best-shaped elements remain on the surface where high stress levels most often occur. After achieving a high quality FEA solid mesh, the next step is to define the model parameters, including element types, material properties, boundary conditions (where realistic) and the duration of the event. At this point, the engineer determines which elements to define as kinematic elements. When working with a mechanism, rigid kinematic elements should be used to represent the stiffer members of the mechanism while flexible elements should be placed in areas of the model in which stress information is important. Because stresses due to motion at the hinges of the fastener mechanism were important in this analysis, the engineer specified flexible elements for each of the nine hinges and kinematic elements for the top and bottom components as well as the fastener arms (see Figure 4).
Figure 4. Kinematic elements are easily applied using the model data entry screens in Superdraw III, Algor’s single user interface for FEA and precision-finite element model building tool. Kinematic elements, which are rigid elements that behave dynamically like flexible finite elements, but do not produce stresses, were applied to the arms and end pieces of the fastener because the engineer was only concerned with the motion and load transfer of these components. The engineer specified flexible elements for the hinges to obtain stress data during a Mechanical Event Simulation of the fastener closing. Engineers often know from field experience what part of an assembly is apt to experience high stresses and flexing; however, if this is unknown, it may be helpful to conduct a trial run using a highly simplified version of the solid model defined entirely with flexible elements. From this trial Mechanical Event Simulation, the analyst can see where stresses will likely occur and decide where to place flexible elements among kinematic elements in a more detailed model. In another approach, an engineer could construct a model made entirely of kinematic elements to assess the kinematic functionality of the event and then selectively insert regular finite elements in areas of stress concern. Kinematic elements can be constrained or loaded with force, traction, pressure and gravity. In addition, they possess mass and transmit forces; therefore, they can produce motion and stress in flexible finite elements. Kinematic elements also have full contact and impact capabilities. They can interact with impact walls and distinct objects made of other element types. Impact between objects is achieved using Algor's contact elements, available as part of the Mechanical Event Simulation technology, which simulate dynamic interaction between objects. In the final step, the engineer processes the Mechanical Event Simulation. The engineer can view the event in Algor while it is processed through two visualization utilities, Monitor and Superview. Monitor, a virtual oscilloscope, is the first visualization program to make results available graphically as line data over time during processing. Engineers do not need to wait for an analysis run to finish before reviewing analysis results. Superview, Algor's visualization program, enables results visualization and provides information about analysis accuracy through a unified interface. In Monitor mode, Superview can be used to view an event as it is processed on the computer screen. Any motion, flexing, buckling, material yielding, deformation or deflections are shown for each timestep as it occurs. In addition, engineers can create analysis replay files in .avi format and bitmaps for presentations or reports to highlight areas of engineering concern. Furthermore, results can be extracted to provide critical step-by-step data for Mechanical Event Simulation applications or automated iterative solutions. In our example (see Figure 5), the engineer used Superview to examine analysis results for the fastener Mechanical Event Simulation. The Mechanical Event Simulation simultaneously yielded motion, dynamic loading and large displacement over time for the entire model. Flexing and stresses were produced at the hinges where regular, flexible elements were specified. Areas with kinematic elements appear gray because stresses were not calculated for those elements.
Figure 5. Engineers can view Mechanical Event Simulations during processing with Algor’s Superview visualization program; therefore, they do not need to wait for an analysis run to finish before reviewing analysis results. The Mechanical Event Simulation of the fastener yielded motion, dynamic loading and large displacement over time for the entire model. Flexing and stresses were produced at the hinges where regular, flexible elements were specified. Areas with kinematic elements are shown in gray. The actual fastener assembly is shown in the top right corner. By performing a Mechanical Event Simulation with kinematic elements, the engineer learned how the fastener assembly design would function during real-world operation. The engineer gained accelerated design experience through building a virtual prototype using a computer, which previously could only have been learned through more expensive and time-consuming physical prototype testing. |
