For years, engineers could only perform static stress analyses on individual components of mechanical systems due to FEA software and hardware limitations. They analyzed simplified models, but couldn't evaluate a system's behavior during motion without building a prototype. Today, engineers are no longer limited by a computer's analysis processing time. Designers can now simulate a mechanical event using a complete CAD solid model or assembly efficiently and quickly on a PC.
Kinematic elements make this possible. Kinematic elements are rigid elements that move like regular, flexible finite elements, but do not produce stresses, which dramatically reduces processing time. They possess mass, transmit forces, and contain full contact capabilities. They can be constrained or loaded with force, traction, pressure, and gravity. Kinematic elements are used in relatively rigid areas of a model while regular elements are used in areas of engineering concern to obtain stresses only where needed.
|This Mechanical Event Simulation of an automotive fastener was performed with Algor's Accupak/VE software. Areas with kinematic elements appear gray because stresses were not calculated for those elements. The actual fastener assembly is shown in the top right corner.|
Preliminary test results performed by Algor, makers of Accupack software, show that kinematic elements can quicken processing speed by as much as 160 times. Because of their power and speed, Algor added kinematic elements to its Accupak family of products. This FEA software realistically simulates motion and flexing in mechanical events and eliminates the need to specify dynamic loads.
When working with a mechanism, rigid kinematic elements should be used to represent the stiffer members of the mechanism while flexible elements should be used in areas where stress information is important. For example, in an automotive fastener assembly (see figure), because stresses in the fastener mechanism's hinges were important in this analysis, the designer specified regular, flexible elements for the nine hinges, and kinematic elements for the top and bottom components and fastener arms. To do this, he opened the "Element Type" data entry screen from Algor's "Model Data Control" window in Superdraw III, a single user interface and precision finite element model-building tool. The engineer simply selected "Brick" as the element type for the hinges and "3-D Kinematic" as the element type for the remaining element groups.
The design engineer then defined the remaining model parameters, including element types, material properties, realistic boundary conditions, and the duration of the event. He did not have to input loading parameters because Accupak software calculates them based on the physics operating during the event. The engineer then ran the analysis and examined the results with Superview, Algor's visualization program. The simulation 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.
The engineer saved time and money by building a virtual, instead of a physical, prototype of the fastener assembly and performing a Mechanical Event Simulation with kinematic elements.