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Modeling Mechanisms for Mechanical Event Simulation

ALGOR’s Mechanical Event Simulation (MES) software calculates motion and stresses with built-in linear and nonlinear material models. The set-up of an MES relies on physical data, rather than calculated or assumed loads and constraints. The combination of motion and stress analysis considering full inertial effects enables engineers to see motion and its results, such as impact, buckling, permanent deformation and displacement. As a result, parts need to be verified by less physical prototype testing, thus reducing costs and time-to-market. The comprehensive analysis method of MES is not only more accurate than linear static FEA, but it provides for full dynamic visualization of the behavior of the part or mechanism as the event transpires.

In this article, some useful techniques will be explored for modeling mechanisms for MES. Topics covered include:


Using KinePak to Create Mechanisms

The KinePak mechanism wizard allows the user to define links and then dynamically examine the motion of various types of basic mechanisms including:

Four-bar     
Toggle
Slider/Crank
Class 1 lever
Class 2 lever
Class 3 lever
Triangle

After selecting one of these types of mechanisms, the lengths of the linkages are specified. As the dimensions are entered or modified, the parametric mechanism is automatically updated and a visual motion range plot indicates the possible range of motion based on the current link lengths. The mechanism may be automatically animated or the user can click and drag the endpoints of the linkages to examine the motion. The dialog also enables the user to specify the angle between member links.

The resulting mechanism can then be transferred to the ALGOR FEA modeling environment. This transferring process maintains any constraints enforced on the original link-based design by automatically applying the corresponding boundary conditions. The mechanism can be saved in one of several geometry formats, enabling complete flexibility for where and how the mechanism is incorporated. A resulting mechanism can be used as the base geometry, as "skeleton" lines of a more complex CAD or hand-constructed mechanism or to drive the motion of a more complex geometric model.

   The 4-bar linkage produced by KinePak served as the template for assembling this mechanism, which drives a paint mixer. ALGOR’s Mechanical Event Simulation software replicated the motion of the mechanism and simultaneously calculated the stresses of the base, coupler and rocker arms based on the rotational motion of the crank arm. The crank arm and mixer plate were modeled using kinematic elements. The assembly of this mixing mechanism was modeled in SolidWorks and captured seamlessly through InCAD technology. 

Modeling a 2-D Joint

This example uses a simulated pin joint in a 2-D loader model. Each of the four pin joints in the model connects loader components to actuator elements, which simulate the hydraulic mechanisms that control the loader’s movement. Each pin joint consists of truss elements configured like the spokes of a wheel. The actuator element connects only to the node at the center of the truss “spokes,” thus permitting its motion.

  

Modeling a Joint with the Joint Creation Utility

To create a joint within an assembly captured from a CAD solid modeler, ALGOR provides a joint creation utility. This utility automatically creates truss elements between the axis of the joint and the rotating part(s). This capability is often used to simulate joints in areas of the model where stress is not a concern. If stress in the joints is a concern, please see the next section, modeling a joint with surface-to-surface contact

After capturing the geometry of the mechanism assembly from a CAD solid modeler using InCAD technology, select all of the surfaces that describe the joint. Then, right click anywhere in the working area to access a pop-up menu of options for the selected surfaces. Choose the "Create Joint" option.   
 

 
After selecting surfaces in this model of a piston assembly with a rotating crank, right clicking in the working area accessed a pop-up menu with the "Create Joint" option.
 

From the "Create Joint" dialog that appears, two types of joints may be selected:

  • A pin joint will connect nodes on the selected surfaces to the opposite end of the joint axis, which will allow the model to rotate around the axis.

  • A universal joint will connect the nodes at either end of the model with the midpoint of the axis, which will allow the model to rotate about the axis as well as swivel about the center point of the axis.

Automatic and manual specification of the axis/center-point can also be chosen. 

After the settings are accepted and the joint is created, the model may be meshed. The resulting mesh will include lines that will be automatically defined as truss elements for each joint. In addition, each joint will be placed in its own part number as shown in the model tree of the image below.

 

The "Create Joint" dialog displays selected surfaces and offers the choice of two types of joints.
 

After meshing, lines that will be defined as truss elements of the joints can be seen in the piston assembly model. The lines of each joint are placed in distinct part numbers as shown in the model tree.
 
Joints can also be added after meshing. To apply a prescribed rotation to a joint, modify the element type of the joint from truss to beam elements in the FEA Editor environment.

Modeling a Joint with Surface-to-Surface Contact

The most intuitive, accurate and realistic method for modeling a joint employs surface-to-surface contact with friction. In this pin joint model, the outside surface of the pin (part 2) and the inner surface of the sleeve (part 1) are defined as different surfaces. The joint stability is provided by surface-to-surface contact between the surfaces, rather than connecting nodes as discussed above.

As this pin example demonstrates, joints can be modeled using any detail level. Pin joint geometry is rather simple. However, other types of joints are more geometrically complicated and there are many cases in which an engineer might decide to suppress detailed joint geometry, as discussed in the previous section, modeling a joint with the Joint Creation Utility.



The Mechanical Event Simulation of this Peaucellier cell uses techniques for modeling mechanisms described in this article. 

Themes and Variations

In this article, we have discussed some basic techniques for modeling mechanisms. When working with CAD solid models, engineers will likely need to combine several of these techniques. In some assemblies, different types of joints may exist at different locations in the assembly.

In addition, some of the examples described may serve to inspire variations on the themes used. Readers are encouraged to familiarize themselves with text books such as Mechanisms & Mechanical Devices Sourcebook (Nicholas P. Chironis, McGraw-Hill Book Company, New York, 1991) and Theory of Machines and Mechanisms (Joseph Edward Shigley and John Joseph Uicker, Jr., McGraw-Hill Book Company, New York, 1980), which provide illustrations and descriptions of common mechanisms and kinematic concepts.



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