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:
The KinePak mechanism wizard allows the user to define links and then dynamically examine the motion of various types of basic mechanisms including:
|Class 1 lever
|Class 2 lever
|Class 3 lever
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.
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.
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.
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"
|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"
From the "Create Joint" dialog that appears, two types of joints may be
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
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
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.
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
a joint with the Joint Creation Utility.
Mechanical Event Simulation of this Peaucellier cell uses techniques for modeling mechanisms described in this article.
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.