Pro Mechanic A

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Dynamic and Structural Analysis with Pro/MechanicaAssociate Professor Jeffrey S. Freeman

October 12, 2004

The slider-crank model shown in Figure 1 has been assembled using Ground as the first body.

Geometric assembly was used to connect the wristpin to the slider. All other assembly

connections are kinematic: using pin-type connections for the crank-to-ground, crank-to-

connecting rod, and connecting rod-to-wristpin; and using a slider-type connection for the slider-

to-ground.

Since kinematic assembly was used in Pro/Engineer, the joint-types are automaticallytransferred to Pro/Mechanica. The types are shown by yellow arrows with a right-handed twistfor the pin-type (revolute) joints, and a yellow box and arrow for the slider-type (translational)

joint. Grounded connections are shown using a green-colored icon.

Figure 1: Slider-crank model in Pro/Mechanica

Defining Model Properties

The first thing we need to define are the material properties to use with the model. Select the

Model-Property-Material menu choice to get to the Materials dialog window. UnlikePro/Engineer, Pro/Mechanica has a pre-defined material library. Select the appropriatematerial from the left column and transfer it to the right column, then assign the material to the

appropriate parts (you can use the Model Tree to select the parts).


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Figure 2: Materials dialog window

The next thing to consider is how the mechanism is driven. This can be done using either a load

(force/torque) or a kinematic driver. Pro/Mechanica can represent loads and drivers using

either a ramp or cosine function, or by tabular data. The joint axis between the crank and the

ground is selected for the kinematic driver. Figure 3 shows the kinematic driver creation dialog,

where a constant velocity driver has been defined.

Figure 3: Kinematic driver dialog

Setting Up the Analysis

Since the slider-crank mechanism has only one degree-of-freedom, the driver defines the motion

of the mechanism and we can solve for the forces required to maintain the motion. This is done

by selecting motion analysis from the Analysis dialog, shown in Figure 4.


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Figure 4: Pro/Mechanica Analysis dialog

Editting the motion analysis parameters, the default parameters, shown in Figure 5, are probablynot correct for what we want. To do motion analysis, Pro/Mechanica will numerically integrate

the equations of motion. This dialog allows us to control how the integration will occur. The

main things most users need to know about this dialog are the duration and increment. Given the

kinematic driver velocity of 6.2832 rad/sec, a duration of 10 seconds means that the crank willrotate 10 times during the simulation, and the results will be output every 36 degrees of crankrotation. For inverse dynamic analysis, the simulation can be performed for a shorter duration,

and a smaller increment (like an output every 2-5 degrees of rotation) is desirable.

Figure 5: Motion analysis options

Running the Simulation

Once the mechanism is defined, the simulation can be executed. In Pro/Mechanica, this is done

by selecting the Run menu choice. Pro/Mechanica is based on SD-Fast, which symbolicallycreates the equations of motion using Kanes method, and then writes out a C subroutine,

which is compiled and linked to the Pro/Mechanica library. In order to run a simulation, youmust have the Microsoft Visual C compiler installed. The compilation/execution proceeds

automatically once Run has been selected.


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Viewing the Results

After a simulation has been completed, the Results menu choice can be selected, and themechanism can be animated to visually verify the motion. The Animate dialog is shown in

Figure 6. It works just like the controls of a VCR. The Capture button will save theanimated output as an MPEG file, which can be included in presentations.

Figure 6: The Animate dialog

The animation looks nice, but to use the results to calculate stress, we need to select the Use InStruct menu option. This brings up a dialog, shown in Figure 7, which queries at what point intime to output the loads. We will then be asked to select the body and the joints where the loads

are applied. By selecting the body, the inertial load will be included in the output, while

selecting the joints includes those loads individually. You should select all the joints attached to

the body of interest.

Figure 7: Use In Struct dialog

Adding and Using Measure Features

One problem is to select the appropriate point in time. To do this, we can add a measure feature

to the Pro/Mechanica model. Returning to the Model menu, select the Measures menu choice todisplay the dialog and then Create. The following choices appear:

Connection: This creates a measure between two bodies. Pro/Mechanica already reports most

joint reaction forces, so this is primarily useful for cams, slots and gears.

Joint Axis: This creates a measure of position, velocity, acceleration or net force at a joint.Load: This creates a measure of a load magnitude. The load can be either a force or a torque.

Body: This creates a measure of a body-related component. Primarily this is used to measure

the orientation, angular velocity or angular acceleration of a body.

Point: This creates a measure of the position, velocity or acceleration of a point of interest on a

body.

Pt to Pt: This creates a measure of the separation distance, speed or acceleration of two points

on different bodies.


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System: This creates a measure of system related variables, such as the total kinetic energy.True Angle: This creates a measure between two vectors, where each vector is associated with

a body (or ground). The angle will be between 0 and pi radians.Contact Pair: This creates a measure between two surfaces of a contact pair.

Clearance: This creates a measure of the distance separating two surfaces, including

penetration. It is especially useful for checking for interference between parts.Computed: This creates a measure that can be computed from other parameters defined for themodel.

If we want to measure when the connecting rod is horizontal, then an orientation body measureshould be added for the connecting rod body. When the simulation is re-run, the orientation of

the connecting rod can be plotted with respect to time, as shown in Figure 8. This was createdusing the default simulation duration (10 sec.) and increment (0.1 sec.) values.

Figure 8: Graph of body orientation using default simulation values

Resolution of the time-orientation relationship can be improved by changing the simulation

parameters. Since the inverse dynamic analysis is identical from cycle to cycle, only one cycle isrequired to analyze the forces. By setting the duration to 1 sec. and the increment to 0.0028 sec.,

the simulation will compute the results of one complete cycle for every degree of rotation. Thisis shown in Figure 9. The graph can be zoomed in to focus on one of the two zero crossings, and

the time of the crossing can be figured from the point information. In this case, the first zerocrossing occurs at t=0.2083 sec. The simulation can be re-run to output results at exactly this

time.


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Figure 9: Graph of connecting rod orientation for one cycle

Returning to the Use In Struct menu option, we can output the forces for the connecting rodbody at the appropriate time. Note that Pro/Mechanica will not interpolate between differentsolution times, so if the mechanism is not solved with the correct output time, Pro/Mechanicawill use the closest time for which it has saved an output.

Stress Analysis in Pro/Mechanica

In order to perform stress analysis for the connecting rod part, we first need to quit

Pro/Mechanica and return to Pro/Engineer to open the part file. This is because the motionanalysis works with assemblies, while the stress analysis works with individual parts. The

connecting rod part is shown in Figure 10.


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Figure 10: The connecting rod part

Again select Mechanica from the Applications menu, but instead ofMotion analysis, selectStructure analysis. In order to analyze the effect of the dynamic loading on the component, wefirst have to setup the loads, constraints and measures. All of the model setup menus are under

the Model menu selection.

Adding ConstraintsFirst determine how the component will be constrained. In this example, the bearing between

the crank and connecting rod will be constrained. The point and edge/curve constraints are not

appropriate for this connection, so a surface constraint will be applied. The surface constraint

dialog is shown in Figure 11.

Select the two surfaces formed by the hole at the crank to connecting rod interface, as shown in

Figure 12. The constraint can be set to control three translations and three rotations

independently. In this example, all six are fully constrained.

It is important to note that the constraints are members of a constraint set. When the analysis is

run, only one constraint set and one load set will be used. We can create multiple constraint and

load sets (most typically just load sets) and define multiple analysis cases.


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Figure 11: Surface constraint dialog

Figure 12: Constraints applied to the interior surface of hole

Adding Loads

Next we want to define the loads. Pro/Mechanica supports many types of loads, but only a

couple of them will work with the results generated from the motion analysis. The types of

loading are:Point: Enables simulation of point loads on the model.

Edge/Curve: Enables the simulation of loads acting on an edge or curve.

Surface: Allows for the distribution of a load over a surface.

Pressure: Enables the simulation of pressure loading on one or more edges/surfaces. Pressure

loading is always oriented normal to the edges/surface(s).

Bearing: Approximates the load distribution which would occur for a bearing.

Gravity: Enables simulation of loading due to acceleration.


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Centrifugal: Enable simulation of loading due to rigid body rotation. This can be due to

angular velocity or angular acceleration.

Temperature: Enables the simulation of temperature changes on the model. The temperature

can be directly defined or generated by the Pro/Mechanica temperature analysis module.

For the example presented here, a bearing load will be applied at the wristpin to connecting rodjoint, and gravitational and centrifugal loads will be applied to the whole body.

First define the bearing load. Once the hole is selected as the load location, the MEC/M Load

button appears in the Bearing Load dialog, as shown in Figure 13. This button is the interface to

loads generated by the Pro/Mechanica motion analysis.

Figure 13: Bearing load dialog

Selecting the MEC/M Load button, we can select the load case saved during the motion

analsysis. Arrows, representing the loading directions, will appear on the component. These

represent all possible bearing loads saved during the motion analysis. Select the one acting on

the wristpin bearing, and its components will appear in the dialog. By selecting Preview, you

can see how the load is distributed.

The gravitational and centrifugal loads are applied in a similar method. However, both loads

will appear at the World Coordinate System (WCS), so there is nothing to select after clicking

the MEC/M Load button in the respective dialogs.

The connecting rod, with both the constraints and loads applied is shown in Figure 14.


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Figure 14: Connecting rod with loads and constraints

Analyzing the ModelThis model can be solved using static analysis. Define a new static analysis using the Analysis-Mechanica Analyses/Studies menu button. The Analyses and Design Studies dialog isshown in Figure 15.

Figure 15: Analyses and Design Studies dialog

Using this dialog, a static analysis study can be created or edited. Selecting to edit the staticstudy, Analysis1, the definition dialog shown in Figure 16 appears. For the constraints and loadsto be used in the study, select the constraint set and load set you have just defined.


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Figure 16: Defining constraint and load sets

The structural model can be checked for any simple problems by selecting the Check Model

menu choice. As long as the material properties, constraint set, and load set are defined and the

static analysis is properly selected, there should be no problems.

Executing the Structural Solution

Select the Run-Start menu choice to solve the model. Pro/Mechanica uses a p-method solver,as opposed to a normal finite element h-method solver. P-method solvers use a large mesh

(which is automatically generated) and varying orders of polynomials to represent the stress

field. The p represents the order of the polynomial. Typically in the solution technique, the

order of the polynomial is increased until changes in the stress field converge to fixed values.

An h-method solver requires a much finer mesh (h represents the size of the mesh) and uses

fixed-order polynomials for each element.

Once the analysis is underway, you can select the Info-Status menu button to view the

progress of the solution. A status file is shown in Figure 17. You should review the run status

file for the information contained.

Viewing the Results

Provided the run was successful, you can examine the results by selecting the Results menu

choice. Pro/Mechanica creates a design study subdirectory, with the same name as the analysis

case, to contain the results. When multiple analysis cases are created, each will have a separate

subdirectory. Select the appropriate design study directory to display (in this case Analysis1).


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Figure 17: Run status file

Figure 18: Results window definition

The measure to display is selected from this dialog, as is the type of display. Pro/Mechanica

supports several types of pre-defined analysis measures. The example shown in Figure 19 shows

a fringe plot of the von Mises stress.

Results can also be shown using exaggerated deformation, as shown in Figure 20. This plot

shows both the deformed and undeformed shapes, and a contour fringe plot of the maximum

principle stress for a different loading case.


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Figure 19: Results window showing von Mises stress fringe plot

Figure 20: Results window showing maximum principle stress with deformation


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Creating Measures

If the stress at a particular point is required, a measure feature must be created before the analysis

is run. Selecting the Insert-Simulation Measure menu button brings up the MeasureDefinition dialog, shown in Figure 21.

Figure 21: Simulation measure definition

A wide variety of different measure types can be defined. Also, many measures are predefined.

This is shown in Figure 22, which details both User-Defined and Predefined measure.

Figure 22: User-Defined and Predefined measures

Defining Parameters for a Design Study

Design studies differ from analyses since parameters are defined which affect the performance of

the simulation. In the case of this example, the parameters were created in Pro/Engineer, andassigned to key dimensions of the part. We can either access these parameter definitions from


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Pro/Mechanica, or we can create new ones, using the Analysis-Mechanica Design Controlsmenu choice. Selecting Design Params, the following dialog appears.

Figure 23: Design parameters dialog

Selecting the Create button presents a new dialog, shown in Figure 24, where thePro/Engineer-defined parameters can be selected. In this case, the SLOT_LENGTH parameterhas been selected. At this point the maximum and minimum limits on this parameter need to be

set. Setting an appropriate range requires some knowledge about design limitations,manufacturing processes, and other higher-level product knowledge.

Figure 24: Selection ofSLOT_LENGTH as a design parameter

Creating an Optimization Design Study

An optimization design study can be created using the Analyses and Design Studies dialogshown previously in Figure 15. In this case, the Design Study Definition dialog, shown in Figure


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25 appears. This dialog allows the definition of both design sensitivity and design optimization.The goal of the optimization can be any measure, either predefined or user-defined. Maximum

and minimum limits can be placed on other model measures. These are activated only if thecheck box is selected. Similarly, the design parameters can be selected from the list of possible

design parameters.

The design study is executed the same way as the static analysis was executed. The differencehere is the length of time spent performing the computations. It is important to recognizewhether a feasible design exists before performing an optimization design study.

Figure 25: Optimization design study definition

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