103 - understanding modal analyses.pdf

8/14/2019 103 - Understanding Modal Analyses.pdf

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StructuralAnalysisIUnderstandingModalAnalysesLecture

UnderstandingModalAnalyses.mp3

Understanding Modal Analyses

Modal Analyses can be used to calculate the natural frequencies and

mode shapes of a Pro/MECHANICA simulation model.

Input:

No Loads No Prescribed Displacements Geometry Material Constraints (optional)

Output:

Frequency (Summary Results) Mode Shape (Fringe Plots)

Model for Modal Analysis


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Mode shape fringe plot.LectureNotes

Modal Analyses

You can use Modal Analysis to determine the vibration characteristics (naturalfrequencies and mode shapes) of a Pro/ENGINEER part or assembly. You can also seethe response to the natural frequencies of your model when it is subjected to time-dependent and/or oscillatory/vibration loads by running any dynamic analysis:Dynamic Time, Dynamic Frequency, Dynamic Random, or Dynamic Shock. A modalanalysis is a prerequisite for a dynamic analysis.

Modal Analyses Input

Modes are intrinsic properties of a structure and they are dependent on the materialproperties (mass, structural damping stiffness) and boundary conditions. Each mode

is characterized by two modal parameters: frequency and shape. These modalparameters change as the stiffness and boundary conditions of the system change.

There are no loads required for a Modal Analysis. Any loads (steady ortime/frequency-dependent) or Prescribed displacement constraints are neglected

when running a Modal Analysis.


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The model can be constrained (surface, curve/edge, and/or points) or not. A ModalAnalysis can evaluate the modal parameters (including the rigid modes) from a

specified minimum frequency on or within a certain frequency range.

Modal Analyses Output

The results from a Modal Analysis can be identified in the summary report and infringe plots. The summary report reports the frequency values for each of the modesand fringe plots can be used to display the mode shapes.

The stress and displacement results from a Modal Analysis are all normalized. That is,the maximum deformation is always 1.0. It is possible to generate results that aremass-normalized when using the Mechanica batch mode for solving purposes.

Best Practices

Units of modal frequency shown in results are always cycles per unit of time.The units of time are affected by the force/length/time units you used to define

the model. Mechanica never reports modal frequency in terms of radians perunit of time.

For Modal Analyses, values for all quantities are not absolute. You cannotcompare them to quantities from any other type of analysis. Mechanica unitnormalizes displacements and rotations to 1.0 by dividing all displacements bythe maximum displacement response.

UnderstandingModalAnalysesDemonstration

UnderstandingModalAnalyses_demo.mp4UnderstandingModalAnalysesProcedure

Procedure: Creating Modal Analyses

ScenarioIn this procedure, you will define a Modal Analysis in a Pro/ENGINEER assembly model for

which materials were already assigned.

CreateModal shears.asm

Task 1.Open the Mechanica Application, investigate the model and existingMechanica Simulation Features, and define a Modal Analysis.

1. Click Applications > Mechanica.

2. Explore and examine the model. From the top of the model tree, click Show >Expand All.


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3. Click Mechanica Analyses/Studies from the main toolbar.

4. Click File > New Modal...

5. In the Name field type Shears_Modal.

Providing a short description of the analysis is not a required step, but it can

be beneficial for other users who would access this data in order to

understand your analysis.

6. Note that there is no Constraint to select. As such, Mechanica defaults to the

Unconstrainedradio button and selects the With rigid mode searchcheck box.

Verify that this is the case as shown in the figure.


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The With rigid mode search option is used when the model will exhibit free

body motion. This option is typically used when running an unconstrained

Modal Analysis. For a multi-degree of freedom system, there are normally

six possible rigid modes of vibration at zero frequency, three Cartesian

translations and three rotations. Also, this option can be used in a

constrained Modal Analysis when the user is unsure if the model issufficiently constrained.

7. Select the Convergencetab and select Single-Pass Adaptivefrom the Methoddrop-down menu if necessary.

8. Select the Modestab. Type 20in the Number of Modes field and verify that theMinimum Frequency field is set to 0.

9. Select the Outputtab and select the Stresses, Rotations, and Reactionscheckboxes, and verify that the Plotting Grid field is set to 4.

In general were interested in the modal parameters and less in the stress or

deformations. But the stress and deformation fringe plots can provide us

with information as to where to set the probes (Mechanicas Measures) in

order to make readings at maxima for a Dynamic

Time/Frequency/Shock/Random Analysis.

10. The dialog box should now appear as shown in the figure. Click OKto complete

the Modal Analysis Definition and close the dialog box.


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Task 2.Save the model and erase it from memory.

1. Return to the Standard Pro/ENGINEER mode by clicking Applications > Standard.

2. Click Save from the main toolbar and click OKto save the model.

3. Click File > Erase > Current > Select All > OKto erase the models from

memory.

This completes the procedure.

UnderstandingModalAnalysesExercise


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Exercise: Modal Analysis

Objectives

After successfully completing this exercise, you will be able to:

Create and run a model analysis. Evaluate the mode shapes and modal parameters from the results of a Modal

Analysis of a 3D Pro/ENGINEER model.

ScenarioIn this exercise, you will use Mechanica to evaluate the mode shapes and corresponding

frequencies of a intake camshaft. You will examine each of the mode shapes and useMechanicas animated results in order to get a better visual understanding of how these

occur.

The Pro/ENGINEER model of the camshaft is longer than it is thick and has a rather stiffstructural component (it is made of FE6 which has a Young's Modulus of E= 131 GPa).

In reality, the camshaft is held in place enabling it to spin and slide along its axis, but is

not allowed to expand radially at the bearing supports. Since you will be examining aworst-case scenario, you will assume that the camshaft is blocked from spinning at one of

its ends.

ModalCam cam_intake_st.prt

Task 1.Open the Mechanica application and explore the existing Mechanicasimulation features.

1. Click Applications > Mechanica.

2. Explore and examine the model. From the top of the model tree, click Show >Expand All. Scroll to the bottom of the model tree and review the existing

Mechanica simulation features.


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Note that a number of Mechanica features have already been created

including:

Surface Regions (for Constraints) Custom Cylindrical Coordinate Systems A Material Assignment Constraints


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3. Right-click Constraint11in the model tree and select Edit Definition.

This constraint is typical of all of the constraints in the model. Each of them

was defined relative to one of the custom cylindrical coordinate systems and

enables twisting and axial translation but constrains radial motion, with the

exception of the Constraints 8 and 9 which remove the twisting and axial

degrees of freedom respectively.

Because the model is a rather slender solid component, it is difficult to use

any element type other than 3D.

4. Click OKto close the Constraint dialog box.


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Task 2.Use AutoGEM to mesh the model.

1. From the main toolbar, click Create > Createto create an AutoGEM mesh.

Note that AutoGEM creates approximately 4000 tetra elements for the

model.

2. Click Closein each of the open dialog boxes, then click Noto avoid saving the

mesh.


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Task 3.Create and run a new Modal analysis.

1. From the Main toolbar, click Mechanica Analyses/Studies .

2. Click File > New Modal...

3. In the Name field, type Modal_Cam.

4. Verify that the Constrainedradio button is selected and that the

modal_constraintconstraint is selected.

5. Select the With rigid mode searchcheck box.

6. Verify that the Number of Modesradio button is selected and enter 6in the

Number of Modes field.

The With rigid mode searchoption is used (if constraints allow free

spinning, radial expansion or translation) when the model exhibits free body

motion. This is not the case in this example, but it can be used as a

precaution in order to avoid any human errors made during the definition of

the constraints.

Due to the calculations involved in finding the mode shapes, modal analyseswith specification of All Modes in Frequency Rangetend to take longer to

complete than analyses with a defined with Number of Modes.

7. The dialog box should now appear as shown in the figure. Click OKto complete the

Modal Analysis Definition and close the dialog box.


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8. Verify that Modal_Camis selected in the Analyses and Design Studies dialog box

and click Start Run > Noto start the design study.

9. Click Display Study Status once the analysis is started.

The analysis should take about five to seven minutes to complete.

Task 4.Investigate the Results of the Modal_Cam analysis.

1. Review the contents of the Run Status dialog box. Note that the primary frequency

is about 1220 Hz, followed by 2470 Hz, 3190 Hz, 3200 Hz, 3380 Hz and 4980 Hz.

So far we have the first information about the camshaft modal parameters

(which are the frequency values for each of the mode shapes), but we dont

know the mode shapes. To examine the mode shape you need to create

Animated result windows for each of the modes.


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2. Verify that Modal_Camis still selected in the Analyses and Design Studies dialog

box and click Results to enter Results mode.

3. Type MODE_1in the Name field and Mode 1in the Title field.

4. Verify that the check box for Mode1is selected in the Study Selection area of the

dialog box.

5. Verify that the Display type field is set to Fringeand that Displacement, mm, and

Magnitudeare selected in the drop-down menus on the Quantity tab.

6. Select the Display Optionstab. Select the Deformedand Animatecheck boxes.

7. Type 3in the Scaling field. Leave all other settings as they are and click OK andShowto display the results.

8. Click Copy from the main toolbar in the Results Window.


10. Select the check box for Mode2and clear all of the other check boxes in the StudySelection area of the dialog box.

11. Click OK and Showto display the results.


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14. Select the check box for Mode3and clear all of the other check boxes in the Study

Selection area of the dialog box.





Selection area of the dialog box.





Selection area of the dialog box.23. Click OK and Showto display the results.



26. Select the check box for Mode6and clear all of the other check boxes in the StudySelection area of the dialog box.


28. Review the mode shape and characteristics of each mode.

Mode 1: 1220

Hz

Mode 2: 2470

Hz

Mode 3: 3190

Hz


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Mode 4: 3200

Hz

Mode 5: 3380

Hz

Mode 6: 4980

Hz

You should be able to make the following

conclusions about the different modes:

Mode 1 at 1220 Hz, exhibits rotation along

the axis of the camshaft.

Mode 2 at 2470 Hz, exhibits translation

along the axis of the camshaft.

Mode 3 and 4 at 3200 Hz are symmetric

modes and exhibit bending at the end that

is constrained for twisting.

Mode 5 at 3400 Hz, exhibits rotation while

restricted at some intermediary supports.

Mode 6 at 4900 Hz, exhibits rotation while

restricted at intermediary supports than

those for Mode 5.

29. When you are finished reviewing the results, click File > Exit Results > Noto

exit the Result Window without saving any results.

30. Click Closefrom the Analyses and Design Studies dialog box.

Task 5.Save the model and erase it from memory.

1. Return to the Standard Pro/ENGINEER mode by clicking Applications > Standard.

2. Click Save from the main toolbar and click OKto save the model.

3. Click File > Erase > Current > Yesto erase the model from memory.


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4. If necessary, click Closefrom the Summary window and click Closeto close the

Diagnostics window.

This completes the exercise.

103 - understanding modal analyses.pdf

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