Free Vibration Analysis

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Free Vibration Analysis

• Workshop 5.1

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Workshop 5.1 - Goals

• Our goal is to investigate the vibration
  characteristics of a motor cover manufactured
  from 18 gage steel. The cover is to be fastened
  to a device operating at 1000 Hz.
• Specifically, we want to look at the effect of
  reducing the number of fasteners used in the
  cover from 5 to 4 for manufacturing efficiency.

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Workshop 5.1 - Assumptions

• The cover is meant to slip over a cylinder and be
  constrained at the bolt hole locations. To
  simulate the area contacting the cylinder the
  surface has been split (see below). We will use
  a frictionless support on this surface to
  simulate the contact area. The frictionless
  support type applies a constraint that is normal
  to the surface, thus axial and tangential
  movement is allowed while radial motion is not.

• To simulate the bolted connections a fixed
  support type will be used on the edges of the
  bolt holes.

Split Surface
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Workshop 5.1 - Start Page

• From the launcher start Simulation.
• Choose Geometry gt From File . . . and browse
  to the file Motor_Cover_5.x_t.
• When Workbench Simulation starts, close the
  Template menu by clicking the X in the corner
  of the window.
• Note Before we begin working with this model we
  want to bring in the 4 hole version of the part
  as well. This will allow us to compare results
  in real time.

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. . . Workshop 5.1 - Start Page

• Before we begin working with this model we want
  to bring in the 4 hole version of the part as
  well. This will allow us to compare results in
  real time.
• Highlight the Model branch and RMB gt Duplicate.
• With Model 2 highlighted choose Geometry gt From
  File . . . .
• Browse to the file Motor_Cover_4.x_t.

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Workshop 5.1 - Setup

• When the Workbench Simulation GUI opens choose
  the U.S. inch pound unit system.
• Units gt U.S. Customary (in, lbm, psi, F, s)

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. . . Workshop 5.1 - Setup

• The 2 models we have chosen are now contained in
  separate Model branches (Model and Model 2). To
  make the comparison more intuitive, rename the
  model branches by right clicking on each Model
  branch. Rename as
• Model 5 Hole Cover
• Model 2 4 Hole Cover

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Workshop 5.1 - Preprocessing

• Since the models consist of surface geometry we
  will be shell meshing the parts. The first
  preprocessing task is to specify the thickness
  for the parts in both branches.
• Highlight the Part 1 branch for the 5 Hole
  Cover.
• Notice the Thickness field is currently
  displayed in yellow to indicate it is undefined.
• Left click in the thickness field and set the
  thickness 0.05 in. Repeat for 4 Hole Cover.

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4
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Workshop 5.1 - Environment

• Highlight the Environment branch for the 5 Hole
  Cover.
• Select the split surface, RMB and apply the
  frictionless support (make sure you are in face
  select mode).
• Insert gt Frictionless Support

Split Surface
5
6
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. . . Workshop 5.1 - Environment

• Switch to edge select mode.
• Hold the control key and select the edges for
  each hole.
• In the graphics window RMB to apply the fixed
  support.
• RMB Insert gt Fixed Support.

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Workshop 5.1 Solution Setup

• To determine the natural frequencies for the
  models we will use the Frequency Finder tool.
• Highlight the Solution branch and insert the
  frequency finder.
• RMB Insert gt Frequency Finder

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. . . Workshop 5.1 Solution Setup

• IMPORTANT steps 6 through 9 completes the setup
  for the environment of the 5 hole cover. Before
  solving we need to setup the 4 hole cover using
  the same boundary conditions.
• Highlight the Environment branch for the 4 Hole
  Cover model.
• Repeat steps 5 to 9 for the second environment.

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Workshop 5.1 - Solution

• With the branches for each model prepared we are
  ready to solve. As a final check verify the
  status symbols next to the branches. All
  branches should have either
• Lightening bolt (ready)
• Green check mark (complete)
• Solve both models.
• Highlight the Project branch and solve both
  models
• RMB Solve
• Note solving from the Project branch causes all
  unsolved branches to be solved. Had we wished to
  solve only one branch we could have highlighted
  only that branch then solved.

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Workshop 5.1 - Results

• After both solutions are complete expand the
  Frequency Finder tools for each branch (click the
  to the left of the branch).
• Highlighting individual frequency results will
  cause a representative plot to be displayed.
• The detail for each frequency shows the modes
  frequency.

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. . . Workshop 5.1 - Results

• To get an overall view of the results we can use
  the worksheet view. Highlight one of the
  frequency finder branches and click on the
  Worksheet tab. Repeat for the other FF tool.

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Workshop 5.1 - Comments

• Remember
• Displacements reported in mode shapes do not
  reflect the actual displacements. Actual
  displacements will depend on the energy input to
  the system.