10 Basic-Latch Spring

8/8/2019 10 Basic-Latch Spring

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Problem:

Shown in the figure is a 12-gauge (0.1094 in) by 3/4 in latching spring which supports aload of F = 3 lb. The inside radius of the bend is 1/8 in. Estimate the stresses at the innerand outer surfaces at the critical section.

Joseph Shigley and Charles Mischke. Mechanical Engineering Design5th ed. New York: McGraw Hill, May 2002.


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Overview

Outcomes1) Learn how to start Ansys 8.02) Gain familiarity with the graphical user interface (GUI)3) Learn how to create and mesh a simple geometry4) Learn how to apply boundary constraints and solve problems

Tutorial OverviewThis tutorial is divided into six parts:

1) Tutorial Basics2) Starting Ansys3) Preprocessing4) Solution5) Post-Processing

6) Hand Calculations

Anticipated time to complete this tutorial: 1 hour

AudienceThis tutorial assumes minimal knowledge of ANSYS 8.0; therefore, it goes into moderatedetail to explain each step. More advanced ANSYS 8.0 users should be able to completethis tutorial fairly quickly.

Prerequisites

1) ANSYS 8.0 in house Structural Tutorial

Objectives1) Model the the latch spring in ANSYS 8.02) Analyze the latch spring for the stresses at the inner and outer surfaces of the

critical section

2


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Tutorial Basics

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In this tutorial:

Instructions appear on the left.

Visual aids corresponding to the textappear on the right.

All commands on the toolbars arelabeled. However, only operationsapplicable to the tutorial are explained.

The instructions should be used as follows:

Bold > Text in bold are buttons,

options, or selections that theuser needs to click on

Example: > Preprocessor > ElementType > Add/Edit/DeleteFilewould mean to follow theoptions as shown to the rightto get you to the ElementTypes window

Italics Text in italics are hints andnotesMB1 Click on the left mouse button

MB2 Click on the middle mousebutton

MB3 Click on the right mousebutton

Some basic ANSYS functions are:

To rotate the models use Ctrl and MB3.

To zoom use Ctrl and MB2 and move themouse up and down.

To translate the models use Ctrl and MB1.


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Starting Ansys

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For this tutorial the windows version ofANSYS 8.0 will be demonstrated. The path below is one example of how to access

ANSYS; however, this path will not be thesame on all computers.

For Windows XP start ANSYS by eitherusing:

> Start > All Programs > ANSYS 8.0> ANSYSor the desktop icon (right) if present.

Note: The path to start ANSYS 8.0 may be different foreach computer. Check with your local network manager tofind out how to start ANSYS 8.0.


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Starting Ansys

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Once ANSYS 8.0 is loaded, two separatewindows appear: the main ANSYSAdvanced Utility Window and the ANSYS

Output Window.

The ANSYS Advanced Utility Window,also known as the Graphical User Interface(GUI), is the location where all the userinterface takes place.

The Output Window documents all actionstaken, displays errors, and solver status.

Graphical User Interface

Output Window


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Starting Ansys

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The main utility window can be broken upinto three areas. A short explanation of eachwill be given.

First is the Utility Toolbar:

From this toolbar you can use the command

line approach to ANSYS and access multiplemenus that you cant get to from the mainmenu.

Note: It would be beneficial to take some time and explore

these pull down menus and familiarize yourself with them.

Second is the ANSYS Main Menu as shownto the right. This menu is designed to use atop down approach and contains all thesteps and options necessary to properly pre-

process, solve, and postprocess a model.

Third is the Graphical Interface windowwhere all geometry, boundary conditions,

and results are displayed.

The tool bar located on the right hand sidehas all the visual orientation tools that areneeded to manipulate your model.


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Starting Ansys

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With ANSYS 8.0 open select> File > Change Jobname

and enter a new job name in the blank field

of the change jobname window.

Enter the problem title for this tutorial.> Ok

In order to know where all the output filesfrom ANSYS will be placed, the workingdirectory must be set in order to avoid usingthe default folder: C:\Documents andSettings.

> File > Change Directory > thenselect the location that you wantall of the ANSYS files to be saved.

Be sure to change the working directory atthe beginning of every problem.

With the jobname and directory set theANSYS database (.db) file can be given atitle. Following the same steps as you didto change the jobname and the directory,

give the model a title.


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Preprocessing

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To begin the analysis, a preference needs tobe set.

> Main Menu > Preferences

Place a check mark next to the Structuralbox. This determines the type of analysis tobe performed in ANSYS.

> Ok

The ANSYS Main Menu should now beopened. Click once on the + sign next to

Preprocessor.> Main Menu > Preprocessor

The Preprocessor options currently avail-able are displayed in the expansion of theMain Menu tree as shown to the right.


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Preprocessing

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As mentioned previously, the ANSYS MainMenu is designed in such a way that oneshould start at the beginning and work

towards the bottom of the menu in prepar-ing, solving, and analyzing your model.

Note: This procedure will be shown throughout the tuto-rial.

Select the + next to Element Type or clickon Element Type. The extension of themenu is shown to the right.

> Element Type

Select Add/Edit/Delete and the ElementType window appears. Select add and theLibrary of Element Types window appears.

> ADD/EDIT/DELETE > Add

In this window, select the types of elementsto be defined and used for this problem.

For this model Tet 10node 187 elements willbe used.

> Solid > Tet 10node 187> Ok

In the Element Types window Type 1

Solid187 should be visible signaling that theelement type has been chosen.

Close the Element Types window.> Close


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Preprocessing

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The properties for the Solid187 elementsneed to be chosen. No real constants need tobe defined, but material properties do.

The material properties for the Solid187 ele-ments need to be defined.

> Preprocessor > Material Props> Material Models

The Define Material Models Behavior win-dow should now be open.

This window has many different possibili-

ties for defining the materials for yourmodel. We will use set the isotropic linearlyelastic structural properties.

Select the following from the MaterialModels Available window:

> Structural > Linear > Elastic> Isotropic

The window titled Linear IsotropicProperties for Material Number 1 nowappears. This window is the entry point forthe material properties to be used for themodel.

Enter 30e6 (30 Mpsi) in for EX (Young'sModulus) and 0.3 for PRXY (Poisson'sRatio).

> Ok

Close the Define Material Model Behavior

window.> Material > Exit


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Preprocessing

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The next step is to define the keypoints(KPs) where loads and constraints will beapplied:

> Preprocessor > Modeling> Create > Keypoints > In Active CS

The Create Keypoints in Active CS win-dow will now appear. Here the KPs will begiven numbers and their respective (XYZ)coordinates.

Enter the KP numbers and coordinates forthe pin definition. Select Apply after each

KP has been defined.

Note: Be sure to change the keypoint number every timeyou click apply to finish adding a keypoint. If you dont itwill just move the last keypoint you entered to the new

coordinates you just entered.

KP # 1: X=0, Y=0, Z=0KP # 2: X=0.1094, Y=0, Z=0KP # 3: X=0, Y=0, Z=0.75KP # 4: X=0.1094, Y=0, Z=0.75

KP # 5: X=0, Y=1.25, Z=0.75KP # 6: X=-0.125, Y=1.375, Z=0.75KP # 7: X=-4.125, Y=1.375, Z=0.75

Select Ok when complete.

In the case that a mistake was made in creat-ing the keypoints, select:

> Preprocessor > Modeling> Delete > Keypoints

Select the inappropriate KPs and select OK.

The created KPs should look similar to theexample to the right (note: the window isrotated slightly).


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Preprocessing

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At times it will be helpful to turn on the key-point numbers.

> PlotCtrls > Numbering > put a

checkmark next to keypointnumbers > Ok

Other numbers (for lines, areas, etc..) can beturned on in a similar manner.

The next step is to create lines between theKPs.

> Preprocessor > Modeling> Create > Lines > Lines> Straight Lines

The Create Straight Lines window shouldappear. You will create 5 lines. Create line 1between the first two keypoints.

For line 1: MB1 KP 1 then MB1 KP 2.

The other lines will be created in a similarmanner. Rotate the screen if needed to aidin creating the lines.

For line 2: MB1 KP 1 then MB1 KP 3.For line 3: MB1 KP 3 then MB1 KP 4.For line 4: MB1 KP 4 then MB1 KP 2.For line 5: MB1 KP 3 then MB1 KP 5.For line 5: MB1 KP 6 then MB1 KP 7.

Verify that each line only goes between the

specified keypoints. When you are donecreating the lines click ok in the CreateStraight Lines window.

> Ok

If you make a mistake, use the following todelete the lines:

> Preprocessor> Modeling> Delete > Lines Only


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Preprocessing

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If while working, the geometry you createddisappears select from the Utility Toolbar.

> Plot > Multi-Plots

Other items (lines, areas, volumes, elements,keypoints, nodes..) can be plotted in thewindow in a similar manner.

An arc needs to be created between KP 6and KP 5.

> Preprocessor > Modeling> Create > Lines > Arcs> By End KPs & Rad

Select KP 6 and KP 5 for the start and endingkeypoints by using MB1.

> Ok

Select KP 3 as the reference for the center ofcurvature side.

> Ok

Type 0.125 for the radius of the arc in theArc by End KPs & Radius box that is nowdisplayed.

> Ok

Rotate the model so that all of the lines canbe seen.


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Preprocessing

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An area will now be created that can beextruded to finish up the geometry.

> Preprocessor > Modeling

> Create > Areas > Arbitrary> By Lines

Select the four lines at that the bottom of thescreen that create a rectangular area. (Thefour lines are near the origin marker.)

> Ok

The new area is now filled in as shown tothe right.

This area will now be extruded along theother three lines.

> Preprocessor > Modeling> Operate > Extrude > Areas> Along Lines

Select the area just created.> Ok (In the Sweep along lines box)

Select the three lines not used to create thearea.

> OK

The geometry is now complete.


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Preprocessing

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Before the model can be meshed for solving,a hard point will be added so that the forcecan be applied to the middle of the latch.

> Preprocessor > Modeling> Create > Keypoints> Hard PT on line > Hard PT by ratio

Select the top line at the end of the latchwhere the force will be applied.

> Ok (In the Hard PT by ratiowindow)

In the Create Hard PT by Ratio window,

enter 0.5 as the length ratio.> Ok

The model will be meshed by first setting asize control for the elements and then mesh-ing the geometry.

> Preprocessor > Meshing> Size Cntrls > ManualSize > Global> Size

In the Global Element Sizes window setthe element edge length to 0.05 and leavethe No. of element divisions set at zero.

> Ok

To mesh the model.> Preprocessor > Meshing > Mesh> Volume > Free

In the Mesh Volumes window select PickAll.

> Pick all

The model is meshed and ready for con-straints and the load to be added.


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Solution

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We will now move into the solution phase.

Before applying the loads and constraints to

the latch, we will select to start a new analy-sis:

> Solution > Analysis Type> New Analysis

For type of analysis select Static and selectOk.

The constraints will now be added.

For this problem, constraints must be addedto fix the latch as it would be if it wereattached to a wall with two bolts.

To apply constraints select:> Solution > Define Loads > Apply> Structural > Displacement> On Areas

Select the area that represents where

the latch would be attached to the wallas shown in purple below.

> Ok


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Solution

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In the Apply U,ROT on Areas windowselect All DOF.

> Ok

The 3 lb load will now be added.

> Solutions > Define Loads > Apply> Structural > Force/Moment> On Keypoints

Select the hard point that was previouslycreated on the top of the latch at its end.

> Ok

In the Apply F/M on KPs window select FYas the Direction of force/mom. Type -3.0 forthe Force/moment value.

> Ok

The model is ready to be solved.


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Solution

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The next step in completion of the tutorial isto solve the current load step that has beencreated. Select:

> Solution > Solve > Current LS

The Solve Current Load Step window willappear. To begin the analysis select Ok.

The analysis should begin and when thesolution is done a Note window should

appear that states the analysis is complete.

Note: Depending on the speed of your computer,it may be several minutes before the solution iscomplete.

Close both the Note window and/STATUSCommand window.


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Post Processing

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Results are viewed by using post processingcommands.

From the ANSYS Main Menu select:> General Postproc > Results Viewer

In the Results Viewer select the down arrownext to Choose a result item and select:

> Nodal Solution > Stress> Von Mises Stress

MB1 the Plot Results button to see theresults for the Von Mises Stresses.

Use the query tool to find the stresses at theinner and outer radius. If you hold downMB1 with the query tool active and move itover the part you can see values in manylocations quickly.

The highest values for the inner radius(shown in red) range from 10,000 to 10,500psi, with an average value of 10,250 psi.

The highest stress values range for theouter radius range from about 6,300 to6,600 psi (excluding the edges).

The values are in the ballpark of the closedform solution shown on the next page. Thismodel assumes that the part of the latchthat touches the wall does not take any ofthe stress (its dark blue color indicates basi-cally no stress). In real life, this section

would take some of the stress as it bendsaway from the wall. A more challengingand accurate model would have includedthe bolt holes and the latches attachment tothe wall with these bolts. In FEA, it is agood idea to make assumptions to simplifythe model, and then, if they adverselyaffect the solution, you can always go backand include them in the model.


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Hand Calculations

ro = radius of outer fiber

ri = radius of inner fiber

h= depth of section

co = distance from neutral axis to outer fiberci = distance from neutral axis to inner fiber

rn = radius of neutral axis

R= radius of centroidal axis

e = distance from centroidal axis to neutral axis

A = area of cross section

M = moment

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