Basic-stress in a Crank - Educating Global Leaderschasek/ME 501/ANSYS Tutorials-Blotter/ANSYS-Bas… · Stress in a Crank Tutorial Basics 3 ... Some basic ANSYS functions are:

Stress in a Crank

Problem:

The figure shows a crank loaded by a force F = 300 lb which causes twisting and bend-ing of the 3/4-in diameter shaft fixed to a support at the origin of the reference system.In actuality, the support may be an inertia which can be rotated, but for the purposes ofa strength analysis this is considered to be a statics problem. The material of the shaftAB is hot-rolled AISI 1018 steel. Find the factor of safety based on the stress state at pointA.

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

Stress in a CrankOverview

Outcomes1) Learn how to start Ansys 8.0 2) 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-Processing6) 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.

Prerequisites1) ANSYS 8.0 in house “Structural Tutorial”

Objectives1) Model the crank in ANSYS 8.02) Analyze the crank for maximum stress

2

Stress in a CrankTutorial Basics

3

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 the user needs to click on

Example: > Preprocessor > Element Type > Add/Edit/DeleteFile would mean to follow the options as shown to the right to get you to the Element Types window

Italics Text in italics are hints and notes

MB1 Click on the left mouse buttonMB2 Click on the middle mouse

buttonMB3 Click on the right mouse

button

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.

Stress in a CrankStarting Ansys

4

For this tutorial the windows version ofANSYS 8.0 will be demonstrated. The pathbelow is one example of how to accessANSYS; 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.


5

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


6

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 commandline approach to ANSYS and access multiplemenus that you can’t get to from the mainmenu.

Note: It would be beneficial to take some time and explorethese 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.


7

With ANSYS 8.0 open select> File > Change Jobname

and enter a new job name in the blank fieldof 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 > then select 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.

Stress in a CrankPreprocessing

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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 toPreprocessor.

> Main Menu > Preprocessor

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


9

As mentioned previously, the ANSYS MainMenu is designed in such a way that oneshould start at the beginning and worktowards 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 Pipe16 and Beam4 elementswill be used.

> Pipe > Elast straight16> Ok> Beam > 3D elastic 4> Ok

In the Element Types window Type 1Pipe16 should be visible signaling that theelement type has been chosen.

Close the Element Types window.> Close


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The properties for the pipe 16 element andbeam 4 element need to be chosen. This isdone by adding Real Constants.

> Preprocessor > Real Constants> Add/Edit/Delete

The Real Constants window should appear.Select add to create a new set.

> Add

The Element Type for Real Constants win-dow should appear. From this window,select Pipe16 as the element type.

> Type 1 Pipe16 > OK

The Real Constant Set Number 1, forPIPE16 window will appear. From this win-dow you can interactively customize the ele-ment type.

The problem statement indicates that theoutside diameter of the first shaft should be0.75 inches. Since the pin is a solid, thethickness of the elements should be equal tothe radius of the outside diameter (shouldbe .375 inches).

Enter the values into the table as shown tothe right.

> Ok


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Select add to create a new set.> Add

The Element Type for Real Constants win-dow should appear. From this window,select Beam4 as the element type.

> Type 2 Beam4 > Ok

The Real Constant Set Number 2, forBeam4 window will appear. From this win-dow you can interactively customize the ele-ment type.

The problem statement indicates that thecross-sectional area is .321 in2, the areamoment of inertia IZZ is .0406 in4 and theArea moment of inertia IYY is .00016 in4.The thickness along the z and y axis alsoneed to be specified.

Enter the values in to the table as shown tothe right.

> Ok


12

Select add to create a new set.> Add

The Element Type for Real Constants win-dow should appear. From this window,select Pipe16 as the element type.

> Type 1 Pipe16 > Ok

The Real Constant Set Number 3, forPIPE16 window will appear. From this win-dow you can interactively customize the ele-ment type.

The problem statement indicates that theoutside diameter of the first shaft should be0.5 inches. Since the pin is a solid, the thick-ness of the elements should be equal to theradius of the outside diameter (should be.25 inches).

Enter the values in to the table as shown tothe right.

> Ok

Close the window.> Close


13

The material properties for the Pipe16 andBeam4 elements now 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 isotropic linearly elasticstructural 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 Behaviorwindow.

> Material > Exit


14

The next step is to define the keypoints(KP’s) 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 KP’s will begiven numbers and their respective (XYZ)coordinates.

Enter the KP numbers and coordinates forthe pin definition. Select Apply after eachKP has been defined.

Note: Be sure to change the keypoint number every timeyou click apply to finish adding a keypoint. If you don’t itwill just move the last keypoint you entered to the newcoordinates you just entered.

KP # 1: X=0, Y=0, Z=0KP # 2: X=0, Y=0, Z=5KP # 3: X=4, Y=0, Z=5KP # 4: X=4, Y=0, Z=6

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 KP’s and select Ok.

The created KP’s should look similar to theexample to the right except the KP’s couldbe labeled with the KP numbers.


15

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 theKP’s.

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

The Create Straight Lines window shouldappear. You will create 3 lines. Create line 1between the first two keypoints.

For line 1: MB1 KP1 then MB1 KP 2.

The other lines will be created in a similarmanner.

For line 2: MB1 KP2 then MB1 KP 3.For line 3: MB1 KP3 then MB1 KP 4.

Verify that each line only goes between thespecified 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


16

Now that the model has been created, itneeds to be meshed. Only meshed modelscan be run to find a solution. Models aremeshed with elements.

First, the element size will be specified.> Preprocessor > Meshing > Size Cntrls > Manual Size > Lines > All Lines

The Element Sizes on All Selected Lineswindow should appear. From this windowthe number of elements per lines segmentcan be defined along with the element edgelength.

Approximately 20 elements along thelength of each line will produce reasonableresults.

Enter 20 into the No. of element divisionsfield

> Ok

Note: you could change the number of element divisionsafter completing the tutorial to a different value and rerunthe solution to see how it affects the results.

With the mesh parameters complete thelines representing the crank can now bemeshed. First assign the correspondingattributes to each line.

> Preprocessor > Meshing > Mesh Attributes > Picked Lines

The Line Attributes window appears. Selectthe line created between KP1 & KP2 andclick OK.

Select 1 for the Real constant set number and1 PIPE16 for the Element type number.

> Ok

Repeat for the other two lines created.

Stress in a CrankSolution

17

For line 2, set Real constant set number to 2and 2 BEAM4 for the element type.

For line3, set Real constant set number to 3and 1 PIPE16 for the element type.

You are now ready to mesh.> Preprocessor > Meshing > Mesh > Lines

From the Mesh Lines window select:> Pick all

This will select all the line segments andmesh them all at the same time.

The meshed line should appear similar tothe image shown to the right.

We will now move into the solution phase.

Before applying the loads and constraints tothe Crank, we will select to start a newanalysis

> Solution > Analysis Type > New Analysis

For type of analysis select static and selectOk.

The constraints will now be added.

For this problem the first key point createdneed to be constrained in all six degrees offreedom.

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

Select KP 1 in the graphics window.> Ok


18

The Apply U, ROT on KP’s large windowshould appear. From this window thedegrees of freedom can be specified.

To the right of DOFs to be constrainedselect ALL DOF.

> Apply.

The constraints now appear at keypoint 1.

The loads will now be applied to the crank.> Solutions > Define Loads > Apply> Structural > Force/Moment > On Keypoints

The Apply F/M on KP’s window shouldappear.

Select KP 4 in the graphics window.> Apply


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The expanded Apply F/M on KP’s windowshould appear. From this window the direc-tion of the force and magnitude can be spec-ified.

Select FY for the Direction of force/mom.Select Constant value for Apply as.

Enter -300 in the Force/moment value fieldwhich will apply a 300 lb force downward.

Verify that all the fields match those of thefigure shown to the right.

> Ok

The fully loaded and constrained modelshould appear similar to the picture shownbelow.


20

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 com-plete a Note window should appear thatstates the analysis is complete.

Close both the Note window and /STATUSCommand window.

Turn on a 3D picture of your model.> Plot Controls > Style > Size and Shape

The Size and Shape window opens. Clickthe check box next to Display of element toturn on the 3D image.

> OK

Stress in a CrankPost Processing

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Results are viewed by using post processingcommands.

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

From the problem statement we will esti-mate the maximum stress due to the forceon KP 4. To obtain the stress, select Nodalsolution, Stress and Von Mises stress.

> Nodal solution > Stress> Von Mises stress

Plot the results.

The max von mises stress valueis shown to be 50,183.

One way to find the factor ofsafety is to divide the yieldstrength by the max von misesstress.

The factor of safety is thenSy/Svon mises= n.

32,000(AISI 1018 steel) / 50183 =0.64

Stress in a CrankPost Processing

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You can also view the X-Component ofstress and the 1 st Principal stress and com-pare them to the hand calculations shownon the next page.

X Component of Stress

1st Principle Stress

Stress in a CrankHand Calculations

23

X Component of Stress

IMc

x =σ

1800)6(300 ==M lb-in.

015.064

4

== dI π in.4

375.02

== dc in.

459,43015.0

)375.0)(1800(==xσ psi

XY Shear Stress

JTc

xy =τ

031.0)2)(015.0(2 ==×= IJ in.4

375.02

== dc in.

486,14031.0

)375.0)(1200(==xyτ psi

Von Mises 22 *3 xyxVonmises τσσ +=

psionmises 8.50181486,14*3459,43 22V =+=σ

Factor of Safety

64.08.50181

32000 ===VonMises

ySn

σ

1st Principle Stress

psixyx 846,47486,14

2459,43

21 =+=+= τσσ