Bulk Head Bolt

Problem:

The figure illustrates the connection of a cylinder head to a pressure vessel using 10 boltsand a confined-gasket seal. The effective sealing diameter is 150 mm. Other dimensionsare A = 100, B = 200, C = 300, D = 20, and E = 25, all in millimeters. The cylinder head isused to store gas at a static pressure of 6 Mpa. ISO class 8.8 bolts with a diameter of 12mm have been selected. This provides an acceptable bolt spacing. Find the stress in thebolts after the pretension load is applied and then again after the external load is applied.In the model use At as the diameter of the bolts across the entire length of the bolt.

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

Bulk Head BoltOverview

Outcomes1) Learn how to set up the problem before starting Ansys2) Increase familiarity with the graphical user interface (GUI)3) Learn how to create and mesh more complex geometries4) Increase familiarity with post processing tools

Tutorial OverviewThis tutorial is divided into six parts:

1) Tutorial Basics2) Problem Planning3) Preprocessing4) Solution5) Post-Processing6) Hand Calculations

Anticipated time to complete this tutorial: 1 hour

AudienceThis tutorial assumes an intermediate knowledge of ANSYS 8.0; therefore, it goes intomoderate detail to explain each step. A problem planning section has been added to helpset up the problem. More advanced ANSYS 8.0 users should be able to complete thistutorial fairly quickly.

Prerequisites1) ANSYS 8.0 in house “Structural Tutorial”2) Completion of three or more Basic Machine Design Tutorials

Objectives1) Use a pretension mesh in ANSYS to apply the bolt preload2) Solve for the preload stress in the bolt3) Apply external loads to the bolt joint4) Solve for the final stress in the bolt

2

Bulk Head BoltTutorial 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.

Bulk Head BoltProblem Planning

4

Take a few minutes to plan out your solu-tion to this problem. Your plan shouldinclude some of the following items:

1) Obtain all necessary known informa-tion.

External Load per Bolt, P:At:Rt (based on At):Rbolt head:Material 1

Young’s Modulus:Poisson's Ratio:Thickness:

Material 2Young’s Modulus:Poisson's Ratio:Thickness:

Preload Force, Fi:

2) Plan a simple, representative geometry.

The entire bulk head does not needto be modeled. One of the 10 bolts will suf-fice. The Create Cylinder by Dimensionsvolume modeling tool is a good tool to cre-ate the volumes used in this problem. Youmight create half of the bolt section so thatyou can see the stresses in the bolt. Afterthe volumes are created, they can be gluedtogether to prevent separation during theanalysis.

3) Contemplate an appropriate elementfor the mesh.

Since you are creating a solid modelfrom a volume, use an element listed underthe Solid category (possibly use Solid92).

4) Plan appropriate loads and constraints.

The bolt preload will be appliedusing the pretension feature in ANSYS. Ifyou model only half of the bolt section, youcan apply symmetrical boundary con-straints to the model. If you model onlyhalf of the bolt section, you also need toapply only half of the preload force andhalf of the external load (since the area ishalf of what it normally would be).Keypoints at the bottom of the bolt can befixed to prevent rigid body displacement.

5) Identify which results will be analyzed.

You are asked to find the pretensionstress in the bolt and to find the final stressin the bolt after the external load is applied.To do this, you can select the nodes in thecenter of the bolt and then make a list of thestresses in those nodes. This can also berepeated after the external load is applied.

Bulk Head BoltPreprocessing

5

1) Change the working directory and job-name.

2) Set Preferences to Structural.

3) Add a Solid92 element.

4) Create the Material Models.

Material 1E: 207e9PRXY: .292

Material 2E: 113e9PRXY: .211

5) Set a scalar parameter for Rt.> Parameters > Scalar Parameters

R=.00518

6) Create the volumes representing the boltshaft, bolt head, nut, and material layers.

> Preprocessing > Modeling > Create> Volumes > Cylinder > By Dimensions

Volume 1: NutOuter Radius: 3*RInner Radius: 0Z1: -.0108Z2: 0Starting angle: 0Ending angle: 180

Volume 2: Bolt HeadOuter Radius: 3*RInner Radius: 0Z1: .045Z2: .0558Starting angle: 0Ending angle: 180

Bulk Head BoltPreprocessing

6

Volume 3: Bolt ShaftOuter Radius: RInner Radius: 0Z1: 0Z2: .045Starting angle: 0Ending angle: 180

Volume 4: Bottom Material LayerOuter Radius: .03Inner Radius: .0065Z1: 0Z2: .025Starting angle: 0Ending angle: 180

Volume 5: Top Material LayerOuter Radius: .03Inner Radius: .0065Z1: .025Z2: .045Starting angle: 0Ending angle: 180

Your model should appear similar to the oneshown to the right.

7) Glue all the volumes together.> Preprocessor > Modeling > Operate > Booleans > Glue> Volumes > Pick All

8) Compress the model numbering. TheNUMCMP command compresses outunused item numbers by renumbering allthe items, beginning with one and continu-ing throughout the model.

> Main Menu> Preprocessor> Numbering Ctrls > Compress Numbers > All

Replot the volumes if necessary.

Bulk Head BoltPreprocessing

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9) Mesh the volumes using the mesh tool.Make sure that smart sizing is turned off.

Under Element Attributes, choose Globaland click Set.

Set the Material number to 1 and click OK.Click Mesh.

Select the bolt shaft, bolt head, nut, and topmaterial layer volumes to be meshed.

In the MeshTool dialog box, choose Globaland click Set under Element Attributes.

Set the Material number to 2 and click OK.Click Mesh.

Select the bottom material layer to bemeshed.

Plot Elements. Your model should appearsimilar to the one shown to the right.

10) Apply the pretension. Select:> Main Menu > Preprocessor> Sections > Pretension > Pretensn Mesh > With Options> Divide at Valu > Elements in Volu

Pick the bolt shaft volume and click OK inthe picker.

Enter the following information in the dia-log box and click OK:

NAME: PretensionKCN: Global Cartesian KDIR: Z-axis VALUE: 0.0225ECOMP: elems

Bulk Head BoltPreprocessing

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11) View the pretension section. Select: > Utility Menu > Select> Comp/Assembly > Create Component

Enter Line for the Component name.Choose Lines for the Entity and click OK.Select:

> Utility Menu > Plot > Components> Selected Components

Your model should look similar to the oneshown to the right.

Bulk Head BoltSolution

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12) Change the Solution Controls for thisproblem. Select:

> Main Menu > Solution > Analysis Types > Sol'n Controls

Click on the Sol'n Options tab.

Choose Precondition CG under EquationSolvers and click OK.

13) Apply symmetric boundary conditionson areas.

> Utility Menu > Select > Entities

Choose Areas, By Location, and Y-coordi-nates and click OK.

Select > Main Menu > Solution > Define Loads > Apply > Structural> Displacement > Symmetry B.C.> On Areas.

Click Pick All.

Select > Utility Menu > Select > Entities

Make sure Areas are still selected and clickSele All. Click OK.

14) Apply a UX displacement constraint onthe middle keypoint at the top of the bolt(i.e. the top of the bolt head). Apply bothUX and UZ displacement constraints on themiddle keypoint at the bottom of the bolt(i.e. the middle keypoint at the bottom of thenut).

Bulk Head BoltSolution/Post Processing

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15) Apply the pretension load.> Main Menu > Solution > Define Loads > Apply > Structural> Pretensn Sectn

Choose the section that you named“Preload” under Pretension Sections. Enter18950 for Force (under Pretension Load) andclick OK.

16) Change the title to:Application of PSMESH - preload only

17) Solve the model.

18) Plot the nodal solution for the stresses inthe z-direction.

19) Select the nodes in the center of the bolt.To do this use the Select Entities option onthe menu bar. The Select Entites toolbarappears. Set up the toolbar to select nodesby location. Click next to Reselect at thebottom. Enter the following ranges, and hitapply after each range (hit OK after the Zrange):

X: -.006,.006Y: 0Z: .1*.045,.045-.1*.045

Plot the nodes. Your model should nowappear similar to the one shown to the right.

20) List the stress results for the selectednodes.

> General Postprocessor > List Results > Nodal Solution

Select the z-component of stress. The nodalsolution list appears. You can save this dataand import it into Excel to calculate theaverage preload stress in the bolt (should benear .449 GPa). When finished go to Selecton the menu bar and choose Everything.

Bulk Head BoltSolution/Post Processing

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21) Now apply the external load to the bolt.First, restart the solution.

> Solution > Analysis Type > Restart

22) Apply a force of 5.3e3 in the z-direction(this is half of the external load because themodel has half the area) to the keypoint cen-tered at the top of the bolt.

23) Change the title to make this solutionunique.

24) Solve.

25) Repeat steps 18-20. The stresses shouldappear similar to those in the model shownto the right.

The average stress in the bolt should bearound .473 GPa.

Bulk Head BoltHand Calculations

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External Load per Bolt

Preload Force

Preload Stress

Bolt Stiffness

Material Stiffness3 frustra

First Frustra

Second Frustra

Third Frustra

Final Stress in Bolt

kNP

eP

6.10

)66()150(.410

1 2

=

= π

GPam

kNAF

preload

preload

t

ipreload

4495.0000843.

9.372

=

=

=

σ

σ

σ

kNFeF

SAF

i

i

pti

9.37600)(0000843(.75.

75.

==

=

mMNk

ek

lEA

k

b

b

t

tb

8.387

045.)9207)(0000843(.

=

=

=

04109.2*)30tan(20

018.)012(.5.1

1

1

=+=

==

DdD

d

w

w

mMNk

eEt

4470

9207020.

1 =

==

mMNk

eEt

59040

91130025.

2 =

==

mMNk

eEt

2343

91130225.

3 =

==

mMNkm 1498=

2056.14988.3878.387

=+

=

+=

C

C

kkk

Cmb

b

GPa

eeAF

ACP

t

i

t

475.0000843.

39.370000843.

)36.10)(2056(.

=

+=

+=

σ

σ

σ