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1 /9UsingtheNXNastran Cbush SpringElement

Thereisnoversus,theonlyspringelementthatIuseistheCBUSH. Why? Takea

lookatthenotebelowwrittenbyawellrespectedNastran architect(TedRose). For

ved

examp e, youuset estan ar astran spr nge ement etweennonco nc ent

nodes,you

have

the

potential

to

suck

force

out

of

your

model;

that

is,

your

OLOAD

wontmatchyourSPCFORCEintheF06fileandyourmodelcouldbeoutofwhack.

ImadethismistakeonceandwasschooledbysenioranalystatNavistarwhenshe

1

All

RightsReser not ce t atmymo e a somestrangestresses. ns ort,t emo e wasgar age.

ThecorrectionwastosimplyswapouttheCELASforCBUSHelements.

201

TheCBUSHelementdoes accountforanyoffsetbetweenGRIDpoints,soifyouusethis,itisacceptabletohave

offsetGRIDpoints.IfyouuseCELASi elements(regularNXNastran springs),youare"addingtermsdirectlyinto

e

s ness

ma r x

an

ere

s

no

o se

e ween

e

po n s.

a

s

e

reason

or

e

recommen a on

intheCommonQ&AthattheGRIDpointsbecoincident.Mostpreprocessorsallowspringelementsbetween

noncoincident points.Inthiscase,ifthespringdoesnotactalongthelineconnectingthepoints,youhavea

"springtoground".Thespringwillnotallowrigidbodyrotation.

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2 /9UsingtheNXNastran Cbush SpringElement

Femap hasseveralgoodlittlesectionsonhowtocreatetheCBUSHelement

ved

1

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RightsReser

201

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3 /9UsingtheNXNastran Cbush SpringElement

Femap hasseveralgoodlittlesectionsonhowtocreatetheCBUSHelement

ved

1

All

RightsReser

201

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4 /9UsingtheNXNastran Cbush SpringElement

TheNXNastran ElementLibrarySection6.2givesyouthemathematicalbackground

thatyouwillneed:

ved

1

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201

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5 /9UsingtheNXNastran Cbush SpringElement

CBUSH Exam le with Solid Elements:

A hexedsphereissuspendedbyfour

ved

.

createthe

minimal

set

for

stability.

TheCBUSHshouldbetreatedlikea

1

All

RightsReser

delicatebeamelement. Eachnode

has6DOF. Whenattachedtoasolid

elementwith

3DOF,

the

rotational

201

modelwillbeunstable.

AtthefreeendoftheCBUSHelement,

therotational

DOF

are

fixed

(456)

and

theverticalDOF(2).

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6 /9UsingtheNXNastran Cbush SpringElement

CBUSH Exam le with Solid Elements:

TheCBUSHelementisselectedas

ved

.

Then,andthisisthestepthat

mostpeoplemiss,isthatone

1

All

RightsReser

needstochangetheFormulation.

Whenyou

press

the

Formulation

201 ,

whereyoucanchangethe

Nastran typeto1.CBUSH

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7 /9UsingtheNXNastran Cbush SpringElement

CBUSH Exam le with Solid Elements:

CBUSHrequiresanorientationvector

ved

.

However,the

spring

constants

can

be

setupintheCBUSHcoordinatesystem

orthatoftheOrientationCSys. For

1

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thisexample,thelatterapproachis

used.

201 ,

keeptherotationalDOFstable. Thisis

necessarywhenthe6DOFCBUSHis

attachedtoa3DOFelement.

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8 /9UsingtheNXNastran Cbush SpringElement

CBUSH Exam le with Solid Elements:

ConstrainingtheCBUSHisthebiggest

ved

.

constraintsright

you

must

use

the

constraintsetandtheCBUSHcard

together.

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WhentheCBUSHisattachedtosolid

elements,DOF123areautomatically

constrainedbut

not

DOF

456.

201

CBUSH, weneedtoconstrainall6

DOFadequately. Thetranslational

(123)arecoveredbythesolidelement

butrotational

(456)

are

problematic.

Onesolutionistofixoneend(456)

andthenusesmallrotationalvaluesin

.

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9 /9UsingtheNXNastran Cbush SpringElement

CBUSH Exam le with Plate Elements:

Thedifferencewithattachedto6DOF

ved

somethingto

anchor

to

for

all

6DOF

(123456). Todemonstratethis,the

modelwillrunwithonlythevertical

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DOFconstrained.

Themodels

used

in

this

example

are

201 .