Tribology in Mech eng_Sierros_WVU_Lec 15_Fall2010

8/8/2019 Tribology in Mech eng_Sierros_WVU_Lec 15_Fall2010

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TribologyinMechanicalEngineeringMAE493N/593T

Dr.Konstantinos

A.

Sierros

WestVirginiaUniversity

Mechanical&AerospaceEngineering

ESBAnnex

263

[email protected]


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Bearings

HydrostaticbearingsIntroduction

Parallelpocketthrustbearings

Bearingdesigns

Velocity

effects

in

hydrostatic

thrust

bearings

Hydrostaticjournalbearings

Materialsselectionanddesignconsiderations


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Hydrostaticbearings

Theloaded

surfaces

of

ahydrostatic

bearing

are

separated

by

afluid

film

which

is

forcedbetweenthembyanexternallygeneratedpressure

Pumpoperatescontinuouslyforsuccessfuloperationandmaintenanceofthe

film Filmformationdoesnotdependonrelativemotionofsurfaces(hydrostatic) Applicationsincludemachineelements


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Hydrostaticbearings

Parallelsurfaces:Padandrunner(Theydonotcomeindirectcontact)

Requireexternalpressurization

Filmis550micronsthick

Tohandleasymmetricloads,bearingsemployseveral

evenlyspacedpads


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Hydrostaticbearings Bearingissuppliedwithfluidunderpressureps

Before

entering

the

recess,

fluid

passes

through

a

restrictor/compensator

and

pressuredropstopr Fluidpassesoutofthebearingthroughthicknesshbetweenbearinglandandopposingbearingsurface(slider/bearingrunner)

Depthof

pocket

>>

gap

h

Restrictorallowspocketpressurepr

tobedifferentthanps

andthisdifference

dependsonloadW

IfWisverylarge,thenhmayfalltozero(noflowthroughbearing)andpr

is

equalto

ps

IfWiszero(noload),hbecomesverylargeandtheonlyresistancetoflowisofferedbytherestrictor


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Hydrostaticbearings

Pumpdraws

fluid

from

areservoir

through

acoarse

filter

or

strainer

Fluidpassesthroughabearingcompensatorviaalinefilterat

pressureps

Asecondfilterallowsdebristobeflushedoutofthesystem


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Hydrostaticbearings

Hydrostaticbearings

are

used

when;

Normalloadsarehigh

Veryprecisepositionalcontrolisrequired

Inacorrectly

designed

hydrostatic

bearing,

surfaces

never

come

into

contact

Therefore,thereshouldbenowearanddamage Accuracycanbemaintainedforthewholelifetimeofthecomponent

Stiffnessand

thus

vibration

characteristics

can

be

controlled

Disadvantagesincludelargesizeandcost Also,externalpumpsmustruncontinuously(safety)

Fluidisliquid(hydrocarbonoil)orgas(air) Aerostaticbearingaremoreexpensivesincehigheraccuracyand

tolerancesare

required


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Relationship

Loadcarrying

capacity

ofbearing

Bearingperformance

parameters

(stiffness,flowrequirements)

Bearingdimensions

Physicalpropertiesofpressurizedfilm

Pocket:The

portion

of

acage

shaped

to

hold

the

ball

or

roller

Thrustbearing:Abearingdesignedprimarilyforthrustloads

Thrustload: Loadexertedparalleltotheaxisoftheshaftonwhichthe

bearingismounted


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Thrustbearing

1.2c;athrust

bearing

allows

rotation

of

the

shaft

and

sustains

aload

parallelwiththeaxisofrotation


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1Dpressureinducedflow

Considerfluid

flow

between

two

stationary

parallel

plates

separated

by

aviscousfilmofthicknessh

PlatesareindefinitelylongintheOy

direction

FlowisparalleltoOxaxisandismaintainedbypressurep(lefthandend)


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Laminarflow

http://www.youtube.com/watch?v=nl75BGg9qdA&NR=1

Laminarflowoccurswhenafluidflowsinparallellayers,withnodisruptionbetweenthelayers.
http://www.youtube.com/watch?v=nl75BGg9qdA&NR=1http://www.youtube.com/watch?v=nl75BGg9qdA&NR=1


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Gaphis

small

compared

to

length

in

Ox

direction

and

fluid

flow

is

laminar

Insmallbearingstheaboverequirementiseasilysatisfied Inverylargehydrostaticbearingssignificanteffectsofturbulencewithinthefilmcanbeobserved


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Volumetricflow

rate

q perunitwidthofthebearing(assumingfluidisincompressible)

dx

dp

n

hudzq

h

==0

3

12

velocitydistribution

in

Ox

direction

viscosity


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Fullydeveloped

pipe

flow

is

described

below;

dx

dp

n

bQ

8

4=

Total

volume

flow

rate

Radiusofcircularpassage


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Singlecircularpadbearing


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Outerradius

ro

and

central

recess

radius

ri

Atr(ri


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Thefall

in

fluid

pressure

across

the

land

is

shown

in

the

figure

below

ThetotalloadWcarriedbythebearingisgivenby;

[ ] drdr

dprW

oo

rr

rp =0

2

0

2


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)/ln(2

)/(12

2

io

oi

rorr

rr

pr

W =


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Loadcarrying

capacity

and

associated

flow

rate

for

ahydrostatic

bearing

can

be

expressedinnondimensionalterms,W*

andQ*

)(*

rppressurepocketxApadofareaWW =

)/(3*

nphQQ r=

W*

andQ*

dependonlyonshapeofbearing;forasinglecircularpadwehave;

)ln(2

)(12

*

io

oi

rr

rrW

=

)/ln(6

1*

io rrnQ =


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Thevariationsinthepreviousquantitieswiththeratioofthe

outertoinnerland

radiiro

/ri

are

shown

below;


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Bearingoptimization

Criteriaof

design

optimization

of

hydrostatic

bearings

Maximizationofloadcarriedforagivenflowrate

Maximizationofbearingstiffnessforagivensupplypressure

Costof

running

abearing

depends

on

the

pumping

power

required tosupplyit

ThepowersuppliedtothebearingHpisrelatedtothesupplypressureandthe

flowrate;

QpH sp = Anefficientdesignshouldminimizethepowerrequiredperunit

loadsupported

Hp/W;

=2*

*

2

32

W

Q

nA

Wh

W

Hp


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Practicalbearingdesigns

Multirecess

bearings

Multiplepocketconfigurationsforimproved resistancetotippingeffects

Opposedpaddesigns


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Velocityeffectsinhydrostaticthrustbearings

Squeezefilm

effects

Generalvelocitymotionofthebearing

Normalcomponent

ofmotionParallelcomponent

ofmotion

Normalvelocities

occur

when

the

load

on

the

bearing

varies

with timeandthesubsequenteffectsareknownassqueezefilmphenomena


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Slidingeffects

Consideracomponentofrelativeslidingvelocitybetweenthesurfacesofthebaring


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Slidingeffects

MaintenanceofthemotionofthelowersliderrequirestheapplicationofahorizontalforceF

h

nUA

F

1areaoftheland


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Slidingeffects

Thefrictionalpowerabsorbedbythebearingisgivenby;

h

AnU

UFHf1

2

==


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Hydrostaticsupport

of

circular

shafts

(journals)

both

stationary

and

rotating

DifferencesfromthrustbearingsFilmsthicknessaroundbearingisnotuniform

Shaftrotationoftenresultsinhighrelativeslidingvelocities

thathavesignificant

effectson

pressure

distribution

Stillhighloadcarryingcapacity Verylowfriction

d l b


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Betweenfourandeightpocketsaroundthecircumferenceofthebearing

Eachpocket

is

controlled

by

its

own

restrictor

Therefore,eachrecessandthesurroundinglandactasanindividualthrustpadWhenjournalrotates,viscousdragleadstoadditionalcircumferentialflowfromonerecesstoanotherwhichhelpsbuildupthesupportingpressureunderload

H d i j l b i


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Defininganon

dimensional

load

W*

LDp

WW

r

=*

DefininganondimensionalflowrateQ*

DLQ

/

3/2* =

H d t ti j l b i


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Relationshipof

Q*

and

L/D

M t i l l ti d th d i id ti


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Materialselectionandotherdesignconsiderations

Thechoice

of

materials

in

hydrostatic

bearings

is

not

as

critical

as

in

other

tribologicalsystems

Innormaloperationthereisnodirectcontactbetweenthebearingsurfaces

M t i l l ti d th d i id ti


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Materialselectionandotherdesignconsiderations

Movementwith

power

disconnected

Materialsshouldbechosentominimizedamageinthiscase Notusinglike

onlike

materials

Highpressures

or

temperatures

Softbearingmaterialsmayextrudeorcreepundertheinfluence

ofhigh

temperaturesandpressures

Thermalexpansionofbearingcomponentscanbeanissueespeciallyinhotgas

lubricationcases

Corrosivelubricants Bearingsurfacesshouldbechemicallystabletothelubricantand/oritsadditives

andcontaminants

Collectionandreturnoftheleakagefluid Preventandcontroltheingressofdirtandforeignsolidcontaminants Preventlossoflubricanttooutsideworld(environmental)usingadequatesealing

Summary


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Summary

HydrostaticbearingsIntroduction


Bearingdesigns

Velocity

effects

in

hydrostatic

thrust

bearings


Materialsselectionanddesignconsiderations

Documents

Tribology in Mech eng_Sierros_WVU_Lec 15_Fall2010