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Factors To Consider When Choosing

An Actuator of Four Feet Or Less

Whetherforpackaging,lifesciences,ofceequipmentorfactoryautomation,smoothmotion,fastac-

celerationsandahighdegreeofaccuracyarehallmarkrequirementsforlinearmovementactuators.Increas -

ingly,miniaturizationhasaddedtheadditionalrequirementofcompactness.

Onesystemthatcanmeetalloftheaboverequirementsinitsconversionofmotortorquetolinearthrustis

thesingleaxisballscrewactuator.Ballscrewsconvertrotarymotiontolinearmotion,ortorquetothrust,and

viceversa.Theballscrewactuatorisacombinationofaballscrewonwhich,toeliminatebacklash,aslide

blockornutsmovementisguidedbyrecirculatingsteelballsthatrollbetweentheblockandracewaysof

guiderails.

What Are The Advantages Of A System Solution?

Traditionally,manyequipmentmanufacturersdesignedandassembledtheirown,custom,ballscrewactuators.Infact,manysemi-conductormanufacturersandmedicalequipmentmanufacturersstilldo.To

designcustomstages,engineershavetodesigntheballscrewsize,i.e.,ndtherightballscrew,carriageand

guiderailsthethreemaincomponents.Theydalsohavetohavesupportsfortheballscrewonbothends

andmotorbrackets.Aftermountingallthecomponentsadjustmentshavetobemade-repeatedly.

ConventionalComponentsOff-The-ShelfBallScrewActuatorSystem

Manycompanieshaverecognizedtheadvantagesofpurchasinganoff-the-shelfsystemsolutionas

opposedtotakingcomponentsandbuildingtheirownsystem.Typically,ballscrewactuatorsystemscome

inatleast5differentstagesizes.Theyhaveseveraltravellengthoptionsandthereareotheroptionsalready

designed.Oncetheengineeridentiestheoptimumsystem,heorshejusthastogettherightmotor.

However,tochoosetherightsystemforanapplication,actuatorsystemsmustbecarefullyanalyzed.

Comparisonsofthedesignandsizingofcomponentssuchastheslideblock,raceway,bearing,guiderail,ball-

screw,nutandhousingmaterialsarecritical.Theyallfactorintoactuatorperformance.

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How A System Solution Affects Compactness

Whenaballscrewactuatorisdesignedintoaproductthatprecludestheuseofoff-the-shelfsystems,

acustomdesignedandassembledsystemmaybetheonlyanswer.However,choosingtoassembleballscrew

actuatorcomponentssuchasalinearguideandaballscrewintoacustomhousingusuallycreatesalargerunit

overallthanusingapre-designedsystem.Forone,apre-designedsystemismuchmorecompactbecausethe

guiderailoftheactuatorisintegratedwiththestructureoftheactuatorandtheslideblockhastheballscrewnut

incorporatedintoit.Generally,ifyouweretobuildanactuatorfromassortedcomponents,youwouldhaveto

haveahousingtoputaballscrewnutinto.Also,therewouldbeaseparatebaseforlinearguides.So,theentire

unitwouldbemuchlarger-asmuchasthirtypercentlarger.

Factors Affecting The Choice of Actuators

Tochoosethemosteffectiveactuatorforaparticularapplication,criticalinformationmustrstbeas-

certained.Factorssuchasloadcapacity,operationspeed,strokelength,environment,orientation,andpositional

accuracyhavetobeidentiedandquantied.

Assumingthesefactorshavebeenascertained,wearenowgoingtoconsidertheaffectsofcomponentdesign

differencesontheoperationofballscrewactuatorsinthefourfootandunderclass.

How Component Design Affects Load Capacity

Inadditiontoballscrewandguiderailsize,loadcapacitydependsonthesizeoftherecirculatingsteelballsthatrollbetweentheblockandtheracewaysintheguiderails,aswellasthenumberofballsincontact

withtheracewaysandthemannerinwhichtheymakecontact.Onewaytomeetloadcapacityisbyincreasing

thesizeoftheballscrewandguiderails.Anotherwaytoincreaseloadcapacity,whichdoesnotincreasethe

overallsizeoftheactuator,istoincreasetheballcircuits.Moststandardballscrewactuatorshaveonesetof

recirculatingballsoneithersideoftheblock(seeFigure1).

Load Capacity Increases With More Ball Circuits

2 Sets of Recirculating Balls On Either Side Of The Block

Doublingthenumberofballcircuitstotwooneithersideoftheblockdoublestheloadcapacityofthe

actuator.Towit,sincewearediscussingactuatorsofuptoaboutfourfootrailtravel,anexampleofmaximum

loadforastandard1,380mmraillengthwithtwoballcircuitsis37kilonewton.Withfourballcircuitsitis74

kilonewton.

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Onthelowendoftheactuatorsizerange,anactuatorwitha100mmlengthrailwithtwoballcircuits

canbeexpectedtohavealoadcapacityof3.945kilonewtonwhereaswithfourballcircuitsitsloadis7.89kilo

newton.

How Component Design Affects Precision

Actuatorsystemsaregenerallyofferedintwoorthreegrades-orlevelsofaccuracy.Commercial

Gradeisthelowest;nextisHighorStandardGrade;PrecisionGradeisthehighest.Tocomparesys -

temslevelsofaccuracy,onecannotassumethatallmanufacturerslowesttohighestgradeshavecomparableaccuracies.Itisnecessarytocomparetheirpublishedrangesfor:PositioningRepeatability;PositioningAccu -

racy;RunningParallelism;Backlash;andStartingTorque.

Aspectsofalinearactuatorthataffectitsprecisionincludehowtrueitsguiderailanditsracewaysare

andhowsmoothlyintheblockandracewaystheballsrecirculate.Asthetravelswerediscussingarefourfeet

andunder,theslightestdeectionorclearanceoftherecirculatingballscansignicantlyaffectaccuratemove -

mentandpositioning.

Inthissizerange,foroptimumaccuracy,itiscriticalthattheguiderailbeprecisionground.Thesame

canbesaidoftheslideblockandballscrewitself.Furthermore,toinsurepositionalaccuracy,theballswithin

theballgroovesoftheracewaysmustnothaveclearancethatallowsthemtodeect.Ofthegroovedesignson

themarket,thestandardchoiceisbetweenballsthatmakecontactwiththeracewaygroovesattwopointsoratfourpoints(seeFigure2).Aslightlyellipticalgroovedesignallowstheballstomakecontactattwoopposing

pointsbutallowsabitofclearanceontheballssidesthatareperpendiculartothecontactpoints.The4-point

contactarchdesignis,becauseofitsshape,calledagothicarch.Thegothicarcheliminatesanyclearancethat

couldleadtodeectionandis,therefore,bestsuitedforapplicationsrequiringmaximumprecision.

Ball Arch Designs Affect Precision

How Component Design Affects Rigidity

Ballscrewactuatorrigidityisaffected,primarily,bythecompositionoftheguiderail.Asthisisthe

outerstructureofthesystem,itistheactuatorssupportanditsrigiditydetermineshowconsistentlytruethe

groovesoftheracewaysare.Thethicknessandstrengthoftheloweredgesoftheguiderailarecriticaltoits

rigidity.AU-shapedouterrailprovidesbetterrigidityagainstmomentloads.

Guiderailspositionedlowerthantheballscrewcenteralsoincreaserailrigidity(Figure3).Whenthe

recirculatingballsgroovesareclosertothebottomoftherail,theblockcancarryheavierloads.Incombination

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withthemorerigidU-shapedstylerail,becausethereslessdeformationandbetteraccuracy,thisdesigneven

allowsone-endsupportedapplications.Anotheradvantagetoguiderailspositionedlowerthantheballscrew

centerisgreatercompactness.

Lower Ball Grooves Increase Rigidity

Alsoaffectingrigidityisthenumberofballcircuits.Fourballcircuitsprovidegreaterrigiditythantwo

ballcircuitsallthingsbeingequal(ball,guiderail,block,ballscrew).

How Application Speed Affects Actuator Sizing

Dependingontheapplication,thespeedatwhichtheactuatormusttravelisadeterminantofthelength

oftheballscrewlead.Thefasterthedesiredtraveltime,thelongertheleadmustbe.However,toachieve

higheraccuracy,itisbesttousetheshortestpossibleleadforthejob.

Thereisadirectcorrelationofspeedtolength.Forexample,assumingtherevolutionofthemotorwere

50rps,witha20mmlead,thespeedwouldbe1,000mm/sandwitha2mmleaditwouldbe100mm/s.

Themeritofashorterleadisthatitcanmoveaheavierloadusingasmallermotor.But,toachievethesamespeed,iftheleadisshorter,themotormustbebigger.

Dealing With Liquids And Particulate Matter In The Environment

Ballscrewactuatorsystemsaretypicallyavailablewithmetalcovers.Howeverthestandardmetalcov-

ershavegapsbetweenthecoverandtheguiderails.Thismakesthemunsuitableinenvironmentswhereliquids

orparticulatemattercouldenterthesystemacommonsituationinmanufacturingorfabricationapplications

whencoolantisused.Thoughusuallyacustomizedoption,accordianpleatedbellows-typecoversareavailable.

Theyaredesignedtobeimpervioustouidsandparticulatematter.Theyreplacethemetalcoverandcoverthe

entireguide.

Naoki Yamaguchi, Assistant Technical Manager, naoki@nbcorporation.com

NBCorporationofAmerica

930MuireldDrive

HanoverPark,IL60133

1-800-521-2045,Fax:(630)295-8881

www.nbcorporation.com