B-6b Aircraft Hardware Student Resource

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StudentResource     SubjectB-6b: AircraftHardware      Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,or otherwisedisposedof,withoutthewrittenpermissionofAviationAustralia. 

                   ThisPageIntentionallyLeftBlank

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CONTENTS Definitions

3

Study Resources

4

Introduction

5

Aircraft Fastening Devices

6.5-1

Springs

6.7-1

Bearings Transmissions

6.8-1 6.9-1

Control Cables

6.10-1

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IssueB:January2008

Revision2

Page1

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Revision2

Page2

DEFINITIONS Define 

Todescribethenatureorbasicqualitiesof.



Tostatetheprecisemeaningof(awordorsenseofaword).

State 

Specifyinwordsorwriting.



Tosetforthinwords;declare. Identify 

Toestablishtheidentityof.

List 

Itemise.

Describe 

Representinwordsenablinghearerorreadertoformanideaofanobjectorprocess.



Totellthefacts,details,orparticularsofsomethingverballyorinwriting.

Explain 

Makeknownindetail.



Offerreasonforcauseandeffect.


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STUDYRESOURCES JeppesenGeneral  JeppesenAirframe  AC43.13-1B/AC43.13-2ACombined–AircraftInspectionandRepair  B-6bStudentHandout  DaleCrane–AviationMechanicHandbook  


Revision2

Page4

INTRODUCTION Thepurposeofthissubjectistofamiliariseyouwithaircrafthardware,fasteningandlocking devices,gears,bearingantransmissions.. Oncompletionofthefollowingtopicsyouwillbeableto:

Listtypesofstandardscrewthreadsusedinaircraftandidentifytheirthreadforms, dimensionsandtolerances. Describemeasuringofscrewthreads.

Identifyvarioustypesofaircraftboltsandmachinescrewsbyspecification,markings andinternationalstandards. Identifyanddescribethefollowingnuttypes: 

selflocking



anchor



standard

Identifytypesofstudsanddescribetheirusesandmethodsofinsertionandremoval. Identifytypesofselftappingscrews. Statethepurposeofdowelsanddescribetheirapplication.

Identifyandstatethepurposeofthefollowinglockingdevices: 

tabandspringwashers



lockingplates



splitpins



pal-nuts



wirelocking



quickreleasefasteners



keys,circlipsandcotterpins.

Identifyvarioustypesofspringsandlistmaterialsusedintheirconstruction. Statecharacteristicsofspringsandlisttheirapplications.

Identifyvariousbearingstypesandstatetheirpurpose. Definebearingapplication,loadsandmaterialsusedintheirconstruction.


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Page5

Identifygeartypesandlisttheirapplication Statethepurposeofthefollowing 

GearRatios.



Reductionandmultiplicationgearsystems.

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Drivenanddrivinggears.

 

Idlergears. Meshpatterns.

Identifybelts,pulleys,chainsandsprocketsanddescribetheirapplication.

Identifycabletypesanddefinethepurposeofthefollowingassociatedcomponents: 

Endfittings,turnbucklesandcompensationdevises.



Pulleysandcablesystems

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Bowdencableandflexiblecontrolsystems


Revision2

Page6

AA Fo rm TO-18 B-6b Aircraft Hardware

Part-6 6 Subj ect

TOPIC6.5.1AIRCRAFTFASTENINGDEVICES isthetermusedtodescribe varioustypesof fastenersandmiscellaneous smallitemsusedinthemanufactureandrepairofaircraft.Thesafeandefficientoperationof anyaircraftisgreatlydependentuponthecorrectselectionanduseofaircrafthardware. Vibrationisalwayspresentduringaircraftoperation. Consequentlytheremust beprovision forsafetyingorlockingoffastenerstopreventthemfromvibratinglooseinflight,andanyone involvedinaircraftmaintenancemustbefamiliarwiththemethodsused.

Therearevariousstandardsusedforhardwarespecificationsintheaircraftindustry. Themostcommonstandardsyouwillencounterare: : 

AN(AirforceNavy)

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MS(MilitaryStandards)

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NAS(NationalAerospaceStandards).



NSA(NATOStandardisationAgency)



BAC(BoeingAircraftCorporation)



FON(Fokker)

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AMS(AeronauticalMaterialsSpecifications)

Standardhardware,whichisavailablefromaviationsuppliers,isidentifiedbyaspecification number.Specialfastenersmustbereplacedwiththosehavingthesamepartnumberand notwithsimilar lookingstandardhardware. Oftenthedifferencebetweena standardanda specialpartisthematerialusedtomanufactureitortheclosertoleranceinitsmanufactureor itmaybeamorecriticalinspectionofthepart. .NASandMSstandard hardwaremustnotbereplacedbyANstandardhardware.

Issue B: January 2008

Revision 2

B-6b.5.1 Screw Threads 1 of 4


Part-6 6 Subj ect

Varioustypesoffasteningdevicesallowquickdismantlingofaircraftpartsthatmustbetaken apartandputbacktogeth eratfrequentintervals. Rivetingorweldingthesepartseachtime they are serviced would soon weaken or ruin the joint.  Furthermore, some joints require greatertensilestrengthandstiffnessthanrivetscanprovide.Boltsandscrewsaretwotypes offasteningdeviceswhichgivetherequiredsecurityofattachmentandrigidity.Generally, boltsareusedwhengreatstrengthisrequired,andscrewsareusedwherestrengthisnotthe decidingfactor. Boltsandscrewsaresimilarinmanyways.Theyarebothusedforfasteningorholding,and each has a head on one end and a screw thread on the other.  Regardless of these similarities, there are several distinct differences between the two types of fasteners.  The threadedendofaboltisalwaysblunt,whilethatofascrewmaybeeitherbluntorpointed. The threadedend ofa boltusually hasa nut securedto itto completetheasse mbly. The threadedendofascrewmayfitintoafemalereceptacle,oritmayfitdirectlyintothematerial beingsecured.Abolthasafairlyshortthreadsectionandacomparativelylonggriplengthor unthreadedportion;whereasascrewhasalongerthreadedsectionandmayhaveaclearly definedgriplength.Aboltassemblyisgenerallytightenedbyturningthenutonthebolt:the head of the bolt may or may not be designed for turning. A screw is always tightened by turningitshead. Whenitbecomesnecessarytoreplaceaircraftfasteners,aduplicateofthesrcinalfastener shouldalwaysbeused.

Aircraftbolts,screwsandnutsarethreadedineitherthe: AmericanStandard 

ANC(AmericanNationalCoarse)



ANF(AmericanNationalFine)



ANEF(AmericanNationalExtraFine)

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ANP(AmericanNationalPipe

 


Revision 2



Part-6 6 Subj ect

UnifiedStandard 

UNC(UnifiedNationalCoarse)



UNF(UnifiedNationalFine)



UNEF(UnifiedNationalExtraFine)

  BritishStandard 

BSW(BritishStandardWhitworth)



BSB(BritishStandardBrass)

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BSP(BritishStandardPipe)

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BSPT(BritishStandardPipeTaper)

  SIMetric TheSIMetricsystemofthreadsisgenerallyusedonequipmentmanufacturedinEurope.All metricthreadswillhavethethreadformasillustrated

 


Revision 2



Part-6 6 Subj ect

Threadsarealsodesignatedbyclassoffit.Theclassoffitofathreadindicatesthetolerance allowedinmanufacturing: 

Class1isaloosefit.



Class2isafreefit.



Class3isamediumfit.





Class4isaclosefit. Class5isatightfit


Revision 2


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Threaded fasteners allow parts to be fastenedtogether withall of the strength unthreaded fasteners provide. However, unlike rivets, threaded fasteners may be disassembled and reassembledanalmostinfinitenumberoftimes.

Mostboltsusedinaircraftstructuresareeithergeneral-purpose,internal-wrenching,orclosetoleranceAN,NAS,orMSbolts.Aircraftbolts,screws,andnutsarethreadedineitherthe AmericanNational Coarse (NC), the AmericanNational Fine (NF), the AmericanStandard UnifiedCoarse(UNC),ortheAmericanStandardUnifiedFine(UNF)series. Inadditiontobeingidentifiedaseithercoarseorfine,threadsarealsodesignatedbyclassof fitfromonetofive: 

AClass1threadisaloosefit,



AClass2isafreefit,



AClass3isamediumfit,



AClass4isaclosefit,and



AClass5fitisatightfit.

AClass1fitallowsyoutoturnthenutallthewaydownusingonlyyourfingers.Wingnutsare agoodexampleofaClass1fit.AClass4and5fitrequiresawrenchtoturnanutdownfrom starttofinish.AircraftboltsareusuallyfinethreadedwithaClass3fit,whereasscrewsare typicallyaClass2or3fit.

  Issue B:January2008

B-6b.5.2 Fastener-Bolts,StudsandScrews Revision 3

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1of24

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An aircraft bolt is given a part code indicatingits diameter in 1/16 inch incrementsand its lengthin1/8inchincrements.Forexample,anAN4-7identifiesaboltthatmeasures4/16or 1/4inchindiameterand7/8inchinlength. For bolts that are longer than 7/8 inch, the code changes. For example, a 1 inch bolt is identifiedbya-10representing1inchandnofraction.Inotherwords,thereareno-8or-9 lengths.Dashnumbersgofrom-7to -10,from-17to-20,andfrom-27 to- 30.Therefore,a boltthatis 11/2incheslong is identifiedby a-14.AboltwiththecodeAN5-22 identifiesan AirForce-Navyboltthatis5/16inchindiameterand21/4incheslong. Threaded aircraft bolts 1/4 inch in diameter and smaller are dimensioned in screw sizes ratherthan1/8inchincrements. TheAN3boltistheexcept iontothisrule .Thesemachinescrewsizesrangefrom0to12.A number10fastenerhasadiameterofapproximately3/16inchandanumber5fastenerhas a1/8inchdiameter.



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Aboltisdesignedtoholdtwoormoreitemstogether.Boltsthataretypicallyusedforair framestructuralapplicationshavehexheadsandrangeinsizefromAN3toAN2O. Boltsareidentifiedbytheirdiameterandlength.Adiameterrepresentstheshankdiameter whilethelengthrepresentsthedistancefromthebottomoftheheadtotheendofthebolt. Abolt’sgriplengthisthelengthoftheunthreadedportionasshown.Ifthegriplengthis slightlylongerthanthisthickness,washersmustbeaddedtoensurethatthenutcanprovide theproperamountofpressurewhenitistightened.Ifthegriplengthissubstantiallylessthan thethicknessofthematerialsthebolt’sthreadswillextendintothematerial,resultingina weakerjoint. Whenjoiningtwopiecesofmaterial,theircombinedthicknessdeterminesthecorrectlength ofbolttouse.

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ThediameterofaboltisindicatedbythenumberimmediatelyfollowingtheprefixsuchasAN. Thedashnumberofstandardboltsindicateslengthin1/8ofaninchincrement. AN3-6A: 

AN=AirForce/Navy.

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3=Diameterin1/16inch

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-6=Lengthin1/8inch

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A=Notdrilledforsplitpin(Noletter=Drilledforsplitpin).

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 Issue B:January2008


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 Aircraft bolts are availablein cadmium-plated nickel steel, corrosionresistant steel, and in 2024aluminiumalloy. Unlessspecified,aboltismadeofcadmium-platednickelsteel.Acorrosionresistantbolt,on the other hand, is identified by the letter “C” inserted between the diameter and length designations.Aluminiumalloyboltsareidentifiedbytheletters“DD.”Forexample,aboltthat is1/4inchindiameter,3/4inchlong,andmadeofcadmium-platednickelsteelisidentifiedby thecodeAN4-6.However,ifthe sameboltis madeof corrosionresistantsteelit carriesthe codeAN4C6,whereasanaluminiumalloyboltwouldbeAN4DD6.

 In addition to the designation code, most aircraft bolts have a marking on their head identifyingwhattheboltismadeofand,inmanycases, themanufacturer.For example,AN standard steel bolts are marked with either a raised dash or asterisk in the centre of its manufacturedhead,corrosion-resistantsteelismarkedbyasingledash,andANaluminumalloyboltsaremarkedwithtworaiseddashes. TheFAAforbidstheuseofaluminiumalloyboltsandalloysteelboltssmallerthanAN3on structural components. Furthermore, since repeated tightening andloosening of aluminium alloy bolts eventually ruins their threads, they are not used in areas where they must be removed and installed frequently. Aluminium alloy nuts can be used with cadmium-plated steelboltsloadedinshear,butonlyonlandaircraft,notbeusedonseaplanes. Whenhardwarewasfirststand ardized,almostallnutswerelocke dontoa boltwithacotter pinand,therefore,allboltshadholesdrilledneartheendoftheirshanktoaccommodatea cotterpin.However,whenself-loc king nutsbecamepopular,manystandard  ANboltswere made withouta drilled shank. Tohelp  you identifywheth eror not a bolthas a hole drilled throughit,theletterAisusedinthepartcode. Forexample,ifan“A”appearsimmediatelyafterthedashnumbertheboltdoesnothavea hole,However,theabsenceofan“A”indicatesaholeexistsintheshank.Asanexample,an AN6C-12Abolt is3/8inchindiameter,made ofcorrosion-resistantsteel,11/4incheslong, andhasanundrilledshank.  Issue B:January2008


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Some AN bolts, such as those used to fasten a propeller into a flanged shaft, must be safetiedbypassingsafetywirethroughholesdrilledthroughthebolt’shead.Aboltdrilledfor this type of safetying has the letter H following the number indicating its diameter. For example,thepartnumberAN6H-34Aidentifiesaboltthatis3/8inchindiameter,madeof nickel-steel,hasadrilledhead,is31/2incheslong,andhasanundrilledshank.

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Tobeloadedinshearonly .Forexample,acontrolcablemustbeattach edtoacontrol horn withaboltthatislooseenoughtoallowthecabletopivotfreelyasthecontrolsurfacemoves, butnotsoloosethatexcessplayexists.Fortheseapplicationsaclevisboltisused. TheAN21throughAN36clevisbolthasadomedheadthatistypicallyslottedorrecessedto acceptascrewdriver.Auniquefeatureofaclevisboltisthatonlyashortportionoftheshank isthreaded,andthereisa smallnotchbetweenthethreadsandtheshank. Thisresultsin a  Issue B:January2008


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longgriplengthwhichincre asesthebolt’sshearstrengthandallowsthebolttorotatemore freelyinitshole. Thediameterofaclevisboltisgivenin1/16inchincrements.Thelengthofaclevisboltis morecriticalthanthatoftheothertypesofboltsand,therefore,itisalsomeasuredin1/16 inch increments with a dash number indicating the length. For example, an AN29-20 identifiesa9/16inchdiameterclevisboltthatis20/16(11/4)incheslong.

These bolts are used in applications where external tension loads are to be applied. The headofthisboltisspeciallydesignedfortheattachmentofaturnbuckle,aclevis,oracable shackle.Thethreadedshankmayormaynotbedrilledforsafetying.



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AN73throughAN81boltsarehex-headednickel-steelboltsthataresimilarinappearanceto the AN3 through AN2O series. However, unlike standard bolts, drilled-head engine bolts haveathickerheadthatisdrilledwithasmallholeineachoftheflatsandinthecentreofthe head.Aswithmostbolts,thediametersofdrilled-headengineboltsarein1/16increments whileboltlengthsarein1/8inchincrements.Thediameterisindicatedbythesecondnumber following the “AN” designation while the bolt length is indicated by a dash number. For example,adrilled-headengineboltdesignatedasAN746hasadiameterof1/4inchanda lengthof3/4inch.

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Close tolerance bolts are designated AN173 to AN186 and are ground to a tolerance of +0.000—0.0005inch.ThisismuchtighterthenstandardAN3throughAN14boltswhichare manufactured with a tolerance of +0.000 —0.0025,or AN16 throughAN2O bolts which are manufacturedwithatoleranceof+0.000—0.0055inch.Closetoleranceboltsmustbeusedin areasthataresubjecttopoundingloadsorinastructurethatisrequiredtobebothriveted andbolted. Closetoleranceboltscarryatriangle markontheirheadsandare groundto amuchtighter  tolerancethanstandardbolts.

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MS20004 through MS20024 internal wrenching bolts are high-strength steel bolts used primarilyinareasthatare subjectedtohightensile loads.A six-sidedholeismachinedinto thecenteroftheirheadstoacceptanAllenwrenchofthepropersize.Theseboltshavea radius between the head and shank and, when installed in steel parts, the hole must be counterboredtoaccommodatethisradius.Whenaninternalwrenchingboltisinstalledinan aluminumalloystructure,aMS20002Cwashermistheusedundertheheadtoprovidethe neededbearingarea. Thestrengthofinterim wrenchingboltsismuchhigherthan thatofa standardsteelAN bolt and,forthisreason,anANboltmustneverbesubstitutedforaninternalwrenchingtype.

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Developed and updated by the National Aerospace Standards Committee Aerospace Standards include many standards on precision fasteners as well as other aerospace hardware. 

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EvenDashnumberindicatethestandardissuelengthin1/8inchIncrements. Griplengthismeasuredfromundertheheadendofthethread. OdddashNumbersisaspecialapplicationbolt=Griplength1/16inchlongerthan evennumber–usedwhereastandardMSboltiseithertoolongortooshort.

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The basic NAS number identifies the part. The suffix letters and dash numbers separate differentsizes,platingmaterial,drillingspecifications,etc. ItisnecessarytorefertoaspecificNASpageintheStandardsbookforthelegend.

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All aircraft bolts, except AN bolts, are measured by their grip length, not by their overall length. Standardissueboltshaveevendashnumbers,inmostapplicationstheseboltswilldobut where the grip length iscriti cal and a (standard) gripis either too long ortooshor t aODD Dashnumberboltisavailable. Aircraftboltsmustbefittedcorrectly,justanylengthboltwillnotdo.Theloadmustbeonthe shankandnotonthethread.Tensilestrengthof160,000psito180,000andashearstrength of95,000psito108,000dependingonthetypeofbolt

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TheEuropeanNSAboltsshou ldnotbe confusedwiththeAmeric anNASBolt.Always refer totheaircraftillustratedpartscataloguewhenpurchasingfasteners,oftenboltsmaylookthe samebuttheyarenot. NSASpecificationisaFOURdigitnumberidentifyingthetypeofboltegNSA5022 ThefirstDashnumbersisthediameterin1/16incheg-4 Theseconddashnumberisthegriplengthin1/16incheg-22 Example:NSA5022-4-22isa1/4x13/8Hexheadbolt NOTE: To prevent dangerous cross use between metric and imperial sizes, all western aerospaceboltsaremanufacturedtoimperialspecifications.



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 Allnutsused inaircraftconstructionmusthave somesort oflockingdevicetopreventthem fromlooseningandfallingoff. Thereare twobasictypesofnuts,self-lo ckingandnonself-l ocking.As thenameimplie s,a self-locking nut locks onto a bolt on its own while a nonself-locking nut relies on either a cotterpin,checknut,orlockwashertoholditinplace.

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Self-locking nuts, or lock nuts, employ a locking device in their design to keep them from coming loose. The two general types of self-locking nuts used inaviat ion are the fiber, or nylontype(LowTemperature),andtheallmetaltype. A self-locking nut must be screwed onto a bolt until all of the chamfer on the bolt’s end protrudes through the insert. Ifthe bolt isnot chamfered, atleas t one thread but not more thanthreethreadsshouldprotrudethroughthenut.If morethanthreethrea dsareexposed, youriskthedangerof“bottomingout”thenutandundertorqueingtheassembly,thuscreating astresspointthatcould fail.Ifmorethanthreethread sareexposed,eitherreplac ethe bolt withoneofthecorrectlengthorinstallawasher. Aself-lockingnut’sdashnumberspecifiesbothdiameterandnumberofthreadsperinch.For example, a -524 represents a self-locking nut that fits a 5/16 inch fine thread bolt with 24 threadsperinch. LOW-TEMPERATURESELF-LOCKINGNUTS Nylon self-locking nuts should not be used in any location where the temperature could exceed250°F.However,youmayusethemonenginesinthoselocationsspecifiedbythe enginemanufacturer. AN365self-lockingnutsareusedonboltsandmachinescrewsandareheldinpositionbya nylon insert above the threads. This insert has a hole slightly smaller than the thread  Issue B:January2008


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diameteronwhichitfits.Thenut’ sClass3fitallow sittorundownonabolt’s threadseasily untiltheboltenterstheinsert. AN364nutsresembletheAN365selflockingnut,buttheyarethinandareapprovedonlyfor shearloads,nottobeusedintension.AN364nutsaretypicallymadetobeusedonclevis boltsthatdonothavedrilledshanks.

 METALSELF-LOCKINGNUTS Inapplicationswheretemperaturesexceed250°F,all-metallocknuts,suchastheAN363, are used. Some of these nuts have a portion of their end slot ted and the slots swaged together.Thisgivestheend ofthenut aslightlysmallerdiameterthanits bodyallowingthe threadstogripthebolt.Othershavetheendofthenutsqueezedintoaslightlyovalshape, and as the bolt screws up through the threads it must make the hole round, creating a grippingaction.

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 AN310CASTLENUT Thesefine-threadnutsaredesigne dtofit onastandardairframeboltwitha Class3 fit,and are used when the bolt is subjected to either shear or tensile loads. The size of a nut is indicatedinthepartcodebyadashnumberwhichdenotesthesizeoftheboltitfits.For example,anAN31O-6nutfitsanAN6boltwhichhasadiameterof3/8inch. Castlenuts areavailable incadmium-platednickelsteel,corrosio n-resistant steel, and2024 aluminum alloy. Unless specified, a castle nut is made of cadmium-plated nickel steel. A corrosionresistantnut,ontheotherhand,isidentifiedbytheletter“C”insertedbeforethe dash number in the part code. Aluminum alloy nuts are identified by the letter “U.” For example, the part code AN31OD-6 identifies an aluminum alloy nut that has an inside diameterof6/16(3/8)inch. AN320SHEARCASTLENUT TheAN320shearcastlenutismadeofthesamematerialandhasthesametypeofthread asaAN31Onut.However,shearcastlenutsaremuchthinnerthanstandardcastlenutsand, therefore,areusedonlyforshearloadsonclevisbolts.AnAN320-6nutisashearcastlenut thatisusedonanAN26clevisbolt.Analuminumalloy(2024)nutisidentifiedasanAN320fl6.

 AN315PLAINNUT The AN315 plain nut has no castellations and, therefore, cannot be held in place using a cotterpin.Sincethesefine-threadnutshavenolockingprovisions,aspring-typelockwasher mustbeusedincombinationwiththenut.Thelockwash erappliesaspringforcetopreven t thenutfromshakingloose.AN315nutsareusedwitheithertensileorshearloadsandare made of either nickel steel, corrosion-resistant steel, and aluminum alloy. The type of  Issue B:January2008


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materialused is indicated inthe designation code inthe same way it isfor bolts. In other words, the absence of an additional letter identifies nickel steel, whereas the letter “C” preceding the dash number identifies corrosion resistant steel, and a “U” identifies 2024 aluminumalloy.Further more,plainnutsaremadewithbothrightandleft-ha ndthreads.For example,an AN315-7Ris anickelsteelnutwithrighthand threadsthatfitsan AN7bolt.An AN315C-4L,ontheotherhand,isa1/4inchdiametercorrosion-resistantsteelplainnutwith left-handThreads. AN316CHECKNUT Insomeinstancesaplainnutislockedinplaceusingachecknut.Achecknutissimplya secondnutthatistightenedagainsttheprimarynutsoitcannotturnoff.AnAN316checknut is made of cadmium-plated steel and is available in both right-and left-hand threads. An AN316-4Risaright-handchecknutthat fitsaquarter-inchthread,whileanAN316-4Lhasa left-handthread. AN340ANDAN345(LIGHTHEXNUTS): Thesenutsareusedinnon-structuralapplicationsrequiringlighttension. LiketheAN315andAN335,theyrequirealockingdevicetosecurethem. AN355SLOENGINENUT Thisnutisdesignedforuseonanaircraftengineandisnotapprovedforairframeuse.Itis made of heat-treated steel and has national fine threads that produce a Class 3 fit. It is availableinsizesfromAN355-3(3/16inch)toAN355-12(3/4inch)andhasslotscutinitfora cotterpin. AN360PLAINENGINENUT This engine nut is similar to the AN355 in that it is approved for use on engines only. However,anAN360differsfromanAN355inthatitdoesnothavecotterpinslotsandhasa blackrustprooffinish.AnAN360-7isaplainenginenutthatfitsa7/16inchbolt. AN350WINGNUT Wingnutsareusedwhenitisnecessarytoremoveapartfrequentlywithouttheuseoftools. Aircraftwingnutsaremadeofeithercadmium-platedsteelorbrassandareavailableinsizes tofitnumber(gauge)sixmachinescrewsupto1/2inchbolts.Allofthesenutshavenational finethreadsthatproduceaClass2fit.Nutsformachinescrewsizesaredesignatedbythe seriesnumber.However,nutsusedonboltshaveaboltsizegivenin1/16inchincrements followedbythenumber16.Forexample,withanAN3SO-616wingnut,the-6indicatesthat thenutwillfita3/8(6/16)inchbolt. 



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Anchornuts are permanentlymounted nut platesthat enableinspectionplatesandaccess doorsto beeasilyremovedandinstal led.Tomaketheinstalla tionofan accessdooreasier  wherethereareagreatnumberofscrews,afloatinganchornutisoftenused.Withafloating anchornutthenutfitslooselyintoasmallbracketwhichisrivetedtotheskin. Sincethenutisfreetomovewithinthebracketitalignsitselfwithascrew.Tospeedthe production of aircraft, ganged anchor nuts are installed around inspection plate openings. These are floating-type anchor nuts that are installed in a channel that is riveted to the structure.Eachnutfloatsinthechann elwithenoughplaysothata screwcanmovethenut enoughtoalignit.

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Tinnermannutsarecost-economicalnutsthatarestampedoutofsheetmetal.Becauseof theirsemirigidconstruction,tinnermannutscanbeadaptedforuseinmanysituations.For example, tinnerman nuts are commonly used on light aircraft to mount instruments to the instrumentpanelaswellasattachinspectionpanelsandcowlings. Tinnermannutsusedto mountinstrumentscaneither beinstalledinan instrumentpanelor in the instrument case itself. To reduce the chance of magnetic interference, the nuts are made of brass and the cage that holds the nut is constructed of phosphor bronze. If the instrument is rear mounted, the legs of the nut are long enough to pass through the instrumentcase.Iftheinstrumentisfrontmounted,thenutfastensintothescrewholeinthe instrumentpanel. Anchor type tinnerman nuts are riveted to a structure to hold screws used to secure inspectionplates. The cowlings on some light aircraft are held on with self-tapping sheet metal screws. To preventthesheetmetalscrewsfromenlargingtheholesinthecowlingbyrepeatedinsertion andextraction,aU-typeTinnermannutisslippedovertheedgeoftheinsidecowlingsothat itstraddlesthescrewhole.Whenascrewistightenedintothenut,thespringactionofthe nutholdsthescrewtight.

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 Goodrich Rivnuts were developed by the BE Goodrich Company to attach rubber de-icer boots to aircraft wing and tail surfaces. To install a rivnut, a hole is drilled in the skin to accommodate the Rivnut, and a special cutter is used to cut a small notch in the circumferenceofthe hole.ThisnotchlockstheRivnutintotheskinto preventitfromtuning whenitisusedasanut.ARivnutofthepropergriplengthisthenscrewedontothepuller and inserted into the hole with its key aligned with the keyway cut in the hole. When the handleofthepullerissqueezed,thehollowshankoftheRivnutupsetsandgripstheskin. ThetoolisthenunscrewedfromtheRivnut,leavingathreadedholethatacceptsmachine  Issue B:January2008


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screwsforattachingade-icerboot.Rivnutsarenowusedinmanyareasonaircr aftandthe automotiveindustry.

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Astudisashaftthatisthreadedatbothends. Theyhaveashortthreadononeendandalongthreadontheother.Theshortthreadisa coarse thread and a much tighter fit than the long thread so the stud will remain in place whenthenutonthelongendisundone.

 Wherejointshavetobebrokenfrequently,studsareusedinplaceofboltsorscrewsto preventdamagetothetappedholes. Aircraftmaintenancemanualwillgivemethodsofstudremovalandinstallation.



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Screwsareprobablythemostcommonlyusedthreadedfastenerinaircraft.Theydifferfrom boltsin thattheyaregenerallymadeoflowerstreng thmaterialsbutnotalway s.Screwsare typically installed with a loose-fitting thread, and the head shapes are made to engage a screwdriver or wrench. Some screws have a clearly defined grip length while others are threadedalongtheirentirelength.

 Therearethreebasicclassificationsofscrewsusedinaircraftconstruction: 

machinescrews,whicharethemostwidelyused;

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structuralscrews,whichhavethesamestrengthasbolts;and

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self-tappingscrews,whicharetypicallyusedtojoinlightweightmaterials.

MACHINESCREWS Machine screws are used extensively for attaching fairings, inspection plates, fluid line clamps and other light structural parts. The main difference between aircraft bolts and machinescrewsisthatthethreadsofamachinescrewusuallyrunthefulllengthoftheshank, whereasboltshaveanunthreadedgriplength.



 18of24

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Screws normally have a Class 2, or free fit and are available in both national coarse and national fine threads. The most common machine screws used in aviation are the fillister headscrew,theflat-headscrew,theround-headscrew,andthetruss-headscrew. STRUCTURALSCREWS

 Structural screws are made ofalloy  steel, are heat treated,and can beused  asstruc tural bolts. They have a definite grip and the same shear strength as a bolt of the same size. Shank tolerances are similar to AN hex-head bolts, and the threads are National Fine. Structuralscrewsareavailablewithfillister,flat,orwasherheads. TheseheadtypesareNOTinterchangeablewitheachother.Thecorrectscrewdrivermustbe usedtoavoiddamagetothescrewhead,especiallytotitaniumscrews. Never use a Philips screwdriver on a Torq-set screw, nor a slotted screwdriver on a HiTorquescrew. SELF-TAPPINGSCREWS Self-tappingscrewshavecoarse-threadsandareusedtoholdthinsheetsofmetal,plastic,or plywoodtogether.Thetype-Ascrewhasagimlet(sharp)point,andthetypeBhasablunt pointwiththreadsthatareslightlyfinerthanthoseofatype-Ascrew.



 19of24

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Therearefourtypesofheadsavailableonself-tap-pingscrews: 

Roundhead,

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Trusshead,

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Countersunkhead,whichisflatontop,and

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Counter-sunkovalscrew.

Thetruss-headisrounded,similartotheroundheadscrew,butisconsiderablythinner. 

Washersprovidea bearingsurfaceareafornuts,andact asspacersor shimsto obtainthe propergrip lengthforaboltandnutassembly.Theyarealsousedtoadjustthepositionofcastellated nutswithrespecttodrilledcotterpinholesinboltsaswellasapplytensionbetweenanut andamaterialsurfacetopreventthenutfromvibratingloose.Thethreemostcommontypes ofwashersusedinairframerepairaretheplainwasher,lockwasher,andspecialwasher.

 PLAINWASHERS All AN washers are in the 900 series. AN960 plain washer provides a smooth surface betweenanutandthematerialbeingclampedThesewashersaremadeofcadmium-plated steel,commercialbrass(B),corrosion-resistantsteel(C),and2024aluminiumalloy(D).They  Issue B:January2008


 20of24

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areavailableinsizesthatrangefromthosethatfitanumbertwomachinescrewtothosethat fitaone-inchbolt. Ifa thinwasheris needed,a lightserie s washer that isone- halfthe thickness ofa regular washer is available. An example of where a light series washer should be used is if the castellationsofanAN31Onutdonotlineupwithacotterpinhole whenthenutisproperlytorqued.Inthissituationalightserieswashercanbesubstitutedfor theregularwashertoaligntheholes.Alightserieswasherisidentifiedbytheletter‘1”added tothecode.Forexample,thecodeANO6OLidentifiesalightserieswasher. When working with wood or composite structures, washers with a large surface area are usedtospreadthefastenerloadoverawiderarea.Theselargeareawasherscarrythecode ofAN970andareallmadeofcadmium-platedsteelwithinsidediametersfrom3116to1/2 inch. LOCKWASHERS In some instances it is not convenient to use self-locking nuts or cotter pins on bolts. For theseapplications,alockwasherisoftenusedbetweenthenutandjointsurfaceifthejointis notstructurallycritical.Lockwashersaremadeofsteelandaretwistedsothatwhenanutis tightenedagainstit,thespringactionofthewashercreatesastrongfrictionforcebetween theboltthreadsandthoseinthenut. Twotypesoflockwashersareusedinaircraftconstruction.ThemostcommonistheAN935 splitlockwasher.Thesewashersareavailableinsizesthatfitfromanumberfourmachine screwto a 1/2 inch bolt. The second typeof lockwash eris the thinner AN936 shakeproof lockwasherwhichisavailablewithbothinternalandexternalteeth. SPECIALWASHERS Some high-strength internal wrenching bolts have a radius between their shaft and the underside of the bolt head. To provide a tight mating surface, MS20002C countersunk washers are used under the heads of internal wrenching bolts. These washers have a countersunk edge to accommodate the radius on the bolt head. Countersunk washers are madeofheat-treatedsteelandarecadmiumplated. Finishing washers are often used in aircraft interiors to secure upholstery and trim. These washers have a countersunk face to accommodate flush screws. Finishing washers bear againstalargeareatoavoiddamagingfragileinteriorcomponents. In many instances, keyed washers can be used as a safety device. Keyed washers have smallkeysorprotrusionstoengageslotscutintoboltsorpanels.



 21of24

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Aclevispinisusedinconjunctionwithtie-rodterminalsandsecondarycontrolswhicharenot subjecttocontinuousoperation. ClevispinsaresecuredwithasplitpinorAN416safetypin. •

Awashermustbeplacedunderthesplitorsafetypin

Thepinisnormallyinstalledwiththeheaduporforward. Thispreventslossshouldthesplitpinfailorworkout.





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A dowel is a solid cylindrical rod, usually made of wood, plastic or metal. In its srcinal manufacturedform,doweliscalleddowelrod.Dowelrodisoftencutintoshortlengthscalled dowelpins. Dowels are used where the precision alignment and correct orientation of two mating surfacesisrequired.  Issue B:January2008


 22of24

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Therearefollowingtypesofdowelsusedinaircraft: •

Smoothsoliddowels

•

Hollowdowels

•

Threadeddowels

•

Splithollowdowels

Smoothsoliddowels:usuallysteeldowelpins,aremadewithhighqualityofmetallicproducts ensuringthesmoothsurfacefinishingandhighperformance.

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 23of24

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   Splithollowdowelsinsomeinstancesareusednotonlytomaintainalignmentbutalsoactas bushingsforrotatingcomponents. Smoothsideddowelsarepressfitinstalled.

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 24of24

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Vibration is alwayspresentduringaircraf t operation. Consequentlytheremust beprovision forsafetyingorlockingallfastenerstopreventthemvibratinglooseinflight. There are various methods of safetying and locking. The most widely used methods are safetywire,splitpins,lockwashers,circlipsandspecialnutssuchasself-lockingnuts,pal nutsandjamnuts.

 

Insomeinstancesitisnotconvenienttouseself-lockingnutsorsplitpinsonbolts.Forthese applications,a lockwasherisoftenusedbetwee nthenut andjointsurfaceifthejointis not structurally critical. Lock washers are madeof steeland are twisted sothat whena nut is tightenedagainstit,thespringactionofthewashercreatesastrongfrictionforcebetween theboltthreadsandthoseinthenut.

  Issue B:January2008

 B-6b.5.3 Fasteners-LockingDevices

Revision 2

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1of12

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The AN935 lock washer may be used between the nut and the surface if the joint is not structurallycritical. TheAN936shakeproofwasheris thinnerthantheAN935lockwash erandisavaila blewith bothinternalandexternalteeth.   Oftenusedforlockinghexheadfasteners Atabmustnotbebentmorethanonce. Youcanre-usemultipletabwashersafterremovingtheusedtab,dressingsharpedgesand carefullyinspectingtheremainingtabsforcracksorscoring.

   Becauseaircraftvibrate,theremustbesomeprovisionforsafetyingorlockingallfastenersto keep them from vibrating loose. Self-locking nuts are used for the vast majority of applicationsinmodernaircr aftconstruction,but therearestillplaces wherelockwireorsplit pinsareneeded.Forexample,drilled-headboltsareoftenusedinvibration-proneareasand aresafetywiredtogether. Lockwiring is a means of securing hardware and components and is a safety method employedinaircraftmaintenanceprocedures. Thetypeoflockwiremostcommonlyusedismadeofstainlesssteel. Wheninstallinglockwire,thewireshoul dpulltheboltheadinthedirectio nof tighteningand shouldbetwistedevenlytothenextbolt.Aftertheendofthewireispassedthroughthehead ofthesecondboltitisagaintwisted,thistimeforaboutthreeorfourturns.Oncethisisdone, theexcessiscutoffandtheendsofthewirearebentbackwheretheycannotcutanyone whopassestheirhandoverthebolts.  Issue B:January2008

B-6b.5.3 Fasteners-LockingDevices Revision 2

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2of12

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 Lockwiringisoftenusedincriticalareas,whereinspectionintervalsmaybeinfrequent.

Doubletwististhemostcommonlyusedmethodasshownabove.

 Whenperformingbyhandpullfirmly.Startthetwistinclosetothefastener,holdendsabout 90degreesapartandtwistinaclockwisedirection(forRHthreads). Inareaswhereanumberofboltsmustbesafetied,suchasapropeller,youmaysafetywire theboltsingroupsofthree.Ifmorethanthreeboltsaresafetiedtogetheritisdifficulttoget thesafetywiretightenoughtobeeffective. Single wire method is used on screws, bolts and nuts in a closely-spaced or closedgeometrical pattern such as a triangle, square, rectangle or circle.  May also be used on electricalsystemsandonpartsthataredifficulttoreach. 

WireTwistersmakelockwiringeasy buttheycandamage thelockwiretherefore,theymust beusedwithcare.  Issue B:January2008


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They can twist wire in one direction only when the knob is pulled. To twist in the other direction,lockthepliersandrotatemanually.

   Thelockingplatesareusuallysecuredtoanadjacentpartofthestructurebyascrew. Lockingplatesmaybeusedrepeatedly,providedtheyremainagoodfitaroundthehexagon ofthenutorbolt.

   Palnutsareusedwithplainnutstolocktheminplace •

Commonlyusedtolockpistonenginecylinderbasenuts

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Usedinmanyaircraftapplications 

Issue B:January2008


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•

Madefromlightpressedalloysteel

   Grubscrewsusedasamethodoflockingtwothreadedcomponentstogether

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QuickReleaseFastenersprovideavibrationresistantjoiningsolutionforquickandrepetitive attachmentandremovalofpanelswithminimumeffort.



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Benefits: 

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-Lockingandunlockingthefasteneronlyrequiresaquarterturnorapush. -Fasteners can be locked and unlocked in a matter of seconds - saving time and reducingcosts. -Vibrationresistant-performswellineventhemostintensiveapplications.

  Acirclipisaspringclipusedforbothinternalandexternallocking. Theyareusedforretainingshafts,seals,bearingsetc. Specialpliers,areusedtoopenorcompressthecirclipforinsertionandremoval.

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Boththeplainandthreadedtaperpinareusedinaircraftstructurestomakeajointthatis designedtocarryshearloads.Thistypeofpindoesnotallowanyloosemotionorplay.The AN385plaintaperpinisforcedintoaholethathasbeenreamedwithaMorsestandardtaper  Issue B:January2008


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pinreamerandisheldinplacebyfriction.Itcanbesafetiedbypassingsafetywirearound the shaft and through a hole drilled in its large end. An AN386 taper pin is similar to the AN385exceptthatitssmall-endisthreadedtoaccepteitheraself-lockingshearnutAN364) orashearcastlenut(AN320). Another type of taper pin is one that has a tapered flat side. Used for locating parts onto shafts–flatsidelinesupwithaflatontheshaft.

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Rollpinsareoftenusedtoprovidelockingforajointwherethepinisnotlikelytoberemoved ortolocksomethingontoashaftsuchasahandleorlever.Arollpinismadeofflatspring steel that is rolled into a cylinder but the two ends are not Joined. This allows the pin to compresswhen itis pressed into ahole and create aspring action that holdsthepintight  againsttheedgeofthehole.Toremovearollpin,itmustbedrivenfromaholewithaproper sizepinpunch.

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Clevis,orfiat-head,pinsareusedforhingepinsinsomeaircraftcontrolsystems.Theyare madeofcadmium-platedsteelandhavegriplengthsin1/16inchincrements,Wheninstalling aclevispinplacetheheadintheupposition,placeaplainwasherovertheoppositeend, andinsertasplit(cotter)pinthroughtheholetolockthepininplace.

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Castellatednutsarelocke dontodrilledboltsbypassin ga splitpinthroughthehole andnut castellationsandthenspreadingtheendsofthesplitpin.Theyaremadeofeithercadmiumplatedcarbonsteelorcorrosion-resistantsteel. Thereare twomethodsof securingsplitpinsthataregenera llyacceptable.Inthe preferred method,onelegofthesplitpinisbentupovertheendofthebolt,andtheotherlegisbent down over one of the flats of the nut. With the second method, the split pin is rotated 90 degreesandthelegswrapped around the castellations. It is important to note that nuts should never be over-torqued to maketheholeintheboltalignwiththecastellations.Ifthecastellationsinthenutfailtoalign withthedrilledbolthole,addwashersunderthenutuntilasplitpincanbeinserted.

  Issue B:January2008

 B-6b.5.3 Fasteners-LockingDevices

Revision 2

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 Keysarehardenedpiecesofmetalthatfitintocutouts(keyways)inwheels,discs,sprockets or gears. The key aligns the wheel, disc sprocket or gear onto a shaft which also has a keywaycutintoit.

 Differenttypes/shapes:

 Theadvantageofawoodruffkeyisonceitisplacedinthepartitwon'tmove.

  SquareKeyshaveatendencytomoveoutoftheirslotunlessretainedbyalockingdevice.  Issue B:January2008


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  ThePrattandWhitneykeyisanotherkeytypethatonceitisplacedinthepartitwon'tmove.

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GibHeadkeyistaperedandisdriveninplacewheninstalled. The Tang governs insertion depth and allows removal. Requires retention in critical applications.   -EndofthisTopic–



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     ThisPageintentionallyleftblank



 12of12

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TOPIC6.7SPRINGS Aspringisanelasticmachinecomponentwhichcanbepressed,stretchedortwistedbya forceandthenreturntoitssrcinalshapewhenthedistortingforceisremoved.Ittherefore storesenergyasafunctionofdisplacement.Althoughspringsarenormallymadeinanalloy ofsteel,oftentermedspringsteel,materialssuchasrubberorplasticmaybeused. Aspringisusedinamechanismtoexertaforce,tostore,ortoabsorbenergy.Familiaruses ofspringsinclude: 

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Supplyingmotivepower,forinstanceintoymechanisms,watchesandclocks. Alsotoclosepistonenginevalves Asacounterbalance,suchasadoorclosingdevice,andtoacertainextentincar suspension

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Asashockabsorber,forinstance,inflexiblecouplings

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Forvibrationcontrol,inenginemountsandinstrumentvibrationpads

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Forcemeasurement,suchasthecalibratedspringsinweighingmachinesand instruments Retention,forinstanceasretainingrings,circlipsorspringwashers

Spring Steel SpringSteelisaspecialclassificationofsteelthathasgreathardness,strengthandelasticity Additionalalloyingelementsareusedinspringsteelinclude: 

Manganese

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Chromium

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Silicon

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Vanadium

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Molybdenum

CommonApplicationsofspringsteelinclude: 

Springs

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Springandshake-proofwashers

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Steelrules

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Measuringtapes

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Feelergauges

 IssueB:January2008

TrainingMaterialOnly Revision3

B6.7Springs 1of8

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Thevarietyofspringconfigurationscanbeclassifiedunderfivemaintypes: 

flatorleaf

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helical

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spiral

 

torsion disk.

Aleafspringisaflatbeamdesignedtodeflectunderload;oneendisusuallyfirmly anchored,whiletheotherislinkedbyamoveableshackletothemovingmachinepart. Loadscanbeappliedineithertensionorcompression(pushorpull);thisisoneofthemain advantagesofthedesign,illustratedinFigure7.1.Anotheradvantageistherelativelylarge amountofenergythespringcanabsorbinasmallspace.

 Theleafspringtypeismostcommonlyseeninautomotiverearsuspension,asshownin Figure7.2.Theyarealsoseenastheundercarriagelegsofsomelightaircraft.Thestress concentrationsinthecentreofthespringcanbecombatedbyaddingmoreleafs(Figure7.2) orbyhavinga‘diamond’designasshowninFigure7.3.

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Sometimestermedacoilspring,thisisprobablythemostcommonspringtype;itiswhat usuallycomestomindwhenyoutalkofsprings.Commonapplicationsincludeautomotive suspension(‘coilovershock’),enginevalvecontrolandsupplyingactuatingforceinaclutch. Thespringisessentiallyawireorbarwoundintoahelix.Theendsmaybemodifiedor groundflatsocompressioncanbeevenlyappliedtothespring,ortheendscouldbeshaped intohooksoreyessothattensioncanbeapplied.ThesecanbeseeninFigure7.4.Thecoil springisdesignedtoactundercompression,ortension,rarelyboth,whichwouldbe detrimentaltothespring'slife.



Compressionspringsfallintothefollowinggeneralcategoriesbasedontheiroverallshape: 





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Cylindrical,StraightorStandard-Allcoilsarethesamediameter.Thesearethe mostcommonandleastexpensivecompressionsprings.Theendscanbeeither openorclosedandtheycanbegroundflat,althoughgrindingsignificantly increasesthecostandisoftenunnecessaryforsmallwiresizes. Conical(Tapered)-Coildiameterdecreasesfromoneendofthespringtoother. Thesespringsareoftenusedwhenthereisnotenoughroomforacylindrical spring.Theycanbemadesothatthesmallercoilstelescopedownintothelarger coilsasthespringiscompressedsothatthespringcompressionsprings. Barrel(Convex)-Taperedsothatbothendsaresmallerthanthemiddle.These springscanhavesomeofthesameadvantagesasaconicalspringwiththe addedadvantagethattheyaresymmetrical. HourglassorConcave-Taperedsothatbothendsarelargerthanthemiddle. Thesespringscanhavesomeofthesameadvantagesasaconicalspringwith theaddedadvantagethattheyaresymmetrical.Theenlargedendcoilsmayalso helpkeepthespringcentredonalargerdiameterhole.




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

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Spiralspringsarecommonlyseenintwomainshapes,bothshowninFigure7.6.Thefirstis aflatspiraltermedan‘Archimedes’spiral,andtheotherisaconicalformwhichisa modificationofthehelicalspring.

 Theadvantageofthespiralspringisitsabilitytobedeflectedinacombinationofways.The flatspiralcanabsorbforceatatangenttoitsaxis;thatis,itcanbewounduptoclosethe  IssueB:January2008







spacebetweenitscoils.Thisisthespringprovidingmotiveenergyinclocksandclockwork engines.Theconicalformcanbecompressedorstretched,andalsoreactstoforcesatright anglestoitsaxis. Thetorsionbarspringisessentiallyashaftofuniformcross-sectionthatstoresenergywhen twisted.Somecarsusetorsionbarsuspension,ortheyareaddedasafter-market accessoriestochangethesuspensioncharacteristics.TheTorsionBartorquewrench utilisesthisprinciple.Thedrivesquareinthetorquewrenchactsasatorsionbarspring, whichactivatesgearstoshowtorqueonadialindicator.Figure7.7illustratesasimple exampleoftorsionbar.



Thisclassificationofspringsincludesalargefamilyofsimilarspringtypes,suchas: 

Singledisk



Multipledisk



Bellvillespring



Lockwasher



Diaphragm

Diskspringsmaybeusedwherespaceislimitedandlargeforcesarepresent;however,they tendtobedifficulttodesignandmanufacture.Thesinglediskhandlessmalldeflections.A stackormultiplediskspring,suchasthatillustratedinFigure7.8,isusedforgreater deflection.

 








Well-knowndiskspringapplicationsincludethediaphragmspringintheautomotiveclutch, illustratedinFigure7.9.

 Anothercommondiskspringvarietyarethefingerwashers,wavespringwashersandthe shakeproofwashersshowninFigure7.10,whichprovidelightlockingofnutsandbolts.

 








Tounderstandspringconstruction,youneedtounderstandsomecommonterms.These termsarelistedbelowandareshowninFigure7.11. 

Coils



Freelength



Groundsection



Insidediameter



Lengthinsidehooks



Lengthovercoil



Meandiameter



Outsidediameter



Pitch



Wirediameter 










Compressionspringscanhaveclosedandgroundendstositsquareonsurfacestoapply pressureevenly,ortheendscanbeopenasshowninFigure7.12. 









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TOPIC6.8BEARINGS Broadlyspeaking,bearingscanbedividedintotwomajorgroups: ,whichhavenomovingpartsandarecomprisedofaplaincylinderor flatwashersurface,ofamaterialgenerallysofterthantheshaftitsupports. , which are an assembly consisting of hardened rolling componentsenclosedininnerandoutercases,calledraces,betweenwhichtheyroll. Thesearefurthersub-dividedintoanothertwomajorcategories: •

•

Agoodbearinghastwocharacteristics: •

•

It must be made of a material that is strong enough to withstand the pressures imposedonit,andyetperm ittheothersurf acetomovewithaminimumofwearand  friction. Thepartsmustbeheldinpositionwithinveryclosetolerancestoprovidequietand efficientoperation,andatthesametimepermitfreedomofmotion.

Designandselectionofabearingandthematerialofitscompositiondependsonthesizeof theforcesactingonit,whethertheyareconstantorintermittentforces,thetypeoflubrication availableandtheenvironmentinwhichitwilloperate.Whenusedinrotatingparts,theforces thebearingmaycopewithare: •

axial

•

radial

ABearingisanysurfacethatprovidessupportfor,orissupportedby,anothersurface.Itisa part in which a journal, pivot, pin, shaft, or similar device turns, revolves, or slides. The Bearings in aircraft components are designed to produce a minimum of friction and a maximumofwearresistance. Gears are used in conjunction with bearings and shafts to transmit power, change drive directions,andincreaseordecreaserotationalspeed.


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B6.8Bearings 1of18


Part-66 Subjec t

Axial,or thrustloads, actalongtheshaftbeingsupported,e.g.:pulling orpushingtheshaft, asshowninFigure8.1.



,actatrightanglestotheshaft,e.g.vertical(hanging)weightona horizontalshaft.ThisisrepresentedinFigure8.2.

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Theseareloadswhichareacombinationofbothradialandaxial.Whenadded,theresult canbeportrayedasforcesdiagonaltotheshaft,asillustratedinFigure8.3.



Abearingismountedinastructure;obviouslythatstructuremustbeadequatetosupportthe bearingrigidlyandwithstandalltheloadingthebearingitselfissubjectedto.Inmanycases italsocarriesoilgalleriesto supplythebearing. Themachinedseatinwhich the bearingis heldmustbecutpreciselytoholdthebearingtotherequiredtoleranceoffit,andnotplace unnecessarystressonthebeari ngbybeingoutofround.Insomecases theseatareamay provideameansoflocatingthebearing,suchasacutoutforatangonabearing‘shell,’as showninFigure8.4,oragrooveforalocatingring.



Bearings are manufactured in many different forms, shapes and sizes to cater for various loads and requirements.  A car is a source of numerous bearing types, from the plain bearingsontheenginecrankshaft,theballbearingsinaccessories,therollersandneedlesin thegearbox,totaperbearingsinthewheelhubs. Thebearingtypesthatwillbecoveredinthistopicare: 



plainbearings(sleeveorslidingbearings),whichincludesbushes anti-friction bearings (rolling or rolling element bearings), which are ball, roller and needlebearings


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Part-66 Subjec t

Inaplainbearing,themostobviousmotionbetweensurfacesisoneofsliding. Aplainbearingisabroadcylindricalcomponent,of amaterialsofter thanthejournalof the shaft it supports.  The softer material is often layered onto a steel backing, or it may be machinedintotheparentmetalofthesupportingcomponent,asisthecasewithitemslike camshafts running in aluminium alloy casings.  A cylindrical plain bearing is illustrated in Figure8.5.

Aplainbearingisusuallydesignedtotakeradialloads,sometimesalsocalled‘journal’loads, which you will recall are those acting at right angles to the axis of the shaft, asshown  in Figure8.6.

 Plainbearingsoftenhavegroovescutinthem,asshowninFigure8.7,tostoreanddisperse lubricant,andaresometimessplitalongtheirlengthtoallowassemblyonashaft.Notealso thelocatingtangsmentionedearlierthatareusedtolocatethebearinginitsmount.

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Asplitbearingisusuallytermedashellbearing;themostcommonare thebigendbearings and crankshaft bearings of piston engines.  These split bearings may be of ‘composite’ construction, to meet the requirements of a surface material that will be self lubricating to taketheloadatstart-upbeforeoilispumpedintotheshell,andsufficientlyrigidtostandhigh radialloads. Aliningmaterialissuperimposedontoasteelbacking,asillustratedinFigure8.8.Thelining materialcouldincludemetalssuchas: whitemetal silver lead Babbitt  

Alloysof: 

copperandlead



aluminiumandtin



leadandbronze.

Babbitt,asoftsilveryalloyoftin,lead,copper,andantimonyisusedformainbearinginserts insomeaircraftreciprocatingengines. Plainshellbearings,suchasthosein Figure8.9, areusedformanycranksha fts;camshaft, con-rodandaccessorydrivebearings.Theirlargesurfaceareahelpsthemwithstandheavy shockloadingsandtheydonotrequirethehighprecisionmachiningthatarollingelement bearingdoes,thereforetheyarerelativelyinexpensivetoproduce.

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Part-66 Subjec t

Someplainbearingsmaybeflangedtotakethrustandradialloadcombinations,suchasthat shown in Figure 8.10 (A); special plain bearings may act purely to bear thrust loads.  An example of a plain thrust bearing is the thrust washer, shown in Figure 8.10 (B).  Thrust washers are often used to take side loads in gearboxes, behind gears and on the end of shafts

  



Comparativelysmallremovableplainbearings,ofaonepiececylindricalsleeveconstruction, maybetermedabushorbushing.Theymaybeusedtosupportthesmallershaftsofsome engine accessories, the ends of control shafts and rods, or supporting the trunnions of varioussystems. Self lubricating bearings such as the ‘Oilite’ bush shown in Figure 8.11 (A), fall into this category.Thesearemadefrom‘sintered’metal,thatis,powderedmetalwhichispressed andheatedtofuseitintoa strongsolidmaterialwithasponge -likestructure. Lubricantcan beimpregnatedintothemetaltomakeupalmost30%ofitsvolume.Anotherimpregnated metal bush variety has grooves and channels cut in it, into which Teflon plastic, Poly-Tetra-Fluro-Ethylene(PTFE)ispacked.ThisisshowninFigure8.11(B).

 BushesmayalsobemadeentirelyofPTFEwhichisselflubricating,forapplicationswhereoil or grease is dangerous, such as in oxygen systems, or in corrosive environments where PTFEischemicallystable. In areas where the temperature is too high for conventional lubricants, a carbon-graphite bushcanbeused,buttheyarebrittleandeasilychippedorbroken.Theyareusedingas turbine engine hot-ends or in food processing machinery where non-toxic materials are essential.


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Bushesandplainbearingsaredesignedtohaveaclearancebetweenthebearingsurface andshaftjournal.Figure8.12showsthisinexaggeratedform.

 Diametricalclearanceis minimalin aircraftbearings,andrarelyexcee ds0.003thousa ndths ofaninch,dependentonbearingsizeanduse.Withintheclearance,afilmoflubricantis maintainedwhiletheshaftismoving.Thefilmisdesignedtocompletelyseparatethemoving surfacestoensurefrictionisreducedtoonlythatpresentinthelubricantitself,andalsoto cushionshockloadings. Duringstart-upofanengine,thejournalsurfacemayrestonthebearingmetalandfriction willberelativelyhigh.Thisshouldbetheonlytimethatwearoccursinthebearing.

Commonly called ‘rolling element’ bearings, roller and ball bearings are grouped as antifrictionbearingsduetotheirabilitytoalmostentirelyeliminatefriction.Thesebearingsare producedinavarietyofformsforvarious uses. Rollingelementbearingsofferanumberof advantagesoverplainbearingsasfollows: 

theyhaveverylowstartingfriction



theycarryheavyloads



theyaresuitablewherereversingoroscillatingmovementsoccur



theyruncoolerthanplainbearings

Theycreatemuchlessfrictionthanaplainbearing. Standardtypesofrollingelementbearingsyoumayexpecttoseeonaircraftare: 



ThesearepicturedinFigure8.13(A)and(B)respectively.

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Inthistypeofbearing,finelymachinedballsofsurfacehardenedsteelrollwithininnerand outer rings. The clearance is closely controlled and a cage (retainer) may be used to separatetheballs. AsseeninFigure8.14,themajorcomponentsofastandardballbearingarethe: 

,orrace



,orrace



,inthiscase,balls











Theouterringmayhaveanexternalgrooveforasnap-ringtoretainitinitsinstalledposition. The outer corners may be chamfered to aid installation.  The manufacturer will precisely machinetheexternalfacesifthebearingistobeinstalledinadouble,orduplex,situationas displayedinFigure8.15. Internally,theracesmaybeespeciallydeepgroovedtotakethrust/radialloadcombinations. Shields,orseals,canbefittedtotheracestoretainlubricantandprotectthebearingfrom contamination.  The racesmay also be machinedin such a way internally or externally to makethemself-aligning. Withawidevarietyofsizes,tolerancesandfeatures,itisvitalthatthecorrectbearingforthe applicationbeused. Partnumbersandotheridentifyingmarkswillusuallybefoundgrou nd oretchedontheouterraceface,asshowninFigure8.16. 

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Ballbearingsareusedinaccessorydrives,gearing,andinanyapplicationwhereanarrow bearingisrequired,whichiscapableofwithstandinghighspeedsandoperatingwithverylow frictiondrag.Theirdrawbacksarecostanddiametricalsize.Becauseoftheir‘point’contact, theyarenotgoodatbearingheavyoscillatingloadswhichwouldcausestressatthetinyarea onwhichthebearingpressesintotherace.

Angularcontactballbearings,dependingontheangle,canhandlehighthrustloadsin combinationwithmoderateradialloads.Thesebearingscanbemountedsinglyback-toback,face-to-faced,orintandem.

Roller Bearing can be Cylindrical, Needle, Tapered or Spherical type. All roller bearings utiliserollingelementsthatarecylindricalandfollowaflatraceway.Aswellascarryinghigh radial loads they also handle shock loads better than ball bearings.  Examples of roller bearingsarepicturedinFigure8.18.

 Guiding landsor shoulders on the inneror outerraces  retain the rollers,and a separating retainerkeepseachrollerapartfromits neighbours. Figure8.19showsatypicalcylind rical rollerbearing,alongwith commonterminologyforrollerbeari ngs.Cylindricalrollerbearings havethehighestradialloadandspeedcapacitycomparedwithotherrollerbearings.

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Atypeofrollerbearinginwhichtherollingelementhasa lengthsignificantlygreaterthanits diameteristermeda‘needle’rollerbearing.AnassemblyispicturedinFigure8.20.

 Needlerollerbearingssupportheavyradialloadsandoccupylessspacediametricallythan anequivalentrollerbearing. Needles have a slightly higher co-efficient of friction because of the number of rolling surfaces, but are used where weight is undesirable and compactness required: e.g. in automotive gearboxes andtransmission shafts.  Needle roller bearingscan run directly on journals. Un-cagedNeedlerollerscannotrunathighspeeds.Becausetherollersarenotseparated, effectivelubricationisdifficulttoachieveathighRPMallowingtherollerstomakemetalto metalcontact.–commonlyusedinuniversaljointsandashingebearings. Caged needle roller bearings can carry heavy radial loads and are used in gearboxes, crankshaftmainandcon-rodbear ings,heavyaccessoriesandareas requiringa heavyload capacitybearing with a diameterless thanan equivalent ballbearing. Caged needle roller bearings that are oil lubricated (either splash or pressure fed) can be run at high speed. Caged needle roller bearingsthat are lubricated withgrease cannot be runat highspeed, effectivelubricationisdifficulttoachieveathighRPMallowingtherollerstoskidandmake metaltometalcontact.

Roller and needle bearings can carry heavy radial loads and are used in gearboxes, crankshaftmainandcon-rodbear ings,heavyaccessoriesandareasrequiringaheavyload capacitybearingwitha diameterlessthananequival entballbearin g. Theirdrawbacksare lowerspeed(moresowithneedlerollers),costandwidth.


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These bearings are very rugged and are capable of supporting heavy radial and thrust combinationloads.A taperedrollerbearing,Figure8.21,hasitscylindr icalrollersarranged withtheiraxisatanangletotheshaftaxis.

 Theouterraceisknown asa cup,theinneracone.Theratioofthrus ttoradialload ingcan becateredforbydesigningthebearingwithavariationintheangleoftaperchosen. ForExample: Aheavythrustloadwithlittleradialloadrequiresalargetaperangle.(Figure8.22A) Slightthrustloadswithheavierradialloadingwouldrequireonlyasmalltaperangle.(Figure 8.22B)

 Purelyradialloadsarenotsupportedbytaperrollerbearingsastheringswouldtendtobe forced apart. The internal clearance in these bearings is adjustable during installation, makingthemtoleranttosomeminormisalignment. Tapered roller bearings are used on car front wheels, aircraft wheels and helicopter rotor masts.Theymaybefittedontheirownorinpairstotakeloadsineitherdirection,suchasin theautomotivedifferentialinFigure8.23.

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Oftenashaftmaybemisalignedduringrotationduetoloadingdeflection,forinstance,onthe mainbearingsofaircraftpistonenginecrankshafts,asindicatedinFigure8.24.  





 

 Anothersituationiswhenmovementisinmorethanoneplane,e.g.:some controlrodjoints.Figure8.25showsacutawaycontrolrodjoint.

Inthese  situations, whenmovem ent isin more than one plane, a bearing whichcan align itselftoachangingaxisisrequired.Anumberofsolutionsareused;Figure8.26showsa curvedouterracewithadoublerowofballs.Becauseofthemovementoftheinnerrace,this type cannot support heavy loads, and they are difficult to fit with end covers for lubricant retentionorprotection.

 Asimilarrollerbearing,whichcancarryloadscomparabletoasimilarsizedrigidbearingand allowconsiderableshaftflexing,alsohasacurvedouterjournal,asshowninFigure8.27.

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Bymakingthe rollersbarrel-shapedor spherical,the innerrace,roller s,andretaine rcanbe made self-aligning to the outer ring.  It can be done in two ways; with a convex roller curvatureasshowninFigure8.28(A),oraconcaverollercurveasinFigure8.28(B).

 Asmentionedbefore,aself-aligningrequirementisfoundincontrolrodendbearings,Figure 8.29showstwosolutions.

Sofarwehaveonlyconsideredbearingsdesignedtosupportcombinationradial/thrustloads suchasdeepgrooveballandtaperedrollersandflangedplainbearings.Wesawinplain bearingsthatathrustwasherisdesignedtotakepurelythrustloads;rollingelementbearings haveacorrespondingequivalent. Ball,rollerandneedlebearingsdesignedforpurelythrustloadshavethetracksoftheirraces alignedtobearthethrustloadingalongtheaxis,asisshownbytherollerbearinginFigure 8.30,andtheballbearingofFigure8.31.   





This type of bearingis oftenused  mountedverti callyto support heavymachi nery on work shop floors e.g. a radial arm drill press, orin variablepitch  propellers which employ them betweenthepropel lerhubbarrelandthebladebuttto bearcentrifugalforceandpermitthe bladetoswivel. Ifradialloadingisexpected,anormalbearingmustbemountedalongsidethethrustbearing. IssueB:January2008

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Aneedlethrustbearinghasaveryhighloadcapacityandcanalsotakelimitedradialloads,if theneedlerollersaretaperedaswellasbeingsphericallycontoured,aspicturedinFigure 8.32.

Thrust bearings are used in engine prop shafts, behind and supporting helical cut gears, (Figure8.33),whic htendtobe forcedoutof engagementbecauseof theirtoothshape,and arealsousedashelicoptermastbearings.



Thisisprovidedwithinverystricttolerancestoallowforlubricantpassage,movement,and heatexpansion,aswellastoallowforfittingmethods,whichwillbeexplainedlaterinthis topic.Ballandrollerbearingsareoftenclassifiedaccordingtotheirtolerancegrouping.

Inorderforrollingelementbearingstooperatecorrectly,theinnerandouterringsmustseat correctlyinorontheirrespectiveshaftsandhousings.Theeasewithwhichtheshaftcanbe placedintoposition,or removed,is termedits‘fit’.A loosefitmeansitcanbemounted and removedwithease,probablywithamanualpush.Howeverinmanycasesafittermedan ‘interference’fitisrequiredbythebearingsoperatingenvironment.


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Thisisdefinedasafitbetweentwopartsinwhichthepartbeingputintoaboreislargerthan theboreitself.AbushdesignedforaslightinterferencefitisshowninFigure8.34.Inorder tofit themtogethertheborecanbe expandedbyheatingand/orthepartshrunkbychillin g. When the twocomponents match again in temperature theyare tightly fitted together. No additionalhardwareisrequiredforfitting;howe vertheareaarou ndtheboremaybesubje ct totensilestress.

 Theturbineshaftofagasturbineenginemayuseabearingthatisfittedafterimmersionina bathof heatedoilwhereitexpands ,whenat aspecifiedtemperatureitis removedfromthe oilbathandquicklyslippedontoitsshaft.Asitcoolstoshafttemperatureitshrinkssecurely ontotheshaft. Heavyinterferencefitsreducetheinternalclearanceofthebearing,whichifnotdesignedfor, will result inbeari ng failure.  Loosefitti ngresul ts inexce sscreep , which may damage the housing,andalsocausesnoisyoperation. Topreventaxialmovementofthebearinginthehousing,acirclipcanbeused.Agroove maybemachinedintheouterringtolocatethecirclip,asshowninFigure8.35.

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Alternately, the circlip may be located in a groove in the housing bore itself, as shown in Figure8.36.

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Aretainingplateissometimesusedifhighstrengthaxiallocationisrequired.Aplateisfitted overtheboreinthehousinginwhichthebearingislocated,andissecuredbystuds,nutsor bolts onto the housing.  It in turn secures the bearing against an internal shoulder in the housing,alsoprovidessealingforlubricant,andpreventsingressofcontaminants.Avariety ofretainingplateisillustratedinFigure8.37.

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Theadvantagesofbearingretainingplatesare: 

highaxialretentioncapacity

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usedwheninterferencefitscannotbeutilised

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noaffectonbearinginternalclearance

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bearingreplacementwillnotdamagehousing

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easytoinstallreplacementbearings

Disadvantagesofretainingplatesare: 

housingrequiresboltholes

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raisesresidualstresslevelsinthehousing

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itaddsweightandrequiresmorespace


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Airframeandcontrolbearingsareretainedbyinterferencefits,butthesemaynotbesufficient toholdthebearinginplacewithsignificantaxialloadsand/orvibration.Thereareanumber of approved ‘staking’ methods that may be used; however these should never be used to compensateforpoorinterferencefits.

This consists of the deformation of the bearing housing using a staking punch (a tool designed forasto the job). The metal isimpre ssed around the circumference bearing in sucha way forceit intothechamf erat the top ofthebore  inthehouofthe singaroundthe bearing outer race, as shown in Figure 8.38.   In some circumstances the shaft may be staked(Figure8.39).



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Figure8.40showsthestakingmethodusedonaflyingcontrolsurfacecontrolrod.

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noaddedweightorspace

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easybearinginstallation


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canaffectbearingclearances

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induceshighresidualstresses

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bearingremovalcandamagethehousing

Aspecialrollertoolrollsthe metalcircumferenceof thehousingoverthebearingouterrace edge;thisisshowninFigure8.41.



Alternately a sleeve fitted betweenthe bearingand housingmay be swaged, as shown in Figure8.42.

 Theadvantagesanddisadvantagesofswagingaremuchthesameasforimpressionstaking. Approvedstakingtoolsmustbeused.Centre-punches,chiselsorscrew-driverswillseverely damagehousingsandbearings.Thestakingprocessmustbecarriedoutinaccordancewith maintenancemanualinstructions.


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TOPIC6.9GEARSandGEARTRAINS-TRANSMISSIONS Gears are used in conjunction with bearings and shafts to transmit power, change drive directionsandincreaseordecreaserotationalspeed.

Thespurgearisyoureverydaygear,theyhavestraightteethwhichareparalleltotheaxisof theshaft,andconnectparallelshaftsonly.Spurgearsaresimpletomanufacturebutcanbe noisyin operation.Whentwo spurgearsof differentsizesmeshtogether,thelargergearis calledawheel,andthesmallergeariscalledapinion.Figure9.1isanexample.

Thegearbeingdriven,suchasthepropellershaftinFigure9.2,willrotateintheopposite directiontothedriving(crankshaft)gear.

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Amodificationofthespurgear,helicalgearsalsoconnectparallelshaftsbuthavetheirteeth cutatanangleorhelixtothegearshaftaxis.ThiscanbeseeninFigure9.3.Theycancarry heavierloadsathigherspeedsthanequivalentsizedspurgearsandrunmoresmoothlyand quietly.

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Singlehelicalgearsproduceendthrustastheymesh,becausethegearwantstoslidealong theshaftinthedirectionofthetoothangle.Theshaftonwhichtheyaremountedrequires thrustbearings,oneoftheapplicationsforthebearingyoulearnedaboutinaprevioustopic. You may see double helical, or ‘herringbone’ gears designed to balance thrust forces and eliminatetheneedforthrustbearings.AsyoucanseeinFigure9.4,the‘V’patternofthe gearteethgivesthegearitsname,supposedlyresemblingtheskeletalremainsofaherring. A more appropriate description might compare them to the tread pattern of mud gripping tyres.

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Bevel gears are used between intersecting shafts and can be designed for any angle of intersection.  Spiral bevel gears, with their curved teeth, are quieter and smoother in operationandcancarrygreaterloadsthanequivalentsizestraightorspurbevelgears. Figure9.5showsexamplesofstraightbevelgears(A)andspiralbevelgears(B). Theteethofbevelgearsareformedonaconicalblank.Thetheoreticalpointofintersection istermedtheapexandtheteethofbevelgearsconvergeontheapex.

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Thesearea progression ofthespir albevel gearthem e. Asshownin Figure 9.6, theyare similarbuttheaxisoftheshaftsarenotonthesameplaneandthereforedonotintersect. Originallydesignedforautomotiveuseindifferentials,theysufferfromheavyslidingcontact andrequireanextremepressure(EP)lubricantinmostcases.Theyarehowever,quieter andsmootherinoperationthanspiralbevelgears.

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Thistypeofgearconnectsnon-intersectingshafts,usuallybutnotalwaysatrightangles.As showninFigure9.7,theyusea‘worm-screw’,shapedlikeathreadcutontheshaft,todrivea worm-wheelwhichisverysimilartoaspurgear.Thespeedratioiscalculatedbydividingthe numberofthreadstartsontheworm-screwintothenumberofteethontheworm-wheelgear. Wormgearsofferhighreduct ionsinsinglesteps,theyarequietandcancarryheavyloads , howevertheyareinefficientcomparedtoothergearsduetothelargeslidingmovementof theirteeth.Atreductionsoverabout20:1,thegear-wheelcannotdrivethewormscrew.

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Part-6 6 Subj ect

This gear system is used to transmit motion between a rotating spur gear and a linear toothedrack.Acommonuseisinmanycarsteeringsystems. Figure9.8isanexampleofarackandpiniongearassembly.       

As shown in Figure 9.9, a sector gear is used whenonly part of the rotationof the output shaftisrequired.A‘sector’issimplypartofacompletecircle,andasillustrated,onlypartof whatwouldbeafullcirculargearisusedforthesectorgear.

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Part-66 Subjec t

Enginesmustoperateatrelativelyhighspeedsformaximumefficiency.Propellersmust operateatlowerspeedsformaximumefficiency Therefore,reductiongearsareusedtoallowboththeengineandthepropellertooperate withintheirmostefficientrevolutionsperminute. Reductiongearsareclassifiedbythenumberofstepusedtobringaboutthespeed reduction.Asinglereductiongearisagearmechanismconsistingofapairofgearsora smalldrivegear(pinion),whichdirectlydrivesalarge(bull)gear. Forexample,ina2-to-1singlereductiongear,thenumberofteethonthedrivengearistwice thatofthedrivingpinion.

Primarilyusedasreductiongearinginaircraftengines,epicyclicgearingisalsofoundina car’sautomaticgearbox.Epicyclicgearingisageararrangementusinganinnersungear, planetarygearsfixedtoaspider,andanoutergearring.Theterm‘epicyclic’isderivedfrom ‘epi’meaning‘upon’,and‘cycle’,acircle.Similarly,‘planetary’givesanindicationofgears circlingacentralgearinthesamemannerasplanetsorbitingasun. Asreductiongearing,ithastheadvantageoveraspurgearsetbykeepingthepropellerofan aircraftengineonthesameaxisastheengineoutputshaft. Twoarrangementswhichmaybeusedare: 

spurplanetary



bevelepicyclic

As shownin Figure9.10, in this system all gears are mountedin the sameplane with the outerringhavinginternalspurteeth.DifferingratiosofinputtooutputshaftRPM(ordirection ofrotation),maybeachievedbylockingtheringgear,sungearorspider;and/orusingthe ringgear,sungearorspiderfortheinput/output.

 Thissystemismostlyusedinreductiongearingassemblies.


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Part-66 Subjec t

Bevelgearsaremountedonthespider(thebranchedshaftformingthemountingaxlesofthe smallgearsateitherend). Thegearsystemislongerbutofsmallerdiameterthanaspurplanetary. Figure 9.11 shows a bevel epicyclic gear assembly.  Again, varying ratios and change in direction of rotation may be achievedif any ofthe sub-assembliesare locked, ormade  to supplytheinput/output,duringthedesignofthesystem.

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

Differential gearing is similar to bevel planetary; however the end gears are usually of the samesize(asshowninFigure9.12). Anyofthethreeshaftscanbeinputoroutput,andtherecanbearrangementsof: 

twoinputsandoneoutput

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oneinputandtwooutputs

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oneinputandoneoutputwiththeothershaftfixed

Doubleinputdiffere ntialgearsare usedinsomeaircra ftcontrolsystem sand instrumentsto eitheraddtwoangularmovementsorfindthedifferencebetweenthem.

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A gear is a toothed wheel transmitting motion from one shaft to another.  If the interconnected or meshed gears are of different radii, and consequently have differing numbersofteeth,thespeedofrotati onoftheshaftswill differ.Forexample ,inFigure9.13,  becausegear(A)has24teethandgear(B)has12teeth,(B)willalwaysrotateattwicethe speedof(A).Shaftspeedisusuallystatedin‘revolutionsperminute’(RPM).

 Thespeedratioofacargearboxisgivenasthenumberofenginerevolutions(input)forone revolution of the tailshaft (output). Similarly the back axle ratio is givenas the number of revolutionsofthetailshaft(input)foronerevolutionofthedrivingwheels(output). Weknowthatwithgear eddrivestherelative speedoftwomeshedgear s(rpm)dependson theratioofteethoneachgear.Theconventionforthe‘gearratio’oftwomeshedgearsis:  

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 

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numberofteethondrivinggear numberofteethondrivengear

Therefore from Figure 9.11 ifgear  A isthe drivinggearandgear  B isthedrive n gear,the gearratiois  

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 

 

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24 12

=2:1 

Howeverthespeedratio(rpm)isdirectlyoppositeandwouldthereforebe1:2.


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Part-66 Subjec t

Thesemay alsobe termedintermediategears. Idlersare usedbetweentwoothergears to either: 

maketheoutputgearrotateinthesamedirectionastheinput

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tolinkgearswhenthereisadistancebetweenthem

Thepresenceofanidlergearmakesnodifferencetotheratioofspeedbetweentheinput andoutput.Inthefollowingdiagram,Figure9.14,gear(B)isanidlergear,itisbothadriven andadrivinggear,howeverif(A)istheinputand(C)theoutput,thegearratioisstill8/24or 1:3(speedratio3:1).Theonlyparttheidlerplaysistotransferthedriveandmake(C) rotateinthesamedirectionas(A).

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However,weareoftenconce rnedwith‘geartrains ’thathavevariou sarrangementsof large andsmallgearsinsequence,asshowninFigure9.15.Butnothingchanges,ifgearAisthe drivingandgearEisthedriven,allthegearsinbetween(B,C,D)areidlergearsandhaveno effectonthegearratio(orspeedratio/rpm)betweenAandE.

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Notice the direction of rotationof gears inFigur e 9.15. Direction of rotationcan besimpl y calculated by counting the number of gears between the driving gear, and the gear in question.Thefollowingcanthenbeapplied: forevennumbersofgearsfromtheinputgear,rotationisoppositethefirstgear 

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foroddnumbersofgears,thedirectionofrotationiswiththefirstgear

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Part-66 Subjec t

 Thissystemismostlyusedinreductiongearingassemblies. Infigure9.16theringgearisthedriveandthesungearisthedriven,inthisinstancethe reductionratio(speedratio)wouldbeasfollows(note,wearenottalkingaboutgearratio):

Iftheringgearhas72teethandthesungearhas36teeth(notetheplanetarygearsare irrelevant),thereductionratio(speedratio)wouldbe 

Howeverreductionratiosareusuallyexpressedinwholenumbersandwouldthereforebe .  Reductiongearsareclassifiedbythenumberofstepsusedtobringaboutthespeed reduction.Forexample,agearmechanismconsistingofapairofgearsorasmallgear (pinion)drivenbytheengineshaft,whichdirectlydrivesalarge(bull)gearonthepropeller shaft,iscalledasingle-reductiongear    


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Part-6 6 Subj ect

Controlchainsarefoundinanumberofapplicationsinaircraftcontrolsystems. Usuallytheyarefoundwherethereisashortdistancebetweentwohardwareitems,(two torquetubesforexample)orthesystemrequiresarelativelylargeamountofforcetooperate anditisnotpracticalforasmallcableruntobeinstalled. Thecockpitpedestalarea,aspicturedbelow,isacommonplacetofindshortchain assemblies.



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Likebicyclechains,aircraftcontrolchainsaremadeupofmultiplelinksandcanbejoined togethertomakeanendlessloop.Theycanalsobeofaterminatingtype–asinglelength withastartandfinish. Aircraftchains,however,tendtobemanufacturedtoclosertoleranceandhavemorerigorous inspectioncriteriaTheyalsohavecomponentssuchaschainkeepers,similartocable guards,tostopthechainfromcomingoffthesprockets Theyhavedrivesprocketsandidlersprocketsandamethodofre-tensioningtheassembly

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Part-6 6 Subj ect

Aircraftcontrolchainsareusuallyoftherollertype.Rollersarefreetorotatearoundbushes whichholdtheinnerlinkplatestogether. Pinsaremountedintheouterplates,whichareclampedtogethertojointhewholeassembly.

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Chains are sized according to pitch, width and roller diameter. Pitch refers to the distance between the axis of each roller.  

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

Non–reversiblechains(i.e.notacontinuousloop)arejoinedorconnectedusingend-fittings similartocableterminals,andcanonlybefittedinonedirection.

 Likecables,chainguardscanbefittedtosprocketstopreventthechainfromcomingadrift whentensionisreduced. Stoppiecesonthechainguardandnon-interchangeableendconnectorscanpreventthe incorrectinstallationofnon-reversiblechains


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Part-6 6 Subj ect

Sprocketsareusedinmanyaircraftsystemsandinavarietyofdifferentapplications.



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Sprockets(dependingontheirapplication)areusuallymadefrom; 

Steelor,



Alloy.

butcanbemanufacturedfrom, 

Hardfibrematerialor,

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Plastic.

Theirprimaryrollistoconvertrotarymotionintoliniermotionand/orliniermotionintorotary motion.Alsowherelongchainsystemsareusedtheyactassupportforthechainintheform ofanidlersprocket.

Anidlersprocketisanon-drivingsprocketusedtosupportthechainrun.

IDLER SPROCKET

 Sprocketteetharespacedtoalignwithchainpitch. Thenumberofteethonthedrivesprocketcomparedtonumberofteethonthesprocket beingdrivendeterminesthegearingratioofthechainsystem. e.g.–adrivesprockethas36teethandthedrivensprockethas12teeth.Thedrivensprocket rotates3timesforeverydrivesprocketrotation.


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Part-6 6 Subj ect

Drive belts are used in a number of applications (example - found in helicopter drive systems).Theycomeinmanydifferentshapesanddesigns

Beltdrives carry less load than chain drives. However, theydo not require lubrication, are moreshockresistant,andtheyarequieterandsmootherinoperation Allbeltsareavariationoftwotypes: 

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Veebelts non-slip‘toothedbelt’

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Beltdrivescancomeinsingleormultiplebeltandpulleysystems.

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Part-66 Subjec t

 Backlashis theclearanceor playbetweentwogearsinmesh.A certainamountisrequired forlubricantpenetrationbetweentheteethofthegears,andtoallowforthermalexpansionof thegeartrain.Figure9.28showsexactlywherebacklashoccurs.

 Gear backlash must be established in accordance with the relevant maintenance manual. Excessive backlash can be caused by worn gear teeth or improper meshing of teeth or bearingswhichdonotsupportthegearsproperly.Someoftheeffectsofincorrectbacklash are: 

Excessivebacklashcanresultinsevereimpactonthegearteethfromsuddenstopping orreversalofload.InFigure9.29,thegearsaresettoofarapartcausingexcessive backlash.Thiswillcausebrokengearteeth,gearbouncingandgearnoise.





Toolittlebacklash,asshowninFigure9.30,willcauseexcessiveloadingonthegear teeth,lubricantwillbeforcedfromthegearsurfaceandprematurefailurewillresult.

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

Ameasurableamountof backlashcanbedetectedbyhandifonegearisheldandthe other rocked.  Usually this is minimal and a Dial Test Indicator (DTI) is set up to measureit.Atypicalgearmayhave.003to.004inchbacklash.Thecorrectbacklash willbegiveninthemaintenancepublicationswhichmustbefollowed.


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Part-6 6 Subj ect

 Ascrewthreadthatcanbeusedtogivehighmechanicaladvantageisknownasascrew jack. Screwjacksconvertrotationalmovementintolinearmovement. Insomeapplicationstheyarecalled‘screwjackactuators’or‘linearactuators’ Thereare3basictypes: 

The‘wormandpeg’ mechanismisnotusedmuch becauseexcessivebacklash oftenresultsfromlocalised wearonthepegandworm screw    The‘wormandnut’mechanism typeisoftenusedonaircraft.It usuallyhasanAcmeorsquare thread.Actuationisbyturningthe shafttodrivethenut,orbyturning thenuttodrivetheshaft.   



 Thistypeisalowfrictionvariationofthewormandnutmechanism.Itissometimesknownas aballscrewactuator.Theclosedpathwayfortheballbearingsallowthemtocontinually circulatealongthematinggroovesoftheshaftandnut,tothusreducefrictionwhenthe mechanismisactuated.Thisformofscrewjackismostcommonlyusedinaircraftformany applicationsinparticulartheflapdrivesystem. IssueB:January2008

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Part-6 6 Subj ect

Push-pullrodshavemanysystemapplications.Forexample,theaileronandtrimtab mechanismabovehasatleastthreeadjustablepush-pullrods.

 Push-pullrodsandtheiradjustableend-fittingscreateatypeofrigidlinkagethateliminates theproblemofvaryingtensioninacontrolsystem.Theypermitthetransferofcompression (push)ortension(pull)forces. Push-pullrodsareusuallymadeofseamlessaluminiumalloytubing.Someareofafixed length,butmosthaveatleastoneadjustableend.   Thetypeofrodshownhas threadedrodendsrivetedto thetube. Adjustablerodendsare screwedontothethreaded endsandlockedwithacheck


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Part-6 6 Subj ect

Adjustablepush-pullrodmayhave; •

left-handthreadedendandaright-handthreadedend.

•

Allowsshorteningorlengtheningtherodwhenalteringsystemrigging.

Thechecknutsareloosened,enablingtherodtobeturnedwithoutdisturbingtherodeye ends

Pullpushrodscanhave fixedoradjustableends.  



Bellcranksareusedto;

•

Transmitforceormotion and

•

Permitachangein directionofthatforceor motion.  

Thebellcrankpivotsonbearingsmountedonashaft. Thereisusuallynoadjustmentpossibletothebellcrank,althoughthepush-pullrodscanbe adjustabletoenablecorrectsystemrigging.


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Thisviewofanaircraftruddershowsatorquetubeandtorquearm. 

 Thetorquearmreceivesthelinearmotioninputfromthecontrolsystemandcausesthe torquetubetorotate.Thusthetorquearmandtorquetubecanbesaidtohaveconverted linearmotiontorotarymotion. Atorquecantransmitforceacrossalongdistance. Thetorquetubeisdirectlyconnectedtothecontrolsurfaceandcausesittomove. Torquetubesandtorquearmsareusuallyoffixedlength,diameterandangle.Theyare usuallynotadjustable. Note-alsotheadjustablepush-pullrodsconnectingtherudderbalancelevertotherudder balancetab.

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AA For m TO-18 B-6b Aircr aft Hardware

Part-6 6 Subj ect

TOPIC6.10CONTROLCABLES Cables are the most widely used linkage in primary control systems because they offer advantagesoverothertypesofcontrol: Strong,lightweightandflexible,theyareeasytoroutethroughsharpturns.

:   

Tensionmustbeadjustedregularlybecausethecablestendtostretchovertime. Pronetowearatpointsofcontacti.e.pulleys,bellcranks,quadrantsetc. Tensionvarieswithtemperaturechanges.

Allaircraftcontrolcableispreformed-thatis,thewiresareshapedinaspiralformbeforethe strandsandthecable sarewoundtoge ther.Thismeansifthecableiscut,thewireswillnot springout. Acableismadeupofstrandswhich,inturn,aremadeupofindividualwires. (RefertoJeppesenAirframeP1-41)

All cable pulleys shouldbe fitted with guards to preventthe cable from jumpingout of the pulleygrooveifthecabletensiondropsoff.

 Figu re 10.1 The two main types of cable guards or cable keepers are shown. Care should be taken to ensure that the guard runs close, but does not contact pulley or cable.


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the

B6.10ControlCables 1of4


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Withtemperature variations, the aircraft structureand control cables expand or contract at differentrates. Whenthishappens,thereisa tendencyforthecabletens iontoalter. Thisiscompensated byfittingcabletensionregulatorswhichuseextensionorcompressionofspringstoadjust thepositionoffree-pivotingcablequadrants.



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Ifthestructureexpandsatagreaterratethanthecables, As cable tension increases past the value of the spring tension, the spring will compress, allowingthequadrantstomovewiththecables. Whenacontrolinp utismade,thecab lesarepulledfromonesideonly.The wholeunitrigid.Thequadrantsthenturninunison. When rigging the cable system, a allowanceforambienttemperature.


keepthe

 on the tension regulator makes

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  FlexiblecablesthatruninsideanoutersleevearecalledBowdenorTeleflexcables. Bicyclebrakecablesareanexample–theoutersleeveisfixedatbothendsandthecableis attachedtoahandleandthebrakecaliper.Whenthehandleispulled,thecablerunsfree insidethesleeveandactuatesthecaliper. Thistypeofcableisusedonaircraftwherethereisnotenoughroomforcablepulleys. Thecable’sflexibilitymakesitidealforanareasuchasacockpitarea,wherethereisnot enoughroomforcablepulleysandpush-pullrodsareimpracticalbecauseofthecurvingpath thecontrolsystemmustfollow.

 Thesteelcablecaneitherbea push-pulltypeor canrotate,drivinga screwjack.Theouter casing of the cables protect the inner cable from damage. If the outer casing is bent, however,theinnercablewillnottravelfreely. Likewise,iftheoutercasingischafedthrough,theinnercableislikelytobedamaged. Boththesesituationsiscauseforrejectionoftheassembly.


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B-6b Aircraft Hardware Student Resource

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