B-6b Aircraft Hardware Student Resource

7/25/2019 B-6b Aircraft Hardware Student Resource

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StudentResource

SubjectB-6b:

AircraftHardware

Copyright 2008 Aviation Australia

Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,orotherwisedisposedof,withoutthewrittenpermissionofAviationAustralia.


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B-6b Aircraft Hardware

IssueB:January2008 Revision2 Page1

CONTENTS

Definitions 3

Study Resources 4

Introduction 5

Aircraft Fastening Devices 6.5-1

Springs 6.7-1

Bearings 6.8-1

Transmissions 6.9-1

Control Cables 6.10-1


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DEFINITIONS

Define

Todescribethenatureorbasicqualitiesof.

Tostatetheprecisemeaningof(awordorsenseofaword).

State

Specifyinwordsorwriting.

Tosetforthinwords;declare.

Identify

Toestablishtheidentityof.

List

Itemise.

Describe

Representinwordsenablinghearerorreadertoformanideaofanobjectorprocess.

Totellthefacts,details,orparticularsofsomethingverballyorinwriting.

Explain

Makeknownindetail.

Offerreasonforcauseandeffect.


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STUDYRESOURCES

JeppesenGeneral

JeppesenAirframe

AC43.13-1B/AC43.13-2ACombinedAircraftInspectionandRepair

B-6bStudentHandout

DaleCraneAviationMechanicHandbook


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INTRODUCTION

Thepurposeofthissubjectistofamiliariseyouwithaircrafthardware,fasteningandlockingdevices,gears,bearingantransmissions..

Oncompletionofthefollowingtopicsyouwillbeableto:

Topic 6.5.1 Fasteners - Screw Threads

Listtypesofstandardscrewthreadsusedinaircraftandidentifytheirthreadforms,dimensionsandtolerances.

Describemeasuringofscrewthreads.

Topic 6.5.2 Fasteners - Bolts Studs and Screws

Identifyvarioustypesofaircraftboltsandmachinescrewsbyspecification,markingsandinternationalstandards.

Identifyanddescribethefollowingnuttypes:

selflocking

anchor

standard

Identifytypesofstudsanddescribetheirusesandmethodsofinsertionandremoval.

Identifytypesofselftappingscrews.

Statethepurposeofdowelsanddescribetheirapplication.

Topic 6.5.3 Fasteners - Locking Devices

Identifyandstatethepurposeofthefollowinglockingdevices:

tabandspringwashers

lockingplates

splitpins

pal-nuts

wirelocking

quickreleasefasteners

keys,circlipsandcotterpins.

Topic 6.7 Springs

Identifyvarioustypesofspringsandlistmaterialsusedintheirconstruction.

Statecharacteristicsofspringsandlisttheirapplications.

Topic 6.8 Bearings

Identifyvariousbearingstypesandstatetheirpurpose.

Definebearingapplication,loadsandmaterialsusedintheirconstruction.


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Topic 6.9 Transmissions

Identifygeartypesandlisttheirapplication

Statethepurposeofthefollowing

GearRatios.

Reductionandmultiplicationgearsystems.

Drivenanddrivinggears.

Idlergears.

Meshpatterns.

Identifybelts,pulleys,chainsandsprocketsanddescribetheirapplication.

Topic 6.10 Control Cables

Identifycabletypesanddefinethepurposeofthefollowingassociatedcomponents:

Endfittings,turnbucklesandcompensationdevises.

Pulleysandcablesystems

Bowdencableandflexiblecontrolsystems


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B-6b.5.1 Screw ThreadsIssue B: January 2008 Revision 2 1 of 4

TOPIC6.5.1AIRCRAFTFASTENINGDEVICES

Aircraft hardware isthetermusedtodescribevarioustypesof fastenersandmiscellaneoussmallitemsusedinthemanufactureandrepairofaircraft.Thesafeandefficientoperationof

anyaircraftisgreatlydependentuponthecorrectselectionanduseofaircrafthardware.Vibrationisalwayspresentduringaircraftoperation.Consequentlytheremustbeprovisionforsafetyingorlockingoffastenerstopreventthemfromvibratinglooseinflight,andanyoneinvolvedinaircraftmaintenancemustbefamiliarwiththemethodsused.

Aircraft hardware standards

Therearevariousstandardsusedforhardwarespecificationsintheaircraftindustry.

Themostcommonstandardsyouwillencounterare:

American Standards:

AN(AirforceNavy)

MS(MilitaryStandards)

NAS(NationalAerospaceStandards).

European or French standards

NSA(NATOStandardisationAgency)

Aircraft manufacturers standards

BAC(BoeingAircraftCorporation)

FON(Fokker)

AMS(AeronauticalMaterialsSpecifications)

Standardhardware,whichisavailablefromaviationsuppliers,isidentifiedbyaspecificationnumber.Specialfastenersmustbereplacedwiththosehavingthesamepartnumberandnotwithsimilar lookingstandardhardware.Oftenthedifferencebetweenastandardandaspecialpartisthematerialusedtomanufactureitortheclosertoleranceinitsmanufactureoritmaybeamorecriticalinspectionofthepart.

AN fasteners can be replaced by NAS and MS equivalent fasteners.NASandMSstandardhardwaremustnotbereplacedbyANstandardhardware.


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Bolts and screws

Varioustypesoffasteningdevicesallowquickdismantlingofaircraftpartsthatmustbetakenapartandputbacktogetheratfrequentintervals.Rivetingorweldingthesepartseachtime

theyare serviced would soonweaken or ruin the joint. Furthermore, some joints requiregreatertensilestrengthandstiffnessthanrivetscanprovide.Boltsandscrewsaretwotypesoffasteningdeviceswhichgivetherequiredsecurityofattachmentandrigidity.Generally,boltsareusedwhengreatstrengthisrequired,andscrewsareusedwherestrengthisnotthedecidingfactor.

Boltsandscrewsaresimilarinmanyways.Theyarebothusedforfasteningorholding,andeach has a head on one end and a screw thread on the other. Regardless of thesesimilarities, thereareseveraldistinctdifferencesbetweenthetwotypesof fasteners. Thethreadedendofaboltisalwaysblunt,whilethatofascrewmaybeeitherbluntorpointed.

The threadedendofaboltusuallyhasa nutsecuredto ittocompletetheassembly. Thethreadedendofascrewmayfitintoafemalereceptacle,oritmayfitdirectlyintothematerial

beingsecured.Abolthasafairlyshortthreadsectionandacomparativelylonggriplengthorunthreadedportion;whereasascrewhasalongerthreadedsectionandmayhaveaclearlydefinedgriplength.Aboltassemblyisgenerallytightenedbyturningthenutonthebolt:theheadof the boltmay ormay not bedesigned for turning.Ascrewisalways tightenedbyturningitshead.

Whenitbecomesnecessarytoreplaceaircraftfasteners,aduplicateoftheoriginalfastenershouldalwaysbeused.

Classification of threads

Aircraftbolts,screwsandnutsarethreadedineitherthe:

AmericanStandard

ANC(AmericanNationalCoarse)

ANF(AmericanNationalFine)

ANEF(AmericanNationalExtraFine)

ANP(AmericanNationalPipe

Figure 5.1


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UnifiedStandard

UNC(UnifiedNationalCoarse)

UNF(UnifiedNationalFine)

UNEF(UnifiedNationalExtraFine)

Figure 5.2

BritishStandard

BSW(BritishStandardWhitworth)

BSB(BritishStandardBrass)

BSP(BritishStandardPipe)

BSPT(BritishStandardPipeTaper)

Figure 5.3

SIMetric

TheSIMetricsystemofthreadsisgenerallyusedonequipmentmanufacturedinEurope.Allmetricthreadswillhavethethreadformasillustrated

Figure 5.4


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Class of fit

Threadsarealsodesignatedbyclassoffit.Theclassoffitofathreadindicatesthetoleranceallowedinmanufacturing:

Class1isaloosefit.

Class2isafreefit.

Class3isamediumfit.

Class4isaclosefit.

Class5isatightfit

NOTE:

Aircraft bolts are almost always manufactured in a Class 3, medium fit.

A Class 4 fit requires a wrench to turn a nut on to a bolt, whereas a Class 1 fit can easily be

turned with the fingers.

Generally, aircraft screws are manufactured with a Class 2 thread fit for ease of assembly.


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B-6b.5.2Fastener-Bolts,StudsandScrews

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B:January2008 Revisi 3

TOPIC 6.5.2 FASTENERS BOLTS, STUDS AND SCREWS

THREADED FASTENERS IN AIRCRAFT

Threaded fastenersallowparts tobefastenedtogetherwithall of thestrengthunthreaded

fasteners provide. However, unlike rivets, threaded fasteners may be disassembled and

reassembledanalmostinfinitenumberoftimes.

THREAD TYPE AND FITS

Mostboltsusedinaircraftstructuresareeithergeneral-purpose,internal-wrenching,orclose-

toleranceAN,NAS,orMSbolts.Aircraftbolts,screws,andnutsarethreadedineitherthe

AmericanNationalCoarse(NC), theAmericanNationalFine (NF), theAmericanStandardUnifiedCoarse(UNC),ortheAmericanStandardUnifiedFine(UNF)series.

Inadditiontobeingidentifiedaseithercoarseorfine,threadsarealsodesignatedbyclassof

fitfromonetofive:

AClass1threadisaloosefit,

AClass2isafreefit,

AClass3isamediumfit,

AClass4isaclosefit,and

AClass5fitisatightfit.

AClass1fitallowsyoutoturnthenutallthewaydownusingonlyyourfingers.Wingnutsare

agoodexampleofaClass1fit.AClass4and5fitrequiresawrenchtoturnanutdownfrom

starttofinish.AircraftboltsareusuallyfinethreadedwithaClass3fit,whereasscrewsare

typicallyaClass2or3fit.


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DESIGNATION CODES

Anaircraftbolt isgivenapartcode indicating itsdiameter in1/16 inch incrementsand its

lengthin1/8inchincrements.Forexample,anAN4-7identifiesaboltthatmeasures4/16or

1/4inchindiameterand7/8inchinlength.

For bolts that are longer than 7/8 inch, the code changes. For example, a 1 inch bolt is

identifiedbya-10representing1inchandnofraction.Inotherwords,thereareno-8or-9

lengths.Dashnumbersgofrom-7to-10,from-17to-20,andfrom-27to-30.Therefore,a

boltthatis11/2incheslongis identifiedbya-14.AboltwiththecodeAN5-22identifiesan

AirForce-Navyboltthatis5/16inchindiameterand21/4incheslong.

Threadedaircraft bolts 1/4 inch in diameter and smaller are dimensioned in screw sizes

ratherthan1/8inchincrements.

TheAN3boltistheexceptiontothisrule.Thesemachinescrewsizesrangefrom0to12.A

number10fastenerhasadiameterofapproximately3/16inchandanumber5fastenerhas

a1/8inchdiameter.


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STANDARD AIRCRAFT BOLTS

Aboltisdesignedtoholdtwoormoreitemstogether.Boltsthataretypicallyusedforair

framestructuralapplicationshavehexheadsandrangeinsizefromAN3toAN2O.

Boltsareidentifiedbytheirdiameterandlength.Adiameterrepresentstheshankdiameter

whilethelengthrepresentsthedistancefromthebottomoftheheadtotheendofthebolt.

Aboltsgriplengthisthelengthoftheunthreadedportionasshown.Ifthegriplengthis

slightlylongerthanthisthickness,washersmustbeaddedtoensurethatthenutcanprovide

theproperamountofpressurewhenitistightened.Ifthegriplengthissubstantiallylessthan

thethicknessofthematerialstheboltsthreadswillextendintothematerial,resultingina

weakerjoint.

Whenjoiningtwopiecesofmaterial,theircombinedthicknessdeterminesthecorrectlengthofbolttouse.

ThediameterofaboltisindicatedbythenumberimmediatelyfollowingtheprefixsuchasAN.

Thedashnumberofstandardboltsindicateslengthin1/8ofaninchincrement.

AN3-6A:

AN=AirForce/Navy.

3=Diameterin1/16inch

-6=Lengthin1/8inch

A=Notdrilledforsplitpin(Noletter=Drilledforsplitpin).


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STANDARD AIRFRAME BOLTS

Aircraftboltsareavailable in cadmium-platednickelsteel, corrosionresistantsteel,and in

2024aluminiumalloy.

Unlessspecified,aboltismadeofcadmium-platednickelsteel.Acorrosionresistantbolt,on

the other hand, is identified by the letter C inserted between the diameter and length

designations.AluminiumalloyboltsareidentifiedbythelettersDD.Forexample,aboltthat

is1/4inchindiameter,3/4inchlong,andmadeofcadmium-platednickelsteelisidentifiedby

thecodeAN4-6.However,ifthesameboltismadeofcorrosionresistantsteelitcarriesthe

codeAN4C6,whereasanaluminiumalloyboltwouldbeAN4DD6.

In addition to the designation code, most aircraft bolts have a marking on their head

identifyingwhattheboltismadeofand,inmanycases,themanufacturer.Forexample,AN

standard steel bolts aremarked with either a raised dash or asterisk in the centre of its

manufacturedhead,corrosion-resistantsteelismarkedbyasingledash,andANaluminum-

alloyboltsaremarkedwithtworaiseddashes.

TheFAAforbidstheuseofaluminiumalloyboltsandalloysteelboltssmallerthanAN3on

structural components.Furthermore,since repeated tighteningandlooseningofaluminium

alloyboltseventually ruins their threads, they are not used inareaswhere theymust be

removedand installed frequently.Aluminium alloynuts can beusedwith cadmium-plated

steelboltsloadedinshear,butonlyonlandaircraft,notbeusedonseaplanes.

Whenhardwarewasfirststandardized,almostallnutswerelockedontoaboltwithacotter

pinand,therefore,allboltshadholesdrilledneartheendoftheirshanktoaccommodatea

cotterpin.However,whenself-lockingnutsbecamepopular,manystandardANboltswere

madewithouta drilledshank.Tohelpyou identifywhetherornotabolthasaholedrilled

throughit,theletterAisusedinthepartcode.

Forexample,ifanAappearsimmediatelyafterthedashnumbertheboltdoesnothavea

hole,However,theabsenceofanAindicatesaholeexistsintheshank.Asanexample,an

AN6C-12Abolt is3/8inchindiameter,madeofcorrosion-resistantsteel,11/4incheslong,

andhasanundrilledshank.


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Some AN bolts, such as those used to fasten a propeller into a flanged shaft, must be

safetiedbypassingsafetywirethroughholesdrilledthroughtheboltshead.Aboltdrilledfor

this type of safetying has the letter H following the number indicating its diameter. Forexample,thepartnumberAN6H-34Aidentifiesaboltthatis3/8inchindiameter,madeof

nickel-steel,hasadrilledhead,is31/2incheslong,andhasanundrilledshank.

CLEVIS BOLTS: AN21 TO AN36

Tobeloadedinshearonly.Forexample,acontrolcablemustbeattachedtoacontrolhorn

withaboltthatislooseenoughtoallowthecabletopivotfreelyasthecontrolsurfacemoves,

butnotsoloosethatexcessplayexists.Fortheseapplicationsaclevisboltisused.

TheAN21throughAN36clevisbolthasadomedheadthatistypicallyslottedorrecessedto

acceptascrewdriver.Auniquefeatureofaclevisboltisthatonlyashortportionoftheshank

isthreaded,andthereisasmallnotchbetweenthethreadsandtheshank.Thisresultsina


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longgriplengthwhichincreasestheboltsshearstrengthandallowsthebolttorotatemore

freelyinitshole.

Thediameterofaclevisboltisgivenin1/16inchincrements.Thelengthofaclevisboltis

morecriticalthanthatoftheothertypesofboltsand,therefore,itisalsomeasuredin1/16

inch increments with a dash number indicating the length. For example, an AN29-20

identifiesa9/16inchdiameterclevisboltthatis20/16(11/4)incheslong.

EYEBOLTS AN42 THROUGH AN49)

Theseboltsare used inapplications where external tension loadsare tobeapplied.Theheadofthisboltisspeciallydesignedfortheattachmentofaturnbuckle,aclevis,oracable

shackle.Thethreadedshankmayormaynotbedrilledforsafetying.


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DRILLED-HEAD ENGINE BOLTS: AN73 TO AN81

AN73throughAN81boltsarehex-headednickel-steelboltsthataresimilarinappearanceto

the AN3 through AN2O series. However, unlike standard bolts, drilled-head engine bolts

haveathickerheadthatisdrilledwithasmallholeineachoftheflatsandinthecentreofthe

head.Aswithmostbolts,thediametersofdrilled-headengineboltsarein1/16increments

whileboltlengthsarein1/8inchincrements.Thediameterisindicatedbythesecondnumber

following the AN designation while the bolt length is indicated by a dash number. For

example,adrilled-headengineboltdesignatedasAN746hasadiameterof1/4inchanda

lengthof3/4inch.

CLOSE TOLERANCE BOLTS: AN173 to AN186

Close tolerance bolts are designated AN173 toAN186 and are ground to a tolerance of

+0.0000.0005inch.ThisismuchtighterthenstandardAN3throughAN14boltswhichare

manufacturedwitha toleranceof+0.0000.0025,or AN16 throughAN2Oboltswhichare

manufacturedwithatoleranceof+0.0000.0055inch.Closetoleranceboltsmustbeusedin

areasthataresubjecttopoundingloadsorinastructurethatisrequiredtobebothriveted

andbolted.

Closetoleranceboltscarryatrianglemarkontheirheadsandaregroundtoamuchtighter

tolerancethanstandardbolts.


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INTERNAL WRENCHING BOLTS MS20004 TO MS20024)

MS20004 through MS20024 internal wrenching bolts are high-strength steel bolts used

primarilyinareasthataresubjectedtohightensileloads.Asix-sidedholeismachinedinto

thecenteroftheirheadstoacceptanAllenwrenchofthepropersize.Theseboltshavea

radius between the head and shankand, when installed in steelparts, the holemust be

counterboredtoaccommodatethisradius.Whenaninternalwrenchingboltisinstalledinan

aluminumalloystructure,aMS20002Cwashermistheusedundertheheadtoprovidethe

neededbearingarea.

ThestrengthofinterimwrenchingboltsismuchhigherthanthatofastandardsteelANbolt

and,forthisreason,anANboltmustneverbesubstitutedforaninternalwrenchingtype.


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PRECISION AIRFRAME BOLTS

NAS: National Aerospace Standards Aerospace Industries Association)

Developed and updated by the National Aerospace Standards Committee Aerospace

Standards include many standards on precision fasteners as well as other aerospace

hardware.

EvenDashnumberindicatethestandardissuelengthin1/8inchIncrements.

Griplengthismeasuredfromundertheheadendofthethread.

OdddashNumbersisaspecialapplicationbolt=Griplength1/16inchlongerthan

evennumberusedwhereastandardMSboltiseithertoolongortooshort.

The basic NAS number identifies the part. The suffix letters and dashnumbers separate

differentsizes,platingmaterial,drillingspecifications,etc.

ItisnecessarytorefertoaspecificNASpageintheStandardsbookforthelegend.


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MS: Military Standard

All aircraft bolts, except AN bolts, are measuredby their grip length, not by their overall

length.

Standardissueboltshaveevendashnumbers,inmostapplicationstheseboltswilldobut

where thegrip length iscriticalanda(standard)gripiseither too longortooshortaODD

Dashnumberboltisavailable.

Aircraftboltsmustbefittedcorrectly,justanylengthboltwillnotdo.Theloadmustbeonthe

shankandnotonthethread.Tensilestrengthof160,000psito180,000andashearstrength

of95,000psito108,000dependingonthetypeofbolt

STANDARD AIRFRAME BOLTS: NATO Standardisation Agency

TheEuropeanNSAboltsshouldnotbeconfusedwiththeAmericanNASBolt.Alwaysrefer

totheaircraftillustratedpartscataloguewhenpurchasingfasteners,oftenboltsmaylookthe

samebuttheyarenot.

NSASpecificationisaFOURdigitnumberidentifyingthetypeofboltegNSA5022

ThefirstDashnumbersisthediameterin1/16incheg-4

Theseconddashnumberisthegriplengthin1/16incheg-22

Example:NSA5022-4-22isa1/4x13/8Hexheadbolt

NOTE: To prevent dangerous cross use between metric and imperial sizes, all western

aerospaceboltsaremanufacturedtoimperialspecifications.


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NUTS

Allnutsused inaircraftconstructionmusthavesomesortoflockingdevicetopreventthem

fromlooseningandfallingoff.

Therearetwobasictypesofnuts,self-lockingandnonself-locking.Asthenameimplies,a

self-locking nut locksontoa bolt on its own while anonself-locking nut relies oneither a

cotterpin,checknut,orlockwashertoholditinplace.

SELF-LOCKING NUTS

Self-locking nuts,or locknuts,employ a lockingdevice intheirdesign tokeep them fromcoming loose.The two general typesofself-lockingnutsused inaviation are the fiber, or

nylontype(LowTemperature),andtheallmetaltype.

A self-locking nut must be screwed onto a bolt until all of the chamfer on the bolts end

protrudes throughthe insert. Ifthebolt isnotchamfered,atleastone threadbutnotmore

thanthreethreadsshouldprotrudethroughthenut.Ifmorethanthreethreadsareexposed,

youriskthedangerofbottomingoutthenutandundertorqueingtheassembly,thuscreating

astresspointthatcouldfail.Ifmorethanthreethreadsareexposed,eitherreplacethebolt

withoneofthecorrectlengthorinstallawasher.

Aself-lockingnutsdashnumberspecifiesbothdiameterandnumberofthreadsperinch.Forexample,a -524 representsaself-locking nut that fitsa5/16 inch fine threadboltwith24

threadsperinch.

LOW-TEMPERATURESELF-LOCKINGNUTS

Nylon self-locking nuts should not be used in any location where the temperature could

exceed250F.However,youmayusethemonenginesinthoselocationsspecifiedbythe

enginemanufacturer.

AN365self-lockingnutsareusedonboltsandmachinescrewsandareheldinpositionbya

nylon insert above the threads. This insert has a hole slightly smaller than the thread


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diameteronwhichitfits.ThenutsClass3fitallowsittorundownonaboltsthreadseasily

untiltheboltenterstheinsert.

AN364nutsresembletheAN365selflockingnut,buttheyarethinandareapprovedonlyfor

shearloads,nottobeusedintension.AN364nutsaretypicallymadetobeusedonclevis

boltsthatdonothavedrilledshanks.

METALSELF-LOCKINGNUTS

Inapplicationswheretemperaturesexceed250F,all-metallocknuts,suchastheAN363,

are used. Some of these nuts have aportion of their end slot tedand the slots swaged

together.Thisgivestheendofthenutaslightlysmallerdiameterthanitsbodyallowingthe

threadstogripthebolt.Othershavetheendofthenutsqueezedintoaslightlyovalshape,

and as the bolt screws up through the threads it must make the hole round, creating a

grippingaction.


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STANDARD NUTS

AN310CASTLENUT

Thesefine-threadnutsaredesignedtofitonastandardairframeboltwithaClass3fit,and

areusedwhen thebolt is subjected to either shear or tensile loads. Thesize of anut is

indicatedinthepartcodebyadashnumberwhichdenotesthesizeoftheboltitfits.For

example,anAN31O-6nutfitsanAN6boltwhichhasadiameterof3/8inch.

Castlenutsareavailableincadmium-platednickelsteel,corrosion-resistantsteel,and2024

aluminumalloy. Unless specified, a castle nut ismadeof cadmium-platednickel steel. A

corrosionresistantnut,ontheotherhand,isidentifiedbytheletterCinsertedbeforethe

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

diameterof6/16(3/8)inch.

AN320SHEARCASTLENUT

TheAN320shearcastlenutismadeofthesamematerialandhasthesametypeofthread

asaAN31Onut.However,shearcastlenutsaremuchthinnerthanstandardcastlenutsand,

therefore,areusedonlyforshearloadsonclevisbolts.AnAN320-6nutisashearcastlenut

thatisusedonanAN26clevisbolt.Analuminumalloy(2024)nutisidentifiedasanAN320fl6.

AN315PLAINNUT

TheAN315 plainnut has no castellationsand, therefore, cannot beheld inplaceusinga

cotterpin.Sincethesefine-threadnutshavenolockingprovisions,aspring-typelockwasher

mustbeusedincombinationwiththenut.Thelockwasherappliesaspringforcetoprevent

thenutfromshakingloose.AN315nutsareusedwitheithertensileorshearloadsandare

made of either nickel steel, corrosion-resistant steel, and aluminum alloy. The type of


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materialusedis indicated inthedesignation code inthesameway it isforbolts. Inother

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 2024aluminumalloy.Furthermore,plainnutsaremadewithbothrightandleft-handthreads.For

example,anAN315-7RisanickelsteelnutwithrighthandthreadsthatfitsanAN7bolt.An

AN315C-4L,ontheotherhand,isa1/4inchdiametercorrosion-resistantsteelplainnutwith

left-handThreads.

AN316CHECKNUT

Insomeinstancesaplainnutislockedinplaceusingachecknut.Achecknutissimplya

secondnutthatistightenedagainsttheprimarynutsoitcannotturnoff.AnAN316checknut

ismade of cadmium-plated steel and is available in both right-and left-hand threads. An

AN316-4Risaright-handchecknutthat fitsaquarter-inchthread,whileanAN316-4Lhasa

left-handthread.

AN340ANDAN345(LIGHTHEXNUTS):

Thesenutsareusedinnon-structuralapplicationsrequiringlighttension.

LiketheAN315andAN335,theyrequirealockingdevicetosecurethem.

AN355SLOENGINENUT

Thisnutisdesignedforuseonanaircraftengineandisnotapprovedforairframeuse.Itis

made of heat-treated steel and has national fine threads that produce aClass 3 fit. It is

availableinsizesfromAN355-3(3/16inch)toAN355-12(3/4inch)andhasslotscutinitfora

cotterpin.

AN360PLAINENGINENUT

This engine nut is similar to the AN355 in that it is approved for use on engines only.

However,anAN360differsfromanAN355inthatitdoesnothavecotterpinslotsandhasa

blackrustprooffinish.AnAN360-7isaplainenginenutthatfitsa7/16inchbolt.

AN350WINGNUT

Wingnutsareusedwhenitisnecessarytoremoveapartfrequentlywithouttheuseoftools.

Aircraftwingnutsaremadeofeithercadmium-platedsteelorbrassandareavailableinsizes

tofitnumber(gauge)sixmachinescrewsupto1/2inchbolts.Allofthesenutshavenational

finethreadsthatproduceaClass2fit.Nutsformachinescrewsizesaredesignatedbythe

seriesnumber.However,nutsusedonboltshaveaboltsizegivenin1/16inchincrements

followedbythenumber16.Forexample,withanAN3SO-616wingnut,the-6indicatesthat

thenutwillfita3/8(6/16)inchbolt.


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ANCHOR NUT

Anchornutsarepermanentlymountednutplatesthatenableinspectionplatesandaccess

doorstobeeasilyremovedandinstalled.Tomaketheinstallationofanaccessdooreasier

wherethereareagreatnumberofscrews,afloatinganchornutisoftenused.Withafloating

anchornutthenutfitslooselyintoasmallbracketwhichisrivetedtotheskin.

Sincethenutisfreetomovewithinthebracketitalignsitselfwithascrew.Tospeedthe

production of aircraft, gangedanchor nutsare installed around inspection plate openings.

These are floating-type anchor nuts that are installed in a channel that is riveted to the

structure.Eachnutfloatsinthechannelwithenoughplaysothatascrewcanmovethenut

enoughtoalignit.


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ThetoolisthenunscrewedfromtheRivnut,leavingathreadedholethatacceptsmachine



TINNERMAN NUTS

Tinnermannutsarecost-economicalnutsthatarestampedoutofsheetmetal.Becauseof

theirsemirigidconstruction,tinnermannutscanbeadaptedforuseinmanysituations.For

example, tinnerman nuts are commonly usedon lightaircraft tomount instruments to the

instrumentpanelaswellasattachinspectionpanelsandcowlings.

Tinnermannutsusedtomountinstrumentscaneitherbeinstalledinaninstrumentpanelor

inthe instrument case itself.Toreduce the chance ofmagnetic interference, the nutsare

made ofbrassand the cage that holds the nut is constructedofphosphor bronze. If the

instrument is rear mounted, the legs of the nut are long enough to pass through the

instrumentcase.Iftheinstrumentisfrontmounted,thenutfastensintothescrewholeinthe

instrumentpanel.

Anchor type tinnerman nuts are riveted to a structure to hold screws used to secure

inspectionplates.

The cowlingson some light aircraft are held onwith self-tapping sheetmetal screws. To

preventthesheetmetalscrewsfromenlargingtheholesinthecowlingbyrepeatedinsertion

andextraction,aU-typeTinnermannutisslippedovertheedgeoftheinsidecowlingsothat

itstraddlesthescrewhole.Whenascrewistightenedintothenut,thespringactionofthe

nutholdsthescrewtight.

RIVNUTS

GoodrichRivnutsweredeveloped by the BEGoodrichCompany toattach 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

circumferenceofthehole.ThisnotchlockstheRivnutintotheskintopreventitfromtuning

whenitisusedasanut.ARivnutofthepropergriplengthisthenscrewedontothepuller

and inserted into the holewith its key alignedwith the keyway cut in the hole.When the

handleofthepullerissqueezed,thehollowshankoftheRivnutupsetsandgripstheskin.


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screwsforattachingade-icerboot.Rivnutsarenowusedinmanyareasonaircraftandthe

automotiveindustry.

STUDS

Astudisashaftthatisthreadedatbothends.

Theyhaveashortthreadononeendandalongthreadontheother.Theshortthreadisa

coarse thread and amuch tighter fit than the long thread sothe studwill remain inplace

whenthenutonthelongendisundone.

Wherejointshavetobebrokenfrequently,studsareusedinplaceofboltsorscrewsto

preventdamagetothetappedholes.

Aircraftmaintenancemanualwillgivemethodsofstudremovalandinstallation.


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SCREWS

Screwsareprobablythemostcommonlyusedthreadedfastenerinaircraft.Theydifferfrom

boltsinthattheyaregenerallymadeoflowerstrengthmaterialsbutnotalways.Screwsare

typically installedwith a loose-fitting thread,and the headshapes aremade toengage a

screwdriver orwrench. Some screws have a clearly defined grip length while others are

threadedalongtheirentirelength.

Therearethreebasicclassificationsofscrewsusedinaircraftconstruction:

machinescrews,whicharethemostwidelyused;

structuralscrews,whichhavethesamestrengthasbolts;and

self-tappingscrews,whicharetypicallyusedtojoinlightweightmaterials.

MACHINESCREWS

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

machinescrewsisthatthethreadsofamachinescrewusuallyrunthefulllengthoftheshank,

whereasboltshaveanunthreadedgriplength.


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Screwsnormally haveaClass2,or free fit and are available inbothnationalcoarse and

national fine threads. Themost commonmachine screws used inaviationare the fillister

headscrew,theflat-headscrew,theround-headscrew,andthetruss-headscrew.

STRUCTURALSCREWS

Structuralscrewsaremadeofalloysteel, are heat treated,andcanbeusedasstructural

bolts. Theyhaveadefinitegrip and the sameshearstrengthasa boltof the samesize.

Shank tolerances are similar to AN hex-head bolts, and the threads are National Fine.

Structuralscrewsareavailablewithfillister,flat,orwasherheads.

TheseheadtypesareNOTinterchangeablewitheachother.Thecorrectscrewdrivermustbe

usedtoavoiddamagetothescrewhead,especiallytotitaniumscrews.

Never use a Philips screwdriver on a Torq-set screw, nor a slotted screwdriver on aHi-

Torquescrew.

SELF-TAPPINGSCREWS

Self-tappingscrewshavecoarse-threadsandareusedtoholdthinsheetsofmetal,plastic,or

plywoodtogether.Thetype-Ascrewhasagimlet(sharp)point,andthetypeBhasablunt

pointwiththreadsthatareslightlyfinerthanthoseofatype-Ascrew.


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Therearefourtypesofheadsavailableonself-tap-pingscrews:

Roundhead,

Trusshead,

Countersunkhead,whichisflatontop,and

Counter-sunkovalscrew.

Thetruss-headisrounded,similartotheroundheadscrew,butisconsiderablythinner.

WASHERS

Washersprovideabearingsurfaceareafornuts,andactasspacersorshimstoobtainthe

propergrip

lengthforaboltandnutassembly.Theyarealsousedtoadjustthepositionofcastellated

nutswithrespecttodrilledcotterpinholesinboltsaswellasapplytensionbetweenanut

andamaterialsurfacetopreventthenutfromvibratingloose.Thethreemostcommontypes

ofwashersusedinairframerepairaretheplainwasher,lockwasher,andspecialwasher.

PLAINWASHERS

All AN washers are in the 900 series. AN960 plain washer provides a smooth surface

betweenanutandthematerialbeingclampedThesewashersaremadeofcadmium-plated

steel,commercialbrass(B),corrosion-resistantsteel(C),and2024aluminiumalloy(D).They


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areavailableinsizesthatrangefromthosethatfitanumbertwomachinescrewtothosethat

fitaone-inchbolt.

Ifa thinwasherisneeded,a lightserieswasherthat isone-halfthethicknessofa regular

washer is available. An example of where a light series washer should be used is if the

castellationsofanAN31Onutdonotlineupwithacotterpinhole

whenthenutisproperlytorqued.Inthissituationalightserieswashercanbesubstitutedfor

theregularwashertoaligntheholes.Alightserieswasherisidentifiedbytheletter1added

tothecode.Forexample,thecodeANO6OLidentifiesalightserieswasher.

Whenworkingwithwood or composite structures, washerswith a largesurfacearea are

usedtospreadthefastenerloadoverawiderarea.Theselargeareawasherscarrythecode

ofAN970andareallmadeofcadmium-platedsteelwithinsidediametersfrom3116to1/2

inch.

LOCKWASHERS

Insome instances it isnot convenient touse self-locking nutsorcotter pinson bolts.For

theseapplications,alockwasherisoftenusedbetweenthenutandjointsurfaceifthejointis

notstructurallycritical.Lockwashersaremadeofsteelandaretwistedsothatwhenanutis

tightenedagainstit,thespringactionofthewashercreatesastrongfrictionforcebetween

theboltthreadsandthoseinthenut.

Twotypesoflockwashersareusedinaircraftconstruction.ThemostcommonistheAN935

splitlockwasher.Thesewashersareavailableinsizesthatfitfromanumberfourmachine

screwtoa1/2 inchbolt.Thesecondtypeof lockwasheris the thinnerAN936shakeproof

lockwasherwhichisavailablewithbothinternalandexternalteeth.

SPECIALWASHERS

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

countersunkedge toaccommodate theradiuson thebolt head.Countersunkwashersare

madeofheat-treatedsteelandarecadmiumplated.

Finishingwashersareoften used inaircraft interiors tosecureupholstery andtrim.These

washers havea countersunk face to accommodate flush screws. Finishing washers bear

againstalargeareatoavoiddamagingfragileinteriorcomponents.

Inmany instances, keyedwasherscan be usedasasafety device.Keyedwashershave

smallkeysorprotrusionstoengageslotscutintoboltsorpanels.


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CLEVIS PINS

Aclevispinisusedinconjunctionwithtie-rodterminalsandsecondarycontrolswhicharenot

subjecttocontinuousoperation.

ClevispinsaresecuredwithasplitpinorAN416safetypin.

Awashermustbeplacedunderthesplitorsafetypin

Thepinisnormallyinstalledwiththeheaduporforward.

Thispreventslossshouldthesplitpinfailorworkout.

DOWELS

A dowel is a solid cylindrical rod, usuallymade of wood, plastic or metal. In its original

manufacturedform,doweliscalleddowelrod.Dowelrodisoftencutintoshortlengthscalled

dowelpins.

Dowels are used where the precision alignment and correct orientation of two mating

surfacesisrequired.


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Therearefollowingtypesofdowelsusedinaircraft:

Smoothsoliddowels

Hollowdowels

Threadeddowels

Splithollowdowels

Smoothsoliddowels:usuallysteeldowelpins,aremadewithhighqualityofmetallicproducts

ensuringthesmoothsurfacefinishingandhighperformance.

Hollow dowels
http://en.wikipedia.org/wiki/File:Steel-Dowel-Pins.jpghttp://en.wikipedia.org/wiki/File:Steel-Dowel-Pins.jpg


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DOWEL PIN APPLICATION

Splithollowdowelsinsomeinstancesareusednotonlytomaintainalignmentbutalsoactas

bushingsforrotatingcomponents.

Smoothsideddowelsarepressfitinstalled.


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B 6b Aircraft Hardware


TOPIC 6.5.3 FASTENERS LOCKING DEVICES

SAFETYING and LOCKING MEDIUMS

Vibration isalwayspresentduringaircraftoperation.Consequentlytheremustbeprovision

forsafetyingorlockingallfastenerstopreventthemvibratinglooseinflight.

There are variousmethods of safetying and locking.The mostwidely usedmethods are

safetywire,splitpins,lockwashers,circlipsandspecialnutssuchasself-lockingnuts,pal

nutsandjamnuts.

LOCK WASHERS

Insomeinstancesitisnotconvenienttouseself-lockingnutsorsplitpinsonbolts.Forthese

applications,a lockwasherisoftenusedbetweenthenutandjointsurfaceifthejointisnot

structurally critical. Lockwashersaremadeof steelandare twistedsothatwhena nut is

tightenedagainstit,thespringactionofthewashercreatesastrongfrictionforcebetween

theboltthreadsandthoseinthenut.


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TheAN935 lock washermaybe used between thenut and thesurface if the joint is not

structurallycritical.

TheAN936shakeproofwasheristhinnerthantheAN935lockwasherandisavailablewith

bothinternalandexternalteeth.

TAB WASHERS

Oftenusedforlockinghexheadfasteners

Atabmustnotbebentmorethanonce.

Youcanre-usemultipletabwashersafterremovingtheusedtab,dressingsharpedgesand

carefullyinspectingtheremainingtabsforcracksorscoring.

LOCKWIRE

Becauseaircraftvibrate,theremustbesomeprovisionforsafetyingorlockingallfastenersto

keep them from vibrating loose. Self-locking nuts are used for the vast majority of

applicationsinmodernaircraftconstruction,but therearestillplaceswherelockwireorsplit

pinsareneeded.Forexample,drilled-headboltsareoftenusedinvibration-proneareasand

aresafetywiredtogether.

Lockwiring is a means of securing hardware and components and is a safety method

employedinaircraftmaintenanceprocedures.

Thetypeoflockwiremostcommonlyusedismadeofstainlesssteel.

Wheninstallinglockwire,thewireshouldpulltheboltheadinthedirectionoftighteningand

shouldbetwistedevenlytothenextbolt.Aftertheendofthewireispassedthroughthehead

ofthesecondboltitisagaintwisted,thistimeforaboutthreeorfourturns.Oncethisisdone,

theexcessiscutoffandtheendsofthewirearebentbackwheretheycannotcutanyone

whopassestheirhandoverthebolts.


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Lockwiringisoftenusedincriticalareas,whereinspectionintervalsmaybeinfrequent.

LOCK WIRING METHODS HAND

Doubletwististhemostcommonlyusedmethodasshownabove.

Whenperformingbyhandpullfirmly.Startthetwistinclosetothefastener,holdendsabout

90degreesapartandtwistinaclockwisedirection(forRHthreads).

Inareaswhereanumberofboltsmustbesafetied,suchasapropeller,youmaysafetywire

theboltsingroupsofthree.Ifmorethanthreeboltsaresafetiedtogetheritisdifficulttogetthesafetywiretightenoughtobeeffective.

Single wire method is used on screws, bolts and nuts in a closely-spaced or closed-

geometricalpatternsuchasa triangle, square, rectangle orcircle. May alsobe usedon

electricalsystemsandonpartsthataredifficulttoreach.

LOCK WIRING METHODS WIRE TWISTERS

WireTwistersmakelockwiringeasybuttheycandamagethelockwiretherefore,theymust

beusedwithcare.


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They can twist wire in one direction only when the knob is pulled. To twist in the other

direction,lockthepliersandrotatemanually.

LOCKING PLATES

Thelockingplatesareusuallysecuredtoanadjacentpartofthestructurebyascrew.

Lockingplatesmaybeusedrepeatedly,providedtheyremainagoodfitaroundthehexagon

ofthenutorbolt.

PAL NUTS

Palnutsareusedwithplainnutstolocktheminplace

Commonlyusedtolockpistonenginecylinderbasenuts

Usedinmanyaircraftapplications


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Madefromlightpressedalloysteel

GRUB SCREWS

Grubscrewsusedasamethodoflockingtwothreadedcomponentstogether

QUICK RELEASE FASTENERS

QuickReleaseFastenersprovideavibrationresistantjoiningsolutionforquickandrepetitive

attachmentandremovalofpanelswithminimumeffort.


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Acirclipisaspringclipusedforbothinternalandexternallocking.

Benefits:

-Lockingandunlockingthefasteneronlyrequiresaquarterturnorapush.

-Fasteners can be locked and unlocked inamatter of seconds - saving time and

reducingcosts.

-Vibrationresistant-performswellineventhemostintensiveapplications.

CIRCLIPS

Theyareusedforretainingshafts,seals,bearingsetc.

Specialpliers,areusedtoopenorcompressthecirclipforinsertionandremoval.

TAPER PIN

Boththeplainandthreadedtaperpinareusedinaircraftstructurestomakeajointthatis

designedtocarryshearloads.Thistypeofpindoesnotallowanyloosemotionorplay.The

AN385plaintaperpinisforcedintoaholethathasbeenreamedwithaMorsestandardtaper


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pinreamerandisheldinplacebyfriction.Itcanbesafetiedbypassingsafetywirearound

the shaftand throughaholedrilled in its largeend.AnAN386taperpin issimilar to the

AN385exceptthatitssmall-endisthreadedtoaccepteitheraself-lockingshearnutAN364)orashearcastlenut(AN320).

Another typeof taperpin is one that hasa tapered flat side.Used for locatingparts onto

shaftsflatsidelinesupwithaflatontheshaft.

ROLL Pin

Rollpinsareoftenusedtoprovidelockingforajointwherethepinisnotlikelytoberemoved

ortolocksomethingontoashaftsuchasahandleorlever.Arollpinismadeofflatspring

steel that is rolled into a cylinderbut the two endsare not Joined. Thisallows the pin to

compresswhen itispressedintoaholeandcreateaspringaction thatholdsthepintight

againsttheedgeofthehole.Toremovearollpin,itmustbedrivenfromaholewithaproper

sizepinpunch.


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CLEVIS PIN

Clevis,orfiat-head,pinsareusedforhingepinsinsomeaircraftcontrolsystems.Theyare

madeofcadmium-platedsteelandhavegriplengthsin1/16inchincrements,Wheninstalling

aclevispinplacetheheadintheupposition,placeaplainwasherovertheoppositeend,

andinsertasplit(cotter)pinthroughtheholetolockthepininplace.

SPLIT PINS

Castellatednutsarelockedontodrilledboltsbypassingasplitpinthroughtheholeandnut

castellationsandthenspreadingtheendsofthesplitpin.Theyaremadeofeithercadmium-

platedcarbonsteelorcorrosion-resistantsteel.

Therearetwomethodsof securingsplitpinsthataregenerallyacceptable. Inthepreferred

method,onelegofthesplitpinisbentupovertheendofthebolt,andtheotherlegisbent

downoverone of the flatsof the nut.With the secondmethod, the splitpin isrotated90

degreesandthelegswrapped

around the castellations. It is important tonote that nuts should neverbeover-torqued to

maketheholeintheboltalignwiththecastellations.Ifthecastellationsinthenutfailtoalign

withthedrilledbolthole,addwashersunderthenutuntilasplitpincanbeinserted.


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KEYS

Keysarehardenedpiecesofmetalthatfitintocutouts(keyways)inwheels,discs,sprockets

or gears. The keyaligns thewheel, disc sprocket or gear onto a shaftwhichalso hasa

keywaycutintoit.

Differenttypes/shapes:

WOODRUFF KEY

Theadvantageofawoodruffkeyisonceitisplacedinthepartitwon'tmove.

SQUARE KEY

SquareKeyshaveatendencytomoveoutoftheirslotunlessretainedbyalockingdevice.


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PRATT AND WHITNEY KEY

ThePrattandWhitneykeyisanotherkeytypethatonceitisplacedinthepartitwon'tmove.

GIB HEAD KEY


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GibHeadkeyistaperedandisdriveninplacewheninstalled.

The Tang governs insertion depth and allows removal. Requires retention in critical

applications.

-EndofthisTopic


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ThisPageintentionallyleftblank


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TrainingMaterialOnly B6.7Springs

IssueB:January2008 Revision3 1of8

TOPIC6.7SPRINGS

Springs

Aspringisanelasticmachinecomponentwhichcanbepressed,stretchedortwistedbyaforceandthenreturntoitsoriginalshapewhenthedistortingforceisremoved.Itthereforestoresenergyasafunctionofdisplacement.Althoughspringsarenormallymadeinanalloyofsteel,oftentermedspringsteel,materialssuchasrubberorplasticmaybeused.

Aspringisusedinamechanismtoexertaforce,tostore,ortoabsorbenergy.Familiarusesofspringsinclude:

Supplyingmotivepower,forinstanceintoymechanisms,watchesandclocks.Alsotoclosepistonenginevalves

Asacounterbalance,suchasadoorclosingdevice,andtoacertainextentincarsuspension

Asashockabsorber,forinstance,inflexiblecouplings

Forvibrationcontrol,inenginemountsandinstrumentvibrationpads

Forcemeasurement,suchasthecalibratedspringsinweighingmachinesandinstruments

Retention,forinstanceasretainingrings,circlipsorspringwashers

Spring Steel

SpringSteelisaspecialclassificationofsteelthathasgreathardness,strengthandelasticity

Additionalalloyingelementsareusedinspringsteelinclude: Manganese

Chromium

Silicon

Vanadium

Molybdenum

CommonApplicationsofspringsteelinclude:

Springs

Springandshake-proofwashers

Steelrules

Measuringtapes

Feelergauges


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Spring Varieties

Thevarietyofspringconfigurationscanbeclassifiedunderfivemaintypes:

flatorleaf

helical

spiral

torsion

disk.

Flat or leaf spring

Aleafspringisaflatbeamdesignedtodeflectunderload;oneendisusuallyfirmlyanchored,whiletheotherislinkedbyamoveableshackletothemovingmachinepart.Loadscanbeappliedineithertensionorcompression(pushorpull);thisisoneofthemainadvantagesofthedesign,illustratedinFigure7.1.Anotheradvantageistherelativelylargeamountofenergythespringcanabsorbinasmallspace.

Figure 7 1

Theleafspringtypeismostcommonlyseeninautomotiverearsuspension,asshowninFigure7.2.Theyarealsoseenastheundercarriagelegsofsomelightaircraft.Thestressconcentrationsinthecentreofthespringcanbecombatedbyaddingmoreleafs(Figure7.2)

orbyhavingadiamonddesignasshowninFigure7.3.

Figure 7 2

Figure 7 3


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Helical spring

Sometimestermedacoilspring,thisisprobablythemostcommonspringtype;itiswhat

usuallycomestomindwhenyoutalkofsprings.Commonapplicationsincludeautomotivesuspension(coilovershock),enginevalvecontrolandsupplyingactuatingforceinaclutch.Thespringisessentiallyawireorbarwoundintoahelix.Theendsmaybemodifiedorgroundflatsocompressioncanbeevenlyappliedtothespring,ortheendscouldbeshapedintohooksoreyessothattensioncanbeapplied.ThesecanbeseeninFigure7.4.Thecoilspringisdesignedtoactundercompression,ortension,rarelyboth,whichwouldbedetrimentaltothespring'slife.

Figure 7 4

ompression spring

Compressionspringsfallintothefollowinggeneralcategoriesbasedontheiroverallshape:

Cylindrical,StraightorStandard-Allcoilsarethesamediameter.Thesearethemostcommonandleastexpensivecompressionsprings.Theendscanbeeitheropenorclosedandtheycanbegroundflat,althoughgrindingsignificantlyincreasesthecostandisoftenunnecessaryforsmallwiresizes.

Conical(Tapered)-Coildiameterdecreasesfromoneendofthespringtoother.Thesespringsareoftenusedwhenthereisnotenoughroomforacylindricalspring.Theycanbemadesothatthesmallercoilstelescopedownintothelargercoilsasthespringiscompressedsothatthespringcompressionsprings.

Barrel(Convex)-Taperedsothatbothendsaresmallerthanthemiddle.Thesespringscanhavesomeofthesameadvantagesasaconicalspringwiththeaddedadvantagethattheyaresymmetrical.

HourglassorConcave-Taperedsothatbothendsarelargerthanthemiddle.Thesespringscanhavesomeofthesameadvantagesasaconicalspringwiththeaddedadvantagethattheyaresymmetrical.Theenlargedendcoilsmayalsohelpkeepthespringcentredonalargerdiameterhole.


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Figure 7 5

Spiral spring

Spiralspringsarecommonlyseenintwomainshapes,bothshowninFigure7.6.ThefirstisaflatspiraltermedanArchimedesspiral,andtheotherisaconicalformwhichisamodificationofthehelicalspring.

Figure 7 6

Theadvantageofthespiralspringisitsabilitytobedeflectedinacombinationofways.Theflatspiralcanabsorbforceatatangenttoitsaxis;thatis,itcanbewounduptoclosethe


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spacebetweenitscoils.Thisisthespringprovidingmotiveenergyinclocksandclockworkengines.Theconicalformcanbecompressedorstretched,andalsoreactstoforcesatrightanglestoitsaxis.

Torsion bar

Thetorsionbarspringisessentiallyashaftofuniformcross-sectionthatstoresenergywhentwisted.Somecarsusetorsionbarsuspension,ortheyareaddedasafter-marketaccessoriestochangethesuspensioncharacteristics.TheTorsionBartorquewrenchutilisesthisprinciple.Thedrivesquareinthetorquewrenchactsasatorsionbarspring,whichactivatesgearstoshowtorqueonadialindicator.Figure7.7illustratesasimpleexampleoftorsionbar.

Figure 7 7

Disk springs

Thisclassificationofspringsincludesalargefamilyofsimilarspringtypes,suchas:

Singledisk

Multipledisk

Bellvillespring

Lockwasher

Diaphragm

Diskspringsmaybeusedwherespaceislimitedandlargeforcesarepresent;however,theytendtobedifficulttodesignandmanufacture.Thesinglediskhandlessmalldeflections.Astackormultiplediskspring,suchasthatillustratedinFigure7.8,isusedforgreaterdeflection.

Figure 7 8


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Well-knowndiskspringapplicationsincludethediaphragmspringintheautomotiveclutch,illustratedinFigure7.9.

Figure 7 9

Anothercommondiskspringvarietyarethefingerwashers,wavespringwashersandtheshakeproofwashersshowninFigure7.10,whichprovidelightlockingofnutsandbolts.

Figure 7 10


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Construction

Tounderstandspringconstruction,youneedtounderstandsomecommonterms.These

termsarelistedbelowandareshowninFigure7.11. Coils

Freelength

Groundsection

Insidediameter

Lengthinsidehooks

Lengthovercoil

Meandiameter

Outsidediameter

Pitch

Wirediameter

Figure 7 11


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Compressionspringscanhaveclosedandgroundendstositsquareonsurfacestoapplypressureevenly,ortheendscanbeopenasshowninFigure7.12.

Figure 7 12


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B6.8BearingsIssueB:January2008 Revision2 1of18

tB-6b Aircraft Hardware

TOPIC6.8BEARINGS

Broadlyspeaking,bearingscanbedividedintotwomajorgroups:

1. Plain bearings,whichhavenomovingpartsandarecomprisedofaplaincylinderor

flatwashersurface,ofamaterialgenerallysofterthantheshaftitsupports.

2. Rolling element bearings, which are an assembly consisting of hardened rolling

componentsenclosedininnerandoutercases,calledraces,betweenwhichtheyroll.

Thesearefurthersub-dividedintoanothertwomajorcategories:

ball bearings

roller bearings

Characteristics

Agoodbearinghastwocharacteristics:

It must be made of a material that is strong enough to withstand the pressures

imposedonit,andyetpermittheothersurfacetomovewithaminimumofwearand

friction.

Thepartsmustbeheldinpositionwithinveryclosetolerancestoprovidequietand

efficientoperation,andatthesametimepermitfreedomofmotion.

Designandselectionofabearingandthematerialofitscompositiondependsonthesizeof

theforcesactingonit,whethertheyareconstantorintermittentforces,thetypeoflubrication

availableandtheenvironmentinwhichitwilloperate.Whenusedinrotatingparts,theforces

thebearingmaycopewithare:

axial

radial

ABearingisanysurfacethatprovidessupportfor,orissupportedby,anothersurface.Itisapart in which a journal, pivot, pin, shaft, or similar device turns, revolves, or slides. TheBearings in aircraft components are designed to produce a minimum of friction and amaximumofwearresistance.

Gears are used in conjunctionwith bearings and shafts to transmit power, change drivedirections,andincreaseordecreaserotationalspeed.


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Bearing Loads and Their Effects

Axial Loads

Axial,or thrustloads,actalongtheshaftbeingsupported,e.g.:pullingorpushingtheshaft,asshowninFigure8.1.

Figure 8.1

Radial Loads

Radial or journal loads,actatrightanglestotheshaft,e.g.vertical(hanging)weightonahorizontalshaft.ThisisrepresentedinFigure8.2.

Figure 8.2


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Combination Loads

Theseareloadswhichareacombinationofbothradialandaxial.Whenadded,theresultcanbeportrayedasforcesdiagonaltotheshaft,asillustratedinFigure8.3.

Figure 8.3

Bearing Supports

Abearingismountedinastructure;obviouslythatstructuremustbeadequatetosupportthebearingrigidlyandwithstandalltheloadingthebearingitselfissubjectedto.Inmanycasesitalsocarriesoilgalleriestosupplythebearing.Themachinedseatinwhichthebearingisheldmustbecutpreciselytoholdthebearingtotherequiredtoleranceoffit,andnotplaceunnecessarystressonthebearingbybeingoutofround.Insomecasestheseatareamayprovideameansoflocatingthebearing,suchasacutoutforatangonabearingshell,as

showninFigure8.4,oragrooveforalocatingring.

Figure 8.4

Bearing Types

Bearingsaremanufactured inmanydifferent forms, shapesand sizestocaterfor variousloads and requirements. A car is a source of numerous bearing types, from the plainbearingsontheenginecrankshaft,theballbearingsinaccessories,therollersandneedlesinthegearbox,totaperbearingsinthewheelhubs.

Thebearingtypesthatwillbecoveredinthistopicare:

plainbearings(sleeveorslidingbearings),whichincludesbushes

anti-friction bearings (rolling or rolling element bearings), which are ball, roller andneedlebearings


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Babbitt,asoftsilveryalloyoftin,lead,copper,andantimonyisusedformainbearinginsertsinsomeaircraftreciprocatingengines.

Asplitbearingisusuallytermedashellbearing;themostcommonare thebigendbearingsand crankshaft bearings of piston engines. These split bearings may be of compositeconstruction, tomeet the requirements ofasurfacematerial thatwill beself lubricatingtotaketheloadatstart-upbeforeoilispumpedintotheshell,andsufficientlyrigidtostandhighradialloads.

Aliningmaterialissuperimposedontoasteelbacking,asillustratedinFigure8.8.Theliningmaterialcouldincludemetalssuchas:

whitemetal

silver

lead

Babbitt

Figure 8.8

Alloysof:

copperandlead

aluminiumandtin

leadandbronze.

Plainshellbearings,suchasthoseinFigure8.9,areusedformanycrankshafts;camshaft,con-rodandaccessorydrivebearings.Theirlargesurfaceareahelpsthemwithstandheavyshockloadingsandtheydonotrequirethehighprecisionmachiningthatarollingelementbearingdoes,thereforetheyarerelativelyinexpensivetoproduce.

Figure 8.9


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Someplainbearingsmaybeflangedtotakethrustandradialloadcombinations,suchasthatshown inFigure 8.10 (A); special plainbearingsmayact purely tobear thrust loads. Anexampleofa plain thrust bearing is the thrust washer, shown inFigure 8.10 (B). Thrustwashersare oftenused to takeside loads ingearboxes,behind gearsand onthe end ofshafts

A Figure 8.10B

Bushings

Comparativelysmallremovableplainbearings,ofaonepiececylindricalsleeveconstruction,maybetermedabushorbushing.Theymaybeusedtosupportthesmallershaftsofsomeengine accessories, the ends of control shafts and rods, or supporting the trunnions ofvarioussystems.

Self lubricating bearings such as the Oilite bush shown in Figure 8.11 (A), fall into thiscategory.Thesearemadefromsinteredmetal,thatis,powderedmetalwhichispressedandheatedtofuseitintoastrongsolidmaterialwithasponge-likestructure.Lubricantcanbeimpregnatedintothemetaltomakeupalmost30%ofitsvolume.Anotherimpregnatedmetal bush variety has grooves and channels cut in it, into which Teflon plastic,Poly-Tetra-Fluro-Ethylene(PTFE)ispacked.ThisisshowninFigure8.11(B).

Figure 8.11

BushesmayalsobemadeentirelyofPTFEwhichisselflubricating,forapplicationswhereoilorgrease is dangerous, such as in oxygen systems, or in corrosive environments wherePTFEischemicallystable.

Inareaswhere the temperature is too high for conventional lubricants, a carbon-graphitebushcanbeused,buttheyarebrittleandeasilychippedorbroken.Theyareusedingasturbine engine hot-ends or in food processing machinery where non-toxic materials areessential.


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Plain Bearing Diametrical Clearance

Bushesandplainbearingsaredesignedtohaveaclearancebetweenthebearingsurface

andshaftjournal.Figure8.12showsthisinexaggeratedform.

Figure 8.12

Diametricalclearanceisminimalinaircraftbearings,andrarelyexceeds0.003thousandthsofaninch,dependentonbearingsizeanduse.Withintheclearance,afilmoflubricantismaintainedwhiletheshaftismoving.Thefilmisdesignedtocompletelyseparatethemovingsurfacestoensurefrictionisreducedtoonlythatpresentinthelubricantitself,andalsotocushionshockloadings.

Duringstart-upofanengine,thejournalsurfacemayrestonthebearingmetalandfrictionwillberelativelyhigh.Thisshouldbetheonlytimethatwearoccursinthebearing.

Anti friction Bearings

Commonly called rollingelement bearings, roller and ball bearingsare grouped asanti-frictionbearingsduetotheirabilitytoalmostentirelyeliminatefriction.Thesebearingsareproducedinavarietyofformsforvarioususes.Rollingelementbearingsofferanumberofadvantagesoverplainbearingsasfollows:

theyhaveverylowstartingfriction

theycarryheavyloads

theyaresuitablewherereversingoroscillatingmovementsoccur

theyruncoolerthanplainbearings

Theycreatemuchlessfrictionthanaplainbearing.

Standardtypesofrollingelementbearingsyoumayexpecttoseeonaircraftare: roller

ball

ThesearepicturedinFigure8.13(A)and(B)respectively.

Figure 8.13


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Ball Bearings

Inthistypeofbearing,finelymachinedballsofsurfacehardenedsteelrollwithininnerandouter rings. The clearance is closely controlled and a cage (retainer) may be used toseparatetheballs.

AsseeninFigure8.14,themajorcomponentsofastandardballbearingarethe:

outer ring,orrace

inner ring,orrace

rolling elements,inthiscase,balls

retainer

Figure 8.14 Figure 8.15

Theouterringmayhaveanexternalgrooveforasnap-ringtoretainitinitsinstalledposition.The outer cornersmay bechamfered to aid installation. The manufacturer will preciselymachinetheexternalfacesifthebearingistobeinstalledinadouble,orduplex,situationasdisplayedinFigure8.15.

Internally,theracesmaybeespeciallydeepgroovedtotakethrust/radialloadcombinations.Shields,orseals,canbefittedtotheracestoretainlubricantandprotectthebearingfromcontamination. The racesmayalsobemachinedin suchaway internallyorexternally tomakethemself-aligning.

Withawidevarietyofsizes,tolerancesandfeatures,itisvitalthatthecorrectbearingfortheapplicationbeused.Partnumbersandotheridentifyingmarkswillusuallybefoundground

oretchedontheouterraceface,asshowninFigure8.16.

Figure 8.16


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Ball Bearing Uses

Ballbearingsareusedinaccessorydrives,gearing,andinanyapplicationwhereanarrowbearingisrequired,whichiscapableofwithstandinghighspeedsandoperatingwithverylowfrictiondrag.Theirdrawbacksarecostanddiametricalsize.Becauseoftheirpointcontact,theyarenotgoodatbearingheavyoscillatingloadswhichwouldcausestressatthetinyareaonwhichthebearingpressesintotherace.

Angular Contact Ball Bearings.

Angularcontactballbearings,dependingontheangle,canhandlehighthrustloadsincombinationwithmoderateradialloads.Thesebearingscanbemountedsinglyback-to-back,face-to-faced,orintandem.

Figure 8.17

Roller Bearings

Roller Bearing can beCylindrical, Needle, Tapered or Spherical type. All roller bearingsutiliserollingelementsthatarecylindricalandfollowaflatraceway.Aswellascarryinghighradial loads they also handle shock loads better than ball bearings. Examples of roller

bearingsarepicturedinFigure8.18.

Figure 8.18

Guiding landsor shoulderson the innerorouterraces retain the rollers,andaseparating

retainerkeepseachrollerapartfromitsneighbours.Figure8.19showsatypicalcylindricalrollerbearing,alongwithcommonterminologyforrollerbearings.Cylindricalrollerbearingshavethehighestradialloadandspeedcapacitycomparedwithotherrollerbearings.

Figure 8.19


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Needle Roller Bearings

Atypeofrollerbearinginwhichtherollingelementhasa lengthsignificantlygreaterthanitsdiameteristermedaneedlerollerbearing.AnassemblyispicturedinFigure8.20.

Figure 8.20

Needlerollerbearingssupportheavyradialloadsandoccupylessspacediametricallythan

anequivalentrollerbearing.Needles have a slightly higher co-efficient of friction because of the number of rollingsurfaces, but are used where weight is undesirable and compactness required: e.g. inautomotivegearboxesandtransmissionshafts. Needlerollerbearingscan rundirectlyonjournals.

Un-cagedNeedlerollerscannotrunathighspeeds.Becausetherollersarenotseparated,effectivelubricationisdifficulttoachieveathighRPMallowingtherollerstomakemetaltometalcontact.commonlyusedinuniversaljointsandashingebearings.

Caged needle roller bearings can carry heavy radial loads and are used in gearboxes,crankshaftmainandcon-rodbearings,heavyaccessoriesandareasrequiringaheavyloadcapacitybearingwithadiameterless thanan equivalentballbearing.Cagedneedleroller

bearings that are oil lubricated (either splash orpressure fed) can be run athigh speed.Cagedneedlerollerbearingsthatarelubricatedwithgreasecannotberunat highspeed,effectivelubricationisdifficulttoachieveathighRPMallowingtherollerstoskidandmakemetaltometalcontact.

Roller Bearing Uses

Roller and needle bearings can carry heavy radial loads and are used in gearboxes,crankshaftmainandcon-rodbearings,heavyaccessoriesandareasrequiringaheavyloadcapacitybearingwithadiameterlessthananequivalentballbearing.Theirdrawbacksarelowerspeed(moresowithneedlerollers),costandwidth.


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Tapered Roller Bearings

These bearings are very rugged and are capable of supporting heavy radial and thrustcombinationloads.A taperedrollerbearing,Figure8.21,hasitscylindricalrollersarranged

withtheiraxisatanangletotheshaftaxis.

Figure 8.21

Theouterraceisknownasacup,theinneracone.Theratioofthrusttoradialloadingcanbecateredforbydesigningthebearingwithavariationintheangleoftaperchosen.

ForExample:

Aheavythrustloadwithlittleradialloadrequiresalargetaperangle.(Figure8.22A)

Slightthrustloadswithheavierradialloadingwouldrequireonlyasmalltaperangle.(Figure8.22B)

Figure 8.22

Purelyradialloadsarenotsupportedbytaperrollerbearingsastheringswouldtendtobeforced apart. The internal clearance in these bearings is adjustable during installation,makingthemtoleranttosomeminormisalignment.

Tapered roller bearingsare usedon car frontwheels,aircraftwheels and helicopter rotor

masts.Theymaybefittedontheirownorinpairstotakeloadsineitherdirection,suchasintheautomotivedifferentialinFigure8.23.

Figure 8.23


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Self Aligning Bearings

Oftenashaftmaybemisalignedduringrotationduetoloadingdeflection,forinstance,onthemainbearingsofaircraftpistonenginecrankshafts,asindicatedinFigure8.24.

Figure 8.24

Anothersituationiswhenmovementisinmorethanoneplane,e.g.:somecontrolrodjoints.Figure8.25showsacutawaycontrolrodjoint.

Figure 8.25

Inthese situations,whenmovement isinmore thanoneplane,abearingwhichcanalign

itselftoachangingaxisisrequired.Anumberofsolutionsareused;Figure8.26showsacurvedouterracewithadoublerowofballs.Becauseofthemovementoftheinnerrace,thistype cannot support heavy loads, and theyare difficult to fit withend covers for lubricantretentionorprotection.

Figure 8.26

Asimilarrollerbearing,whichcancarryloadscomparabletoasimilarsizedrigidbearingandallowconsiderableshaftflexing,alsohasacurvedouterjournal,asshowninFigure8.27.

Figure 8.27


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Bymakingtherollersbarrel-shapedorspherical,theinnerrace,rollers,andretainercanbemade self-aligning to the outer ring. It can be done in two ways; with a convex rollercurvatureasshowninFigure8.28(A),oraconcaverollercurveasinFigure8.28(B).

Figure 8.28

Asmentionedbefore,aself-aligningrequirementisfoundincontrolrodendbearings,Figure

8.29showstwosolutions.

Figure 8.29

Thrust Bearings

Sofarwehaveonlyconsideredbearingsdesignedtosupportcombinationradial/thrustloadssuchasdeepgrooveballandtaperedrollersandflangedplainbearings.Wesawinplainbearingsthatathrustwasherisdesignedtotakepurelythrustloads;rollingelementbearingshaveacorrespondingequivalent.

Ball,rollerandneedlebearingsdesignedforpurelythrustloadshavethetracksoftheirracesalignedtobearthethrustloadingalongtheaxis,asisshownbytherollerbearinginFigure8.30,andtheballbearingofFigure8.31.


This typeofbearingis oftenusedmountedverticallyto supportheavymachineryonworkshop floors e.g.a radial arm drillpress, orin variablepitch propellerswhich employ them

betweenthepropellerhubbarrelandthebladebutttobearcentrifugalforceandpermitthebladetoswivel.

Ifradialloadingisexpected,anormalbearingmustbemountedalongsidethethrustbearing.


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Aneedlethrustbearinghasaveryhighloadcapacityandcanalsotakelimitedradialloads,iftheneedlerollersaretaperedaswellasbeingsphericallycontoured,aspicturedinFigure8.32.

Figure 8.32

Thrust bearingsare used inengine prop shafts, behind and supporting helical cut gears,(Figure8.33),whichtendtobeforcedoutofengagementbecauseoftheirtoothshape,andarealsousedashelicoptermastbearings.

Figure 8.33

Diametrical Clearance

Thisisprovidedwithinverystricttolerancestoallowforlubricantpassage,movement,andheatexpansion,aswellastoallowforfittingmethods,whichwillbeexplainedlaterinthistopic.Ballandrollerbearingsareoftenclassifiedaccordingtotheirtolerancegrouping.

Bearing Retention Methods

Inorderforrollingelementbearingstooperatecorrectly,theinnerandouterringsmustseatcorrectlyinorontheirrespectiveshaftsandhousings.Theeasewithwhichtheshaftcanbeplacedintoposition,orremoved,is termeditsfit.A loosefitmeansitcanbemountedandremovedwithease,probablywithamanualpush.Howeverinmanycasesafittermedaninterferencefitisrequiredbythebearingsoperatingenvironment.


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Interference Fit

Thisisdefinedasafitbetweentwopartsinwhichthepartbeingputintoaboreislargerthantheboreitself.AbushdesignedforaslightinterferencefitisshowninFigure8.34.Inorder

tofitthemtogethertheborecanbeexpandedbyheatingand/orthepartshrunkbychilling.When thetwocomponentsmatchagain intemperature theyare tightlyfitted together. Noadditionalhardwareisrequiredforfitting;howevertheareaaroundtheboremaybesubjecttotensilestress.

Figure 8.34

Theturbineshaftofagasturbineenginemayuseabearingthatisfittedafterimmersioninabathofheatedoilwhereitexpands,whenataspecifiedtemperatureitisremovedfromtheoilbathandquicklyslippedontoitsshaft.Asitcoolstoshafttemperatureitshrinkssecurelyontotheshaft.

Heavyinterferencefitsreducetheinternalclearanceofthebearing,whichifnotdesignedfor,will result inbearing failure. Loosefittingresults inexcesscreep,whichmaydamagethehousing,andalsocausesnoisyoperation.

Circlips

Topreventaxialmovementofthebearinginthehousing,acirclipcanbeused.Agroovemaybemachinedintheouterringtolocatethecirclip,asshowninFigure8.35.

Figure 8.35


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Alternately, the circlipmaybe located in a groove in thehousingbore itself, as shown inFigure8.36.

Figure 8.36

Retaining Plates

Aretainingplateissometimesusedifhighstrengthaxiallocationisrequired.Aplateisfittedovertheboreinthehousinginwhichthebearingislocated,andissecuredbystuds,nutsorboltsonto the housing. It in turn secures the bearingagainst an internalshoulder in thehousing,alsoprovidessealingforlubricant,andpreventsingressofcontaminants.AvarietyofretainingplateisillustratedinFigure8.37.

Figure 8.37

Advantages

Theadvantagesofbearingretainingplatesare:

highaxialretentioncapacity

usedwheninterferencefitscannotbeutilised

noaffectonbearinginternalclearance

bearingreplacementwillnotdamagehousing

easytoinstallreplacementbearings

Disadvantages

Disadvantagesofretainingplatesare:

housingrequiresboltholes

raisesresidualstresslevelsinthehousing

itaddsweightandrequiresmorespace


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Staking

Airframeandcontrolbearingsareretainedbyinterferencefits,butthesemaynotbesufficienttoholdthebearinginplacewithsignificantaxialloadsand/orvibration.Thereareanumber

ofapprovedstakingmethods thatmay be used;however theseshould never beused tocompensateforpoorinterferencefits.

Impression Staking

This consists of the deformation of the bearing housing using a staking punch (a tooldesigned for the job). Themetal isimpressedaround the circumferenceofthebearing insuchawayastoforceit intothechamferat the topofthebore inthehousingaroundthebearingouter race, as shown in Figure 8.38. In some circumstances the shaftmay bestaked(Figure8.39).


Figure8.40showsthestakingmethodusedonaflyingcontrolsurfacecontrolrod.

Figure 8.40

Advantages

noaddedweightorspace

easybearinginstallation


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Disadvantages

canaffectbearingclearances

induceshighresidualstresses

bearingremovalcandamagethehousing

Swaging

Aspecialrollertoolrollsthemetalcircumferenceof thehousingoverthebearingouterraceedge;thisisshowninFigure8.41.

Figure 8.41

Alternatelyasleeve fittedbetweenthebearingandhousingmaybeswaged,asshown in

Figure8.42.

Figure 8.42

Theadvantagesanddisadvantagesofswagingaremuchthesameasforimpressionstaking.Approvedstakingtoolsmustbeused.Centre-punches,chiselsorscrew-driverswillseverely

damagehousingsandbearings.Thestakingprocessmustbecarriedoutinaccordancewithmaintenancemanualinstructions.


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B6.9TransmissionsIssueB:January2008 Revision2 1of18


TOPIC6.9GEARSandGEARTRAINS-TRANSMISSIONS

Gears are used in conjunctionwith bearings and shafts to transmit power, change drivedirectionsandincreaseordecreaserotationalspeed.

Spur Gears Straight cut

Thespurgearisyoureverydaygear,theyhavestraightteethwhichareparalleltotheaxisoftheshaft,andconnectparallelshaftsonly.Spurgearsaresimpletomanufacturebutcanbenoisyinoperation.Whentwospurgearsofdifferentsizesmeshtogether,thelargergeariscalledawheel,andthesmallergeariscalledapinion.Figure9.1isanexample.

Figure 9.1

Thegearbeingdriven,suchasthepropellershaftinFigure9.2,willrotateintheoppositedirectiontothedriving(crankshaft)gear.

Figure 9.2

Spur Gears -

Helical Cut

Amodificationofthespurgear,helicalgearsalsoconnectparallelshaftsbuthavetheirteethcutatanangleorhelixtothegearshaftaxis.ThiscanbeseeninFigure9.3.Theycancarryheavierloadsathigherspeedsthanequivalentsizedspurgearsandrunmoresmoothlyandquietly.

Figure 9.3


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Singlehelicalgearsproduceendthrustastheymesh,becausethegearwantstoslidealongtheshaftinthedirectionofthetoothangle.Theshaftonwhichtheyaremountedrequiresthrustbearings,oneoftheapplicationsforthebearingyoulearnedaboutinaprevioustopic.Youmayseedoublehelical,or herringbonegearsdesigned tobalancethrust forcesand

eliminatetheneedforthrustbearings.AsyoucanseeinFigure9.4,theVpatternofthegearteethgivesthegearitsname,supposedlyresemblingtheskeletalremainsofaherring.Amore appropriatedescriptionmight compare them to the tread pattern of mud grippingtyres.

Figure 9.4

Bevel Gears

Bevelgears are used between intersecting shafts and can bedesigned for any angle ofintersection. Spiral bevel gears, with their curved teeth, are quieter and smoother inoperationandcancarrygreaterloadsthanequivalentsizestraightorspurbevelgears.

Figure9.5showsexamplesofstraightbevelgears(A)andspiralbevelgears(B).

Theteethofbevelgearsareformedonaconicalblank.Thetheoreticalpointofintersectionistermedtheapexandtheteethofbevelgearsconvergeontheapex.

Figure 9.5


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Hypoid Gears

Theseareaprogressionofthespiralbevelgeartheme.AsshowninFigure9.6, theyaresimilarbuttheaxisoftheshaftsarenotonthesameplaneandthereforedonotintersect.

Originallydesignedforautomotiveuseindifferentials,theysufferfromheavyslidingcontactandrequireanextremepressure(EP)lubricantinmostcases.Theyarehowever,quieterandsmootherinoperationthanspiralbevelgears.

Figure 9.6

Worm Gears

Thistypeofgearconnectsnon-intersectingshafts,usuallybutnotalwaysatrightangles.AsshowninFigure9.7,theyuseaworm-screw,shapedlikeathreadcutontheshaft,todriveaworm-wheelwhichisverysimilartoaspurgear.Thespeedratioiscalculatedbydividingthenumberofthreadstartsontheworm-screwintothenumberofteethontheworm-wheelgear.

Wormgearsofferhighreductionsinsinglesteps,theyarequietandcancarryheavyloads,howevertheyareinefficientcomparedtoothergearsduetothelargeslidingmovementoftheirteeth.Atreductionsoverabout20:1,thegear-wheelcannotdrivethewormscrew.

.

Figure 9.7


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Rack and Pinion

This gear system is used to transmit motion between a rotating spur gear and a lineartoothedrack.Acommonuseisinmanycarsteeringsystems.

Figure9.8isanexampleofarackandpiniongearassembly.

Figure 9.8

Sector Gear

Asshown inFigure9.9,asectorgear isusedwhenonlypartof the rotationof theoutputshaftisrequired.Asectorissimplypartofacompletecircle,andasillustrated,onlypartofwhatwouldbeafullcirculargearisusedforthesectorgear.

Figure 9.9


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Reduction Gear Assembly

Enginesmustoperateatrelativelyhighspeedsformaximumefficiency.Propellersmustoperateatlowerspeedsformaximumefficiency.

Therefore,reductiongearsareusedtoallowboththeengineandthepropellertooperatewithintheirmostefficientrevolutionsperminute.

Reductiongearsareclassifiedbythenumberofstepusedtobringaboutthespeedreduction.Asinglereductiongearisagearmechanismconsistingofapairofgearsorasmalldrivegear(pinion),whichdirectlydrivesalarge(bull)gear.

Forexample,ina2-to-1singlereductiongear,thenumberofteethonthedrivengearistwicethatofthedrivingpinion.

Epicyclic Planetary) Gears

Primarilyusedasreductiongearinginaircraftengines,epicyclicgearingisalsofoundinacarsautomaticgearbox.Epicyclicgearingisageararrangementusinganinnersungear,planetarygearsfixedtoaspider,andanoutergearring.Thetermepicyclicisderivedfromepimeaningupon,andcycle,acircle.Similarly,planetarygivesanindicationofgearscirclingacentralgearinthesamemannerasplanetsorbitingasun.

Asreductiongearing,ithastheadvantageoveraspurgearsetbykeepingthepropellerofanaircraftengineonthesameaxisastheengineoutputshaft.

Twoarrangementswhichmaybeusedare:

spurplanetary

bevelepicyclic

Spur planetary

AsshowninFigure9.10, in thissystemallgearsaremountedin thesameplanewith theouterringhavinginternalspurteeth.DifferingratiosofinputtooutputshaftRPM(ordirectionofrotation),maybeachievedbylockingtheringgear,sungearorspider;and/orusingtheringgear,sungearorspiderfortheinput/output.

Figure 9.10

Thissystemismostlyusedinreductiongearingassemblies.

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