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StudentResource
SubjectB-6b:
AircraftHardware
Copyright 2008 Aviation Australia
Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,orotherwisedisposedof,withoutthewrittenpermissionofAviationAustralia.
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Part 66 Subject
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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IssueB:January2008 Revision2 Page4
STUDYRESOURCES
JeppesenGeneral
JeppesenAirframe
AC43.13-1B/AC43.13-2ACombinedAircraftInspectionandRepair
B-6bStudentHandout
DaleCraneAviationMechanicHandbook
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IssueB:January2008 Revision2 Page5
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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B:January2008 Re 2 Page
Issue vision 6
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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AA Form TO-18
B-6b Aircraft Hardware
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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AA Form TO-18
B-6b Aircraft Hardware
B-6b.5.1 Screw ThreadsIssue B: January 2008 Revision 2 2 of 4
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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AA Form TO-18
B-6b Aircraft Hardware
B-6b.5.1 Screw ThreadsIssue B: January 2008 Revision 2 3 of 4
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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AA Form TO-18
B-6b Aircraft Hardware
B-6b.5.1 Screw ThreadsIssue B: January 2008 Revision 2 4 of 4
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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AA Form TO-18
B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 1of24
B-6b Aircraft Hardware
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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B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 2of24
B-6b Aircraft Hardware
B:January2008 Revisi 3
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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AA Form TO-18
B-6b.5.2Fastener-Bolts,StudsandScrews
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B-6b Aircraft Hardware
B:January2008 Revisi 3
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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AA Form TO-18
B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 4of24
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B:January2008 Revisi 3
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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B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 5of24
B-6b Aircraft Hardware
B:January2008 Revisi 3
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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B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 6of24
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B:January2008 Revisi 3
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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AA Form TO-18
B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 7of24
B-6b Aircraft Hardware
B:January2008 Revisi 3
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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AA Form TO-18
B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 8of24
B-6b Aircraft Hardware
B:January2008 Revisi 3
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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AA Form TO-18
B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 9of24
B-6b Aircraft Hardware
B:January2008 Revisi 3
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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B-6b.5.2Fastener-Bolts,StudsandScrews
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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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B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 11of24
B-6b Aircraft Hardware
B:January2008 Revisi 3
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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B-6b.5.2Fastener-Bolts,StudsandScrews
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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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B-6b.5.2Fastener-Bolts,StudsandScrews
Issue on 13of24
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B:January2008 Revisi 3
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
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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
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DOWEL PIN APPLICATION
Splithollowdowelsinsomeinstancesareusednotonlytomaintainalignmentbutalsoactas
bushingsforrotatingcomponents.
Smoothsideddowelsarepressfitinstalled.
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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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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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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.
Figure 8.30 Figure 8.31
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).
Figure 8.38 Figure 8.39
Figure8.40showsthestakingmethodusedonaflyingcontrolsurfacecontrolrod.
Figure 8.40
Advantages
noaddedweightorspace
easybearinginstallation
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B6.8BearingsIssue on 18of18
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B:January2008 Revisi 2
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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AA Form TO-18
B6.9TransmissionsIssueB:January2008 Revision2 1of18
tB-6b Aircraft Hardware
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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