ROLLINGCUTTERBITSConesrotateabouttheiraxisasthebitisrotatedaboutitsownaxis.ByfarthemostcommonbittypecurrentlyusedSuitedforawidevarietyofformationcharacteristicsAvailablewithalargevarietyofToothdesign

BearingtypesThebiggestlimitationindesigningRCBistat:

Thebitmustfitinsidetheborehole.ThedesignerisrequiredtomakemaximumuseofaverylimitedspaceThesizeofeverycriticalpartcanbeincreasedonlyattheexpenseofanotherpart.ThedrillingactionofaRCBdependstosomeextentontheoffsetofthecones.Theconesaremovedsothattheiraxesdonotintersectatacommonpoint.Offsetangle:Changesfrom0o(hard)to

4o(soft)formations.Causestheconescrapetheholebottomlikeadragbit.TendstoincreasetheROPForsoftformationsLong,WidelySpaced,Steelteeth.Longteetheasilypenetrateandplowingactionoftheoffsetconeremovesthematerial.Italsopromotesfastertoothwearinabrasiveformations.Thewidespacingoftheteethontheconepromotescleaning.TheshapeofbitteethalsohasabigeffectonthedrillingactionofaRCB.ForHarderRocksThetoothlengthandconeoffsetmustbereducedtopreventtoothbreakage.Thedrillingactionofabitwithzeroconeoffsetisessentiallyacrushingaction.Thesmallerteethalsoallowmoreroomfortheconstructionofstrongerbearings.Milledtoothcuttersaremanufacturedbymillingtheteethoutofasteelcone.Milledtoothbitsdesignedforsoftformationsusuallyarefacedwithawearresistantmaterial,suchastungstencarbide,ononesideofthetooth.Hardfacingononesideallowsmorerapidwearononesideofthetooththantheotherandthetoothstaysrelativelysharp.Themetallurgyrequirementsofthebitteethalsodependsontheformationcharacteristics.Themilledtoothbitsdesignedtodrillharderformationsareusuallycasehardenedbyspecialprocessingandheattreatingthecutterduringmanufacturing.Thiscase-hardenedsteelshouldwearbychippingandtendtokeepthebittoothsharp.Tungstencarbideinsertcuttersaremanufacturedbypressingatungstencarbidecylinderintoaccuratelymachinedholesinthecone.Theinnerrowsofteetharepositionedondifferentconessothattheyintermesh.Theintermeshingallowsmoreroomforastrongerbitdesign.Providesaself-cleaningactionasthebitturns.Allowsmaximumcoverageoftheholebottom.70%Thebottomholecoverageofmostbitsisabout70%.Theouterrowofbitteeth,heelteeth,hasthehardestjob.Duetothecirculargeometry,morerockmustberemovedfromtheoutermostannularringoftheholebottom.Thisrockismoredifficulttoremovebecauseittendstoremainattachedtotheboreholewall.Theheelteethmaywearexcessively,causingthebittodrillanout-of-gaugehole.Thiscausesagrossmisalignmentoftheloadonthebearingsandprematurebitfailure.Prematurefailureofthenextbitisalsolikelyiftheholeremainsundersized.ThecommonbearingassembliesusedforRCBare:StandardRollerBearing,Gas-CooledBearingdesignfordrillingwithAirorN/G,SealedRollerBearing,Journalbearing.ThestandardrollerbearingsThemostinexpensiveoneAllbearingsarelubricatedbythedrillingfluidThestandardrollerbearingsconsistsof:Aroller-typeouterbearing,Aball-typeintermediatebearing,Afriction-typenosebearing.Theroller-typeouterbearingThemostheavilyloadedmember.Usuallytendstowearoutfirst.Wearonthebottomsidewheretheweightappliedtothebitistransmittedfromthepintothecone.Theintermediateballbearings

Carryprimarilyaxialloadsonthecones.Alsoservetoholdtheconeinplaceonthebit.ThenosebearingsDesignedtocarryaportionoftheaxialloadsaftertheballbearingsbegintowear.Thenosebearingisafriction-typebearinginmostbitsizes,butinthelargerbitsizesanotherrollerbearingisused.Gas-CooledBearing

designFordrillingwithAir/N/GAmodifiedbitisavailablewithpassagewayspermittingaportionofthegastoflowthroughthebearingassemblyThesealedbearingassemblyTheintermediate-costbearingassemblyThebearingsaremaintainedinagreaseenvironmentbygreasesealsandareservoir.Acompensatorplugallowsthegreasepressuremaintainedequaltothehydrostaticfluidpressureatthebottomofthehole.Thegreasesealsrequiresomespaceand,thus,areductioninbearingcapacity.Theeliminationofabrasivematerialfrombearingsusuallymorethancompensatesforthisdisadvantage.Asthebitwears,thegreasesealseventuallyfailandthedrillingfluidcanenterthebearingsandacceleratethebearingwear.Thejournalbearingassembly

ThemostadvancedbearingtypeTherollerbearingsareeliminatedandtheconerotatesincontactwiththejournalbearingpin.Byeliminatingoneoftherollers,additionalspacebecomesavailableforstrengtheningtheremainingcomponents.HastheadvantageofgreatlyincreasingthecontactareathroughwhichtheWOBtransmittedonthecone.Journalbearingbitsrequireeffectivegreaseseals,specialmetallurgy,ande