Lessons Learned Duringthecourseofmanufacturing andtest,asignificant number ofLessons Learned wereaccumulated. Thefollowing isadescription ofsomeoftheseissuesandthelesson(s) learned. 1)Piniontipandrootrelief&material chanae TheDLAoutputpinionwassubmitted toalifetestwiththeSTAunit(Development SARJunitwithaflight likeracering).Thislifetestusedaflightpinionbutwaspowered usingSupport TestEquipment (STE)with adirectdriveoffofthepinionshaft.Thetestwasstartedwithapiniongearmadeofnitrided, goldplated155PHthathadcrowned teeth.TheSTAbullgearlikewise wasmadeof15-5phandwasnitrided. Thetest wasinitiated withagoalofdemonstrating a13-year life(75,972 revolutions). Therateofrotation was accelerated fromnormaloperation toguarantee atimelycompletion. Normal SARJrotational rateis approximately 0.011revtmin. Duringthetest,theratewasincreased 186timesto2.05rev/min. Afteronly1500revolutions, asignificant quantity ofdebriswasdiscovered uniformly spreadacrossthe circumference oftheSARJbullgear.Thetestwasstopped andthedebrisexamined. Alargeportionofthe debriswasfoundtobecoming fromtheMagnetic Particle BrakemadeoutofDeldn.Thehardware was cleaned andtestingwasresumed. After4500revolutions, testingwasagainstopped duetoasignificant increase intheformation ofdebris. Thedebriswasthistimefoundtobeinpart,coming fromthepinion gear.Thepiniongearwasinspected andthetipswerefoundtocontain patches wherethenitridehadbeen blownoff.Athorough investigation wasinitiated todetermine thecauseofthegeneration ofdebris. Many itemswereconsidered including gearalignment, backlash, andSTEaffectsandtheireffectsonthegear's surface-to-surface contact, itwasdiscovered thatifthecontact patchwerenotcontrolled, thetipsofthe pinionweresusceptible tocontact withthematinggear,introducing highcontact stresses andallowing chipping ofthecasehardened surface. Everyattempt tocontrolthecontact patchincluding maintaining a tightbacklash hadseemingly beenmade.Although theprescribed centerdistance wasmaintained, the manufacturing errorswouldrequirerunning atamuchlargerbacklash. Thiswasconsidered asapotential fix,however, thiswouldhavereduced theloadcarrying capabilities atthehightorque(stall)condition. Therootcausewasdetermined tobemanufacturing errorontheSARJracering(asaresultofitssize)and lackofcenterdistance control. Afterasignificant amount ofdetailed modeling andanalysis, itwasdecided thattheintroduction ofatipandrootreliefwouldeliminate theinterference. Byincorporating atipandroot relief(Figure 9),thecontact patchwasbettercontrolled andopportunities forhighcontact stresses were eliminated. Thebasematerial wasreviewed aswellduetothechipping discovered onamajority ofthepinionteeth. Thepinion,whennitrided, formsahardened outershell.Thiscasingwhensubmitted tohighcontact stresses duetopointloading issusceptible tochipping. Achange fromthe15-5Stainless nitrided pinionto a13-8Stainless wouldeliminate theneedforacasehardening process asthe13-8hasapotential for higherstrength. Byeliminating thecase,thepinionwillbemoreforgiving tomanufacturing errorandfailure modeswillbelesscatastrophic sincenocasewouldbepresent toflakeoff. Thedecision wasmadetoreplace the15-5outputpinionwithatipandrootrelieved 13-8pinionandrestart thetest.Thetestwassubsequently completed withnosignificant debriscreated fromthemeshing ofthe gears.Allflightunitsnowusethe13-8pinions. Thistipandrootreliefisnotbeingrecommended fornewdesigns asitmayincrease wearduetothe reduced contact ratio.Itwas,however, inthiscaseaviabledesignalternative. Lessons,- -Payattention tomanufacturing techniques onlargegearassemblies. -Useofthrough-hardened 13-8asabasematerial isareasonable designaltemative. 68