WIXLIB

performance of the LTU. This testing also consisted ofMotor Threshold Voltage (MTV) and running torquetesting at temperature and ambient. After successfullycompleting these tests, the LTU was setup in a bell jarvacuum chamber and programmed for a long durationlife test, using precise power control to bring thethermally isolated LTU actuator to between 55 C to 89.5 C running temperature in a set profile byheating the winding and the two external resistors.For a five-year lifetime, at a 10% duty cycle, the motorbearings will experience approximately 100 millionrevolutions. This is based on 4,380 hours of operationat 6.25 revolutions/sec.The bearings are small(0.375” OD), 440C SS, Conrad deep groove bearingswith eight 1/16” diameter balls. The ball-raceconformity is 56% and the free contact angle is 15.6degrees. The final selected preload was 1.00 0.25 lbf.Assuming a maximum preload of 1.25 lbf, the meanHertzian stress is 113 ksi. For the eight ballcomplement, one shaft revolution will yield four ballpasses. Therefore, 100 million revolutions yields 400million ball passes. Multiplying this value with themean Hertzian stress yields a CDF of approximately4.5 x 1013 ball passes-psi. For comparison, measuredCDF values for several bearing life tests appear inTable 1.Periodic measurements using Lab View softwaremeasures the MTV automatically to monitor changesto the internal running friction, giving an indication asto the state of the lubricant inside the LTU. The LTUlife test was completed in May 2006, and the LTUdisassembled at JPL. The gearmotor was returned tothe vendor for further disassembly.2.Table 1, Measured Cumulative Degradation FactorRESULTS2.1 Cumulative Degradation Factor PredictionThe Cumulative Degradation Factor (CDF) analysisinitially indicated “Very Low Probability of Success”as designed at the CDR. The proposed lubricants forthe DAWN TGA are all based on Brayco 815Z, linearperfluoropolyether oil. This oil and its greasevariations have been the lubricants of choice for spacemechanisms for many years. The gears and gearboxwas to be lubricated with Braycote 602EF grease.Braycote 601EF grease and Brayco 815Z oil were tobe used in the motor, motor bearings, and output shaftbearings. Both greases contain a PTFE thickener andBraycote 601EF also contains a bentonite claycorrosion inhibitor and sodium nitrite rust inhibitor.The Braycote 602EF only contains MoS2, a solidlubricant. . The TGA ball bearings are made of 440Cstainless steel and the gears are made of Vascomax C300, maraging steel containing Ni, Co, and Mo.Only measured CDF by testing in vacuum were usedfor comparison as even a small amount of moisturefrom GN2 or LN2 can greatly reduce the lubricantconsumption rate [7].This prediction was then used as the tool to convincethe Dawn project to accept the spacecraft vendor’s(Orbital Science Corp., Dulles, VA) mission duty cycleestimate of 1%. With this realistic value of 1%mission duty cycle, and in combination with the lowestpractical rotor bearing preload, the prediction using theCDF technique changes to “High Probability ofSuccess” for meeting the updated mission requirementwith a revised CDF of 4.5 x 1012 bp-psi. This revisedCDF is in the midrange of Lockheed Martin’s failurerange of 2.2 to 8.7 x 1012 bp-psi based on bearing lifetests [6].After examining the relative number of stress cycles ofall TGA bearings, it was determined the motorbearings were at greatest risk because they undergo themost cycles. Planet bearings see reduced cyclesbecause of the various gear ratios in the TGA.Therefore, calculations are based only on the motorbearings.Although the CDF for bearing tests with Pennzane oilformulations is an order of magnitude greater than forBrayco 815Z, Pennzane was not chosen for the MMAsince the unit would be operating at elevatedtemperature. The vapor pressures of Pennzaneformulations are about two orders of magnitude greaterthan Brayco 815Z at temperatures approaching 90 C.The actual consumption and evaporation rates shouldbe measured with the SOT and in actual bearing lifetests at these higher temperatures. The operatingWhen the 10% mission duty cycle, proposed by JPL,was used in the calculation of the CDF, it clearlyindicated that the lubricant in the motor bearing wouldnot survive. With full cooperation from the gearmotorvendor, a different design was implemented to lowerthe motor rotor bearing preload, to 25% of the initialvalue at 1.0 0.25 lbf. However, this redesign CDFcalculation still yielded the same failure prediction.NASA/CR—2009-2156813

temperature for the Dawn TGA, while the Ion Engineis running, is predicted to be between 55 to 89.5 C,and is beyond life test and on-orbit experience forPennzane formulations. It was felt that the risk ofusing Pennzane at this temperature is high without athorough investigation and test program.2.4 EM 1 ConfigurationThe first EM had the original 4 lbf preload and thestandard (larger -1) gear with the revised steppermotor. The rationale is that the first path finding EMshould be built as quickly as possible with existinghardware. This EM was used in the thermal dynochamber setup, resulting in the discovery of gearbinding at temperatures below –55 C.2.2 Vascomax C-300 Consumption RateThe SOT results (Figure 7) indicated Vascomax C-300did not have an accelerated lubricant consumption ratewith Braycote 601EF grease. However, the data didindicate a reduced lifetime on both Vascomax and440C SS with 602EF when compared to 601EF. Thegearmotor vendor was quickly directed to change fromthe 602EF lubricant (used on MER) to 601EF tomaximize life of the gears used on Dawn. Also, allMMA processes for the Dawn TGA at JPL use the601EF grease with Braycote 600 used for greaseplating.2.5 EM2 ConfigurationThe second EM, EM2, was initially intended to be theLTU. The rotor bearing preload was lowered to 2 lbfeasily by using different thickness shim washers. EM2was assembled with the smaller -2 gears with lighthand lapping. The CDF with the 2 lbf was in anacceptable range, but on the high side of the failurerange. EM2, with 2.0 lbf and -2 gears, was delivered toJPL and went through the full qualification thermaldyno test profile for 6-1/2 thermal cycles whilemodifications to the rotor bearing preload was beingengineered.This EM2 produced acceptableperformance, but with more reduction in output torqueat temperatures below –55 C, indicating high internalfriction.2.6 LTU ConfigurationSubsequently, the target rotor bearing preload waslowered even further, to 1.0 0.25 lbf for the third unitbeing built. This unit, the LTU, used a differentpreload bearing cup design, with each wave springmeasured and the cup match machine to bring thepreload to the design range. The rotor bearings aregrease plated and four additional drops of Brayco 815Zoil added to every other ball for each rotor bearing.The gearbox used the smaller -2 gears with very lightlapping to a newly defined “end play” target range forthe gearmotor output. After grease plating the gearsand the bearings in the planetary gearbox, additionalgrease strips with Braycote 601EF were applied on theinside of the gearbox housing. After bake out, theLTU was brought down to –108 C cold survivaltemperature, then 6-1/2 thermal cycles between –70 Cto 120 C, with dyno runs at the first and last thermalcycle. Below (Figure 8) is the thermal dyno test resultfor the LTU in April 2005:Figure 7 – Vascomax C-300 SOT Test( 10 -68 Pa, 40 RPM, 1.5 GPa mean Hertzian Stress)2.3 Modifications to the Stepper MotorThe initial gearmotor specification omits the allowablemaximum torque and the first generation stepper motorwas designed to the maximum capability of the twostage planetary gearbox. This excessive torque willdamage the harmonic drive, producing a calculatedtorque of more than 1,500 in-lbs. The gearmotorspecification was then clarified with clear definition ofthe winding configuration (shorting of un-poweredwinding), two missions specified fixed pulse width at15 and 20 ms, and allowing the addition of externalresistors to stabilize motor current (constant voltagedriver) over the wide temperature range. The revisedmotor also incorporated the reduced preload afterdelivery of the first Engineering Model (EM).NASA/CR—2009-2156814

Figure 8 – LTU Pre-Vibe Thermal Dyno Test, 50 ppsThe pulse width is commandable from the ground tofull pulse width (20 ms) if needed, with an increase ofapproximately 30 to 40 in-lb of output torque from the15 ms value.against a titanium gearmotor output adaptor hub.Titanium could, and in this case did, acceleratedegradation of the Braycote lubricant. Futuregearmotors should fabricate this output adaptor hubwith 440C SS materials to prevent this defect so thatthe retainer, with the Braycote lubricant, will slide on aknown material that will not accelerate break down ofthe lubricant.Post-Vibe thermal dyno test of the LTU in July 2005produced similar torque output, MTV and nomovement due to vibration by measuring the numberof steps to actuate the microswitches at the hardstopposition.2.8 HD Wave Generator Bearing Materials2.7 HD Wave Generator Bearing SeparatorOrientationThe off-the-shelf HDS harmonic drive uses 52100bearings. Much effort was spent on run in anddifferent techniques to prevent the bearing fromrusting after precision cleaning. Future gearmotordesigns should specify 440C SS bearings to simplyprocessing. The additional lead time using the 440CSS bearing is well worth the wait.An EM TGA was built in the summer of 2004, withflight configuration titanium struts, actuator bracket,and all flight like hardware as the vibration test fixtureto qualify the three flight ion engines. Instead of agearmotor, a simple mass model took its place, withmanual crank to drive the input to the wave generatorof the harmonic drive. The separator was installed onthe “closed” side of the flex spline of the harmonicdrive. In order to keep the separator from popping offwhen load was applied, a set of custom shims and alarge 400 series SS wash

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