ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
© 2020 Sierra Nevada Corporation   404 The UUT was returned and disassembled at SNC. T he interior condit ion of the motor , particularly the front  bearing,  was evaluated by both SNC and JPL personnel.  The following observations were made regarding  the condition of the ShEl gearbox and motor:   • No significant anomalies  were found within the ShEl gearbox .  • No significant anomalies  were found within the M45S brake assembly .  • In disassembly of the motor, the r ear bearing rotated  smoothly  and residual, dark , and slightly wet   lubricant remained.   • A significant amount of wear debris was present on both sides of the fr ont motor bearing: motor  rotor and motor pinion. See Figure 8.   • The f ront bearing rotated rough ly and caught  occasionally. Two fingers of the ball retainer were  visibly damaged.  The bearing was v ery dry and empty of both lubricant and wear material  (appeared to have been ejected into motor rotor and motor pinion areas). The inner (piloting)  diameter of the crown steel retainer inc luded a rolled burr. Further inspection found that this inner  diameter was measurably enlarged, allowing radial movement of the retainer  to the point where the  outer diameter of the retainer could contact the inner diameters of the outer race. Additionally , there  were indications  that the retainer spine contacted the interior surface of the bearing shield ,  indicating that the retainer ball pockets had worn to allow excessive axial movement of the retainer .  See Figure 9 for the appearance of this damaged front bearing.  

Figure 8 – Wear Debris Surrounding the M45S Front Motor Bearing from ShEl DV Unit  

Figure 9 – Interior Inspection and Retainer Observations on M45S Front Motor Bearing from ShEl DV Unit
© 2020 Sierra Nevada Corporation   407   Figure 10 – M45S Cold Performance with Phenolic Bearing Retainer and Various Lubricant Schemes  

The timeline of recovery for the M45S motor, including assembly, test, and teardown inspection  events is  shown in Figure 11 with the following details:   • Testing, including random vibration,  began with the baseline bearing configuration in SN019 and  SN032, modifying the operating temperature range  to -35°C to +25° C to be less severe than the  original  -70°C to +70° C.  • Refurbishment of SN012 and SN016 was completed after receiving bearings with phenolic  retainers and the noted grease fill  from JPL.   o Cold temperature drag testing revealed that the bearings in SN012 with 58.1 mg grease fill  had excessively high drag. That unit,  therefore,  was not selected to continue to  life testing.   o SN016 did proceed to life testing  over the - 35°C to +25 °C temperature range in the event  that the SN032 with the baseline bearing configuration f ailed.   • SN032 life testing successfully completed with favorable results over the -35°C to +25° C  temperature range, indicating that the baseline bearing configuration was capable of meeting the  margined  life requirements with a minimum operating temperature of -35°C.  o As a result, life testing of  SN016 was halted and the unit was made ready for a different  set of bearings.  By this point, SN016 had accumulated 75 million revolutions (71%  of  margined l ife).  • With successful results  of the baseline bearing configuration at -35°C, the program set out to  determine if either the baseline or a refurbished bearing configuration could successfully meet the  margined  life requirements with a lower  minimum operating temperature of - 55°C, still warmer than  the original requirement of - 70°C. A lower minimum operating temperature would reduce the  needed motor heater power, conserving the rover’s limited power supply for mission operations .  This testing was performed  with three units :  o The slow  speed , long -duration SN019 life test , already in progress , was modified to make  up ground  at the -55°C plateau.   o SN016, refurbished to include the baseline bearing configuration, was run at full speed.   o SN012, refurbished with bearings JPL modified to include phenolic retainers and the   22-mg slurry fill, was run at full speed as well .  • SN012 and SN016 were exposed to both the random vibration environment and the Planetary  Protection (PP) Bake- out, which require d at least 122 hours at + 114°C and a pressure of less than  1x10-5 torr. The bake- out was included  to ensure that the bearing lubricant would be conditioned in  a manner consistent with the flight motors.   • All three motors finished testing within a week of each other. A Technical Interchange Meeting  (TIM) was held to review all  motor performance data and visual bearing condition s.
© 2020 Sierra Nevada Corporation   408  

Figure 11 – Timeline of M45S Motor Recovery Assembly, Test, and Teardown Events  

In the TIM, the decision was made to proceed with the baseline bearing configuration for the qualification  and flight mechanisms, provided the minimum mission life test  temperature was  increased from -70°C to   -55°C. Furthermore, since the M45S was capable of meeting performance requirements  at colder  temperatures  and the concern was regarding reduced mechanism life, it was recommended that the power  allocat ion for robotic arm motor heating be in proportion to the exp ected life of each joint . This allocation  would conserve limited rover power  resources  while maximizing overall robotic arm life. Practically, this  means that  a joint that is expected to experience minimal revolutions could be operated colder than a joint  that is expected to experience the most revolutions.  Additionally , it was recommended that heater power  continue to be used as available to maximize life , not just to achieve a minimum - 55°C operating   temperature.  

Motor performance was acceptable for all five of the M45S recovery test configurations. A summary of the  variables  tested and the outcome of those tests  are shown in the matrix of configurations and environments  in Table 2: 

Table 2 – M45S Motor Bearing Recovery Test  and Results  Matrix   Motor  SN Retainer /  Lubricant  I&T Temp erature    / Motor Revolutions  Mission Temp erature  /  Motor Revolution s Total  Revs  Speed   (rpm)  Visual  Cond .  SN032  Crown  Steel 1     -33°C +25°C 106M  7000  OK     53.0M  52.8M   SN016  Crown  Phenolic  2     -33°C +25°C 76M 7000  Good      42.4M  33.1M   SN012  Crown  Phenolic  3 -70°C +25°C +70°C -55°C -35°C +25°C 106M  7000  Good  1.86M  3.88M  1.79M  39.3M  29.5M  29.9M   SN016  Crown  Steel 1 -70°C +25°C +70°C -55°C -35°C +25°C 106M  7000  OK 1.86M  3.87M 1.79M  39.2M  29.4M  29.8M   SN019  Crown  Steel 1 -70°C +25°C +70°C -55°C -35°C +25°C 105M  1500  OK 1.85M  3.85M 1.81M  39.2M  29.4M  29.3M   1  Baseline Lubrication: Grease Plate of Braycote 600EF Grease with 5%  to 10% fill with 1:1 slurry by  volume of Braycote 600EF grease and Brayco 815Z oil   2  No grease plate. Filled with 38.7  mg Braycote 600EF Grease, or ~ 20% fill  3  No grease plate. Filled with 22 mg of 2:1 slurry of Braycote 600EF grease:  Brayco 815Z oil , or  ~11.5%  fill
© 2020 Sierra Nevada Corporation   409 Figure 12 is a comparison of the post-life test condition of the front motor bearing  from various units:   • Left: O riginal M45S DV motor -only life test with mission temperatures of - 70°C to +70° C. Bearing  spun roughly and the lubricant/wear debris was dry and dark.   • Center: M45S SN016 full-speed baseline bearing life test with modified mission temperatures of    -55°C to +25° C. Bearing spun smoothly and lubricant was still present and slightly discolored  around the retainer ball pocket areas.   • Right: M45S SN012  full-speed life test with modified mission temperatures of - 55°C to +25° C and  a phenolic retainer with 22 mg grease/oil slurry fill . Bearing spun smoothly and lubricant was  plentiful and minimally discolored.  

Figure 12 – Post-Life Condition of Front M45S Bearing s 

Conclusion s and Lessons Learned  

Conclusions   While the end- of-life condition of the bearings with phenolic retainers was superior to thos e with baseline  crown steel retainers, the level of risk and schedule impact associated with replacing bearings installed in  flight motors was deemed too significant. Multiple life tests confirmed  that increasing the minimum mission  life test  temperature from - 70°C to -55°C result ed in the baseline bearing configuration meeting the  margined  life requirements. The team planned to implement this change in minimum  operating temperature  by allocating the motor heating power budget proportionally based on the life requirement of each M45S  motor  application. By October 2 019, the robotic arm joint  assemblies , using SNC gearmotors containing  the M45S motor with the baseline bearing configuration , were qualified for flight after successfully  completing their margined life tests utilizing  an increased  minimum mission life test temperature of -55°C,  providing additional  confidence in the chosen solution . 

Lessons Learned   • Programs  should evaluate the appropriateness of applying a 15°C thermal uncertainty margin in  combination with the 2X margin on required revolutions during life testing, especially when the  temperature margin results in a significant viscosity change in the lubric ant.  • Given the - 72°C pour point  of Brayco 815Z oil , operation at - 70°C result ed in a starvation condition.  Future work should evaluate newer lubricant formulations with lower pour points if long duration  operation at - 70°C is unavoidable.   • Bearing drag t esting over temperature may be useful as a means of determining the low -end  temperature limit for effective wet lubrication.  Bearing drag normally increases with decreasing  temperature, but drag torque was shown to decrease  below this low -end temperature l imit as the  lubricant cannot flow and the bearing operates  in a lubricant starvation condition.
© 2020 Sierra Nevada Corporation   410 • Bearing lubricant life depends on many  factors in addition to industry accepted Hertzian contact  stress and lubricant stress cycles. A developmental life test  of the flight configuration of bearings is  recommended as early as possible to ensure succes s while  accounting for as many other variables  as possible: bearing retainer design and material, lubricant fill, test temperature and duty cycle, film  thickness at design speed(s) , etc.  • Engineering should identify and resolve conflicting design requirements early in the program: low  drag torque and long life requirements are potentially in conflict with a wet lubrication scheme.   • In this design and environment, phenolic retainers would have provided  longer life than crown steel  retainers.  

Acknowledgements  

The authors wish to express appreciation to the Motion Control Subsystem (MCS) team at JPL for their  collaboration in this investigation and resolution. Specific contributors of importance were Mark Balzer,  Andrew Kennett, and Alex Ferreira.  This work was performed for the Jet Propulsion Laboratory, California  Institute of Technology, under the Prime Contract NNN12AA01C between Caltech and NASA under  subcontract number Subcontract 1541135.  Government sponsorship acknowledged.  

References  

1. Herman, J. and Davis, K., “Evaluation of Perfluoropolyether Lubricant Lifetime in the High Stress and  High Stress -Cycle Regime for Mars Applications”, Proc. 39th Aerospace Mech. Symp.,  NASA CP2008- 215252, Huntsville, AL (2008)   2. Suffern, D. and Parker, J., “Developmental Bearing and Bushing Testing for Mars Gearboxes”, Proc.  44th Aerospace Mech. Symp. , NASA/CP- 2018- 219887, Cleveland, OH (2018)   3. Conley, P.  L. and Bohner, J.  J., “Experience with Synthetic Fluorinated Fluid Lubricants”, Proc. 24th  Aerospace Mech. Symp.,  NASA CP- 3062, Kennedy Space Center, FL (1990)   4. Zaretsky, E . V. “Lubricant  Effects on Bearing Life,” NASA -TM-88875, December 1986   5. Mobley, J.; R obertson, M.; Hodges, C., “Extended Life Testing of Duplex Ball Bearings”, Proc. 43rd  Aerospace Mech. Symp. , NASA/CP- 2016- 219090, Santa Clara, CA (2016)   6. Zaretsky, E . V., “STLE Life Factors for Rolling Bearings.” STLE SP -34, Park Ridge, IL, 1999.   7. Lo, C. J., et al. , “Use of Cumulative Degradation Factor Prediction and Life Test Result of the  Thruster Gimbal Assembly Actuator for the Dawn Flight Project.” NASA/CR -2009- 215681.   8. Jones, W. R., et al., “The Effect of Stress and TiC Coated Balls on Lubricant Lifetimes Using a  Vacuum Ball -on-Plate Rolling Contact”, Proc. 33rd Aerospace Mech. Symp., NASA/TM -1999 -209055,  Pasadena, CA (1999)
412 rover , the project re quired  new actuators  that then needed to be baselined from proven/existing  technologies to meet the launch schedule.  

The Curiosity r over had major actuator development issues , which  contribut ed to the launch slipping from  2009 to 2011. The development of several new motors was highlighted as a major reason for the actuator  schedule slips  [4,11]. The JPL Mars 2020 actuator team considered the lessons  learned from the earlier  rover mission and decided to leverage an existing COTS design to reduce the flight hardware schedule  risk.  

maxon has an extensive catalog of  motors for industrial use and has proven flight heritage from the MER  mission that, with suitable modifications, the motor designs are robust for space applications.  maxon COTS  flat motors were utilized in the SCS testbeds and baselined in the original derivation of the system volume  and functional requirements. A close collaboration between maxon and JPL was essential in developing an  up-screened COTS -to-robust , flight flat motor mounted on a $2B mission. Two motor types, referred to here  as M20 and M32 ( with 2 0 and 32 being the COTS motor diameter in millimeters) , were based on existing  maxon COTS products and chosen to meet the project envelope constraints  (Figure 1). The M20 gearmotor  include s an integrated gearbox  (Figure 1) . Additionally, a detent module was added to the M32 to provide  a passive holding torque to a static motor. When the motor is spinning  above a minimum speed, the inertia  is sufficient to overcome any subtractive/additive torques from each detent step, with  no significant  performance reduction from the motor.  

Figure 1. Top Left: M32 motor cut -away showing detent brake;  Top Right : M20 FM gearmotor ;   Bottom: Exploded view of M20 gearmotor  

Mars 2020 Mechanism Descriptions  

The SCS includes a rotary percussive drill located on the turret at the end of the large robotic arm and the  sample tube processing plant internal to the rover (the ACA, Figure 2). A 3-DOF  arm in the processing plant  passes the tubes and samples  to multiple science stations and hermetically seals the tubes. Additionally , a  bit carousel passes tubes and bits between the internal ACA and the external drill.  Several of the  mechanisms in the SCS subsystem are driven using the M32 detent motors described in this paper, which  are mated to three different  torque amplifying planetary gearboxes to provide a family of actuators ( named
413 SHACD, SAS , and Chuck) , designed and manufactured by Sierra Nevada Corporation in Durham, NC. The  motors provide the active driving torque and holding torque for the following applications: the 3- DOF  Sample  Handling Assembly (SHA) , which consists of a linear stage and two rotary (shoulder/elbow) joints; the tube  gripper and sealing ram within the tube sealing station of the ACA; the central rotating axis of the bit  carousel; the mechanism that shifts the coring drill between high- speed drilling and high- torque core break - off modes; and the opening and closing of the chuck at the end of the drill. All of these mechanisms are  single- string devices performing a serial task in the se quence of capturing a rock core sample.  

The M20 gearmotor utilized similar modifications as developed in the M32 design effort into the smaller  motor diameter for the most volume constrained component of the ACA, the end effector. This component  is mounte d to the end of the SHA and physically grabs the sample tube for transport within the ACA.  

Figure 2. The Adaptive Caching Assembly (ACA)  

The end effector has some of the strictest planetary protection and handling requirements ever levied on a  mechanism.  The Mars Helicopter technology demonstrat or mounts to the underbelly of the rover in its  stowed configuration and has a one- time actuation of the deploy ment  arm. A summary of the four new  designs utilizing the M20 and M32 motors for 10 flight locations o n the rover are listed in Table 1.  

Table 1.  Gearmotor assignments   Application  Gearmotor  Name  Motor   SHA Elbow  SHACD  M32  SHA Linear  SHACD  M32  SHA Shoulder  SHACD  M32  Bit Carousel  SHACD  M32  Tube Drop -off SHACD  M32  Sealing Station  SAS M32  Core -Break Lock Out SAS M32  Helicopter Deploy ment  Arm SAS M32  Chuck  Chuck  M32  End Effector  DEE (M20)  M20
417 the additional advantage of allowing shorter and lower mass assembly and also eliminated any unreliability  in the connection. The decision required a significant reordering of the assembly sequence, but this did not  cause any further problems. T he manufacturing of the part itself, performed within maxon, involved a  number of challenges  to address, in particular th e deburring of the bottom of the ring gear teeth after  shaping, which  required an external supplier for an electrochemical machining step and another external  supplier for sand blasting the many detailed features on the motor flange.      Flight Design Issue s Investigated  

Detent Design   Two versions of the M32 motor were designed, with different detent strengths, each sharing components  except for the magnets generating the field. The axially charged magnet in the two designs had different  lengths, creating a 10 mN •m detent and a 20 mN •m detent, with the larger length baselined to enable the  same housing to be used for both motor types. The step size of 24 detents per revolution was chosen to  correspond to the total commutation state changes of the motor design. When paired with a gearbox, the  detent size produces holding torque at a small angle at the gearmotor output.  

The Hall sensors measure the edge field of the 4- pole pair continuous ring NdFeB magnet.  The Hall sensors  are on the output side of the motor for maxon’s flat series designs, which is the same side as the detent  mechanism module. When the detent was added to the motor, the performance of the detent mechanism  did not match the strength of the standalone unit and motor switching error was encountered. Measurement  of the magnetic field revealed the single axial magnet had significant stray flux in the vicinity of the Hall  sensor, causing the switching error and reduced detent strength ( Figure 5).  

   Figure 5. Left : Compensation magnet measurements for stray flux ; Right : Computed T omography ( CT)  scan cross section of final assembly  

Two corrections were made. First, a compensation magnet axially charged in the opposite direction was  added to the detent magnet stack, strengthening the field through the detent wheel. Second, the axial gap  between the edge of the rotor cap ring magnet and the Hall sensor was reduced by shimming each motor  individually after measuring subassembly dimensions. The change in axial gap increased the magnetic field  strength and hence the signal -to-noise ratio in the H all sensor.
418 Low-Quantity Grease Dosing   For the M20 gearmotor, JPL required r apid acceleration to the commanded speed without an excessive  current draw over the entire specified t emperature range. This is a problem due to the high viscosity of  Braycote at low temperatures. The standard quantities of grease used per gearbox stage by maxon for  COTS products are determined by the requirement to deliver the longest possible lifetime wi th standard  environmental conditions. The use of Braycote required the standard quantities to be reassessed. A series  of tests were performed with gearboxes built using differing quantities of grease. In each case, high  frequency resolution data of the start -up current draw was collected. It was necessary to reduce the grease  load by a factor of 10 compared to the standard dose in order to obtain acceptable start -up performance.  Subsequent life testing demonstrated that the require d life (low compared to industrial applications) could  still be achieved. For the flight units , application using the grease plating method was used to ensure the  low quantity of grease was uniformly distributed, a process that is unnecessary for industrial applications,  where only  manual dosing with a syringe at defined locations is sufficient.  

The mechanism driven by the M20 actuator has a limited linear stroke. The cold drag performance  of the  actuator  had significant transience in the first 5 seconds of motion; this large variation in torque production  was outside the system- level requirements for controllability. JPL tested three actuators at several low  temperatures in order to characterize the start -up current as a function of temperature. Figure 6 shows a  knee in the drag happening between -30°C and -40°C for this gearbox . The actuator still exhibited margined   torque production at the - 70°C operation temperature limit, but due to controllability and other friction effects  in the mechanism at - 70°C, the heater was modified to increase the low -end operating temperature.  

Figure 6. M20 drag current temperature dependence  

Shock and Motor Bearings   The M32 motor was tested (along with the mating gearboxes) to withstand a pyroshock environment  simulated by a tunable beam. The pyroshock spectrum had a peak level at 3,000- g with a knee frequency  at 1,600 Hz. After testing, the motor operated with a significant rattle, a noise equated to that of a coffee   grinder. Although CT  scans did not reveal gross damage in the assembly, sever al other non- destructive  tests, including a noise frequency analysis and rotating shaft deflection test, indicated motor bearings as  the damaged component. The frequency analysis had peaks at the motor bearing inner ring frequency that  would scale with mot or speed and the deflection test had a signature reminiscent of dimples in the raceway  allowing up to 0.0127 mm of deflection. The motor bearings were disassembled and inspected. The motors ’  front bearing inner ring, outer ring, and balls were deformed in both directions from the tunable beam test  (Figure 7).  The back bearing did not have any damage.