ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
10-mm Motor/Gearbox Life-Test-to-Failure Determination of Failure Source 10-mm Motor/Gearbox Application Hardware Description The 10-mm motor and gearbox assembly is comprised of a three-phase brushless dc motor attached to a  16:1 reduction ratio planetary gearbox. The motor has ball bearings on the rotor and the gearbox planet  gears use plane bearings. The gearbox is lubricated with Bray grease, type 601. There are two preloaded  ball bearings on the output of the gearbox to provide a moment carrying capacity for offset loads on the  output shaft. The face width of the planet gears is approximately one millimeter and the ring gear is made  from bronze.  The 10-mm actuator drives the filter wheel of the Mast cameras on the Curiosity rover operating on the  surface of Mars. The actuator has a pinion gear attached to the output shaft and is mounted into a housing  that contains the larger filter wheel for the camera. The pinion gear on the actuator output drives the filter  wheel directly with a gear mesh on the outside diameter of the filter wheel disc. The motor is operated as a  stepper motor in this application, so no rotor position sensors are mounted within the motor housing. There  is a magnetic detent wheel attached to the back of the rotor to provide the necessary magnetic detent for  proper stepper motor function. The filter wheel assembly is shown in Figure 17. Magnets on the filter wheel  itself are sensed with hall sensors in the housing to confirm filter position during operation of the mechanism.  Figure 17. Filter Wheel Assembly Application of the 10-mm Actuator  Life Test Setup  Two Filter Wheel Assemblies were used on the Curiosity Rover mission. Two Filter Wheel Assemblies are  also being put in the new camera build for the Mars 2020 mission. Since the Mars 2020 mission is a longer  duration than the Curiosity mission, the life test that was performed on the Filter Wheel mechanism for the  Curiosity mission was not adequate for the newer mission requirements. The life test Filter Wheel Assembly  from the Curiosity hardware build was resurrected and placed into a life test to understand if any  fundamental limitations of the hardware design existed that might affect its use for the Mars 2020 mission.  NASA/CP—2018-219887 314
From the x-ray images there is no apparent problem within the actuator at all. The gearbox looks to be in  good shape, the bearings at the gearbox output show no indication of extreme wear or debris, the motor  bearings do not show any sign of wear or debris, and the motor windings show no indication of being  damaged. The failure mechanism needed to be consistent with a wear phenomenon since the test required  a slowly increasing drive current to keep the mechanism operating properly. This would indicate an  increasing load torque as opposed to a sudden change. The only item detected with the x-ray images was  the offset of the detent wheel from the detent stator. This appeared to be a manufacturing defect since it  would not explain a slowly increasing load torque.  The Culprit is Exposed!  The next possible inspection method involved opening up the gearbox and motor to directly inspect the  components for wear and debris. The problem with this method is that the 10-mm actuator is not built to be  disassembled. The component items are bonded together and machining would be required to separate  the parts. There was a significant concern that a machining process may dislodge the source of the drag  torque and the failure would be permanently lost. To eliminate this concern, the CT Scanning method was  employed. The volume of the actuator is small and the resolution of the machine is a fixed value for the  scan volume. If the volume size is made smaller, the image dimensional resolution is increased. The  actuator was CT Scanned and the model was queried to determine the source of the drag torque. The  gearbox was the suspected guilty party, so it was inspected in detail first.  Figure 21 shows the overall view of the motor and gearbox assembly for the 10-mm actuator. The CT Scan  detailed computer model allows the cross-sectioning of the image so internal components can be inspected.  In this actuator, the planetary gear faces are one millimeter wide. The bright portions of the image are the  permanent magnets due to their high density. Figures 22 and 23 show the planetary gear stages in a crosssection of the computer model at each stage location. No debris is visible and no wear of the gear tooth  Figure 20. X-Ray Image Showing the Detent  Magnet Section of the Motor Assembly.  Magnets are on the Rotor and the Teeth are  on the Stator Figure 19. X-Ray Image Showing the  Gearbox Section of the Actuator.  Output Ball Bearings are at the Top of the  Image and the Front Motor Bearing is at the  Bottom  NASA/CP—2018-219887 316
form is evident from the image. The gear teeth and tooth form look as if there is no wear at all. The input to  the first stage of the planetary gears had been through 258 million revolutions at this point.  Figure 21. Overall Image Cross-Section of the 10-mm Actuator Assembly  Figure 22. Second Planetary Gear Stage Figure 23. First Planetary Gear Stage with the  Motor Pinion at the Center NASA/CP—2018-219887 317
The view of the first stage of the planetary gears  seemed to show an offset of the pinion gear  toward the motor. This necessitated an  inspection of the front motor bearing. Looking at  a section of the model from the side in Figure 24  shows that the motor bearing appears to have  shifted downward in the image and the motor  pinion gear has done the same. Scrutinizing the  bearing assembly itself shows no unusual wear  or debris. All other elements of the motor  assembly in this region seem fine.  The offset of the rotor was a mystery and  required further investigations. Looking at an  isometric view of the section of the motor through  the shaft at the front motor bearing did not  expose anything unusual. Figure 25 shows the  view with the front motor bearing fully intact with  no significant wear or debris.  Moving down to the rotor detent assembly, the  detent magnets are positioned properly within  the inside diameter of the detent stator and no  anomalous behavior appears at this location.  Figure 26 shows this region of the motor  assembly with no visible indication of a torque  drag anomaly.  Inspecting the CT Scan model further finally showed a source of the drag. The rear motor bearing is shown  in Figure 27 and the ball separator is missing. The balls have moved within the bearing assembly to a  position where they have jammed and the bearing will not rotate. While the jammed bearing appeared to  be the guilty party, it could not explain an increasing torque load over time. The problem with the bearing  jam being the source of the drag is the jam does not allow the bearing to rotate. This should have stopped  the actuator suddenly and not in the manner of a slowly increasing drag torque.  Figure 24. Side View of the First Planetary Gear  Stage with the Motor Pinion at the Center.  The Bearing is Displaced Downward in the Image Figure 25. Front Motor Bearing Closest to the  Gearbox InputFigure 26. Detent Rotor and Stator NASA/CP—2018-219887 318
Closer inspection provided the needed clues to the solution. The shaft in the bore of the bearing in Figure  27 appears to be smaller than the bore. Figures 28 and 29 show a side-by-side comparison of the end of  the motor with an as-built beginning of life motor on the left and the failed life test motor on the right.  Figure 27. View of the Rear Motor Bearing Showing the Balls Have Displaced Around the Circumference  of the Bearing Races to a Jamming Position. This Makes the Bearing Unable to Rotate at All.  Figure 28. Healthy Motor at the Beginning of  Life Showing Bearing Placement and Shaft Fit.  The measurements in the images are in inches  (0.0095 in = 241 microns)Figure 29. Life Test Motor Showing Shaft  Wear and the Bearing Having Moved Toward  the Rotor. The Ball Retainer is Seen Located  Under the Bearing Assembly.  (0.0229 in = 582 microns) NASA/CP—2018-219887 319
Figure 30 shows a close-up of the rear ball bearing and where it is dragging on the rotor (from Figure 29).  The bearing has cut through the magnet end cap (aluminum) and started cutting into the magnet material.  The rotor diameter has also worn significantly smaller than the bearing bore. The root cause of the failure  was the bearing seizing and stopping rotation. This was followed by the rotor shaft rotating in the bore of  the bearing inner race. Since a stepper motor does not provide current feedback that is related to load  torque, the jamming of the ball bearing was not detected. This event was missed because the motor rotor  was assembled with a small clearance to the ball bearing bore and that allowed the rotor to rotate within  the bore of the bearing. This motion started the wearing of the rear section until too many components were  dragging on each other.  Figure 30. Close-up of the Motor Rear Bearing Showing the Wear of the Components of the Rotor  An interesting point is the components all have a thrust load on them that draws them toward the rotor. This  thrust load is from the magnet flux at the end of the magnets. This is often not a significant player in the  performance of a motor, but the thrust load is enough to draw the component into the rotor such that, if  there is sliding contact, significant wear can occur.  Conclusions and Lessons Learned  The CT Scan inspection method is extremely powerful in diagnosing failures in mechanical assemblies  where access to the suspect regions of the hardware is very difficult or impossible. It is also great to use  where the accessing of the internals has the risk of losing the failure. Since the most important tenet of  troubleshooting is to NEVER LOSE THE FAILURE, the CT Scanning method is a fantastic resource.  Additionally, thermal cycling of motor windings for extreme cases is paramount to having confidence in  mission success. Continuous monitoring during the temperature cycling of the item being tested is critical  since the failure may not be present at room temperature, as was the case with the InSight IDA motors.  Lastly, making the rotor free to rotate within the motor bearing bores, if possible, provides a robust and  graceful degradation path for actuator motors. This is especially important at the lowest torque location  within the mechanism – the motor rotor.  Shaft diameter  smaller than  bearing bore Magnet in Contact  with Bearing Race Magnet End Cap  fully cut through by  the bearing race  NASA/CP—2018-219887 320
Acknowledgements  This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a  contract with the National Aeronautics and Space Administration. Reference herein to any specific  commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not  constitute or imply its endorsement by the United States Government or the Jet Propulsion Laboratory,  Pasadena, California. Copyright 2018 California Institute of Technology. All rights reserved. Government  sponsorship acknowledged.  The author wishes to acknowledge the great contributions provided by Raytheon SAS Failure Analysis Lab  in El Segundo, California for expertise operating the North Star Imaging machine and the InSight failure  analysis team and associated review boards in uncovering the root cause of the winding anomalies. On the  10-mm Actuator failure analysis effort, Malin Space Science Systems performed the life test and provided invaluable information and access to the Filter Wheel Assembly actuator at the conclusion of the life test. Malin Space Science Systems facility is located in San Diego, California. NASA/CP—2018-219887 321
Figure 5. Gouging and wear on base plate caused by gear interference and coil irregularities  The final contributor was poor material selection. All wearing surfaces were aluminum to aluminum,  producing a high friction coefficient (approximately 1.0) and covered surface areas around 50 cm2. The  flanges were not structural and could have easily been changed to plastic or a polytetrafluoroethylene  (PTFE) embedded material, reducing friction by a factor of 2 at least. Though the interference was an offnominal case, lower friction materials would have reduced the torque requirement in the event of physical  contact between these components.  The interferences were removed by making larger gaps between the rotating components. The flange  damage was repaired and the base plate was redesigned to accommodate the new motor design. The  gear’s height was reduced by about 0.2 mm and 0.2-mm shims were added between the top flange and  top plate. These changes nearly tripled the gap widths, allowing for spooling irregularities and reduced the  wear observed at the next deployment tests.  Much of the mechanical interference and wear investigation uncovered other problems that were addressed  in the SSD’s redesign and test development. These causes were related to the internal friction assumptions,  boom spooling irregularities, and 3D prototype comparisons.  Poor Understanding of Friction  The SSD is designed to slowly release the spring energies of four tightly wound 7.2-meter stainless steel  booms. The spring forces naturally produce friction during boom deployment and retraction. The booms  are rolling over numerous thermoplastic bushings, sliding along two thin aluminum surfaces and rubbing  between their concentric coils as they “bloom” [2]. These loads are relatively straightforward to calculate  individually, but as a system, there was no accurate way to represent behavior other than with prototype  testing. The half scale plastic model provided some torque estimations and allowed the team to optimize  contact surface designs, but the measured torques were not properly scaled to represent a full-scale  system. This will be expounded upon in the “Inaccurate Correlation” section.  The three friction sources noted above—rolling, sliding, and blooming—are great contributors to the torque  load, but there were many more that had to be considered during the redesign (Figure 6). First was the  motor transmission inefficiencies. The vendor datasheets noted that the 91:1 motor operated at about 70%  efficient, meaning only about 70% of the motor shaft output was transmitted to the gearbox shaft [3]. This  means the speed was knocked down by a factor of 91, but the torque only increased by a factor of 63. This  was a small oversight in reading a datasheet with huge ramifications. Furthermore, friction in the rulon  bushings and between gear teeth was ignored and given that there was no quality inspection on the  individual parts, these could have been either non contributors or primary causes of our system failure.   NASA/CP—2018-219887 327
Figure 6. Three sources of friction in the deployer system Lastly, there was significant external friction that had yet to be considered. As the booms deploy, small  sliding feet were attached every couple of meters to prevent contact with the floor and boom flanges. These  friction forces react back into the deployer and act as resistance loads against the motor torque. We had  never considered this and even if the motor could have initiated deployment, there was a good chance that  the ground testing would fail due to the test-induced loads as the booms deployed across a surface. The  slider and test design impacts will be covered in the “Sail Deployment Development and Testing” section.   Boom Spooling Irregularities  One of the primary focuses during the deconstruction activities were the spooled boom configurations and  their impacts on surrounding components. The coiled booms were expected to be 35-mm tall, but as they  were deployed and retracted over time, small irregularities produced an inconsistent spool height. Within  the concentric coils, the spool height may rise or fall by 0.3 mm. This inconsistency pressed against the  spool flanges and gears, causing the deformation and wear shown in Figures 4, 5, and 7. The lack of  alignment features for the boom spooling operation damaged surrounding components on retraction and  deployment.   This design flaw was remedied during spooling operations. The retraction speed was reduced to about half  the deployment speed and required two people to closely monitor the booms’ entrance angles and  clearances between the upper flanges and lower gears. There was not enough structure or volume to  include passive alignment features, and the hands-on solution proved to be acceptible. After properly  spooling the booms, flange and gear damage was greatly reduced and deployment torque requirements  were noticably different.  NASA/CP—2018-219887 328
Figure 7. Visible Wear after a few boom-only deployments due to boom irregularities and depiction of  spool cross section  Inaccurate Correlation between Prototype and EDU  The team’s false confidence supplied by a full-scale 3D printed prototype is determined to be the failure’s  root cause. The prototype testing was critical to the development pace for the SSD, but where it did provide  wonderful design inputs, it also gave a deceiving sense of familiarity to a different EDU system. The ½ scale  prototype was made of 3D printed ABS. Though the plastic parts may have resembled the aluminum parts  on the EDU, the mechanical properties were incomparable. The similarities and differences can be  observed in Figure 8. Furthermore, the booms’ lengths on the prototype were 4 meters as compared to the  7.2 meters on the EDU.  Figure 8. Prototype and EDU SSD  NASA/CP—2018-219887 329
deployment dynamics and produced inconclusive results. The air bearings were never used again on  account of their high inertias and hose-induced loads.  Figure 9. First sail deployment test: half-scale 3D-printed prototype.  After abandoning the air bearing system, the half-scale sails were deployed using low friction furniture  sliders as boom supports. Though this method was cruder in nature, the results were more symmetric and  predictable, helping baseline progress during the prototype phases. Despite the less flight-like test system,  the prototype deployer helped determine intermittent sail shapes, reflective areas, and deployment failure  modes. After about 10 boom-only and 3 half-scale sail deployments, the prototype was set aside as the  team began focusing on EDU testing.  After the SSD redesign work, the NEA Scout team re-entered test activities and completed its final EDU  deployment test in the fall of 2017. Full-scale SSD testing was a logistical and technical challenge for the  team. The deployed sail is 9.3 x 9.2 m, 2.5-microns thick and incredibly fragile (Figure10). The sheer size  and sensitivity of the material limited the team to available facilities for large-scale testing. Even after looking  to other NASA centers for possible deployment tests, the team determined the Flat Floor to be the most  suitable facility based on its area, ease of access, and low cost.   Figure 10. Full-scale EDU deployment  Throughout the full-scale deployment test suite, the team studied and applied lessons learned from more  than a dozen off-nominal deployment cases, characterized the booms and sail in a 1G environment and  produced a detailed deployment procedure. Sliding friction forces were reduced by 50%, a simple and  effective method to gravity offload the boom tips developed. Furthermore, the team characterized boom  and sail-related failure modes and operational methods to avoid them. These careful experiments  developed a new test scheme that would remove some of the complicating factors present in deploying  NASA/CP—2018-219887 331
flexible gossamer structures in 1G and allowed the booms and sail to behave in manners more similar to  the zero-G flight environments.  Sail Development Test Lessons Learned  The first lesson learned relates to the air bearings. The air bearings were a large mass at the end of each  boom (Figure 9). Even after allowing 3 degrees of freedom, the bearing masses dominated the boom tips’  natural paths. The air hoses connected to each air bearing also introduced significant friction forces into  the system. Though the air bearings were frictionless, their air supply hoses had to drag along the floor and  the hoses bending resistance completely overwhelmed the booms’ restoring forces. This problem only  worsened as the booms increased in length until the booms eventually buckled. For large sail systems,  large masses at the boom tips will dominate deployment dynamics and lessen the booms’ abilities to selfcorrect. Avoid large masses and force inputs near boom tips during deployment testing.  The second deployment test was conducted on a basketball court. The team explored using the basketball  court out of necessity of needing available floor space. The air bearings could not be used on the hardwood  surface, requiring the team to get creative in the test approach. The wax finish on the floor coupled with the  introduction of felt sliders proved to be a successful combination. The team introduced a felt slider design  that would support the booms at multiple locations only grams of extra mass added to each boom. This  removed the concern of ground support equipment mass overwhelming the dynamics of the test.   The next lesson learned came during the first successful SSD prototype deployment on a basketball court.  This test showed a sail and boom shape progression that seemed more indicative of a flight deployment.  Initially, the team assumed the sail would deploy in a uniform, rectangular manner. However, testing  showed that the sail would form in a “bow-tie” shape during deployment (Figure 11 A-B). At half scale, the  booms were able to compensate and correct at the end of deployment (Figure 11 C-E).   Figure 11. Sail shapes during nominal deployment.  With the success of the half-scale deployment tests, the full-scale tests followed the same procedure of  integrating felt sliders on the booms at predetermined points. Originally the team thought the only human  intervention needed was to monitor when to initiate and conclude the test. Additionally, the team was  confident the boom’s self-correcting behavior observed in the half-scale test would repeat in the full-scale  test.  The third lesson learned through the deployment tests was boom buckling. During half-scale tests, when  the sail was fully removed from the central spool, the tension along the perimeter of the sail assisted the  NASA/CP—2018-219887 332