ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
205 lubrication tasks would have a significant impact on the vibrations observed on ISS. The accelerometer  data after ULF2 showed that the peak accelerations had been reduced significantly. Data plots showing pre and post ULF2 accelerometer data are displayed in Figure 20 and show the dramatic decrease in vibrations to the ISS due to SARJ rotation.  

    Figure 20. ISS Acceleration Data Pre and Post ULF2 [9]   Lessons Learned

The dramatically improved performance of the SARJ mechanism due to the cleaning and addition of grease to a damaged and contaminated race ring is an important finding for space systems mechanism design. Most directly, these observations can be applied to the design of mechanisms that will, by nature of their intended use, be exposed to high levels of contamination (e.g., mechanisms on lunar or Martian based systems). It is recommended that designers consider adding the capability of re-lubrication and/or  cleaning of bearing systems, especially those which be exposed to high debris filled environments.

Conclusions   Anomalous performance of the Starboard SARJ was noted shortly after the mechanism was activated on-orbit. An inspection found that there was debris covering the bearing race of the rotating mechanism . An  anomaly team was immediately formed to investigate the issue . Through a series of tests, analysis, and  simulations the team determined that the most likely cause of the damage was high friction at the bearing/race ring contact coupled with a susceptibility of the bearing mechanism to an overturning moment on the bearing rollers . Dynamic analysis simulating the contact conditions of the SARJ  mechanism, both lubricated and un-lubricated, confirmed that in the un-lubricated condition sufficient  stresses occur to damage the race ring . Grease lubricant was applied to the mechanism in order to  mitigate the existing damage as well as prevent further damage. Additionally, astronauts removed the debris noted during previous inspecti ons. These actions proved effective as overall joint drag significantly  decreased as did structural vibrations caused by operating the mechanism.  The SARJ anomaly investigation and recovery provides several lessons learned both in the arena of diagnosing on-orbit anomalies and in complex aerospace mechanism design and verification . Mechanism  design should incorporate proven and verifiable features whenever possible . Features that do not lend Anomaly  Post Cleaning and Lubricating  Pre-Anomaly   NASA/CP-2010-216272
206 themselves readily to analysis cannot easily undergo the rigorous verification process that aerospace  mechanisms demand; for example the TBA roller profile gives rise to non-Hertzian contact effects, which requires advanced analytical methodology in order to recover the stresses . The acceptance program for  complex mechanisms should include a run-in period during which the mechanism is operated in the flight environment . This run-in period should be used to correlate analytical models of the mechanism as well  as provide a baseline of expected performance for the mechanism while on-orbit . Test correlated and  verified models can be used to determine system level sensitivities and potential design problems .  Additionally, understanding the sensitivities of the system will allow for the most effective use of instrumentation and monitoring techniques of the mechanism while on orbit . A baseline performance  characterization provides ground operators an effective means for gauging the severity of changes in performance after activation . Finally, mechanisms that operate in an environment susceptible to debris  contamination should consider inclusion of re-lubrication and cleaning capabilities . These lessons should  be utilized by architects of future aerospace mechanisms to yield more robust systems.    References 

1. Leger, L . J. & Dufrane, K. (1987). “Space Station Lubrication Considerations.”   21 st Aerospace  Mechanisms Symposium Proceedings . NASA- CP-2470 .  2. Wright, M. C., Salazar, T., Kim, H., Tucker, B., Lubas, D., Marciniak, P., McDanels, S. (2008,  February) . “International Space Station (ISS) Starboard Solar Alpha Rotary Joint (SARJ)  Expedition 16 EVA 13 and 14 Debris Analysis .”  KSC Engineering Directorate, Materials Science  Division.  3. Favero, J., Frazier, B., Nielsen, P. (1998, October). “SARJ Dynamic Simulation Analysis and Results.”  The Boeing Company. A03-J092- PN-M-9801474.   4. Moore, L.E. “DRAFT SARJ Anomaly Report for Review – STATUS.” Message to Carlos Enriquez . 13  July 2009. E-mail attachment.  5. Krantz, T., DellaCorte, C., Dube, M. (2010). Experimental Investigation of Forces Produced by  Misaligned Rollers . 40th Aerospace Mechanisms Symposium Proceedings . Manuscript in  preparation .  6. Enriquez, C.F. (2009, September). International Space Station Solar Alpha Rotary Joint Anomaly  Report . The Boeing Company, International Space Station Program. D684- 13412 -01.  7. Loewenthal, Stu, Scotty Allen, and John Golden. "Solar Alpha Rotary Joint Anomaly Test Findings:  Mechanism Perspective." Aging Aircraft 2009 . (2009): Print.  8. Johnson, K.L. (2003). Contact Mechanics . Cambridge, United Kingdom: Cambridge University Press.  9. Laible, M. (2009) . “SSARJ Autotrack Test 5 Results.”  The Boeing Company, Houston, Loads and  Dynamics.  10. Martinez, J.E. “SARJ DIGS % Area Calculations.” Message to Dr. John Goldon. 01 December 2009.  Email attachment.  NASA/CP-2010-216272
207 Test Validation of the Repair to the Space Station Solar Alpha Rotary Joint 

Curtis Allmon*, Will Wilkinson * and Stu Loewenthal * 

Abstract   The Solar Array Alpha Joint Lubrication Interval Test (SARJ LITE) test rig was built as a method to evaluate the performance of the grease repair on the Starboard SARJ of the International Space Station  (ISS). The on-orbit SARJ was temporarily parked after re ceiving significant damage on one of its race ring  surfaces as a result of inadequate lubrication (high dry contact friction) and unaccounted for roller traction kinematics. In a scaled down rig, flight-like roller bearings were preloaded and cycled on a nitrided 15-5  race surface. Grease was added to the track and wi th instrumentation monitoring performance, trending  data will be extracted and used to determine lubrication intervals for bot h Port and Starboa rd ISS SARJ’s.  The grease lubrication was found to be effective in e liminating the high friction that contributed to the onorbit race damage.    Introduction   The ISS is powered by eight solar arrays that trac k the sun for optimum power generation. Each Solar  Array is mounted to a Beta Joint that provides a degree of rotational freedom and, further inboard, a  cluster of four arrays is mounted to SARJ, one starbo ard and one portside, providing an additional degree  of rotational freedom (see Figure 1). The Alpha and Be ta joints by design provide continual year round  tracking of the sun. This paper covers the ground te sting used to determine lubrication intervals for the  Space Station SARJ’s. Included is an overview of the SAR J, a description of the on-orbit anomaly that led  to the greasing of the SARJ, a thorough description of  the test rig used to determine the intervals for  relubrication, and a summary of the perform ance to date, and the plan forward.   

Figure 1: Space Station on-orbit 

                                                  * Lockheed Martin Space Systems Company, Sunnyvale, CA 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010   Port SARJStarboard SARJ  NASA/CP-2010-216272
208 Background 

The Space Station utilizes two SARJ’s, a Port and a Starboard, whose primary purpose is to rotate the outer most truss segments and allow the solar arrays to track the sun. Sun tr acking is achieved through  rotation of two orthogonal joints, the Alpha and the Beta. The loss of a SARJ would mean significantly reduced rotation of the solar arrays as the Beta joint would then be the only joint left; power harnessing  would be significantly reduced.  Each SARJ consists of 12 trundles, 2 race rings, a nd 2 drive motors called DLA’s (Drive Lock Assembly).  The trundles are the bearing assemblies that allow one race ring to rotate relative to the other. The DLA is the motor that drives the ring using a pinion gear  that engages the race rings bull gear. In primary  operation, the trundles and DLA’s are mounted to the inboa rd side and the outboard race ring is driven. In  its redundant mode, or outboard operations, the trundles and DLA’s are flipped and mounted to the  outboard side and the inboard race ring is driven. The redundant mode of operation is only used if there is  a failure or issue with the inboard mode of operation. 

Figure 2: SARJ with blowup of a trundle and a race ring segment 

Each trundle bearing assembly consists of three rollers that are made of 440C with an ion-plated gold coating. The DLA assembly consists of two trundle-like  bearing configurations called followers, which also  each consist of three 440C gold-coated rollers. The rollers between the trundle and the followers differ in  TRUNDLE RACE  RING Nitrided  raceway  surfaces  Damaged Surface  (Outer Canted) Datum A  NASA/CP-2010-216272
209 several minor respects such as contact width and edge radius. Although the follower rollers have a  slightly larger overall width, their contact width is slightly less due to their larger radius. Both the trundles  and the followers have a datum A roller that rides on the bottom surface of the race ring (see Figure 2)  that is different than the other two rollers. The datum A roller is wider and also has  a different taper angle.  Its corresponding surface on the race ring was also diffe rent than the other two surfaces as it was ground  and the other two surfaces were not.  In the fall of 2007, an increase in the drive current  was observed on the Star board SARJ. After further  monitoring, it was decided to park the joint in or der to prevent any further damage. The Starboard SARJ  had only been operating for approximately 5 months compared to the Port SARJ, which had been operating nominally for approximately 14 months. Shortly thereafter, an EVA ensued, which included an inspection of the ring. It was then discovered that a significant amount of damage had been done to the  outer 45 deg canted race surface with debris being observed in high quantity on much of the adjacent hardware all around the race ring.

Figure 3: Damaged on-orbit SARJ race ring 

A root cause investigation was then initiated. A fault tree was built wi th all conceivable possibilities of  cause being included. Each item was closely reviewed and rated on likelihood based on results from data review, analysis, test, and on-orbit inspection. It wa s determined that roller mistracking coupled with high  surface friction from dry roller contact was the root cause of the failure. The high contact stress due to  roller edge loading as a result of mistracking caused the roller to spall the nitride case. Mistracking is the result of the rollers being slightly misaligned (<0.5 deg) relative to the race as shown in Figure 4. A side  friction force is generated at the roller/race contact since the rollers are slipping slightly as they roll. The side force creates a tipping moment abo ut the camber pivot axis. This in turn causes the roller edge to dig  into the race as illustrated in Fi gure 4. The details of the failure investigation are discussed in a  companion paper (Reference 1).  A subsequent EVA inspection of the Port SARJ revealed traces of grease on the race surfaces. It was  discovered that the trundle bearings had most li kely leaked out a sufficient  amount of grease to  adequately lubricate the ring surfaces and prevent the rollers from edge loading and causing the damage  to the race surface. A review of  the differences between the Port a nd Starboard SARJ’s had revealed that  Damaged Surface  (Outer Canted) 

NASA/CP-2010-216272
210 the Starboard SARJ never had a ther mal vac test performed, which may have created the environment  for the Port SARJ trundles to leak some grease. 

Figure 4- Mechanics of mistracking causing tipping and edge loading   As part of the recovery effort, both the Port and  Starboard SARJ’s were greased using Braycote 602  during mission STS-126 Nov 2008. Since then, the Po rt SARJ has been under continuous operation with  nominal performance while the Star board SARJ remains parked except for an occasional test run for  evaluation. Several questions remained unanswered including: a) at what rate would the damaged ring continue to degrade after greasing during normal au to-tracking (continuous operation), and b) at what  time intervals would relubrication operations be ne eded for both joints? Furthermore, how long could the  relubricated damaged ring last before a transition to outboard operations be required?   Rig Design   These unanswered questions led to the need for some additional ground testing. It was decided that a high-fidelity test rig dubbed the SARJ LITE (Lubrication Interval Test) would be created to determine lubrication life on the SARJ. This was accomplished by building a scaled-down ve rsion of the flight SARJ  that simulated flight trundle bea ring kinematics such as mistracki ng along with flight roller and race  geometries/materials.   Its purpose was threefold. First, to use the rig to damage the test plate to generate flight-like race  damage. Secondly, to test for ring  durability by making sure runnin g on the damaged surfac e doesn’t lead  to catastrophic breaks in the ring; and thirdly, to determine grease relubrication intervals that would reveal how often an EVA is needed to relubricate the rings. The SARJ LITE data in addition to data from other  component test rigs are being used to steer de cisions for future Space Station operations.   In the on-orbit configuration, 12 se ts of trundle rollers and 2 sets of DLA follower rollers ride on the race  ring surface. With the race ring diam eter being nearly 10 feet (3 m), it was decided t hat a full-scale test rig  would not be feasible so a smaller size was chosen  to be compatible with a thermal chamber but the  spacing between trundle rollers would be maintained. The DLA follower rollers were left out due to their significantly reduced preload that  lessened the likelihood of their contribution to ring damage. A  configuration of three equally spaced rollers was chos en for the rig. It was additionally decided that since  NASA/CP-2010-216272
211 each trundle assembly has two different roller types, a wide and a narrow, two different race tracks would  be utilized to run simultaneous tests, one for the wide and one for the narrow. The rig was setup so each track and preload system was independent of the other and could be run by itself or in parallel with the  other. The preload system initially used a stack of  Bellville washers to achi eve the desired preload  (roughly 13.3 kN (3,000 lb)). After analysis of the roller support structures (called spiders) however, it was  determined that the spiders had su fficient compliance and were soft en ough to act as the Bellville stack  themselves, so the Bellville washers were taken out  of the design. Instead of modeling the trundle  assembly, which includes a cluster of three rollers, the test rig simply used a single test roller backed with  a support roller (see Figure 5). The preload was to be set with an individual preload bolt for each test setup, evenly distributing the load between the three rollers.   Roller geometry was maintained close to flight with the exception of the taper a ngle, which was increased  to accommodate the reduced size of the test plate. The camber axis was also included in the test trundle  design. The camber axis was intended to allow the roller to tilt, which causes the roller to lay flat against the race ring. However, it was this very camber f eature that ended up contributing to the race damage by  allowing the roller to tip and initiate the damage to the ring. A feature was added to the rig design to lock the roller housing at a particular camber angle. The roller was then maintained with edge loading for extended periods during attempts at damaging the race in a flight-like manner. 

Figure 5- SARJ LITE Rig (vacuum chamber cover not shown) 

Adjustment of the mistracking angle was another feature built into the rig (see Figure 6). The mistracking  element was important because it played into the root cause. Some of the rollers from actual on orbit trundles were measured and found to have a certain amount of mistracking built into them due to tolerancing buildup. This data was used to aid the final setting for the lube life test. Additionally, since the rig would be used to cause initial damage to the plate, a roller adjusted to a high mistrack angle could quicken the damage runs. The mistracking was set using a known zero, which was achieved using  instrumentation that defined a perfect alignment of the roller relative to the track. An LVDT as shown in  Support           roller Flight like roller assembly with instrumentation  Nitrided test plate with 2 tracks   Rollers  Load   Cell 

Outer track Inner track  NASA/CP-2010-216272
212 Figure 6 was used to aid in defining the motion requir ed to obtain a particular mistrack angle. Once the  angle was achieved, the three faste ners holding the bearing mount to the spider were tightened. This  process was performed on each bearing assembly.       

Figure 6: Test roller with its respective instrumentation 

Pre-Test Damage Phase   The first task was to damage the outer track to simulate the damage condition of the on-orbit Starboard race. This race had significant pitting both across the track and along the entire circumference. The nitrided layer was broken through and only islands, re mains of the surface, were left with the majority of  the nitride case turned into debris (see Figure 3). It was originally thought that the rig could be used to damage the test plate using hardened rollers and purpo sely mistrack them to high angles to induce  tipping, or even to pin them to maintain an extreme tipped state. The damage runs were made initially with only the three outer track rollers engaged so the entire inner spider, including its rollers, was disengaged. Difficulty was encountered when attempting to spall the test plate. The rig was then reconfigured (see Figure 5) with extensions added to the inner spider, which allowed all six bearing assemblies to run on the outer track and double the num ber of stress cycles the ring would see. This  helped speed up the damage but the damage was co nfined to a narrow region (see Figure 7).    It was suspected that the roller tipping was too high (see Figure 8) to propagate the damage across the  track. So the tipping angle was reduced to get more roller contact across the wi dth of the track. Early  results looked promising as some additional spalling was observed, but over time roller and track wear  Sideload   cell  Mistrack  setting  sensor   Tipping  sensor  (LVDT) 

Spider  

NASA/CP-2010-216272
213 became the dominating factor (see Figure 7). Wear simply produces debris that eventually becomes  pancake-like in appearance. It differed from the on-orbit pitting and spalling observed. The 440C rollers  would wear at the tips (see Figure 9) where th e contact was made and become dull and a trough would  gradually form on the track. 

            Figure 7: Example of damage generated on outer track 

Additional attempts were made using alternate tip angles, mistrack angles, and preloads, with mostly  wear being created. Harder tool steel rollers were also implemented but they also began to wear and lose  their edge over time.   It seemed that no matter what the configuration was, the result was more of the same. It was later determined using a comparison of samples that t he on-orbit case hardened nitride layer had more  subsurface defects (porosity, etc.) than that of the test plate. Although the test plate was nitrided at the  same vendor as the flight race ri ngs, its superior metallurgical stru cture made it less susceptible to  damage. This was perhaps the key factor fo r our inability to damage the test plate.    Other techniques for damaging the race had been pursued and some looked promising. These methods included EDM'ing, etching, pressing pins, and use of a fr iction stir welder. The friction stir welder at JSC  used a single roller on a linear path and could cause damage on a single chord. Sixteen chords would cover the entire plate. The initial damage was done wi th a mistracked and tipped roller. The stir welder  would then be radially offset inward for more damage passes and the process of shifting inward repeated  until a strip of nearly 5 cm (2 in) was produced (see Figure 10). Once the damage runs were complete, the plate was shipped to Sunnyvale and mounted onto the test rig.  

Edge cracks  and spall s  Wear  NASA/CP-2010-216272
216 sufficient time to flow back into the contact between roller passes after being pushed to the side (see  Figure 12). The time required is a function of the length of travel, which is significant for our test rollers being relatively wide. This grease flow phenomenon is  relatively well documented, e.g., see Reference 1.  Further observations allowed determi nation of the 0.4 rpm to be the threshold at which the lube life test  should be performed. It provided an approximate 8 to 1 test acceleration factor, which was tolerable as 3  years of on-orbit data could be gat hered in just over 4 months.  

Figure 11: Datum A sideload friction data showing the benefit of grease and low test speed 

Figure 12: Damaged Outer track with grease  NASA/CP-2010-216272