ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
191 Prior to the anomaly peak motor currents of 0.25 ampere  were nominal. This corresponds to joint drag of  approximately 790 N·m (590 ft·lbf). During the initial investigation, the peak currents increased to levels  as high as 0.60 ampere, or 1910 N·m (1410 ft·lbf) of joint drag . The system capacity is 1.4 amperes.  While the increase in drag was over 100% of the nominal value, it was still well within system capacity  with a torque margin of 1.33 . The drive current readings remained steady for two weeks at the increased  level . The system experienced  another drastic increase in drive motor current in the first week of October .  The multiple changes in drive motor current throughout the course of the investigation are shown in Figure 5 . After the 3 rd step change drive motor currents were over 1.2 amperes, or 3810  N·m (2810 ft·lbf)  of joint drag . In a one week period, the torque margin for the mechanism had decreased from 1.33 to  0.17. There was a risk that with another step change in required current there would be an unrecoverable  stall of the mechanism.  

Figure 5. Starboard SARJ Drive Motor Current Changes  during the Course of the Anomaly Investigation   In order to fully characterize the joint drag, an on-orbit test was executed. The objectives of the test were to take the controller software out of the loop and directly measure joint drag for all angular positions of the joint . The test objectives were accomplished by operating the mechanism in a mode of operations  called “Torque as Stepper” (TAS), which does not utilize the controller loop. In TAS mode, the operator  keys in a level of commanded current. The procedure for the test had the operators iteratively step up the amount of current requested until the mechanism began rotating . This was done in 30 degree increments  so as to cover all angular positions of the joint. The TAS test confirmed conclusively that the controller was not contributing to the current spikes and that the high drag condition existed across the entire circumference of the joint, although some areas were worse than others. During the test, drag levels  ranged between 2380 N·m (1760 ft·lbf) and 2860 N·m (2110 ft·lbf). 

NASA/CP-2010-216272
192 The anomaly investigation team was confident that increased joint drag was the source of the data  anomalies, however, the source of the drag was unknown. An EVA task was planned on the upcoming  Space Shuttle mission to investigate the nature and source of the suspected mechanical drag.  Extra Vehicular Activity and Mechanism Inspection

The goal of the EVA inspection was to look for a “smoking gun” that could have led to the off-nominal  data trends. Specifically, the crew was asked to inspect the thermal covers shrouding the joint (Multi Layer Insulation, or MLI, covers) and the bolts that serve as the structural attachment between the MLI covers and the SARJ. This inspection would have revealed any evidence of a micro-meteoroid strike or of  some interference between the MLI covers and the rotating half of the SARJ mechanism. Additionally, the  crew was asked to inspect the launch restraint fittings to determine if there was any interference with the MLI covers. If time permitted, the crew was also asked to remove a MLI cover to inspect the bearing races and TBAs.   Nothing off nominal was noted during the external survey . The crew had sufficient time to remove a MLI  cover for additional inspection. Upon removing the cover the astronaut immediately noticed that there  were fine metal shavings across the outboard bearing race surface . Additionally the astronaut noted that  the TBA roller housing was acting as a magnet and collecting metal shavings. Samples of the debris collecting around the TBA housing were gathered and returned to ground for analysis (results discussed  below) . A characteristic picture of the condition of the mechanism can be seen in Figure 6. The outboard  race ring, and specifically the outer canted surface, appeared discolored and mottled and there was debris on much of the surrounding hardware .   

Figure 6. Astronaut Inspecting the Starboard SARJ During an Extra Vehicular Activity  

The root cause of the damaged bearing race was not immediately known. I t was clear, however, that the  mechanism was mechanically damaged and operating in a significantly degraded condition. There was  concern that continued operation could exacerbate the problem. Also, the vibrations caused by rotating the joint had become severe enough that accelerated hardware fatigue was a concern. The anomaly investigation team recommended that operation of the Starboard SARJ be halted immediately until the root cause of the anomaly was known.  After seeing the damage on the Starboard SARJ,  engineers requested an inspection of the Port SARJ. It  appeared to be operating as expected based on telemetry review. The inspection would provide a point of  comparison to the Starboard SARJ as well as a baseline image of the mechanism. The Port SARJ  NASA/CP-2010-216272
193 inspection was executed on the following EVA . The astronaut determined that the Port SARJ race rings  looked pristine . The inspection confirmed that the damage was confined to the Starboard SARJ . Figure 7  shows a picture of the mechanism taken during the inspection . There is no apparent damage or debris  accumulating around the bearing race surfaces or the TBA. 

Figure 7 . Astronaut Inspection of the Port SARJ During an Extra Vehicular Activity  

Lessons Learned   The source of the anomalous data signature was determined less than eight weeks after its genesis. During this period of time the mechanism continued to operate and damage to the bearing race propagated around the entire circumference of the ring . It is possible that if the source of the anomaly had  not been discovered, the damage would have progressed further  (affecting one of the undamaged  bearing surfaces). In hindsight, the anomaly investigation deserves further scrutiny to assess which diagnostic approaches were most effective, and whether any improvements could have been be made to the anomaly investigation process or the health monitoring system.  The best data came from a hands-on visual inspection of the suspect mechanism. For m any spaceflight  mechanisms, this is not feasible, or comes at an extremely high cost. On-orbit testing also provided an  effective means of exonerating specific components as well as characterizing the mechanism performance. A systemic review of all possible contributing factors and appropriate test strategies should be developed immediately after an anomaly is identified.  Additional instrumentation on the mechanism would have aided in diagnosing the anomaly. Strain gauges  and accelerometers mounted directly to the TBAs would have been extremely useful. There are strain gauges on the SARJ system but none that measure local deflections at the bearing housings. The SARJ system is susceptible to high tractive forces (addressed in detail in the proceeding sections) which would  have been picked up on locally mounted strain gauges. There are accelerometers on the ISS truss but  these are not part of the SARJ monitoring and diagnostic system. Ground development testing indicated that the SARJ system was susceptible to debris generation. Accelerometers would provide an indication that debris in excess of expectations was present. A failure modes analysis should lead to the most effective instrumentation and monitoring criteria for a mechanism. Given the susceptibility to debris  generation and sensitivity to tractive forces additional instrumentation would have been appropriate for the SARJ.  NASA/CP-2010-216272
196 The mistrack or “toe” angle of a TBA roller is the misalignment of the roller due to rotation about an axis  normal to the race ring surface. The existence of some small mistracking angle is inherent in the hardware build process. The metrology laboratory at MSFC performed detailed measurements of the mistrack angle of the Starboard SARJ TBA rollers after they had been returned from orbit . Mistracking  between the roller and race creates a friction force on the roller in the thrust direction, denoted Q in Figure 9. This yields a moment about the camber axis, which is reacted by the normal load between the roller and race . As the magnitude of the thrust friction increases, the load distribution on the roller becomes  unevenly distributed to react the induced moment . The distance d in Figure 9 denotes the lateral distance  from the camber axis to the center of normal pressure of the contact . In stable tilting, the distance d  increases with increasing camber tilt, offsetting the camber moment generated by an increasing thrust friction Q. However, after a certain camber angle threshold is reached, d begins to decrease with  increasing camber angle, causing the roller to enter into unstable tilting . The analysis and inspection of  the Starboard SARJ TBAs shows that the outer canted rollers remained in the regime of stable tilting during the time when the race ring was being damaged. 

Figure 9. Trundle Bearing Roller Stability   In order to investigate the effect of roller mistracking on the contact forces, a numerical boundary element analysis tool was developed by researchers at Purdue University and Boeing . This tool was rigorously  validated against closed form solutions and also shows excellent agreement with the traction results obtained through testing at Glenn Research Center (GRC)  [5]. GRC was able to quantify the frictionmistracking relationship in a Vacuum Roller Rig (VRR) . The VRR replicates the flight-like rolling interface  and materials in a vacuum environment. A comparison between traction curves developed by test at GRC and via analysis is shown in Figure 10. One will note that the analytical results closely match the test  data. It is also worth nothing that the VRR rollers do not have identical degrees of freedom to the TBA rollers and therefore the TBA rollers must be addressed by a modified thrust curve  (discussed below, see  Figure 13).  NASA/CP-2010-216272
197 Figure 10. Comparison of Thrust Curves from Analytical Results and VRR Test Data [5] 

Figure 11 shows the transverse shear traction distribution on the GRC test rollers for a representative misalignment case . The plot on the left shows that the contact area is divided into regions of stick and  slip. The stick region, shown in green, is located at the leading edge of the contact . As the unstressed  material of the test rollers enters into the contact patch, the shear deflections and tractions between the two rollers build until the shear tractions reach the limiting value of friction defined by the coefficient of sliding friction, µ, times the normal pressure . As material moves through the trailing end of the contact,  slipping occurs between contacting points on the rollers, and the shear tractions remain at the limiting value of friction. 

Figure 11. Shear Traction Distribution for GRC Test Rollers, Q/P = 0.4   For increasing mistrack angles, the stick region gradually decreases in size until the total thrust load equals µ times the normal load, at which point the entire contact region is in slip . For small mistrack  angles such as those observed on the Starboard SARJ TBAs, the rollers remain in partial sliding, and the total frictional force is less than the limiting value of sliding friction.  The tractive phenomena in the flight TBA rollers are analogous to those in the GRC test rollers, with the exception that the TBA rollers have an additional camber degree of freedom . The flight TBA rollers also  have a flat profile that transitions to a 1.5-mm (0.060-in) blend radius at the edges of the contact . As a  NASA/CP-2010-216272
198 result, the size and shape of the contact patch for a TBA roller vary dramatically depending on the thrust  loading and camber angle of the roller . The contact patch dimensions are plotted in the left portion of  Figure 12 for increasing values of thrust load . This plot shows that as the TBA roller tilts on edge due to  the thrust loads, the area in contact with the race ring is reduced . The right portion of Figure 12 shows  that the average normal contact pressure increases dramatically as a result of the reduced contact area.  

Figure 12. Contact Patch Dimensions and Average Contact Pressure for TBA Roller    In the study of rolling contact mechanics, the relationship between the transverse rolling creep (related to the mistrack angle) and the traction ratio Q/P is known as the traction curve . The analysis of the SARJ  TBA rollers found that the shape of the contact patch caused by cambering has an influence on the traction curve for the roller . This effect is such that as a TBA roller begins to tip about its camber axis, the  frictional loads are less severe than they would otherwise be, thereby mitigating the tipping phenomenon .  The blue dotted line in Figure 13 shows the traction curve for the flight TBA roller if the camber axis were fixed at 0° . The red dotted line shows the traction curve if the camber axis were fixed at 2° . In the actual  TBA, where the camber axis is free to rotate, the traction curve follows the 0° fixed camber curve for small mistrack angles . However, for larger mistrack angles the changing geometry of the contact patch causes  the thrust friction loads to be less severe than in the fixed camber case, as shown by the solid black line in the figure . Despite this phenomenon, thrust loads as high as 0.4 times the normal load can be  generated with mistrack angles less than 0.5° .    Detailed measurements and tolerance analysis of the Starboard SARJ TBAs identified an approximate worst case mistrack angle of 0.36° . The normal and shear pressure distributions at the contact are shown  in Figure 14 for that mistrack case . As a result of camber tilting, only about one quarter of the width of the  roller is in contact with the race ring surface . The resulting stresses at the interface between the race ring  case material and the parent material were found to exceed the yield strength of the parent material .  

NASA/CP-2010-216272
199  

Figure 13. Traction Curves for Outer Canted TBA Roller 

Figure 14. Distribution of Normal and Shear Pressure for 0.36°   Mistracking, TBA Outer Canted Roller 

In addition to concentrating the contact pressures at the roller edges, frictional thrust loads due to mistracking also have the effect of increasing or decreasing the normal loads on adjacent rollers .  Depending on the specific combination of roller mistrack angles and the direction of SARJ rotation, it is possible for the race ring’s triangular cross-section to be wedged in between two of the TBA rollers . This  wedging action is analogous to a positive feedback loop . Frictional loads on the Datum A and outer  canted rollers lead to increased normal loads on those rollers, which in turn allow the contacts to generate  NASA/CP-2010-216272
200 higher frictional loads . Photographs of the damaged race ring surface taken by the on-orbit crew support  the hypothesis that the initiation of damage occurred under these wedging conditions.   Contact Stress for Flat and Crowned TBA Rollers

The root cause team further found that the bearing edge stresses were exacerbated by the flat, uncrowned profile of the TBA rollers. The roller geometry made the system sensitive to non-Hertzian effects, which gave rise to high pressure points at the edges of the rollers during the initial run-in period of the  mechanism . These high pressure points are visible in the contact stress profile of the TBA rollers shown  in the left side of Figure 15 . Part of the recurrence control plan for the SARJ is to modify the rollers on any  spare TBAs to include logarithmically crowned rollers . The logarithmic roller profile is a shape that has  been mathematically optimized to eliminate the high pressure points that ordinarily occur at the edges of cylindrical roller bearings. 

Figure 15. Normal Contact Stress Distribution for TBA Roller, P = 4.36 kN (980 lb)   Root Cause Investigation Conclusions

On-orbit video photographic, visual evidence, and debris samples analyses indicate that the SARJ race  ring nitrided layer spalled from the base 15-5 PH steel. The spalling occurred over a period of two and one half months after the Starboard SARJ on-orbit activation. The first signs that the spalling was taking  place were manifested through telemetry data that indicated the  average operating current of the SARJ  was rising from nominal values of 0.15 amp to as high as 0.8 amp.   During this time, the Structural Dynamic Measurement Data System of the ISS was indicating vibrations  that were anomalous. These vibrations were later confirmed to be linked to the degrading condition of the SARJ race ring surface. As the race ring surface became progressively rougher over time as a result of the spalling, the vibrations the ISS was sustaining while the Starboard SARJ was rotating were a concern to the structural life of the Space Station. These vibrations were inducing load cycles on ISS hardware  that were high enough to be counted in the nominal ISS loads spectrum for structural fatigue.   Testing and analysis indicate that the SARJ race ring surface damage was caused by tractive forces (normal and shear) applied to the race ring surface by the TBA and DLA rollers. Severe loading cases are observed with mistracking angles smaller than the as-measured TBA mistracking angles. Mistracking  loads are sensitive to roller edge loading, multiple roller mistracking, direction of mistracking and race ring rotation direction. Analysis indicates that worst loads occur on the gear side of outer canted surface and Datum A surface.    NASA/CP-2010-216272
203   Figure 16. Tools used in SARJ Clean and Lube Operations 

While the cleaning methods were developed based upon the on orbit trials, the method to lubricate th e  cleaned race ring was developed on the ground. A 60-degree section of race ring was used to determine  the pattern and quantity of grease to be applied . Based on testing and crew input, it was decided that the  best way to apply lubrication to the race ring was to place a single bead of grease down the middle of the roller track. The outer 45 and datum –A- surfaces were lubricated in this manner. The inner 45 race surface was lubricated using the J-Hook nozzle. The J-Hook was designed so the grease would be  smeared along the width of the race ring surface and would encompass the roller track of the TBA and DLA rollers. The final grease configuration required residual grease dams on either side of the roller track after the TBA roller had passed over and spread the grease. These repositories of grease on the sides of the track serve as a source of oil that will constantly re-wet the roller track and provide lubrication during subsequent SARJ operations .    Lab tests were also run where grease was added to a race ring surface contaminated with metallic particles similar to the debris retrieved from the SARJ on orbit. These tests showed that the hydrodynamic force generated by the roller passing over the grease was enough to push a majority of the debris out of the roller track. As a result, the addition of lubricant to the damaged race ring serves a dual purpose as a cleaning fluid for any debris left on the SARJ after the cleaning operation. The clean and lubrication operations were successfully completed during ULF2. Figure 17 shows the final application  method and grease configuration .  

Figure 17. Final Grease Application Methods 

Grease Application Trials  Single Bead of Grease in the Center of the Roller Track   Residual Reservoir of Grease  on Both Sides of the Roller  Track after Spread by the TBA   Scraper Tool  Grease Gun w/ J -Hook  Nozzle   EVA Wipe  Grease Gun w/ Straight Nozzle   NASA/CP-2010-216272
204 Results of Corrective Action   Following the successful cleaning and lubrication of the starboard SARJ, the joint was rotated and the  motor drive current was monitored. The motor current  decreased immediately from a pre-cleaning and  lubrication average of 0.242 a mpere to 0.174 a mpere. The more critical benefit was the significant  reduction in the maximum current levels . The large swing in motor drive current observed prior to t he  clean and lube operations was mitigated significantly, resulting in a reducti on in the maximum current  from 0.870 ampere to 0.384 a mpere. These results demonstrate that the clean and lube operations were  successful in increasing the stall margin in the SARJ which, in turn, maintains operational flexibility of the  ISS. These post lube data also compare well with the performance of the starboard SARJ telemetry prior  to the anomaly when the SARJ had an average motor current of 0.153 a mpere and a maximum of 0.221  ampere. Data plots showing the motor current prior to the clean and lube operations (data taken during  the 10A mission), during ULF2 just after the clean and lube operations, and data taken during ULF2 stage operations are shown in Figure 18.  

Figure 18. Starboard SARJ Motor Drive Current Comparison Pre and Post Clean & Lubrication   The port SARJ was also lubricated with Braycote 602EF © during Flight ULF2 using the same procedures  and tools as were developed for the starboard SARJ. The lubricant was intended to protect the mechanism against damage by the same mechanism as was experienced by the starboard SARJ. Pre and post lubrication data for the port SARJ shows a 20% decrease in average drive motor current, shown in Figure 19. This drop is significant as it indicates that the mechanism pre lubrication was operating in a regime of higher roller/race friction than post lubrication and was therefore more susceptible to damage  initiation prior to the application of the grease lubricant. 

Figure 19.  Port SARJ Average Motor Drive Current Comparison Pre and Post Clean & Lubrication   As noted earlier, anomalous structural vibrations were one of the first hints of a problem with the SARJ. The surface of the race ring had degraded significantly and it was not anticipated that the clean and  NASA/CP-2010-216272