ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
179   Figure 2:  Surface Finish error, burr; 500x magnification 

During this anomaly investigation, it was found that a PBN thread dimension call-out was incorrect. The depth of threads called out exceeded the thread area of  the encapsulated nut. Only 15 to 16 threads were  able to be machined into the PBN barrel given the amount  of depth allotted for the threads. However, the  thread dimension call-out stated 18 threads should be machined into the nut barrel. This was discovered when running a go/no-go gauge in the PBNs before lubr ication to check for correct thread dimensions.  The gage would only go 15 to 16 turns depending on the PBN. Repercussions of this incorrect thread dimension not being found would have been seen during the lubrication process while running a thread cutting tap into the nut in order to clear cured lubricant out of each thread groove. The cutting tap would have passed the last thread of the PBN barrel and cut into the base metal at the bottom. Shavings of the  base metal could later be caught between the thread s of the PBN and Powered Bolt during mating. This  extra metal between threads could cause failures due to higher torque limits and damaged lubrication. With this information being discovered before acceptance testing, the correct amount of turns was able to be used during the PBN lubricant tapping process descri bed in the next section. Since a thread cutting  tap was never used on PBNs before, there were no shavings of loose base metal and no failures of this type in the past. This incorrect call-out was never not iced during mating of the PBN with the Powered Bolt  because the Powered Bolt only enters the PBN with 14 turns.   

Lubrication 

Dry film lubricants are used in space applications and sp ecifically with the Powered Bolt Nuts for their  thermal stability, low abrasivity and low shear stre ngth. Dry Film Lubrication of the PBNs is another  important aspect of manufacturing which ensures a smooth running sy stem with no galling issues and  consistent torque-tension characteristics of th e mechanism. Each PBN should have values above   90.4 N-m (800 in-lb) throughout cycles 2 through 24 of  acceptance testing. Unacceptable test data from  the PBNs manufactured in 2007 can be seen in Figure 3. If the PBNs are not fabricated or lubricated  correctly, testing torque limits above 90.4 N-m (800 in-lb) will be seen during cycles 2 through 24 of  acceptance testing resulting in a failed acceptance test. The Acceptance test for the PBNs is a very  rigorous test. A Powered Bolt is cycled into a PBN 28 times under a normal operating load. Cycles 0 and  NASA/CP-2010-216272
180 1 are at ambient pressure and temperature. Cycles 2 through 21 are under vacuum at ambient  temperature. Cycle 22 is under vacu um and at cryogenic temperatures. Cycle 23 is under vacuum and at  hot temperatures. Cycle 24 is at vacuum at ambient temperatures. To finish the test, Cycles 25 through 28 are at ambient temperature and pressure. Figure 4 shows complete acceptance test data of all of the  2009 PBNs. The 2009 Acceptance test data shows more  consistent torque-tension behavior of the  correctly fabricated PBNs compared to the failed 2007 PBNs.   The lubrication process for an encapsulated nut is very difficult due to its confined area. Most machinery is unable to enter the nut barrel and spray lubricant evenly over all threaded areas. The common process for applying dry film lubricant to small diameter internal threads is to use a spray gun with a long nozzle. The lubrication thickness and uniformity are depen dent on several variables; fan speed, speed of the  nozzle through the threads, nozzle pressure, agitation rate of lubrication slurry, and nut temperature. If  any of the variables are miscalculated or misused, the nut will be processed with agglomerations of the  lubricant or areas without lubricant. PBNs may contain both agglomerations of lubricant and areas of no lubricant simultaneously. Each of these defects is  hard to detect without proper equipment and  knowledge. A borescope is commonly used to look at  the internal threads of nuts, but without the  knowledge of what to look for, these defects can go undetected.  

Figure 3: Acceptance test data from failed PBN lot of 2007  NASA/CP-2010-216272
181 Figure 4: Acceptance test data from passed PBN lot of 2009 

Dry film lubricant, which is applied to internal threads using common practices, has an unpredictable thickness. Thickness is almost impossible and usua lly too expensive to measure in an indestructible  manner. If the lubricant is too thick, this will cause high torque limits during acc eptance testing leading to  failure of the nuts. Thickness of the PBN lubricant was also a reason for failure for some of the previously  built PBNs. A way to defer from dry film lubricant being too thick is to use a thread cutting tap. On previously built PBNs, a roll forming tap or a specia lly manufactured forming tap was used. Both the roll  forming tap and specially manufactured forming tap we re not sufficient enough to cut through the cured  ceramic base coat of the lubricant; this problem will be similar for most ceramic-based dry film lubricants.  During this process, the roll forming tap became stuck in the nut barrel on various PBNs and had to be cycled multiple times into the nut. Much force had to  be applied to the tap to smooth and cut out lubricant.  This process increases th e risk of damaging the lubricant or the threads of the PBN. A thread cutting tap  should only be used on the fully cured ceramic base coat of the lubricant. Dry film lubricants may be applied in multiple coats and a thread cutting tap should not be used on the top coat. The thread cutting tap will strip the top coat off the base coat and the lu brication properties of the top coat and the lubrication  system will be lost.      When deciding on a thread cutting tap, the end thickness of the lube must be taken into consideration.  Adding the end thickness of the lubricant and subtract ing the top coat not yet applied to the nut pitch  diameter will ensure the thread cutting tap only clears out the unneeded base coat  of the lubricant and  leaves an even and smooth base coat on the threads. Figure 5 is an agglomeration of ceramic particles  after curing of the lubricant and before using the th read cutting tap. Another benef it of using a thread  cutting tap is cutting out debris which may come loose later during acceptance test ing. This loose debris  gets caught between the threads and causes torque spike anomalies in acceptance test data. This debris can also lead to false failu res. Spraying the t op coat of lubricant over a loos e base coat will cause a loss  of both top coat and base coat as de bris during testing. Losing the top coat and base coat in areas will  cause the lubrication system to function improperly, leading to higher torque readings and possibly galling. For each PBN, the thread cutting tap was used only one time to mitigate cutting into too much lubricant or causing damage to the lubricant. Using a thread cutting tap on dry film lubricant is not an easy process and difficult to do by hand. The PBNs were pl aced in a vice while a hand tool was used to gain  NASA/CP-2010-216272
182 cutting torque in order to cut through the cured ceramic base coat. After the correct thickness of the base  coat was obtained through proper tapping of the nut, the top coat of the lubricant system was applied. The variables stated earlier in this section are now controlled to ensure a consistent uniform lubrication  throughout production.    

Figure 5:  Agglomerations of base coat ceramic of Dry Film Lubricant 

If dry film lubricants are applied in a liquid form, the substrate must be heated to ensure proper evaporation of the carrier liquids (commonly known as fl ashing). If proper flash is not reached, the carrier  liquid will migrate with the lubricating particles and pool at its potential energy equilibrium. This pooling  will cause uneven lubrication. A result of uneven lubric ation can be seen in the case of the Powered Bolt  Nuts acceptance test failures. The coefficient of friction was not met during these tests and high torque  limits were reached, leading to failure. The dry film lubr icant used in this process, Vitro-lube NPI-1220C, is  a resin bonded ceramic coating, theref ore if the lubricant is not proper ly flashed, an amber coloration can  be seen in resin rich areas (Figure 6). This specific dry film lubricant was chosen because of its thermal vacuum stability and because of its ability to perform  reliably under the high load rates of the Powered  Bolt assembly; 1.03 MPa of contact stress. Vitro-lube  NPI-1220C was also selected due to its superior  wear life as compared to other dry film lubricants. This  specific dry film lubricant is made up of two parts;  a ceramic base coat and a resin top coat. Some dry film lubricants are not resin bonded and the resin rich areas in Figure 6 will not be seen. Areas of high ceramic content can also be seen if Vitro-lube NPI1220C is not flashed properly; these areas form where the majority of the ceramic base coat pool during  lubricant application. Ceramic pooling of a failed PBN can be seen in Figure 7. This result of improper  flashing of dry film lubricant can be used as a guide for non-resin bonded lubricants. When using a non-resin bonded lubricant the surface characteristic s will be different between base metal and dry film  lubricant.          NASA/CP-2010-216272
183  

Figure 6: Amber coloration of resin binder; 50x magnification 

Oxidation protection given to the substrate ceramic by  the phenolic resin was a main factor in choosing  resin bonded dry film lubricant for the powered bolt nuts. In the case of PBNs, Molybdenum disulfide  (MoS 2) will not oxidize to Molybdenum trioxide (MoO 3), a poor lubricating material, in air due to the use of  the top coat resin. A downfall to using MoS 2 over other dry film lubricants is it will fail due to high  coefficients of friction when testing in high humid atmospheres under high loads. MoS 2 will oxidize to  MoO 3 in high humid atmospheres under high loads. Most  non resin-bonded dry film lubricants will slowly  oxidize in air environments, such as storage warehouse s, leading to early failures of the lubricant.   

Figure 7: Two 180° views of a failed PBN; 50x magnification 

   NASA/CP-2010-216272
184 A way of analyzing lubricant for composition is usi ng an Energy Dispersive Spectrometer (EDS). This  equipment will show if lubricant is missing from the threads. The EDS indicated that the ceramic base  coat was applied to the threads before the resin t op coat. Figure 8 shows EDS images of a PBN which  failed acceptance testing. Spectrum 1 is of a non-amber colored area and Spectrum 2 is of an amber  colored area. The chemistry shows the absence of the Molybdenum disulfide and Antimony which make  up the ceramic base coat of Vitro-lube NPI-1220C. This absence of ceramic base coat is a direct cause of  not preheating the PBNs before lubrication.    

Figure 8: Differences in coating chemistry of PBN   Depending on a substrate’s thickness and material type,  the temperature of the substrate itself may differ  in order to properly flash the carrier liquid. The fl ash temperature will also differ for lubricants depending  on the carrier liquid used. The 2009 batch of PBNs was preheated in an oven/furnace for a specified amount of time at a temperature predetermined based on the PBNs material thickness and the carrier liquid (in this case isopropyl alcohol). A thermocouple was attached to the internal threads of the PBN while in the oven to evaluate the surface temperature wh ere the flashing of the lubricant would take place.  During production, the PBNs were taken out of the ov en and prepared for lubrication; wheeled to station,  set-up properly, etc... Depending on  the technician, some of the PBN s would not be lubricated for 10  minutes, losing their flash temperature. It was realized  that the PBNs needed to retain flash temperatures  during production. To retain this temperature, mitigati ng risk of improper lubrication, a 3-minute time limit  was placed on the PBNs to be out of the oven before being lubricated. Some dry film lubricants, for  example Vitro-lube NPI-1220C, are applied in multip le coatings. To ensure proper flashing throughout all  coatings, mechanisms must have an out of oven time limit placed upon them to retain their flash temperature during lubrication. This added preheat process to the lubrication method of the PBNs  ensured proper flashing of the dry film lubricant leading to even lubrication of the nut threads and less production variation. At completion of the lubricat ion process, the PBNs exhibi ted dark spots throughout  the nut. Further analysis of these dar k spots showed it to be excess top coat. This excess top coat was  deemed acceptable because it was not thick enough to cause mating problems and the excess top coat  will not cause a lack of lubrication of the system. During acceptance  testing this excess top coat will be  burnished into the ceramic base coat as is the rest of the top coat.     

   NASA/CP-2010-216272
185 Conclusions 

For future manufacturing of PBNs and other Aerospace mechanisms that have dry film lubrication similar  to the Powered Bolt Mechanism, proper processes mu st be established to ensure a reliable part is  produced. These process enhancements will minimize par t-to-part variation and acceptance test failures  while avoiding added costs and delays in delivering a final product to the customer.   Key process enhancements discovered during this manufacturing anomaly resolution include:   Thread Fabrication  • When lapping a surface, a method that is capabl e of obtaining the desired surface finish in every  thread, groove, etc, must be determined and controlled. In this case, manual thread lapping with a “softer” tool and lapping compound, lapped multiple times worked well to wear down any burrs left from machining.  • Polishing with a lathe and sponge method was eliminated.  • Use of a go/no-go gauge before lubrication to check for correct thread dimensions was  implemented.   Lubrication  • To maintain consistent lubrication thickness a thread cutting technique was implemented over a  thread rolling tap. Consideration of final lubricant dimensions is a key aspect of choosing a cutting  tap.  • A preheating process was implemented to obtain proper flash temperature. Due to the migrating  capabilities of dry film lubricants that are applied on to substrates as liquids, it is beneficial to  preheat all mechanisms to defer and risk of lubric ant pooling or lack of lubricant in less energy  potential areas.  • Imposing an out of oven time limit during the lubr ication process to keep the mechanisms at flash  temperature.   All of these process enhancements placed upon dry film lubricated aerospace mechanisms will ensure proper system operation of equipment, minimize the risk of failure, and minimize product variation during production.   

Acknowledgements 

The authors of this document would like to thank the employees of NASA Mars hall Space Flight Center  for their support during this anomaly resolution, especi ally Phil Hall, and the employees of Boeing for their  support and information especially Chuck Domby. We would also like to thank Mark Haley of National Process Industries and Fletcher Byrd of Amer ican Mechanical and Electrical Services.   

   NASA/CP-2010-216272
187 The International Space Station Solar Alpha Rotary Joint Anomaly Investigation 

Elliot P. Harik*, Justin McFatter* , Daniel J. Sweeney**, Carlos F. Enriquez*,  Deneen M. Taylor** and David S. McCann* 

Abstract 

The Solar Alpha Rotary Joint (SARJ) is a single-axis pointing mechanism used to orient the solar power generating arrays relative to the sun for the International Space Station (ISS). Approximately 83 days after its on-orbit installation, one of the two SARJ mechanisms aboard the ISS began to exhibit high drive motor current draw. Increased structural vibrations near the joint were also observed. Subsequent inspections via Extravehicular Activity (EVA) discovered that the nitrided case-hardened steel bearing race on the outboard side of the joint had extensive damage to one of its three rolling surfaces . A farreaching investigation of the anomaly was undertaken. The investigation included metallurgical  inspections, coupon tests, traction kinematics tests, detailed bearing measurements, and thermal and structural analyses . The results of the investigation showed that the anomaly had most probably been  caused by high bearing edge stresses that resulted from inadequate lubrication of the rolling contact . The  profile of the roller bearings and the metallurgical properties of the race ring were also found to be significant contributing factors . To mitigate the impact of the damage, astronauts cleaned and lubricated  the race ring surface with grease . This corrective action led to significantly improved performance of the  mechanism both in terms of drive motor current and induced structural vibration.  

Figure 1. International Space Station as of August 2009       Introduction 

International Space Station Overview

The International Space Station (ISS) is a research facility currently being assembled in low Earth orbit .  The ISS project is a multi-national effort led by the United States, with partners from Russia, Canada, the  European Union, Japan, and others . Construction of the ISS began in 1998 and is scheduled to be  complete by 2011 with operations continuing until 2015 . The ISS is the largest artificial satellite in Earth                                                   * The Boeing Company, Houston, TX  ** NASA Johnson Space Center, Houston, TX 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  NASA/CP-2010-216272
189 (30 years) and the assumed exposure of the mechanism to atomic oxygen, lubricant selection criteria of  the day [1] led designers to select a metallic film lubricant .  

Figure 3. Trundle Bearing Assembly 

The roller assembly is fitted into the trundle bearing via a camber pivot axis . This fitting is free to rotate  approximately three degrees to allow for proper alignment of the outer roller with the bearing race under mechanical and/or thermal deflections. This rotational axis of the roller assembly leaves the roller  susceptible to an overturning moment caused by thrust loads at the roller/race interface. This effect is  discussed in detail below. The design intent was that the solid film lubricant on the rollers would mitigate these thrust loads.  

On Orbit Anomaly Investigation 

Approximately eleven weeks after the Starboard SARJ was activated on-orbit, the mechanism began  exhibiting anomalous operational data . For the following two months,  engineers on the ground reviewed  on-orbit telemetry and worked with the ISS operators and ISS crew to determine the most likely cause of the anomalous signature through a series of on-orbit tests . Eventually, an inspection of the mechanism  during an EVA found that the bearing race was damaged and covered in debris. 

Initial Anomaly Investigation

The SARJ software provides continuous status on most of the system’s performance parameters . These  parameters include, but are not limited to, position, speed, motor current draw, target tracking accuracy, and hardware temperatures. This telemetry is reviewed continuously to ensure the health and effective operation of the mechanism. The first indication of anomalous behavior came from unexpected changes in the Starboard SARJ telemetry. In early September 2007, the ISS operations team raised a concern that  the difference between the commanded and actual velocity of the SARJ was increasing. The SARJ controller software uses the difference between commanded and actual velocity to determine how much  current to provide to the mechanism’s drive motor. The change in velocity profile prompted a detailed review of the SARJ operational data. From this review, engineers determined that subtle changes in SARJ performance could be noted starting on September 1 st. Figure 4 shows the onset of the anomalous  data signature. There are two pieces of telemetry shown. The first is the joint position and the second is the commanded velocity of the mechanism. The three plots depict ten minutes periods of time from three consecutive orbits . Two observations can be made from these data: (1) the irregular data signature  initially occurred at a specific angular position of the joint, and (2) the magnitude of the irregularity is  NASA/CP-2010-216272
190 increasing with time . The frequency of the data spikes increased with time such that after a few days the  commanded velocity at all SARJ angles was off nominal. 

Figure 4. Anomalous Data Signature from the SARJ Controller 

This anomalous signature appeared similar to a signature encountered previously on the Port SARJ . The  Port SARJ data signature was caused by a problem with the SARJ controller software . With that in mind,  the controller and manager software for the system were reset. The intent was to clear out memory  buffers and re-initialize controller variables that could be accumulating and leading to the anomalous data signature. The software resets did not lead to any change in the anomalous signature . The SARJ  controller did not appear to be the primary cause of the anomaly.  Efforts to exonerate additional components of the system were attempted by switching logical command  strings . The SARJ controller architecture utilizes two fully redundant command strings . Each string is  composed of a multiplexer/de-multiplexer (MDM) where the SARJ controller software is housed, a RJMC where the velocity control loop and motor power source is located, and a DLA which interfaces with the driven gear of the SARJ and houses the drive motor . Changing commanded strings did no t have an  appreciable effect on the anomalous data signature . This action demonstrated that the hardware from  each drive string was not solely responsible for the data signature.   After clearing the software, MDMs, RJMCs, and DLAs, the primary focus of the investigation turned to the controller logic. While specific anomalies in the software had been ruled out, it was postulated that the controller might be over correcting for slight changes in mechanism performance. If this were the case,  then controller parameters could be modified to optimize system performance. A thorough review of the controller logic did not produce any evidence that the anomalous signature could be a controller effect .    Five weeks into the anomaly investigation engineers had eliminated a number of likely causes for the anomaly  but still did not understand the root cause . Then a significant change in the drive motor current ,  approaching system limits, led engineers to the conclusion that the problem was most likely mechanical in nature . An increase in joint drag appeared to be the cause of the anomaly.    Joint Drag Changes During the Anomaly Investigation

The drive motor current is directly related to the torque required to overcome internal drag and applied load in order to rotate the joint . Assuming a benign loading environment, the SARJ torque is a direct  measure of the fiction in the joint . The relationship between torque and drive motor current is shown in  Equation 1 (torque constant and SARJ gear ratios can be assumed to be constants) .             SARJ Torque = Drive Motor Current ● (Torque Constant ● SARJ Gear Ratio) (Eq. 1)  NASA/CP-2010-216272