ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
217 Lubrication Interval   The goal of the lubrication interval portion of t he test was to determine how long the SARJ could run  without needing a resupply of grease . Each relube would mean an additi onal EVA, which is added risk  and cost to the Space Station program. Data for si deload, tipping, drag torque, temperature and preload  was gathered and plotted against time. The sideload friction data (as shown in Figure 13) provided the  most relevant feedback to aid in determining the need to relubricate as an increase would indicate a  drying track. As shown, the sideload curve remains fl at with no sign of any significant trend increases  after 125,000 total revs on the damaged outer track and nearly 95,000 revs on the Datum A track at the time of this writing. This is equivalent to about 3.5 ye ars on-orbit since the last greasing. The local up and  down variations in side load appearing in Figure 13 closely follows the temperature of the load cell mounting structure, which is also plotted. Apparently, the differentia l thermal expansion of the mounting  structure alters the load cell reading  since the load cell is so stiff.  

Figure 13: Sideload vs. test plate revs   Roller tipping angle and roller drag are also indicators of increased friction and the lubricant drying out. As  shown in Figure 14, the tipping LVDT curve and drag torque curve for the outer track is also relatively flat over time. This indicates that the grease is still e ffective. However, the drag on the inner track seems to  be increasing (more negative) slightly at this  point and will continue to be monitored.    This is consistent with intermediate visual insp ections of the tracks where the outer track remains  relatively well greased throughout its entirety. The voids and pits in the damaged outer track were filled  with the grease as the rollers plowed through the tra ck, thus creating a reservoir and perhaps allowing the  roller to track interface to remain somewhat wetted (s ee Figure 12). On the other hand, the inner track  looked relatively dry as the grease had been pushed out of the way, leaving only a thin film of oil that  would be sufficient enough to keep the roller interface wet. The inner track does show a propensity to dry out faster than the outer track.  This portion of the test is still young and as it progre sses, data will continue to be collected and monitored  for shifts and changes. The outer track has passed the 3.5 year on-orbit mark since the last lubrication  and the inner track is closing in on three year s. Testing will likely continue to mid 2010.  Inner Datum A Track since last regreassing 405060708090100 40000 50000 60000 70000 80000 90000 100000 Test Plate RevsTemp, deg F & Side Load, LbTem p Side Load 4, 5 & 6 0.4 to 1.0 rpm errorOuter 45 deg Track  0102030405060708090100 50000 60000 70000 80000 90000 100000 110000 120000 130000 Test Plate RevsTemp, deg F & Side Load,  LbTem p Side Load 1, 2 & 3 NASA/CP-2010-216272
218  

Figure 14: Tipping & drag vs. revs 

Conclusion 

A test rig was developed that closely simulates the roller/race contact conditions of the SARJ trundle  rollers. The test plate was artificially damaged to re present the on-orbit damage race. The test results  show that grease lubrication is an effective way of combating the high friction that led to roller tipping and  high edge contact stress that led to the on-orbit failur e. On-orbit relubrication intervals in excess of 3  years are reasonable based on the data generated to date. The authors would like to acknowledge the contribution from the experts at Boeing, NASA, NESC , and ATK in the design an d formulation of test  requirements for the SARJ LITE rig.    References 

1. .Harik, E.; Sweeney, D.; Enriquez, C., and McFatter,  J.; “Diagnosing On-Orbit Failures International  Space Station (ISS) Solar Alpha Rotary Joint (SARJ) Bearing Failure,”  40 th AMS Proceedings (to be  published), 2010  2. Cann, P. M., "Starvation and Reflow in a Grease-Lubricated Elastohydrodynamic Contact," Tribology  Transactions, vol 39, no. 3 (1996), 698 - 704.  Outer Tipping  -5-4-3-2-10123 50000 60000 70000 80000 90000 100000 110000 120000 130000 Test Plate RevsTipping, milTip 1 Tip 2 Tip 3 Drag  -9-8-7-6-5-4-3-2 50000 60000 70000 80000 90000 100000 110000 120000 130000 Outer Track Test Plate RevsDrag, lbOuter 45 deg Datum A NASA/CP-2010-216272
220  

Figure 1 - SARJ ring and roller schematic, top view. (a)  Overall view, roller axis aligned with ring  axis; (b)  close-up view near roller, aligned axis;  (c)  close-up view near roller, roller axis  misaligned (misalignment magnitude greatly exaggerated). 

The axial force that develops in response to the misa lignment of the axes can have a significant influence  on the operating conditions of the SARJ hardware. The axial force and the associated moment that arise  from misaligned axes are carried by a pair of taper ed roller bearings via the roller shaft to the roller  housing. The axial force interacts through a pivot point in the roller housing (Figure 2). In the absence of misalignment, the pivot point allows for uniform contac t of the nominally flat ro ller profile contacting the  nominally flat raceway surface. But for the case of misaligned axes, the axial force acts via the pivot point  and produces a non-uniform contact pr essure across the roller profile. Note from Figure 2 the moments  created by the normal load and axial force acting  via the pivot point must be balanced for static  equilibrium. Thereby, the axial force acts to shift the po sition of the resultant normal load along the face of  the ring and roller contact. The axial force arising from  misaligned axes can, if of sufficient magnitude,  cause tipping of the roller. Also note that the axial forc e acting on a roller is carried to the roller shaft via a  pair of lightly-preloaded tapered roller bearings. The magnitude of the axial force influences the drag torque of the bearings and thereby influences the torque required to rotate the mechanism. 

Figure 2 - Schematic of the SARJ roller and ring in contact, front view. Illustrated are the forces  imposed by the ring on the roller and the reaction forces at the pivot point in the roller housing. 

To help understand the SARJ mechanism operation,  a set of dedicated experiments were completed  using roller pairs. The purpose of the experiments was to quantify the relationship of the misalignment of  roller axes to the resulting forces that develop. The relationship of shaft axis misalignment magnitude to  axial force magnitude was determined for the material combination used in the SARJ mechanism for a variety of surface and environment conditions as can influence the friction and, thereby, the behavior of the mechanism.      (a) (b) (c) (a) (b) (c) normal loadroller pivot axis of roller housingreaction forces at pivot point ringrotational axis of the roller axial force normal loadroller pivot axis ofroller housingreaction forces at pivot point ringrotational axis of the roller axial force NASA/CP-2010-216272
222 The load that presses the rollers together is termed herein the “normal load” (Figure 5). The normal load  is applied via an air-pressure actuated piston. The air piston acts through a load cell against the drive motor plate that is gimbal-mounted relative to the test  chamber (Figure 4(a)). In this way the air cylinder  moves the roller on the input shaft in an arc motion towa rd the test roller. Once the rollers are in contact,  additional force commanded from the air cylinder increases the normal load between the test rollers. The force sensed by the load cell located between the gimbaled motor plate and the air piston is a linear combination of two sources, the unbalanced mass relati ve to the gimbal point and the normal load on the  test roller.  (a) 

(b) 

Figure 4 – Schematic views of the vacuum roller rig. (a)  Schematic, side view.  (b)  Schematic, overhead view with shaft misalignment depicted and exaggerated. 

The following calibration procedure was used so that the two sources influencing the load cell output  during testing could be separated. First the load  cell was removed from the rig, calibrated using  deadweights, and reinstalled on the test apparatus. Next, a LVDT was used to monitor the displacement of the motor plate. With no test roller installed on the output shaft, the air piston was used to move the  motor plate through the full range of motion while record ing the output of the calibrated load cell. In this  manner the force as sensed at the load cell due to the unbalanced mass of the gimbaled motor plate was determined as a function of the motor plate position. Next, the end of the input shaft where the test roller is mounted was attached by a highly-rigid link to the apparatus frame. The rigid link included a calibrated  reference load cell in the load path. The rigid link was carefully positioned to be oriented in the position and direction of the normal load created between the cont acting test rollers. By increasing the pressure drive motor ferrofluid seals and bearings“axial” force/thrust sensor brakepivot point for drive motor  plate relative to turntable bedplate air piston“normal load” force sensorvac chamber turntable bedplatebellowsdrive motor ferrofluid seals and bearings“axial” force/thrust sensor brakepivot point for drive motor  plate relative to turntable bedplate air piston“normal load” force sensorvac chamber turntable bedplatebellows ferrofluid seal and bearing“axial” force/thrust sensor brakedrive motor bedplateturntable on bearingload cell for “tangential” force, mounted to turntable ferrofluid seal and bearing“axial” force/thrust sensor brakedrive motor bedplateturntable on bearingload cell for “tangential” force,mounted to turntable pivot point for drive motor plate relative to turntable NASA/CP-2010-216272
224 The pressure in the chamber was monitored by an ioni zation gauge at the top of the main test chamber.  The typical pressure in the testing chamber during vacuum testing was 5x10-6 Torr. Vacuum is provided  by a diffusion pump with a liquid nitrogen cold trap. The diffusion pump is assisted by a mechanical  vacuum pump. The diffusion pump and cold trap a rrangement prevents oil vapor s from the mechanical  vacuum pump to enter the test chamber so as to maintain the desired tribological test condition.    Test Specimens

The test specimens used for this research had a nominal geometry of 35.6-mm (1.4-inch) outer diameter and a 12.7-mm (0.5-inch) width. The roller on the drive motor (input) shaft was made from 15-5 alloy (matching the SARJ raceway material). The roller on  the brake (output) shaft was 440C alloy, matching  the SARJ roller material. A set of nitrided 15-5 ro llers were manufactured to match the processing  parameters of the SARJ ring. An additional set of 15- 5 rollers without nitriding were manufactured for  research purposes. In the remainder of this document we use the term “un-nitrided” to refer to a 15-5 roller that does not have the nitride surface layer. The profile across the roller width for the 15-5 rollers was nominally flat. The mating 440C rollers used for th is project had a crown radius profile across the  roller width. For the test apparatu s used, at least one of the two ro llers must be crowned to have a  controlled contact condition. The nominal crown radi us of the 440C test rollers for this project was  approximately 42 mm (1.65 inch). The 440C rollers  had a measured surface hardness via a Rockwell  tester of typically 56 HRC. The SARJ mechanism makes use of gold-plated rollers. A subset of the 440C  rollers were provided with gold plating. All of t he gold-plated test rollers were done in a single batch  process. The plating vendor reported the applied gold layer thickness as 2300 angstroms.     A photograph of a pair of rollers installed and undergoing test is provided in Figure 6. The upper roller is a  15-5 roller with a nominally flat profile. The bottom roller is the 440C roller having a crown radius. The localized contact provided by the crowned roller is evident in Figure 6. 

Figure 6 – View of tested rollers in the test apparatus showing localization of the roller contact  and crowned profile of the lower roller.     NASA/CP-2010-216272
225 To document the surface condition of the new rollers, rollers were inspected via a stylus profilometer  using a diamond-tipped stylus. The data were processed to assess the roughness features. The roughness of a 440C roller was typically 0.14 mi crometer roughness-average. The roughness of a  nitrided 15-5 roller was typically 0.62 micrometer roughness-average. Plots of the roughness profile for a typical 440C and nitrided 15-5 roller are provided in Fi gures 7-8 (note the differing automatic scaling used  in these two figures).  

Figure 7 - Typical roughness of a 440C roller. 

Figure 8 - Typical roughness of a nitrided roller. 

Procedure to Install Test Rollers   Test specimens were cleaned and installed using carefu l procedures to provide a clean test surface. The  15-5 test rollers were cleaned just prior to installation into the rig using de-ionized water and 0.05 micron alumina powder. After appropriate hand scrubbing, the cleaning powder was rinsed with deionized water making sure that the entire roller surface wetted unifo rmly to confirm complete cleaning of surface oils.  The water was removed from the roller using dried pr essurized nitrogen. Bare (w ithout gold-plating) 440C  specimens were cleaned in the same manner as the 15-5 specimens. The 440C test rollers with gold-plating were vacuum-sealed in plastic bags by the pl ating vendor, and so cleaning with alumina powder  was not needed. The bags remained closed until re ady for installation. Test rollers and mounting  hardware were handled only with gloved hands and clean tools to complete installation into the test apparatus.  Procedure for Testing Rollers

The first step for testing after installation of the test  rollers was to immediately isolate the testing chamber  and provide a vacuum, using the mechanical roughing pump, to approximately 50x10-3 Torr chamber  pressure. This isolation step was done even if test scheduling required some delay between the time of  installation of rollers and the time for testing to  minimize exposure of the cleaned surfaces to any  contaminants that might be present in the atmosphere. Just prior to testing the diffusion pump was  energized and the pressure in the testing chamber established to approximately 5x10-6 Torr.  NASA/CP-2010-216272
227 practical engineering purposes of this investigation, the functional relationship of the axial force/normal  load ratio to shaft misalignment angle is not strongly influenced by changes in the normal load. To simplify the plotting and discussion of data, the results fo r the three levels of normal load were treated as  a single dataset for the remainder of this report.   Table I – Test Conditions 

Roller on input shaft Roller on output  shaft Grease  lubrication? Chamber  condition  un-nitrided 15-5 440C – no plating no vacuum  un-nitrided 15-5 440C – no plating yes vacuum  nitrided 15-5 440C – no plating no vacuum  nitrided 15-5 440C – gold plating no vacuum  nitrided 15-5 440C – no plating yes vacuum  nitrided 15-5 440C – no plating no ambient air 

Figure 9 – Ratio of axial load to normal load for three levels of normal load. Test conditions were  nitrided 15-5 roller vs. bare 440C (no plating) roller, no grease, in vacuum.   The results of testing at three levels of normal load fo r the case of a 440C roller mated with a nitrided 155 roller and operated in a vacuum are provided in Figure 10. For the case of a bare (un-plated) 440C roller against a nitrided 15-5 roller and without lubrication, the axial force can be as great as 0.4 times the normal load for a shaft angle of 0.5 degree. Exper iments and analysis done by others have shown that  such a magnitude of force on a SARJ roller would cause roller tipping and contact pressures much greater than anticipated by the designers. However, fo r the same base materials but provided lubrication  via a solid gold film or via grease, the axial forc e does not exceed about 0.2 times the normal load even  for the largest misalignment angles tested. For practica l purposes, for the case of lubricated rollers the  maximum attainable axial force develops for shaft angles  of about 0.5 degree. These test results highlight  the critical role of lubrication for the SARJ mechanism.  If the raceway of the SARJ mechanism becomes damaged, it is possible that the un-nitrided substrate will  be exposed and will interact with the hardened 440C roller surface. Therefore, it was desired to study the  case of un-nitrided material mating with 440C rollers in a vacuum environment. The results of these tests misalignment angle (degrees)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5axial force / normal load -0.7-0.6-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.799 lb preload 149 lb preload 202 lb preload440 N normal load 663 N normal load 899 N normal load NASA/CP-2010-216272
228 with the un-nitrided material are provided in Figure 11.  The data for nitrided roller is also provided on the  chart for comparison. The axial force that develops fo r the case of un-nitrided vs. 440C with no lubrication  is somewhat less than can be obtained for the nitrided  surfaces. Still, with no lu brication the axial force  can be high, an undesirable condition for the SARJ mechanism. Providing grease lubrication to the unnitrided 15-5 material greatly reduces the maximum attainable axial force with the value limited to about 10 percent of the applied normal load. Comparing the dat a for testing with grease (Figures 10 and 11),  regardless of the nitrided or un-nitrided condition the functional relationship of axial load to shaft angle is  approximately the same, and the axial force is limited to about 15 percent of the normal load.     

Figure 10 – Axial force as a ratio of the normal load as a function of the shaft misalignment angle  when testing nitrided rollers. The data were reco rded as three levels of normal load (445, 667, and  890 N) {100, 150, and 200 lb}. misalignment angle (degrees)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5axial force / normal load  -0.7-0.6-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.7gold plated 440C vs nitrided 15-5 bare 440C vs nitrided 15-5 bare 440C vs nitrided 15-5 with grease NASA/CP-2010-216272
229   Figure 11 - Axial force as a ratio of the normal load  as a function of the shaft misalignment angle  when testing un-nitrided rollers. The data were reco rded as three levels of normal load (445, 667,  and 890 N) {100, 150, and 200 lb}. Data for nitrided rollers from Figure 10 are included for  comparison. 

Certain testing of the full-scal e SARJ mechanism was done in an air environment. To provide some  insight about the relative behavior of rollers operating in vacuum or air, a set of tests were conducted in  air using a bare 440C roller and a nitrided 15-5 roller with no lubrication. The testing was done for 3 levels of normal load (445, 667, and 890 N), {100, 150, and 20 0 lb}. The results of the testing are provided in  Figure 12. The maximum attainable axial force is slightly  less when testing in air as opposed to testing in  vacuum. The test was conducted using a pair of rollers  that was first exposed to vacuum for testing and  then exposed to air for approximately 20 minutes before st arting the testing. It is recognized that this lab  procedure does not necessarily recreate the surfac e condition of the SARJ mechanism during full-scale  testing. The test data show that high magnitudes of axial force can be created when operating 440C rollers against nitrided 15-5 rollers in air.  The data of Figures 9-12 provide insight about the op eration of the SARJ mechanism. Large axial forces  can develop even for small magnitudes of shaft misalignm ent. These axial forces in the case of the SARJ  mechanism act via a pivot point in the housing to pr oduce non-uniform contact pressures across the roller  width, and if the forces are of sufficient magnitude can cause roller tipping. The experiments and data demonstrate the critical role of good lubrication. Th e gold films provided good lubrication for the short  duration testing done and reported herein. It was noted that the gold did wear away during testing, and by cursory visual inspection the gold appeared to have been removed for the contact path. However, the axial forces remained low indicating good lubrication.  Close inspection of the rollers after removal from  the rig with the aid of magnification revealed that th e gold, while depleted in depth and coverage, was not  completely worn away during these tests. The grease lubrication limited the magnitude of the axial forces  that could develop to even smaller magnitudes than was achieved with the gold-film lubrication.   misalignment angle (degrees)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5axial force / normal load  -0.7-0.6-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.7bare 440C vs nitrided 15-5 bare 440C vs un-nitrided 15-5 bare 440C vs un-nitrided 15-5 with grease NASA/CP-2010-216272
230   Figure 12  - Axial force as a ratio of the normal load as a function of the shaft misalignment angle  when testing nitrided rollers in vacuum and in air. The data were recorded as three levels of  normal load (445, 667, and 890 N) {100, 150, and 200 lb}. 

Axial Force as a Function of Braking Torque with Shaft Misalignment   The data results presented in the pr evious section was for the conditi on of a small amount of torque  transmitted by the rollers. To gain additional insight about the behavior of rollers in contact with shaft  misalignment, a test was completed to assess the relationship of axial force as a function of the torque transmitted by the rollers for a condition of shaft misa lignment of 1.5 degrees. The rollers used for this  test were a bare 440C roller and a previously run ni trided 15-5 roller. The rig was operated at a speed of  approximately 15 rpm. The test was done in vacuum of 5x10 -6 Torr. The normal load between the rollers  was 434 N (98 lb). With the test operating, the braking toque on the output shaft was steadily increased. The axial force (normalized to the normal load) as a function of the braking torque applied to the output  shaft is provided in Figure 13. As the braking torque increased, the axial force decreased as should be  expected and will be explained in discussion to follow. The axial force changes in a non-linear fashion with respect to the braking torque on the output shaft. The contacting region betw een the rollers includes  both a stick and slip zones. The transition from stic k to slip depends on the total traction force comprised  of two orthogonal components, the axial and tangential forces. The application of additional braking torque increase the tangential component of the trac tion force and thereby alters the contact conditions,  with the stick zones decreasing in size and the slip zones increasing in size. This test highlights that the axial force that develops when shafts are misaligned at small angles is largely the result of strains that develop in the stick region of the contact. The total traction force that can be supported by the contact is limited by the frictional condition of the mating surfaces. As was highlighted in the previous section, lubrication limits the total traction capability of t he contact and thereby limits the magnitude of the axial  force in response to shaft misalignment.  The trends of the data of Figure 13 demonstrate that the axial force to normal load ratio investigated  herein, although having a mathematical form matching that of coefficient of friction, is not a direct  measure of the coefficient of friction of the contacting surfaces. The friction condition indeed plays a primary role influencing the behavior of the contact. The axial force to normal load ratio is also influenced by the design details and by the operating conditions  including the torque transferred by the roller.   misalignment angle (degrees)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5axial force / normal load  -0.7-0.6-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.7bare 440C vs nitrided 15-5 in vacuum bare 440C vs nitrided 15-5 in air NASA/CP-2010-216272