ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
further as the supply of oil from the thickener is reduced. For reasons explained below, we surmise that  this indicates that our lubricant has not fully run in after 400 hours.  In the final set of experiments reported here, a thin film of Rheolube grease (0.14 gm per row) was  applied to the bearing components before it was again operated at 6000 rpm. These results are shown in  Figure 6. We found that the film began with a thickness that was comparable to the apparent steady state  condition with the full grease pack (~19 /g80in). However, the film thickness was observed to fall  continuously over the next 500 hours to approximately 9 /g80in. The final data points suggest that the rate of  decline was reducing at this point, but it is unclear whether the thickness would have continued to fall  below 9 /g80in with extended operation.    0 100 200 300 400 500 6000510152025Total Film Thickness (uin) Run Time (hrs) Figure 6. Total Film thickness (in microinches) as a function of cumulative time operating at 6000  rpm with a grease pack of 0.14 g Rheolube. Triangles are the results from the load cell and circles  are the results from the displacement sensors.  The continued maintenan ce of a nearly 20 /g80in film thickness in Figure 5 was likely due to some prolonged  retention of the grease thickener in the EHD contact. Figure 6 shows that the total film thickness will fall to  9/g80in if starvation is driven further by restricting lubricant resupply. Figure 4 shows that if the film is  resupplied only with neat oil, then the thickness will also fall to approximately 9 /g80in. We conclude that the  robust film of Figure 5 must be due to some enduring intermittent resupply of grease to the contact rather  than just oil. We surmise that a thin film of grease persists on the balls outside the running band,  providing fresh grease to the contact when the ball spin axis is disturbed. This represents a transient condition that would likely diminish with continued operation. This run-in transient may have been  artificially prolonged by a lack of disturbances to the bearing during our controlled testing. Therefore,  Figure 6 suggests that the film thickness shown in Figure 5 would eventually fall to below 10 /g80in after  prolonged use in a typical bearing application.  Conclusion  We have demonstrated a technique to  measure the total EHD film thickness in an operating angular  contact bearing. Excellent correspondence between the two independent methods of measuring outer  ring deflection provides confidence in this measure. Our procedure for measuring the instantaneous  114
6. Zhang, J., Drinkwater, B.W., and Dwyer-Joyce, R.S. “Acoustic Measurement of Lubricant Film  Thickness in Ball Bearings” J. Acoustic. Soc. Am.  119(2) (2006), 863-871  7. Hamrock, B.J. and Dowson, D. “Isothermal Elasohydrodynamic Lubrication of point contacts, Part  IV, Starvation Results”  ASME J. Lubr. Technol.  99 (1977), 15-23  8. Chiu, Y.P. “An Analysis and Prediction of Lubricant Film Starvation in Rolling Contact Systems”  ASLE Trans.  17 (1974), 22-35  9. Coy, J.J., and Zaretsky “Some Limitations in Applying Classical EHD Film Thickn ess Formulas to  a High-Speed Bearing” ASME J. Lubr. Technol.  103 (1981), 295-301  10. Cann, P.M., Damiens, B., and Lubrecht, A.A. “The Transition between Fully Flooded and Starved  Regimes in EHL” Tribology International  37 (2004), 859-864   116
Scanning System Development and Associated Bearing Cage Instability Issue Manfred Schmid* and Christian Hehr*  Abstract  The Scan equipment used to control th e MicroWave Radiometer Imager (MWRI) of the polar orbiting FY3  meteorological satellite was developed and delivered to the customer by Astrium Satellites GmbH. The  instrument detects and monitors meteorological and biosphere environmental anomalies.  The key elements of the design and the most important design requirements and design features are  described hereunder.   Special attention is paid to the bearing cage anomaly observed on the Scan Compensation Mechanism  during the test campaign and to the subsequent root cause investigation performed.   Introduction  The scanning equipment consists of a Scan Drive Mechanism (SDM) rotating the instrument package, of a  physically independent Scan Compensation Mechanism (SCM) used to eliminate disturbance moments  induced by the SDM and of a closed loop Scan Control Electronics (SCE) controlling and synchronising  the two mechanisms.  The heavy MWRI instrument package mounted on top of the Scan Drive Mechanism requests conical  antenna scanning. This requirement leads to a continuously rotating mechanism of high scan rate stability. One EQM and two FM sets of scanning equipm ent were developed, qualified and delivered to the  customer.   During the mechanism test campaign a randomly occurring bearing noise was observed in the SCM. Apart  from the mechanism design, the pa per explains the way up to identification and elimination of the bearing  noise issue under the given technical and programmatic constraints and provides the lessons learnt from this experience. Key Equipment Design and Performance Requirements  The mass of the instrument package mounted on top of the SDM is 60 kg at an instrument Moment of  Inertia of 2.7 kg-m 2. The instrument has to be accelerated within 180 s to the nominal spin rate.   The nominal spin rate is 1.7 s/rev. In order to achieve the required radiometer performance, the spin rate  shall stay stable within an error bandwidth of ±0.34 ms compared to the nominal value. In order to achieve  this stability, closed loop control is implemented.  For compensation of the momentum induced by the SDM into the S/C, a Scan Compensation Mechanism  is provided. The SCM is physically independent from the SDM and it is designed as a separate  mechanical unit mounted in line with the SDM rotation axis to the instrument structure. Both units, the  SCM and the SDM are controlled by the digital controller implemented in the FPGA of the Scan Control  Electronics (SCE).   The SCM rotation rate is directly synchronised to the SDM scan rate via the controller. The SDM scan rate  is multiplied by a factor of 16, which leads to a rotation speed on the Scan Compensation Mechanism of  about 565 rpm.   * EADS Astrium GmbH Satellites, Friedrichshafen, Germany  Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  117
The chosen SCM speed ratio is the result of a trade-off between bearing life margin and flywheel inertia at  minimum resulting flywheel mass (and at the given maximum allowable Flywheel diameter of 400 mm).  The equipment life is 3 years in orbit; this yields nominally 890 million revs on the SCM over life without  margin. The operational temperature for the mecha nisms is -30 to +50 deg C.  SCE SCMSDM Figure 1.  Scan Drive Equipment mounted to the Transportation Jig. Scan Drive Mechanism Design  The SDM consists of a drive module using a set of inclined ball bearings, of a redundant brushless DC  Motor and of a redundant optical encoder for closed loop velocity control. On the rear end of the drive Module, a slipring unit for power and signal transfer from/to the rotating Radiometer Instrument package is attached. The Harness is guided through the hollow drive shaft to the slipring rotor and from there via the slipring brushes to the slipring stator.   The I/F to the rotating instrument mounted on top of the mechanism is formed by a launch off-load device.  The launch off-load device consists of axially pre-loaded flexible metallic bellows and of an accurately  shaped conical Interface. The combination of both elements provides instrument alignment accuracy and  torsion stiffness during mission as well. During launch the conical I/F is lifted off from the mechanism I/F, so to de-couple the instrument load path from the mechanism. After release of the Instrument HRMs on  S/C side, the instrument is pulled by the preloade d SDM bellows back into the conical I/F in order to allow  transfer of the drive torque from the SDM to the instrument at accurate alignment of the instrument rotation axes. The Rotor shaft is mounted via two pairs of hard preloaded angular contact ball bearings in back to back  arrangement into the SDM housing. The bearings are manufactured from Stainless Steel 1.4125 (440 C),  the cages are manufactured from Phenolic Resin, vacuum impregnated with oil.  The motor is of the brushless DC type with completely cold redundant independent Stators. Its maximum  operation torque is 2.3 Nm.   The redundant optical Encoders provide absolute position data with 16-bit  resolution. Its output data are  used for motor commutation on the one hand a nd for high accuracy motor velocity control on the other.   The Slipring is composed of 19 solid gold tracks and redundant gold alloy brushes to comply with the  expected 3 years orbit life corresponding to about 55 million revs.  118
Launch  Offload Device and  Instrument I/F  Drive and  Bearing Module Optical Encode r Slipring Module  Figure 2. SDM Design  Scan Compensation Mechanism (SCM) Design  The function of the SCM is to compensate for the momentum induced by the rotating instrument into the  S/C. The SCM consists of a drive module using  a brushless DC Motor with redundant motor stators. The  SCM motor is similar to the SDM, however it is op timised for the higher operation speed. Its maximum  operation torque is 0.9 Nm. For rotation velocity feedback 3 Hall Sensors are included in each motor  stator. To the motor output shaft a flywheel unit is attached. Trim masses for fine-tuning of static imbalance are mounted to the Flywheel.   Concerning the bearing configuration it has to be noted that the bearings and its lubrication concept were  kept identical to the SDM in order to gain synergy by similarity during the mechanism development and  bearing procurement process.   Flywheel Drive and  Bearing  Module Figure 3. SCM Design  Scan Control Electronics  The Scan Control Electronics (SCE) is built up as a completely cold redundant unit, each supporting the  main respectively redundant motor windings of each mechanism and receiving feedback from the  redundant high-resolution encoders respectively from the Hall sensors. The controller is implemented into the FPGA. 119
Figure 4. Scan Control Electronics Block Diagram Mechanism Performance Test Summary  Apart from the standard verification approach applied to space mechanisms, two main critical functional  performance data had to be verified. One of them was the rotation period of 1.7 s/rev at a stability of ±0.34 millisecond; the other one was the maximum residual momentum of <0.1 Nms induced into the S/C by the  Scan equipment during start-up and operation.   Figure  5 shows the equipment performance test setup. In Figure 6 the typical scan rate deviation from the  nominal value over a time of 2000 seconds is shown, while in  Figure 7 the residual torque profile over time is depicted. The scan rate deviation was measured by  reading out the high-resolution encoder signal; the residual momentum was measured via a Kistler torque  test device.  Figure 5.  Functional Performance Test Set-up Instrument Dummy (mass and  Inertia simulator)  Launch Offload Device  SDM SCM Torque Measurement Device  120
Figure 6. Typical deviation from nominal Scan Rate over time  Figure 7. Typical induced torque profile during instrument start-up (180 seconds)  Bearing Unit Characteristics  Both, the Drive Mechanism rotating at slow angul ar velocity and the Scan Compensation Mechanism  rotating at higher velocity, make use of an identical bearing configuration. The only major difference  between the SCM bearing and the SDM bearing is defined by the higher bearing pre-load applied to the  SDM bearing as a consequence of the higher operational forces acting to the SDM due to a potential later instrument CoG shift out of the nominal rotation axis.  By design, TiC-coated balls were prefer red, however due to procurement issu es originated by the transfer  of the TiC coating process from Europe to the US, the availability of TiC-coated balls was limited at the time of bearing procurement. The finally delivered TiC-coated balls were mounted into the bearings by the  121
bearing supplier; however, scratches of unknown origin were identified during bearing inspection on the  balls. This led to the implementation of the back-up bearing design using steel balls and to a subsequent  change of the lubrication system. In the following table (Table 1) the bearing characteristics are listed.  Table 1: Bearing Characteristics Bearing Characteristics SDM Bea ring Differences for SCM  Bearing Type Thin Ring  Arrangement  Back to Back  Material 440 C Cage Material  Phenolic Resin  Ball material TiC-coated steel balls  (was changed later to  steel only)  Vacuum impregnated  Z 25 (was changed  later to MAP grease  and Nye oil)  Cage guidance Inner Race riding  Number of Bearings 2 pairs, back to back  Outer Diameter 98.425 mm  Inner Diameter 77.788 mm  Height of Bearing Pair 15.874 mm  Ball diameter 4.762 mm  Number of balls 42 42 (original number, reduced later)  Contact angle  27 deg  Preload  750 N 400 N Max Contact Stress 1200 MPa 1000 Mpa   Axial Stiffness 250 N/μm 330 N/μm  Radial Stiffness 450 N/μm 550 N/μm  Angular Stiffness 330 Nm/mrad 410 Nm/mrad  Running Torque <0.08 Nm <0.04 Nm  Calculated Life >1.18E+10 revs  >18.6E+8 revs  Rotation velocity 35.3 revs /min 564.7 revs/min  Figure 8: Bearing Dimensions  122
Figure 9. SCM Bearing Details  Observed Bearing Noise Issue on the SCM  In order to gain design and procurement synergy (considering the long lead times of ball bearings) within  the extremely tight development schedule of 10 month for the EQM and of 12 month for the first FM  equipment, an identical bearing concept was chosen for both mechanisms. It was confirmed by analysis, that at the given rotation speed of about 565 rp m on the SCM (performing  momentum compensation)  compared to 35 rpm on the SDM (su pporting the MWRI instrument), the bearing life margin at the given  load conditions was still very good.   Dynamic cage instability effects at the comparably low maximum rotation speed of about 9.5 revs/s were  not expected. This latter assumption was supported by references where similar bearings had been used in Helicopter Subsystems at much higher speed.  After integration of the EQM equipment, functional testing was started. After some hours of perfect  operation a sharp scratching noise was observed, obviously coming from the bearing unit of the fast  running Scan Compensation Mechanism. The good mechanism performance was not affected by the phenomenon; the performance values were stable and as good as before.  The noise was not reprodu cible and it occurred only from time to time. After start up periods of noise free  operation were observed, followed by periods of sharp randomly occurring noise. During mechanism  speed up to the nominal velocity, the issue was typically observed at rotation velocities above 500 revs /  minute, while during deceleration the effect was observed do wn to about 100 rev/min.   Consequently the mechanism was dismantled and all bearing seats and bearing Interfaces were checked  again in order to identify any in-correct integration, tolerance or cleanliness issues and to verify the lubrication and cage condition. All tolerances were found as expected and no misalignment, debris, bad  lubrication, bearing degradation or faulty integration could be detected.   The mechanism was re-integrated an operated again. All friction values were as expected, the operation  was smooth. However, after some hours the noise came back. Again it was found that the issue did not  jeopardise in any way the mechanism performance or power consumption, however the remaining general lifetime concern was quite severe.   During one subsequent performance test, a quite hard noise anomaly was detected, which suddenly  disappeared. From this time onwards, the phenomenon was never detected any more throughout all the  123
qualification test campaigns, neither after vibration, nor during hot or cold operation. The unit operated  perfectly.  Therefore the origin of the observed tem porary anomaly could not be clearly identified at this stage on the  EQM unit. Consequently the H/W wa s delivered to the customer for system testing activities according to  the contract and still within the required lead time of 10 month.   Three month after delivery, an alert was received from the cu stomer, who had observe d again noise in the  mechanism.  After receiving the mechanism back on site, the orig in of the anomaly could be investigated in detail. At  that time the FM1 mechanism was already integrat ed and functionally tested without any issue. However  after an extended storage time of some days prior to environmental acceptance testing, also on FM1 the  noise issue could be observed, however in a less severe manner than on the EQM. Therefore it was clear  from now that a systematic effect was occurring.  Investigations and Results  Due to the missing characterisation possibility of all potentially influencing parameters in the specific  application, a comprehensive empiric investigation was started in order to identify the major drivers for the  anomaly. By design, the SCM integration concept was kept simple. This design feature was identified as a key  advantage during all following H/W investigations, since it was possible to dismantle and re-integrate the  mechanism at least once a day for check of function and identification of the success of a corrective  measure performed.   Basically four different causes for the anomaly were considered:  1) Bearing mounting conditions (in the mechanism) 2) Bearing internal design and load conditions  3) Cage design  4) Lubrication concept  The most important modifications performed on mechanism and bearing level are listed in the following  table (Table 2) and the observed effects on the noise anomaly are described:  Table 2: Modifications performed on the EQM Mechanism  Potentially  Influencing Parameter Modification  Observed Effect on noise  reduction Comment 1) Bearing mounting conditions Potential Bearing  integration / tolerance  issueDismantling of  mechanism and  detailed inspection No improvement possibility All bearing seats and  mounting interfaces were in  nominal condition  Potential alignment issue due to two pairs  of ball bearings in back  to back arrangement. Dismounting of one  bearing pair,  operation with only  one pair of bearings No improvement observed to avoid static overdetermination  2) Bearing internal design Too low bearing  preload  Increase of pre-load  compared to the  nominal value No improvement observed  124
Potentially  Influencing Parameter Modification  Observed Effect on noise  reduction Comment Too high bearing  preload decrease of pre-load  to about 300 N Cage noise was eliminated  during ambient pre-testing,  however this was not confirmed  after vibration.  It was observed after vibration  that the bearing outer rings  tended to rotate in the bearing  seat w.r.t. each other at  reduced bearing preload  After Vibration test at reduced pre-load, bearing  noise was observed again.   Ball diameter variation out of nominal range  with the effect of  “slippage” of individual  ballsNew balls used  No improvement  possibility The ball diameter variations  were in the nominal range. A  new set of bearing balls was  mounted, no noise reduction  observed  Degradation of races  due to potentially wrong  ball to race contact line Inspection / oil  filtrationNo improvement possibility The races were inspected at  supplier premises by  microscope and were found  to be all nominal  Degradation of ball surfacesInspection No improvement possibility The surface conditions of the  balls were nominal  Potential inner / outer race circularity out of  tolerance measurement of  race circularity No improvement possibility The circularity tolerances  were measured. The values  were within the specified  toerance.  3) Cage design Too weak cages (large  cage diameter at low  cross section) Manufacturing of  stiffer cages with  higher cross section The noise production became  worse after implementation of  stiffer cages The stiffer cages did not  solve the issue. The  observed noise became even  worse.  The original cage cross  section was maintained  Cage guidance. The  cage design and cage  tolerances were  reconsidered and  different inner and  outer race riding cages  were alternatively  manufactured. Change of cage  design from inner to  outer ring riding  configuration,  Change of cage  tolerances. No improvement observed Nominal ly the bearing cages  were inner ring riding, they  were changed to an outer  ring-riding configuration.   Since a positive effect on the  bearing behavior was not  identified, the inner riding  configuration was maintained  as the baseline.  Number of Cage  pocketsThe even number of  pocket cages (42)  was changed to an  odd number of  pockets (39)No improvement observed According to theory high  velocity bearings gain cage  stability if an odd number of  balls is used.  The number of 39 was  maintained throughout the  tests though it did not yield  any improvement w.r.t.  observed cage behavior.  4) Lubrication Lack of lubricant Re-lubrication of the  bearing Noise disappears for some  hours only,   no long term improvement  observed Fomblin Z-25, Vacuum  impregnated cages  125