ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
404 Down Selection Testing for Best Fretting Wear Performance 

The team devised a test for the worst case design configuration; the design that induced the most stress  on the bearings. The FWA bearing design supported the higher mass spread out over large wheels and required the most stiffness to keep the dynamic deflections to a minimum. It was impractical, however, to build four filter wheel assemblies. Therefore, the team developed a surrogate design that would replicate the FWA bearing mount design to the exact dimensions and tolerances. The FWA design consisted of two independently driven optic wheel assemblies mounted on a common fixed shaft. Each optic wheel assembly was configured with the same flexible bearing mount and incorporated the same duplex bearing  pair as used on the PIL, but with a thicker set of bearing spacers as seen in Figure 7 . 

Figure 7.  FWA dual wheel bearing and motor mount configuration on solid titanium shaft. 

The test configuration replicated only half of the FWA design and included a surrogate wheel of the same  mass that induced the same bearing forces and moment of inertia. The test configuration also replicated many of the other parts to exact dimensions and tolerances. The difference of each of the four test configurations was the unique change that presented possible solution to the fretting wear issue. Because the hardened 440C steel of the bearing was already very resistant to fretting, no changes to the bearings were incorporated. The test configurations focused only on changes to the titanium shaft and anodic coated titanium bearing sleeve. Three coatings and two material changes were chosen as candidate solutions for the test.  The coatings were chosen for their wear resistance and adherence properties on titanium down to cryogenic temperatures. The coatings selected were titanium-nitride (TiN), ion-plated gold (Au), and an amorphous carbon-based tungsten-carbon/carbide (WCC). The Au and TiN coatings have been used successfully as wear-resistance coat ings at near ambient aerospace applications  and have demonstrated  good adherence to titanium. Despite having no direct aerospace heritage, the excellent wear and adherence data demonstrated by the WCC coating warranted a spot on the candidate list.  Bearin g OD Clam p Ring  Shaft end ca p (Ti) Solid Shaft (Ti)  Bearing & bearing  spacer (440C)  Cryo Release Sleeve (Ti and Pol ymer) ½-28 Shaft Nut (Ti)  Clam p Disk (Ti)  FWA H ousing  Motor Rotor (455 CRES )
405 The design team was very hesitant to change materials due to the differential CTE issues, but the wear  resistant properties of Nitronic-602 and MP35N3 were hard to ignore. A previous aerospace application  demonstrated the wear characteristics of an MP35N shaft and a Nitronic-60 sleeve, and the FWA solid  shaft design could benefit from the higher strength of the MP35N. So for the fret wear test configuration, all sleeves were to be made of Nitronic-60, and one shaft was to be made from MP35N. However, because of the differential CTE issue, and the lack of direct CTE measurements of this material down to 35 K, samples of each of these materials were tested with the results shown in Figure 8.   

Figure 8.  Measurements of thermal expansion down to 35 K for both Nitronic-60 and MP35N.  The  dL/L of Nitronic-60 at 35 K is 0.28% and 0.21% for MP35N. 

Although each unique candidate solution was representative of the original design, the team judged that  there were enough differences in the test that a control configuration was needed. Therefore, a fifth configuration was added to include the original, un-coated titanium shaft and anodic coated sleeve. After completing the assembly of each of the configurations, each was mounted onto a common vibration block that was to be used to subject all units to the required three-axis vibration test. A thorough analysis of the five-wheel test block, as shown in Figure 9, was performed to confirm block input values would induce representative loads into the system. 

   Figure 9.  Diagram of test configuration and picture of the five test configurations mounted to a  vibration block and sealed for contamination prevention. 

                                                  2 Nitronic-60 is an austenitic stainless steel alloy known for its wear and galling resistance and is used in many high  temperature and corrosion resistant applications.  3 MP35N is a nickel, cobalt, chromium, and molybdenum, allow which has a inherent high strength,  outstanding corrosion resistance, and excellent cryogenic properties.
406 In preparation for testing the candidate solutions, the team recognized that the fret wear contamination  was observed after only one 3-axis vibration test, but the FWA and PIL flight units will undergo four vibration tests during ground preparations and then launch. Additionally, the flight units will be subjected to a number of cryogenic test cycles in between the various vibration tests. Furthermore, the JWST review team volunteered that the launch capsule may be purged with dry nitrogen through ascent, and it is documented that WCC coating friction levels increase and TiN coatings friction levels decreases in low humidity environments. With all these factors in mind, the team developed a test plan that would subject the candidate solutions to all representative atmospheric and vibration conditions and be subsequently dis-assembled to determine which configuration produced minimum fretting wear contamination. 

Test Results   The test configuration was subjected to the equivalent of five 3-axis vibration tests, a cryo test, and one final vibration test with all units purged to less than 5% relative humidity. The data review from the set of five 3-axis vibration tests yielded no anomalies with first mode frequencies at 200 Hz, maximums of a little over 30 Gs rms, and only small variations from predictions. Additionally, signature run comparisons throughout the test showed modest variations in amplitude and no changes in frequency. The tests were deemed successful, and before progressing to the cryo testing, the assemblies were disassembled for inspection of the bearing mounts.  The control configuration with a titanium shaft and anodic coated titanium sleeve showed similar fretting wear debris as before, shown in Figure 10. The ion-gold coated titanium shaft with Nitronic-60 sleeve also fared poorly due to apparent coating failure and fretting wear debris as shown in Figure 11. The TiN coated titanium shaft showed improved fret wear performance, but a nominal amount of fret wear debris was still found on this configuration as shown in Figure 12. The WCC-coated titanium shaft, however, showed no indication of coating wear, and very little fret wear debris as shown in Figure 13. Finally, the MP35N shaft showed very small amounts of wear and little wear debris. After initial inspection, the WCC-coated titanium shaft showed the best performance. 

   Figure 10. Scanning electron microscope (SEM) anal ysis of the titanium shaft and anodic-coated  titanium sleeve.  Titanium and iron debris (1 & 2), and conglomeration of fine particles are  characteristic of the fret wear seen before on original design.
407    Figure 11. SEM analysis of ion-plated gold ti tanium shaft and Nitronic-60 sleeve.  The photo  shows the titanium shaft with gold coating failure.  The SEM micrographs showing bare Ti surface  (1), iron debris (2), and ion gold plating and Ti debris (3). 

Figure 12.  Photo and SEM analysis of TiN-coated titanium shaft and Nitronic-60 sleeve.  The TiNcoated titanium shaft showed only small amounts of wear.  The SEM analysis indicates iron, Ti,  and TiN debris (1 & 2), and TiN plating on the shaft bearing seat (3).  The small particulates  observed on the shaft consisted of conglomerates of sub-micron sized iron and TiN particles  typical of fret wear.  The last micrograph shows an example of the numerous voids and  protrusions prevalent in TiN coatings.
408    Figure 13. SEM analysis of the WCC-coated shaft and Nitronic-60 sleeve.  The WCC-coated  titanium shaft showed no wear marks on the shaft surface.  Only small amounts of tungsten and  iron debris were found on the shaft bearing seat. 

   Figure 14. SEM analysis of the MP35N shaft and Nitronic-60 sleeve.  The MP35N shaft surface  showed wear only at the microscopic level, and very little wear debris.  The debris consisted  predominately of iron with some cobalt alloy constituents. 

The next test was to subject the different shafts to cryogenic temperatures. Of the four initial candidate  shafts, only three were placed in a cryogenic chamber and brought down to 35 K at a maximum of 40 K per hour as shown in Figure 15. The ion-plated gold shaft was not advanced due to poor performance. After returning them to ambient conditions, the shafts were reviewed under visual magnification as well as SEM analysis. The results of the inspection after cryo yielded no adhesion anomalies, and the shafts were re-installed into their respective surrogate FWA configuration for the last vibration test.
409    Figure 15. Cryogenic test fixture with candidate shafts mounted to cold head (shrouds removed).   The gold shaft was replaced with a spare titanium shaft for temperature instrumentation. 

The last test for the candidate configurations was a shortened version of the previous three-axis vibration test but in dry air. During this vibration test, the assemblies were only subjected to one random and one sine vibration test per axis. However, these vibration tests required that each configuration be plumbed to dry air to ensure the tests were conducted in less than 5% RH dry air. This was stipulated by the customer to address the possibility that the JWST observatory would be launched in dry air conditions combined with the fact that the friction coefficient of WCC coatings increases in dry environments. As shown in Figure 16, each configuration was plumbed with independently controlled dry air input and monitored with independent humidity meters. As a side note, the MP35N configuration was eliminated from the test due to the higher density (i.e., weight) of this material. After this vibration test was completed, the units were disassembled and inspected.  

Figure 16.  Second vibration test of the TiN-coated, WCC-coated, and bare (control) titanium shaft  configurations.  Note the dry air input as well  as the humidity meters for each configuration.     Conclusion   After finding contamination in the bearing mounts of the FWA and PIL assemblies through testing, the team was able to identify the root cause and modify the design to resolve the issue. Fretting wear between the titanium shaft and bearings and sleeve was found to be the source of the contamination. The team selected a set of candidate designs that could be implemented to minimize the fretting issue after conducting a study of possible remedies. Due to limited cryogenic data on the candidate solutions, a series of random vibration tests on a simulated wheel assembly were run to verify the improved fretting
410 wear performance at 35 K. The original configuration was included in the tests for a control comparison. A  clear winner was found in the WCC-coated titanium shaft with Nitronic-60 sleeve and original 440C bearings, as shown in Figure 17. The cobalt alloy (MP35N) configuration also showed little wear and only small amounts of debris, but it did not perform as well as the WCC-coated shaft. It also had the disadvantage of a differential CTE and higher density. The titanium-nitride coated titanium shaft showed more wear than the WCC-coated shaft and more debris was observed with this configuration. The micro-pits and protrusions were also a detractor for this configuration. Lastly, the ion-plated gold configuration showed the poorest performance with significant adhesion problems and large amounts of wear debris. The hard WCC-coating proved to be the best solution to minimize the risk of wear contamination for this application.  

Figure 17.  Four candidate configurations showing relative fret wear performance.  The WCCcoated titanium shaft demonstrated the least susceptibility to fretting wear. 

References 

1. Clark, Charles S., “Redesign and Test of Cryogenic Mechanism for Improved Stiffness”,  Proc. SPIE  Paper 8150-19 (August 2011).  2. Clark, Charles S., “NIRCam Pupil Imaging Lens Actuator Assembly”, Proc. SPIE Paper 7439C-46  (August 2009).  3.  Sean McCully, "Experimental Development Tests of a Cryogenic Filter Wheel Assembly for the  NIRCam Instrument and James Webb Space Telescope", Proc. Aerospace Mechanisms Symposium (May 2006).
411 Design and Manufacturing Considerations for Shockproof and Corrosion-Immune  Superelastic Nickel-Titanium Bearings for a Space Station Application 

Christopher DellaCorte* and Walter A. Wozniak*  

Abstract 

An intermetallic nickel-titanium alloy, 60NiTi (60wt%Ni, 40wt%Ti), is a promising tribological material for space mechanisms. 60NiTi offers a broad combination of physical properties that make it unique among bearing materials. 60NiTi is hard, electrically conductive, highly corrosion resistant, readily machined prior to final heat treatment, easily lubricated and is non-magnetic. It also falls within the class of superelastic alloys and can elastically endure large strains (beyond 5%) making it highly resistant to excessive and unexpected (shock) loads. Key material properties and characteristics such as elastic modulus, tensile fracture sensitivity and residual stress behavior, however, differ from conventional alloys such as steel and this significantly affects bearing design and manufacturing. In this paper, the preliminary design and manufacture of ball bearings made from 60NiTi are considered for a highly corrosive, lightly loaded, low speed bearing application found inside the International Space Station’s water recyling system. The information presented is expected to help guide more widespread commercilization of this new technology into space mechanism and other applications.    Introduction   Recent research on binary nickel-titanium (Ni-Ti) alloys has identified them as promising candidates for bearings and mechanical components [1]. The nickel-rich alloy, 60NiTi in particular, exhibits a remarkable combination of properties and characteristics relevant to rolling element bearings for space mechanism applications. For instance, 60NiTi is hard, electrically conductive, highly corrosion resistant, readily machined prior to final heat treatment, easily lubricated and is non-magnetic [2,3]. 60NiTi is also in the family of superelastic alloys and can elastically endure large strains (beyond 5%)  making it highly resistant to shock loads. Table I lists many of these properties as they are currently known alongside the conventional shape memory alloy 55NiTi and more traditional bearing materials. As an emerging material, some of these published properties are estimated or preliminary.  

Table I – Representative Thermophysical and Mechanical Properties of Bearing Materials  Property  60NiTi 55NiTi 440C Si3N4 M-50  Density 6.7 g/cc 6.5 g/cc 7.7 g/cc 3.2 g/cc 8.0 g/cc  Hardness 56-62 Rc 35-40 Rc 58-62 Rc 1300-1500Hv 60-65 Rc  Thermal Cond.  W/m-°K 18 9 24 33 ~36  Thermal Expansion ~10x10 -6/°C ~10x10-6/°C 10x10-6/°C 2.6x10-6/°C ~11x10-6/°C  Magnetic Non Non Magnetic Non Magnetic  Corrosion Resistance Excellent Excellent Marginal Excellent Poor  Tensile/Flexural Strength ~1000 MPa ~900 MPa 1900 MPa 600-1200 MPa  (Bend Strength) 2500 MPa  Young’s Modulus ~114 GPa ~100 GPa 200 GPa 310 GPa 210 GPa                                                    * NASA Glenn Research Center, Cleveland, OH  

Proceedings of the 41st Aerospace Mechanisms Symposium, NASA Jet Propulsion Laboratory, May 16-18, 2012
413 requirement for a rapid quench during heat treatment which leads to significant residual stresses. These  three key differences along with other second order attributes affect bearing design and manufacturing processes.  The following sections describe the general design and manufacturing process for 60NiTi bearings by considering a specific space mechanism bearing application; the Distillation Assembly (DA) centrifuge that is part of the ISS Environmental Control System (ECLSS) on the International Space Station (ISS). The ball bearings used in this application operate in a highly corrosive environment at low speed under very low average loads thus fatigue is not a great concern. However, the bearings must endure high launch loads without damage. Based upon preliminary data, 60NiTi appears to be a viable candidate bearing material. This paper lays out the design process and the results of pathfinder manufacturing investigations to develop a more generalized methodology for incorporating superelastic materials into space mechanisms.   Materials and Procedures 

Bearing grade 60NiTi is manufactured via a proprietary high-temperature powder metallurgy (PM) process roughly similar to that described in the literature [5]. Pre-alloyed 60NiTi powder is hot iso-static pressed (HIPed) into various shapes and sizes depending upon the desired end product. To make 60NiTi balls, the powder is HIPed into rough, spherical ball blanks that were then ground, polished and lapped. Because the PM process yields ball blanks that have isotropic mechanical properties high quality (Grade 5) bearing balls can be readily produced. The finished 60NiTi ball specimens, shown in the photograph in Figure 1, are bright and shiny in appearance and resemble conventional polished steel balls.   

Figure 1.  60NiTi Grade 5 test balls.   To make other shapes such as bearing races and mechanical and thermo-physical property measurement specimens, 60NiTi rods and ingots were first made using the same PM process. Figure 2 shows such specimens produced by the PM process.
414   Figure 2.  60NiTi ingots and blanks produced by PM process. 

Once the ingots and other shapes are produced, a series of steps that include wire electrode discharge machining (EDM), conventional machining using carbide tools and grinding are employed. A multi-step thermal process (heat treatment) generally occurs after rough machining to near final dimensions but before final grinding and polishing in the case of balls and bearing raceways. For parts that are not dimensionally critical, simple wear plates for example, final grinding after heat treatment may be unnecessary.   A typical heat treatment includes solution treating at 1000 C in vacuum or inert gas atmosphere followed  by a rapid quench in water. The solution treating dissolves precipitate phases like Ni 4Ti3 and Ni 3Ti forming  the preferable NiTi phase. Rapid quenching locks in the dominant NiTi phase and discourages the  formation of the other phases that can lead to brittleness and low hardness. Details regarding the processing and resulting properties are the subject of ongoing research and are partially described elsewhere [6]. Using the generalized processes described above, specimens are being fabricated to enable rolling contact fatigue stress limits, bend strength, Charpy impact toughness, compressive strength and thermal properties such as thermal expansion, thermal conductivity and thermal diffusivity. This data will be available in the future but initial bearing design can proceed using information available and estimated.   Design and Manufacturing Considerations 

A baseline bearing can be designed using the preliminary and estimated 60NiTi materials properties. This design can then be reviewed and analyzed for its general appropriateness the DA bearing application. Using the preliminary design, a manufacturing method must be then developed and tested. Since key structural properties of 60NiTi differ from bearing steel (notably the elastic modulus is low and high machining forces are needed for material removal), the manufacturing process and tooling may differ from the norm. If the manufacturing investigation identifies design deficiencies it can be revised in an iterative manner. Further, as additional material properties are obtained, such as rolling contact fatigue stress limits, the design can be revisited. The following sections expand upon the preliminary design analysis and a pathfinder manufacturing trial.   Results and Discussion   The DA bearing application currently under consideration is characterized as highly corrosive and mechanically benign. The bearings are nominally 50-mm bore deep groove ball bearings operating at low speed (a few hundred rpm) under very modest axial preload. In space, there is virtually no imposed radial load. Table III gives the representative baseline bearing conditions.
415 Table III - Representative Application Bearing Parameters   And Operating Conditions   Parameter Value or Condition  Outer Diameter, OD ~80 mm   Inner Diameter, ID ~50 mm   Width, W ~16 mm  Ball Size, D ~9 mm  Ball Material Si3N4  Race Material Cobalt Alloy  Cage  Snap fit, polymer  Lubricant Lithium based grease  Ball-Race Stress Limit ~2 MPa  Ball-Race Mean Stress ~1 MPa  Axial Preload ~200 N  Radial Load (terrestrial) ~100 N/bearing  Speed 100-300 rpm  Environment Warm, highly acidic aqueous solution  Ambient Pressure Slight vacuum 

When reviewing the data and parameters in Table III it becomes clear that the environmental stresses on  the bearings, namely the warm and highly acidic environment, far outpace the modest mechanical loads and stresses. In fact, during early system development, the originally specified martensitic stainless steel bearings (440C) experienced unacceptable surface corrosion. Following bench top corrosion studies in a simulated environment, the steel bearings were replaced with the cobalt race-Si 3N4 ball hybrid bearing  design as a baseline.    Figure 3a shows a photograph of the DA assembly during ground tests. This device is essentially a rotating drum evaporator that utilizes a belt drive to provide a low pressure, warm internal chamber that boils wastewater on the ISS. The steam coming off of the drum section is collected and condensed and is then further treated before re-use. Further details of this system can be found in the literature [7-9]. In cross section (Figure 3b shows a representative design), the subject rotor system is a simple configuration of two shielded ball bearings that are lightly spring preloaded and operate in the warm, moist and acidic environment.
417 Figure 4 shows a photograph of the centrifuge bearings after removal from service. The bearings are a  straight ball bearing design and utilize a cobalt alloy for the races and silicon nitride for the balls to achieve maximum corrosion resistance.  

Figure 4. ISS Distillation Assembly centrifuge bearings.   The performance of the current DA bearings has not been completely satisfactory. The very first system launched suffered from raceway damage that occurred during bearing assembly and installation. Though this damage did not result in system failure, long life could not be assured. The second system launched was returned to earth after a short period of use for repair of an unrelated system component. During a routine bearing inspection significant race wear was observed. Figure 5 shows a photograph of the surface of a worn inner race in which the ball has created a pronounced wear track.  

Figure 5.  Photograph of an ISS Distillation Assembly centrifuge bearing showing wear track  following operation on orbit.    The wear was attributed to the lack of hardness of the cobalt-based races compared to the ceramic balls. The bearings still functioned, but again, long-life was not assured. It is these shortcomings of the current bearing race materials that provide the impetus to consider 60NiTi. Among the first material selection criteria to be assessed is corrosion resistance to the process fluid.