Shingo Obara

Bearing Fatigue Life Tests in Advanced Base Oil and Grease for Space Applications

Two synthetic base oils (815Z and 2001A) and two greases (601EF and R2000) used for space applications have been studied at ground level. Rheological tests were performed in order to characterize the behavior of each of the base oils versus the pressure and the temperature. Next, the effect of base oils and greases on ball bearing fatigue life was carried out using thrust ball bearings. The results of L 10 life tests showed a bearing life order from the highest to the lowest of grease 601EF (blended with base oil 815Z), base oil 2001A, grease R2000 (blended with the base oil 2001A) and, the lowest, the base oil 815Z. The general tendency of the base oils and the greases shows that the bearing life increases with the EHL film parameter. However, the anticipated beneficial effect of an apparently high film parameter for the base oil 815Z was not seen due to permanent viscosity loss in the EHL contact. The results showed that the viscosity of the base oil 815Z, which contains the acetal group (-OCF 2 O-), was decreased by mechanical shear at the high shear rate in the EHL conjunction and the EHL oil film was thinner than expected. Hydrogen fluoride is released with the decomposition of the acetal group. As a result, the permanent viscosity loss at high Hertzian pressure and the hydrogen fluoride generation shortened bearing life with the 815Z base oil. In the case of grease 601EF with base oil 815Z, the permanent viscosity loss did not occur in ball bearings and the bearing life is extended.

Friction and Wear Characteristics of Advanced Space Lubricants

Synthetic oils and greases are used for space lubricant. So, it is important to know the performances of these lubricants. The base oil 815Z and 2001A and the greases 600EF, 601EF and 602EF with base oil 815Z and grease R2000 with base oil 2001A were considered as the test lubricants in this study. The highest wear scar has found for base oil 815Z but it showed the lowest coefficient of friction, whereas greases 600EF, 601EF and 602EF showed lower wear scar and considerable friction coefficient. Investigating these phenomena, authors found that the base oil 815Z contains the acetal group (-OCF2O-). At high shear rate in EHL conjunction the viscosity was decreased by mechanical shear. Hydrogen fluoride occurred with the decomposition of acetal group. It increases the wear rate of the contact surfaces. But that decomposition does not occurred in the greases with base oil 815Z and showed better result as space lubricant.

Vibration isolation system for cryocoolers of Soft X-ray Spectrometer (SXS) onboard ASTRO-H (Hitomi)

Soft X-ray Spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from micro-vibration from cryocoolers mounted on the dewar. This is mitigated for the flight model by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the flight model was verified before launch of the spacecraft in both ambient condition and thermal-vac condition, showing no detectable degradation in energy resolution. The in-orbit performance was also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the micro-vibration could degrade the cryogenic detector is shown.

The Lifetime of Boundary Lubrication Performance of Small-Quantity-Applied Liquid Lubricants for Space Mechanisms Evaluated with a Vacuum Reciprocating Tribometer

A small quantity of additive-free multiply alkylated cyclopentane was applied to the specimen to evaluate the boundary lubrication performance and its lifetime under high vacuum condition. A ball-on-disk type vacuum reciprocating tribometer was used to measure the friction coefficient and the separation voltage during the tribotest operated under high vacuum. The thickness of the liquid lubricants on the specimen was altered from less than 1 μ m to 165 μ m. The lubrication lifetime of the additive-free MAC increased with the lubricant thickness. At the initial duration of the test, the friction coefficient at both edges and a center position of the reciprocating motion was stable and low. After a certain sliding distance it was observed that the separation voltages became zero, the friction coefficient gradually increased with testing time, and finally a seizure-like high friction occurred. Generally, the friction coefficient at the edge of the stroke was larger than that at the center. In contrast, when the film thickness was large and the lubrication lifetime became long, the friction coefficient value at the edge and the center inverted after a certain sliding distance, and finally the authors observed a seizure-like high friction. The friction coefficient inversion was possibly due to the insufficient supply of the lubricant from the surrounding due to the viscosity increase. The occurrence of oxidative polymerization under high vacuum catalyzed by a nascent surface was suggested.

Bearing Fatigue Life Tests of Two Advanced Base Oils for Space Applications Under Vacuum and Atmospheric Environments

Four series of rolling-element bearing fatigue tests were conducted with 51104 size thrust ball bearings with three balls made from SUJ2 (AISI 52100) steel lubricated with two advanced synthetic base oils used for space applications. The test lubricants were perfluoropolyether (PFPE) and multiply alkyated cyclopentane (MAC). Each oil was tested with bearings under vacuum and atmospheric environments. The bearings were tested at a maximum Hertzian stress of 4 GPa on the inner and outer races. The outer race was rotated at a speed of 250 rpm. A pool lubrication system was used. Fresh lubricant was used for each test bearing. Testing in vacuum conditions was at 5 × 10−2 Pa. The test oils were analyzed to determine whether changes occurred as a result of operating in air and in a vacuum. In a vacuum environment, the PFPE 815Z oil exhibited a longer fatigue life than the MAC 2001A oil. However, in an air environment, the MAC 2001A oil had a longer L10 fatigue life than the PFPE 815Z oil. The fatigue life tests of PFPE 815Z oil in vacuum resulted in a longer L10 life than when tested in an air environment. In an air environment, hydrogen fluoride was generated in the bearing tests with the PFPE 815Z oils. Under vacuum conditions, hydrogen fluoride was not generated with the PFPE 815Z oil, resulting in longer bearing fatigue lives. The fatigue life tests of MAC 2001A oil in a vacuum resulted in shorter L10 fatigue life than in an air environment. The shorter life was attributed to the lower elastohydrodynamic oil film formation with the MAC 2001A oil because of a higher operating temperature and decomposition of the oil in vacuum.

New developments in rotating and linear motion mechanisms used in contamination sensitive space telescopes

We have been developing a rotating mechanism and a linear motion mechanism for their usage in contamination sensitive space telescopes. They both are needed for ~1.4 meter optical telescope and its focal plane instrument onboard SOLAR-C, the next-generation spaceborne solar observatory following Hinode. Highly reliable long life performance, low outgassing properties, and low level of micro-vibration are required along with their scientific performance. With the proto-type mechanisms, the long life performance and outgassing properties of the mechanisms have been evaluated in vacuum chambers. The level of micro-vibration excited during the operations of the rotating mechanism was measured by operating it on the Kestler table. This paper provides the overall descriptions of our mechanism developments.

Vibration isolation system for cryocoolers of soft x-ray spectrometer on-board ASTRO-H (Hitomi)

Abstract. The soft x-ray spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from microvibration from cryocoolers mounted on the dewar. This is mitigated for the flight model (FM) by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the FM was verified before launch of the spacecraft in both ambient condition and thermal-vacuum condition, showing no detectable degradation in energy resolution. The in-orbit detector spectral performance and cryocooler cooling performance were also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the microvibration could degrade the cryogenic detector is shown. Lessons learned from the development to mitigate unexpected issues are also described.

Ultra-sonic motor for the actuators of space optical communications terminal

The main advantages of space optical communication technologies compared with RF communications are 1) Wide bandwidth that enables a much higher data rate and 2) Smaller antenna and hardware due to the ultra-short wavelength characteristics. The cost and weight of each spacecraft has been decreasing year by year. Space optical communication technologies, that are being established, have been required to reduce cost and weight recently. The general rotational actuators of spacecraft are magnetic motors. However, it is difficult to reduce it’s weight and cost dramatically since magnetic motors include iron core and metal coil. In addition, we do not have the flexibility of magnetic motor’s shape. JAXA is interested in optical data relay including LEO-GEO optical communication. In this application, space optical communication equipment must equip rotational actuators as a coarse pointing mechanism. Therefore, the authors have focused on ultra-sonic motors (USM) for the equipment of space optical communication so that we will achieve lower cost, lower weight and a more-flexible-shape of actuators than magnetic motors. In this presentation, the authors propose applications of USM as actuators of space optical communications. USM has been widely used in our life and industry. Usage in industry includes vacuum environments of the semiconductor manufacturing process. So, the authors estimated the usage of USM can be applied to actuators of spacecraft. At first, the authors discuss the advantages and disadvantages of USM compared to traditional magnetic motors. Then, driving performance of USM under vacuum, high and low-temperature conditions are shown. At last, results of life estimation test of USM are discussed.

Behavior of Advanced Space Lubricant Under Impact Load

The squeeze film formation ability of advanced space lubricant is studied under impact load by falling bearing steel ball against a flat anvil made of mild steel. Two synthetic base oils (PFPE815Z and MAC2001A) and six greases (600EF, 601EF, 602EF, R2000, ML and MU) are investigated for the EHL dimple film thickness and a breakdown of lubricant film under elastic-plastic impact. The results showed that the viscosity of the base oil 815Z, which contains the acetal group, was decreased by the high shear rate under the EHL squeeze action. To prevention of surface damage under impact load, EHL dimple film thickness is shallow, and becomes as efficient as the grease of which oil film formation is high. From the viewpoint, MU-G and ML-G are good.Copyright

Dynamic Analysis of Minimally Lubricated Ball Bearings for Space Applications

A computer program for dynamic analysis of ball bearings, in which a simple mixed lubrication model for the retainer/race and retainer/ball contact is incorporated, is developed and retainer instability in reaction wheel bearings is investigated. Results show that an increase in the amount of oil promotes the instability and that a retainer with rectangular pockets is more stable than one with circular pockets.Copyright