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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.

Tribological Properties and Film Formation Behavior of Thermoreversible Gel Lubricants

The tribological properties of a new and unique thermoreversible gel lubricant (TR gel lube) were investigated. TR gel lube, which includes 10–40% of amide-type gelling agent in base fluid, is able to repeatedly alter or convert phase from gel state to liquid state at the melting point of the gelling agent. High-pressure rheological tests were performed in order to characterize the behavior of base oil and TR gel lube as a function of pressure and temperature. The effect of TR gel lube on ball bearing fatigue life was carried out by systematic tests using thrust ball bearings. The results of L 10 life tests of TR gel lube showed a longer life than the conventional greases. This result was investigated from the oil film formation. It was found that the gelling agent played a key role in the lubricating properties. Some mechanisms such as adsorbed film formation and solid-like formation are proposed.

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.

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

Tribological Properties and Applicability of Thermo-Reversible Gel-Lubricants

The tribological properties of a new unique thermo-reversible Gel-Lubricant (TR Gel-Lube) were investigated. TR Gel-Lube which includes 10–40% of amide type gel agent in base fluid, is repeatedly able to change from gel-state to liquid-state at the melting point of its gel agent. High-pressure rheological tests were performed in order to characterize the behavior of base oil and TR Gel-Lube as a function of pressure and temperature. The effect of TR Gel-Lube on ball bearings fatigue life was carried out by systematic tests using thrust ball bearings. The results of L10 life tests of TR Gel-Lube showed a longer life than the conventional greases. This result was investigated from oil film formation. It was found that gel agent played a key role in the lubricating properties. Some mechanisms such as adsorbed film formation and solid like formation are proposed.Copyright