Shingo Obara

Fluid-rigid body interaction is a significant topic in research on particle methods. This study developed a fluid-rigid body coupling method based on a physically consistent particle method, i.e., the moving particle hydrodynamics (MPH) method, incorporating the passively moving solid (PMS) model. When the discrete particle system satisfies the fundamental laws of physics, i.e., mass conservation, linear and angular momentum conservation, and the second law of thermodynamics, the method is asserted physically consistent, and this feature is important for robust dynamic calculations. The PMS model is a pioneering approach that is practical for particle methods in which fluid and rigid-body particles are initially calculated as a fluid. Then, only rigid-body particles are modified to restore the initial shape by applying rigid-body constraints. Thus, combining the MPH method and the PMS model realizes a fluid-rigid body coupling method that satisfies fundamental physical laws. The proposed method was first verified via the fundamental rigid body and fluid-rigid body coupling problems: the Dzhanibekov effect on a T-shaped rigid body, a floating rectangular solid, a floating cylinder, and water entry of a two-dimensional cylinder. Second, the proposed method was validated via calculating a cylinder rolling on a liquid film as a fluid-rigid body coupling problem with rotation. By using a potential-based surface tension model, the computed results showed reasonable agreement with the experimental data obtained in this study. Overall, it was confirmed that the proposed method is a promising fluid-rigid body coupling approach, in which the surface tension and wettability can be considered as well.

New bearing units having lubricant supply structure were devised for realizing longer lifetime and better performance of rotation mechanisms of space machineries. The spacer sandwiched by two bearings was filled with grease and porous body was established between grease and the bearing for both preventing thickener transferred into bearings and supplying only oil for bearings. The oil supply function was verified by friction tests and bearing tests using fluorescent agent. The contribution for long lifetime of the oil supply system was confirmed by the lifetime tests under a vacuum which showed stable rotation for 2×10⁸ revolutions at a room temperature and 1×10⁹ revolutions at a temperature of 60℃. Moreover, large size bearing units with the oil supply structure were evaluated under high rotation speed for utilized in larger spacecraft requiring larger attitude control torque and observation systems requiring agile attitude control. Using larger size bearings and rotating with higher speed would cause to promote oil scattering by centrifugal force and oil evaporation by temperature rise, whereas the oil supply structure was conducted the feasibility to realize both larger size bearings, higher rotation speed, longer lifetime and low torque.

The Bingham fluid simulation model was constructed and validated using a physically consistent particle method, i.e., the Moving Particle Hydrodynamics (MPH) method. When a discrete particle system satisfies the fundamental laws of physics, the method is asserted as physically consistent. Since Bingham fluids sometimes show solid-like behaviors, linear and angular momentum conservation is especially important. These features are naturally satisfied in the MPH method. To model the Bingham feature, the viscosity of the fluid was varied to express the stress-strain rate relation. Since the solid-like part, where the stress does not exceed the yield stress, was modeled with very large viscosity, the implicit velocity calculation was introduced so as to avoid the restriction of the time step width with respect to the diffusion number. As a result, the present model could express the stopping and solid-like behaviors, which are characteristics of Bingham fluids. The proposed method was verified and validated, and its capability was demonstrated through calculations of the two-dimensional Poiseuille flow of a Bingham plastic fluid and the three-dimensional dam-break flow of a Bingham pseudoplastic fluid by comparing those computed results to theory and experiment.

In this study, we improve a multiresolution method to reduce the computation time of fluid lubrication simulation based on a particle method by applying an implicit algorithm for viscosity calculation. The present method is based on the moving particle simulation (MPS) method and the overlapping particle technique (OPT), which is a multiresolution method for particle methods under unsteady states. The MPS method is used to solve the Navier–Stokes equation. The OPT is used to reduce the number of required particles and reduce the computation time. We improve the OPT by applying an implicit method for viscosity calculation to eliminate restrictions regarding time increment due to the diffusion number. In addition, we enable the particle size to be changed significantly between subdomains in the OPT. To validate the proposed method, we simulate the fluid lubrication of line contact in two dimensions until the flow reached a steady state. Consequently, it is shown that the pressure obtained using the proposed method agrees well with that obtained using the Reynolds equation. The computation time of a particle simulation using the improved multiresolution method is significantly shorter than that of a single-resolution simulation. In addition, the effects of particle size and subdomain size are validated in the fluid lubrication simulation using the OPT.

Impulsive vibration during rocket launch has the risk of causing indentations inside the bearings of reaction wheel mounted on a satellite. The indentations induce the vibration disturbance during the bearing rotation, which is harmful to precision equipment. In order to investigate the mechanism of the indentation formation, we carried out the experiments and finite element simulation for simplified reaction wheel. Based on the results of simulation, we estimated the depth of plastic deformation and wear using Palmgren’s formula and PV value, respectively. The indentation depth of simulation shows good agreements with that of experiment. The results also show that the wear between balls and rings is dominant factor of indentation formation, as compared with the plastic deformation. The amount of wear is greatly affected by the location of ball. In the location where the contact direction between ring and ball is parallel to the vibration direction, contact pressure contributes the wear as compared with slip velocity. Inversely, in the location where the contact direction is perpendicular to the vibration direction, the slip velocity contributes the wear and induces the larger wear region.

Pointing mechanism installed on GOSAT/TANSO-FTS is one of the most essential techniques to realize optimal observation patterns on the earth and a long-term operation period. In orbit, the continuous increase of angular rate sensor cable torque was occurred, and this induced the limited pointing angle operation for the along track direction. In this paper, we report reproducing experimental results for the degradation of the pointing mechanism in orbit.

For satellite, it is necessary to reduce the vibration of rotating bearings in reaction wheel. In order to investigate the mechanism of the small vibration caused by bearings and its control scheme, we have conducted the multi-body dynamics analysis for angular ball bearing and reaction wheel. We focus on the effect of mechanical factors such as non-circularities of the inner and outer rings and the angle misalignment between them to the rotating secondary vibration component in the axial direction. It was found that the angle-misalignment enhances the rotating secondary vibration component when the inner ring involves second-order waviness. Also, axial-direction vibration is enhanced in the case that the waviness orders of inner and outer rings are coincident each other and angular-direction vibration is enhanced in the case that the difference in waviness orders of inner and outer rings is equal to 1 when the inner and outer rings involve the waviness. These vibrations are caused by the non-linear elastic contact between ball and raceway. Finally, from the parameter design, it was found that the effect of angular misalignment to axial vibration is greater than that of non-circularity of outer rings and these mechanical factors independently affect to the vibration.

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.

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 objective of this study is to evaluate the boundary lubrication performance of ionic liquids under high vacuum and low temperature by taking the cosmic space environment into consideration, as a screening stage prior to evaluating lubrication performance in actual space mechanisms. The boundary lubrication performance of ionic liquids was evaluated at room temperature with a reciprocating linear motion tribometer, and at low temperature (from –80°C to room temperature) with a unidirectional rotation tribometer. Low-temperature rheometry was also carried out. Ionic liquids showed a supercooling state and crystallization. This crystallization was prevented by mixing different ionic liquids together. The equimolar mixture of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (EMI-TFSA), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (BMI-TFSA), and 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (HMI-TFSA) showed no crystallization in our experiment. The antiwear performance of this sample oil mixture was similar to that of MAC and much better than that of PFPE at low temperatures. This mixture prevented metal contact at –80°C, most likely due to its high viscosity and high adsorption of molecules.

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.

The lubrication performance of imidazolium-based ionic liquids was evaluated under high vacuum using a ball-on-disk tribometer. A ball and a flat disk made of SUS440C stainless steel were used as specimens. A surface of the as-received flat disk specimen was covered with a thick oxide layer (>40 nm). For an examination of the effect of the surface oxide layer on the tribological performance of the ionic liquids under high vacuum, another specimen with a thin oxide layer (approx. 4 nm thick) was prepared. The ionic liquids with the TFSA anion showed better lubrication performance with the thicker oxide layer specimen, whereas the ionic liquids with the BF4 anion showed superior performance with the thinner oxide layer specimen. These results are discussed based on the HSAB principle. It is shown that the mechanism of the opposite tribological characteristics can be reasonably explained in terms of the chemical hardness by the HSAB principle.

Four multiply alkylated cyclopentane (MAC)-based greases, used for space applications, were used for rolling contact fatigue tests employing thrust ball bearings. These greases were R2000, ML, MU, ...

The tribological performance of two types of additives—alkylated phenyl phosphate and lead naphthenate—dissolved in multiply-alkylated cyclopentane was evaluated under a high vacuum using two types of ball-on-disk tribometers: a reciprocating motion tribometer under mild loading conditions and a unidirectional rotating motion tribometer under heavy loading conditions. A ball and a flat disk made of SUS440C stainless steel were used as specimens for both tribometers. The surface of the as-received flat disk specimen was covered with a thick (>40 nm) oxide layer. For the examination of the effect of the surface oxide layer on the tribological performance of the liquid lubricants under a high vacuum, another specimen with an oxide layer (approx. 4 nm thick) was prepared. The alkylated phenyl phosphate additive showed better lubrication performance with the specimen with the thicker oxide layer, but the lead naphthenate additive showed superior performance with the thinner oxide layer specimen. It is also shown that these opposite tribological characteristics are explained by the hard and soft acids and bases principle.

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...

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.

<jats:p>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.</jats:p>

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 ...

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-), wa...

Lubrication is one of the most important key technologies for reliable and durable machines in ultra high vacuum. Solid lubricants, which are pre-coated before operation by a certain amount thickness, are commonly used for the system. The life of present system is, therefore, determined by the wear life of the coating of lubricant.<br>   To extend its lifetime, a new lubrication method for in-situ and on-demand restoration of the lubricants called as “Tribo-coating” had been introduced, and its effectiveness as practical solid lubrication method for sliding element has been clearly shown.<br>   This paper introduces the unique advantage of excellent tribological performance of tribo-coating film for ball bearings, and the usefulness of the tribo-coating for friction control of ball bearings in ultra high vacuum is clearly shown.<br>

<jats:p>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.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Sliding friction experiments under vacuum lubricated with additive‐free multialkylatedcyclopentane (MAC) were carried out using a ball‐on‐disc tribometer with 440C stainless steel as the material of the specimen. Antiwear characteristics of additive‐free MAC during the steady friction region under vacuum were shown to be better than those of perfluoropolyether (PFPE). However, it was found that the friction coefficient lubricated with the additive‐free MAC under vacuum showed an initial seizure‐like high friction of approximately 0.2 at the beginning of the test, followed by a steady low friction of approximately 0.1. It was also found in contrast that PFPE did not show the initial seizure‐like high friction but maintained a lower friction coefficient than 0.1 throughout the experiment. The initial high wear volume lubricated with the additive‐free MAC was found to be related to the initial seizure‐like high friction. The tricresylphosphate formulation prevented the initial seizure‐like high friction. Copyright © 2007 John Wiley & Sons, Ltd.</jats:p>

Strain wave gearing (SWG) is a precise gearing mechanism composed of three mechanical elements; the wave generator (WG), the flexspline (FS) and the circular spline (CS). The SWG is widely used for the space applications because of its suitable features, such as lightweight and high precision. In this study, the lubrication mechanisms of SWG lubricated with MAC (Multiply-Alkylated Cyclopentane) or MAC based grease were investigated in vacuum and in air. To examine the lubricating conditions of SWG, the contact electric resistances between the mechanical elements were measured in different environments. The experimental results showed that in all the contacting interfaces the contact electric resistance measured in vacuum was lower than that measured in air, which means that metal-to-metal contact occurred more easily in vacuum operation. A mixed lubrication analysis at the WG/FS interface, where the lubricating condition changed most drastically depending on the environment, was conducted. The calculation results revealed that for in-air operation the oil amount inside the WG/FS clearance is preserved by flowing-in by the ambient pressure and flowing-out by the squeeze motion between WG and FS, whereas for in-vacuum operation the lubricant flows out only, leading to lubricant starvation. The drastic change of the lubrication conditions in air and in vacuum is probably caused by this mechanism.

An unbranched perfluoropolyether (PFPE) 815Z is the current ball bearing lubricant for space applications. Measurements of elastohydrodynamic lubrication (EHL) oil film thickness have been carried out to assess the lubricating performance of PFPE with average molecular weight of 9200 using an optical interferometric technique under mean Hertzian pressure 0.45 GPa. The film thickness of 815Z became less than predicted film thickness from Hamrock and Dowson formula for EHL central film thickness. There are two main explanations why PFPE is inferior to mineral oil in their ability to form EHL films, temporary viscosity loss and permanent viscosity loss. In order to elucidate the results, measurements of permanent viscosity loss under mean Hertzian pressure from 0.41 GPa to 2.67 GPa have been carried out using the thrust ball bearing. There results show that the degree of the permanent viscosity loss depends on Hertzian pressure, occurrence of permanent viscosity loss of 36 % with 2.67 GPa and 2 % with 0.41 GPa.

Adhesion between two clean surfaces was investigated using a surface-activated bonding method in an ultrahigh vacuum (UHV) at room temperature. Adhesion was observed in all combinations of SUS440C/Ti-6Al-4V, INCONEL718/Ti-6Al-4V, A7075/SUS440C, SUS304/Al and INCONEL718/SUS440C. To understand the basic problems of UHV bonding, the influence of the surface roughness on bonding strength and interface structure has been studied. The surface roughness at the nanometer scale strongly influenced the bonding strength. In the case of SUS304 and Al, surfaces with less than l nm (RMS) had a high bonding strength of over 80 MPa. The interface does not have a thick reaction layer, only an amorphous layer on the order of several nanometers. Separation of the SUS304/Al interface by heat treatment has been also studied. A specimen heated at 823 K for 2 hr in a vacuum separated spontaneously without any external mechanical force. Separation occurred at the interface between the Al and the layer of the reaction products. This separation occurred only if the specimen was heated in a vacuum or argon atmosphere. The specimen did not separate when it was heated in air, nitrogen or oxygen atmospheres. The separated SUS304 could be re-bonded to Al at room temperature.

This paper represents a mixed lubrication analysis for the interface between the Wave Generator (WG) outside surface and the Flexspline (FS) inside surface in Strain Wave Gearing (SWG). The analytical model consists of the FEM model of the WG outer race and FS and the mixed lubrication model combined with the pressure-compliance equation of surface roughness contact developed by Greenwood and Williamson and the average Reynolds equation by Patir and Cheng. The lubricant film thickness, the hydrodynamic and asperity contact pressures and the lubricant flow vectors at the WG/FS interface are calculated as a function of the input rotational speed. The numerical results show that in the present operating conditions the SWG works primarily in the regime of mixed lubrication and the roughness contacts are made at both edges of the WG outside surface. In addition, axial lubricant flow from the inside to the outside at WG/FS interface is found at the relatively low rotational speed of the WG.

Longitudinal microgrooves were assumed on the circular journal bearings and static and dynamic characteristics were investigated by solving the modified Reynolds equation for a rough bearing surface. It was found that the dynamic characteristics of the journal bearings were improved by longitudinal microgrooves or truncated micro-grooves on the bearing surface while the static characteristics, such as load-carrying capacity and friction coefficients, were not changed by the microgrooves. Calculations regarding the linear stability of a symmetrical rotor supported by two journal bearings were also carried out and it was found that the stable region was expanded on the stability chart by microgrooves. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

<jats:p>The worn surface profile of a composite structure was experimentally and numerically investigated focusing on the effects of sliding conditions. Wear tests on composites made of an oxide ceramic and an amorphous metal against a tetragonal zirconia polycrystals-alumina were carried out under various mean contact pressures, P, and sliding velocities, V. The test results showed that the worn surface profiles of the composites changed with the PV value. A new numerical method for simulating the worn surface profile of a composite structure has been developed. The present method is based upon the assumption that the profile of a worn surface is changed by thermal distortion of the sliding bodies due to frictional heating and by elastic deformation due to normal pressure and friction traction. The calculated results were compared with the test results, and the comparison showed that the elastic deformation plays an important role in forming the worn surface profile and that the effect of thermal distortion becomes remarkable with an increase in PV values. The numerical results clarified the contribution of the thermal distortion to the change in the worn surface profile of the composite.</jats:p>

<jats:p>A numerical analysis has been carried out for a three-dimensional frictional heating problem of a composite material, in which an asperity contact (heat source) moves across the boundary of the constituents of a composite. The mathematical model adopted here is that a surface asperity on a semi-infinite body slides on a composite which consists of two semi-infinite bodies. Expressions of temperature distribution both in the composite and on the moving surface are derived. Then the temperature distribution and its change with time are obtained by a numerical procedure. It is shown that the temperature rise caused by frictional heating remarkably depends on the thermal properties of constituents of the composite, and temperature distributions in the vicinity of the asperity contact both on the moving surface and in the composite rapidly change when the asperity passes over the boundary of the constituents. The effect of the frictional heating on the worn surface profile of a composite is also discussed.</jats:p>

A numerical analysis has been carried out for the three-dimensional frictional heating problem of composite materials, where an asperity contact (heat source) moves across the boundary of constituents of the composite. The mathematical model adopted here is that of a surface asperity on a semi-infinite body sliding on the composite which consists of two semi-infinite bodies. Theoretical equations with respect to the temperature distribution in both the composite and the moving surface are derived. The equations are solved numerically and the temperature distribution change with time is also obtained. It will be shown that the temperature rise caused by frictional heating depends markedly on the thermal properties of constituents of the composite, and the temperature distribution in the vicinity of the asperity contact in both the moving surface and the composite changes rapidly when the asperity passes over the boundary of constituents. The effect of the frictional heating on the worn surface profile of the composite is also discussed.

The oil flow in the porous media is experimentally investigated to explicate the mechanism of lubrication in the porous journal bearings. For the test bearing, a simplified model of the porous bearing is used, whose matrices are composed of the packed glass spheres having small uniform diameter. To visualize the flow in the porous media, a dyed oil is fed into the porous media impregnated with uncolored oil and the fluorescent flow is rendered visible by the ultraviolet flash lamp. It is found that there exists the circulation of the Oil flow through the porous media, which the flow contributes to the lubrication in the porous bearings. In hydrodynamic lubrication conditions, the oil in the porous media flows away from the position of the load line towards the unloaded region. However, in boundary lubrication conditions, when the oil supply pressure is relatively small, the oil in the porous media flows from the loaded region towards the site where the oil film pressure would be negative.