Hydrodynamic journal bearings experience significant variation in oil film temperature. Reliable data of operating temperatures in these journal bearings are very useful and important for practical bearing designers and mathematical modellers. Here, an elliptical journal bearing has been tested to access the temperature rise at oil-bush interface with three grade oils at loads varying from 100 N to 600 N and speeds = 3000, 3500, 4000 rpm at constant oil supply pressure. The results show that with increase in load at constant speed and pressure, and with increase in speed at constant load and pressure, the oil film temperature increases in the central plane of both the lobes of the bearing for all grade oils under study. Further, it is observed that under the given operating conditions, oil 2 gives the coolest operation of the elliptical bearing under analysis.

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One-dimensional textured parallel bearings have been successfully optimized for the maximum load capacity or the minimum friction coefficient using a unified computational approach. However, there is no efficient approach allowing for the optimization of two-dimensional (2D) bearings. The work conducted is, in most cases, by “trial and error”, i.e. changes are introduced and their effects studied, either experimentally or through numerical parametric studies. This is time consuming and costly. In this paper, a uniform approach to the optimization of surface textures in 2D bearings, based on nonlinear programming routines, is proposed. The approach aims at finding the optimal textured surfaces that support the maximum load and/or minimize friction coefficient. Examples of parallel hydrodynamic bearings with surfaces textured by rectangular or elliptical dimples and governed by Reynolds equations, considering mass-conserving cavitation and decrease in viscosity due to temperature change are optimized. Results of the optimization are comparable to those obtained using an exhaustive search and found in literature.

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The authors have clarified that trace impurities such as water and oxygen inevitably contained in a hydrogen environment influence friction and wear of metallic materials substantially and even often govern them. In this study, an experimental technique was devised to enable sliding tests in a hydrogen environment which contains controlled concentration of water and virtually no oxygen as impurities. By comparing data of Fe sliding tests between this study and our previous studies, the influences of water and oxygen were understood separately. Water without oxygen as impurity in a hydrogen environment decreased wear of pure Fe while oxygen in additional to water increased the wear. Sliding of pure Fe consumed water when a hydrogen environment contained only water while it produced water and consumed oxygen when the environment contained both water and oxygen. The number of water molecules consumed by sliding in a hydrogen environment with water more than 5,000 ppb and without oxygen was larger than the estimated number of nascent Fe atoms by sliding. This suggests that multi-layer adsorption of water molecules decreased the wear of pure Fe.

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Influence of the moist air humidity (10-70%) on the fretting characteristics of steels under grease lubrication is experimentally investigated. Ball (bearing steel, JIS SUJ2) on disk (carbon steel, JIS S15C) type fretting tests for 0-20 mg of grease supplied before each test at the punctual contact between ball and disk were conducted at 50 μm slip amplitude and 6.7 Hz frequency. From the shape of the friction hysteresis one calculates the slip index, and based on it, determines the type of fretting regime against the number of fretting cycles. Influence of the grease amount and relative humidity on the friction coefficient, slip index and wear rate is interpreted based on the occurrence of the grease film breakdown. Fretting wear rate appears as unaffected by variation of the environmental humidity for an amount of grease exceeding 3 mg, this representing the minimum amount necessary to avoid the grease film breakdown. Results obtained are useful to find the amount of grease necessary but sufficient for proper lubrication, and hence to protect the environment by grease saving. In order to generalize the results obtained for ball- bearings, screws, joints, guide-ways, etc., a tribological analysis based on the Stribeck curve was performed and then, the amount of grease necessary but sufficient to obtain fully lubricated fretting regime in moist air for ball bearings was discussed in detail.

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A molecular dynamics simulation is performed to clarify the energy dissipation process in the atomic-scale friction, particularly with the two-dimensional atomic-scale stick-slip phenomenon as observed in the friction experiment by AFM/FFM. In the present simulation, a well defined Cu{1 0 0} surface is scanned in the direction of <1 0 0> with different scan position by a carbon atom probe using a simple model where the effect of cantilever stiffness of the atomic force microscope is taken into consideration. From the simulation results, it is clarified that the dissipated mechanical energy thorough the one or two-dimensional atomic-scale stick-slip phenomenon can be calculated more precisely from the stored elastic energy using the amplitude of stick-slip force signal in each direction than the work calculated using the average friction force.

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Deployment of a safe, efficient hydrogen production and delivery infrastructure on a scale that can compete economically with current fuels is needed in order to realize the hydrogen economy. While hydrogen compression technology is crucial to pipeline delivery, positive displacement compressors are costly, have poor reliability and use oil, which contaminates the hydrogen. A completely oil-free, high-speed, efficient centrifugal compressor using 4th generation compliant foil bearings and seals has been designed for hydrogen pipeline delivery. Using 6-12 MW drives operating at speeds to 56,000 rpm, a modular, double entry compressor was configured to deliver 500,000 kg/day at pressures greater than 8 MPa. Each of the two or three multi-stage compressor frames operate above the bending critical speed of the rotating group since speeds are 5 to 7 times faster than conventional compressors. To assure a structurally and economically feasible design, the rotor of each compressor frame spins at the same speed with blade tip velocities near 600 m/s. An iterative aerodynamic/structural/rotordynamic design process was used, including both quasi-three dimensional inviscid internal flow and Computational Fluid Dynamic (CFD) analyses. The flow field was carefully analyzed for areas of excessive diffusion, sudden velocity gradients and flow separation. Excellent correlation between the preliminary design and CFD analysis was obtained. Structural and rotor-bearing system dynamic analyses were also completed to finalize the compressor system configuration. Finite element analysis of the compressor impeller was used to verify structural integrity and fatigue limits for selected materials. Rotor-bearing system analysis was used to define acceptable bearing locations and dynamic coefficients, system critical speeds and dynamic stability. Given the high speeds, supercritical operation, and required reliability, efficiency and freedom from contaminants, compliant foil gas bearings and seals were designed and evaluated. Since hydrogen will be used as the lubricant for the foil bearings, substantially lower power loss than oil lubricated bearings will be experienced and the auxiliary supply or scavenge system is eliminated.

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The motored engine friction torque test has been developed for the purpose of evaluating various additive formulations. Difference in frictional behavior using different additive chemistries were observed and the importance of the additive chemistry for fuel economy was indicated. Sequence VID engine testing was conducted and the correlation between the motored engine friction torque test and Sequence VID was considered. Excellent correlation between the motored friction torque tests and Sequence VID FEI 1 was demonstrated. However, excellent correlation with FEI 2 was not observed at any conditions of the motored engine friction torque test. Also, the motored engine friction torque test correlates with vehicle fuel consumption with JC08H and 10.15 modes on a chassis dynamometer test.

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This study presents the effects of the type of polymer and gas on blister fracture in terms of visualizing O-ring behavior under high-pressure gas. To visualize blisters (internal cracks) by optical microscopy, transparent EPDM and VMQ O-rings were molded, and a special viewable high-pressure vessel with a glass viewport was developed. The O-ring specimens were exposed to high-pressure hydrogen, helium, and nitrogen gases at 10 MPa under room temperature, 25°C, after which these gases were rapidly decompressed for 0.3 seconds. The blister fracture occurred in EPDM but not in VMQ even though the two materials had the same Young's modulus. It is presumed by AFM observation that the difference in microstructure at sub-micrometer level between EPDM and VMQ influenced their blister initiation. The blister damage of the transparent EPDM O-ring in nitrogen gas was the most serious, while that in the helium gas was the slightest. The reason why the blister damage of the O-ring in the helium gas was the slightest is considered to be because solute helium gas diffused out from the O-ring after decompression faster than the other gases.

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This paper describes an experimental study on quantitative evaluation of correlation between the structure formed by thickeners and yield stress of greases. In order to evaluate the thickener structures, untreated surface of greases was observed by using an atomic force microscope, and an index was obtained from AFM images to describe the degree of dispersion of the thickeners. The index physically represented both factors of shape and distribution of thickeners. The results for twelve types of lithium soap greases with different base oils and thickener concentrations showed good correlation between the index of the degree of dispersion and the yield stress of the greases. This relationship indicated that uniform distribution of thickeners in greases caused higher yield stress.

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Comprehensive interpretations of grease properties are presented. Dropping point, penetration and rheological properties are examined for a wide variety of greases involving both lithium soap greases and diurea greases. Types and viscosity of the base oils, types and content of the thickeners and manufacturing conditions are extensively varied in the greases examined. Grease exhibits the rubbery region over a wide range of frequency, indicating the network structure formed by thickener fibers. The magnitude of storage modulus, G', in the region is related to the network density of the thickener fiber in analogous to polymer rheology. Two regions are found in the dropping point of grease. In the Region A, dropping point decreases as the network density decreases. In the Region B, the dropping point is almost independent of the network density, but dominated by network collapse. Moreover, the loss modulus, G'', at non-linear regime, representing viscous dissipation of energy, is found to be a controlling factor for the penetration, and it allows for excellent correlation with the extensive body of penetration data.

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This paper describes an exploratory study on the effects of temperature on the formation of oxide film and rolling contact fatigue life in hydrogen, argon and air. Rolling contact fatigue tests were conducted at 333 K and 363 K by using a three-ball-on-disk type apparatus. The rolling contact fatigue life in hydrogen was shorter than that in argon, and life in air was the longest. Relationship was found between fatigue life and hydrogen concentration in steel. Cross sections of the specimens show that iron oxide grew to larger grain size in the subsurface in hydrogen environment, which may have resulted in shorter fatigue life. It was also found that fatigue failure occurs on ball surface in hydrogen at 363 K.

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Tribological properties of a vertically aligned carbon nanotube (CNT) film with a thickness of approximately 700 μm under millinewton and micronewton level loads were investigated using a 1 mm radius ball. At the beginning of frictional slidings, friction forces were high, and then decreased and seemed to reach relative steady state values, in both millinewton and micronewton load experiments. After the initial high frictions, top surface CNTs contacting with the counterpart ball on the CNT film were plastically deformed and bundled each other, which were observed to be flat by scanning electron microscopy (SEM). In the relative steady friction states, the friction forces basically increased with increasing loads. Under millinewton level loads, curves of the friction force as a function of the loads seemed to be parabolic. Surprisingly, although no adhesion forces were detected, frictions existed even at zero loads under micronewton loads.

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The effect of lip surface microtopography on lubrication of radial shaft seals is studied numerically. Numerical analysis of hydrodynamic flow at the lip is conducted with a simple sinusoidal wave as a lip surface asperity model. In order to understand the effects of lip surface microtopography on micro EHL, amplitude, wavelength, curvature and asperity ratio on film formation are investigated. It is found that the wavelength, the curvature and the aspect ratio significantly affect film formation and the coefficient of friction, and that coefficient of friction is influenced not only by minimum film thickness but by the average film thickness.

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TThe friction characteristics of a mechanical seal for rotary blood pumps were investigated to evaluate the volume percentage of blood in the sealing face. These mechanical seals separate blood and cooling water, so the fluid film in the sealing face is composed of two different solutions. Therefore, the volume percentage of each solution is not known. In the experiments, various viscosities of an ethylene glycol solution were used as a substitution for blood, and the friction characteristics were compared for different Ra values for the seat rings: 0.009 and 0.088 μm. These friction characteristics were investigated based on the two different assumptions: a completely mixed state and a completely separated state. As a result, the volume percentage of the sealed fluid in the completely mixed state was larger than that in the completely separated state. In fact, it was considered reasonable that the practical state in the sealing face was between two states. For example, the volume percentage of the sealed fluid showed 65-73% at a viscosity of the sealed fluid of 3.53 mPa·s and at a rotational speed of 3000 min^-1. Additionally, it was found that smaller volume percentage of the sealed fluid was evaluated for 0.088 μmRa than for 0.009 μmRa.

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A novel one-way clutch was developed utilizing self-locking property of belt and experimental study was carried out to confirm the fundamental property of the clutch. Unlike conventional couplings or clutch, the belt-type clutch works effectively even in an off-centered condition. Experiments were carried out changing the parallel distance between the shafts under constant braking torque condition. It was confirmed experimentally that the rotation of driver shaft was transmitted to the follower shaft without any slip even in some off-centered conditions. An effect of off-centered distance between the two shafts on angular velocity was evaluated experimentally and compared with the theoretical value. The fluctuation in angular velocity increases with an increment of the off-centered distance as predicted by the theory.

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Poly-vinyl-alcohol (PVA) hydrogel shows unique characteristics, especially self lubrication function and future application is expected such as components in artificial joints. One of the problems for future application is the lack of damage process information on this material. PVA-gel shows strong visco-elasticity and low friction coefficient so that it is difficult to measure volume change and measure mass change directly. Then, this study focus on the damage process itself. Surface damaged by friction was observed and a particulate analysis was performed to obtain basic information on damage process. For this purpose, followings were conducted; discriminated analysis, rectangular bounding, and elliptical shape fitting. Through these treatment, it is suggested that the smaller particle generation is the starter, and there is critical particle size remaining on the bulk surface, and elongation tendency changes during friction.

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Burwell and Strang supposed that the transition from mild to severe wear mode would occur when the total area of plastic regions on a soft surface contacting with a hard surface reached to the apparent contact area. This paper describes a modification of the theory in the previous paper which made approximately a stochastic analysis of the total area of plastic regions on a flat surface contacting with a rough surface employing the roughness model by Greenwood-Williamson (GW) and Johnson' cavity model to obtain the plastic area surrounding an asperity contact. The analytical results show that the contact severity defined by the ratio of the total plastic area to the nominal contact area is affected only by the ratio of the standard deviation of roughness summit heights to the mean radius of the summits which relates to the plasticity index defined by GW and as a result this supports the experimental results on the transition load for wear mode by Hirst and Hollander. In addition, the effect of temperature characteristics of materials hardness on the contact severity is shown and it can be seen that the softening temperature is significant under operating condition of high pV (pressure-velocity) value.

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A novel method for friction measurements has recently been developed: the so-called “relaxation tribometer”. It is based on a lubricated contact submitted to a constant normal load, with one solid being supported by a 1-D mechanical oscillator, able to return to its equilibrium position through damped oscillations. The complete analysis of the time response of amplitude during elastic recovery is compared to a simple mechanical model. This allows us to identify and quantify precisely two different contributions of friction: the linear velocity-dependent contribution (typically “viscous-type” damping, by the parameter zeta), and the solid-like contribution (typically “solid” friction coefficient at zero speed, mu). The electric contact resistance (ECR) signal, which confirms the lubrication regime, is obtained by using an electric resistance detection device. In this study, tribological properties of pure glycerol (GL) and 1,3-buthylene glycol (13BG) are characterized and compared. The ECR signal of GL gives evidence for a fully fluid contact, which is characteristic for a full film lubrication regime. Interestingly, this lubrication regime is maintained when increasing the normal load from 50 to 500 mN. For 13BG, evidence is given for a boundary lubrication regime. Emulsions with GL and 13BG are also investigated. In addition, lubrication regimes of the emulsions are found to be close to those of the raw materials (GL and 13BG). The capabilities of this new technique are expected to bring new insights into friction measurement and characterization of contacts lubricated by complex fluids.

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Biological materials that make up the body organs and tissues are soft, wet and visco-elastic. Hydrogels can mimic these aspects and show promise for bio-medical applications. Their tribological properties are very important for promising applications such as artificial cartilage and bio-models for endovascular surgery training. The present study investigates the friction of polyvinyl alcohol (PVA) hydrogel against nine different metallic and non-metallic bio-compatible engineering materials likely to occur as countermaterials in these two applications. All the materials exhibited a characteristic velocity-dependent peak friction coefficient. Comparing the peaks, we find that lowest peak friction coefficient is produced by ceramics and glass (μ < 0.05), followed by metal alloys (μ < 0.05-0.08) and highest for polymers (0.4 < μ < 1.5), including PTFE which typically has very low-adhesion. Our results suggest that to achieve low friction, polymers should be avoided as a counter-material to PVA-hydrogels. It is also shown that PVA surface roughness is critical for achieving early transition to elasto-hydrodynamic lubrication and low friction, as shown in our comparative calculations with two different values of surface roughness of the gel. In the mixed lubrication region, the general trend is for friction to increase with roughness of the counterbody, but internal damping properties and adhesion also play important role, as shown by a simple linear model fit.

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