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Failures in tribological components such as bearings can significantly affect the performance and lifespan of machinery, necessitating the implementation of effective condition monitoring technologies. This study verified the feasibility of dielectric spectroscopy (DES) as a method for detecting abnormalities before damage occurs. A comparative evaluation was conducted between dielectric relaxation parameters and measurements from size exclusion chromatography, total acid number, and viscosity for oxidatively degraded poly (α-olefin) oils. The results confirmed that DES is an effective method for assessing the oxidative degradation state of lubricants. These findings suggest that DES could be applied to oil film condition monitoring and predictive maintenance.

This study demonstrates that the robustness of friction dampers can be enhanced by using the positive velocity dependence of the kinetic friction force. First, we show that the kinetic friction coefficient can be reduced under extremely low sliding velocity conditions by using oleyl acid phosphate (OLAP) as an additive to the lubricant on the sliding surfaces between bronze-filled polytetrafluoroethylene (PTFE) and steel. Next, based on the results of the numerical simulations, we show that the use of a sliding surface with a positive velocity dependence of the friction coefficient is effective in improving the vibration reduction ability of the friction dampers. Finally, our findings are demonstrated through a simplified experiment.

Grease-lubricated rolling bearings transition from the churning phase to the channeling phase. This transition property affects grease life and torque properties. Therefore, the relationship between grease yield stress and grease degradation during operation, which affects this transition, has been investigated. However, there have been few studies on grease flow that affects the transition. In this study, the mechanism of grease reduction on the races was investigated for small bearings operated at low speeds, where thermal degradation and softening of the grease are less likely to occur. It was inferred that the grease transfer to the cage affects the channeling transition and that the amount of transfer varies depending on the initial grease viscosity. These findings can be applied to grease composition and cage design and are useful in providing bearings with excellent low-torque characteristics, such as in industrial motor applications.

Titanium alloy is widely used in different industrial applications. For the surface modification to improve the tribological properties, laser irradiation is a promising technology, which offers high efficiency, and high automation potential and geometrical flexibility. In this study, a novel method of surface modification for titanium alloys by carbonization using low fluence of laser irradiation in the atmosphere of PAO oil is proposed. Results show that the carbon content on the surfaces significantly increases with the laser shot number, with the laser irradiated spot showing little change of Ra. XPS analysis confirms that the carbon from the oil has bonded to the Titanium inside the alloy. By comparison with that irradiated without oil, the hardness of that irradiated in oil is much higher, demonstrating the feasibility of the surface modification of titanium carbide layer generation. To investigate the tribological properties, laser scanning irradiation in oil with different laser pulses were carried out to create laser modified areas and reciprocating ball-on-disk friction tests under oil lubrication were conducted. The laser modified areas show friction of 0.13, much lower than that of the unirradiated which is approximate 0.55, and the sliding lifetime of low friction is also increased with the laser pulse number. Moreover, by introducing patterning laser irradiation onto the uniformly irradiated area, the wear resistance can be further greatly improved, and the sliding lifetime can extend to 13 times of the optimal result of uniform irradiation.

Energy-resolved HAXPES is proposed here as a method to analyze boundary layer formation on rough frictional surfaces, in which the vertical OLAP boundary layer structure containing organic components is non-destructively analyzed. In addition, HAXPES analysis was performed with high incident X-ray energies to obtain information about the entire boundary layer, and to distinguish the organic and inorganic layers for calculating the effective thickness of each. The results revealed that the vertical structure of the OLAP boundary layer consists of a surface layer of organic molecules, including esters and phosphates, and an inorganic iron phosphate layer at the substrate interface, with each component exhibiting a vertical concentration gradient. Furthermore, the effective thickness of the organic layer formed under the present sliding conditions was estimated to be about 50 to 80 nm, which became thicker and exhibited lower friction at increased running-in times, and is greater than the inorganic layer thickness of 10 to 15 nm. It is reasonable to attribute the observed low-friction and velocity-dependent properties to the thicker organic layer that preferentially supports the load during friction, demonstrating that this method is useful for determining the effects of boundary layers on tribological properties.

Cylindrical roller bearings used in traction motors for railway vehicles are used at high rotational speeds and under light loads. Under these operating conditions, the life due to cage wear is much shorter than the life due to raceway fatigue. Therefore, bearing life can be extended by reducing cage wear. The authors thought that to reduce cage wear, it is necessary to establish a dynamic analysis method for the contact between the roller and the cage, and to identify the wear mode of the cage. If cage wear follows Archard’s equation, then cage wear is proportional to the impulse caused by the contact between the rollers and the cage. Therefore, in this paper, a simple model consisting only of a roller and a cage was constructed, and the impulse was obtained via dynamic analysis. The impulses calculated by the dynamic analysis were in good agreement with those measured. In addition, the experiments showed that cage wear is proportional to the impulse and revealed the wear mode of the cage. These allow the method proposed in this paper to be used to predict cage wear and to determine bearing specifications to reduce cage wear.

When rolling bearings are used under light loads and high rotational speeds, their life may be determined by cage wear rather than raceway flaking. Under these operating conditions, the life and reliability of the bearings can be improved by reducing cage wear. To reduce cage wear, it is necessary to measure and analyze the contact forces acting on the cage due to the interactions between the roller and the cage pocket. In this paper, a measurement system capable of measuring “the contact forces between the roller and the cage pocket” and “the cage center displacement” at the same time has been constructed. Measurements have been conducted using this system for a cylindrical roller bearing at various rotational speeds. As a result of measuring the contact forces, it has been found that “the force to accelerate the cage” is generated continuously in the load zone of the bearing, while “the force to decelerate the cage” is generated intermittently in the non-load zone of the bearing. Since the contact forces are not constant during one rotation of the cage, the impulses due to the interactions have been calculated to evaluate not only the contact force but also the contact time. Results indicate that the impulses per rotation of the cage caused by each force show a tendency to decrease up to 2000 min-1 of rotational speed and not to almost change at higher speeds. In addition, it has been found that the cage center displacement measured becomes a constant velocity circular motion when the rotational speed exceeds 2000 min-1. These findings suggest that the contact between the roller and the cage can be easily modeled when the rotational speed exceeds 2000 min-1. The analysis based on this model created is in general agreement with the impulses obtained from the measurement results.

Abstract Adhesion is one of essences with respect to rubber friction because the magnitude of the friction force is closely related to the magnitude of adhesion on a real contact area. However, the real contact area during sliding depends on the state and history of the contact surface. Therefore, the friction force occasionally exhibits rate-, state-, and pressure dependency. In this study, to rationally describe friction and simulate boundary value problems, a rate-, state-, and pressure-dependent friction model based on the elastoplastic theory was formulated. First, the evolution law for the friction coefficient was prescribed. Next, a nonlinear sliding surface (frictional criterion) was adopted, and several other evolution laws for internal state variables were prescribed. Subsequently, the typical response characteristics of the proposed friction model were demonstrated, and its validity was verified by comparing the obtained results with those of experiments conducted considering the contact surface between a rough rubber hemisphere and smooth acrylic plate.

In this study, we discussed the occurrence condition stick-slip model based on a 2DOF system, in which a 1DOF system model commonly used in stick-slip analysis was attached on an elastic foundation. Specifically, the effects of the mass, stiffness, and damping coefficient of the elastic foundation on the occurrence and non-occurrence of stick-slip were investigated. It was found that when the elastic foundation parameters were determined based on the optimal parameter tuning method of the dynamic vibration absorber (DVA) theory, the range of stick-slip occurrences reduced compared to the sliding system without the elastic foundation.

To enhance the cutting performance of chemical vapor deposit (CVD) diamond-coated tool, a short pulse laser grinding technique is applied. However, the thermal impact of a nanosecond laser damages the diamond crystallinity of the processed surface. To reduce this thermal impact, a femtosecond laser is innovatively used in this study to conduct the pulse laser grinding (PLG) of a CVD diamond-coated cutting tool, to achieve a sharpened tool edge with high quality. Furthermore, the CVD diamond tool edges processed by femtosecond and nanosecond lasers are compared based on sharpness, smoothness, and microstructure changes. The results show that a sufficient laser fluence higher than the threshold and a reduction in the pulse overlapping rate of the laser fluence of the femtosecond laser PLG could ensure a better tool edge shaping. A laser power of 7 W, processing angle of 20°, and scanning speed of 60 mm/s with roughness reduced to approximately half, are the suitable processing conditions of the femtosecond laser. From the observation of a scanning electron microscope, the tool edge processed by femtosecond laser PLG has a relatively sharp edge, with a radius of curvature around 1 μm, similar to that of a nanosecond laser. The further magnified images reveal a distinct processed surface characteristic. The nanosecond laser-processed surface has obvious longitudinal machining marks while that of the femtosecond laser has ablated debris. Moreover, the surface microstructure change of CVD diamond by femtosecond and nanosecond laser PLG are compared using Raman spectroscopy, further confirming that femtosecond laser could successfully suppress unfavorable structural effects in CVD diamond. Based on the results, femtosecond laser has a great potential for processing higher-quality CVD diamond tool edges.

To deepen the understanding of the frictional sliding phenomena of rough surfaces, advances are required in numerical analysis methods at various spatial scales. In this study, to examine the microscopic behavior of a rough asperity contact corresponding to a bulk contact on the macroscopic scale, a loop-type meso–macro coupled analysis scheme is proposed. A mesoscale numerical model and a macroscopic friction model are required for the proposed multi-scale analysis. A friction model was adopted based on the multipoint contact model for the mesoscale model, and the pressure- and state-dependent elastoplastic analogy friction model was used for the macroscale model. In the proposed meso–macro coupled analysis, the parameter set for the elastoplastic analogy friction model was first identified via a numerical friction test using the mesoscale multipoint contact model assuming various conditions. Then, a macroscale finite element analysis incorporating the elastoplastic analogy friction model was performed for the macroscopic analysis of contact between a rough rubber hemisphere and a smooth plate. Here, the information from the mesoscale rough surfaces were reflected in the macroscale finite element analysis. Finally, a mesoscale localization analysis was performed in which the macroscopic histories of several typical locations were obtained by finite element analysis and used as boundary conditions for the mesoscale model. It is suggested that the microscopic sliding process of rough surfaces represented by the finite element analysis can be examined using the proposed method.

Pulse laser grinding (PLG), as a cutting tool processing method, can not only achieve edge sharpening with high precision, but it can also produce surface modification. For example, polycrystalline cubic boron nitride (PCBN) tools processed by PLG can show increased hardness due to the reduction in defects. However, the mechanism of edge formation under PLG processing remains unclear. In this study, by observing the plasma generated during processing using a high-speed camera, the elementary process for each laser pulse of the PLG process was visualized. The plasma luminescence moved successively through four stages: multipoint luminescence, uniform luminescence, the downward movement of the luminous center, and faint luminescence. By comparing the results of three different laser pulse pitches (0.2, 2, and 20 μm), it was found that the pulse pitch had a significant influence on the PLG processing mode. When the pulse pitch was too small, the sidewall effect was likely to lead to local excess machining. The large pulse pitch resulted in processed surfaces that could not be fully covered by laser irradiation, and it was preferred to remove the decrease threshold subsequently. Thus, the moderate pulse pitch condition showed a superior processed surface compared to the others.

This study discussed design criteria for suppressing stick-slip using dynamic vibration absorber. First, a simplified analytical model that can quantify the effect of the dynamic vibration absorber on the occurrence and nonoccurrence conditions of stick-slip was proposed. The model comprises of two vibration systems, i.e., the primary vibration and additional mass systems. In the primary vibration system, stick-slip occurs due to the difference of static and kinetic friction acting on a sliding surface between a primary mass and driven rigid plane. In the additional mass system, an additional mass is connected to the primary mass in the primary vibration system by a linear spring and a dashpot. From the numerical simulations using this analytical model, it was verified that the presence of the additional mass is effective in suppressing stick-slip occurring in the primary vibration system. Secondary, based on the previous studies, i.e., the theories of stick-slip and dynamic vibration absorber, the discriminant equation of the boundary conditions for the occurrence and nonoccurrence of stick-slip was analytically derived. Through some numerical simulations, the validity of the discriminant equation proposed was discussed. It was confirmed that the proposed discriminant equation can accurately formulate the design guideline of the dynamic vibration absorber for suppressing stick-slip. Finally, as a conclusion of this study, the basic design strategy of the dynamic vibration absorber for suppressing stick-slip occurring in mechanical systems were discussed.

It is known that detachment waves are generated on the surface of a soft material when it is slid on a hard surface under certain conditions. In the present study, the variation in the contact area that arises when a plate of a cross-linked polymer, poly-dimethylsiloxane (PDMS), is slid in contact with a glass hemisphere has been measured every 2 ms using a microscope and high-speed camera. The measurements elucidate that the behavior of the PDMS surface under sliding friction is divided into three modes. In mode I, a detachment wave is generated and it propagates across the contact zone. In mode II, a detachment wave is generated but does not propagate perfectly. In mode III, no detachment waves are generated. The Young modulus and load control the generation of detachment waves, but the contribution of the former is approximately 8 orders of magnitude larger than that of the latter.

This study demonstrates optical measurements of a real contact area and tangential contact stiffness in a rough contact interface between a soft adhesive elastomer and a glass plate. A friction tester developed in this study employs a rough contact interface between a rough rubber plate that is made of cross-linked poly-dimethyl siloxane (PDMS) and a smooth optical glass (BK7) hemisphere. This friction tester also equips a transmission optical system that is composed of a white light-emitting diodes (LED) light source, an objective lens system, and a charge coupled device (CCD) camera, for the in situ observation of the space distribution of the real contact area in the apparent contact region. Based on a simplified analysis, in accordance to the measurements of the transmitted light intensity, the amount of the real contact area was determined without any complicated calibrations. As a result, it was found that the amount of the real contact area linearly increased with normal load. Furthermore, using the digital image correlation (DIC) method, the time changes in the surface displacement field within the apparent contact region were visualized from the onset of the sliding motion to the onset of the steady sliding. These results provided a direct estimation of the tangential contact stiffness that resulted from the tangential deformation at the contacting asperity layer.

High-speed cutting has various advantages such as high material removal rates and reduction of machining time. However, the greatest problem of high-speed cutting is its excessive tool wear rate. This is attributed to the fact that high-speed cutting causes a large cutting temperature rise and then, promotes the occurrence of tool material diffusion. In this study, we focused on extremely short-duration cutting. If the cutting duration for each edge undergoing the intermittent cutting process is designed to be considerably smaller than the time constant of thermal diffusion, the temperature rise at a tool face can be suppressed to a low value; i.e., the diffusion wear due to temperature can be reduced. In this method, since the cutting volume of a single pass is small, in order to cover the total cutting volume as in is the case of ordinal cutting, the number of cutting cycles is increased. Thus, mechanical wear will increase with a shortening of the cutting period. Therefore, to reduce the total wear volume in extremely short-duration cutting, we need to design tool edge geometries that can lessen mechanical wear and investigate the optimum cutting period in which the tool wear can be minimized. In this study, we investigated the effect of the tool rake angle and the inclination angle on the wear volume. Then, we measured the tool temperature and the wear volume for various cutting periods during a single pass and at different cutting speeds. Consequently, we found that the smaller the cutting duration, the smaller are the temperature rise at the flank face and the tool flank wear. Several experiments suggested that extremely short-duration cutting has a high potential for achieving high-efficiency metal removal.

In this study, in-situ observations of a contact interface between a transparent rubber hemisphere made of PDMS (poly dimethyl siloxane) and a smooth plate made of PMMA (polymethyl methacry- late) were performed. Using a reflective optics, the space distribution of real contact regions formed in the apparent contact region was visualized. Additionally, the normal load dependence of friction force that acts on the contact interface was also investigated. As a result, it was found that the classical Amontons' friction law breaks down in this system; friction coefficient gradually decreased with increasing normal load. Based on Persson's contact theory, we discussed the mechanism of the normal load dependence of real contact area and friction coefficient. Finally, a novel method for estimating the real contact area and shear strength in sliding friction of a rubber like material was developed. The use of this method allows to estimate the real contact area and shear strength without any optical measurements of contact interfaces.

This paper describes a novel design concept for a tool for cutting carbon-fiber-reinforced plastic (CFRP) composites. The cutting tool, which is termed a two-layer tool, was fabricated from two materials with a spatial distribution of hardness around the tool edge. In the two-layer tool, the rake face is made of a material with a relatively high wear resistance, i.e., polycrystalline diamond (PCD), whereas the flank face is made of a material with a relatively low wear resistance, i.e., tungsten carbide (WC-Co). The results of milling tests conducted with a unidirectional CFRP laminate and the two-layer tool showed that the existence of a hardness distribution works to reduce cutting forces, because the wear process of the two-layer tool develops with a constant roundness at the tool edge over relatively long cutting distances. A simplified model was developed to describe how friction force is reduced. Increasing the difference between the wear resistances of the PCD layer and the WC-Co substrate and increasing the clearance angle of the tool edge were found to be effective in reducing the cutting forces. The finding of this study will be helpful in the development of novel design concepts for extending the life of tools for cutting CFRP composites.