In this study, spur gear driving tests were conducted with the ultimate goal of developing a calculation method for predicting tooth flank wear. To determine how initial roughness of tooth surface affects tooth wear, three types of test gears were manufactured and subjected to wear tests in which only the tooth surface roughness was different and other parameters, such as surface hardness, were kept as identical as possible. The tooth profile of each test gear was measured using a gear measuring machine. Wear was quantified by comparing the tooth profiles before and after testing. In this paper, regression analysis was used from the experimental results to search for a formula that expresses the relationship between wear, tooth surface roughness, and number of meshing cycles. First, the relationship between the wear rate and the number of load cycles was formulated. Next, the wear rate and the initial roughness of the tooth surface were formulated. Then, the two formulas were combined to construct an empirical formula. Finally, the values obtained from the formula were compared with the experimental results to verify the validity of the proposed formula. This empirical model is expected to be useful for predicting the amount of tooth wear from the tooth surface roughness and usage time.

A smart gear system capable of non-contact monitoring of the characteristics of sensors formed on plastic gears was proposed. However, the effect of the changing internal structure of the plastic gear during operation on the monitoring antenna had not been understood. In this study, the effects of various changes occurring during the running test of a PA6 gear was investigated, which has no embedded circuitry, on the frequency characteristics of an adjacent monitoring antenna. First, a heating test was conducted on the PA6 gear to clarify how changes in gear surface temperature during running test influence the frequency characteristics of the monitoring antenna. Next, an intermittent running test was performed, during which the internal molecular structure at the stress concentration region of the PA6 gear was analyzed using FT-IR, and the capacitance of a capacitor utilizing the plastic gear as a dielectric was measured with an impedance analyzer. This allows us to elucidate the effects of increasing driving time on the molecular structure and dielectric properties of the PA6 gear. Finally, continuous running tests were conducted to measure the time-dependent changes in the frequency characteristics of the monitoring antenna. Based on the results of the heating and intermittent running tests, the mechanisms were discussed through which prolonged operation influences the frequency characteristics of the antenna. The results revealed that, immediately after the start of running test, the rapid increase in gear surface temperature raised the dielectric constant of the gear, causing a decrease in the peak frequency of the monitoring antenna. Subsequently, the repeated loads applied during running test led to the change of amide bonds of the gear material, reducing the dielectric constant of the gear. Consequently, the peak frequency of the monitoring antenna gradually increased with the number of meshing cycles. This study suggests the potential for detecting prediction of fatigue failure in PA6 gears by sensing various operational changes through an adjacent monitoring antenna.

This study investigated the influence of parasitic capacitance on the progression of electrical pitting damage in motor ball bearings. To simulate this effect, capacitors with values of 0, 0.1, 1, and 10 µF were connected in parallel between the inner and outer rings of a thrust ball bearing. The tests were conducted under controlled conditions: a constant load of 5 N, a rotational speed of 450 rpm, a DC voltage of 6.1 V applied across the bearing, and a total test duration ranging from 500 to 505 hours. During the tests, the charging and discharging behavior of the system was monitored, and the surfaces of the bearings were examined using a confocal microscope after the experiments. The results showed a clear correlation between the capacitance value and the severity of surface damage. Ridge marks—typical indicators of electrical pitting—became more distinct as the capacitance increased. No significant ridge marks were observed when no capacitor was connected. However, as the capacitance increased to 0.1, 1, and 10 µF, the formation of ridge marks became increasingly prominent. This trend is attributed to the greater energy stored and released during each discharge cycle as capacitance increases, which intensifies the electrical pitting effects on the bearing surface. These findings highlight the critical role of parasitic capacitance in accelerating electrical pitting and provide important insights for the design and protection of motor systems against bearing damage caused by electrical discharge phenomena.

Endurance tests of plastic gears are typically conducted under continuous operation, similar to metal gears, and their fatigue strength is evaluated based on the results. Since the intermittent operating conditions in actual products are highly diverse, evaluating load-carrying capacity through this continuous test is a practical approach. However, due to factors such as the high temperature dependence of plastic material properties, the lifetime under continuous operation may differ significantly from that under intermittent operation. Therefore, if the lifetime of plastic gears used in actual products with intermittent operation can be estimated, this information can be effectively utilized in the design process. To this end, we first developed an endurance test system capable of arbitrary intermittent operation. Then, we presented examples of various operating conditions during relatively simple intermittent operation. Second, as a first step toward estimating the actual lifetime of plastic gears used in intermittent operation similar to actual products, we investigated the effect of the operating time per intermittent operation on gear lifetime. Experiments were conducted under conditions where applied torque, rotational speed, operating time, and idle time remained constant, with only the rotation direction changing. As a result, under the limited intermittent operating conditions of this study, we confirmed that shorter intermittent operating times lead to reduced lifetimes. Furthermore, experiments were conducted in which the gear material and load conditions (rotational speed and applied torque) were varied. As a result, we found that the lifetime ratio, defined as the intermittent operation lifetime divided by the continuous operation lifetime under the same load conditions, is an effective index for relatively easily estimating these different lifetimes. This result suggests that the lifetime of a gear in intermittent operation, where only the direction of rotation changes, can be estimated from only endurance test results obtained under continuous operation conditions.

With the shift toward electric vehicles, automotive power transmission gears are increasingly required to operate under high-load and high-speed conditions. In particular, friction and wear phenomena occurring at high sliding velocities on gear tooth surfaces, as well as ghost noise induced by surface characteristics, have become important concerns. In general, gear vibration exhibits clear periodicity, and time-synchronous averaging (TSA) is widely employed for vibration analysis. The TSA signal mainly reflects macroscopic information, such as variations in tooth-profile geometry. In contrast, recent studies have focused on analyzing the residual signal obtained by removing the synchronous component from the original signal. In such approaches, the stochastic properties of the residual signal are treated as second-order cyclo-stationarity (CS2), which provides microscopic information related to fine surface asperities. However, second-order cyclo-stationary analysis has been mainly applied to gear tests conducted under low- and medium-speed operating conditions, and its application to high-speed gear tests involving high sliding velocities has been considered difficult due to sensor sensitivity limitations. To clarify the key features and limitations of CS2 analysis under high-speed operating conditions, this study applies second-order cyclo-stationary analysis to gearbox acceleration signals acquired during high-speed gear scuffing tests. In addition, in this study, to extract vibration components in frequency ranges above 10 kHz, where sensor sensitivity significantly decreases, the slope of a CS2 evaluation indicator (ICS2) during the test was introduced. This evaluation was applied to scuffing test results obtained at rotational speeds of 8,000–10,000 rpm using gear pairs with different finishing conditions. As a result, a good correlation was observed between the sliding-induced excitation frequency associated with scuffing scars and the CS2 analysis results. This finding indicates that second-order cyclo-stationary analysis is applicable to high-speed gear vibration for relatively large surface asperities, whereas detecting finer surface features, such as machining marks, requires higher-sensitivity sensors.

This study experimentally investigates the influence of lubricant kinematic viscosity on the initial wear behavior at the pin–bush interface of a bush chain. Although lower viscosity is generally associated with increased wear, the wear progression during the initial wear stage in actual bush chain contacts has not been sufficiently clarified. Wear tests were conducted using additive-free oils with kinematic viscosities ranging from VG22 to VG1000 in order to isolate the physical effect of viscosity under mixed and boundary lubrication conditions. The maximum wear depth on the pin surface was evaluated using axial surface roughness measurements. Oil film thickness was estimated based on elastohydrodynamic lubrication (EHL) theory, and the film thickness ratio was calculated using the composite surface roughness of the contacting surfaces. The results show that wear depth decreases with increasing lubricant viscosity during the initial wear stage. Although the calculated film thickness ratio reached values corresponding to hydrodynamic lubrication, measurable wear was observed, indicating that the actual lubrication regime remained predominantly mixed or boundary lubrication. These findings clarify the role of lubricant viscosity in early-stage wear progression and provide experimental insight for improving durability and predictive wear modeling of bush chain systems.

Ultrasonic cavitation peening is one of surface treatments to improve surface hardness. In the ultrasonic cavitation peening, the surface was treated using shock waves generated when the cavitation bubbles break. In this study, we have applied the ultrasonic cavitation peening to aluminum alloy bolts in order to improve its fatigue strength. The ultrasonic cavitation peening was conducted to the root of the first thread of the aluminum alloys A5056 and A6061 bolts in ethanol. In the fatigue tests, after we applied the tensile force 2 kN for A5056 bolt and 4 kN for A6061 bolt at first, the cyclic load was applied to the bolts with the stress ratio R=0.1. Vickers hardness and surface roughness R_z of the root of the first thread were also measured after the ultrasonic cavitation peening. Results showed that the fatigue strengths of the aluminum alloys A5056 and A6061 bolts which were treated the ultrasonic cavitation peening became higher than those without the peening. However, the excessive ultrasonic cavitation peening has a negative effect on the fatigue strength although the ultrasonic cavitation peening is effective in improving the fatigue strength. This is because that the fatigue strength depends not only on the peening but also the surface roughness after peening.

An effective approach to tooth-profile expression using normal polar coordinates has been proposed by Watanabe in 1949, but not applied to practical design. This method represents the tooth profile using the variables obtained by drawing a normal line from any point on the tooth profile and finding the intersection with the operating pitch circle—that is, the length of the normal line from the point on the tooth profile to the intersection, the inclination angle of the normal line, and the coordinate value of the intersection. Previous research has shown that it is relatively easy to obtain and analyze the generated tooth profile by applying normal polar coordinates to external gears. In this research, this method was applied to internal gears including non-involute gears, and a method to analyze the trimming problem was proposed. First, for each rotation angle of the pinion cutter, the distance from the line passing through both centers of the internal gear and the pinion cutter to the farthest point on the pinion cutter and the closest point on the internal gear was calculated. These values could be calculated from the variables of the normal polar coordinates. Next, these two values were compared for each rotation angle of the pinion cutter, and if there was a point at which the distance to the farthest point on the pinion cutter was greater, it could be determined that trimming would occur. This method was applied to a specific arbitrary tooth profile to obtain the limit number of pinion cutter teeth at which trimming would not occur when machining an internal gear with a specific number of teeth.

Impingement of a cavitating jet on a solid surface causes erosion, resulting in problems in fluid machinery. To avoid degradation and breakdown of the machinery, cavitation erosion must be reduced, and the erosion damage should be estimated. In this study, which focuses on hydraulic equipment, erosion experiments and damage measurements were conducted using a cavitating jet rig and solid and hollow cylindrical aluminum-alloy specimens (20 mm diameter and 20 mm height). Some hollow specimens with side holes drilled across the center hole were prepared. The upstream and downstream absolute pressures were set to 10.1 and 0.2 MPa, respectively; the test liquid was hydraulic oil with a viscosity grade of 32; and the oil temperature was maintained at 40°C. Erosion mass and surface damage were measured using a precision balance and an X-ray computed tomography (CT) scanner. The hollow specimens showed less erosion on the impingement surface but exhibited partial damage on the inner surfaces of the holes, whereas the side holes sometimes showed opposite effects. The precision balance was advantageous due to its ease of use, but local evaluation was difficult. By contrast, measurement using the X-ray CT scanner was beneficial for internal observation, but operation was difficult. The precision balance and X-ray CT scanner proved ideal for measuring the whole mass loss and evaluating partial volumetric changes, respectively. Nonetheless, the erosion losses determined by both measurement approaches coincided except under specific conditions.

The purpose of this study is to clarify the meshing mechanisms of all gears conjugated with involute cylindrical gears, including configurations with nonparallel axes, and to bring these mechanisms into practical application. The authors previously reported that face gears have straight meshing contact lines of action in three-dimensional space, along which the meshing point between an involute cylindrical gear and conjugate gear moves at a constant speed along the straight line-of-action. These straight lines of action are continuous and collectively form a curved surface, referred to as the meshing contact surface of action. The unified theory proposed in this report extends the meshing-contact-surface-of-action based principle from face gears to configurations with arbitrary shaft angle and crossed angles. Based on this unified theory, we present a method for deriving the tooth surfaces of bevel-shaped conjugate gears generated using inclined shaft angles. The resulting tooth surfaces are compared to those obtained through numerical tooth generation analysis, confirming a perfect match within the effective contact area. By applying proposed unified theory, gears with arbitrary shaft angle and offset can be designed, enabling mechanical designers to arrange gears within power transmission systems at optimal angles during the mechanism design process.

To comprehensively verify the safety of driving operations by automated driving systems, a vehicle-in-the-loop simulation that combines a chassis dynamometer and a sensor emulator was devised. This approach ensures equivalence using actual vehicles and guarantees reproducibility and coverage through simulation. In this study, we aimed to reproduce the real-world vehicle behavior, including roll, during bench testing with respect to steering. To achieve this, we constructed a system that allows steering on a chassis dynamometer using actual tires, thereby eliminating the need for a model to be developed. We then examined the impact of the test vehicle restraints on vehicle roll.

This paper presents the development of a wearable, passive shoulder–elbow assistive exoskeleton that incorporates a remote center of motion mechanism and utilizes a gravity compensation–based approach to assist workers. Physical workers in factories, construction sites, farms, and similar environments are commonly at risk of injuries and physical disabilities, and consequently, various assistive devices have been developed to prevent injury and improve work efficiency. Compared with other body parts, less attention is paid to upper-limb assistive devices, which results in inadequate assistance and insufficient injury prevention. Therefore, a wearable, low-cost, and lightweight upper-limb assistive device that can be used in multiple scenarios is highly desirable. To achieve this, we first analyzed the anatomical structure and assistive torque requirements of the shoulder and elbow joints, explored several mechanical design strategies for passive assistance, and ultimately selected a practical design. Electromyography experiments were conducted with healthy participants to evaluate the effectiveness of the exoskeleton. The experimental results demonstrated that the proposed exoskeleton significantly reduced users muscular effort. This study highlights the potential of a passive upper-limb assistive exoskeleton to provide functional assistance in everyday scenarios without requiring electrical actuation. The proposed exoskeleton offers a promising foundation for future developments in upper-limb assistive exoskeletons.

Injection molding is a highly efficient method for producing plastic gears. However, the effects of weld lines formed during the injection molding process on the characteristics of gear teeth have not yet been sufficiently clarified. In this study, injection-molded polyacetal gears were investigated, and the characteristics of teeth located near gates and weld lines were comparatively evaluated. Tooth helix deviations were measured at multiple transverse sections along the tooth height, and tooth helix deviation networks were subsequently constructed to visualize and comparatively evaluate the relative deviation characteristics among tooth surfaces. In addition, the molecular structure near the surface of the gear side faces was analyzed using the FT-IR/ATR method, and the hardness in the subsurface region was measured by nanoindentation. Furthermore, gear running tests were conducted to investigate the distribution of crack initiation sites associated with tooth root bending fatigue in teeth located between the gate and weld positions. The results revealed that teeth located directly above weld lines exhibit tooth helix deviations with phase characteristics different from those of teeth located at gate positions. Moreover, the molecular orientation of teeth above weld lines differs from that of other teeth, and a reduction in hardness within the subsurface region was confirmed. In addition, selective tooth root crack initiation was observed in teeth located above weld lines in the injection-molded polyacetal gears, whereas, for gears subjected to hobbing after injection molding to remove the surface layer of the tooth flanks, the distribution of tooth root crack initiation sites tended to become more dispersed. These results indicate that the surface layer of tooth flanks located directly above weld lines contains critical surface-layer characteristics associated with weld-line-induced molecular orientation and subsurface softening.

In high-throughput gear manufacturing, traditional inspection methods, such as magnetic particle inspection, ultrasonic testing, and penetrant testing, are unsuitable owing to their reliance on manual processes and specialized equipment. To address these problems, we propose a laser-based non-contact system for rapid gear surface scanning. A single-channel laser sensor captures reflection data from rotating gears, enabling efficient damage detection and vertical position measurement. In addition, the challenge of precise two-dimensional damage localization along the vertical and horizontal directions is addressed. Specifically, we introduce a two-dimensional diagnostic strategy, during which the system repeats measurements along diverse laser paths across the tooth flank. By analyzing variations in these measurements, horizontal damage positions are inferred along with vertical positions. This approach enables precise mapping of damages, such as pitting on any tooth flank region. Moreover, a convolutional neural network trained on paired measurement data and actual damage coordinates predicts the corresponding two-dimensional locations from one-dimensional data. Unlike conventional deviation-based methods, the proposed system substantially improves the horizontal positioning accuracy. Experimental results suggest that the integrated system combining rapid laser scanning and two-dimensional diagnostics provides a scalable solution for real-time gear inspection in automated production lines.

Fluid-film journal bearings are widely used as essential components in rotating machinery due to their high damping properties. In recent years, operating journal bearings under starved lubrication has been considered as a strategy to reduce bearing losses from the perspective of energy efficiency. However, precisely evaluating dynamic behavior is essential to achieve a balance between energy efficiency and reliability. In this study, the nonlinear vibration characteristics of a flexible rotor system supported by a circular journal bearing were investigated under starved lubrication. First, the Elrod-Adams model was used to analyze the dynamic characteristics when the rate at which oil was supplied was reduced. This model is based on the conservation of mass, and can represent ruptures and reformation regions of the oil film under starved conditions. We adopted the shooting method to investigate the onset speed of instability (OSI) induced by the journal bearing accurately. This approach is an analytical technique used to determine periodic solutions and evaluate their stability. The results confirm the previously reported tendency of the OSI to increase with reduced oil supply, and additionally show that the strength of the stability decreased. This implies that the OSI of a given system may be inversely lowered by destabilizing forces originating from other turbomachinery elements such as impellers or turbines.