• Tribology and Lubricants
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• v.34 no.4
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• pp.132-137
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• 2018

Ultrasonic waves are used in various applications in multiple devices, sensors, and high-power machinery, such as processing machines, welders, and cleaners, because the acoustic vibration frequencies are above the human audible frequency range. In ultrasonic machining, electrical energy at a high frequency of 20 kHz or more is converted into mechanical vibration by a vibrator and an amplifier. This technique allows instantaneous separation between a tool and a workpiece during machining, machining by pulse impulse force at the time of re-contact and minimizes the minute elastic deformations of the workpiece and machine tools due to the cutting effect. The Al7075 alloy used in this study is a typical aluminum alloy with superior strength that is mainly used in aircrafts, automobiles, and sporting goods. To investigate the optimal conditions for machining aluminum alloy using ultrasonic vibration, the present experiment utilized the Taguchi orthogonal array method, and the coefficient of friction was analyzed using the characteristics of the Taguchi technique. In ultrasonic friction and abrasion tests, the changes in the friction coefficient were measured in the absence of ultrasonic vibrations and at 28 kHz and 40 kHz. As a result, the most considerable influence on the friction coefficient was found to be the normal load, and the frequency of ultrasonic vibrations increases, the coefficient of friction increases. It was thus confirmed that the amount of wear increases when ultrasonic vibration is applied.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.19 no.5
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• pp.38-44
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• 2020

Surface texturing of micro-dimples has been used in many manufacturing industries to reduce friction between two sliding contacted surfaces. Surface texturing decreases the frictional force owing to minimizing of the sliding contact area. In this paper, micro-dimples have been fabricated on an Al6061-T6 surface using a two-frequency elliptical vibration texturing (TFEVT) method. A high-frequency of 18 kHz and low-frequency of 250 Hz were applied to an elliptically-vibrated tool holder. The Stribeck curve was plotted to analyze the friction coefficient trends. Furthermore, the representative wetting index, such as the water contact angle (WCA), was measured by considering the friction coefficient. WCA is associated with micro-dimple density and associated parameters. Consequently, the dimpled surfaces with a low friction coefficient exhibited a relatively high WCA in the feed direction. According to the Stribeck curve, the dimpled surfaces demonstrate superior friction performance for mixed-film lubrication compared to the non-textured surface.

• Journal of the Korean Society for Railway
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• v.12 no.1
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• pp.72-80
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• 2009

The tribiological characteristics such as friction coefficient, friction stability, wear rate and braking temperature between various types of metallic friction materials and heat resistant steel disk, were investigated by using lab-scale dynamometer. Friction materials for high speed train have higher friction coefficient and friction stability as compared to aircraft friction materials even though friction materials for high speed train have lower wear rate. In addition. Cu-matrix friction materials have higher temperature increase rate than Fe-matrix friction materials. All friction surfaces have Fe-base oxide layer after completing test.

• The Journal of Korea Robotics Society
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• v.13 no.3
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• pp.174-181
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• 2018

For a mobile robot that travels along a terrain consisting of various geology, information on tire force and friction coefficient between ground and wheel is an important factor. In order to estimate the lateral force between ground and wheel, a lot of information about the model and the surrounding environment of the vehicle is required in conventional method. Therefore, in this paper, we are going to estimate lateral force through simple model (Minimal Argument Lateral Slip Curve, MALSC) using only minimum data with high estimation accuracy and to improve estimation reliability of the friction coefficient by using the estimated lateral force data. Simulation is carried out to analyze the correlation between the longitudinal and transverse friction coefficients and slip angles to design the simplified lateral force estimation model by analysing simulation data and to apply it to the actual field environment. In order to verify the validity of the equation, estimation results are compared with the conventional method through simulation. Also, the results of the lateral force and friction coefficient estimation are compared from both the conventional method and the proposed model through the actual robot running experiments.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.20 no.2
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• pp.122-126
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• 1984

Tube drawing process is more sensitive to the friction coefficient but it is a hard task to analyze it's disposition qualitatively. This paper aims to calculate the mean friction coefficient from author's direct equation which amended from general rod drawing process, and compare the result with SACHS' or WILLIAM'S. From this study, the thickness change was inspected in multi-pass drawing. The thickness affected with the mean friction coefficient and the thickness was deformed more thicker than initial thickness on the condition of good lubrication. For the sake of a good estimation of mean friction coefficient, the radial force of die must be considered when the drawing process has some more area reduction about more than 10 percents.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.10b
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• pp.155-156
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• 2002

During the sliding between a-CNx and $Si_3N_4$, applying nitrogen as environmental gas provided very low friction as the level of 0.01 in friction coefficient. In order to know the effect of the running-in process on the reduction of the friction, the effect of surface roughness of mating surface on friction was investigated. It was shown that smooth surface in wear scar of ball provided low friction coefficient. Friction coefficient after running-in was proportional to the Ry value of wear scar of ball. Also smooth thin transferred layer was observed on the wear scar of balls with an AFM after sliding test. Those results showed the smoothing of wear scar of ball, the generating of the transferred layer from CNx was necessary for low friction.

• Journal of the korean Society of Automotive Engineers
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• v.12 no.4
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• pp.56-65
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• 1990

Wet-friction materials have been widely used for clutches and brakes of automotives over past several decades. In order to enhance its performance, its friction behaviour should be fully understood. It is, however, still not at hand and therefore an attempt was made to have some more understanding of friction behaviour of wet-friction materials. Measurements of coefficient of friction were made with the variation of lubricants, lub. temperature, sliding velocity, and contact pressure. In addition, the effects of both the viscosity of lubricants and the porosity of materials on the coefficient of friction were also investigated. It can be concluded that the coefficient of friction is decreased as the concentrations of the resin and inorganic fillers are increased, and it tends to decrease with the increase in the lubricant temperature and sliding velocity.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1998.10a
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• pp.144-148
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• 1998

Using a test apparatus developed Lrt the laboratory, frictional characteristics of natural rubbers have been analyzed by experimental study. Friction coefficient has been calculated from the measured normal force and friction force under various speeds, loads, and temperatures. The relations between the various operating conditions and friction coefficients have been verified. Especially, drag friction due to the visco-elastic behavior of the rubber has been observed in this analysis.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.945-950
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• 2004

Surface texturing of tribological application is another attractive technology of friction reducing. Also, reduction of friction is therefore considered to be a necessary requirement for improved efficiency of machine. In this paper attempts to investigate the effect of density for micro-scale dimple pattern on bearing steel flat mated with pin-on-disk. We demonstrated the lubrication mechanism for a Stribeck curve, which has a relationship between the friction coefficient and a dimensionless parameter for lubrication condition. It is found that friction coefficient is depended on the density of surface pattern. It was thus verified that micro-scale dimple could affect the friction reduction.Lubrication condition regime has an influence on the friction coefficient induced the density of micro dimple.

• Earthquakes and Structures
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• v.11 no.1
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• pp.25-43
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• 2016

The uneven distribution of rolling friction coefficient may lead to great uncertainty in the structural seismic isolation performance. This paper attempts to improve the isolation performance of a spring-damper-rolling isolation system by artificially making the uneven friction distribution to be concave. The rolling friction coefficient gradually increases when the isolator rolls away from the original position during an earthquake. After the spring-damper-rolling isolation system under different ground motions was calculated by a numerical analysis method, the system obtained more regular results than that of random uneven friction distributions. Results shows that the concave friction distribution can not only dissipate the earthquake energy, but also change the structural natural period. These functions improve the seismic isolation efficiency of the spring-damper-rolling isolation system in comparison with the random uneven distribution of rolling friction coefficient, and always lead to a relatively acceptable isolation state even if the actual earthquake significantly differs from the design earthquake.