Tsunamitsu Nakahara

Adhesion between rail/wheel under water lubricated contact

This paper describes the behavior of adhesion coefficient, i.e. the maximum traction coefficient in the rolling direction between rail and wheel under a water lubricated condition for various values of rolling speed, slip ratio, contact load, surface roughness and water temperature by means of a simplified theory. The adhesion coefficient has been analyzed here by combining the elasto-hydrodynamic lubrication (EHL) theory and the contact theory of rough surfaces. The results show that the adhesion coefficient is greatly influenced by the water temperature and asperities height of rough surface.

Relationship Between Temperature Distribution in EHL Film and Dimple Formation

This paper describes an experimental study on dimple formation under elastohydrodynamic lubrication (EHL) conditions. The oil film thickness between a ball surface and a glass disk was measured using optical interferometry, and the temperatures of both the surfaces and of the oil film averaged across it were measured using an infrared emission technique. It was found that the temperature profile across the oil film varies abruptly along the sliding direction, and the Couette flow decreases due to the viscosity wedge action as the oil flow is close to the dimple zone. The maximum temperature rise in the dimple zone sometimes reached over 400 K, thus, the phase transition of the oil from liauid to glass may not occur.

Experimental investigation of effects of slip ratio on elastohydrodynamic lubrication film related to temperature distribution in oil films

Abstract This article describes the mechanism of variations of elastohydrodynamic lubricated oil film under high slip ratio conditions. Experiments were conducted using a ball-on-disc apparatus. The thickness of the oil film was measured by optical interferometry. Temperature increase in the ball surface, disc surface, and oil film was determined by an improved infrared technique using a transparent disc without any coating and non-transparent disc with a comparatively thick Cr coating. The results reveal that the film thickness decreases with an increase in the slip ratio at constant entrainment velocities. The shape of the oil film indicates an increase in the thickness gradient at the inlet region and a slight increase in the oil film just behind the exit region. The temperature at the inlet region increases with the slip ratio and entrainment speed. Both the inlet region and the contact area exhibit a significant temperature difference between the surfaces. The variation of the shape of the oil film can be explained on the basis of the viscosity wedge action. The viscosity wedge action, heating at the inlet region, and inflow of heat from a slower surface to the inlet region reduce the film thickness considerably.

Measurements of Temperature Distributions around Longitudinally Grooved Rough Surfaces in Sliding Elastohydrodynamic Point Contacts

This paper describes the temperature measurements in the EHL conjunction area comprising a longitudinally grooved steel ball and a sapphire disk under high slip conditions. The authors measured the temperatures of the oil film as well as both the disk and ball surfaces; furthermore, they estimated the temperature profile across the oil film by means of experimental values. The experimental results show that the temperature of the grooved ball surface increased considerably compared with that of a non-grooved ball. The temperatures of the faster surface for the grooved ball became sensitive to the slip ratio, whereas that for the non-grooved surface was almost constant. The temperature distribution had a higher value at the land zones and a lower one at the grooved zones. The temperature rise in the grooved zones varied qualitatively depending on the thermal conditions of both the sliding surfaces.

Method of determination of truncation parameters from measured surface profile

Abstract The probability density function of the roughness height of a sliding surface is not always Gaussian like that of a truncated surface caused by running-in or mild wear. Therefore, it is important for obtaining contact pressure or frictional characteristics to estimate the truncation level of the non-Gaussian distribution function. This paper presents a method of determination of the two truncation parameters in the truncation model presented by King et al. [Proceedings of the 4th Leeds–Lyon Symposium on Tribology, MEP, London, 1978, p. 333]. The two truncation parameters p and β can be determined by plotting the values of skewness Sk and kurtosis K obtained from a measured profile of surface roughness on the Sk–K diagram calculated with the truncation model for various given values of parameters p and β. The height distributions reproduced by the truncation model with the truncation parameters p and β identified by the present method is in good agreement with the original ones of the measured surfaces.

A model of seizure based on Burwell and Strang's concept of wear mode transition

This paper offers a simplified model of seizure process that provides a foundation to develop the effective friction control in tribo-systems in one of a great National R&D program to prevent global warming that has been presented by Japanese Ministry of Economy, Trade and Industry and is supported financially by New Energy and Industrial Technology Development Organization. The seizure is defined as severe wear mode in this paper and thus the critical condition of seizure is that of the transition from mild to severe wear mode. In the severe wear mode, plastic deformation called ‘wedge’ by Cocks or ‘prow’ by Antler is considered to take place in considerably large area due to the combination of contact pressure and shear stress. The concept, with which Burwell and Strang accounted the effect of load on the transition of wear mode, is employed for the critical pressure. The hardness distribution, which relates the critical pressure, is assumed in a simple function estimated roughly from the temperature distribution in the direction of depth from surface. The relation between seizure and the product of contact pressure and sliding velocity, i.e., pV value, is discussed.

Measurements of Two-Dimensional Distribution of Refrigerant Concentration in EHL Film Using Micro FT-IR and Effect of Variation of Concentration on Oil Film Thickness

The dissolution of refrigerant into a lubricant causes a decrease in viscosity of the oil and it gives a large effect on the lubrication of sliding parts in a refrigerant compressor. This paper describes an application of micro FT-IR to measure the two-dimensional concentration distribution of refrigerant held in solution in the EHL film surrounded by the refrigerant gas and discusses the refrigerant concentration variation in the vicinity of the Hertzian contact area. In order to measure the concentration distribution, an apparatus which can observe the EHL film in a point contact in the refrigerant atmosphere was developed. The refrigerant concentration was measured using micro FT-IR through a CaF2 window from outside of the apparatus with polyol ester as a base oil in an atmosphere pressurized with HFC-134a refrigerant gas. The results indicate that the concentration of HFC-134a refrigerant reduces in the inlet boost region of EHL contact and the Hertzian contact area but in the side region of Hertzian contact area it is greater than that in the bulk fluid. In addition, the effect of the variation on the oil film thickness is discussed.

Analysis of Traction Curve in Linear Region Considering Volume Viscoelasticity

Measurements of the gradient in the linear region of traction curve, where the effects of heat generation and nonlinearity of shear viscosity can be neglected were made for two kinds of synthetic oils, DOP and 5P4E, using a traction tester improved to a precision less than 0.01 percent in slip ratio. The measured results were compared with the theoretical results based on a viscoelastic model for shear stress and on an elastic model for compressibility using primary data of the rheological properties which were obtained not by means of traction test; they were markedly less than the theoretical results under low speed for both oils and also under high speed for DOP. A new analysis of the traction was performed considering volume viscoelasticity for compressibility which relates shear viscoelasticity under high pressure. The theoretical results agree fairly with the measured results.

The influences of the fluid capacitance in the oil feed line system on the transient response of hydrostatic guideways

Abstract A machine tool table supported by externally pressurized guideways can be frequently found to exhibit an underdamped response to squeeze loads when the supply pressure for the hydrostatic guideways is changed by means of the restrictor or the pressure regulator in the pressurized oil feed line in order to obtain suitable bearing characteristics. It is known that the resulting overshoot is largely affected by the oil feed line between the restrictor and the recess, because the oil feed line has some fluid capacitance induced by the compressibility of the pressurized oil, the elastic deformation of the pipe and the intermixing of air into the overall system. To clarify the influences of the fluid capacitance in the oil feed line on the dynamic characteristics of the hydrostatic guideway, the overshoots resulting from step loads have been investigated. Experiments were carried out with a simple model hydrostatic guideway system having a pipe line with large fluid capacitance and the changes of oil film thickness have been measured. From the experimental results, it is shown that the overshoot ratio which is defined as the ratio of the overshoot to the steady displacement of table increases with increasing fluid capacitance in the pipe line and shows a maximum value when the oil film stiffness is a maximum. In the case of constant bearing load condition, the maximum oil film stiffness can be found when the value of pressure ratio pr/ps is not equal to 2 3 but 1 3 , where pr is the recess pressure and ps is supply pressure. To confirm these experimental results, the motion of a table supported on an externally pressurized oil film has been studied theoretically. The results are in good agreement with the experimental results and show that the overshoot rate is governed by a dimensionless characteristic Tc which is the ratio of static oil film stiffness Ks to compressibility stiffness Kc defined as Kc = A2e/Ca and dimensionless static oil film stiffness K s is defined as K s = K s /(W/h) , (where, Ca is the fluid capacitance in the pipe line between the recess and the restrictor, Ae is the effective area of the pad, W is the bearing load and h is the oil film thickness). The term Tc determines the overshoot rate and the overshoot appears in the case when Tc > 1. Furthermore, in order to indicate that the optimum pressure ratio pr/ps is equal to 1 3 in the case of constant load, the relationship between the oil film stiffness Ks and the pressure ratio pr/ps considering the various bearing conditions have been calculated and it is shown that there are four optimum conditions.

The Effect of Interaction Between Neighboring Asperity Contacts on Wear Mode Transition in Sliding Contact of Rough Surfaces

Concerning the final aim, that is to make clear the mechanism of the wear mode transition from mild to severe belong to the increase of contact pressure in adhesive wear, estimation of the yielding condition in subsurface under contact boundary between rough surfaces was attempted in this study. Especially the effects of interaction between neighboring contacts as well as relative sliding on the yielding area were focused. The contacts between asperities were modeled by the contact between two neighboring hard wedges with the parallel axes and soft plane to be simplified. These models were calculated by commercial FEM solver in 2-dimension. Additionally, the effect of the difference between the heights of two wedges on the yielding region was evaluated by comparing the result with that obtained from the simple model of two wedges with the same height in non-sliding contact. The results showed that the effect of the interaction between adjacent contacts and existence of relative slip motion give significant factors to the yielding state, on the other hand, the height difference between neighboring asperities affects little the determination of the yielding region.Copyright