A thermo-hydrodynamic lubrication (THL) theory is applied to journal bearings that support large-sized high-speed rotary machineries and it is confirmed that the maximum pad surface temperature under operation is below an allowable limit value. The temperature predicted by the traditional THL model is in good agreement with the measurement. However, the predicted journal surface temperature is not comparable with the measurement. In the present paper, it is firstly shown that the amount of heat, which is generated by the shear flow in the oil film and is discharged out of the film, is evaluated appropriately in the film near the pad surface while it is evaluated excessively in the film near the journal surface. Secondly, a new THL model is proposed that the oil film temperature near the pad surface and the temperature near the journal surface are obtained independently, that is, the conventional energy equation for the oil film is solved for the former temperature while the equation described in the relative coordinate system with the journal surface as the reference one is used for the latter temperature. Lastly, it is shown that the THL model can predict the journal surface temperature comparable with the measurement without deteriorating the accuracy of the predicted maximum pad surface temperature.

In situ observation of lubricant films formed with greases was carried out at EHL conditions with a micro-FTIR. Three urea compounds were used as a thickener and 12 mass percent of the thickener was added in PAO, respectively. From the IR spectra, CH and NH stretching vibration peaks were observed from urea greases. The film thickness and the concentration of thickener were estimated from the absorbance of CH and the ratio of absorbance of NH and CH. Two-dimensional distribution of the thickness and the concentration around Hertzian contact was obtained at the resolution of 50 μm. The thickener of urea grease was concentrated on ball and disk surfaces. It was found that the concentrate on thickener at Hertzian contact region was dependent on thickener itself. Time dependence of the thickness and the concentration at Hertzian contact were obtained at the resolution of 45 s under shearing conditions. The film thickness of urea greases at Hertzian contact was greatly decreased by shearing and the film thickness and the concentration were recovered when the condition was changed to rolling. This behavior was supposed with detachment and growth of the adhered film of the thickener. Lubricating characteristics of urea greases were discussed based on the concentration of thickener at Hertzian contact.

The Japanese Society of Tribologists is pleased to announce that the Best Paper Award of the Society was awarded to the following papers from Tribology Online:

◊ “In Situ Raman Observation of the Graphitization Process of Tetrahedral Amorphous Carbon Diamond-Like Carbon under Boundary Lubrication in Poly-Alpha-Olefin with an Organic Friction Modifier,” by Hikaru Okubo, Chiharu Tadokoro, Yuki Hirata and Shinya Sasaki, Tribology Online, Vol. 12, No. 5 (2017) 229-237.

◊ “Study and Comparison of Lubricity of Green and Commercial Cutting Fluid Using Tool-Chip Tribometer,” by Suvin Parayantayyathu Somarajan and Satish Vasu Kailas, Tribology Online, Vol. 13, No. 6 (2018) 340-350.

and

◊ “A Study of the Lubrication under Impact Loading - Experimental and Analytical Application to Push Belt CVTs,” by Yuki Ono, Kenji Matsumoto and Yuji Mihara, Tribology Online, Vol. 14, No. 2 (2019) 18-23.

Congratulations!

The Award Medals and the Certificates were mailed to all the recipients instead of presented by the president of the Japanese Society of Tribologists at the award ceremony in the JAST Annual Meeting, which was supposed to be held in Tokyo on 26th May, 2020. We regret that the award ceremony was cancelled due to COVID-19.

This paper presents the tribometric characteristics analysis of a journal bearing system lubricated with ferrofluid (FF) considering different shaft materials. Two magnetically permeable materials namely, EN-19 and mild steel were used to manufacture the journal. These journals were made magnetic by inserting neodymium magnet inside. It was seen that the magnetic shaft made from EN-19 offered a better dimensional accuracy in comparison with the magnetic shaft made from mild steel. Thus, magnetic shaft made from EN-19 was used in a brass bearing to evaluate various tribometric characteristics namely, fluid film pressure, fluid temperature rise, bearing vibrations, friction torque and wear at various conditions. The experimental results showed that the maximum pressure enhanced up to 84% in the case of FF based bearing system in comparison with conventional lubricant based system. Also, the film pressure was measured for newly acquired FF and the three years old FF to check the stability of the FF. It was found that almost no change in the pressure profile was observed. Further, the temperature rise was found to be less in FF based system while marginal difference in damping characteristic was observed. Moreover, tribometric characteristics such as friction and wear were also investigated and compared between FF based and oil based bearing system. A significant drop in friction torque was found in the case of FF based system. In addition, the wear loss due to the machine starts and stops was measured in lubricated condition for several systems using weight loss and stylus method. However, wear loss was found to be marginally increased in the FF based system, in comparison with the low viscous conventional fluid based system. It has been clearly established that the FF based system presented here, may turn out to be a better option from efficiency and stable system point of view.

Mechanical seals are composed of two annular flat rings in contact and relative motion to ensure sealing of a rotating shaft. Because of friction in the sealing interface, a significant temperature rise can be experienced in the contact. Two decades ago, it has been shown that creating a network of shallow dimples on one sealing surface can help to create a lubricant film in the interface and thus reduce friction. Since then, many research works have been carried out on the so-called surface texturing, showing the interest of surface modification for mechanical seals. In the present work, several surface patterns, defined by numerical simulations and machined by plasma etching on stainless steel rings were tested. The rings were mounted on a test bench in which they slide against a sapphire disk counter face. This disk is transparent to infrared radiation and allows interface temperature measurements by infrared thermography. It is shown that all the tested surface texture patterns exhibit a temperature rise at least 2 times lower than with flat smooth surfaces. A slight difference between the different dimple shapes is obtained indicating that the temperature and friction are more controlled by texture area and surface roughness rather than by the texture pattern.

In order to attain high recording density in a hard disk drive, reduction of the head-media spacing (HMS) is crucial. The contribution of lubricant thickness to the HMS is a significant factor in performance. Because the lubricant thickness is on the order of angstroms, molecules of long-chain lubricants, such as perfluoroether, must lay parallel to the magnetic disk surface to reduce lubricant thickness. In this study, new lubricants were designed using ionic liquids (ILs). The ILs had smaller molecular weight than that of current perfluoropolyether (PFPE) lubricants but with lower evaporation loss. The molecular conformation of the ILs was controlled by two hydroxyl groups, which were placed in a cation moiety at both terminal ends (IL-1) or placed on one side of the cation (IL-2). The monolayer thickness of IL-1 and IL-2 was seen to be 0.6 nm and 1.2 nm, respectively. The adhesion force at a lubricant thickness of 1.0 nm decreased in the order: PFPE (Z-Dol 2000) > IL-2 > IL-1. The small adhesion force, which is one of the essential parameters for low flight of a magnetic head, was achieved by changing the molecular conformation of the IL.

Using a bearing tester that can control the relative positional relationship between the shaft and the bearing in parallel, the seizure and friction property of the combination of a-C:H:Si DLC coated journal and the aluminum alloy bearing were verified. In case of the shaft rotation speed was 5200 rpm under the commercial 0W-20 engine oil at 110°C lubrication, the coefficient of friction of the steel journal rose sharply and seizure occurred when the pressure reached 113 MPa. In contrast, no seizure occurred with DLC journal at 121 MPa of the upper limit of the device. As a result of the observation of the sliding surface after the seizure, the large wear and the deposition of Sn which was seemed to be melted were confirmed for the bearing combined with the steel journal. The surface roughness of the DLC journal and bearing was not changed clearly after the test. This is probably due to the poor affinity with aluminum of DLC. Also, the EPMA result showed that Mo, Zn and P elements in lubricant were clearly not detected in both steel and DLC journal. Also, in the EHL region, the friction coefficient of DLC journal was reduced up to 10% co MPared with that of the steel journal.

The aim of this study is to investigate the effects of applied loads and temperatures on the tribological properties of MBS oil, which is a bio-oil extracted from banana peel waste of Musa Aluminata Balbisiana, MBS. Tribological evaluation of MBS oil was conducted using pin on disc tribometer as per G 99 ASTM standard. In this method, hemisphere pin was loaded against the rotating disc containing the MBS oil . The test was implemented by dropping 2 ml of MBS oil as a lubricating oil on sliding surface at varying loads from 20 to 100 N at 27°C, 40°C and 100°C. The results showed that, at 80 N, the coefficient of friction (COF), volume wear loss (V_loss), wear scar diameter (WSD), and wear rate (Ws) values were lower at all the tested temperatures. Microscopic analysis revealed that, the above results is due to the formation of tribo-chemical film which existed as protective layer on sliding surface thus preventing metal to metal contacted each other, hence contributed to the favor of frictional reduction.

Volume and issue: Vol. 10 , No. 1 (2015)

Page: pp. 56-63

Title: Lubricities of Environmentally Acceptable Lubricants with Zinc Dialkyldithiophosphate and Dibenzyl Disulfide on Tribological Properties of Plasma Electrolytic Oxidation Coated A6061-T6 Alloy under Mixed/Boundary Lubrication

Author(s): Takuya Nakase, Shinji Kato, Mathias Woydt, Shinya Sasaki

The paper focuses on examining agreement of the adaptive smoothed particle hydrodynamics (ASPH) in the investigation of the sliding friction of silica at micronscale throughout observation of several friction characteristics. It is found that the ASPH approach well presents the friction of micronscale silica due to agreement of the friction coefficient and the applied load-friction coefficient relationship between the present results and the previously experimental reports. The shape of the particle modeled in the ASPH almost does not effect on the detected results for the hard system due to the very slight variation of the particles during the sliding. However, the variation of the particles can explain for the discrepancy between the stick time and the slip time and the unsharp change between the stick state and the slip one. The study is also extended for the contacts of the two sinusoidal rough surfaces and finds that the friction coefficient is almost independent of the wavelength while it linearly increases with the amplitude.

Numerous published studies on the non-wettable surfaces with surface texture and slip conditions have been reviewed comprehensively to evaluate the performance of hydrodynamic bearings from last two decades. The recent numerical and experimental studies have been explored to observe the individual and combined influence of surface texture and slip. A variety of texture shapes and optimized slip zone profiles have also been discussed in this article. It has been observed from the literature that a suitable combination of modified slip and the conventional no-slip zone is effective in improving dynamic and static performance of hydrodynamic bearings. Many experimental studies also revealed that micro-scale textures are significantly capable of improving the tribological performance. Moreover, slip boundary condition is more beneficial at low and medium eccentricity ratios.

In this study, the reciprocating pin-on-plate sliding test was conducted between ultra-high molecular weight polyethylene (UHMWPE) and CoCrMo alloy with test lubricants containing representative synovial fluid constituents, such as proteins, hyaluronic acid, and phospholipids, to understand their effects on the in vivo friction and wear characteristics of implanted joint prostheses. To explore some details of the friction and wear process, the wear track remaining on the UHMWPE plate specimens was morphologically analyzed by laser microscopy and chemically analyzed by Fourier transform infrared (FTIR) spectroscopy. The influence of each constituent on the rheological properties of the test lubricant was also evaluated using a rotational rheometer. We observed that proteins had a definitive impact on the friction and wear process; proteins adsorbed on the UHMWPE surface and increased friction and wear by changing the predominant friction and wear mechanism from abrasive to adhesive. Phospholipids also increased friction while hyaluronic acid decreased friction by increasing the viscosity of the lubricants and enhancing the fluid dynamic effect. However, when phospholipids and hyaluronic acid were mixed with proteins, they enhanced the entrainment of the protein molecules into the contact area and increased the wear of UHMWPE.

The current research focused on the tribological properties of flake-like WS₂ particles dispersed into PAO₄ base oil with oleic acid as surfactant. Various concentrations of 0.2 mass%, 1 mass% and 3 mass% were applied in the preparation of WS₂-OA lubricant. The wear tests were carried out using a ball-on-plate reciprocating sliding test configuration. It was found that the highest COF of 0.578 was obtained using PAO₄ base oil with significant fluctuations. The COF could be reduced to approximately 0.34 and stabilized after adding the OA. When introducing the WS₂ articles, the COF was around 0.158 under 0.2 mass%, 0.143 under 1 mass% and 0.145 under 3 mass%, and became almost constant after 4 min. In addition, the highest wear rate in width of 61.5 µm/m and in depth of 2.2 µm/m was found using PAO₄ base oil. The wear rate can be reduced to 51.6 µm/m in width (16.1%) and 0.9 µm/m (59.1%) in depth under 3 mass% OA. While using the WS₂ particles, the wear rate in width could be further reduced to 45.2 µm/m using 1 mass% but with slightly higher wear rate in depth than that of OA. Based on the analysis in SEM and EDX, the abrasive grooves became less after introducing the OA, some dark spots composed of carbon acted as tribo-films on the worn area. While using WS₂ particles, it could be found that WS₂ flakes were adhered on the surfaces to reduce the friction and wear.