Bearings are one of the essential mechanical elements indispensable for machine tools. In particular, hydrostatic bearings are high motion accuracy and static stiffness. In addition, it is the most significant advantage of hydrostatic bearings that the copying principle be negligible. Therefore, many ultra-precision machine tools are equipped with hydrostatic bearings. First, this commentary will introduce water hydrostatic bearings characteristics and design methods compared to oil hydrostatic bearings. Next, we will introduce the structure, specifications, and performance evaluation of the developed rotary table equipped with constant flow water hydrostatic bearings. Finally, we will introduce an example of actual processing using this rotary table.

A saturated journal bearing is defined as a bearing that operates in the saturated vapor pressure of the lubricant, which is 1 kPa for water at room temperature. When the ambient pressure of a water-lubricated bearing is lowered to the saturated vapor pressure, a large vaporous cavitation is generated on the bearing surface. The bearing is cooled by the latent heat of evaporation. Also, the bearing loss is significantly reduced due to low viscosity of the vapor in the cavitation. The structures of a saturated water-lubricated bearing are described. Nonlinear vibrations such as rigid rotor whip are observed at high-speed rotation. Since they occur as Hopf bifurcations of a two-dimensional dynamical system, it requires approaches different from those for a one-dimensional forced vibration system.

Phenolic bearings have been used as water lubricated resin bearings for the main shaft of hydroelectric power turbines since around 1970, contributing to the reduction of environmental pollution. They enable improved power generation efficiency and compact equipment design due to their low coefficient of friction and high load capacity under water lubrication. Recently, a new phenolic bearing with dramatically small dimensional changes due to water absorption, which has been a problem, has been developed. The new material shows excellent self-lubricating property not only under fluid film lubrication conditions but also under boundary lubrication conditions where it is difficult to form water lubrication film when a turbine starts and stops.

In this commentary the potential of two types of partial slip water lubricated thrust bearings, which generates a pressure due to the discontinuity of shear flow rate between a slip region and a non-slip region, is introduced. The first is a combination of a water-repellent region and a hydrophilic region, and then a combination of a porous region containing bubbles and a dense region. Both partial slip bearings function as the bearing when the film thickness is increase and the friction coefficient is decrease with the addition of this simple bearing structure. Further, even for a conventional bearing having a wedge-shaped bearing surface, it can be expected to improve the bearing characteristics by replacing a certain region with a slip surface, if the pressure gradient is low.

2-Methacryloyloxyethyl phosphorylcholine (MPC) has a molecular structure consisting of a phospholipid (phosphorylcholine group) and a methacloyl group. The polymerization of methacloyl groups results in MPC polymers, which have a chemical composition similar to that of cell membrane surfaces and thus provide biocompatible surfaces with excellent protein adsorption inhibition and anti-thrombogenic properties. It has been applied as a coating agent for artificial blood vessels, artificial hearts, stents, artificial joints, and contact lenses. When it was applied to an artificial joint, it was reported that MPC polymer films have not only biocompatibility but also have good wear resistance and high lubricity. This lubricity is explained to be due to the hydration lubrication achieved by water retention in the MPC polymer, but the detailed mechanism has not been elucidated. This review article presents an experimental study that quantified the dynamic viscoelasticity of a nanometer-thick MPC polymer film in a hydrated state for elucidating the lubrication mechanism. Nanorheological measurements developed by Itoh et al. were used for the measurements. Such a measurement method is expected to become a fundamental technology for establishing low-friction technology by coating hydrated polymer thin films.

High performance lubricants based on next generation basestock technology can help achieve the emission reduction targets and improve fuel economy/energy efficiency. Poly-alpha-olefins (PAO) are synthetic basestocks which have been used in a variety of high-performance lubricants and greases since the 1960ʼs, and its performance has continuously been improved through manufacturing technology upgrades. In this paper, the performance benefits of a novel low viscosity low volatility (LVLV) PAO technology are introduced. Compared to other PAO products, LVLV PAO provides with an excellent balance of viscosity and volatility, improved oxidation stability, low temperature fluidity and traction properties. LVLV PAO can enhance the flexibility of lubricant manufacturers when developing high performance low viscosity automotive lubricants, such as 0W-8 engine oil and next generation driveline fluids.

Many studies on friction of rubber materials revealed the effect of power spectrum of surface roughness on friction. On the other hand, quantitative comparison of the effects of specific surface roughness parameters has not been sufficiently conducted as well as the effects of rubber deterioration on friction. In the present paper, the effect of rubber deterioration on static friction and its variation with surface profile (hard chromium (HCr) treatment, salt-bath nitrocarburizing (SBN) treatment, and gas nitrocarburizing (GN) treatment is clarified. The friction test results showed interesting results that the static friction coefficient of SBN surfaces decreased with rubber degradation, whereas the static friction coefficient of GN surfaces increased. Subsequently, we analyze the main characteristics of surface roughness parameters and explore the mechanism. First, due to the small skewness of the GN surface, the blockage of leakage channels is developed, resulting in the formation of oil reservoirs and a squeeze effect due to micro-slip of the rubber. Second, the surface of SBN has high peaks and the bearing area ratio of the peaks is high, which prevents the occurrence of leak-channel blockage and allows the peaks to penetrate rubber. The present paper specifically clarifies that these surface properties provide the obtained friction results.

Focusing on HFO (Hydro Fluoro Olefin) refrigerant, which is expected to be applied as a green refrigerant, the effect of R1234yf on the tribological characteristics was compared with HFC (Hydro Fluoro Carbon) refrigerant R32. Tribological tests were carried out with and without a model lubricant containing triethyl phosphate as an EP additive. R1234yf showed a lower friction coefficient and better wear resistance than R32. XPS analysis of lubricated tracks revealed that metal fluoride was formed from R1234yf and R32 and higher amount of metal fluoride was observed on the track formed under R1234yf than that under R32. The frictional characteristics of R1234yf were affected by refrigerant pressure and temperature of specimen. The higher the pressure of refrigerant and the temperature of the specimen, the better the tribological characteristics were observed. In order to clarify adsorption of refrigerant on metal surface, scratching tests were carried out under refrigerant atmosphere using a mass spectrometer. Although R32 did not adsorb, R1234yf adsorbed on nascent steel surface formed by scratching. The adsorption rate of R1234yf increased linearly with scratching speed. Since propylene also adsorbed on nascent steel surface, π-electrons in R1234yf play an important role on the adsorption. It can be concluded that R1234yf adsorbs easily on nascent steel surface resulting in formation of iron fluoride which acts as a boundary lubrication layer.