The Japanese Society of Tribologists organized the WTC World Tribology Congress 2009, held in Kyoto on 6-11 September 2009, in which 12 Mini Symposia were also held concurrently. Tribological Aspects of Fluid Power was one of those Symposia.

The brief scope of this Symposium, designated as Tribological Aspects of Fluid Power, was: “Tribology is the key technology of fluid power components. Not only bearings and seals but also sliding parts of hydraulic pumps and motors as well as cylinders and valves are extremely important elements of such components. Their performance and characteristics strongly affect the reliability and efficiency of those related components.”During this Symposium session, the basics and application of fluid power were discussed.

At the Symposium, lubrication models and numerical analyses for hydraulic pumps, motors, valves, and seals as well as tribological characteristics and development of hydraulic oils were presented among other topics. They were then discussed in two sessions. The papers submitted to Tribology Online have subsequently undergone a normal peer review process by multiple reviewers. In this special issue, six papers—four articles and two short communications—are published following the decisions of the authors themselves to submit their papers for peer review.

On behalf of the Symposium members, I, the Main Organizer and the Guest Editor, would like to express my sincere gratitude to the Presenters, the Participants, and the Authors for the success of this Symposium and preparing their manuscripts for this special issue. I also thank the Reviewers and the Publication Coordinators for their great efforts undertaken to complete the peer-review processes and perform the difficult and time-consuming tasks of publication. Finally, I extend my special thanks to Dr. Heinrich W. Theissen of RWTH Aachen University, Co-Organizer of this Symposium, as well as Professor Masabumi Masuko, the Editor-in-Chief of this Journal, for their expert help and thoughtful consideration.


Main Organizer and Guest Editor
Toshiharu Kazama

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In order to promote biobased lubricants, the German government has supported the conversion of existing or new equipment from mineral oil to biobased oil through financial grants, technical advice, and publicity, during the years 2001 through 2008. The author has been responsible for providing technical support and scientific evaluations to the program.
95% of the conversions were mobile hydraulic equipment and waterway installations. In an overview, market data and specifics are given, as well as typical properties of biobased hydraulic fluids, cost calculations, results of contamination experiments, and technical instructions for conversion of equipment.

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Hydraulic fluids of new concept, oily high bulk modulus fluids, have been developed. The new fluids are expected to improve hydraulic system performances. Unique properties under high pressure of the fluids have been studied.

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In order to investigate the influence of a single tribo system on the ageing of lubricants, a so-called Tribo-Oxidationtest has been designed at IFAS. It consists of a disc on disc tribometer integrated into the pressure vessel of an oxidation test according to ASTM D 2112. The coefficient of friction μ of different friction partners is discussed in this paper. The attempts are done with biological esters which are synthesized in a collaborative research centre. Due to biological aspects they contain no additives. It has been shown that there is an influence of oil ageing on friction coefficient but also a big influence of viscosity on friction coefficient.

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The spools in most all the hydraulic spool type control valve have several circumferential grooves to prevent well known hydraulic locking problems which result in high friction force and excessive wear. In this paper, a commercial computational fluid dynamics (CFD) code, FLUENT is used to investigate the effects of groove sectional shapes and its sizes on the flow and lubrication characteristics of single grooved hydraulic spool valve. The streamlines, velocity and pressure distributions in the groove, and leakage flow through the clearance are obtained for various groove depths and sectional shapes. For shallow groove, the streamlines and leakages are highly affected by groove shapes and depths. However, for relatively deep groove, the pressure distribution and leakage are nearly uninfluenced by groove depth. It is newly found that there occurred vortices inside groove beyond a certain ratio of groove depth to width. The vortex can play as contaminants collector so contribute to reduce abrasive wear. The numerical method adopted in this paper and results can be used in optimum design of spool valves.

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Examinations have been carried out to determine suitable coatings and fluids for hydraulic displacement units. Based on investigations of the tribological system piston / bushing of an axial piston pump unit tests have been carried out with geometrically optimised components with PVD-coated surfaces while using environmentally friendly fluids. The aim of these unit tests was the determination of efficiency, performance of the tested oil as well as its ecological properties depending on time.

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A thermohydrodynamic lubrication (THL) model of a hybrid (hydrostatic and hydrodynamic) thrust bearing is developed. It is applicable to a slipper of swashplate-type axial piston pumps and motors. The generalized Reynolds equation, three-dimensional energy equation, and the heat conduction equation are derived. Physical properties such as density, viscosity, specific heat at constant pressure, thermal conductivity, and thermal expansivity of a hydraulic oil are considered as functions of temperature and pressure. The effects of the operating conditions on the temperature rise, clearance shape, and the power loss are shown. The numerical parameters are specified for the fluid-a hydraulic oil with ISO VG 46-supply pressure 7-21 MPa and rotational speed 15-60 rps. The solutions between the slipper model and the circular hydrostatic thrust bearing as well as between the THL and isothermal (ISO) solutions are compared. Increases in the supply pressure, rotational speed, and the revolution-radius increase the maximum temperature and the power loss. Furthermore, the discrepancies between the THL and ISO solutions increase. The rotational speed affects characteristics more than the supply pressure.

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