Former Chief, Metallic Materials G., Materials Technology Development Div.,
Technological Development Dept., RTRI
Currently, Chief-Engineer, Railroad Vehicle G., Roller Bearing Technology Dept.,
Bearing Engineering Center, NSK Ltd.
"Tribology" is defined as "a science and technology concerning two planes in relative motion and interference with each other, and several problems and practical applications". It was defined in such campaigns as "to avoid troubles and wastes caused by lubrication, and save national economy" in England in 1966. It is noteworthy that the definition refers to practical applications and involves technological viewpoints. It includes the boundary fields between mechanical engineering, physics, chemistry, metallurgy and other disciplines. The objects of tribology are to control friction, traction and lubrication, and reduce surface damage such as wear, seizure and contact fatigue.
2.Tribological problems in railways
Many problems, which coincide with the above-mentioned purpose, are observed in the railway technology in Japan, which has remarkably developed since JR companies were established. They are actively promoting development of technologies based on their management strategy. It is thought that safety, speed-up, maintenance, preservation of environment and saving energy are common themes. The tribological problems are also seen in their themes on concrete. There are a number of important problems in speed-up and maintenance fields, in particular.
In railway systems, there are various parts accompanying a tribological phenomenon. Characteristics of typical tribological problems in the railway fields, for example, wheel - rail system and trolley wire - pantograph system, include high-speed operation and diversity of environment exposed to air, water and contamination. Materials such as carbon steel, copper alloy etc., used in those interfacial systems are common and inexpensive, because they need large quantities, and must be economic and easy to acquire.
This paper describes an outline of tribological problems, mainly concerning with the speed-up and maintenance, for each component of railways, trolley wire - pantograph system, components of rolling stock and wheel - rail system. Safety is a common and basic theme. It is needed to ensure safety at accidents and calamities in recent years. For environmental preservation and saving energy, it is thought that railways are relatively advantageous as compared with other transportation modes. Important subjects include decrease of wear loss in various morphological forms and recycle of worn parts.
1)Tribology of trolley wire - pantograph system
It is important to prevent wear of contact strip by arc during current collection and corrugate trolley wires. It is a key subject to ensure dynamic stability of the system for speed-up. For this purpose, the strength of trolley wire has been increased. Improvement of anti-arc properties and reduction of weight of sliders are also useful. For those purposes, high strength composite contact wires, high strength precipitation hardened copper alloy contact wires and metallized carbon sliders have been developed. Efforts are required to find an optimum combination of contact wire and slider materials.
2)Tribology of rolling stocks components
To ensure running stability at high speeds, experimental and computational examinations of bogie have been done for lightening bogie weight, changing the bogie structure and developing new components. Machine elements, such as gear units, couplings, bearings, dampers and resilient rubber etc., are investigated to improve their performance. In the future, it will become more important to maintain stable performance.
The main target in the brake field has been to develop a technology corresponding to speed-up and improve the durability of friction materials. The improvement of brake performance and durability of cast iron shoe has been achieved by using alloys and compound materials. In the disk brake system, the improvement of durability for heat burden of brake disk has mainly been promoted and, at the same time, an optimum combination of brake disk/ lining materials has been inspected including introduction of non-asbestos lining. After this, it is prospected that many kinds of light weight composite materials will come into use to develop new brake systems, in order to cope with speed-up.
For bearings and gears, weight reduction and performance improvement have been needed for speed-up of rolling stock. Manpower saving and mechanization of maintenance work have also been strongly needed. Taking those two viewpoints into consideration, for example, the form of axle journal bearing has been changed from cylindrical roller bearing to taper roller bearing, susceptibly involving an alternation of lubrication method for the case of the latest high-speed rolling stock. To establish a new journal bearing inspection method without dismantling it, a diagnosis method using vibration for the traction motor bearing as a main object and an evaluation method for rolling bearings by acoustic emission technique have been applied. It is desired to improve the reliability of inspection standard based on the past experience, when new inspection methods are applied. As there are some cases where bearings are replaced mainly due to electrical pitting, it is needed to apply an insulated bearing and a countermeasure for anti-electrical pitting in drive control systems. Recently, the replacement standard of roller bearing is reappraised. The development of countermeasure for anti-seizure of plane bearing is an important problem for the maintenance of freight cars.
The improvement of heat and oxidation resisting lubrication oil is strongly needed to correspond to speed-up. As a diagnosis method for condition monitoring of machine elements by using lubrication oil, on the other hand, an analysis method of wear particles in lubrication oil, especially such as ferrography, is highly expected. To ensure resistance against the heat generated by stirred lubricants, laboratory and field tests are being promoted to change the base oil from the conventional mineral oil to a synthetic oil. Researches are also being promoted to improve characteristics of grease by using different thickeners. The theme of ferrography is arrangement of experiential and verification data to judge abnormal value.
3) Tribology of wheel-rail system
It is a key technology for speed-up to ensure the adhesion between wheel and rail. The adverse effects of contaminants, water and other substances on adhesion have been clarified. Technologies to effectively control and improve adhesion have progressed. As part of those effects, researches are being promoted to improve adhesion by jetting ceramics particles and utilize adhesion in the slipping zone by effectively combining high-precision slide detection and brake control methods. It is well known that flats on wheel tread decrease remarkably when slide detection apparatus are used, though they haven't yet completely been eliminated, because there are various causes. To avoid occurrence of flats, it is necessary to find an optimum method from the viewpoints of wheel maintenance, train operation, rail conditions and materials as well.
Extending the life of rail is an important theme of tribology concerning maintenance. As the material quality of rail has been improved in recent years, rail defects caused by material faults are decreasing, while those related to laying and use, namely welding part fracture, shelling, corrugation, head check and other defects are steadily increasing. For shelling and corrugation, investigation of mechanism of occurrence, based on the available knowledge makes a theme, establishing practical countermeasures, such as development of rail material, optimum profiling and repairing methods etc., are required. Prevention of corrugation is needed from the viewpoint of environmental protection, in particular.
3. Concluding remarks
As shown above, there are a number of important tribological problems of railways mainly for speed-up and reduction of maintenance. We are making efforts to solve these problems to continue improving railways into the 21st century.