In the first of a two-part article on railway axlebox design, we look at the evolution of this key railway subsystem from an historical perspective. As a company with a long tradition of developing axlebox solutions, SKF has always addressed environmental issues such as energy conservation and lubrication saving for this market.
Since their earliest use in railway applications, axlebox bearings have offered energy and lubrication saving opportunities. Today most railway vehicles are equipped with increasing advanced designs based on wheelset axlebox assemblies, comprising the wheelset bearings or bearing units, the axlebox housing and integrated sensors. SKF now serves the railway market with additional solutions such as condition monitoring and service packages including service engineering, remanufacturing and training.
The essential design concept for railway wheels has remained unchanged throughout railway history. The assembly of two railway wheels and an axle is commonly called a “wheelset,” which rotates and is supported by bearings that are called “axlebox” or “journal” bearings. These are housed in axleboxes or supported by special adapters that are connected directly or via springs with the running gear, in most cases designed as a bogie. An axlebox is one of the safety-critical subsystems in railway vehicles.
Through the application of tribology, the study of friction, lubrication and wear, engineers and scientists have learned much about the interaction of surfaces in relative motion. Examples of applied tribology can be found in transportation. For railways, the humble beginnings of wheel and axlebox designs has evolved through early antifriction axlebox bearings to the latest development of highly sophisticated axlebox bearing units and complex solution packages covering bearings, seals, lubrication, mechatronics (e.g., sensors to detect operational parameters) and a comprehensive range of services.
The first railways
One of the early energy-saving examples is the Linz/Austria – Budweis/Czech Republic horse railway, mainly built for the transportation of salt, which was then very expensive. The 80-mile-long line was opened in 1832. At that time it was by far the world’s longest railway connection. Fig. 1 shows that a rail system could carry eight to 10 times the load of a road transportation.
Some early patents exist, but there is no evidence that they were all applied. Three-axle passenger cars, launched in 1903, were one of the first well-documented antifriction axlebox bearing applications. The cars were equipped with axleboxes, each of which incorporated two deep groove ball bearings (fig. 2a). The traction effort for a two-car set with a total weight of 33.15 tons was 4.4 kN with sliding bearings and only 0.62 kN with ball bearings, which is a reduction of 86% (fig. 2b). The bearings and axleboxes were manufactured by Deutsche Waffen- und Munitionsfabriken A.G. (DWF) in Berlin, Germany. This company later became part of the Vereinigte Kugellagerfabriken (VKF), which in turn was acquired by SKF.
A further test was carried out in 1905 by Professor Graham of the Syracuse University in New York. He conducted research into energy consumption in the form of a comparison field test of two trams, the first equipped with sliding bearings and the second with roller bearings (fig. 3a). Energy consumption over the route for the tram using sliding bearings was 6.45 kWh; compared to 3.10 kWh for the tram with roller bearings – an energy saving of 52% (fig. 3b). In 1907, the Syracuse Rapid Transit Cooperator told the Standard Roller Bearing (SRB) Co. in Philadelphia that after 4.5 years of operation and some 250,000 miles, the roller bearings showed no wear. The annual saving in coal to generate the electrical power needed was $260 per year per vehicle, equal to about 14 oz of gold. The Standard Roller Bearing Co. later became part of the Marlin Rockwell Corporation (MRC). SKF acquired MRC in 1986.
Meeting the need for speed
Speed has been the essence of railways since the first steam locomotive made its appearance in 1804. SKF remains at the forefront of high-speed train design, providing some of the most safety-critical components of railway vehicles – the wheelset axlebox assemblies, comprising the wheelset bearings or bearing units, the axlebox housing and integrated sensors. SKF has always been active in developing solutions to meet the challenging requirements of high-speed train builders and operators, for the development, design and testing of wheelset bearings (figs 4a and 4b). By the 1930s, trains in Europe and North America had already reached traveling speeds of about 80 mph, with top speeds of 100 mph. Today high-speed rail transportation is defined in some European standards as vehicles with a maximum speed of more than 125 mph.
In addition to energy-saving cap-abilities, further contributions to the environment can be achieved by reducing the amount of lubricant consumption. Bearing lubricants such as oil and grease have to be refined from mineral oil. During maintenance, after many years of long service, the used lubricant has to be collected during axlebox dismounting and specially treated as waste disposal, like other used mineral oil-containing products. It is obvious that a minimizing lubricant quantity is a positive contribution to the environment.
At the beginning of rail transportation oil-lubricated sliding bearings were used. The initial oil fill in the axleb