Interface - The Journal of Wheel/Rail Interaction

RAIL GRINDING & RE-PROFILING

Wheel Re-Profiling and Rail Grinding Strategies on Wiener Linien (Part 1 of 2)
By Edgar Fischmeister, Markus Ossberger, Roman Pongracz and Paul Mittermayr • October, 2007

Rail grinding and wheel re-profiling are essential elements of track and vehicle maintenance. These maintenance procedures have been shown to play a major role in the removal of surface defects or irregularities, and in controlling the shape and surface conditions of wheels and rails. Although the benefits are well known, in many cases, they have not been translated into daily practice.

Many urban transit systems that provide metro and tramway services continue to grind rails, for example, based on pre-determined maintenance plans, rather than in response to a specific rail condition or strategy to optimize or improve wheel / rail interaction. Pre-determined grinding plans tend to concentrate on removing short- and medium-wavelength corrugations, and undulating deformations of longer wavelengths. Most of the grinding equipment used on urban transit systems does not incorporate non-contact measurement technology and, therefore, is unable to identify and remove longer wavelength corrugations. Nor can the grinding equipment provide any consistent comparison of the pre- and post-grind rail conditions.

While interchange railways operate a wide range of vehicles with wheels in varying maintenance conditions, urban and suburban passenger systems typically operate a captive fleet of vehicles, about which, wheel wear patterns and conditions are well known. In some cases, wheel wear can be controlled by operating specific vehicles on specific lines. Wiener Linien, Vienna's public passenger transport organization, realized the need to match vehicle-to-track interaction when it created the Rad-Schiene-Ausschuss ("wheel/rail committee") 20 years ago. Initially, work focused on establishing a dialogue between technical units to promote a mutual understanding of the issues. Today, efforts are directed toward matching the technical units' maintenance strategies. The committee's efforts have resulted in a well-defined rail grinding and re-profiling strategy that is based on properly matched wheel and rail profiles.

As part of its wheel / rail optimization program, Wiener Linien examined the development of corrugation on various track designs. A system of highly resilient discrete supports installed in tight (110 m) curves on the U2 metro line was shown to reduce secondary airborne noise generated by vibrations. This system also virtually eliminated stick-slip action and corrugation in these curves. Seeing the influence that the track-form had on inhibiting the development of medium-wavelength corrugations, the Austrian Federal Railways (ÖBB), the Bureau of Applied Mechanics and Mathematics (BAMM) and the Ingenieurbüro für Angewandte Technologie mbH (IAT) launched a "track dynamics" project to investigate the phenomenon.

Data collected from 296 sensors that were installed to measure extension, acceleration and displacement in varying track-forms (including ballasted track with wooden sleepers, ballasted track with concrete sleepers, ballast-less track with plastic sleepers, and highly resilient discrete supports), indicated that the type of track-form had a significant influence on the development of medium-wavelength corrugations. Medium-wavelength corrugations developed in conventional ballasted and non-ballasted track-forms, and, most severely, in unpadded concrete sleepers embedded in ballasted track. Corrugations did not develop, however, in track with highly resilient discrete supports. The tests also revealed the benefits of separating the outer rail from the inner rail in the sleeper region — an approach that led to the use of "bi-bloc" sleepers in newly built metro tracks.

Profile Optimization Program
While specialized approaches to dealing with corrugation or other wheel / rail problems were helpful, Wiener Linien recognized the need for comprehensive measurement of all portions of the system. In order to do so, it purchased two non-contact, track measurement vehicles — one for the tramway system and one for the metro system (see Profile Optimization in the Urban Rail Context). Data from these measurement vehicles, which record track geometry and rail profile data every 50 - 100 cm, enabled Wiener Linien to examine the interaction between "traditional" wheel and rail pairings and actual wheel and rail profiles on the system. An unworn "old" standard profile, several stages of worn profiles and a "new" wheel profile were paired with rail profiles collected on the individual metro and tramway lines and assessed, using equivalent conicity as a suitable parameter.

Hollow-worn wheels represented one of the major wear categories (see Figure 1). Hollowing, which is common to railways around the world, is caused by two mechanisms: particle removal and plastic deformation. The task of tire material is to have worn particles lift off without any other negative impact, such as the early development of radial crack nuclei, on the tires. Appropriate alloys, hot rolling and heat treating produce a structure in which wear particles scale off almost horizontally without leaving micro-cracks perpendicular to the tread. This process coincides with plastic deformation of "tire material close to the surface," meaning that the material will "flow," slowly but steadily, in a lateral (axial) direction away from the wheel / rail contact patch area toward the wheel flange and the outer face of the tire. ("Tire material close to the surface" refers to tire steel down to a depth of about 4 mm, i.e. the depth where the material structure is still influenced by mechanical stresses.) The wheel tread in a cross-section of a Wiener Linien metro tire is worn hollow between the flange root and the outer long bevel of the wheel profile (see the area bracketed in green in Figure 1) after 150,000 - 200,000 km, and approximately 1.5 - 2 years of service.

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