INTRO: Use of automatic wheel monitoring on DSB’s S-bane network in København has reduced wheelset damage through the prompt detection of surface defects. Rolling stock maintenance costs have fallen, as well as the levels of noise and vibration generated by train operations
BYLINE: Ulrik Danneskiold-Samsøe and Ove Ramkow-Pedersen
IN JANUARY 2000, Danish State Railways put into service a new wheel defect detector at Østerport on the København S-bane network. DSB has been continuously monitoring wheel defects on its S-bane EMU fleet since 1986, and the new detector incorporates a number of improvements based on experience with earlier systems. The location on the principal cross-city route close to København’s main station ensures that the equipment will monitor the majority of units, as most services are funnelled through this area with 2min headways at peak hours.
Before the introduction of automatic monitoring the S-bane fleet suffered heavily from wheel flats. At that time the underfloor wheel lathe at the S-bane depot was working three shifts a day. Its replacement now works less than one shift a day, significantly reducing maintenance costs.
Causes of wheelflats
Climatic conditions in København unfortunately stimulate the creation of flats. The humid environment and rich vegetation cause a reduction in the adhesion coefficient, which in turn increases the likelihood of wheelslip particularly during the leaf fall season. Slipping causes spot heating of the contact area on the wheel, and when it has ceased the perlitic alloy is quenched and converted to more brittle martensite. Surface deformation also takes place, and cracks are formed in the contact area between the small layer of martensite and the surrounding perlitic steel.
As the wheel moves, the martensite layer delivers a strong impact to the rail causing a rounding of the geometry and crack growth. This impact will therefore be at its largest just after the flat has been created, with the peak impact value eventually starting to fall to a certain level after the rounding process has stabilised. When the cracks have grown sufficiently large, small pieces of the contact surface may begin to work loose, and the overall impact level may start to increase again. The wheel monitoring system detects this trend or defect pattern by analysing the energy impact level to determine how recently the defect was caused.
One means of tackling this problem at its source is to implement measures that will prevent train wheels from slipping. In order to maintain adhesion on the S-bane network during the leaf fall season, the railhead is cleaned outside traffic hours using a high pressure flush of water mixed with lime. This technique has been in use for at least four years, and has caused a significant reduction in the number of wheelflats. Other measures include ensuring that brake blocks are released from the wheels after parking in very cold weather, and avoiding emergency brake applications as much as possible through the use of better signalling systems.
Fitting modern wheelslide prevention systems to rolling stock forms another possible solution. But despite investment in preventive measures, flats are often still generated. Once wheelflats do appear, however, wheel monitoring systems have proven to be successful in limiting their impact.
The physical impact generated by a wheelflat on the railhead is proportional to the speed of the train. In order to obtain comparable data, it is therefore necessary to install the wheelflat detector in a section of track where trains operate at a fairly constant speed. Tests performed at the Transportation Technology Center in Pueblo, USA, and experience in general have show that a passing speed in the range of 60 to 80 km/h is a practical choice, but reliable measurements can also be obtained above and below these speeds.
Flats usually form on the wheel tread, and this is most pronounced on railways with mainly tangent track and precise wheel/rail profiles. Curved track will increase the likelihood of flats occurring close to the flange or the wheel edge, and worn profiles will increase the likelihood of hunting. Detection accuracy declines as the degree of hunting or the percentage of flats forming outside the wheel tread increases.
The detection system can be configured so as to allow the bogie to move at least one half-wavelength of the hunting movement for a normal two-axle bogie while passing the detector. This increases the chance of getting at least one ’direct hit’ of the flat as the train passes the detector site. In specifying the new detector for Østerport, this feature was highlighted by DSB. The length of the measuring section was increased to cover at least four full wheel rotations, whereas the old detector was only long enough to cover one wheel rotation.
Another important requirement was the need to detect wheelflats at an early stage, while the rounding effect is still at its peak. The new equipment has therefore been installed on the busiest section of the S-bane network, where most trains pass and new flats can be detected at the earliest possible stage.
A feature retained from the old system has been smooth data distribution through a local area network to all departments that may find the detection results useful. Wheel data is currently received by DSB’s maintenance planning and engineering functions, and may in future be transmitted to the S-bane wheel shop, maintenance department and infrastructure owner Banestyrelsen.
Benefits to date
DSB’s main reason for installing a monitoring system has been to reduce wheel maintenance costs, which for its second and third-generation S-bane EMUs is principally a matter of achieving the best whole-life cost. The fourth generation - the innovative wide-bodied articulated sets built by Alstom-LHB (RG 1.96 p19) - impose a further demand. Wheel diameters must be maintained within very close tolerances on each half-set of these trains, which each have four out of five single-axle bogies powered by inverter-fed AC traction motors.
A single wheelset can only be reprofiled within very narrow tolerances if reprofiling of the other four is to be avoided, which increases the importance of detecting surface defects at an early stage. The wheelflat detection system is therefore of vital importance to the economic operation of these latest EMUs, even though they are equipped with a better wheelslide prevention system than their predecessors.
The wheelflat detection system also allows better maintenance planning and can therefore improve fleet availability. Fitted with AVI tags, EMUs can be called in for scheduled maintenance rather than being sent for attention when a defect is discovered, which can lead to a sudden influx of vehicles requiring attention. Manual tread inspection has been superseded by the continuously-operating system, reducing costs and helping to prevent damage to bogie-mounted components.
Due to København’s coastal climate where the differences between night and daytime and summer and winter temperatures are not great, wheel tread defects rarely cause track damage on the S-bane network. In other countries, however, wheel defects have been known to cause cracking of rail and sleepers and even rail breaks.
Levels of noise and vibration inside the cars and at the lineside have been reduced, and wheel defect monitoring can thus form part of strategies to reduce the environmental impact of railway operations. Reducing levels of wheel/rail noise through wheel monitoring has enabled DSB to do without costly lineside noise barriers, but precise figures for these notional savings are not available.
The ongoing trend to divide national railways into separate companies responsible for infrastructure and operations has seen those bodies responsible for the permanent way develop an increased awareness of the damage caused to track, signalling and other lineside equipment by defective wheels. In some countries, infrastructure authorities are considering the installation of wheel condition monitoring systems, in order to monitor standards of rolling stock maintenance by operators who might be incentivised by a system of penalties and rewards.
The obvious interest to train operators of reducing maintenance costs has already been mentioned, but some are also concerned about the environmental impact of defective wheel treads, particularly the noise levels generated by freight and high speed trains. On these environmental grounds, many countries enforce severe restrictions on ’noisy’ operations such as freight, and continuous wheel condition monitoring helps operators to stay within the limits prescribed by local regulations.
Operators, infrastructure authorities and environmental protection agencies therefore have an obvious common interest in investing in wheel condition monitoring technology. It would be to their advantage to share the costs and mutual benefits of detecting and promptly rectifying wheel defects before they become a problem.
CAPTION: DSB’s wheelflat detector at Østerport on the cross-city S-bane corridor has been made long enough to monitor four complete rotations of each wheel
CAPTION: Surface deformations and martensite are formed on the tread by spot heating of the contact area as a result of wheelslip
CAPTION: The three-phase drives and steerable single-axle bogies used on the latest generation of S-bane EMUs require wheel diameters to be maintained to very close tolerances
ØDS-Caltronic A/S, Denmark
Reader Enquiry Number 128