The Influence of Route Characteristics, Train Design and Maintenance Policy on Wheel Tread Damage, Wheel Life and Costs for Multiple-unit Trains

In the UK, the use of similar vehicle types by a range of privatised operators gives the opportunity to assess the influence of different route conditions and maintenance practices on wheel tread damage, wheelset life and costs. This paper investigates these influences, using data obtained directly from the train operators and maintainers. By disseminating best practice it is expected that wheelset life can be improved on many fleets, with resultant cost savings. 1. Background: Through the life of a rail vehicle, the wheelsets are one of the most expensive components. Wheelset manufacturing costs are high (especially when both financial and environmental costs are considered). They require regular maintenance activities [1] including:  Inspection for safety-critical damage to wheel and axle  Profile measurement to ensure that flange dimensions remain within standards  Reprofiling on the wheel lathe (typically about once a year)  Renewal of wheelset (typically every 4 to 5 years) These activities have significant labour and material costs, but also require the train to be taken out of service which impacts fleet availability and service provision . Consequently there is a strong demand to reduce the rate of wheel damage, and thereby to extend wheel profiling intervals and wheelset life. Privatisation of railways in the UK has led to a situation where similar train fleets are operated and maintained by many different organisations around the country. Each operator‟s fleet is dedicated to operation on a particular route, and maintained locally. Limited communication between the competing companies means that maintenance practices often differ between fleets. Performance targets and financial pressures often mean that fleets are intensively used; resulting in limited opportunities for reprofiling wheels or undertaking detailed fault-finding and investigation. These constraints can limit the optimisation of wheelset maintenance practices. 2. Methodology: A wide-ranging survey of wheel tread damage types and maintenance practices has been funded by the Rail Safety and Standards Board (RSSB) and carried out by the Institute of Railway Research at the University of Huddersfield. The work has been supported by other industry organisations, and aimed to investigate the influences of train design, route conditions and maintenance practices on wheel tread damage. The first stage of this process was a questionnaire review of wheel tread damage and wheelset maintenance on the passenger rolling stock fleets. The questionnaire was designed to be on a single page so that it would appear straightforward to fill in and is reproduced in the Appendix of this paper. Communications through industry colleagues and organisations also encouraged fleet engineers to complete the questionnaire. An excellent response was achieved, covering more than 90% of all the passenger vehicles in the UK. Some useful data was also obtained for freight vehicles and locomotives, although this paper focuses on the passenger diesel multiple-unit (DMU) and electric multiple-unit (EMU) fleets, which total about 10,000 vehicles. The questionnaire was followed up with visits to many maintenance depots and wheel lathes, enabling detailed discussions and observations of damaged wheels. Telephone conversations were also held where visits were impractical. Once the responses had been collated and analysed, the results were fed back to all the responders, who were able to check their data and identify any inconsistencies. Because the questionnaire had been completed by different engineers around the country, there may have been different interpretations or variations in sample size or confidence in the data. This stage helped to check that the data was consistent, and led to revisions of some entries and increased confidence in the dataset. On completion of the analysis, workshops were held at maintenance depots around the country, presenting the results of the work. This provided an opportunity for further discussion and feedback. The authors therefore have good confidence in the data presented and the conclusions drawn; it is possible that some individual figures remain inaccurate but these will not influence the over-all picture presented here. 3. Overview of Results 3.1 Reprofiling Intervals The typical life of a wheelset on a UK multiple-unit train involves reprofiling 3 or 4 times at approximately annual intervals, followed by renewal of the wheelset when it reaches the minimum permitted diameter. On a few fleets it is necessary to renew axle bearings during this period but the wheel pans and the axle may be re-used. On some fleets, most reprofiling is carried out as a planned maintenance activity at a given distance interval. Other fleets use automated or manual condition monitoring to determine when the condition of the wheel tread requires reprofiling. The average reprofiling interval can be a useful indicator of the wheel life. This is shown in Figure 1 with the vehicle types grouped by their dominant duty. Figure 1: Average Reprofiling Intervals for Various UK Train Types A number of conclusions can be drawn from this graph. Firstly, there is an order of magnitude difference between best and worst performance. Several intercity (long distance) vehicle types achieve a reprofiling interval over 400,000km, with the best being close to 1,000,000km. However, some intercity DMU vehicles do not perform so well. Most UK passenger vehicles are now fitted with disc brakes, many also having some form of dynamic brakes. However, some older DMU vehicles remain tread-braked. These generally have shorter reprofiling intervals, driven by the rate of tread wear and limits on flange height and hollow wear on the profile. Intercity trains typically run on higher-speed routes with straighter alignment, so are potentially less prone to wheel damage related to curving (such as flange wear and rolling contact fatigue). They are also less prone to wheel damage related to braking (such as flats and thermal damage), because they tend to have fewer stops, and often more vehicles in the train so traction and braking is distributed over more axles. Some intercity fleets also use premium wheel steels [3], , which offer greater resistance to rolling contact fatigue (RCF) damage, to extend wheelset life. In contrast, regional and commuter trains have many stop/start cycles and may run on more sharply curved routes. The regional and commuter fleets show a wide range of reprofiling intervals. There are a wide variety of train designs, duties and maintenance practices in this group, which will be discussed in more detail in the next section. 3.2 Reasons for Reprofiling It is interesting to compare the significant reasons for reprofiling, for each group of train types, and this is shown in Figure 2. Other damage types are relatively rare and tend to occur on individual defective wheels, rather than as a fleet-wide damage mechanism. For tread-braked vehicles, the tread wear from braking causes an exceedence of the flange height limit, and this drives about 40% of reprofiling. Another 35% of wheels require reprofiling after they have slid during braking, causing flats and cavities. These vehicles are not fitted with any form of wheel slide protection (WSP) system. Figure 2: Dominant Reasons for Reprofiling for Various UK Train Types Most DMU types have driving axles linked by cardan shafts, requiring the wheelsets to be the same diameter. This „parity‟ requirement can drive a proportion of wheel reprofiling. The main reason for reprofiling on the regional and intercity DMU vehicles is RCF. This directly accounts for about 40% of wheels reprofiled, but is also the reason why a mileage (distance) based reprofiling interval is used on some fleets, giving a total of around 60% for each group. Some fleets with poor WSP suffer from flats, while other fleets have problems with tread rollover. Some disc-braked regional DMU fleets are also fitted with „scrubber brakes‟ to clean the tread to improve adhesion and train detection, and these can cause some tread wear. Many commuter EMU fleets are maintained with a mileage (distance) based reprofiling interval, usually to control RCF growth which is a significant reason for reprofiling on these vehicles – together comprising 65% of the total. Some fleets with poor WSP suffer badly from flats. Those operating on sharply curved routes are reprofiled because their flanges have worn down to the minimum thickness, while those on fairly straight routes tend to wear the treads hollow and may be reprofiled for exceeding limits on flange height or tread rollover. The two large fleets of intercity EMU trains both achieve long reprofiling intervals. On the tilting train fleet, the planned interval is limited by conicity and RCF crack growth, although trials of a new lower conicity wheel profile (P12) and the use of a premium wheel steel (RS8T, Superlos) have shown promise [4] in reducing both problems. On the non-tilting articulated train fleet, the wheels eventually become out-of-round, while some of the leading vehicles suffer from flange wear.