ON today's railway infrastructure, managing friction in the wheel/rail interface includes considering the top of the rail-wheel tread interface, as well as the more traditional gauge face-wheel flange contact. The former is still a relatively new concept, despite a history of treatment that goes back well over 10 years.
Top of rail (TOR) treatment by friction modifiers or traction enhancers is not yet covered by British or international standards, although it is accepted and practiced by both Network Rail and London Underground and many other infrastructure managers and railway operators around the world.
The primary reason for lubricating the wheel flange is to reduce wear, but it can also treat flanging contact noise in curves. In Britain, TOR friction modifiers are primarily used to reduce noise and rail head corrugations. They are also known to improve curving behaviour, as well as reducing wear and damage to both wheel and rail. In North America one of the main drivers for use is to improve fuel efficiency.
In simple terms, a friction modifier adjusts the friction between the TOR and the wheel tread to an intermediate level that is lower than dry rail but significantly higher than lubricated conditions. This latter criterion is to ensure that the traction and braking capabilities of the vehicles are not compromised.
Over the last year, LB Foster has been working with Network Rail to improve the function and benefits of its various applications of friction management systems.
Lubricating wheel flanges has been practised for many years. So what is new and how hard can it be? Not surprisingly, the challenge is not how you start doing it, but how you keep it going and maximise the benefits. Recent decades have seen both evolution of rolling stock and the grades of steel used for wheels and rails. This has altered some of the operating parameters quite significantly.
In Britain lubrication has traditionally been provided as part of the infrastructure. Mechanical and hydraulic lubricators were placed in curves at the point where wheels commenced flanging contact. Improvements in steering made possible by advances in modern suspension mean that this point of contact is now less predictable, varying considerably between a typical freight wagon and a multiple unit passenger train.
Modern lubricator equipment typically comprises electrically-powered cabinets with large grease reservoirs and the potential to provide sufficient grease at one point of pick-up to suffice for several curves, thereby covering considerable distances. Placement of the application bars in straight track enables more convenient and safer location of equipment. Pick-up of grease in this situation relies upon the grease building up to the extent that a train which is not in flange contact with the rail will still pick up lubricant through its natural oscillating deviation from the track centre line.
The advantages of modern rolling stock come with a downside. Mixed traffic operation means that one train type naturally deviates more than others, taking up most, if not all, of the available grease. This phenomenon has been observed with freight trains taking large quantities of lubricant.
Work is currently underway to determine what needs to be done to ensure modern multiple unit passenger trains pick up the right amount of grease. This is a collaborative effort between Network Rail, LB Foster and the University of Sheffield, which has involved measuring the size and shape of grease 'bulbs' on the rails and using a scale test rig at the laboratory in Sheffield.
Figure 1 shows a calculated deviation from the track centre-line of an axle running on typical track geometry and illustrates how much a typical freight wheelset is predicted to deviate compared with a passenger wheelset.
The investigations have considered and compared passenger train grease pick-up at several sites, as well as grease output and settings, and how to evaluate the in-curve performance. Further work is still required in a range of areas, but the conclusions may not rule out consideration of onboard lubrication in certain circumstances.
One proposal that has emerged is for the wider adoption of LB Foster's improved grease application bar, which - when deployed appropriately - is predicted to increase efficiency of lubricant pick-up and reduce waste.
The bar's foam pad acts as a shelf, below which grease is not pushed – and thus lost - by the action of the wheel flange. Laboratory tests also showed that the amount of grease picked up could be expected to be proportionally higher and that larger grease "bulbs" could be developed.
While deployment of lubrication on-board trains is far more common in mainland Europe, a number of train operators in Britain are using LB Foster's Kelsan solid stick LCF lubrication and solid friction modifier systems. London Underground represents a typical metro customer, while other deployments range from commuter multiple unit fleets to freight wagons. All have the common aim of prolonging wheel life through preserving wheel flanges.
Key reasons for deploying onboard friction management systems include cost and safety aspects associated with upkeep and refilling. In particular, as track access time becomes more restricted, the option of refilling and maintaining equipment in depots has become increasingly attractive.
LB Foster's Keltrack On Board (KOB) technology delivers the Keltrack friction modifier to the top of the rail without compromising the traction or braking behaviour of the vehicle. The system can be controlled via GPS or directly by vehicle data, to allow application of the required amount of friction modifier in relation to speed and location. Key benefits include mitigation against rail head corrugation, combating curve squeal, and a reduction in horizontal creep forces between wheel and rail, leading to enhanced wheel and rail life.
Switches are a vital feature of railway infrastructure and a key consideration for friction management strategies. Repair to damage caused at the thin end of the switchblade means cleaning, welding and grinding work, with a frequency that is unpredictable and often disruptive. The damage itself is also critical, as fracture of a switchblade can render a junction impassable.
Based on the observed improved steering behaviour from the application of Keltrack to the top of rails in advance of junctions, a trial application was deployed at Cemetery Junction in Nuneaton near Birmingham. Switchblades at this location were being repaired at 3-4 month intervals and replaced every two years. Following the Keltrack activation, no repairs were required for two years because the change to the wheel contact path on the rail and the amount of contact was significantly reduced with the switch rail.
LB Foster is currently working with Network Rail engineers to further improve understanding of how this really works and how best to deploy it elsewhere.
One key question is to establish whether the same treatment would work at other sites. If regular repairs are being undertaken at a particular location, could an improved wheel path make a difference? This can be validated with a relatively simple test application and spray paint to indicate the wheel/rail contact. LB Foster's Sheffield-based team has also been experimenting with a range of measurement tools to help provide further answers.