Railway trains are one of the planets major modes of transportation, moving large amounts of people and cargo across distances short and long. Like other modes it has tribological issues – primarily lubrication of the engines that power, along with other moving parts.
But trains have an extra challenge where the mode meets its medium – at the interface between wheels and rail. According to an industry official, conditions at this interface are a challenge to manage.
The wheel-rail interface is of utmost importance for operation, as all of the forces are necessarily transmitted through metal-to-metal contact, Xabier Perez, of Spanish train manufacturer CAF, told the Lubmat conference in Bilbao, Spain, in June. He went on to explain that significant wear occurs at that interface and that it costs operators sizeable expense to monitor and address. As a result, industry continues looking for better lubrication solutions.
The design of railcar wheels and their interface with rails play an important role keeping the train on its track, said Perez, a senior technology advisor with Guipuzcoa-based CAF, which is formally known as Construcciones y Auxiliar de Ferrocarriles. The cone-shaped, solid wheel sets provide a simple but effective means of guiding the [car] on straight and curved tracks….
First of all, because the wider end of the wheels sit inside the rails, the wheel and axel sets keep the cars from slipping off the rails. The wheel geometry also helps the cars lean into turns. As train enters a curve – say, to the left – centrifugal force tries to push cars to the right. By doing so, the wheel on the right rides up on the rail so that the rail touches the wheel closer to its wider end. At the same time, the wheel on the left is pulled away from the rail, allowing the interface to slip toward the small end of the cone and therefore for that wheel to ride lower on the rail.
The combined effect is that the wheel sets and the car tilt slightly toward the direction of the curve, giving the train more stability.
The designs of wheels and rails may be simple, but the interactions between them are not, Perez said. In terms of physics, much is going on, from the downward force of the weight of the car, to the sideways force experienced during curves, friction that operators would like to eliminate and some friction that is needed.
The interface has to transmit static and dynamic forces, traction and braking forces, Perez said, adding that some of those forces are tremendously large. Considering typical static loads per wheel somewhere between five and 12 tons, this is a demanding working environment.
These forces cause significant wear, most of it occurring along the upper inner corner of the rails and on the wheels where they flare out to the flange. The wear alters the profiles of both wheel and flange, which could cause problems if allowed to proceed too far.
The guidance properties of the [rail car], and therefore its dynamic behavior on the track, are deeply affected, Perez said. Flange thickness is a major factor in guaranteeing against derailment.
Recognizing the potential danger, the industry uses rail grease in an attempt to minimize wear. Rail maintenance companies apply lubricant using stationary applicators placed at spots where wear is mostly likely to occur – often at curves in the track. In addition, train operators us applicators mounted on the underside of cars. Rather than apply the grease continually, these systems are also programmed to apply the grease only where it is needed.
Perez said the grease must be applied with precision – only to the wheel flange and the corner of the rail. It is important that grease not come into contact with the top of the rail; in fact operators apply friction modifiers there to help provide sufficient traction for the wheels and braking. If grease does invade the top of the rail it can cause slips or even accidents.
From our perspective, the main challenge is simply to make sure that the lubricant is exactly where it should be at all times, Perez said. The problem is that the railway environment is vast and very uncontrolled. You do not know for sure at which place the contact with the flange will take place, and you need to make sure that the lubricant is at the right place at the right time, but nowhere else in order to limit contamination and cost.
Despite the attention to lubrication, significant wear does occur. This requires the industry to keep track of wear so that it can redress the situation when necessary.
Maintenance of wheel/rail interface is a big cost driver for the railway business, Perez said. The first step is monitoring. In stations wheel profiles are regularly checked with automatic or hand-held gauges. A newer tool is dynamometric wheel sets that measure the force between wheels and rails while the train is moving.
When wheels become sufficiently worn, they are reconditioned to restore their profile or else they are replaced. Worn rails are also replaced.
Perez said wear is a significant problem for the industry, although severity varies from track to track and train to train.
It is very dependent on infrastructure and operation, he explained. Some administrations have few problems despite using not very sophisticated systems. Others cannot solve their problems with any kind of system.… It is clear that the overall size of the problem – in terms of wheel reprofiling, rail grinding and replacement costs – is very big.
Industry is working to improve the situation, and much of the current focus is on establishing a more scientific basis for understanding and addressing wear.
Lubrication of the wheel-rail interface is just starting to boom, said Peter-Paul Mittertreiner, a founding member of the European Lubricating Grease Institutes Railway Lubricants Working Group. In the past, the communication between lubricant suppliers, applicator suppliers and railway companies was not optimal, resulting in less [cooperation] than would be optimal. But the financial crises and environmental laws are now pushing us together and thus working to our benefit.
He explained that railway operators lubricate wheel-rail interfaces not only to reduce wear but also to improve fuel economy and to cut down on noise.
A number of joint research projects are now underway in Europe, focused not only on specifications to define rail lubricant performance but also on measuring methods, test protocols and other standards that can provide a common language between railway, lubricant and lube applicator companies.
Mittertreiner said it may take time before such work yields results on trains and railroads. Tests on a railroad track take a long time and are very expensive to make sure the millions of passengers are not endangered, he said.