Reducing the risk of derailment as a result of track buckles or rail breaks has long been a priority for railway companies and track engineers. Measuring the stress-free temperature of continuously welded rail is a key part of this, enabling weaknesses and risks to be identified and managed. However, in the past this was a time-consuming, expensive and often unreliable process that involved cutting the rail.
Pandrol’s innovative VERSE® system offers a non-destructive method for measuring the stress-free temperature of rails. Introduced in 2000, it is already enhancing safety, efficiency and cost-effectiveness across railways worldwide.
Increases and decreases in temperature cause rails to lengthen and shorten. The rail fastenings, sleepers and ballast are all designed to prevent expansion and contraction, so temperature changes result in the rail going into compression when the temperature rises and into tension when the temperature falls. If there is excess compression, the track will buckle. If there is excess tension, the rail will break (usually at a weld, where the rail is weakest).
Stress-free (or neutral) temperature is the point at which the rail is not in tension or compression. The track engineer identifies the stress-free temperature – usually 5° or so above the mid-point between the lowest and highest temperature the rail is likely to reach. Rail companies then need to monitor the stress-free temperature of the rail to identify risks, plan effective maintenance and maintain safety and operating performance.
The VERSE® system – developed jointly by Pandrol and AEA Technology – is an accurate, non-destructive method of stress-free temperature measurement. As there is no need to cut the rail, it takes much less time than traditional approaches, at less than 5% of the cost.
Taking a measurement involves unclipping around 30 m of track. The VERSE® equipment incorporates a clamp to the rail head, which allows the rail to be lifted to 10 kN (monitored by a load transducer). The system then deflects the rail and a displacement transducer logs the load and displacement at regular increments, accurately measuring the force and displacement profile of the deflection. The higher the tension in the rail, the more force will be required to deflect it.
Output from the transducers is routed to a handheld computer and combined with other data, including ambient rail temperature, rail profile and height of the rail (so that rail head wear and rail grinding can be taken into account). The VERSE® software then uses this data to return an instant stress-free temperature trackside, with a high level of accuracy. The data files are stored on the handheld computer and can be downloaded to PC software for further analysis.
The VERSE® equipment is simple and efficient to use. Preparation is minimal – there is no need to know the inherent stress and stress history of the length of rail, and measurements do not need to be taken at different rail temperatures. The rail temperature at the time of measurement needs to be lower than the stress-free temperature, which makes VERSE® ideal for use at night and early in the morning. The user inputs critical data into the handheld computer and is guided through the rail deflection process. Typical testing time is 20 to 30 minutes for an experienced crew, depending on the type of rail fastening.
VERSE® is now in use in over 25 countries worldwide. By identifying immediate problems, the system has prevented many unnecessary re-stressing operations and enabled the correction of high-risk situations that would have been missed in the past. In the longer term, it is improving rail companies’ understanding of the effects of time, traffic, maintenance, weather cycling and track component condition.
Some VERSE® users in the UK have taken as many as 1,000 measurements in a matter of weeks, giving them new insight into the state of their track and providing valuable information to improve standards and procedures. In some cases, problems have been identified at up to 30% of sites measured, including significant differences of stress between rails in the same track and large variations between sections only 400 m apart. On one newly-laid track, more than 5% variation was found between one rail and the other.
Canadian National Railways have used VERSE® to study the effect on stress-free temperature of ballast cleaning machines and other mechanical track maintenance procedures. Dallas Area Rapid Transit have used VERSE® to check brand new track before commissioning and found problems with previously used stressing techniques. One company has used VERSE® in conjunction with a tamper to correct the stress-free temperature through a curve. The tamper moves the curve outward to put the tension back and measurements are checked with VERSE®. This is reported to make considerable savings compared to conventional de-stressing.
Looking to the future, it is clear that VERSE® has a key role to play in enabling railways worldwide to become more efficient, cost-effective and, above all, safer.