A long-standing and recurring problem in the rail industry, wheel flats can cause extensive damage and trigger safety-related events if undetected. (*) They are caused by slippage between the wheel and rail that usually occurs during braking as the required friction is reduced. The faster wheel flats are detected, the easier it is to limit damage to wheel bearings, axles, and additional infrastructure such as rails and sleepers.
Continuous monitoring is on the rise
The industry uses sound or vibration sensors mounted on tracks or bogies to detect flat spots. Rail infrastructure operators increasingly opt for bogie-mounted vibration sensors, as they offer numerous benefits:
- They enable continuous monitoring so rail professionals can detect wheel flats at an early stage when and where they occur.
- As part of a condition-based maintenance system, the sensors will indicate where on the track network wheel flats most commonly occur, allowing operators to investigate potential track problems that can reduce friction leading to wheel slips.
- In combination with the appropriate condition-based maintenance software algorithms, the same vibration sensor can be used to both analyse wheel bearing wear and detect track anomalies, which offers interesting economic benefits.
The steel-on-steel wheel track connection will always have a low level of friction, but a specialised bogie-mounted system that enables continuous monitoring will give operators the information they need to make informed decisions and take mitigating actions at the lowest cost.
The primary causes of wheel flats
Flat spots are usually the result of excessive breaking or conditions that reduce friction between the wheels and tracks, such as diesel spills, chemicals or stormy weather conditions.
Wheel flats are more common in autumn and winter because of seasonal factors. Leaves on the line can pose serious challenges for rail operators, causing trains to be delayed and even temporarily taken out of service. This situation is perhaps most difficult in countries where the tracks are close to densely forested areas such as North America, the UK and mainland Europe. To illustrate the extent of the problem, a mature European tree sheds between 20,000 and 50,000 leaves and, each autumn, literally thousands of tonnes of leaves fall onto railway lines across the UK alone.
Smart detection is the best prevention
Network Rail, the owner and operator of most of the UK’s rail infrastructure, alleviates the problem by trimming trees and vegetation along the tracks and by deploying ‘leaf busters’ which spray powerful jets of chemicals directly onto the train lines to clear leaves away. Still, it is impossible to keep tracks completely clear, particularly following storms or high winds. A train-mounted system with GPS capabilities that detects wheel overspeed or breaks in traction are the first indicators of potential problem areas.
(*) (The first known reference to the problem can be found in the diary of George Stephenson, noted in 1831 after his construction of the world’s first public intercity railway, the Liverpool and Manchester Line in the UK.)
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