Wheels are critical components of trains. Wheel-set faults are the most important cause of train accidents, according to a comparison between the mechanical components of the train. Many defects affect the smooth revolutions of wheels. Eccentricities, discrete defect, periodic non-roundness, non-periodic (stochastic) non-roundness, corrugation, roughness, flat, spalling and shelling are sort of the wheel defects. These imperfections give rise to high-impact forces in the wheel–rail interface, inducing damage to the rail and train components. Modern trains with faster speeds and larger axle loads have greater wheel–rail contact forces. For this reason, wheel and rail maintenance managers are keen to keep wheels in an adequate condition and detect potential failure as soon as possible. As a result, wheel defect prediction and prevention are essential issues for rolling stock safety that help reducing the system-wide maintenance costs.
For a dynamic system such as a railway wheel, there are different ways of estimating the condition: physical modelling, statistical modelling and condition monitoring. Using numerical, analytical and statistical models is therefore not applicable to in-service wheel condition assessment, and accordingly, condition monitoring can be the most convenient method for condition estimation. This article describes ultrasonic measuring system as an available condition monitoring approach to the detection of railway wheel defects. Nowadays, ultrasonic techniques are being used for non-destructive evaluation. The ultrasonic method is one of the main non-destructive tests used in the railway industry to evaluate the train wheel during manufacture procedures and maintenance inspections. For more information, you can also read “Laser-Based Railway Track Inspection”.
Below, you can access the headings of this article:
- What is Ultrasonic Measuring system?
- Applications of Ultrasonic Measuring system in the railway industry
- Overview of train wheel inspection with ultrasonic measuring system
The ultrasonic measuring system works with ultrasound for distance measurement in air, unlike ultrasound for material testing, uses low frequency acoustic converters to keep down attenuation that it travels through air until it hits a surface that reflects it. The acoustic converters operate according to two different principles, as pie zo-ceramic oscillators of a narrow bandwidth or as capacitive oscillators of a wide bandwidth without prominent resonance frequencies. The ultrasonic distance measuring system facilitates non-contact distance measuring in gaseous mediums on resounding surfaces, such as metal, glass, wood, liquids, and many types of fabrics. Ultrasonic measuring systems have advantages and disadvantages, which include:
- Non-destructive technique
- Does not require access to both sides of the sample
- Can be engineered to cope with coatings, linings, etc.
- Good accuracy (0.1 mm and less) can be achieved using standard timing techniques
- Can be easily deployed, does not require laboratory conditions
- Relatively cheap equipment
- EMAT (Electromagnetic acoustic transducer) can conduct thickness measurements through corrosion and other surface coatings on metals
- No need to remove the coating of the metal.
- Usually requires calibration for each material
- Requires good contact with the material
- Cannot take measurement over rust (Does not apply to EMAT)
- Requires coupling material between the measured surface and the probe. (Does not apply to EMAT)
- Interpretation needs experience
Ultrasonic measuring system is a universal measuring instrument for railways due to their small size, high measuring frequency and accuracy, as well as their ability to measure under almost any ambient conditions. Ultrasonic inspection is used mostly for finding internal cracks within the rail for measuring their size. The principle behind ultrasonic measurements is somewhat comparable to the more commonly known radar and sonar techniques. A signal is sent out into the structure, after which the reflection of the signal is captured and analyzed. Some of the crucial applications of ultrasonic measuring system in the railway industry are:
1- Rail profile measurement
2- Rail cross section measurement
3- Wheel profile measurement
4- Wheel set measuring stands
5- Contact wire measurement
Ideally, wheel/rail contact should be confined to the wheel tread/rail head where the geometry is such that the loaded is relatively mild. However, during curving contact can occur between the wheel flange and the rail gauge corner. Contact conditions are more severe in this location because the geometry is less conformal, and the sliding is greater. In such situations, it is common to have two point contacts, at the wheel, flange and tread. The contact is typically 1cm in size and supports a large load, therefore high contact stresses are generated. These combined with the slip in the contact are primarily responsible for driving the processes that lead to wheel and rail damage, whether it be by deformation, wear or rolling contact fatigue. An approach that has shown promise is the use of reflected ultrasound. This makes use of the fact that ultrasound will be transmitted through a rough surface interface where there is asperity contact and be reflected where there are small air pockets. Thus, a scan of reflected ultrasound across an interface can be achieved.
The wheel inspection systems using ultrasonic technology seek out flaws and cracks by means of the propagation of Rayleigh ultrasonic waves produced electromagnetically on the rolling surface. The Rayleigh waves propagation through the surface defects determines the development of reflected waves that are automatically identified by the signal acquisition and processing software. The inspection system needs no coupling medium and its performance is independent of the surface conditions of the wheel, allowing proper operation even with rough or dirty surfaces.
ADOR Tech, with the help of its international partners, offers a full range of Non-Contact Measuring Systems for the Canada and US Railway Industries. For more information, please contact us!