Enabling repeatable wheelset damage identification

Source: RTM Aug/Sep 16

Surface crack measurement technology has been used for rail defect inspections for nearly 10 years, but Dr Adam Bevan, head of enterprise at the University of Huddersfield’s Institute of Railway Research (IRR), and Stephanie Klecha, UK engineering manager at MRX Technologies, tell RTM how this has been expanded for assessing wheelsets.

Rail vehicle wheelsets are regularly maintained to ensure safe operation on track and prolong life. This is achieved through measurements to inspect roundness, profile shape (wear), rim thickness and visual inspections of surface damage. If necessary, wheels are reprofiled on a lathe to preserve the optimal wheel shape and remove any visible surface damage. 

Surface damage is difficult to classify visually, leading to highly subjective results. It is also not possible to establish defect depth through visual inspections. Magnetic flux leakage (MFL) technology has been successfully applied to the detection of defects in rails. This technology has been adapted and validated for the evaluation of wheel damage resulting in a fast, repeatable method of quantifying damage on railway wheels. 

The IRR’s Centre for Innovation in Rail supported MRX Technologies in bidding for funding through the Rail Operator Challenge Competition that was being run by the RSSB’s Future Railway Programme. This resulted in funding for an 18-month project in which the handheld Surface Crack Measurement (SCM) device was further developed by MRX Technologies and validated by the IRR through a series of wheel lathe trials and laboratory assessments of damaged wheels. 

Assessing RCF defects 

The SCM technology uses MFL measurements to assess the depth of rolling contact fatigue (RCF) defects. This is achieved by magnetising the specimen and measuring the remnant magnetic flux with an array of sensors. In a defect-free specimen, the flux lines travel undisturbed through the specimen, but if a defect is present, the flux cannot easily travel through it, causing some flux to leak at the position of the defect. This flux leakage is measured by sensors located in the SCM device as is it moved around the circumference of the wheel. 

The measured signals are assessed using algorithms developed to relate the magnitude and frequency of the leaking flux signatures to the type and depth of the damage. The SCM technology reports the depth of the deepest defect in the scan (at depths of up to 10mm) and has been calibrated to quantify the amount of material to remove from the wheel to eliminate the damage. The damage measured by the SCM hand-held unit is output from the software as a damage map, which can be used to determine both the position and severity of the damage on the wheel tread. The maximum depth of all the defects identified during the scan is also reported. 

Through a series of depot trials, the handheld SCM device was demonstrated to have significant advantages over visual inspection. In many cases, especially for cracking and damage masked by grease and dirt, the SCM output has been shown to give an instant and clear indication of the severity and position of the damage on the wheel tread for a range of common wheel damage mechanisms. In contrast, visual inspection required a detailed and time-consuming assessment to identify the same damage. 

Damage was also detected on a number of wheels by the SCM device which was not visible on the surface and therefore impossible to detect during visual inspection. In these cases the damage was revealed during reprofiling on the wheel lathe and included examples of clusters of RCF and near-surface, thermally initiated cavities. Knowing this information prior to wheel reprofiling allows the wheel lathe operator to programme the required cut depth to remove the measured damage, reducing the time that the vehicle is over the wheel lathe. 

The SCM has also been shown to provide confirmation when a wheel is defect-free. This is useful for identifying if a wheel actually requires reprofiling and also to confirm that all damage has been removed following reprofiling, as there is some evidence that damage returns more quickly if not completely removed. 

Potential uses and benefits of SCM technology for quantifying wheel damage include: replacing visual inspection during routine maintenance exams; optimisation of cut depths at wheel lathe; trending to understand RCF development and growth rates; and supporting specific case studies and engineering decisions.

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