WHEELSET maintenance accounts for a large proportion of rolling stock whole-life costs. Regular maintenance is achieved by inspecting roundness, profile shape (wear), rim thickness and surface damage. Wheels are usually reprofiled on a lathe annually to preserve the optimal wheel shape/profile and remove any visible surface damage with renewal every four to five years. These activities have significant labour and material costs and require the train to be taken out of service which impacts fleet availability.
Surface damage is difficult to classify visually, leading to highly subjective results, while the depth of defects cannot be determined through visual inspections. Wheel turning on a lathe removes this damage, but there is a crucial balance between removing enough material to eliminate the defects whilst taking the minimum cut to preserve rim thickness and prolong wheel life. Wheel lathe operators make multiple small cuts to prevent excessive material removal, but this increases the time that the vehicle is on the wheel lathe.
Magnetic flux leakage (MFL) technology, which has been successfully applied to the detection of surface and sub-surface defects in rails, has recently been adapted to evaluate wheel tread damage to produce a fast, repeatable method of quantifying wheel damage. This reduces inspection times and optimises wheel turning thereby saving time and increasing wheelset life. Management and trending of the recorded data also enables maintainers to identify problem vehicles or wheelsets and plan maintenance in advance, which will also assist train operators when evaluating wheelset performance and costs.
The contact between the wheel and rail provides a harsh operating environment. The loading conditions and contact geometry cause major stresses that are significantly higher than the yield stress of the as-manufactured wheel material. Additionally, the transmission of traction, braking and steering forces apply tangential stresses and thermal inputs resulting in material flow, wear and cracking damage such as rolling contact fatigue (RCF) to the wheel tread. This makes it difficult to prevent all forms of damage occurring on wheel treads.
Optimising wheel life is therefore a matter of limiting the rate of damage, managing the consequences, and preventing the development of unsafe conditions. This can be achieved through careful wheelset inspections combined with effective data recording at the wheel lathe to help determine the root cause of the damage and optimise wheel life.
Typically a wheelset is reprofiled three or four times during its life and renewed when it reaches the permitted minimum diameter. Reprofiling is carried out either as planned maintenance or at a given interval, based on an understanding of the damage rates for a particular fleet, or when the surface condition of the wheel tread has degraded to an extent that requires reprofiling.
Wheel tread condition in terms of wear - typically flange height and thickness, out-of-roundness and surface condition is inspected regularly using a combination of automated and manual monitoring techniques to identify when reprofiling is needed. But technology is not yet available for inspecting and quantifying the surface condition of wheels. Maintainers are reliant on visual inspections which are highly subjective, lead to non-repeatable results and cannot establish damage depth, which makes consistent corrective action, data assessment and trending difficult.
The amount of material removed from the wheel during reprofiling is governed by the level of flange wear and severity of any tread damage (Figure 1).
Figure 1a shows a worn P8 wheel profile (red profile) after around 98,000km of running with 1mm of flange wear. Restoring the full flange requires the removal of approximately 2.5mm of material from the wheel radius, as illustrated by the green profile and shaded area. In this case the cut depth required to restore the profile removes all the RCF damage (cracks indicated by the angled red lines on the field side of the wheel). In comparison, Figure 1b shows a worn P8 wheel profile (red profile) after approximately 286,000km.
Flange wear generally occurs early in the life of the profile and then stabilises. Therefore only a small increase in flange wear is seen (1.1mm), resulting in about 2.7mm of material being cut from the wheel radius to restore the profile (green line and shaded area). As the wheelset has run a greater distance, the depth of RCF damage has also increased, requiring a greater cut depth to remove it. As the wheel lathe operator does not know the depth of the damage, he will either make many smaller cuts or an excessively large cut based on experience to ensure that all damaged material is removed. This increases the time that the vehicle is at the wheel lathe and generally results in more material being removed than necessary, thereby reducing wheel life.
Previous research investigated the interaction between the amount of material loss at the lathe to recover the profile and to remove RCF damage. Due to the trend in flange wear, the depth of cut required to restore the wheel profile remains fairly constant with mileage after the higher initial flange wear rate when the profile is new. As the mileage increases, RCF cracks propagate more rapidly, so the depth of RCF damage increases, which necessitates a deeper cut. Therefore, there is an optimum turning interval where the material removal needed to restore the profile shape due to flange wear is the same as that required to remove the RCF damage.
Crucial balance
This example highlights that a crucial balance exists for a wheel lathe operator between removing enough material to eliminate the damage, minimising the cut depth to preserve the rim thickness, and minimising the time at the wheel lathe by not taking multiple smaller cuts. Having the ability to reliably and accurately measure the depth of damage would significantly assist in the decision making of the lathe operator and optimise wheel surface damage management.
MRX’s Surface Crack Measurement (SCM) technology has been successfully used to quantify the severity of rail defects for over eight years, and has recently been adapted to measure wheel surface damage using a specially developed prototype hand-held unit (HHU).
SCM technology uses MFL measurements to assess the depth of RCF defects by magnetising the specimen and then measuring the remnant magnetic flux with an array of sensors. In a defect-free specimen, the flux lines travel undisturbed through the specimen, whereas 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. The wheel SCM HHU uses 16 magnetic field sensors to measure and record the flux leakage (Figure 3). The sensors are positioned at a 5mm pitch across the wheel tread and data is sampled every 0.5mm as the unit is moved around the wheel circumference to give a high resolution scan of the wheel tread surface condition.
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. SCM technology reports the depth of the deepest defect in the scan and is calibrated to quantify the amount of material to remove from the wheel to eliminate the damage. Experience from similar rail SCM products shows that the technology can detect and quantify both macro (such as cracks and cavities) and micro (such as thermal damage resulting in martensite or an abrupt change in metallurgical grain structure) discontinuities in the material. Work is currently on-going to optimise and validate the algorithms to assess both micro and macro defects in wheels.
MFL can detect surface and near-surface defects up to 10mm into the material, whereas eddy current technology can only inspect the top 3mm of the material. MFL also gives a direct measurement of the damage depth, whereas eddy current is reliant on the operator assuming the defect angle into the material which is often unknown. Using the MFL technique the HHU provides an upper and lower detection limit of 10mm and 1mm respectively and a system accuracy of ±0.5mm.
The damage measured by the SCM HHU is output as a damage map (Figure 2). The vertical axis shows the length of the scan (in metres) around the wheel circumference, and the horizontal axis shows the position of the damage on the wheel tread (flange to field side of the wheel) in mm.
Measured data which has been classified as a defect is presented as a coloured output. The colour scale ranges from minor damage (blue) to severe damage (red), with grey indicating no detected defects. The map can be used to determine the position and severity of the damage. The maximum depth of all the defects assessed is shown beneath the damage map.
EMU and DMU wheelsets have been magnetised and scanned using the HHU to identify the severity of the identified damage. In a number of cases these scans were conducted on a wheel lathe and included a pre-cut scan to assess the severity of the damage in the un-cut wheel, and a post-cut scan to ensure that all the measured damaged material had been removed from the wheel.
An initial cost:benefit analysis has been undertaken to demonstrate the use of HHU during regular inspection to optimise cut depths. By using the HHU the cut depth can be identified prior to reprofiling resulting in less material being removed from the wheel diameters.
This has been demonstrated by tracking the life of a wheelset based on the observed wear rates and cut depths with and without the use of the HHU. Figure 3 shows the reduction in wheel diameter due to wear and reprofiling with running distance. Optimising the cut depth by using HHU results in two additional wheel reprofiling activities equating to about 595,000km of additional running prior to reaching the assumed wheel scrapping distance.
Taking into account the typical costs associated with replacement and inspection and the cost of having a train out of service suggests a potential cost saving of around 25% in wheelset life.
The ability to measure the defect depth offers several benefits:
- repeatable assessment of the severity and location of wheel surface damage and less reliance on judgement associated with visual inspection, while reducing the risk of surface damage being missed or classified inconsistently
- identification of the depth of cut required which might not be obvious or visible
- reducing the risk of removing too much material during reprofiling and the variability between wheel lathe operators
- confirmation that all damage has been removed
- the ability to trend the type and severity of damage on a vehicle or wheelset, and highlight problems with wheels/vehicles
- improved planning and scheduling of wheelset maintenance, and
- reducing vehicle downtime.
Further work is currently on-going to examine a number of scrap wheels to optically determine the deformation depth, crack length and crack depth. This information will be correlated with the damage measured by the HHU from the scrap wheels to provide additional confidence in the defect depth algorithms. A business case detailing the benefits of the HHU for trending and maintenance planning will also be developed.
