The operational reliability of railway rolling stock, in particular passenger trains, is key in maximising availability and is highly dependent on the health of the drive system (i.e. traction motor and gearbox).
Traction motors are used to power the wheelset of electrically operated rail vehicles (EMU – Electrical Multiple Unit). An EMU requires no separate locomotive, as electric traction motors are incorporated within one or a number of carriages. Drive to the wheel set is normally achieved through a reduction gearbox.
Rolling bearings are a key part of the drive system of rail vehicles. If bearings fail unexpectedly, this can result in serious damage to other components and equipment, as well as loss of operation in-service. During operation, equipment reliability depends heavily on the type of bearing selected as well as on correct installation, operation and maintenance. Due to improvements in manufacturing technology and materials, bearing fatigue life, which is related to sub-surface stresses, is not generally the limiting factor and probably accounts for only a very small percentage of failures.
Reactive versus predictive
Equipment degrades with age and usage and the commonly used 'reactive' approach to maintenance by fleet operators involves fixing problems only after they occur. While this may appear to be the simplest and cheapest approach in terms of upfront costs for maintenance, when problems do occur, these can often result in costly secondary damage, along with costly unplanned service outages, recovery costs in the case of serious faults/failures, as well as loss of reputation and asset availability.
In Preventive Maintenance (PM) strategies, equipment is overhauled on a regular basis regardless of the condition of the parts. This normally involves scheduling of the train in the depot where equipment is inspected, removed and replaced, or overhauled irrespective of whether it is needed. This type of approach may reduce failures before they happen but it also leads to increased maintenance costs as parts are replaced when they don't necessarily need to be. There is also a risk of "infant mortality" due to human error during the time the train is taken out of service for repair, adjustments or replacing parts.
If key equipment on the train can be monitored in such a way as to obtain advance warning of a problem, significant cost savings can be made by avoiding unnecessary repairs and removing the train from operational service. This type of approach is known as Predictive Maintenance.
Remote condition monitoring
Rail operators are increasingly adopting Remote Condition Monitoring (RCM) to monitor railway assets, including equipment condition onboard the train as it operates in-service, in order to predict which parts are likely to fail and when. In this way, maintenance can be planned and there is an opportunity to change only those parts that are showing signs of deterioration or damage. This means that problems can be detected in advance and maintenance is performed only when needed.
However, while the use of RCM is receiving much attention, these types of systems are often expensive to install and interpreting the data can be difficult. The interpretation of the data is just as important as collecting data in the first place. A misdiagnosis can lead to the unnecessary removal of rolling stock from in-service operation, poor asset availability, lost revenues, high costs and customer dissatisfaction. A loss of confidence in such systems can be just as bad as not having confidence in the first place and potentially can be more disruptive to in-service operation.
Consequently, Schaeffler UK decided to investigate whether depot-based vibration measurements, using an underfloor wheel lathe to rotate the wheelset, could be used to assess the condition of the traction motor and gearbox.
Routine wheel turning
Underfloor wheel lathes are used by fleet operators to maintain the condition of the wheel tread, allowing machining of wheel profiles without the need for these to be removed from the vehicle.
Wheel lathes generally operate in the range 60-100m/min. This means that for a wheel diameter of 800mm this gives an axle speed of between 24 and 40rpm. For a typical reduction gearbox ratio of 4:1, the traction motor speed would therefore be between 96rpm and 160rpm respectively. The advantage of this type of measurement is that it allows the condition of the drive system to be easily assessed during routine wheel turning. This simplifies the whole process and is more cost effective as large capital investment, installation of equipment and extensive training are no longer necessary.
Unique approach to CM
Working closely with a number of different fleet operators, Schaeffler UK adopted this unique approach by using vibration measurements to assess the condition of traction motors without the need to remove equipment from the bogie.
Six separate studies were undertaken on a variety of high-speed passenger trains. The studies involved a wide range of traction motor makes and sizes, from 8MW high speed trains down to light rail-vehicles.
In these studies, all vibration measurements were undertaken with the wheel tread unturned. While turned wheels would result in a substantial reduction of background vibration due to the interaction between the contacting surfaces of the wheel tread and lathe drive wheel, the wheel turning frequency may have been too long for trending of vibration. It was therefore decided to carry out all measurements with the wheel tread unturned.
Each of the studies successfully identified potential failures of rolling bearings in traction motors early, thus avoiding any catastrophic failures or repairs. Potential failures were identified on ball bearing and cylindrical roller bearing components such as inner and outer ring raceways, cages and rolling elements. These signs of localised damage (and widespread damage in some cases) appeared in many forms including abrasive wear, adhesive wear, spalling, fatigue, corrosion, fretting, cracks, indentations, discolouration, false brinelling and degradation/starvation of grease.
In other cases, electrical erosion, high axial loads and contamination were found to be the primary causes of bearing damage. In addition, it was discovered that some of the traction motors in the studies had been fitted with non-premium bearing brands, which although may have seemed attractive at the time from a purchasing viewpoint, the performance and reliability were seriously compromised and resulted in a shortened service life, premature failure and significantly higher operating costs.
In summary, the studies undertaken provided valuable information to fleet operators (and Schaeffler) about the condition of bearings and other rotating components on traction motors. If left undetected, these worn or damaged components may have resulted in catastrophic failures of traction motors, with possible disruption to operation in service.
