18 | the rail engineer | august 2012
feature
P12 has thicker, slightly steeper flange
P12 has smoother reverse slope transition P12 has anti-RCF relief
Blue line - P8 Red line - P12 (WRISA2)
The difference between P8 and P12 wheel profiles.
is mainly undertaken to remove wheel tread cavities and wheelflats, although it can also correct flange wear, tread wear and rollover. Southern uses the WheelChex system to detect wheelset problems. WheelChex is a brand name for a Wheel Impact Load Detector (WILD) system developed by DeltaRail. The system has been used effectively by Southern and many other operators within the UK and overseas. At the WheelChex site at Salfords on the Brighton main line, about 175,000 Southern trains pass each year, making up 74% of the total fleet. Southern trains account for only 9% of the level 1 warnings generated, the balance being overwhelmingly due to freight operations. In his presentation, Iain Nairne outlined how he has undertaken a case study on Southern’s class 377 Electrostar fleet which has highlighted the fact that a small but significant number of wheel bearings require premature exchange due to defects. Class 377 was the last fleet to utilise 120mm bearings, the new standard being 130mm. Iain has established that lateral load is the primary cause of defects, although electrical damage (traction return current) is also a factor. To detect this, train riders have travelled on every class 377 unit on a 28 day cycle, but using human perception to detect wheel bearing faults
has proved difficult. It was even harder to pinpoint them to specific wheelsets, with half of the wheelsets removed having no defects. Iain went on to describe how a complementary system to WheelChex, known as RailBAM®, can be used to detect wheel bearing defects and pinpoint them to specific wheelsets within a train consist. Australian company Trackside Intelligence Pty Ltd (Track IQ) developed the RailBAM (Rail Bearing Acoustic Monitor) system and is now working with Siemens to lead sales in the UK and Continental Europe. Using acoustic principles it is able to detect bearing defects up to 100,000 miles in advance of final bearing failure. RFID (radiofrequency identification) tags allow trains to be identified and defects to be monitored over time. RailBAM was successfully trialled on Southern over a 5-month period in 2007, during which 24 bearing defects were reported, with just one false report. The mature system has since been successfully deployed at Swaythling (2009) and Mortlake (2011) on the Wessex Route to monitor the whole of the South West Trains fleet, as well as trains from other TOCs and FOCs passing the sites.
Weibull analysis Chris Tait, fleet projects and contracts manager First ScotRail, has made use of a concept that might not be familiar to many of us - that of Weibull Analysis. Sometimes termed reliability life data analysis, it attempts to make predictions about the life of all products in a population. It does this by fitting a statistical distribution to the “life data” gained from a representative sample of units. Chris has successfully utilised the Weibull analysis technique to improve wheelset management on the First ScotRail class 170 DMU fleet. ScotRail has four maintenance depots but just one wheel lathe, located at Shields depot, Glasgow. The class 170 fleet comprises 59 3-car sets - 41 Express units and 18 Suburban units. The class 170 express units average 178,000 miles a year while the suburban units cover about 132,000 miles. Both have a planned wheelset renewal periodicity of 715,000 miles and a planned re-profile cut every
140,000 miles. The wheels are designed to the standard P8 profile. Wheel lathe data from Shields depot was used to determine the average number of days from known good wheel to damaged wheel. This was then converted into average mileage. Ten samples from each vehicle wheel position were used in the analysis. An average wear rate per millimetre was determined for each wheel position, allowing a wheel wear predictor to be established. The Weibull characteristic life result for class 170 shows a clear distinction between driving vehicles and centre vehicles. Axle
Driving Vehicle
Centre Vehicle
1
176,588 miles
255,546 miles
2
188,143 miles
239,195 miles
3
227,874 miles
232,664 miles
4
243,376 miles
235,689 miles
The results were compared to WheelChex wheel impact alerts from January 2010 to August 2011. These supported the findings of the Weibull analysis, demonstrating that the middle vehicle wheels exhibit fewer wheel impacts and therefore require less tyre turning than the outer vehicle’s wheels. Chris Tait says, “Weibull and WheelChex data analysis has identified that a staggered tyre turning regime is more applicable for the class 170 fleet. The review of tyre turning periodicity using Weibull analysis has identified the current 140,000 miles is no longer optimal. A staggered periodicity is more suitable, at 150,000 miles for driving vehicles and 185,000 miles for middle vehicles.” Implementation of the staggered tyreturning regime will result in each unit requiring seven wheel lathe operations from bogie overhaul to overhaul. There are currently five operations with the 140,000 mile periodicity and it is thought that the increase in visits to the Shields Depot wheel lathe will be compensated by quicker turnaround times and flexibility within depot work patterns.
Evolution Wheelsets are of course important drivers for vehicle availability and therefore for customer service. They can also be key components in causing damage and cost to infrastructure. Safely prolonging the life of these expensive items has already paid dividends, but this brief insight belies the extent of the ongoing work. Clever engineering, pragmatic problem solving and the use of new techniques and technology will continue to optimise the balance between wheelset safety, performance and cost, underlining what we’ve known for a long time - that engineering excellence needs to lie at the heart of any modern, efficient railway system.