A combination of gyroscopic curve detection and vibration measurement ensures enough wheel lubricant is supplied to prevent high speed trains from derailing on curves.
Anyone who’s lived close to railway sidings and goods yards will be familiar with the terrible squeal of metal wheel flanges against the heavily worn inner surfaces of curved track. At low speeds, such rail squeal represents little more than an annoyance but at increased speed and train weight, it could be the prelude to a major derailment.
Reducing wheel friction
Most of the history of the world’s railways has passed by without rails being lubricated in curves but as locomotives became heavier and speeds increased dramatically, both rail and wheel wear were climbing exponentially. With the cost and disruption of track and rolling stock maintenance becoming a significant problem, something had to be done and a number of friction reduction methods were examined resulting in two railway innovations – inclined tracks and wheel flange lubrication.
The dynamics a the wheel/track interface for inclined (or banked) tracks is complex but such geometry is thought to work well by some experts for reducing friction on high speed curves by exerting less centrifugal force on the train causing lateral pressure between the track and the flange. The disadvantage is that the degree of inclination is optimised for specific speeds so if the section of inclined track is used for mixed traffic, not only is the advantage lost but the problem is made even worse for low speed heavy traffic like goods trains which exert unacceptably high loads on the inner rail of the curve.
Train mounted wheel flange lubrication systems provide a more optimum answer to track friction and can reduce wear to both wheel and rail by as much as 78 times, according to a study done in 2006 by the Department of Civil Engineering Technology at the University of Johannesburg. Railway bearing and lubrication specialists at SKF believe that up to half a million kilometres can now be travelled before having to re-profile wheel-sets. However, lubrication also has its challenges including curve detection and dosing amount. Using bogie-mounted vibration sensors in conjunction with curve detection provides one answer.
Mounted onto the train’s bogies, the sensors detect levels of vibration beyond those that would be normally expected and which would indicate friction between the wheel flanges and the inner radius of the rail. As vibration levels increase, the risk of the wheel “climbing” the rail and dismounting becomes larger. Excessive vibration leads to automatic train braking and the detection of a curve determines how much lubricant should be delivered to the wheel flange.
The train bogie or wheel-set is a highly vulnerable environment exposed to harsh weather conditions, multi-directional shock and vibration forces and flying water and debris from beneath the track. The sensors therefore need to be highly robust and mounted in a suitable protective enclosure to prevent damage and spurious readings.
Saudi Arabian project
A system incorporating these principles is being used in the construction of the Haramain high speed rail project, an express link between Medina and Mecca, which will be operated by the Saudi Railway Organization after its inauguration towards the end of 2015.
The Spanish built AVE Class 102 and Talgo coaches will be protected by curve detection and vibration sensors supplied by German company, Micro-Epsilon as they hurtle across the 453km (281 mile) stretch of track in Saudi Arabia at speeds approaching 300kph (186mph).
The IP68 and EN50155 rated, stainless steel mounted vibration sensors are mounted on the train’s Bombardier wheel-sets and interact with the two innovative gyroscopic curve detectors which are mounted fore and aft on the train.
The front sensor detects when the train moves into a curve and the rear sensor detects when the train moves out of the curve. The wheel lubrication system is activated on and off by these two sensors, providing lubricant to the wheels precisely when (and in the volume) it is needed.
The gyroscope measures the angular velocity of the train on a continuous basis and provides a current and voltage output to the train’s central electronic control or onboard telemetry system. As vibration increases and the curve tightens, more lubricant is delivered to the interface between the wheel flange and the rail. Delivering exactly the right amount of lubricant not only reduces the likelihood of derailment, but also improves comfort, reduces noise levels and lessens the amount of wear on the rails and wheels whilst optimising lubricant costs.
In addition, the sensor can be used for collecting data for predictive maintenance of the wheels, which increases maintenance intervals and train availability.
According to European railway industry regulations, trains which have the capability of travelling over 160kph (99mph) need to have derailment safety protection systems fitted to them. Although the technology for detecting the potential for derailment and preventing it can take many forms, the parametric approach of measuring wheel vibration and detecting track curvature is an effective and relatively simple means of preventing one set of circumstances that could lead to derailment.
By continuously monitoring curvature and wheel vibration and metering lubricant in the appropriate quantities and to the correct wheels, the system is self-sufficient and doesn’t rely on the expertise or intervention of the driver.
With longer intervals between track and wheel repairs, there is also the advantage that potentially threatening defects will be spotted earlier at routine inspections before a catastrophic failure occurs.
The bogie-mounted sensors and curvature sensors at the front and rear of the train don’t form an integral part of its structure and so can be easily retro-fitted to existing rolling stock to bring similar advantages to today’s inventory of high speed trains.
The existence of different bogie types, however, means that the sensors often need to be customised before being fitted. According to Berndt Opitz, Development Manager at Micro-Epsilon, the company has a number of industry-specific robust MEMS-based (Micro Electro Mechanical Systems) sensors which can be housed in stainless steel or high thickness aluminium.
These products can be custom-engineered with a variety of connections, output signals and mounting options. Sensors supplied to the rail industry are characterised by their low failure probability, maximum operating life, high functional safety and are designed to meet the highest requirements in terms of system stability.