Low adhesion and sanders

AUTHORS INTRODUCTION WHY IS LOW ADHESION IMPORTANT TO THE RAILWAY? Andrew Lightoller is a This paper will introduce the concept of low Chartered Mechanical adhesion between train and rail, including Low adhesion can cause safety incidents (such Engineer, working for why it is so important to the railway, and will as Signals Passed At Danger, or SPADs) and DB ESG, a rolling stock show how sand is used to improve low adhesion significant performance issues (such as station consultancy. Andrew braking performance. Using sand is not a new overruns or delays). In low adhesion, train drivers started his career in concept, indeed steam trains were using sand for will naturally drive more cautiously; perhaps the rail industry as a traction purposes over 100 years ago. However, resulting in a lower maximum speed between graduate engineer at by increasing sand delivery it is possible to deliver stations and braking early and gently. There are Bombardier where consistent train braking performance regardless also issues with traction, particularly affecting he took over as Brakes engineer on the of the underlying adhesion conditions. Solving freight trains with limited motored . Classic project. After various experience the braking issue builds driver confidence in poor Each year, approximately 350,000 delay minutes including a six month placement at Chiltern adhesion conditions and opens the door to using are attributed to low adhesion. The direct impact depot commissioning 168s and EU testing sand in traction to achieve higher performance. of low adhesion is estimated to cost the GB of aluminium foam, Andrew moved to Atkins railway industry and wider society £345m per as a Brakes & Pneumatics Engineer in 2001. This paper describes how the braking year. The cost is made up of delay compensation, In 2014, Andrew moved to DB ESG after performance of different sanding configurations as well as the costs to mitigate low adhesion their acquisition of WSPER and works as a was quantified in a real railway environment, and (including rail head cleaning) and the operational Principal Mechanical Engineer involved with how an innovative new analysis methodology and reputational damage which occurs when WSPER, brake testing and other general was developed (since wheel/rail adhesion cannot train reliability and punctuality are degraded. Low consultancy work. be measured directly, the ‘Reference Adhesion’ adhesion issues are also a significant barrier to provides a method of inferring effective adhesion maximising future network capacity increases, Liam Purcell, CEng during a brake event). since predictable and reliable braking is a key FICE FPWI requirement for running trains closer together. INTRODUCING LOW ADHESION Liam is currently AVAILABLE MITIGATIONS working as a Principal In order to accelerate or decelerate normally, Consultant at Ricardo railway vehicles rely on adhesion between Although low adhesion exists mainly on the Rail, and leads the and rail. A typical ‘step 2’ service brake rail (the permanent way engineer’s domain), it Intelligent Infrastructure demand (representing a requested deceleration of primarily affects the rolling stock (the rolling stock team. Liam is a 0.6m/s2 or 6%g) requires a coefficient of adhesion engineer’s domain). There are several types of Chartered Civil Engineer and Fellow of the between wheel and rail of µ ≥ 0.06. Table 1 shows available mitigations against low adhesion: ICE and PWI, with sixteen years’ experience a range of typical braking demands, however in the railway industry. please note that some trains have different brake • infrastructure-based mitigations include rates and/or more granular control. vegetation management (to reduce After initially joining AEA Technology contaminant getting on to the rails in the first Rail /DeltaRail as a graduate in 2003, he For the majority of the year, this level of wheel/ place) and rail head treatments (to clean the worked as a consultant focussing on rail adhesion is readily available. However, each contaminant off the rails). vehicle-track interaction and derailment autumn, deciduous trees shed their leaves with investigation. Liam joined Lloyd’s Register some being crushed under the pressure of train • rolling stock mitigations include sanders (the Rail (now Ricardo Rail) in 2012 to specialise wheels, bonding to the rail to produce a very hard subject of this paper), as well as braking in wheel-rail adhesion, asset management and smooth layer. When combined with moisture, systems independent of wheel/rail adhesion and independent assurance of infrastructure the resulting coefficient of adhesion between (magnetic or eddy-current track brakes). projects. wheel and rail can be as low as µ = 0.01; six times There are also potential future technologies lower than that required to sustain normal braking. being evaluated, including using high- During braking, if the demanded adhesion powered lasers or plasma arcs to burn exceeds the available adhesion, the train wheels contaminant off the rails, or using frozen start to rotate more slowly than the actual train’s carbon dioxide to freeze the leaf film and speed; the wheels are starting to slide. With a blow it clear of the track. pneumatically actuated friction braking system, the Wheel Slide Protection (WSP) system will This paper focusses on enhancing the use of intervene to control the slide before any wheel sanders. Since sanders already have a history of damage occurs. in service use on the railway, the technology is available now and the benefits are proven. Sand This is generally done on an -by-axle basis is also relatively inexpensive. by rapidly venting pneumatic pressure in the brake cylinders, thus removing the braking force INTRODUCING SAND (AND and allowing the wheel speed to increase back SANDERS) to true train speed. Some slide is beneficial as it can create heat, drying and scrubbing the track, Sand, as shown in image 1, has been used on improving adhesion for the following axles. This the railway for almost as long as railways have effect, known as wheelset conditioning, tends to existed; historically steam drivers work best at improving medium levels of adhesion could open a valve to activate steam-powered - however it is less beneficial at very low adhesion sanders to aid traction. On a modern train, the levels. WSP system controls the sander (automatic sanding) - although the driver often has a manual override button to obtain sand on-demand.

28 Sanders deliver sand to both wheels of an are equipped with fixed rate sanding ahead axle either by having a pair of sanders (one of the third axle, delivering a nominal 2 kg of for the left wheel and one for the right wheel) sand per minute (to each wheel). The sander or by having two delivery hoses from a single is often inhibited at speeds below 10mph, to sander. Since sanders are directional, Multiple avoid laying a high density of sand which could Units are typically equipped with sanders at cause interference with track circuits. (2 kg / axle three in each direction. min of sand, deposited at 10 mph, is equivalent to a sand density on the track of 7.5 g/m. At When the WSP system detects wheel slide, this density, sand is unlikely to interfere with it calls on the sander to eject sand. A typical track circuits.) The above configuration, herein sander configuration is shown in image 2; referred to as Single Fixed Rate Sanding the sand is delivered from a hopper and (SFRS) undoubtedly has low adhesion braking compressed air carries it along a flexible sand performance benefits. However, by using Image 1: Railway sand (note coarse grains). delivery hose, ejecting the sand just ahead more sand, there is potential to deliver greater of the wheel. The sand is aimed at the wheel/ benefits. rail contact point, as shown in image 3. As the sand passes through the wheel/rail interface, DELIVERING MORE SAND the abrasive nature of the sand helps transmit the braking shear forces from the wheel into A greater quantity of sand could be delivered the rail, and the contamination layer is also through: damaged by the abrasive sand particles. The sand is crushed and turns to dust (see • Distributed sanding (multiple sanders image 4); the dust probably helping to absorb distributed along the train), and moisture. Sanders are considered to have a • Variable rate sanding (speed-dependant two-fold benefit by: sand discharge rate, with higher sand Image 2: Typical sander configuration (note rates at higher speed). sand hopper and flexible delivery hose). • Providing a temporary boost to the level of adhesion experienced by the sanded The key benefit of the variable rate sander axle (and subsequent axles to a lesser is that it can deliver a higher rate of sand at extent), and higher speeds (4 kg / min at speeds above • Abrading or breaking down the leaf film, 20 mph), yet it is able to vary its flow rate at assisting in its removal. lower speeds to respect the 7.5 g/m guidance. A comparison of sand flow rates for fixed and It should be noted that automatic sanders do variable rate sanders is given in image 5. A not predict where to lay sand: they need wheel train equipped with variable rate sanders at slide to occur first to trigger the sand delivery. axle 3 is referred to as Single Variable Rate Automatic sanders were initially introduced on Sanding (SVRS), and when two axles are Image 3: Sand being ejected from the sand multiple units by British Rail Research, starting equipped (typically axles 3 & 7) it is known as delivery hose, directed at the wheel/rail around 1996. Most modern multiple units Double Variable Rate Sanding (DVRS). interface (train is stationary; test button used to eject sand for photograph).

Image 4: Sand being delivered at speed (≈ 40 mph) during wheel slide.

Table 1: Relationship between brake demand and adhesion.

Image 5: Comparison of fixed rate (orange) and variable rate (green) sanders; sand flow rate (left Image 6: Test site for sander testing in low graph) and sand density on the track (right graph). adhesion.

29 is derived from tree pulp and is therefore a A working assumption in this was that adhesion similar composition to real leaves. This artificial would remain constant between individual (but representative) low-adhesion zone tests. In reality this was not experienced. enables consistent and reasonably repeatable After the first few testing sessions, it became tests. A bespoke trolley was used to lay the clear that the changing environmental and paper tape (which is gummed on one side, so it rail conditions produced substantial adhesion adheres to the railhead), as shown in image 7. changes between consecutive tests, and the planned test strategy was not appropriate. A TEST TRAIN new approach was needed.

Image 7: Paper tape being laid at the test site. Two brand new four-car Class 387 EMUs were REFERENCE ADHESION used for the testing, and were modified by: QUANTIFYING THE BENEFITS A new strategy, Initial Reference Adhesion, OF ENHANCED SANDING: RSSB • provision of variable rate sanders in was devised whereby the first two (un-sanded) T1107 PROJECT advance of axles 3, 7, 11 and 19; axles of the train were used to estimate the • installation of water spray system ahead initial underlying wheel/rail adhesion that the The T1107 project aimed to determine the of axle 1 (to moisten the paper tape to train was running over. This novel approach to benefits of enhanced sanding that could be create low adhesion); inferring real-time adhesion was a significant achieved in low adhesion conditions, using • installation of transducers to measure and breakthrough for the project because not only a real train running on real track. The test record parameters including axle speeds, did it provide a suitable method of comparing objectives were: brake cylinder pressures, train speed, etc. one test with another, but it also removed the need to establish identical rail conditions • quantify the benefit of distributed sanding; SANDER TESTING between tests. • quantify the benefit of variable rate METHODOLOGY sanding; This new approach relies on the assumption • determine if it is possible to deliver The paper tape was ‘bedded’ by coasting that the WSP system regulates the brake ‘assured’ 6%g braking in low adhesion over it and then ‘conditioned’ by undertaking force (in this case brake cylinder pressures) conditions. full-service brake applications while using to maximise the use of the available adhesion the water spray to moisten the tape. Once to stop the train in the shortest distance. As TEST SITE sufficient slide was apparent, an un-sanded a result of this assumption, differences in test was undertaken to verify that a suitable WSP system performance may lead to small Testing was carried out at Network Rail’s RIDC low adhesion layer had successfully been differences in the results but comparison of (Rail Innovation and Development Centre) created. Finally, a sanded braking test was stops from the same train (and possibly of Melton facility; the line used for testing was undertaken, and the resulting data analysed to different trains using the same WSP system) approximately 7 km in length - an aerial view determine basic statistics of the test. should be directly comparable. of the test site is shown in image 6. The site was divided into a 2 km acceleration zone (for Initially, the test methodology was based on Reference Adhesion is an approximation of accelerating to 55 mph); a 1 km low adhesion quantifying the low adhesion train braking the wheel/rail adhesion that each axle of the zone (to undertake the sander testing) and a performance without sand (the control test) train was able to utilise over the course of the 4 km run-off zone (for train deceleration and a followed by a sanded test; with the control test brake application. Initial Reference Adhesion safety buffer). effectively providing a benchmark from which is the average Reference Adhesion for the first to compare the improved deceleration of the two axles of the train, and provides a snapshot To create the low adhesion zone, 1 km of track sanded test. of the incoming track condition, before it is was ‘contaminated’ with paper tape. The tape changed significantly by the train. is typically used in the packaging industry and

Image 8: Mean deceleration achieved for DVRS, SFRS and un-sanded Image 9: Stopping distance achieved for DVRS, SFRS and un-sanded. stops. stops.

Image 10: Average results from all the sander tests. Image 11: Average stopping distance chart from 55mph for sander configurations with µ = 0.02

30 Image 12: Reference Adhesion plots for axles along the train, each grey Image 13: Reference Adhesion plots for axles along the train, each grey line indicates an SFRS test run and the orange line shows the mean line indicates a DVRS test run and the green line shows the mean DVRS SFRS. result.

Image 14: Range of stopping distance results from 55 mph for differing Image 15: Examples of theoretical maximum sand density of 7.5 g/m, if sander configurations (Initial Reference Adhesion between 0.02 and all the sand were to land on the railhead in a narrow bead (left) or more 0.045). spread out (right).

Equation1.

Calculation of the Reference Adhesion of an RESULTS ADHESION IMPROVEMENTS axle is relatively simple, as indicated above - ALONG THE TRAIN (equation 1) where, The T1107 testing project carried out 147 sanded tests and 78 un-sanded tests. The Looking at the calculated Reference Adhesion • ‘Average BCP’ is the mean Brake Initial Reference Adhesion methodology along the train enables an understanding as Cylinder Pressure measured over the provided an understanding of the adhesion to how the improvement in stopping distance brake application for that axle; levels at each axle and allowed: (or deceleration) is being achieved. Image 10 • ‘Dry Rail BCP for Brake Demand and shows the average of all the tests for each Load’ is the BCP pressure in good (non- • a comparison of achieved deceleration sander configuration tested. The red line slide) adhesion conditions after automatic and stopping distances against Initial indicates the average un-sanded adhesion adjustment for passenger loading; and Reference Adhesion for un-sanded and improvement along the train resulting from • ‘Dry Rail Deceleration for Brake Demand’ sanded stops; wheelset conditioning only. The other three is the straight & level track retardation • a comparison of adhesion levels axle-by- results demonstrate the improvement from rate in ‘g’ for the selected brake demand. axle along a train; and the existing SFRS (orange) to SVRS (yellow) • a demonstration of the benefits of and finally to DVRS (green). It is the significant The above calculation assumes that the enhanced sanding. improvement in adhesion for all of the axles automatic adjustment of brake cylinder that creates the significant additional braking pressures for passenger load assures a similar DECELERATION & STOPPING force of DVRS and thus reduced stopping retardation rate independent of vehicle payload DISTANCE distances. - otherwise the deceleration rate should also be factored for load. Image 8 demonstrates the effectiveness of DVRS BENEFITS DVRS (green squares) over SFRS (orange The Reference Adhesion for each axle circles) and un-sanded stops (red diamonds). From the above results we can see that the provides a method of demonstrating how The smaller amounts of sand from existing stopping distances with DVRS are significantly adhesion changes on an axle-by-axle basis sanders (SFRS) may be seen to be reasonably reduced when compared to un-sanded and along the train. The Initial Reference Adhesion effective at medium adhesion levels (Initial existing SFRS stops. The trendlines shown (average of first two axles), provides a Reference Adhesion >0.05) but its effect is in image 9 can be used to estimate an benchmark of the starting adhesion conditions limited and more variable at lower adhesion average stopping distance for any given Initial and enables comparison of one test with levels. Increasing the sand output to that of Reference Adhesion; image 11 shows stopping another. DVRS provides a significant benefit at the very distances from 55 mph at adhesion = 0.02. low adhesion levels approaching 0.02. In terms of stopping distance, the vertical axis of the As well as the significant improvement in graph can be changed to distance and instantly average stopping distance, enhanced sanding it can be seen that in very low adhesion, delivers much more consistency in the stopping DVRS returns stopping distances that are distance. The variance in Reference Adhesion approaching those of dry rail (image 9). along the train and between tests may be

31 / representatives to familiarise themselves with the enhanced low adhesion braking performance afforded by the DVRS equipment. During conversations with the drivers, they were impressed with the performance that the DVRS system gave them. Driver feedback included “anything that stops me a metre quicker, I’m happy with” and “this DVRS system is brilliant, absolutely brilliant, a marked improvement in stopping”. In addition, both WMT’s Operations Director and Network Rail’s Head of West Midlands Operations were very happy with the performance, describing the new way of using sand as a “game changer” and “ground-breaking”. Image 16: Things to consider when retrofitting DVRS. Enhanced sanding is the simplest technical solution to low adhesion, but the human factors aspect must not be overlooked. It will take time for drivers to become accustomed to the enhanced braking that sanding provides and to gain confidence to let the train deal with low adhesion conditions. A train driver’s primary concern is the safety of the train and its passengers; changing driving style sufficiently to make use of the extra sand will take time.

CONCLUSION Image 17: DVRS retrofitted on Class 323. The standard which controls the use of sanders seen with the existing SFRS results (image 12) However, if turnouts are located in braking on multiple units is GMRT2461 ‘Sanding and the greater consistency of DVRS results areas, there is a chance that enhanced Equipment’. This standard was updated in (image 13). The variation in stopping distance sanding could lead to a slightly higher wear 2018 to include the results of the T1107 testing can be simplified into another graph; image 14 rate of switch slide bearing surfaces. work, to encourage the GB rail industry to illustrates the shortest and longest stopping adopt distributed and variable rate sanding. distance for each sander configuration. This IN-SERVICE PILOT The work described in this paper represents a further demonstrates the benefits of the significant advancement in the understanding additional sand that DVRS brings. Following the T1107 testing, train operators of enhanced sanding. The work has quantified were interested in realising the potential the benefit of different sanding configurations, AND AN ‘ASSURED’ 6%g? benefits of DVRS to improve autumn and has shown how fitting DVRS on a modern performance; however, the challenges of four car unit can consistently deliver 6%g The final objective was to determine if it is retrofitting were not clear. To support industry braking irrespective of underlying adhesion possible to deliver ‘assured’ 6%g braking in low adoption of DVRS, RSSB commissioned a conditions. The work has also shown how it adhesion conditions (since train timetables are project to implement an in-service pilot of is feasible to retrofit DVRS to existing multiple typically designed around 6%g deceleration). DVRS on the mainline railway. This project units and has provided guidance for those The Class 387 brake step 3 results with DVRS, aimed to provide dedicated support for the first doing so. The general trend is that, as the sand plotted as deceleration (vertical axis) in image in service retrofit of DVRS and to verify that is increased, the stopping distances get shorter 8 show that for every test undertaken, the the benefits of DVRS demonstrated on the but (possibly more importantly) the variation mean deceleration was greater than 6%g. test track would translate into similar benefits in stopping distances is reduced, indicating a This result is important for maximising future for normal in-service running of a passenger greater predictability and consistency for the network capacity, since predictable and train. In early 2018, several train fleets were train driver. DVRS also has the capability to reliable braking (regardless of underlying identified as candidates for the in-service unlock future network capacity by enabling adhesion conditions) is a key requirement to pilot. A feasibility study was conducted for control systems to run trains closer together. enable future train control systems to run trains each and it was considered feasible to retrofit closer together. DVRS to all the fleets. However, all had some ACKNOWLEDGEMENTS technical challenges to overcome; image 16 INFRASTRUCTURE shows the main considerations. Equipping a With thanks to our client Rail Safety and CONSIDERATIONS unit with DVRS typically requires upgrading Standards Board (RSSB) for developing, the existing fixed rate sander at axle three to a managing and funding the programme of low To reassure the permanent way engineer, variable rate sander, and adding an additional adhesion work. Thanks to project partners, sanders only dispense sand during wheel variable rate sander at an intermediate axle Great Western Railway, Porterbrook, Network slide events. By this definition, sand is only (typically axle seven or eleven depending Rail, Freightliner, Serco Rail Technical deposited where low adhesion is present on on unit formation and available space). West Services, Knorr Bremse, Bombardier, the track and is causing the wheels to slip. Midlands Trains Class 323 units were selected Bridgeway, West Midlands Trains, AB Hoses Sanders are designed to discharge a maximum for the in-service pilot and were equipped with and Instrumentel. 7.5 g of sand per meter of rail – as seen in DVRS (image 17); these were introduced into image 15. At 60 mph the sand density will be passenger service on the Cross-City Line in BIBLIOGRAPHY three times lower than this, at just 2.25 g/m Birmingham in 2019. The trial units were also (refer back to image 5). The enhanced sanders fitted with remote condition monitoring, to T1107 ‘Trial of Sander Configurations and Sand will help to clean the track, which should capture key performance data. Laying Rates’ improve the track conditions for the following IMP-T1107 ‘Class 323 DVRS In-Service Pilot train; this ought to lead to fewer instances DRIVER FEEDBACK Issue’ of slide and wheel flats which are known to COF-BRP ‘Class 323 DVRS Driver damage the track. The authors are not aware Following the introduction of DVRS equipped Familiarisation’ of any evidence that enhanced sanding leads units, a series of low adhesion driver All reports are available through RSSB’s to any higher wear rate of the rail, nor that it familiarisation sessions were conducted in repository, www.sparkrail.org will lead to sand-contamination of the ballast. order to provide an opportunity for train drivers

32 TECHNICAL ARTICLE

AS PUBLISHED IN The PWI Journal April 2020

VOLUME 138 PART 2

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