Swedish tests of ll-brake blocks under winter conditions - winter 2020–2021

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"The Swedish Transport Agency arranged testing of LL brake blocks in the northern part of Sweden. The tests were performed from January through to April 2021, using a train built-up by one locomotive and five (mostly) unloaded (empty) test wagons. The locomotive was unbraked during the tests. Two sets of five wagons were used, where one wagon set was equipped with organic composite blocks and one set was equipped with sinter blocks. Both wagon sets had a detailed sensor instrumentation on three of the wagons during the test campaign. During the test campaign, four stop braking tests per day were performed from 100 km/h. Three different locomotive driver instructions were employed that prescribed the brake applications to be performed in-between the stop braking tests and one aim was to investigate braking performance for these instructions. The first instruction is denoted “normal” brake conditioning and the brakes are then applied for 13 s every 10 minutes with main brake pipe pressure lowered by 0.6 bar. The “enhanced” brake conditioning is that the brakes are applied for 10 s every 15 minutes with main brake pipe pressure lowered by 1.0 bar. The third one is denoted “provocative” brake conditioning which means that no brake applications are performed for some prolonged periods between stop braking tests. A total of 127 stop braking cycles were performed by the instrumented test train, 78 stops with organic composite LL-blocks and 49 with sinter LL-blocks. For organic composite brake blocks and braking with loaded wagons (15 tonnes axle load), very strong braking action was found, which resulted in locking-up of wheel axles and formation of wheel tread damage in the form of wheel flats. Because of this, tests with loaded wagons were only performed on the first day of testing and all remaining tests were performed with unloaded wagons. The wheel flats were machined by an angle grinder so that acceptable levels of wheel-rail contact forces were achieved, although it can be presumed that some test wagons had somewhat increased vibration levels for the remainder of the test campaign. For organic composite blocks and unloaded wagons with an axle load of 6.5 tonnes it is found for perfectly bedded-in blocks that the stopping distances when using the normal brake instructions are the shortest, followed by the ones for the enhanced instructions, and the longest are found to result when employing the provocative driver instructions. No extremely long stopping distances (longest stopping distance is 910 m) are resulting, which can be compared to a nominal braking distance (based on information in UIC leaflet 544-1) for the train being 850 m. Ice and snow were present on the blocks during testing to a high degree for all three driver instructions, often in such amounts that the actual blocks could not be seen. For sinter blocks and unloaded wagons with an axle load of 6.5 tonnes it is found for not perfectly bedded-in blocks that they behave rather well when employing normal brake instructions; in fact, the performance is somewhat better than for “enhanced driver instructions”. This finding is the same as for the organic composite blocks. Braking employing the provocative instructions causes occurrences of substantially prolonged braking distances, with longest stopping distance 1100 m that can be compared to the nominal braking distance 850 m. For one such stop, two wagons more or less lost their braking abilities as an effect of massive amounts of ice and snow, that had accumulated between blocks and wheels, fell off at the test. The stopping distance for the train under these conditions was 1550 m, which constitute more than a doubling of the braking distance as compared to the shortest stopping distances registered. The sinter blocks are not very well bedded-in at the tests; the degree of bedding-in is between about 60% and 100% for the test wagons. One effect of a low degree of bedding-in is that it seems to help keeping the brake blocks frictionally active, a conclusion drawn from the fact that they are less prone to high reductions in friction and the related low block temperatures at tests. One reason for the weaker braking for the sinter brake blocks as compared to the organic composite blocks is that the time until the friction force reaches a significant level, after the brake is applied, is almost the double. "