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Dr. Eng. Mandeep Singh Walia, Green Cargo AB, SWEDEN
Dr. Eng. Gaël Le Gigan, Volvo Car Corporation, SWEDEN
Mr. Bjarke Raaby, Green Cargo AB, SWEDEN
Dr. Eng. Tore Vernersson, Chalmers University of Technology, SWEDEN
Prof. Roger Lundén, Chalmers University of Technology, SWEDEN
The maintenance procedure for railway brake discs is highly complicated, time-consuming and costly. Firstly, the entire wheelset is separated from the wagon and then the bearings and the wheels are separated from the axle followed by the brake discs. For this reason, brake discs are preferably changed at the same time as the wheels. This procedure involves a risk of damaging the wheels and the axle during removal and reinstallation. The axle might develop scuffs or scratches, leading to replacement of the entire axle. Such incidents cause increased life cycle costs of these wagons. A segmented brake disc can provide improved maintainability, since they can be exchanged with the wheelset still under the wagon. Thus, there is no need for dismounting the wheels from the axle. Only the external friction ring is replaced, effectively reusing the brake disc hub mounted on the axle. Another important factor for the life cycle costs of these wagons is the wear of brake discs and brake pads. Highly wear resistant brake discs and brake pads will be favourable.
Studies already exists where brake disc temperatures and wear are studied via laboratory experiments and simulations. However, studies are rare that compare different types of brake discs and pads by field experiments and simulations. In the present study, the performance of two different types of brake discs and brake pads used on Swedish postal wagons is investigated through a combination of field experiments and numerical simulations. Thus, one traditional solid grey cast iron brake disc with organic composite brake pads and one segmented NiCrMo alloyed cast iron brake disc with sintered brake pads are compared. The focus is on disc and pad temperatures and wear at revenue traffic and on calculated thermomechanical fatigue performance of the two brake disc types.
Field tests were carried out for a postal wagon equipped with two different friction pairs. One consists of an enhanced grey cast iron material “segmented” brake disc (friction rings built from five identical sectors) with sintered friction material. The other consists of a standard grey cast iron “reference” disc with organic composite friction material as originally used on this wagon. During revenue service with speeds up to 160 km/h, temperatures for brake discs and brake pads were recorded along with train braking data in the form of train speed, friction forces and pneumatic brake pressures. A compact thermocamera setup was used to record temperature distributions over disc friction surfaces. Wear of discs and pads were measured intermittently by use of a laser scanning device and precision scales, respectively. Simulation models for predicting brake disc and pad temperatures are established. Heat partitioning factors pertaining to the disc and pad interface and convection cooling models are calibrated using data acquired for the two studied friction pairs. Finally, the thermomechanical behaviour and the fatigue of the two brake disc types are simulated for a simplified load case with uniform deceleration from maximum speed at maximum axle load for the vehicle. The results from the revenue service show that the disc wear is six times and pad wear is four times lower for the segmented disc than for the reference disc. The thermomechanical simulations show that for the studied stop braking cycle, the segmented brake disc has two times longer calculated fatigue life (until initiation of cracks on the brake disc friction surfaces) than the reference brake disc.
In conclusion, the segmented brake disc assembly shows a better performance than the reference disc assembly in terms of temperature, build-up of residual tensile stresses, fatigue life and wear. Finally, the segmented disc can provide the possibility to replace the friction ring “condition based”, avoiding disassembling the wheelset components when not necessary. This will reduce the maintenance cost and all these aspects in combination can reduce the life cycle cost of a wagon.
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