RTR Eurailpress

RTR 4/2010 33 Self-energising electro-hydraulic brake (SEHB) „ been chosen for guiding the brake pads. This is characterised, inter alia, by a relatively simple structure, the avoidance of tilting torques thanks to a symmetrical arrangement, a low bearing friction and good possibilities of integrating it in existing bogies. 4 Parking-brake function and hydraulic principle The review of the hydraulic circuitry led to optimisation of the oil leakages from it, better functional dependability after long periods of inaction and a higher brake-pad application force in the absence of selfenergisation. The structure of this system is discussed below. The mechanical and hydraulic circuit diagram is shown in Fig. 1, in which alternative concepts are also marked. Attention is drawn to the open or closed system and the addition of an ancillary pump. 4.1 Parking-brake function The SEHB’s self-energisation is limited by friction in the cylinders, friction in the rod guides and flow losses through the valves. Experience has shown that this can turn out to be problematical, especially if there are low friction values between the brake pad and brake disc. To react to that, the brake pads then need to be pressed harder to produce the necessary parking-brake force. For this reason, the parking-brake force is applied in two stages. The first stage consists in a spring on the brake actuator. The spring produces a pressure load mechanically, which is limited by the available installation space. When the brake is released, the brake actuator is held back hydraulically by operating the valve, and an air gap forms between the pad and the disc. A pump is used for the second stage. This pre-pressurises the intermediate-pressure accumulator and supports the pressure applied by the spring. In critical friction conditions and when the train is standing still, it is thus possible to build up a high friction force even in the absence of self-energisation. A reliable, buffered power supply is envisaged for this purpose. 4.2 Hydraulic circuitry Once a braking action has been completed, the support cylinder must be moved back to its starting position with the brake released. In a closed system it is possible to do without the spring-loaded resetting of the support cylinder. In cases like this, it can be done with the pre-pressurised low-pressure accumulator, whose pressure level must, however, be higher than the surrounding air pressure. Two of the advantages of this are that the support cylinder can be kept extremely compact and can thus be reset faster. The drawback to an open circuit is that it is possible for water and dissolved air to precipitate in its tank. Small quantities of water may also be drawn in through the piston rods during the cylinder’s return stroke. Precipitation reduces corrosion and cavitation damage from occurring on the system’s components and thus makes it possible to practice very long maintenance intervals on the self-energising electro-hydraulic brake. The open circuit is the most suitable configuration for meeting the requirements of railway traffic and that is why it is being pursued further. If opting for the open system, it makes sense to have a spring-loaded return of the support cylinder. If the SEHB has not been in use for a longish period of time, it must be assumed that pressure will have been lost inside it as a consequence of minimum leakage at the valves. In systems that combine the open hydraulic system with a pump, it is possible to rectify this by applying the brake as a normal action in a brake test and then releasing it again before the train departs. A pump is used to provide the necessary pressure for the brake actuator at this working point. 5 Support cylinder Up until the time of writing, the support cylinder used on the test rig has been of a two-chamber type. With this arrangement, the friction force is supported by one of the chambers, while the other sucks in replenishments from the low-pressure oil. Given that the pressurised chamber changes when the railway vehicle changes its direction, it is necessary to have a hydraulic rectification circuit. This ensures that it is always the pressurised chamber that is connected with the high-pressure part of the SEHB and the refilling chamber that is connected with the low-pressure or tank level. A support cylinder with only one chamber that is pressurised regardless of the vehicle’s direction of movement offers the advantage of minimising the volume of oil and thus the necessary hydraulic capacity. Quite apart from the simplification of the hydraulic circuitry, this configuration offers the advantage that there is no need for a suction action when building up the braking force. Fig. 3: Circular guidance of the brake calliper by a four-fold ar ticulated lever drive Brake pad Friction face Axle-mounted brake disc Flange Support cylinder Brake callipers Gear-unit swing bases Brake actuator Bogie testing and maintenance Machines for tough railway conditions www.nencki.ch/railway

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