RTR 4/2010 32 Self-energising electro-hydraulic brake (SEHB) 3 Mechanical principle of brakepad guidance The brake calliper has to be suspended in such a way that it is free to move, permitting the friction force to be directed to the support cylinder. The calliper’s range of movement is determined by the stroke of the support cylinder. This is decided on at the time of design. Given that braking torque is the product of the frictional force and the friction radius, it is advantageous to be able to guarantee a more or less constant friction radius. This can be achieved by guiding the calliper either through an internal connection to the wheel-set axle or through an external connection to the running-gear frame. 3.1 Internal circular guidance of the brake calliper An internal circular guidance (Fig. 2) is characterised by the fact that the brake calliper is attached to the wheel-set axle by means of a rotating bearing. The friction radius remains constant throughout the whole of the brake’s working range. Split bearings are envisaged for installing the self-energising electro-hydraulic brake and they must be designed for a long service life. The disadvantages of this variant include not just the high costs for the suspension but especially the installation of the brake on the unsuspended wheel set. In this situation, both transverse and longitudinal accelerations are transferred directly to the brake. The limited space available for installation to the side of the axle-mounted brake disc must also be regarded as a critical point. This might make it essential on some bogies to install the suspension on one side. The asymmetrical force transmission, which would thus be inevitable, would cause a tilting torque. The guidance mechanism would then have to be correspondingly massive in its design to compensate for this. 3.2 External guidance of the brake calliper In order to get round the whole problem of axle suspension, it is possible to place the guidance mechanism for the friction length on the bogie frame. Six-piece lever drives permit precise circular guidance, but they are large components. Less space is required for the installation of four-piece lever drives. These permit a good approximation of a circular path. Figure 3 illustrates an external brake-calliper guidance attached to a runninggear frame, with the brake pads being guided virtually along a circular path by means of a four-piece lever drive. The self-energising electro-hydraulic brake illustrated here is flange-mounted to the bogie, as is the case with conventional air brakes. There is adequate space available for this arrangement. In this instance, an external system has due to an eccentric arrangement of the support cylinder, the need to ensure that the necessary coefficient of friction for self-energisation is available even in unfavourable operating conditions. That makes it desirable to have an adequately high baseline contact pressure acting on the brake pad when there is no self-energisation, and optimisation of the whole package installed in the vehicle. possible corrosion and cavitation damage due to water in the hydraulic circuit, possible functional impairment of the brake after long periods of inactivity, since the principle underlying the hydraulic valves makes it impossible for them to be entirely sealed, and thus the limited amount of available oil for creating pressure in the support cylinders may be used up, higher tilting torques and bending moments on the mechanical components Fig. 1: Principle of the self-energising electro-hydraulic brake (source of all figures: the authors) Guidance Guidance Brake disc Wheel-set axle Contact force Brake pad Support cylinder Brake calliper Brake actuator Support force Friction force Brake actuator Brakeactuator force Control valve Pump (optional) Intermediatepressure reservoir Low-pressure reservoir (closed system) Reservoir (open system) Fig. 2: Circular guidance of the brake calliper by connecting it to the axle Brake pad Friction face Axle-mounted brake disc Flange Support cylinder Brake actuator Brake callipers
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