Anti-skid
The anti-skid is a device on rail-bound vehicles. Originally it was used to avoid flat spots on wheelsets, but in the meantime also generally for (selective) control of the brake slip of an axle in order to achieve optimal use of the adhesive value between wheel and rail. The railway worker speaks of sliding as soon as an axle blocks, but the vehicle continues to move along the rail, i.e. the blocked wheel at the contact point slides on it; therefore the name. The system works in a similar way to the anti-lock braking system in a car , but above all it serves to maintain the maneuverability in the event of emergency braking.
functionality
If the braking force exceeds the coefficient of friction between wheel and rail, the respective axle rotates with reduced speed ( slip ) or blocks completely. The wheel slide protection recognizes this and intervenes by reducing the brake pressure in the brake cylinder by means of bleeding impulses or limiting it by blocking the air supply. In the case of dynamic brakes, the control of the brake is accordingly intervened or the brake u. U. completely switched off.
In order to detect slippage or sliding of an axle, the speed of the wheelsets must be recorded. In the case of mechanical wheel slide protection, there is a flywheel on each wheelset shaft in the wheel slide protection housing, the inertia forces of which act on a vent valve when the wheelset speed suddenly drops, with which the brake pressure is reduced directly or indirectly. The electronic wheel slide protection records and compares the speeds of all wheel sets of a vehicle via electronic sensors . Sudden reductions in speed are recognized on the basis of specified characteristics, and the electronic wheel slide protection also reacts to speed differences depending on the technical level. Some systems also use external sources such as B. Radar for actual speed detection. Depending on how big the slip is, how long it lasts and how suddenly it occurs, the pressure in the brake cylinder is kept constant, reduced or increased again via solenoid valves .
The probability with which a slip condition occurs on the axles of a vehicle depends largely on the currently prevailing coefficients of friction between wheel and rail. Depending on whether the rail is dry, wet or even soiled (e.g. by leaves), different coefficients of friction occur. Tests for wheel slide protection devices are carried out with soft soap, among other things, in order to test the reaction of the devices in the worst case. Such series of tests can only inadequately simulate the real conditions, especially since there is no generally applicable coefficient of friction characteristic, rather the characteristic curve varies depending on many influencing factors. Depending on the situation and the condition of the rail, the maximum frictional connection can be at very different (relative) slip values, so that an optimally acting anti-skid device must be able to automatically determine the individual characteristic curve and use it to control the brake.
By applying brake sand by the leading vehicle, the coefficient of friction can be selectively improved if necessary.
In Germany, the UIC leaflet 541-05 is generally used as the basis for the functionality and safety of wheel slide protection devices.