Spin (wheel-rail)
In railway engineering, skidding refers to the spinning of a driven wheel on a rail. Devices called anti-skid devices are intended to prevent this.
The decisive factor for the transmission of tensile and braking forces from the wheel to the rail is the friction ratio between wheel and rail. This is described by the coefficient of static friction , which indicates the ratio of the maximum tensile or braking force to the normal force . On dry rails, the coefficient of static friction is around 0.3 (even higher on sanded rails), while on wet rails, especially in combination with other contamination, it is sometimes significantly lower.
If a tensile force acts on the wheel circumference, there is basically a relative movement, known as slip , between the wheel and rail surface, the amount of which depends on the force acting and the speed. If the tensile force approaches the maximum frictional force, the slip initially increases (macroslip) until, when the maximum frictional force is exceeded, the wheel and rails are decoupled and the wheel rotates in an uncontrolled manner faster than the driving speed. This condition is called skidding. Skidding leads to increased wear on the rail heads and wheel running surfaces; in addition, overturning or sudden catching of the wheelset can damage the drive.
The same applies to the transmission of braking forces; Accordingly, the following movement states can be distinguished between wheel and rail:
- Rolling (wheel circumferential speed ≈ driving speed)
- Skidding (wheel circumferential speed ≫ driving speed)
- Gliding (wheel circumferential speed ≪ driving speed)
Anti-skid
If the wheel sets skid, this is counteracted initially by reducing the tractive force and, above all, in the case of vehicles with block brakes, by applying the brakes with low pressures of a maximum of 1.0 bar (anti-skid brake). As a result, skidding axles can be quickly intercepted, and the wheel running surfaces are cleaned of the lubricating film and roughened. Then (or as a preventative measure), in order to increase the coefficient of static friction, sand can be blown under the wheels on certain axles from a sanding device .
In older vehicles, for example the 110 series , the driver is responsible for adapting the drive power to the maximum coefficient of friction and operating the anti-skid brake. Newer vehicles, for example the DB series 218 and 111 , can automatically detect a skid process and then temporarily limit the drive power. A further functionality is provided by an adhesion value detection which detects the maximum friction value and adjusts the tensile force accordingly. The first vehicles with a relatively weak version of such a device were the 143 series . Nowadays it is possible (such as with the 101 series ) to fully exploit the entire static friction potential with the adhesion value measurement and drives that can be regulated for each wheel set, sometimes even by taking advantage of higher friction values in the super slip range. With an optimized traction control, wheel-rail friction coefficients of 0.366 can be achieved reproducibly.
literature
- Michael Nold, Martin Jung, Oliver Klar: Optimized traction in the Gmf 4/4 II . In: ZEVrail , 11–12 / 2018, pp. 477–481.