Driver's brake valve
The driver's brake valve (abbreviated to FbV) is a valve block for controlling the compressed air brake on railway vehicles. Depending on its position, it regulates whether the brake line of a compressed air brake is vented, and thus braking is initiated or maintained, or whether the brake line is connected to a compressed air generator and compressed air reservoir, and thus the brake is released again or remains released. This device is located in the driver's cab of locomotives , railcars or control cars , which explains its name.
Positions
A driver's brake valve usually has the following latched service brake positions of the operating lever, the arrangement of which may differ depending on the design:
- Filling position - In this position, the main air line (HLL) of the train is connected to the main air reservoir (for locomotives) or the main air reservoir line (for control cars ), bypassing the pressure regulator . This causes the main air line to fill up quickly and thus release the brakes.
- Driving position - The main air tank is connected to the HLL via the pressure regulator in the FbV, which regulates a constant 5.0 bar in the HLL. Air losses due to leaks or the replenishment of the auxiliary air tanks on the vehicles are compensated for.
- Middle position - In this position the FbV completely blocks its connection to the main air line. Pressure losses in the HLL are not compensated. The unused FbV can be shut down on traction vehicles with several driver brake valves. The middle position is also used for the HLL leak test as part of the brake test .
- Final position - In the case of FbV with continuous venting in the braking position, this position is used to maintain a certain HLL pressure permanently. For this purpose, air losses in the train are compensated by the pressure regulator. This had been the state of the art since the beginning of the 20th century. Older FbV had to be placed in the middle position instead, which in turn increased the braking effect due to leaks in the train over time.
- Service brake position - This is either a position in which the main air line can be continuously and steplessly vented ("time-dependent actuation"), or several latched levels with which a specific HLL pressure can be selected directly ("position-dependent actuation") ). Combinations of continuously adjustable pressure in the area of weak braking effect and graduated preselection in higher braking levels are also common. Flat slide and rotary slide FbV always work time-dependent, self-regulators are position-dependent, electronic FbV are available in both time and position-dependent designs.
- Quick braking position - The HLL is quickly vented to ambient air pressure over a large cross-section, so that the highest possible braking force is quickly available.
Technical development
Flat slide valves
In the simplest case, the position of the operating lever regulates how fast the compressed air escapes from the brake line or how quickly the compressed air can flow from the reservoir into the brake line via the size of the valve openings. This FbV was named after the built-in flat slide valve with the different sized bores. Driver brake valves of this type had the following disadvantages: Lack of make-up in the final position, which resulted in a slowly self-reinforcing braking effect due to leaks in the train. The single-moving locomotive also had to be braked with the indirectly acting and single-release brake, which was extremely unsatisfactory in the shunting service. The installation of an additional, direct-acting, multi-release brake that acted on the existing brake linkage of the indirect locomotive brake was not possible because of the flat slide. By staying in the filling position for too long, the brakes in the train could be overloaded and were then permanently effective until they were completely deflated. Furthermore, the length of the train determined how long the driver's brake valve had to remain in the braking position.
The latter problem was solved at the beginning of the 20th century by adding an expansion tank of a defined size to the FbV. The FbV then first lowered the pressure in the expansion tank, which was independent of the length of the train in terms of time. The HLL pressure was only adjusted to the pressure in the expansion tank via a relay valve.
Rotary slide valves
The next development step was the use of a rotary valve instead of the flat valve. This made it possible to install an additional brake valve for the direct-acting brake. As early as 1910, flat slide valves were therefore considered technically obsolete.
Another improvement to the rotary slide valve was the addition of the end position. This was made possible by integrating a make-up in the compensation mechanism so that any desired HLL pressure could be retained.
Driver's brake valves of the rotary slide type are in some cases still in use today. In German-speaking countries, these are in particular the types Knorr 8 (for traction vehicles with one driver's cab) and Knorr 10 (for vehicles with two driver's brake valves).
Self regulator
The so-called self-regulator was the latest in the development of purely pneumatic driver brake valves. With FbV of this type, the pressure regulator is integrated into the FbV and is no longer set to a fixed value of 5.0 bar, but rather designed to be adjustable. It replaces the make-up mechanism of the old rotary slide valves. Overloading of the brakes is prevented because a filling surge is only possible depending on the pressure and is limited in time. For fixed brakes whose control valve does not fully release at 5.0 bar HLL pressure, the HLL pressure can be temporarily increased to up to 5.3 bar with a component called an "equalizer". The control valves can be prevented from responding by slowly lowering the increased pressure to 5.0 bar. These then adjust to the control pressure of the FbV so that the adjustment process only has to be carried out once. Although the self-regulating valve had been developed by the Knorr company before the Second World War , it did not gain acceptance until the post-war period and was installed in practically all newly built locomotives. In Germany, the types Knorr D2, its variant D5 (with the option to carry an actuator for a dynamic brake coupled with or to brake independently of it) as well as the Knorr EE4 for railcars are widespread.
Electronic driver brake valves
The latest generation of driver's brake valves no longer works purely pneumatically, but taps the lever positions electrically with the help of potentiometers . The required main air line pressure is generated by a brake computer outside of the FbV control unit with the help of exhaust and filling solenoid valves during normal operation. Automatic adjustment functions by the brake computer are common. When the FbV is moved to the driving position, a slightly higher pressure is generally applied to the HLL before it is slowly lowered to the usual 5.0 bar. A pneumatic stage is only available in the driver's brake valve as a fall-back level, especially for the quick brake position. A widespread representative of this design is the FS42 from Knorr, which is used on many three-phase AC locomotives.
literature
- Brakes . In: Deutsche Bundesbahn (Hrsg.): Railway teaching library of the Deutsche Bundesbahn . 4th edition. tape 122 . Josef Keller Verlag, Starnberg 1962.