The present invention relates to hydrodynamic brakes and particularly to the type of hydrodynamic brake comprising a toroidal work chamber with a stator and a rotor housed therein.
It is known that the braking torque produced by a hydrodynamic brake increases with the square of the speed of rotation of the rotor for a constant degree of filling of the brake housing with pressure fluid. As a rule, however, the braking of, for instance, a car should take place with an at least approximately constant braking torque. Therefore, a control device must be provided to ensure that upon the application of the brake, a small degree of filling occurs initially and that the degree of filling increases continuously as the speed of rotation of the rotor decreases, possibly until the housing is completely filled.
FIG. 2 of German Unexamined Application for Patent (Offenlegungschrift) No. 21 20 743 discloses a hydrodynamic brake in which an overflow valve is arranged in the outlet conduit of the brake in order to regulate the degree of filling. The valve body of the overflow valve is displaceable in the "open" direction by a force produced by the hydraulic pressure in the outlet conduit and is displaceable in the "close" direction by a suitable opposing force. Pressure fluid present in a pressure chamber acts on the valve body of the overflow valve in the "close" direction. The pressure fluid is maintained by a precontrol valve at an adjustment pressure which is proportional to the desired value. As long as the brake operation takes place with the working chamber partially filled, the forces acting on the valve body of the overflow valve are in equilibrium. This means that the force produced by the hydraulic pressure in the brake outlet conduit is equal to the opposing force produced by the adjustment pressure. (If necessary the opposing force can be supplemented by the force of a spring.) Thus, as a result of the overflow valve, the pressure prevailing in the outlet conduit of the fluid brake is set to a value corresponding to the desired value.
However, it is primarily desirable that the hydrodynamic braking torque be maintained substantially constant at a desired value. This, however, is not possible with all brake designs, particularly designs including an overflow valve, as described above. For example, in brakes of some designs, it may happen that when a desired torque value is set, the hydrodynamic braking torque decreases when the speed of rotation decreases rather than, as is generally desired, remaining constant or increasing slightly.
FIG. 2 of West German Pat. No. 2,408,876, which corresponds to U.S. Pat. No. 3,989,127, discloses a hydrodynamic brake in which a regulating valve is provided instead of the above mentioned precontrol valve. A regulated adjustment pressure is produced in the regulating valve by comparison of the desired value with a control variable. The regulated adjustment pressure displaces to a greater or lesser extent the valve body of an adjustment valve, for controlling the inlet and outlet cross-sections of the brake. The control variable can be given a precisely determinable relationship to the hydrodynamic braking moment. Thus for instance the control variable can be made exactly proportional to the hydrodynamic braking moment over the entire range of speeds of rotation of the rotor.
This known hydrodynamic brake has been proved suitable for use in railway or highway vehicles, in which it is not necessary to obtain excessively high decelerations. On the other hand, in military cross-country vehicles, and particularly in heavy track-laying vehicles, extremely high decelerations are at times required. For example, a hydrodynamic brake may have to decelerate a vehicle having a weight of 50 tons from a speed of 70 km/h to a speed of 20 km/h within less than 3 seconds. The known hydrodynamic brake cannot satisfactorily perform such a task, because the known brake design requires a strong spring to give the adjustment valve body a high reset speed, and the adjustment pressure must operate against this strong reset spring. A large part of the adjustment pressure available is required merely to counterbalance the force of the spring, and the portion of the adjustment pressure which remains available for accelerating the valve body is relatively small. This fact reduces the reaction speed of the entire control device.