Motor vehicles – Steering gear – With fluid power assist
Patent
1995-08-23
1997-08-26
Johnson, Victor E.
Motor vehicles
Steering gear
With fluid power assist
180428, 180441, 1375961, 1376275, 91375A, B62D 522
Patent
active
056602472
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a servo-steering unit for motor vehicles. The servo-steering unit includes a steering valve which has two inlet seat valves and two outlet seat valves the closing members of which are movable by actuating tappets against the force of centering springs depending on a rotation of a steering spindle. The closing members of the inlet seat valves are here actuatable in a closing direction by the force of pressure springs so that the inlet seat valves are closed in the neutral position of the steering valve. The outlet seat valves are open in the neutral position of the steering valve. The inlet seat valves do not open until the respective outlet seat valve closes.
BACKGROUND OF THE INVENTION
Such a servo-steering unit with closed center has been disclosed in DE-A1-41 26 020. That kind of servo-steering unit has a linear course of a characteristic line. This means that, in case of elevated servo pressures, that actuating torque can greatly increase.
However, in the servo-steering units with an open center used at present it is customary, starting from a specific pressure in the servomotor, once again to increase slightly the actuating torque. This means that starting from said specific point the characteristic line extends more abruptly. This effect is called "interception" or "actuating-torque limitation".
SUMMARY OF THE INVENTION
The problem on which the invention is based is to improve a servo-steering unit according to the preamble, that is, a servo-steering unit with closed center, in a manner such that with the steering valve thereof characteristic lines can be produced which in their shape approximate as much as possible the characteristic lines of servo-steering units with an open center used at present. The solution particularly results from a limiting piston being provided for each side of the steering valve which is actuated on one side by one of the centering springs and by the first pressure prevailing in a working chamber of a servomotor and on the other side by a limiting force. The limiting force, in the neutral position of the steering valve, is greater than the force of the centering springs so that the limiting piston abuts against a shoulder of a housing that contains the steering valve. Thereby, starting from a specific servopressure, the mechanical centering forces of the steering valve are reduced.
Convenient and advantageous embodiments of the invention are provided in the sub-claims. But the invention is not limited to the combination of features of the claims. Other logical possible combinations of claims and separate features of the claims result for the expert depending on the existing problem.
The limiting force can be produced by a limiting spring and/or a second pressure extracted from an accumulator and acting in a limiting chamber.
If only one limiting spring is used to produce the limiting force, then the pre-load force of said spring must be greater than the force of the centering spring. It is then convenient, by means of a passage bore in the actuating tappet, to connect the limiting chamber actuated by the reverse pressure with the tank of pressurized fluid. For said purpose, the actuating tappet has a coaxial extension which is snugly passed through the limiting piston.
If, on the contrary, importance is attached to smaller dimensions of the springs and, in particular, it is going to be prevented that the limiting springs have to be designed stronger than the centering springs, then the limiting chamber is actuated by a second pressure produced by a pressure-reducing valve from the pressure prevailing in an accumulator. The limiting springs can be completely eliminated in theory by adequate selection of said pressure. Besides, it is possible with said arrangement, by an adequate electronic control, to change said pressure, for instance, depending on the speed of the vehicle or other travel parameters such as the wheel skid and thus to produce a reaction force dependent on speed or another parameter.
If a specific, firmly defined pressure is desi
REFERENCES:
patent: 3750836 (1973-08-01), Kristof
patent: 4607717 (1986-08-01), Nakayama
patent: 5028851 (1991-07-01), Wilder
patent: 5529137 (1996-06-01), Lang
Johnson Victor E.
ZF Friedrichshafen AG
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