Motor vehicles – Steering gear – With fluid power assist
Patent
1996-03-12
1999-09-14
Johnson, Brian L.
Motor vehicles
Steering gear
With fluid power assist
180421, B62D 522
Patent
active
059507594
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a servo-assisted steering system for motor vehicles The servo-assisted steering system includes a steering valve that has two intake seat valves and two outlet seat valves, which can be adjusted via an actuating device as a function of a rotary motion of a steering shaft. The closure bodies of the outlet seat valves can be acted upon via their actuating tappets by the force of centering springs in the direction toward the actuating device, so that the actuating tappets contact the actuating device. The closure bodies of the intake seat valves can be acted upon in the closing direction by the force of compression springs, so that the intake seat valves are closed in the neutral position of the steering valve. The outlet seat valves are open in the neutral position of the seat valve. The intake seat valves are not opened until after the closure of the respective associated outlet seat valves.
2. Description of the Prior Art
This kind of servo-assisted steering system with a closed center is known from German Patent Disclosure DE-A1 41 26 020. Such a servo-assisted steering system has a linear characteristic curve course. This means that at high servo-pressures, the actuating moment can rise sharply.
In modern servo-assisted steering systems with an open center, however, it is usual for the actuating moment to be increased only insignificantly beyond a certain pressure in the servomotor. This means that beyond this certain point, the characteristic curve has a steeper course. This effect is known as "cutoff" or "actuating moment limitation".
The object of the invention is to improve a servo-assisted steering system of this generic type, that is, a servo-assisted steering system having a closed center, in such a way that with its steering valve, characteristic curves can be generated that in their form come as close as possible to the characteristic curves of conventional modern servo-assisted steering systems with an open center.
SUMMARY OF THE INVENTION
This object is attained by the servo-assisted steering system of the present invention. It is attained in particular in that in a generic servo-assisted steering system, one limiting piston is provided for each side of the steering valve, and the limiting piston is acted upon on one side by the pressure prevailing in a work chamber of a servomotor and on the other by a limiting spring. The limiting spring, in the neutral position of the steering valve, keeps the limiting piston at a distance from the closure body of the intake seat valve. As a result, beyond a certain servo-pressure, the mechanical centering forces of the steering valve are reduced. The limiting piston then acts upon the closure body of the intake seat valve, if the force of the pressure acting on the limiting piston exceeds the force of the limiting spring. An essential feature here is that the limiting piston acts on the closure body of the intake seat valve, and not on the closure body of the outlet seat valve. As a result, on steering back to a previous position, a valve characteristic curve is produced in accordance with which on a drop in the pressure in the servomotor, the actuating moment at the steering wheel decreases gradually.
The function of the limiting piston is improved by making the cross section of the limiting piston smaller than the cross section of the closure body of the intake seat valve.
Expedient and advantageous features of the invention are described herein. However, the invention is not limited to the combination of characteristics of the claims. For one skilled in the art, additional logical possibilities for combining claims and individual claim characteristics will become apparent from the stated object.
In a preferred embodiment, the limiting piston is kept in contact with a stop shoulder of a valve housing by the limiting spring, in the neutral position of the steering valve. As a result, the pressure at which the limiting piston begins to move can be defined precisely. In addit
REFERENCES:
patent: 4607717 (1986-08-01), Nakayama
patent: 5509493 (1996-04-01), Lang et al.
patent: 5529137 (1996-06-01), Lang
patent: 5590566 (1997-01-01), Joerg et al.
Knodler Helmut
Lang Armin
Johnson Brian L.
Lerner Avraham
ZF Friedrichshafen
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