Power plants – Pressure fluid source and motor – With control means for structure storing work driving energy
Reexamination Certificate
1999-09-16
2001-07-17
Ryznic, John E. (Department: 3745)
Power plants
Pressure fluid source and motor
With control means for structure storing work driving energy
C060S470000
Reexamination Certificate
active
06260355
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a hydraulic control arrangement which is used for a mobile working machine, especially for a wheel loader.
It is known from DE 39 09 205 C1 to make use for the purpose of damping the pitching vibrations of wheel loaders which occur, especially, with a full loading shovel and at a high driving speed of a damping system which is a component of the hydraulic control arrangement of the wheel loader. For the purpose of vibration damping, the generally two hydraulic lifting cylinders for raising and lowering the loading shovels can be connected via a shutoff valve to a hydraulic accumulator which can be charged by a hydraulic pump via a fill line which branches off from the pump line upstream of the directional control valve block. The shutoff valve, which is arranged between the hydraulic accumulator and the lifting cylinders, is closed as long as the loading shovels are working, and can be opened by the driver or automatically as soon as pitching vibrations occur during driving, or as soon as the driving speed is above a specific value, for example above 6 km/h.
As a consequence of the fact that the filling line branches off upstream of the directional valve control block, the hydraulic accumulator is charged not only upon actuation of the directional control valve assigned to the lifting cylinders, but upon actuation of any direction control valve which leads to a build up of pressure in the pump line. For example, the actuation of a steering valve belonging to a hydraulic steering system of the working machine can also lead to an inflow of pressure fluid to the hydraulic accumulator. If the shutoff valve is then also open, an uncontrolled movement of the lifting cylinders can take place.
Another damping system against pitching vibrations, which is likewise part of the hydraulic control arrangement of a working machine, is known from DE 41 29 509 C2. In this case, the filling line branches off from a working line which runs between the lifting cylinders and the directional control valve assigned thereto. The shutoff valve arranged in the filling line is pressure-controlled and can be opened by the load pressure, prevailing in the working line, of the lifting cylinders against the accumulator pressure, which can be applied to a rear control chamber on the valve element of the shutoff valve, and against the force of a weak compression spring. The accumulator pressure is thus in each case only slightly lower than the highest load pressure of the lifting cylinders which occurs during a working cycle. In order to damp the pitching vibrations, the rear control member of the shutoff valve is unloaded via a pilot valve to the tank, with the result that the shutoff valve can be opened and pressure fluid can be pushed back and forth freely between the hydraulic accumulator and the lifting cylinders.
It has emerged that because of special kinematic relationships on a wheel loader when the shovel is being pushed into the earth, a higher pressure can be built up in the lifting cylinders than prevails in the hydraulic accumulator, and then pressure fluid is displaced into the hydraulic accumulator from the lifting cylinders even when the assigned directional control valve is not actuated. The wheel loader is raised at the front, with the result that the front wheels turn in the air and the rear wheels dig in. This is disadvantageous for the loading operation.
SUMMARY OF THE INVENTION
It is the object of the invention further to develop a hydraulic control arrangement of the introductory-mentioned type so as largely to avoid uncontrolled movements of the hydraulic cylinder which can be connected to the hydraulic accumulator in order to damp pitching vibrations of the working machine.
According to the invention, this object is achieved in the case of a hydraulic control arrangement having the features from the preamble of claim
1
by virtue of the fact that the hydraulic accumulator can be filled up only when the directional control valve, which is assigned to the hydraulic cylinder, is actuated. Consequently, upon actuation of another directional control value no pressure fluid can flow into the hydraulic accumulator, with the result that the actuation of another directional control valve cannot lead to a movement of the hydraulic cylinder. It is also prevented that pressure fluid is displaced into the hydraulic accumulator from the hydraulic cylinder when the directional control valve is not actuated. The enclosed pressure fluid therefore prevents a movement of the cylinder piston and thus a change in the relative position between the boom and the body of the working machine, with the result that raising of the front axle is prevented.
The hydraulic cylinder is usually a double-acting one and can be controlled with the aid of a directional control valve which can be adjusted into working positions in two opposite directions starting from a middle neutral position. In accordance a feature of the invention it is provided that the hydraulic accumulator can be filled up only when the directional control valve is actuated in one direction. The control arrangement is thereby simplified. In the case of a wheel loader, for example, it suffices when pressure fluid can flow into the hydraulic accumulator only given an actuation of the directional control valve in the direction of raising the lifting cylinders.
In accordance with another feature of the invention, use is advantageously made for the purpose of controlling the capacity of the hydraulic accumulator to be filled of a control valve which can be switched over by actuating the directional control valve from a first control position, in which it prevents filling of the hydraulic accumulator, that is to say in which the filling line is blocked off such that no pressure fluid flows into the hydraulic accumulator even when the pump pressure or the load pressure is higher than the accumulator pressure into a second control position, in which the hydraulic accumulator can be filled.
The control valve which can be switched over by actuating the directional control valve is advantageously a seated valve, thus ensuring the filling line is blocked off in a largely leak-free fashion.
It is conceivable in principle for the control valve which can be switched over by actuating the directional control valve to be coupled mechanically to the latter. Directional control valves which are used in mobile hydraulics are currently ever more frequently driven no longer directly mechanically but with the aid of precontrol signals which can be prescribed with the aid of a precontrol unit. In accordance with still another feature of the invention, a precontrol signal for the directional control valve is also used for the purpose of producing a control signal for switching over the control valve from the first control position into the second control position.
Particularly preferred is a refinement according to which the filling line branches off from a working line running between the directional control valve and the hydraulic cylinder, and there is located in the filling line a pressure-controlled shutoff valve with a valve element to which the load pressure prevailing in the working line can be applied in the opening direction and the pressure in a rear control chamber can be applied in the closing direction. The rear control chamber is connected to the hydraulic accumulator in order to fill the hydraulic accumulator. An open connection between the hydraulic accumulator and the hydraulic cylinder must be present in order to damp the pitching vibrations of the working machine. The rear control chamber is unloaded with respect to the tank for this purpose. The rear control chamber can be blocked off against the outflow of pressure fluid in order to prevent filling of the hydraulic accumulator when the directional control valve is not actuated. The shutoff valve then cannot be opened by the load pressure.
REFERENCES:
patent: 3717995 (1973-02-01), Case
patent: 3971215 (1976-07-01), Baron et al.
patent: 50
Rausch Georg
Roth Dieter
Farber Martin A.
Mannesmann Rexroth AG
Ryznic John E.
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