Fluid handling – Systems – Supply and exhaust
Reexamination Certificate
2000-04-17
2001-04-24
Buiz, Michael (Department: 3753)
Fluid handling
Systems
Supply and exhaust
C091S420000, C091S447000
Reexamination Certificate
active
06220289
ABSTRACT:
The invention concerns a hydraulic valve arrangement with locking and floating function, with a control valve, which, for two operating positions connects one motor connection with a pump connection and a second motor connection with a tank connection and vice versa, in a floating position connects both motor connections with the tank connection and in a locking position separates both motor connections from the pump and tank connections, and with two closing valves, each being connected between the control valve and a motor connection and each having a piston loaded by a spring and limiting a spring chamber, the spring chambers being pressure releasable.
Known hydraulic valve arrangements of this kind are mainly, but not exclusively, used in tractors, which can be provided with attachment tools, for example snow clearance or street cleaning vehicles. The pressure release of the spring chambers, which is required to keep the closing valves open in the operation positions and in the floating position of the control valve, occurs in that release channels open into the bore of the control valve and the opening is oversteered by its slide. However, sometimes it has turned out that errors occur in the function, particularly in the locking function.
The invention is based on the task of providing a hydraulic valve arrangement of the kind mentioned in the introduction, which has a better operating behaviour.
According to the invention, this task is solved in that lifting valves are provided for the pressure release, which valves are operated by a control pressure depending on the position of the control valve.
This design is based on the assumption that leakages are responsible for the failures, which leakages can never be completely avoided in slide controls. For high requirements, the invention provides the use of lifting valves for pressure release, which can without problems be dimensioned so that they are completely tight in closed position. The dependence of the valve operation of the position of the control valve is effected in that the lifting valves are operated by a position dependent control pressure. Both lifting valves are open in the floating position and closed in the locking position, whereas in the operating position one lifting valve or the other is open optionally. Therefore, corresponding control pressure levels can be provided, to which the lifting valves can be connected in dependence of the position of the control slide. An additional advantage is that leakage losses in connection with the pressure release are avoided.
In this connection, it has turned out to be advantageous that the lifting valves open at high control pressure and close at low control pressure. This gives a high certainty that the release process runs properly.
Expediently, pressures, which are available in the control valve anyway, are used as control pressure. Thus, the control pressure in the operating positions can be the load pressure. In the locking position the control pressure can be the tank pressure. And in the floating position the control pressure can be a floating signal pressure or the pump pressure.
With regard to design it is recommended that the lifting valves have a closure piece which is pressed against its valve seat by means of a spring, and a piston acting upon the closure piece in the opening direction, which piston is loaded by the control pressure in the opening direction and by the tank pressure and a spring in the closing direction. Such a valve is normally closed and opens when a sufficiently high control pressure appears.
In a further embodiment of the invention it is provided that the pistons of each pair are arranged in series two and two, and in the operating position the control pressure is led to a control pressure chamber between the pistons, in the floating position commonly to control pressure chambers at the ends of both pairs. In the operating position, therefore, one of the two pistons facing the closure piece is operated, in the floating position, however, both pistons of each pair are displaced.
In this connection it is recommended that the control pressure line leading to the control pressure chambers at the ends of both pairs be connected via a throttle with a low-pressure line carrying low pressure. The pressure building up in the piston operating chambers in the locking position can therefore be dissipated via the line carrying low pressure (or tank pressure).
A preferred alternative provides that in each case one piston with a control pressure chamber is available, to which, in the operating position, the control pressure is led separately, in the floating position via a closing valve, from a common control pressure line. For each lifting valve one piston will be sufficient, which saves costs.
Besides, it is expedient that the control pressure line carrying load pressure in the operating position is con nected to a low pressure in the locking position. One and the same control pressure line can here be used for two operating positions. A normal control valve can be used. Further, automatic pressure dissipation occurs, when the control pressure line is connected with the low pressure.
REFERENCES:
patent: 4716933 (1988-01-01), Stoever et al.
patent: 5000001 (1991-03-01), Christensen et al.
patent: 5036750 (1991-08-01), Katayama
patent: 5211196 (1993-05-01), Schwelm
Christensen Carsten
Christensen Thorkild
Zenker Siegfried
Buiz Michael
Danfoss Fluid Power A/S
Krishnamurthy Ramesh
Lee Mann Smith McWilliams Sweeney & Ohlson
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