Motors: expansible chamber type – With motive fluid valve – Relatively movable serial valves
Reexamination Certificate
2001-02-27
2003-11-11
Look, Edward K. (Department: 3745)
Motors: expansible chamber type
With motive fluid valve
Relatively movable serial valves
C137S596170, C251S367000
Reexamination Certificate
active
06644169
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a valve system for the hydraulic work system of a work vehicle, in particular an industrial truck, with an elevating drive system to raise and lower a load and a tilting drive system, each of which can be actuated by respective control valves, in particular by electrically actuated control valves, the opening width of which determines the speed of movement of the respective users, wherein the elevating drive system is realized in the form of a single-action hydraulic cylinder and a check valve that is realized in the form of a seat valve and opens toward the hydraulic cylinder and is located in a hydraulic line that leads from the control valve to the hydraulic cylinder, and wherein the tilting drive system is realized in the form of a double-action hydraulic cylinder.
2. Description of the Currently Available Technology
Valve systems of the type described above are used in industrial trucks, for example, and in fork lift trucks in particular. The elevating drive system, in the form of one or more lifting cylinders connected in series, is used to raise and lower loads. To raise the load, the lifting cylinder, which is realized in the form of a single-action hydraulic cylinder, can be connected by means of a control valve to a pump line, and for lowering the load it can be connected to a return line that is connected to a reservoir. In such a case, the control valve is realized in the form of a proportional valve, the opening width of which determines the quantity of hydraulic fluid that is admitted into the lifting cylinder or is discharged from the lifting cylinder, and thus determines its speed of movement. A pilot-controlled check valve in the hydraulic line that leads from the control valve to the lifting cylinder blocks off the lifting cylinder with no leakage of hydraulic fluid. In systems of the prior art, during the lifting operation, the check valve is switched into the open position by the pressure of the hydraulic fluid flowing into the lifting cylinder. For lowering, the check valve is unblocked, whereby the check valve is opened by the pressure in the pump line. In that case, however, to open the check valve to lower the load, a pressure must first be built up in the pump line that at least equals the load pressure of the elevating drive system to switch the check valve into the open position. That results in energy losses which, on battery-operated fork lift trucks in particular, lead to a reduced operating time of a charged battery.
When electrically actuated control valves are used, such as control valves actuated by electromagnets, for example, it must also be taken into consideration that the control valve for the elevating drive system cannot be opened too wide when a load is being lowered to prevent an excessively rapid descending movement and to guarantee that there is a sufficient braking distance available for the deceleration of the load. For this purpose, on electrically actuated control valves of the prior art, the opening width of the control valve is limited in the “descent” position, which means that small loads are lowered at a slow speed of descent since, as the result of the presence of only a small pressure differential at the throttle point of the control valve, the speed of descent that can be used to lower a large load cannot be achieved.
The hydraulic cylinders of the tilting drive system are generally realized in the form of double-action hydraulic cylinders. To achieve a firm restraint of the hydraulic cylinders in tilting drive systems of the prior art, there is a pilot-controlled check valve in each of the hydraulic lines that leads to the hydraulic cylinders. The check valve located in the discharge line is thereby switched into the open position by the pressure in the respective admission line. However, such a system is susceptible to oscillations. To prevent oscillations, high restraining pressures of the check valves are set that must accordingly be overcome by the admission pressure to achieve a high stability. However, especially when the loads to be moved by the tilting drive system are small, there are large losses that lead to a shorter battery life in industrial trucks that are powered by rechargeable batteries.
Therefore, it is an object of the invention to make available a valve system of the type described above that has an improved function with low energy losses.
SUMMARY OF THE INVENTION
The invention provides a valve system in which the check valve has a control surface that acts toward the opening position, which control surface can be pressurized by the load pressure of the elevating drive system, and a control pressure chamber of the check valve that acts toward the closed position, which control pressure chamber can be pressurized with the load pressure of the elevating drive system and, when the control valve is actuated into the “descent” position, can be placed in communication with a reservoir, and/or that there are flow regulators located in both the admission line and in the discharge line respectively of the control valve of the tilting drive system, which flow regulators have an open and a closed position, whereby the flow regulators are respectively pressurized toward the closed position by the pressure upstream of the throttle point of the control valve, and toward the open position by the pressure downstream of the throttle point of the control valve and by a spring.
The control pressure chamber of the check valve that is pressurized by the load pressure of the elevating drive system and is active in the closed position can thus be relieved when a load is lowered. The check valve is thereby opened by the load pressure of the elevating drive system that is applied to the control surface that acts in the opening direction. When the user is idle, the hydraulic line that leads to the lifting cylinder is blocked off by the check valve without any leakage of hydraulic fluid. When the load is lowered, the control pressure chamber of the check valve pressurized by the load pressure is thereby relieved and the check valve is actuated by the load pressure applied to the control surface that acts in the opening direction. No pressure needs to be built up in the pump line to open the check valve, and consequently there are no energy losses.
The hydraulic cylinders of the tilting drive system are restrained by means of respective flow controllers in the admission and discharge lines of the tilting drive system, whereby the admission-side flow controller is controlled by the pressure decrease at the admission-side throttle point, and the discharge-side flow controller is controlled by the pressure decrease at the discharge-side throttle point of the control valve. The flow controllers thereby maintain the incoming and outgoing flow of hydraulic fluid, regardless of the load, at the value specified by the opening width of the control valve. The tilting drive system can thereby be operated at the speed of movement specified at the control valve, independent of the height of the load. The tilting drive system is thereby restrained in both directions of movement between the flow regulators, whereby the only loss is the pressure drop that is necessary for the control of the flow regulators at the throttle points of the control valve.
The switching systems of the invention for the elevating drive system and the tilting drive system can be used individually or in combination with one another. Their combined use increases the efficiency of the overall system.
It is particularly advantageous if the control pressure chamber of the check valve can be placed in communication with a relief valve that is realized in the form of a seat valve and can be connected to a reservoir, whereby the relief valve can be moved into a closed position by the load pressure of the elevating drive system, and into an open position when the control valve of the elevating drive system is actuated into the “descent” position, in which position the control pressure
Deininger Horst
Schulze Eckehart
Leslie Michael
Linde Aktiengesellschaft
Look Edward K.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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