Patent Publication Number: US-7900445-B2

Title: Hydraulic supply systems

Description:
BENEFIT CLAIM 
     This application is based on, and claims the benefit of priority to, UK application GB 0606997.5, filed 7 Apr. 2006, which priority application is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system for utility vehicles, in particular agricultural tractors, for supplying primary and auxiliary pressure medium consumers with different system pressures, which are built up as a function of the load pressure of the pressure medium consumers and which exceed the load pressure by a pre-determined control pressure differential. In order to produce a first control pressure differential for operating a primary pressure medium consumer, its load pressure is used directly for building up the system pressure and in order to produce a second, higher control pressure differential for operating an auxiliary pressure medium consumer, its load pressure is converted in an amplifying device to an increased pressure, which is used for building up a higher system pressure. 
     2. Description of Related Art 
     European Patent EP 10 70 852 A2 describes a hydraulic system, which can be equipped with a load sensing pump or a fixed displacement pump and whose system pressure and flow rate are adapted to the respective need by a pressure and flow controller. If a fixed displacement pump is used the pressure and flow controller is adjusted as a function of the maximum load pressure of the primary pressure medium consumers so that the system pressure is always higher by a pressure differential of approximately 10 bar than the maximum load pressure. For operating auxiliary pressure medium consumers due to the greater pressure losses in the supply lines the pressure and flow controller is adjusted in such a way that the system pressure is always higher by a pressure differential of approximately 20-23 bar than the maximum load pressure. To create the higher control pressure differential for auxiliary pressure medium consumers an amplifying device is provided, which produces an actuating pressure exceeding the maximum load pressure, to which the pressure and flow controller is subjected. To this end for operating both the primary and auxiliary pressure medium consumers the actuating pressure between two throttling restrictions of the amplifying device is reduced. To create different actuating pressures, as they are needed to produce the various control pressure differentials for these pressure medium consumers, the line containing the throttling restrictions is blocked by means of an additional pressure regulator whenever a primary pressure medium consumer is in operation and unblocked whenever an auxiliary pressure medium consumer is in operation. A disadvantage here is that the load pressure of the primary pressure medium consumers, which is utilized as actuating pressure for operating said pressure medium consumers is subject to restriction when passing through the throttling restrictions. As a result the actuating pressure takes longer to build up and finally the system dynamics are lower as a result. 
     A further disadvantage of the prior art hydraulic system is apparent if no implement is mounted on the vehicle, that is to say no auxiliary pressure medium consumer is connected to the hydraulic system of the vehicle. In this case it is possible that due to thermal expansion of the pressure medium inside the load pressure line of the auxiliary pressure medium consumers, which conducts the load pressure, or due to a leakage, pressure medium undesirably flows to the pressure and flow controller of the pump. The effect of this is automatic restriction of the pump even as far as actuation of the assigned pressure relief valve (pump short-circuit). 
     Hydraulic systems of the type described are predominantly used in high performance agricultural tractors because of the high cost of the load-sensing pump. In less powerful agricultural tractors hydraulic systems with cost-favourable fixed displacement gear pumps as well as load sensing control valves are normally used. A pressure regulator is assigned to such a pump, which in the case of non-operating pressure medium consumers and a control pressure differential of approximately 5 bar returns the conveyed pressure medium to the tank virtually unpressurized. On the other hand if at least one pressure medium consumer is in operation, a system pressure, which is higher by a control pressure differential of approximately 13 bar than its load pressure is built up as a function of the load pressure. Therefore all primary pressure medium consumers mounted on the vehicle can be adequately supplied with pressure medium. As is the case of the prior art hydraulic system however this does not apply to auxiliary pressure medium consumers, in the long supply lines of which substantial pressure losses take place. 
     It is therefore desirable to provide a hydraulic system of the kind described at the beginning, which, irrespective of whether the hydraulic system is equipped with a variable displacement (load sensing) pump or fixed displacement (gear) pump, can supply sufficient volume and pressure to both the primary pressure medium consumers and the auxiliary consumers without any significant time delay. 
     BRIEF SUMMARY OF THE INVENTION 
     This objective is achieved by the fact that the amplifying device consists of a pressure reducing valve, which is permanently set at a pre-determined reduced pressure and has a first pressure inlet, a second inlet, an outlet and a control piston, whereby a line conducting the system pressure is connected to the first pressure inlet, a load reporting line conducting the load pressure of the auxiliary pressure medium consumer is connected to the second inlet and a load reporting line conducting the increased load pressure is connected to the outlet, and the control piston on its one side is subjected to the force of a permanently adjusted spring, which determines the pre-determined reduced pressure as well as the load pressure prevailing at the second inlet of the auxiliary pressure medium consumer and on its other side to the pressure at the outlet. 
     This arrangement in the case of hydraulic systems with the most varied system pressure sources enables the cost of the amplifying device to be minimized by using only one economical standard component and the auxiliary load pressure to be superimposed with a permanently pre-determined pressure. So that this permanently pre-determined pressure, when no auxiliary pressure medium consumer is in operation, cannot have any undesirable effects on the pressure controller of the pump, a solenoid-operated switching valve can be arranged in the line, which is switched to the passage position whenever auxiliary pressure medium consumers are in operation. 
     The load reporting line conduction the load pressure of the auxiliary pressure medium consumers is connected via a flow control valve to the tank. This reliably prevents pressure from building up in the amplifying device due for example to thermal expansion of the pressure medium, which may affect the build up of the required system pressure in an undesirable way. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
       The invention is described below in detail with reference to the accompanying drawing showing a circuit diagram for a hydraulic system in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The circuit diagram shows a hydraulic system of an agricultural tractor having a fixed displacement pump  1 , which sucks pressure medium via a suction pipe  2  from a tank  3  and supplies this medium via pressure pipes  4 ,  5  to a tractor-mounted control block  6 . From here the pressure medium is distributed to primary pressure medium consumers  7 , directly connected to the hydraulic system. By means of an auxiliary control block  11 , connected via hydraulic couplings  8 ,  9 ,  10  to the hydraulic system of the tractor, further distribution to auxiliary pressure medium consumers  12 ,  13  is realized. “Pressure medium consumers” here are understood as single and double acting hydraulic actuators (linear actuators and rotating actuators) for driving different implements such as for example the primary power take off cylinder of the implement attachment device or the auxiliary hydraulic motors for the undercarriage and the blower of a sowing machine for example. 
     The primary control block  6  consists of an inlet section  6   a , a valve section  6   b  and a sealing plate  6   c , which are bolted together to form a unit. Several valve sections  6   b  can be provided, depending on the number of pressure medium consumers  7  to be actuated. 
     The inlet section  6   a  houses a pressure relief valve  14 , a pressure regulator  15  and a pressure sequence valve  16 , whose common task consists in keeping the system pressure of the hydraulic system at a level, at which on the one hand as low as possible power losses arise for example in neutral circulation, that is to say if no pressure medium consumer is in operation, and on the other hand each actuated pressure medium consumer is operated at optimum pressure. Such arrangements have been known for a long time and have been described in detail. 
     The pressure relief valve  14  limits the system pressure to a maximum permissible value. Customary values for agricultural tractors are 200 bar. The pressure scale  15  as a function of the load pressure of the operating pressure medium consumers  7  communicated via a load reporting line  17  is adjusted so that a defined pressure gradient, also called control pressure differential, always prevails between the pressure pipe  5  and the load reporting line  17 . If no pressure medium consumer is actuated and therefore no load pressure prevails, the pressure regulator  15  switches to neutral circulation. For this purpose the pressure regulator  15  is pre-tensioned by means of a spring  18  to a pressure of approximately 5 bar, so that pressure medium conveyed from the pump  1  flows back virtually unpressurized and with low losses to the tank  3  via the return pipes  19 ,  20 . If the load reporting line  17  is unpressurized, the pressure sequence valve  16  is in the blocked position. If a pressure&gt;10 bar prevails in the load reporting line  17 , the pressure sequence valve  16  changes to the passage position. In this case a control pressure differential of 13 bar arises at the pressure regulator  15 . 
     The valve section  6   b  contains a solenoid-operated main slide valve  21  of the load sensing type, a section pressure regulator  22  and a shuttle valve  23 . The primary pressure medium consumer  7  is connected to the main slide valve  21 . Its pressure medium is supplied via the pressure pipe  5  and its load pressure is supplied to the pressure regulator  15  via load reporting line  24 , shuttle valve  23  and load reporting line  17 . The section pressure regulator  22  lies in a pressure pipe  25  branching off from the pressure pipe  5  to the main slide valve  21  and by corresponding pre-tensioning of a spring  26  permits a desired differential pressure to be adjusted between the pressure pipe  25  and the load reporting line  24 . A customary value for the pressure differential is approximately 8 bar. 
     For normal operation of primary pressure medium consumers  7  a differential pressure of approximately 13 bar is necessary. The pressure regulator  15  builds up this differential pressure, as the result of the previously closed pressure sequence valve  16  being switched to the passage position by means of the maximum load pressure of the actuated primary pressure medium consumers  7  via the load reporting lines  24 ,  17 . Thus a differential pressure of approximately 13 bar arises between the load pressure pipe  17  and the junction  27 , which is sufficient to compensate for any flow losses between the pump  1  and the valve section  6   b . Such adjustment of the pressure gradient ensures low-loss and reliable operation of the primary pressure medium consumers  7 . 
     The auxiliary control block  11  is arranged on an implement, a potato digger for example, and consists of an inlet section  11   a  and several valve sections  11   b , whereby a valve section  11   b  is present and a sealing plate  11   c  for each pressure medium consumer  12 , 13  operated with the implement. The auxiliary valve section  11   b  has a solenoid-operated main slide valve  28  of the load sensing type, a section pressure regulator  29  and a shuttle valve  30  corresponds in structure and working method to that of a primary valve section  6   b . Load reporting lines  31  leading from the main slide valve  28  conduct the load pressure of the auxiliary pressure medium consumers  12 ,  13  to the shuttle valve  30 . From these the respective maximum load pressure is transmitted to the auxiliary load reporting line  32 , which leads to the hydraulic coupling  10 . From here a primary load reporting line  33  conducts the load pressure to an amplifying device  34  integrated in the sealing plate  6   c.    
     The amplifying device  34  consists of a mechanical pressure reducing valve  35  permanently set to 10 bar, solenoid-operated switching valve  36  and a flow control valve  37  set to a nominal flow rate of approximately 0.5 liters per minute. In the embodiment described the pressure reducing valve  35  is arranged in such a way that its pressure inlet  38  is connected to the pressure pipe  5 , the inlet  39  to the load reporting line  33   a  branching off from the load reporting line  33  and the outlet  40  is connected to a load reporting line  41 . The switching valve  36  is arranged in the load reporting line  41 . It is only switched to the passage position when auxiliary pressure medium consumers  12 ,  13  are in operation, so that the load pressure applied to the outlet  40  can be conducted to the pressure regulator  15  via the shuttle valve  23  and the load reporting line  17 . If no auxiliary pressure medium consumer is actuated, the switching valve  36  is switched to its closed position. The load reporting line  41  is then connected to the return pipe  19  by the switching valve  36  and thus safely vented. Therefore any undesirable influence of the pressure regulator  15  is ruled out with certainty. 
     The control piston  42  of the pressure reducing valve  35  is in a state of equilibrium, whereby the permanently set force of a spring  43  as well as the load pressure at the inlet  39  act upon one side of the control piston  42  and the return pressure at the outlet  40  acts upon the other side of the control piston  42 . 
     Typically such pressure reducing valves are used to reduce the pressure at the pressure inlet  38  to a permanently pre-determined value and to make the reduced pressure available at the outlet  40 . Differently than proposed in the present embodiment therefore with conventional arrangement of the pressure reducing valve  35  the connection actually used as inlet  39  for the load pressure of the auxiliary pressure medium consumers  12 ,  13  represents a tank inlet, while the pressure at the outlet  40  is used to actuate further valves. 
     The flow control valve  37  lies in a branch line  33   b  of the load reporting line  33 , which communicates with the return pipe  19 . As a result it is guaranteed that if attachments are not in use no unintentional load pressure reporting occurs through thermally-related pressure increase in the load reporting line  33 . 
     The load pressure of the control block  11  according to the circuit diagram lies on the inlet  39  of the pressure reducing valve  35 . If no auxiliary pressure medium consumer  12 ,  13  is in operation, a pressure of 10 bar prevails at the outlet  40  depending on the permanent setting of the pressure reducing valve  35 . So that this constant pressure does not lead to inadvertent reaction of the pressure regulator  15 , the switching valve  36  is switched to its position blocking the load reporting line  41 . 
     As soon as an auxiliary pressure medium consumer  12 ,  13  is put into operation, the switching valve  36  is switched to its position unblocking the load reporting line  41 . The load pressure lying on the inlet  39  is looped in the ratio 1:1 through the pressure reducing valve  35  to the outlet  40  and is superimposed onto the always present permanently set differential pressure of 10 bar. The load pressure increased by the differential pressure is now communicated via the load reporting lines  41  and  17  to the pressure regulator  15 , which as a result of the increased load pressure is induced to further raise the system pressure of the apparatus compared to the operation of primary pressure medium consumers  6 , which guarantees trouble-free operation of the auxiliary pressure medium consumers  12 ,  13 . 
     The invention has been described by the example of a hydraulic system with a fixed displacement pump. If the invention is to be used with a hydraulic system with a variable displacement pump, then the load reporting line  17  has only to be connected to the corresponding load inlet of the pressure and flow controller of the pump. Since such pressure and flow controllers are generally known in structure and working method, these are not described in greater detail.