Patent Publication Number: US-6907728-B2

Title: Load sensing hydraulic system

Description:
BACKGROUND TO THE INVENTION 
   This invention relates to a load sensing hydraulic system for an agricultural vehicle, and in particular to a load sensing hydraulic system that delivers hydraulic fluid according to demand and that limits power losses. 
   In agricultural vehicles, many of the vehicle functions are hydraulically actuated. For example, on a tractor, the steering and clutch are often hydraulically actuated. Additionally, implements attached to the tractor often comprise hydraulic actuators, which require pressurized hydraulic fluid to operate them. 
   As tractors and the implements drawn by such tractors increase in size, the capacity of the tractor&#39;s hydraulic system must also be increased to meet the possible demands made on it. 
   It has been noted that increasing the flow of hydraulic fluid within the system to a level that meets the requirements of any demand placed on it gives rise to significant power losses and can lead to the hydraulic fluid overheating. 
   While some of the hydraulically powered functions of the vehicle are used frequently, others are used less frequently. It would therefore be desirable to provide a hydraulic system to match the supply of hydraulic fluid to the demand therefor. 
   SUMMARY OF THE INVENTION 
   The invention provides a load sensing hydraulic system comprising a first and second pump means, at least one hydraulic fluid consumer, and a pressure balance circuit, wherein: in the case of no demand from the consumers, hydraulic fluid flows from the first and second pump means through the pressure balance circuit and to tank; in the case of a demand from the consumers for hydraulic fluid at a flow rate less than or equal to the flow rate provided by the first pump means, hydraulic fluid from the first pump means flows to the consumers, and hydraulic fluid from the second pump means flows to tank; and in the case of a demand from the consumers for hydraulic fluid at a flow rate greater than the capacity of the first pump means, hydraulic fluid from both pump means flows to the consumers. Preferred features of the invention are described in the dependent claims, and the description following. In the context of the invention, tank means hydraulic fluid at a substantially lower pressure then hydraulic fluid exiting the pumps. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a hydraulic circuit diagram of a first embodiment of the system. 
       FIG. 2  is a hydraulic circuit diagram of a second embodiment of the system. 
       FIG. 3  is a hydraulic circuit diagram of a third embodiment of the system. 
       FIG. 4  is a block diagram of part of an assembly comprising a pressure balance circuit of the type described with reference to  FIGS. 1  to  3 . 
       FIG. 5  is a schematic representation of the assembly shown in FIG.  4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1 , there is shown a load sensing hydraulic circuit for an agricultural tractor, the hydraulic circuit providing hydraulic fluid to hydraulic fluid consumers, namely: spool valves  1 ,  2 , the hitch valve  3 , the power steering  4 , and the trailer braking system  5 . 
   When there is no demand for hydraulic fluid from the consumers  1  to  3 , the fluid from the first stage of the pump goes to the pilot heads of the valves blocks  8  and  13 , switching them on and opening them to allow fluid from the first and second stages  6 ,  7  of the pump to pass to tank. 
   When there is a demand for hydraulic fluid from the consumers  1  to  3 , but at less than the capacity of the first stage  6  of the pump, the load sensing pressure signal from line Y partially switches off the path through the valve block  8  to tank  15 , the flow demanded by the spool valves being delivered via line Z. A pressure balance is created between the load-sensing signal Y, the pressure exerted by the first stage  6  of the pump and the force exerted by spring  9 . The load-sensing signal Y and pressure from the first stage  6  of the pump act on the valve block  13  as well as the block  8 . The valve block  13  is kept open due to the different forces acting on the block (note, spring  14  exerts a smaller force on valve block  13  than spring  9  does on valve block  8 ). 
   When the load sensing signal from line Y indicates that the demand from the consumers  1  to  3  is greater than the first stage  6  of the pump can meet, all the flow from the first stage  6  of the pump is directed to the spool valves. The first stage  6  is then overloaded and cannot maintain the pressure balance between the load sensing signal from line Y, the force exerted by spring  9 , and the pressure exerted by the first stage  6 . The pressure exerted by the first stage  6  falls to assume a new pressure balance between the load sensing signal Y, the pressure exerted by the first stage  6  of the pump, the pressure exerted by the second stage  7  of the pump, and the force exerted by the spring  14 . The result is that the path through the valve block  13  is partially switched off, diverting the required flow of hydraulic fluid from the tank to the consumers  1  to  3 . 
   When the demand from the consumers reaches the maximum deliverable by the first and second stages  6 ,  7  of the gear pump, the path through the valve block  13  to tank  15  is completely blocked off, and the hydraulic fluid flows only to the consumers  1  to  3 . 
   Hydraulic fluid flowing from the first and second stages  6 ,  7  of the gear pump to the consumer  1  to  3  passes through the filter  12 . 
   Referring now to  FIG. 2 , where like numerals are used to indicate like parts, the circuit comprises a two-stage gear pump having first and second stages  6 ,  7 . The first stage  6  provides hydraulic fluid at a flow rate of thirty-three liters per minute, the second stage  7  providing hydraulic fluid at a flow rate of fifty-five liters per minute. 
   The circuit responds to the pressure in signal line Y, which is indicative of demand so that when there is a low demand on the hydraulic system, only the first stage  6  of the pump supplies hydraulic fluid to the consumers  1  to  3 , with the second stage  7  of the pump delivering hydraulic fluid directly to tank. As the demand increases, flow from the second pump  7  is diverted from the tank to the consumers. The system therefore provides hydraulic fluid either at a flow rate of up to forty-two liters per minute, or at a flow rate between thirty-three and eighty-eight liters per minute, depending on demand. 
   The hydraulic circuit comprises a pressure balance arrangement generally indicated by the letter A. The pressure balance comprises first and second valve blocks  8 ,  13 . The valve blocks  8 ,  13  each comprise a spring biased valves including springs  9 ,  14  respectively. 
   In the case of no demand from the consumers  1  to  3 , there is no pressure in line Y, and the first and second stages of the gear pump delivers hydraulic fluid to tank. The first stage  6  of the pump delivers pressurized hydraulic fluid, which switches the pressure balance on. Pressurized fluid acts against the force of spring  9  to open the valve, thereby permitting hydraulic fluid to pass though the valve block  8 . The hydraulic fluid passing through the constriction  10  and check valve  11  to tank  15  and generates a hydraulic pressure in the line U greater than the force of spring  14  so the valve  13  opens and hydraulic fluid from the second stage  6  of the pump flows through hydraulic line V. 
   Upon demand from the consumers  1  to  3  for hydraulic fluid at a flow rate of less than thirty-three liters per minute, a pressure signal is generated in the broken line Y. This pressure signal partially closes the valve  8  against the pressure generated by the first stage  6  of the pump. The demand at the spool valve is met via hydraulic line Z, and is filtered by filter  12 . Whilst demand from the consumers  1  to  3  is less than forty-two liters per minute, there is a continued flow (the difference between the demand level and forty-two liters per minute) of hydraulic fluid through the constriction  10  and check valve  11 . This continued flow causes a pressure drop across the constriction  10  and check valve  11  such that the pressure in line U is sufficient to overcome the force exerted by the spring  14 , and hence hydraulic fluid from the second stage of the pump continues to flow to tank. The check valve  17  prevents flow from stage  6  to stage  7  of the gear pump. 
   In the case where the demand exceeds thirty-three liters per minute, the full capacity of the first stage  6  of the pump passes through the hydraulic line Z, and the flow through the constriction  10  and check valve falls to zero. As a result, there is no pressure drop across the constriction  10  and check valve  11 , and hence no hydraulic pressure is exerted against the spring  14 . The spring  14  closes the valve  13  and the hydraulic fluid delivered by the second stage  7  of the pump flows through line Z to the consumers via the filter  12 . 
   Pressurized hydraulic fluid will always be available from the second stage  7  of the pump for the trailer brakes  5 . 
   Demand by the power steering system  4  for pressurized hydraulic fluid is met by the hydraulic pump  16 . 
   Referring again to  FIG. 1 , the pressure balance of the hydraulic circuit will be described in more detail. The pressure balance B is comprised of a two-stage gear pump having a first stage  6  and a second stage  7 . The outputs of the stages  6  and  7  are connected to the valve blocks  8  and  13  respectively. The spring  9  exerts a greater force than does the spring  14 . The output to the spool valves is via line Z. A check valve  17  is located in line Z in order to avoid flow of hydraulic fluid from the first stage  6  to second stage  7  of the pump. 
   The valve block  8  includes two control inputs, one on each side. The first control input is the pressure exerted by first stage  6  of the pump, the second being the load sensing signal, received via line Y from the consumers  1  to  3  (see FIG.  1 ). 
   The valve block  13  includes two control inputs, one on each side thereof, i.e. the load sensing signal, from the consumers  1  to  3  (via line Y), and the pressure signal from the first stage  6  of the pump. 
   Demand for pressurized hydraulic fluid for the power steering system  4 , and low pressure consumers such as clutches, is met by a separate pump  16 . Demand for hydraulic fluid from the trailer brake valve is met by the second stage  7  of the gear pump. 
   Referring now to  FIG. 3 , there is shown a load sensing hydraulic circuit for an agricultural tractor, the hydraulic circuit providing fluid to hydraulic consumers, namely: spool valves  1 ,  2 , hitch valve  3 , power steering  4  and the trailer brake valve  5 . 
   The circuit comprises a two-stage gear pump having first and second stages  6  and  7 . The first stage provides pressurized hydraulic fluid at a flow rate at thirty-three liters/minute, the second at fifty-five liters/minute. 
   The circuit responds to demand so that when there is no demand from the consumers  1  to  3 , both stages of the pump deliver to tank; when there is a low demand on the hydraulic system, only the first stage  6  of the pump supplies hydraulic fluid to the consumers  1  to  3 , the second stage delivering hydraulic fluid to tank; and as the demand from the consumers  1  to  3  increases, flow from the second stage  7  is diverted from tank to the consumers  1  to  3 . 
   The hydraulic circuit comprises a pressure balance indicated by the letter C. The pressure balance block comprises a four-way, three position valve  8 , three check valves  17 ,  22 , and  23  and two pressure relief valves  20  and  21 . 
   In case of no demand from any consumer  1  to  3 , the two-stage pump delivers pressurized hydraulic fluid, which switches the main valve  8  to the position E. Hydraulic fluid from both stages  6 ,  7  of the pump passes through the valve  8  directly to tank. 
   Upon demand from consumers  1  to  3  for hydraulic fluid at a flow rate of less than the capacity of stage  6 , a pressure signal is generated in the broken line Y. This pressure signal moves the valve  8  to position F and partially closes the line from stage  6  to tank to provide flow under pressure at the spool valve via the hydraulic line Z. The oil from the stage  7  continues to pass through the valve  8  to the tank. 
   The check valves  17  and  22  prevent flow from the stage  6  of the gear pump to stage  7  thereof. 
   In the case where there is a demand from hydraulic fluid greater than the capacity of the first stage  6  of the pump, the pressure generated by the first stage  6  falls slightly, unbalancing the spool valve  8 , which moves to position G and closes the lines from each stage to the tank to deliver the flow from the two-stages to the consumers  1  to  3  through the line Z via the filter  12 , which protects all the spool valves and hitch valves from contamination. 
   On the block, a main pressure relief valve  21  limits the maximum pressure in the hydraulic line for each pump stage to protect the trailer brake valve ( 5 ) (flow passes through the check valve  22 ), the spool valves and the hitch valve (flow passes through check valves  17  and  23  for stage  7 ; flow passes through check valve  23  only for stage  6 ). This main pressure relief valve is a safety valve for the circuit and limits the pressure peaks in the consumers. 
   A second relief valve  20  is situated on the load sensing line Y of the valve block. The relief valve  20  limits the maximum pressure of the pump by limiting the demand from the consumers  1  to  3 . 
   In the case of maximum pressure demand without flow (i.e. a cylinder at the end of its stroke), the relief valve  20  limits the pressure from line Y and balances the valve  8  in position F instead of G to allow the flow from the second stage  7  of the gear pump to pass through the valve directly to tank and to limit the power losses, since just one pump is under pressure, rather than two. 
   Referring now to  FIGS. 4 and 5 , there is shown a part of the transmission casing  30  of an agricultural tractor. The tractor is not shown since such machines are well understood by those skilled in the art. With reference to these figures, like reference numerals are used to indicate like parts. 
   Two gear pumps indicated generally by the reference numeral  31  for pumping hydraulic fluid are mounted in the transmission casing. A cover plate  32  is removably attachable to the transmission housing  30  in order to give access to the pumps  31  and other components mounted within the transmission housing  30 . A pressure balance  33  is mounted on the cover plate  32 , the pressure balance being hydraulically connected to the pumps  31 . A trailer braking valve  34  is attached to one side of the pressure balance  33 . A spool valve  35  is arranged downstream of the pressure balance  33 , and in fluid connection therewith. Hydraulic fluid passing through the pressure balance  33  passes through a filter  12  en route to the spool valve block  35 . 
   By arranging the pressure balance  33  on the cover, and placing the filter  12  between the pressure balance  33  and the spool valves  7 , only one filter is required as opposed to two if the pressure balance is mounted on the input plate of the spool valve block. 
   The load sensing hydraulic system of the invention uses considerably less energy than many known systems, because the flow rate of hydraulic fluid is matched to the need placed on the system by the consumers. Another feature of the invention is that the hydraulic fluid in the system is not heated as a result of being pumped around at a flow rate which may be well above that which is required. 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.