Patent Publication Number: US-7222484-B1

Title: Hydraulic system with multiple pressure relief levels

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulic systems that independently control the operation of a plurality of hydraulic actuators, and more particularly to such hydraulic systems in which various hydraulic actuators have different operating pressure limits as determined by separate pressure relief valves. 
     2. Description of the Related Art 
     Numerous types of machines have components that are moved by a hydraulic system. For example, a lift truck is vehicle for transporting objects throughout a factory or warehouse and has an engine which drives a pump to provide pressurized fluid for powering different functions, such as driving wheels to propel the vehicle or lifting the objects. 
     An exemplary lift truck  10  is shown in  FIG. 1  and includes a body  12  with an operator compartment  14 . A multiple section, telescopic mast  16  is attached to the front of the body and includes a base section  18  and one or more telescopic sections  20  nested within the base section. A fork carriage  22  with load carrying forks  23  is slidably mounted to one of the telescopic sections and is moved up and down by a lift cylinder  24 . Typically the lift cylinder  24  is connected to a mechanism (not shown) comprising chains which pass over pulleys to extend and retract the telescopic sections  20  relative to the base section  18 . A tilt cylinder  26 , horizontally mounted between the front wheels  25  of the lift truck  10 , is attached to the body  12  and the lower end of the mast base section  18 . The tilt cylinder  26  pivots the telescopic mast  16  about a horizontal shaft  28  to tilt the ends of the forks  23  up and down to hold the load thereon. The hydraulic fluid that drives the lift and tilt cylinders  24  and  26  is controlled by valves that are operated by controls in the operator compartment  14 . 
     Use of the lift truck  10  often requires that the lift and tilt cylinders  24  and  26  operate in unison to tilt the mast  16  as the fork carriage  22  is being raised. However, each of those functions has a unique pressure characteristic which dictates that its maximum pressure be limited to a different magnitude. Because the fork carriage  22  can carry a relatively heavy load, the maximum pressure limit for the lift cylinder  24  may be 200 bar, whereas the maximum pressure limit for the tilt cylinder is on the order of 140 bar, for example. These maximum pressure levels are determined by the setting of pressure relief valves at various locations in the hydraulic circuit. 
     Quite often each hydraulic function on a machine had a separate pressure relief valve that was set to a specific pressure limit for the associated function. This type of control was relatively expensive as a pressure relief valve was required for each function even though several of them had the same pressure limit. 
     U.S. Pat. No. 4,561,463 describes an alternative hydraulic system that has a multiple section valve assembly with a pair of relief valves, one for a single high pressure function and another for two lower pressure functions. A high pressure relief valve governed the pressure at the inlet to the valve assembly and that of the first valve section for the high pressure function. The pressure in the subsequent valve sections was governed by a second relief valve having a lower setting. However, when the second valve section provided pressurized fluid to its associated hydraulic actuator, the third valve section was rendered inoperative. Thus the second and third valve sections were serially connected and all the hydraulic functions could not operate simultaneously. 
     Therefore, a need still exists to enable three or more hydraulic functions, that require different pressure limits, to operate simultaneously without having to provide a separate pressure relief valve for each function. 
     SUMMARY OF THE INVENTION 
     A hydraulic system has a supply conduit that receives fluid under pressure from a source and has a return conduit through which fluid is sent back to the source. A pressure relief passage also is provided. A primary pressure relief valve limits pressure in the supply conduit to less than a first pressure limit. 
     A first control valve is connected to both the supply conduit and the return conduit and has a first workport for connection to a first hydraulic actuator. In a first position, the first control valve connects the supply to the first workport and in a second position the return conduit is connected to the first workport. 
     A second control valve is connected to the supply conduit and to the return conduit and has a second workport for connection to a second hydraulic actuator. A first outlet port is coupled to pressure relief passage. The second control valve has a first position in which the supply conduit is connected to the second workport and has a second position in which the return conduit is connected to the second workport. 
     A third control valve is connected to the supply conduit and the return conduit and has a third workport for connection to a third hydraulic actuator. A second outlet port of the third control valve is coupled to pressure relief passage. The third control valve has a first position in which the supply conduit is connected to the third workport and has a second position in which the return conduit is connected to the third workport, 
     A secondary pressure relief valve connected between the pressure relief passage and the return conduit. The connection and the operation of the secondary pressure relief valve limits both the pressure that the second control valve applies to the second workport and the pressure that the third control valve applies to the third workport to less than a second pressure limit. Typically the first pressure limit is greater than the second pressure limit. 
     The primary pressure relief valve ensures that the pressure in the hydraulic system never exceeds the first pressure limit and the second pressure relief valve prevents pressure in the hydraulic system from exceeding the second pressure limit when either or both of the second or third hydraulic functions is active. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a lift truck that incorporates a hydraulic system according to the present invention; 
         FIG. 2  is a schematic diagram of a version of the hydraulic system that uses open center manually operated valves; and 
         FIG. 3  is a schematic diagram of another version of the hydraulic system that uses electrically operated valves. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in the context of a hydraulic system for a lift truck, such as the one shown in  FIG. 1 , with the understanding that the inventive concepts can be applied to hydraulic systems for a wide variety of other types of equipment and machines. 
     With reference to  FIG. 2 , the hydraulic system  30  for the lift truck  10  has a source  33  of hydraulic fluid which includes a reservoir  32  from which hydraulic fluid is drawn by a pump  34  and forced under pressure to a pressure control valve  31 . The pressure control valve  31  responds to the hydraulic fluid pressure demands of the propulsion function  40  that drives the wheels  25  of the lift truck  10 . Typically the vehicle propulsion tales priority over other hydraulic functions, and the pressure control valve ensures that those pressure demands are met to propel the lift truck. Thus the pressure control valve  76  directs the pump output fluid to the propulsion function  40  and any of that output fluid remaining after satisfying the propulsion function is furnished via a supply conduit  35  to the other hydraulic functions  41 ,  42 , and  43 . 
     The supply conduit  35  is divided into a plurality of sections  36   a - 36   e  between those other hydraulic functions  41 - 43  and into a parallel branch  37 . A primary, or first, hydraulic function  41  operates the lift cylinder  24  while the second hydraulic function  42  operates the tilt cylinder  26 . The third and fourth hydraulic functions  43  and  44  provide fluid to auxiliary devices connected to the lift truck  10 . Hydraulic fluid returns from the hydraulic functions  40 - 44  to the tank  32  via a return conduit  38 . 
     The pressure in supply conduit  35  is limited to a maximum level by a primary pressure relief valve  39  which opens when the pressure exceeds a level set by an adjustable spring, although other types of relief valves can be employed. In the case of the lift truck  10 , the primary pressure relief valve  39  is set to a relatively high pressure level, such as 200 bar, for example. When that pressure limit is exceeded, a path is created through the primary pressure relief valve  39  from the supply conduit  35  to the tank return conduit  38 . The relatively high pressure fluid is required by the first hydraulic function  41  in order to raise the fork carriage  22  when carrying a very heavy load. 
     The first hydraulic function  41  is controlled by a first control valve  46  that is manually operated by a lever within the operator compartment  14  of the lift truck  10 . The first control valve  46 , as are the control valves for the other hydraulic functions, is a three-position, open-center valve. In the center position, the lift cylinder  24  is disconnected from both the supply conduit  35  and the return conduit  38 . However, in this position of the first control valve  46 , an open center passage allows hydraulic fluid to flow through the valve between supply conduit sections  36   a  and  36   b.    
     When the first control valve  46  is shifted upward in  FIG. 2  into an open first position, the first supply conduit section  36   a  is disconnected from the subsequent sections  36   b - 36   e  that are connected to the downstream hydraulic functions  42 - 44 . The first supply conduit section  36   a  is now connected to the workport of the first control valve  46  which is coupled to the head chamber of the lift cylinder  24 . It should be noted that pressurized fluid need only be applied to the lift cylinder  24  to raise the fork carriage  22 , because gravity provides the force for lowering the fork carriage. As a consequence, a hydraulic connection is not provided to the rod chamber of the lift cylinder  24 . A load check valve  50  is connected between the workport of the first control valve  46  and the lift cylinder  24  to prevent an excessive load force from driving hydraulic fluid backwards through the first control valve  46  and into the supply conduit  35 . 
     When the first control valve  46  is shifted into an open second position (downward in the orientation in  FIG. 2 ), the first supply conduit section  36   a  is connected to the next section  36   b  leading to the downstream functions  42 - 44 . In this open position the valve&#39;s workport is connected to the return conduit  38  so that fluid can be exhausted from the lift cylinder  24 , thereby lowering the fork carriage  22  under the force of gravity. 
     The second, third and fourth hydraulic functions  42 - 44  are designated as secondary hydraulic functions because each requires lower pressure hydraulic fluid as compared to the first hydraulic function  41  for the lift cylinder  24 . The second hydraulic function  42  operates the tilt cylinder  26  that pivots the mast  16  by means of a second control valve  52 , That valve has an inlet port connected the supply conduit branch  37  and another port that is connected to the tank return conduit  38 . Note that the tilt cylinder  26  is double acting in that, depending upon the pivoting direction, pressurized fluid is applied to one of the head or rod chambers of that cylinder. Those cylinder chambers are connected to different workports of the second control valve  52 . 
     The second control valve  52  also is a three-position valve having a center-off position with an open center such that in this position supply conduit sections  36   b  and  c  are interconnected while the tilt cylinder  26  is disconnected from both the supply and return conduits  35  and  38 . When the second control valve  52  is in a open position in pressurized fluid from the supply conduit branch  37  is fed to one of the chambers of the tilt cylinder  26 , and fluid is exhausted from the other cylinder chamber to the return conduit  38 . The load check valve  54  for this hydraulic function is located between supply conduit branch  37  and the control valve inlet. Therefore, regardless of which chamber of the tilt cylinder  26  is being powered, the load check valve  54  prevents the backward flow of hydraulic fluid from the cylinder into the supply conduit. In the open positions of the second control valve  52 , the pressurized fluid flowing through the valve also is applied via a pressure relief port and check valve  56  to a pressure relief passage  58 . 
     The third and fourth functions  43  and  44  can be utilized to provide fluid to control auxiliary hydraulic devices on the lift truck  10 . For example, the fork carriage  22  could be replaced with a work device that requires hydraulic power. Alternatively, a hand tool or other external apparatus can be powered by the lift truck&#39;s hydraulic system  30 . Each of the third and fourth functions  43  and  44  has a control valve  60  and  62 , which are similar in structure to the second control valve  52 . Of significance is that the third and fourth control valves  60  and  62  load sense passage have a relief pressure port that is connected by a separate check valve  64  or  66  to the pressure relief passage  58 . Thus, the highest pressure among the secondary hydraulic functions  42 - 44  is conveyed through the respective check valve  56 ,  64  or  66  into the pressure relief passage  58 . 
     The pressure relief passage  58  is connected by a secondary pressure relief valve  68  to the tank return conduit  38 . The secondary pressure relief valve  68  is set to open at a lower pressure level than the primary pressure relief valve  39 . As noted previously, the tilt function performed by the tilt cylinder  26 , as well as the auxiliary third and fourth functions  43  and  44  require hydraulic fluid at a maximum pressure which is significantly less than the maximum pressure required by the lift cylinder  24  to raise the fork carriage  22  and its load. As a consequence the secondary pressure relief valve  68  has a pressure setting determined by the variable spring which will allow it to open at approximately 140 bar, for example. 
     The key feature of the configuration of the hydraulic system  30  is that all of the hydraulic function are connected in parallel to the supply and return conduits  35  and  38  and are governed by two pressure limits determined by only a pair of pressure relief valves  39  and  68 . When one or more of the secondary hydraulic functions  42 - 44 , is active, even when the first hydraulic function  41  is active, the secondary pressure relief valve  68  limits the maximum supply conduit pressure. When only the primary, or first, hydraulic function  41  is operating, the primary pressure relief valve  39  limits the maximum pressure that may occur in the supply conduit  35 . In this latter mode, the control valves  52 ,  60  and  62  of all the secondary hydraulic functions  42 ,  43  and  44 , respectively, are all in the closed center position in which the pressure relief passage  58  and the secondary pressure relief valve  68  are disconnected from the supply conduit  35 . Even when more than one primary hydraulic function is included in the hydraulic system, when only the primary hydraulic functions are active only the primary pressure relief valve  39  governs the supply conduit pressure. 
     As noted above, when both primary and secondary hydraulic functions are active simultaneously, the hydraulic system defaults to the low pressure limit of the secondary pressure relief valve  68 . This may restrict the performance of the primary hydraulic function when higher pressure is required. If the system was configured to default to the higher pressure limit of the primary pressure relief valve  39 , the maximum pressure rating of the secondary hydraulic functions could be exceeded, which might result in failure of hydraulic components or structural members of the machine. Regardless of whether the high or low pressure limit is used as the default, when both primary and secondary hydraulic functions operate simultaneously, one type of function can be adversely affected. 
     That adverse condition can be avoided by employing the second hydraulic system  70  depicted in  FIG. 3 . This second hydraulic system  70  is based on a present trend toward electrical controls by which the machine operator manipulates a joystick  71  that produces an electrical signal indicating the desired motion for a component of the machine. The joystick signals are applied as inputs to an electronic controller  73  which then produces output signals for activating solenoid operated hydraulic valves that control the flow of fluid to the cylinders  24  and  26  on the lift truck  10 . 
     The second hydraulic system  70  comprises a pump  72  which draws fluid from a tank  74 . A pressure control valve  76  responds to the pressure demands of the propulsion function  78  of the lift truck  10  to ensure that those pressure demands are met. Any pump output fluid remaining after satisfying the demands of the propulsion function  78  is furnished via a supply conduit  80  to the other hydraulic functions  81 ,  82 , and  83 . On this exemplary machine, there is a single primary hydraulic function  81  which operates the lift cylinder  24  to raise and lower the mast  16 , and there are two secondary hydraulic functions  82  and  83 . However, other machines may have other numbers of primary and secondary hydraulic functions. 
     A conventional pressure compensation valve  79  ensures that the pressure within the supply conduit  80  is sufficient to meet the highest pressure demanded by the other hydraulic functions  81 ,  82  or  83 . The pressure compensation valve  79  responds to the difference between the pressure in the supply conduit  80  and in a load sense passage  84  that indicates the greatest pressure demanded by those hydraulic functions. A primary pressure relief valve  85  limits the load sense conduit pressure signal in conduit  84  to a maximum pressure level (e.g. 200 bar) which is the primary pressure setting for the hydraulic circuit. 
     The first, or primary, hydraulic function  81  controls the operation of the lift cylinder  24  and employs a control valve  86  formed by a pair of proportional, pilot operated poppet valves, such as are described in U.S. Pat. No. 6,745,992. However, it should be understood that other types of valves may be used. The first of these pilot-operated poppet valves  88  is coupled in series with a load check valve  90  between the supply conduit  80  and the head chamber of the lift cylinder  24 . As with the previous hydraulic circuit, pressurized fluid is only applied to the head chamber of the lift cylinder  24  because the force of gravity is used to lower the mast  16 . The second pilot-operated poppet valve  89  is coupled between the lift cylinder  24  and a return conduit  87  which leads to the tank  74 . Because this system is electrically controlled, a manual valve  77  is provided between the lift cylinder  24  and the tank return conduit  87  as a safety measure to lower the mast  16  when electrical power is unavailable to operate the hydraulic system. 
     Hydraulic system  70  has two secondary functions  82  and  83 . The second hydraulic function  82  controls the tilt cylinder  26  on the lift truck  10  and employs a second control valve  91  having a spool that is operated by the hydraulic pressure at each end. Those pressures are controlled by a pair of solenoid valves  95  and  96 . Applying pressurized fluid to one end of the second control valve spool and relieving the pressure at the opposite end to the return conduit  87  moves the spool into one of two open states, thereby sending fluid from the supply conduit  80  to one chamber of the tilt cylinder  26  and exhausting fluid from the other chamber to the return conduit. A conventional load check valve  92  prevents the flow of fluid backward from the tilt cylinder  26  to the supply conduit  80 . 
     The third function  83  is similar to the second function and is provided to power an auxiliary device on the lift truck  10 . The third function  83  has a third control valve  100  with a spool that moves in response to pressure applied to its ends by a pair of solenoid valves  102  and  104 . 
     The second control valve  91  has a port  93  that is coupled by a check valve  94  to a pressure relief passage  97  and the third control valve  100  has a port that is connected by a check valve  106  to the relief pressure passage. The pressure relief passage  97  is coupled by a secondary pressure relief valve  98  to the tank return conduit  87 . In addition, the pressure relief passage  97  is connected to one input of a conventional load sense shuttle valve  99 . The other input of the load sense shuttle valve  99  is connected to the outlet of the first control valve  88  for the primary hydraulic function  81 . The output pressure of the load sense shuttle valve  99  corresponds to the greater load pressure from either the first hydraulic function  81  or the pressure relief passage  97  which carries the greater load pressure from the second and third hydraulic functions  82  and  83 . The output pressure of the load sense shuttle valve  99  is applied via a load sense passage  84  to the pressure compensation valve  79 . 
     When only the secondary functions are active, the pressure from the pressure relief passage  97  is conveyed by the load sense shuttle valve  99  through the load sense passage  84  to the pressure compensation valve  79 . That pressure from the secondary functions controls operation of the pressure compensation valve  79 , thereby governing the pressure in the supply conduit  80 . Specifically, supply conduit pressure is equal to the load sense passage plus a margin established by the pressure compensation valve. When only the primary function  81  is active, its load pressure is applied through the load sense shuttle valve  99  and the load sense passage  84  to the pressure compensation valve  79 . In situations where both the primary and secondary functions are active, the greatest load pressure from among them is conveyed by the load sense shuttle valve  99  and the load sense passage  84  to the pressure compensation valve  79  for governing the pressure in the supply passage. 
     The primary and secondary pressure relief valves  85  and  98  independently limit the maximum pressure that is applied to the primary and secondary hydraulic functions, respectively. The output pressures of the secondary hydraulic functions  82  and  83  are conveyed from the respective port  93  or  105  of the second and third control valves  91  and  100  into the pressure relief passage  97 . If both secondary hydraulic functions are simultaneously active only the greater output pressure is passed by the check valves  94  and  96  into the pressure relief passage  97 . When the pressure relief passage pressure exceeds the setting of the secondary pressure relief valve  98  that valve opens releasing the pressure to the return conduit  87 , thereby limiting the maximum output pressure of the secondary hydraulic functions  82  and  83 . 
     The primary pressure relief valve  85  prevents the output pressure of the first, or primary, hydraulic function  81  from exceeding its maximum permitted limit. Because the maximum permitted pressure at the first hydraulic function  81  is greater that the maximum pressure allowed at the secondary functions  82  or  83 , that maximum load pressure will be conveyed through the shuttle valve  99  and the load sense passage  84  to the primary pressure relief valve  86 . That relief valve opens when its pressure setting is exceeded, thereby releasing the pressure to the return conduit  87 . This limits the pressure in the load sense passage  84  that in turn controls the operation of the conventional pressure compensation valve  79  to limit pressure which can occur in the supply conduit  80 . The shuttle valve  99  blocks the output pressure of the first hydraulic function  81  from reaching the secondary pressure relief valve  98 . Therefore the secondary pressure relief valve  98  governs only the secondary hydraulic functions  82  and  83  and the primary pressure relief valve  86  effectively governs only the primary hydraulic function  81 . 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.