Patent Publication Number: US-8966891-B2

Title: Meterless hydraulic system having pump protection

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a hydraulic system and, more particularly, to a meterless hydraulic system having pump protection. 
     BACKGROUND 
     A conventional hydraulic system includes a pump that draws low-pressure fluid from a tank, pressurizes the fluid, and makes the pressurized fluid available to multiple different actuators for use in moving the actuators. In this arrangement, a speed of each actuator can be independently controlled by selectively throttling (i.e., restricting) a flow of the pressurized fluid from the pump into each actuator. For example, to move a particular actuator at a high speed, the flow of fluid from the pump into the actuator is restricted by only a small amount. In contrast, to move the same or another actuator at a low speed, the restriction placed on the flow of fluid is increased. Although adequate for many applications, the use of fluid restriction to control actuator speed can result in flow losses that reduce an overall efficiency of a hydraulic system. 
     An alternative type of hydraulic system is known as a meterless hydraulic system. A meterless hydraulic system generally includes a pump connected in closed-loop fashion to a single actuator or to a pair of actuators operating in tandem. During operation, the pump draws fluid from one chamber of the actuator(s) and immediately discharges pressurized fluid back into an opposing chamber of the same actuator(s). To move the actuator(s) at a higher speed, the pump discharges fluid at a faster rate. To move the actuator with a lower speed, the pump discharges the fluid at a slower rate. A meterless hydraulic system is generally more efficient than a conventional hydraulic system because the speed of the actuator(s) is controlled through pump operation as opposed to fluid restriction. That is, the pump is controlled to only discharge as much fluid as is necessary to move the actuator(s) at a desired speed, and no throttling of a fluid flow is required. 
     An exemplary meterless hydraulic system is disclosed in U.S. Pat. No. 4,369,625 of Izumi et al. that issued on Jan. 25, 1983 (the &#39;625 patent). The &#39;625 patent describes a multi-actuator meterless-type hydraulic system, wherein each actuator is paired with a pump in a closed-loop manner. As described above, a speed and rotational direction of each actuator is controlled by controlling a displacement angle of its paired pump. 
     Although an improvement over open-loop hydraulic systems, the closed-loop hydraulic system of the &#39;625 patent described above may still be less than optimal. In particular, the system of the &#39;625 patent may be prone to pump failure caused by shock-loading from the actuators. That is, during operation, each actuator can induce pressure spikes within the associated circuit when loading on the actuator suddenly changes. If these pressure spikes are allowed to travel in reverse direction through a discharge passage back to the paired pump, the spikes can create damaging loads on the pump. The system of the &#39;625 patent does not provide protection against shock loading. 
     The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art. 
     SUMMARY 
     In one aspect, the present disclosure is directed to a hydraulic system. The hydraulic system may include a pump having variable displacement and over-center functionality, an actuator, and first and second passages extending between the pump and the actuator to create a closed-loop circuit. The hydraulic system may also include a first check valve disposed within the first passage to allow fluid flow only from the pump to the actuator, and a second check valve disposed within the second passage to allow fluid flow only from the pump to the actuator. The hydraulic system may further include a first bypass line connecting the first passage at a location between the actuator and the first check valve to the first passage at a location between the first check valve and the pump, and a second bypass line connecting the second passage at a location between the actuator and the second check valve to the second passage at a location between the second check valve and the pump. The hydraulic system may additionally include a valve configured to control fluid flow through the first and second bypass lines. 
     In another aspect, the present disclosure is directed to a method of operating a hydraulic system. The method may include pressurizing fluid with a pump, and directing the fluid to an actuator in two different directions via a closed-loop circuit formed by a first passage and a second passage. The method may further include preventing return flow from the actuator to the pump via a first check valve in the first passage, and preventing return flow from the actuator to the pump via a check valve in the second passage. The method may also include selectively allowing return flow from the actuator to bypass the first or second check valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial illustration of an exemplary disclosed machine; and 
         FIG. 2  is a schematic illustration of an exemplary disclosed hydraulic system that may be used in conjunction with the machine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10  having multiple systems and components that cooperate to accomplish a task. Machine  10  may embody a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, or another industry known in the art. For example, machine  10  may be an earth moving machine such as an excavator (shown in  FIG. 1 ), a dozer, a loader, a backhoe, a motor grader, a dump truck, or another earth moving machine. Machine  10  may include an implement system  12  configured to move a work tool  14 , a drive system  16  for propelling machine  10 , a power source  18  that provides power to implement system  12  and drive system  16 , and an operator station  20  situated for manual control of implement system  12 , drive system  16 , and/or power source  18 . 
     Implement system  12  may include a linkage structure acted on by linear and rotary fluid actuators to move work tool  14 . For example, implement system  12  may include a boom  22  that is vertically pivotal about a horizontal axis (not shown) relative to a work surface  24  by a pair of adjacent, double-acting, hydraulic cylinders  26  (only one shown in  FIG. 1 ). Implement system  12  may also include a stick  28  that is vertically pivotal about a horizontal axis  30  by a single, double-acting, hydraulic cylinder  32 . Implement system  12  may further include a single, double-acting, hydraulic cylinder  34  that is operatively connected between stick  28  and work tool  14  to pivot work tool  14  vertically about a horizontal pivot axis  36 . In the disclosed embodiment, hydraulic cylinder  34  is connected at a head-end  34 A to a portion of stick  28  and at an opposing rod-end  34 B to work tool  14  by way of a power link  37 . Boom  22  may be pivotally connected at a base end to a body  38  of machine  10 . Body  38  may be connected to an undercarriage  39  to swing about a vertical axis  41  by a hydraulic swing motor  43 . Stick  28  may pivotally connect a distal end of boom  22  to work tool  14  by way of axes  30  and  36 . 
     Numerous different work tools  14  may be attachable to a single machine  10  and operator controllable. Work tool  14  may include any device used to perform a particular task such as, for example, a bucket (shown in  FIG. 1 ), a fork arrangement, a blade, a shovel, a ripper, a dump bed, a broom, a snow blower, a propelling device, a cutting device, a grasping device, or any other task-performing device known in the art. Although connected in the embodiment of  FIG. 1  to pivot in the vertical direction relative to body  38  of machine  10  and to swing in the horizontal direction about pivot axis  41 , work tool  14  may alternatively or additionally rotate relative to stick  28 , slide, open and close, or move in any other manner known in the art. 
     Drive system  16  may include one or more traction devices powered to propel machine  10 . In the disclosed example, drive system  16  includes a left track  40 L located on one side of machine  10 , and a right track  40 R located on an opposing side of machine  10 . Left track  40 L may be driven by a left travel motor  42 L, while right track  40 R may be driven by a right travel motor  42 R. It is contemplated that drive system  16  could alternatively include traction devices other than tracks, such as wheels, belts, or other known traction devices. Machine  10  may be steered by generating a speed and/or rotational direction difference between left and right travel motors  42 L,  42 R, while straight travel may be facilitated by generating substantially equal output speeds and rotational directions of left and right travel motors  42 L,  42 R. 
     Power source  18  may embody an engine such as, for example, a diesel engine, a gasoline engine, a gaseous fuel-powered engine, or another type of combustion engine known in the art. It is contemplated that power source  18  may alternatively embody a non-combustion source of power such as a fuel cell, a power storage device, or another source known in the art. Power source  18  may produce a mechanical or electrical power output that may then be converted to hydraulic power for moving the linear and rotary actuators of implement system  12 . 
     Operator station  20  may include devices that receive input from a machine operator indicative of desired maneuvering. Specifically, operator station  20  may include one or more operator interface devices  46 , for example a joystick (shown in  FIG. 1 ), a steering wheel, or a pedal, that are located proximate an operator seat (not shown). Operator interface devices  46  may initiate movement of machine  10 , for example travel and/or tool movement, by producing displacement signals that are indicative of desired machine maneuvering. As an operator moves interface device  46 , the operator may affect a corresponding machine movement in a desired direction, with a desired speed, and/or with a desired force. 
     As shown in  FIG. 2 , each hydraulic cylinder  26  may include a tube  48  and a piston assembly  50  arranged within tube  48  to form a first chamber  52  and an opposing second chamber  54 . In one example, a rod portion  50 A of piston assembly  50  may extend through an end of second chamber  54 . As such, each second chamber  54  may be considered the rod-end chamber of the respective hydraulic cylinder  26 , while each first chamber  52  may be considered the head-end chamber. 
     First chambers  52  and second chambers  54  of each hydraulic cylinder  26  may be selectively supplied with pressurized fluid from a pump  80  in parallel with each other, respectively, and drained of the pressurized fluid in parallel to cause piston assembly  50  to displace within tube  48 , thereby changing the effective lengths of hydraulic cylinders  26  in tandem to move boom  22  (e.g., to raise and lower boom  22 ) relative to body  38  (referring to  FIG. 1 ). A flow rate of fluid into and out of first and second chambers  52 ,  54  may relate to a translational velocity of hydraulic cylinders  26 , while a pressure differential between first and second chambers  52 ,  54  may relate to a force imparted by hydraulic cylinders  26  on boom  22 . 
     Although not shown in detail, it is contemplated that one or more of hydraulic cylinder  32 , hydraulic cylinder  34 , left travel motor  42 L, right travel motor  42 R, and/or swing motor  43 , may also be connected to pump  80  in parallel with hydraulic cylinders  26 , if desired. Hydraulic cylinders  32 ,  34  may each embody linear actuators having a composition similar to hydraulic cylinders  26  described above. Left travel motor  42 L, right travel motor  42 R, and swing motor  43 , however, may embody rotary actuators. Each rotary actuator, like hydraulic cylinders  26 , may include first and second chambers located to either side of a pumping mechanism such as an impeller, plunger, or series of pistons. When the first chamber is filled with pressurized fluid from pump  80  and the second chamber is simultaneously drained of fluid, the pumping mechanism may be urged to rotate in a first direction by a pressure differential across the pumping mechanism. Conversely, when the first chamber is drained of fluid and the second chamber is simultaneously filled with pressurized fluid, the pumping mechanism may be urged to rotate in an opposite direction by the pressure differential. The flow rate of fluid into and out of the first and second chambers may determine a rotational velocity of the rotary actuator, while a magnitude of the pressure differential across the pumping mechanism may determine an output torque. The rotary actuator(s) may be fixed- or variable-displacement type motors, as desired. 
     Machine  10  may include a hydraulic system  72  having a plurality of fluid components that cooperate with the linear and rotary actuators described above to move work tool  14  (referring to  FIG. 1 ) and machine  10 . In particular, hydraulic system  72  may include, among other things, a circuit  74  fluidly connecting pump  80  with the different actuators of machine  10 , an over-pressure protection arrangement (OPPA)  76  associated with pump  80 , and a load-holding valve arrangement (LHVA)  78  associated with hydraulic cylinders  26 . It is contemplated that hydraulic system  72  may include additional and/or different circuits or components, if desired, such as a charge circuit, an energy storage circuit, switching valves, makeup valves, relief valves, and other circuits or valves known in the art. 
     Circuit  74  may include multiple different passages that fluidly connect pump  80  to hydraulic cylinders  26  and, in some configurations, to the other actuators of machine  10  in a parallel, closed-loop manner. For example, pump  80  may be connected to hydraulic cylinders  26  via a first pump passage  82 , a second pump passage  84 , a head-end passage  86 , and a rod-end passage  88 . 
     Pump  80  may have variable displacement and be controlled to draw fluid from its associated actuators and discharge the fluid at a specified elevated pressure back to the actuators in two different directions (i.e., pump  80  may be an over-center pump). Pump  80  may include a stroke-adjusting mechanism, for example a swashplate, a position of which is hydro-mechanically or electro-hydraulically adjusted based on, among other things, a desired speed of the actuators to thereby vary an output (e.g., a discharge rate) of pump  80 . The displacement of pump  80  may be adjusted from a zero displacement position at which substantially no fluid is discharged from pump  80 , to a maximum displacement position in a first direction at which fluid is discharged from pump  80  at a maximum rate into first pump passage  82 . Likewise, the displacement of pump  80  may be adjusted from the zero displacement position to a maximum displacement position in a second direction at which fluid is discharged from pump  80  at a maximum rate into second pump passage  84 . Pump  80  may be drivably connected to power source  18  of machine  10  by, for example, a countershaft, a belt, or in another suitable manner. Alternatively, pump  80  may be indirectly connected to power source  18  via a torque converter, a gear box, an electrical circuit, or in any other manner known in the art. It is contemplated that pump  80  may be connected to power source  18  in tandem (e.g., via the same shaft) or in parallel (e.g., via a gear train) with other pumps (not shown) of machine  10 , as desired. 
     Pump  80  may also be selectively operated as a motor. More specifically, when an associated actuator is operating in an overrunning condition (i.e., a condition where the actuator is driven by a load, the fluid discharged from the actuator may have a pressure elevated above an output pressure of pump  80 . In this situation, the elevated pressure of the actuator fluid directed back through pump  80  may function to drive pump  80  to rotate with or without assistance from power source  18 . Under some circumstances, pump  80  may even be capable of imparting energy to power source  18 , thereby improving an efficiency and/or capacity of power source  18 . 
     OPPA  76  may include components that cooperate to protect pump  80  from damaging pressure spikes that can move through circuit  74  in reverse direction relative to an output direction of pump  80 . Specifically, OPPA  76  may include, among other things, first and second check valves  87 ,  89 , first and second bypass lines  90 ,  92 , and a control valve  94 . 
     First check valve  87  may be disposed within first pump passage  82  and configured to allow fluid flow in only one direction away from pump  80  and toward first chambers  52  of hydraulic cylinders  26  (i.e., first check valve  87  may inhibit reverse flow from hydraulic cylinders  26  back into pump  80  via first pump passage  82 ). Similarly, second check valve  89  may be disposed within second pump passage  84  and configured to allow fluid flow in only one direction away from pump  80  and toward second chambers  54  of hydraulic cylinders  26  (i.e., second check valve  89  may inhibit reverse flow from hydraulic cylinders  26  back into pump  80  via second pump passage  84 ). 
     First bypass line  90  may connect at one end to first pump passage  82  at a location between hydraulic cylinders  26  and first check valve  87 , and at a second end to first pump passage  82  at a location between first check valve  87  and pump  80 . In other words, first bypass line  90  may allow return fluid within first pump passage  82  to bypass first check valve  87  and enter pump  80 . Second bypass line  92  may connect at one end to second pump passage  84  at a location between hydraulic cylinders  26  and second check valve  89 , and at a second end to second pump passage  84  at a location between second check valve  89  and pump  80 . In other words, second bypass line  92  may allow return fluid within second pump passage  84  to bypass second check valve  89  and enter pump  80 . 
     Control valve  94  may be configured to regulate fluid flow through first and second bypass lines  90 ,  92 . In particular, control valve  94  may be a solenoid-operated, spring-biased valve configured to move between a first discrete position at which fluid may freely flow through first bypass line  90  but is substantially blocked in second bypass line  92 , and a second discrete position (shown in  FIG. 2 ) at which fluid may freely flow through second bypass line  92  but is substantially blocked in first bypass line  90 . It is contemplated, however, that control valve  94  could alternatively be a variable-position valve instead of discrete position valve, or embody a hydro-mechanical valve instead of a solenoid-operated valve, if desired. For example, control valve  94  could be pilot operated via a signal from a swashplate control valve (not shown). Control valve  94 , as a variable position valve, could be useful in some situations for controlling a speed of hydraulic cylinders  26 , a load on pump  80 , and/or for facilitating regeneration wherein some fluid returning from hydraulic cylinders  26  may be passed directly back to hydraulic cylinders  26  via control valve  94 , without the fluid first passing through pump  80 . 
     (LHVA)  78  may be configured to selectively lock hydraulic cylinders  26  in place when an operator ceases to request movement of hydraulic cylinders  26 .  FIG. 2  illustrates (LHVA)  78  as having two different types of load-holding valves, including a hydro-mechanical valve  96  and an electro-mechanical valve  98 . It should be noted, however, that the two different valves are shown only to illustrate that different types of load-holding valves could be utilized in conjunction with hydraulic cylinders  26  and two substantially identical load-holding valves  96  or  98  would normally be utilized in most applications. 
     Load-holding valve  96  may be a poppet-type valve having a poppet element  100  moveable within a bore  102  between a flow-blocking position (shown in  FIG. 2 ) at which a nose portion of poppet element  100  engages a seat within bore  102 , and a flow-passing position at which the nose portion is away from the seat. Poppet element  100  may be spring-biased toward the flow-blocking position and moved toward the flow-passing position when a pressure of fluid acting on the nose portion exceeds a combined force of fluid acting on an opposing base portion and the spring-bias. Second pump passage  84  and rod-end passage  88  may be in fluid communication via bore  102  at the nose portion of valve element  100  such that movement of valve element  100  between the flow-blocking and flow-passing positions controls fluid flow between passages  84  and  88 . A restricted passage  104  may connect rod-end passage  88  with the base portion of valve element  100  to help regulate motion of valve element  100 . A bypass passage  106  having a check element  108  may allow fluid to be pushed by the base portion of valve element  100  out of bore  102  and into rod-end passage  88  during initial retracting movements of hydraulic cylinders  26  (i.e., after hydraulic cylinders  26  have been locked by load-holding valve  96 ). 
     In some situations, it may be necessary to drain fluid from the base portion of poppet element  100  to allow poppet  100  to move away from the seat within bore  102 . For this purpose, a two-position (e.g., flow-passing, flow-blocking) valve  109  may be disposed between the base portion and a low-pressure tank  112  to control selective draining of the base portion. 
     Load-holding valve  98  may also be a poppet-type valve having poppet element  100  moveable within bore  102  between the flow-blocking and the flow-passing positions. First pump passage  82  and head-end passage  86  may be in fluid communication via bore  102  of load-holding valve  98  at the nose portion of valve element  100  such that movement of valve element  100  controls fluid flow between passages  82  and  86 . In contrast to load-holding valve  96 , however, load-holding valve  98  may include a control passage  110  in place of restricted and bypass passages  104 ,  106 . Control passage  110  may be selectively fluidly communicated with fluid from head-end passage  86  or with low-pressure tank  112  via a solenoid valve  114 . When control passage  110  is communicated with the fluid from head-end passage  86 , valve element  100  of load-holding valve  98  may be urged toward its flow-blocking position, thereby hydraulically locking hydraulic cylinders  26 . When control passage  110  is fluidly communicated with low-pressure tank  112 , valve element  100  may be allowed to move toward its flow-passing position, thereby allowing free movement of hydraulic cylinders  26 . 
     During operation of machine  10 , the operator may utilize interface device  46  to provide a signal that identifies a desired movement of the various linear and/or rotary actuators to a controller  140 . Based upon one or more signals, including the signal from interface device  46  and, for example, signals from various pressure sensors (not shown) and/or position sensors (not shown) located throughout hydraulic system  72 , controller  140  may command movement of the different valves and/or displacement changes of the different pumps and motors to advance a particular one or more of the linear and/or rotary actuators to a desired position in a desired manner (i.e., at a desired speed and/or with a desired force). 
     Controller  140  may embody a single microprocessor or multiple microprocessors that include components for controlling operations of hydraulic system  72  based on input from an operator of machine  10  and based on sensed or other known operational parameters. Numerous commercially available microprocessors can be configured to perform the functions of controller  140 . It should be appreciated that controller  140  could readily be embodied in a general machine microprocessor capable of controlling numerous machine functions. Controller  140  may include a memory, a secondary storage device, a processor, and any other components for running an application. Various other circuits may be associated with controller  140  such as power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other types of circuitry. 
     Industrial Applicability 
     The disclosed hydraulic system may be applicable to any machine where improved hydraulic efficiency and pump protection are desired. The disclosed hydraulic system may provide for improved efficiency through the use of closed-loop technology. The disclosed hydraulic system may provide for pump protection through the use of OPPA  76 . Operation of hydraulic system  72  will now be described. 
     During operation of machine  10 , an operator located within station  20  may command a particular motion of work tool  14  in a desired direction and at a desired velocity by way of interface device  46 . One or more corresponding signals generated by interface device  46  may be provided to controller  140  indicative of the desired motion, along with machine performance information, for example sensor data such a pressure data, position data, speed data, pump or motor displacement data, and other data known in the art. 
     In response to the signals from interface device  46  and based on the machine performance information, controller  140  may generate control signals directed to the stroke adjusting mechanism of pump  80  and to valve  94 . For example, to drive hydraulic cylinders  26  at an increasing speed in an extending direction, controller  140  may generate a control signal that causes pump  80  of circuit  74  to increase its displacement in the first direction that results in pressurized fluid discharge into second pump passage  84 , rod-end passage  88 , and first chambers  54  at a greater rate, while simultaneously moving control valve  94  to the first position. When control valve  94  is in the first position, return fluid from first chambers  52  of hydraulic cylinders  26  and/or from the other linear or rotary actuators of hydraulic system  72  may flow through head-end passage  86 , first pump passage  82 , first bypass line  90 , and control valve  94  back into pump  80 . 
     Similarly, to drive hydraulic cylinders  26  at an increasing speed in a retracting direction, controller  140  may generate a control signal that causes pump  80  of circuit  74  to increase its displacement in the second direction that results in pressurized fluid discharge into first pump passage  82 , head-end passage  86 , and first chambers  52  at a greater rate, while simultaneously moving control valve  94  to the second position (shown in  FIG. 2 ). When control valve  94  is in the second position, return fluid from first chambers  54  of hydraulic cylinders  26  and/or from the other linear or rotary actuators of hydraulic system  72  may flow through rod-end passage  88 , second pump passage  84 , second bypass line  92 , and control valve  94  back into pump  80 . 
     OPPA  76  may help to protect pump  80  from a shock load traveling in reverse direction through first and second pump passages  82 ,  84 . That is, during operation of hydraulic cylinder  26 , most commonly when another of the linear or rotary actuators (i.e., hydraulic cylinder  32 , hydraulic cylinder  34 , left travel motor  42 L, right travel motor  42 R, or swing motor  43 ) is simultaneously being actuated with hydraulic cylinders  26 , it may be possible for a pressure wave to be generated that travels in reverse direction through the one of first and second pump passages  82 ,  84  currently functioning as the high-pressure supply passage back to pump  80 . If left unchecked, this pressure wave could damage pump  80 . Accordingly, check valves  87 ,  89  may be situated to inhibit the reverse-traveling pressure wave from passing through first or second pump passages  82 ,  84  and into pump  80  in the reverse direction. With check valves  87 ,  89  in place, however, pump  80  may have difficulty drawing in fluid to pressurize for hydraulic cylinders  26 . To remedy this situation, bypass lines  90 ,  92 , together with control valve  94 , may fluidly connect pump  80  to the correct low-pressure feed from first or second pump passages  82 ,  84 . 
     When an operator stops requesting movement of hydraulic cylinders  26  (e.g., when the operator releases interface device  46 ), controller  140  may cause the displacement of pump  80  to move to the zero displacement position (i.e., to destroke). When pump  80  is destroked, the pressure within first and second passages  82 ,  84  may be reduced, while the pressure within head- and/or rod-end passages  86 ,  88  may still be high. In this situation, pressure may naturally build at the poppet base portion of load-holding valve  96 , causing valve element  100  to move to its flow-blocking position. In the embodiment of hydraulic system  72  that utilizes load-holding valve  98 , the pressure at the poppet base portion may be controlled to build via solenoid valve  114  when pump  80  is destroked, similarly causing the corresponding valve element  100  to move to its flow-blocking position. When valve elements  100  are in their flow-blocking positions, hydraulic cylinders  26  may be hydraulically locked from substantial further movement. Operation may be similar when machine  10  is turned off and/or the operator activates a hydraulic lock-out switch (not shown). 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed hydraulic system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed hydraulic system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.