Patent Publication Number: US-9845590-B2

Title: Hydraulic system for an earth moving machine

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to hydraulics and, more particularly, to a hydraulic system configured to both enable steering of an earth-moving machine and to actuate various implements disposed on the machine. 
     BACKGROUND 
     Earth-moving machines are used to move and relocate various materials and other objects about a worksite. An example of an earth-moving machine may be a loader, which may be propelled on wheels or continuous tracks, having a lift arm and bucket for lifting, transporting, and dumping material. Other possible examples of earth-moving machines may include bulldozers, dump trucks, and the like. To power the earth-moving machine, the machine may include a prime mover such as an internal combustion engine, e.g., a diesel compression ignition engine, which combusts hydrocarbon-based fuels to convert the potential chemical energy therein into a mechanical or motive force. The generated power can be utilized by a number of components on the earth-moving machine including drive components to propel the machine, steering assemblies for direction control, and work implements connected to the machine. 
     To distribute the generated power, the prime mover may be associated with a hydraulic system that distributes pressurized hydraulic fluid about the earth-moving machine to actuate the various components. A typical hydraulic system may include a common reservoir for containing low pressure hydraulic fluid, one or more pumps operatively coupled to the prime mover to pressurize the fluid, and a plurality of hydraulic actuators disposed about the earth-moving machine that convert fluid pressure into physical force and motion. High pressure hydraulic hoses or pipes can be included to direct the hydraulic fluid about the machine and between the hydraulic components. The components and hoses may be arranged in one or more hydraulic circuits organized to direct hydraulic fluid from the reservoir through the components and back to the reservoir for reuse. 
     Because the hydraulic system may include a single prime mover and possibly a common reservoir, the system needs to be designed to allocate the utilities of these common resources to meet the requirement of the different components and subassemblies on the earth-moving machine. Further, the power requirements of the earth-moving machine may change as the machine performs different operations at different times. U.S. Pat. No. 8,336,232 (“the &#39;232 patent”), assigned to the assignee of the current application, describes an arrangement or architecture for a hydraulic system to selectively allocate the pressurized hydraulic fluid between different applications on the earth-moving. In particular, the &#39;232 patent describes a wheel loader having a lift circuit operatively associated with the lift arm and a tilt circuit operatively associated with the bucket. A combiner valve is disposed between and in fluid communication with the lift circuit and the tilt circuit to selectively redirect hydraulic fluid between the circuits based on the requirements and capacities of the hydraulic system. The present disclosure is directed to addressing similar considerations to those described in the &#39;232 patent. 
     SUMMARY 
     The disclosure describes, in one aspect, an earth-moving machine for loading, hauling, and dumping earth. The earth-moving machine includes a frame supported on a plurality of traction components that are steerable with respect to the frame by operation of a hydraulic steering assembly. The earth-moving machine also includes a lift arm pivotally connected to the frame and adapted to be raised and lowered with respect to the frame. A bucket is pivotally connected to the end of the lift arm and can be tilted with respect to the lift arm to dump material. To power operation of the lift arm, bucket, and hydraulic steering assembly, a first hydraulic pump is operably associated with the lift arm to actuate the lift arm and a second hydraulic pump is operably associated with both the plurality of traction components to actuate steering of the plurality of traction components and the bucket for tilting the bucket. 
     In another aspect, the disclosure describes a method for hydraulically operating an earth-moving machine. The method involves pressurizing low pressure hydraulic fluid into a first pressured hydraulic charge by use of a first hydraulic pump and into a second pressurized charge by use of a second hydraulic pump. The first pressurized hydraulic charge is directed to a lift circuit to raise a lift arm of the earth moving machine while the second pressurized hydraulic charge is directed to at least one of a tilt circuit operably associated with a bucket tiltable with respect to the lift arm and a steering circuit to actuate a steering assembly operably associated with a plurality of traction components that are steerable with respect to the earth-moving machine. 
     In yet another aspect of the disclosure, there is described a hydraulic system including a lift circuit, a tilt circuit, and a steering circuit. The lift circuit includes a first hydraulic pump in fluid communication with a first hydraulic actuator that is operably connected with a lift arm. The first hydraulic pump is adapted to direct a first pressurized hydraulic charge from the first hydraulic pump to the first hydraulic actuator. The tilt circuit includes a second hydraulic pump in fluid communication with a second hydraulic actuator operably connected to a bucket that can pivot with respect to the lift arm. The second hydraulic pump is also included as part of the steering circuit and is in fluid communication with a third hydraulic actuator operatively connected with a hydraulic steering assembly. The tilt circuit and the steering circuit are adapted to direct a second pressurized hydraulic charge from the second hydraulic pump to at least one of the second hydraulic actuator and the third hydraulic actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view of a representative earth-moving machine, in particular, a wheel loader constructed in accordance with the present disclosure. 
         FIG. 2  is a schematic diagram of a hydraulic system for operating the sub-assemblies and components of the wheel loader of  FIG. 1  in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to earth-moving machines that can be used for lifting, hauling, and dumping material about a worksite, such as a wheel loader, excavator, dozer, dump truck, or the like. As used herein, the term “machine” may refer to any machine that performs some type of operation associated with an industry, such as mining, construction, farming, transportation, or any other industry known in the art. Moreover, the machine may include one or more work implements connected thereto that may be utilized for a variety of tasks, including, for example, loading, compacting, lifting, brushing, and may include, for example, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and other tools. Referring to  FIG. 1 , wherein like reference numbers refer to like elements, there is illustrated an embodiment of an earth-moving machine in the form of a loader  100 . 
     The loader  100  may be of the wheeled variety in which the frame  102  of the loader is supported by a plurality of traction components  104  that contact the ground or work surface  106  of the worksite. In the illustrated embodiment, the plurality of traction components  104  can be wheels that are rotatable with respect to the frame  102  of the loader  100  by way of bearing assemblies. The wheels can be further categorized as drive wheels  108  that are power-driven to propel the loader  100  over the work surface  106  and steer wheels  110  that can turn to change the direction of travel of the loader. The frame  102  can be an articulated frame with a two-part construction including a rear end  112  and a front end  114  that are connected together by an articulating joint  116  to enable the rear and front ends to pivot with respect to each other. The drive wheels  108  may be disposed on the rear end  112  of the loader  100  and the steer wheels  110  may be disposed on the front end  114  to form part of a hydraulic steering assembly  118 . When suitably directed, the steer wheels  110  may pivot the front end  114  of the frame  102  with respect to the rear end  112  to turn the loader  100 . In other embodiments, however, the frame  102  may be a unitary design with the steer wheels  110  associated with a hydraulic steering assembly of a different configuration to turn the steer wheels with respect to the frame, or the loader  100  may be supported on different types of traction components  104  altogether, such as continuous tracks. 
     To accommodate an operator responsible for directing and controlling operation of the loader  100 , an operator station  120  may be supported on the frame  102  in an elevated position. Disposed in the operator station  120  proximate to the operator may be one or more operator interface devices, such as a steering wheel, joysticks, levers, knobs, pedals, switches, or other devices used to direct operation of the loader  100 . In particular, the operator interface devices  122  may be used to direct propulsion and maneuver the loader  100  with respect to the work surface  106  and to operate any work implements associated with the loader. As the operator moves or manipulates an operator interface device  122 , the device may affect a corresponding motion or action by the loader in a desired direction, or with a desired speed or force. The operator station  120  may also include one or more displays, screens, dials, gauges, and the like to provide the operator with information about the operation of the loader and its components. 
     For performing operations about the worksite, the loader  100  includes one or more work implements  130  connected to the frame  102 . For example, the work implement  130  may be a lift arm  132  and a bucket  134  to lift, haul, and dump materials. The lift arm  132  may be an elongated, rigid structure extending between a first end  136  and a second end  138 . The lift arm  132  may be pivotally connected to the front end  114  of the frame  102  at its first end  136  by a first pivot joint  140  so that it may be pivotally raised and lowered with respect to the frame  102  and the work surface  106 . The bucket  134  can be pivotally connected to the distal second end  138  of the lift arm  132  by a second pivot joint  142  and configured to tilt with respect to the lift arm. Hence, when the lift arm  132  is lowered and the bucket  134  engages the work surface  106 , the loader  100  can be moved in the forward direction to fill the bucket with material. The lift arm  132  can then be raised to disengage the bucket  134  from the work surface  106  so the loader  100  can haul the material therein about the worksite. The bucket  134  can be tilted with respect to the lift arm  132  to dump the material at a desired location. The lift arm  132  may be shaped to support the weight of the bucket  134  in a cantilevered manner and, in an embodiment, two lift arms may be provided that are pivotally connected to either side of the bucket for improved support. Examples of other work implements include booms, blades, shovels, and the like. 
     To generate power for maneuvering the loader  100  and for operating the work implements  130 , the loader can include a power system based around a prime mover  150  disposed on the rear end  112  of the frame  102 . As indicated above, the prime mover  150  can be an internal combustion engine, e.g., a diesel compression ignition engine, which combusts a hydrocarbon-based fuel to convert the potential energy therein into usable mechanical or motive forces, typically embodied by a rotating output shaft or drive shaft protruding from the engine. The prime mover  150  can be mechanically connected to the drive wheels  108  through transmissions, differentials, and the like to forcibly rotate the drive wheels  108  with respect to the frame  102 . However, to enable the work implements and other components disposed about the loader  100  at locations remote from the prime mover  150  to utilize the generate mechanical power, the loader can be associated with a hydraulic system  152  that uses the motion to pressurize a hydraulic fluid. The pressurized hydraulic fluid can be directed to or circulated to the work implements  130  where, for example, the hydraulic pressure can be used to power one or more hydraulic actors associated with the work implements. 
     For example, to raise and lower the lift arm  132  with respect to the frame  102 , the pressured hydraulic fluid can power a first hydraulic actuator  154  operatively connected between the lift arm  132  and the front end  114  of the frame. The first hydraulic actuator  154  can extend and retract in a telescoping manner to cause the lift arm  132  to pivot with respect to the first pivot joint  140  to raise and lower the bucket  134 . Likewise, to tilt the bucket  134  with respect to the frame  102 , a second hydraulic actuator  156  can be operatively connected between the bucket and the lift arm  132  and can also extend and retract to pivot the bucket about the second pivot joint  142 . In the present embodiment, the hydraulic system  152  can also be associated with a third hydraulic actuator  158  that is part of the hydraulic steering assembly  118  to pivot the front end  114  with respect to the rear end  112  of the loader to enable steering of the loader. In other embodiments, the third hydraulic actuator  158  may be associated with different types of steering assemblies such as rack-and-pinion designs. Of course, other work implements, sub-assemblies, and hydraulic devices, and any associated hydraulic actuators may be operatively associated with the hydraulic system  152 . 
     Referring to  FIG. 2 , which shows a schematic representation of the components of the hydraulic system  152 , the first hydraulic actuator  154 , the second hydraulic actuator  156 , and the hydraulic actuator  158  can, in various embodiments, be double acting hydraulic cylinders. The hydraulic cylinder includes a piston  160  slidably received in an enclosed housing or barrel  162  and disposed to reciprocate back and forth within the barrel. The piston  160  and barrel  162  may be circular or cylindrical in shape to facilitate relative sliding movement between the parts. The piston  160  can include a rod  164  extending from one side of the piston that protrudes from the enclosed barrel  162  to connect with the work implement or frame. Further, the piston  160  can divide the internal volume of the barrel  162  into a head end  166  corresponding to the side of the piston from which the rod  164  extends and a cap end  168  corresponding to the side of the piston  160  opposite the rod. The head end  166  and the cap end  168  can communicate with respective ports disposed through the barrel  162  to receive and discharge hydraulic fluid from the volume defined by the barrel. If pressurized hydraulic fluid is directed into the cap end  168  of the barrel  162 , it can force the piston to slide toward the head end  166  causing the rod  164  to protrude farther out of the barrel  162 . Conversely, if pressurized hydraulic fluid is directed into the head end  166 , the piston  160  moves toward the cap end  168  retracting the rod  164  into the barrel  162 . Although only one hydraulic cylinder is shown associated with each of the lift arm  132 , bucket  134 , and steering assembly  118 , it should be appreciated that in other embodiment multiple cylinders may be associated with each of the devices. In other embodiments, the hydraulic actuators may include pistons of a different construction or operation or may be different actuation devices such as hydraulic motors or the like. 
     To supply the hydraulic fluid that actuates the hydraulic actuators, the hydraulic system  152  can include a tank or hydraulic reservoir  170 . The hydraulic reservoir  170  contains a volume of relatively low pressure hydraulic fluid and may be vented to the atmosphere or may be enclosed so that the contents can be maintained in a slightly pressurized state. The hydraulic fluid can be any suitable type of incompressible fluid such as lubrication oil or the like and may have a sufficient viscosity to enable the fluid to readily flow in the hydraulic system. As indicated in the schematic, the hydraulic reservoir  170  may be disposed at a lower relative elevation compared to the other components of the hydraulic system  152  so the hydraulic reservoir  170  can function as a sump to which the system returns and collects the hydraulic fluid. To facilitate its use on the mobile loader  100  and to simplify filling and fluid replacement, a single hydraulic reservoir  170  may be include with the hydraulic system  152  but in other embodiments, multiple smaller reservoirs may be include. 
     To pressurize and direct hydraulic fluid from the hydraulic reservoir  170  to and from the first, second, and third hydraulic actuators  154 ,  156 ,  158 , the hydraulic system  152  can include a first hydraulic pump  172  and a second hydraulic pump  174 . The first and second hydraulic pumps  172 ,  174  may be any suitable type of pump for pressurizing and positively displacing hydraulic fluid to flow in a circuit, including piston pumps, rotary gear pumps, vane pumps, gerotor pumps, swash plates, and the like. The first and second hydraulic pumps  172 ,  174  may be fixed displacement pumps or, as indicated, variable displacement pumps capable of changing or adjusting the output volume or flow rate the pump. The first and second hydraulic pumps  172 ,  174  can include an inlet  176  in fluid communication with the hydraulic reservoir  170  to receive or draw low pressure hydraulic fluid and an outlet  178  from which the pressurized hydraulic fluid is discharged. In various embodiments, the first and second hydraulic pumps  172 ,  174  may be reversible to enable hydraulic flow both to and from the hydraulic reservoir  170 . To drive the pumps, the first and second hydraulic pumps  172 ,  174  may be coupled to the driveshaft of the prime mover  150  by, for example, a respective pump shaft  179  as indicated. 
     To selectively direct and control the flow of pressurized hydraulic fluid to and from the hydraulic actuators, the hydraulic system  152  may include one or more flow control or direction control valves. In a illustrated embodiment, each of the first, second, and third hydraulic actuators  154 ,  156 ,  158  can be associated with a first flow control valve  180 , a second flow control valve  182 , and a third flow control valve  184 , respectively. To regulate flow of hydraulic fluid to the hydraulic actuators, the flow control valves  180 ,  182 ,  184  can be positioned between the respective hydraulic actuators and the first and second hydraulic pumps  172 ,  174  and in fluid communication with the actuators and pumps. The flow control valves  180 ,  182 ,  184  may be three-position, two-way valves that can selectively direct pressurized hydraulic fluid to or from the head end  166  or the cap end  168  of the respective hydraulic actuator to facilitate moving the piston  160  inside the barrel  162 . In an embodiment, the flow control valves  180 ,  182 ,  184  may be solenoid operated spool valves including an electromagnetic solenoid  186  for changing the position of an internal spool biased against a spring  188 . When the solenoid  186  is electromagnetically activated, the solenoid moves or configures the spool to unseal and seal various ports in the respective flow control valve that directs fluid to and from the actuator moving the piston  160  and rod  164  in a manner that actuates the associated work implement. 
     In the illustrated example, the three-position flow control valves  180 ,  182 ,  184  may include a first position  190  in which the internal spool is moved to direct hydraulic fluid to the cap end  168  and remove hydraulic fluid from the head end  166  to facilitate double action of the respective actuator. The flow control valves  180 ,  182 ,  184  can also include a second position  192  in which hydraulic fluid is directed to the head end  166  and removed from the cap end  168  to facilitate two-way flow. The flow control valves  180 ,  182 ,  184  may also include a neutral third position  194  in which hydraulic flow to the respective hydraulic actuator is cut off and the hydraulic actuator is isolated from the rest of the hydraulic system  152 . The neutral third position may lock and hold the hydraulic actuator and its associated work implement in an intermediate position. In other embodiments, the flow control valves  180 ,  182 ,  184  may be of a different construction such as a two-position valves, three-way valves, pilot actuated valves, etc. In the schematic shown in  FIG. 2 , to direct hydraulic fluid between the various pumps, valves, and actuators, the solid lines between those components represent hydraulic lines, hoses, or tubes per convention. 
     To direct and allocate the hydraulic fluid from the common hydraulic reservoir  170  to the first, second, and third hydraulic actuators  154 ,  156 ,  158 , using the first and second hydraulic pumps  172 ,  174 , the hydraulic system  152  can be configured into a plurality of distinct hydraulic circuits. For example, the first hydraulic actuator  154  that is operatively connected to the lift arm can be associated with a lift circuit  200  as indicated by the dashed lines. The first hydraulic pump  172  can also be associated with the lift circuit  200  and is primarily dedicated to providing pressurized hydraulic fluid to the first hydraulic actuator  154 . Accordingly, the first hydraulic pump  172  is in direct fluid communication with the first flow control valve  180  that is associated with the first hydraulic actuator  154 . The first hydraulic pump  172  and the first hydraulic actuator  154  can form an independent and isolated lift circuit  200  so that a first pressurized hydraulic charge generated by the first hydraulic pump may be exclusively directed to the first hydraulic actuator. Additional circuits can include a tilt circuit  202  associated with the second hydraulic actuator  156  that is operatively connected to the bucket and a steering circuit  204  operatively associated with the third hydraulic actuator  158  that is operatively associated with the hydraulic steering assembly. To provide pressurized hydraulic fluid to the tilt circuit  202  and the steering circuit  204 , the second hydraulic pump  174  can be in fluid communication with both the tilt circuit  202  and the steering circuit  204  and is primarily dedicated to pressurizing and directing hydraulic fluid to the second hydraulic actuator  156  and the third hydraulic actuator  158  of those circuits respectively. Hence, a second pressurized hydraulic charge generated by the second hydraulic pump  174  may be exclusively directed to the second and third hydraulic actuators  156 ,  158 . 
     To enable the second hydraulic pump  174  to selectively direct pressurized hydraulic fluid to both the tilt circuit  202  and the steering circuit  204 , a direction control valve  210  can be disposed between and communicate with the second hydraulic pump  174  and the second and third flow control valves  182 ,  184 . The direction control valve  210  may be a two-position, two-way valve which can control the direction of flow of the pressurized hydraulic fluid from the second hydraulic pump  172  and back to the hydraulic reservoir  170 . The direction control valve  210  may include an electromagnetically activate solenoid  212  and a biasing spring  214  that can move or configure an internal spool of the valve between a first position  216  and a second position  218  to selectively change the flow direction of the hydraulic fluid. The direction control valve  210 , when operated in conjunction with the second and third flow control valve  182 ,  184 , can introduce or remove hydraulic fluid from either the head end  166  or cap end  168  of either the second or the third hydraulic actuators  156 ,  158  to selectively extend or retract the respective rod  164 . Hence, the bucket associated with the second hydraulic actuator  156  and the hydraulic steering assembly associated with the third hydraulic actuator  158  can be operated independently of each other with pressurized hydraulic fluid from the second hydraulic pump  172 . 
     As illustrated in  FIG. 2 , the lift circuit  200  and the tilt and steering circuits  202 ,  204  are generally independent and isolated from each other with each having an independent source of pressurized fluid due to the distinct and dedicated arrangement of the first hydraulic pump  172  and the second hydraulic pump  174 . In an embodiment, the work requirements of a particular circuit may be of a magnitude that the respective first or second hydraulic pumps  172 ,  174  are unable to provide the required pressure, quantity, or flow rate of hydraulic fluid. To address such circumstances, the hydraulic system  152  can be configured so that the first hydraulic pump  172  and the second hydraulic pump  174  can cooperate to combine their respective fluid outputs. A combiner valve  220  can be disposed between the lift circuit  200  and the tilt and steering circuits  202 ,  204 , downstream of the outlets  178  of the first hydraulic pump  172  and the second hydraulic pump  174  and in fluid communication with both pumps. The hydraulic lines to and from the combiner valve  220  hence function as a bridge between the lift circuit  200  and the tilt and steering circuits  202 ,  204 . The combiner valve  220  can be an adjustable restrictor that can be selectively adjusted from a setting preventing any flow and isolating the circuits to a setting allowing substantially unimpeded flow between the circuits. When operated in conjunction with the selective opening and closing of the first, second, and third flow control valves  180 ,  182 ,  184 , the combiner valve  220  can direct hydraulic fluid from the first hydraulic pump  172  to the tilt and steering circuits  202 ,  204 , or can direct hydraulic fluid from the second hydraulic pump  174  to the lift circuit  200 . The first and second hydraulic pumps  172 ,  174  can assist each other in providing pressurized hydraulic fluid as required by their respective circuits by operation of the combiner valve  220 . 
     To further leverage the capacities of the hydraulic system  152 , an energy recovery system  230  can be included to recover and recycle the potential energy of the pressurized hydraulic fluid from at least one of the first, second, and third hydraulic actuators  154 ,  156 ,  158 . The energy recovery system  230  can include an accumulator  232 , which may be a pressure tank of a particular volume into which pressurized hydraulic fluid from one of the hydraulic actuators can be directed for temporary retention. In other words, instead of returning pressurized hydraulic fluid from a hydraulic actuator to the hydraulic reservoir  170 , the accumulator  232  can hold pressurized hydraulic fluid temporarily for reuse in the hydraulic system  152 . To redirect the pressurized hydraulic fluid to the accumulator  232 , a charge valve  234  can be disposed in fluid communication with at least the first flow control valve  180  associated with the first hydraulic actuator  154 . The charge valve  234  can be a two-position, one-way valve including a solenoid  236  and a spring  238  for selectively switching between opened and closed positions. The charge valve  234  can be located upstream of the accumulator  232  so that, when the charge valve is opened, pressurized hydraulic fluid from the first hydraulic actuator  154  flows to the accumulator. 
     To recycle the pressurized hydraulic fluid contained in the accumulator  232 , a discharge valve  240  can be disposed downstream of the accumulator and in fluid communication with, for example, the inlets  176  of the first hydraulic pump  172  and the second hydraulic pump  174 . The discharge valve  240  can be a two-position, one-way valve having a first opened position  242  and a second closed position  244 . When the discharge valve  240  is in the second closed position  244 , the discharge valve isolates the accumulator  232  and retains the pressurized hydraulic fluid therein. However, if the discharge valve  240  is moved to the first opened position  242 , the discharge valve puts the accumulator  232  in fluid communication with the lift circuit  200  downstream of the first hydraulic pump  172  so that hydraulic fluid contained in the accumulator can flow to the lift circuit. The pressurized hydraulic fluid can assist the first hydraulic pump  172  in pressurizing low pressure hydraulic fluid from the hydraulic reservoir  170 , reducing the work expended by the first hydraulic pump and enabling the pressurized hydraulic fluid to be reused in the lift circuit  200 . Likewise, if the discharge valve  240  is placed in the second opened position  244 , the discharge valve can direct pressurized hydraulic fluid from the accumulator to the tilt and steering circuits  202 ,  204  across the combiner valve  220  to assist the second hydraulic pump and the associated tilt and steering circuits. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to a hydraulic system  152  to operate multiple work implements  130 , components, and sub-assemblies on an earth-moving machine such loader  100 . Referring to  FIGS. 1 and 2 , the work implements  130  may include a lift arm  132  for raising and lowering a bucket  134  that is configured to tilt with respect to the lift arm to hold or dump material. Other hydraulically powered assemblies may include a hydraulic steering assembly  118  operatively associated with traction components  104  for changing the direction of travel of the loader. To pressurize and distribute pressurized hydraulic fluid to actuate these implements and sub-assemblies, the hydraulic system  152  can be configured as a plurality of distinct, separate hydraulic circuits primarily served by either the first hydraulic pump  172  or the second hydraulic pump  174  and arranged to beneficially allocate and utilize the components and resources of the hydraulic system. 
     For example, the lift arm  132  is associated with a distinct lift circuit  200  for supplying pressurized hydraulic fluid to a first hydraulic actuator  154  operatively connected to the lift arm. The first hydraulic pump  172  can be dedicated to generating and directing a first pressurized hydraulic charge to the first hydraulic actuator to raise and lower the lift arm  132 . The bucket  134  and the hydraulic steering assembly  118  are associated with a respective tilt circuit  202  and steering circuit  204  which are partially combined and overlap certain utilities. In particular, the second hydraulic pump  174  can be primarily dedicated to generating and directing a second pressurized hydraulic charge to at least one of the second hydraulic actuator  156  associated with the bucket  134  or the third hydraulic actuator  158  associated with the hydraulic steering assembly  118 . The first hydraulic pump  172  and the second hydraulic pump  174 , while physically separated and independently operable with respect to each other, can be in fluid communication with a common hydraulic reservoir  170  containing low pressure hydraulic fluid and can be coupled to the same prime mover  150  to receive motive power. Separating the hydraulic system  152  into distinct circuits at the first and second hydraulic pumps  172 ,  174 , enables the hydraulic system to leverage the common resources of the hydraulic reservoir  170  and the prime mover  150 , facilitates conservation of hydraulic fluid, and enables independent and selective operation of the first and second hydraulic pumps to improve performance of the associated circuits. 
     For example, a possible advantage of the foregoing arrangement is the performance improvements from partially combining the tilt and steering circuits  202 ,  204 . The loader  100 , in normal operation, will typically not actively adjust the hydraulic steering assembly  118  and tilt the bucket  134  at the same time. Because the two devices are typically not used concurrently, the second pressurized hydraulic charge from the second hydraulic pump  174  can be selectively directed to either the second hydraulic actuator  156  associated with the bucket  134  or third hydraulic actuator  158  associated with the hydraulic steering assembly  118  as appropriate. This allows for a reduction in size or capacity of the second hydraulic pump  174 . If, on occasion, the hydraulic steering assembly  118  and bucket  134  are used concurrently in a manner overwhelming the second hydraulic pump  174 , the combiner valve  220  can be opened to direct a portion of the first pressurized hydraulic charge output from the first hydraulic pump  172  to assist the second hydraulic pump  174  with actuating the second and third hydraulic actuators  156 ,  158 . 
     Associating the lift arm  132  as a separate lift circuit  200  with the first hydraulic pump  172  dedicated thereto further improves the hydraulic system  152  by enabling the lift arm to operate concurrently with either the hydraulic steering assembly  118  or the bucket  134 . Additionally, raising and lowering the lift arm  132  may require more power than adjusting the hydraulic steering assembly  118  or tilting the bucket  134 . The required power may be provided more easily by exclusively directing the first pressurized hydraulic charge from the first hydraulic pump  172  to the first hydraulic actuator  154 . In the event additional power is required, the combiner valve  220  can be opened to redirect a portion of the second pressurized hydraulic charge output from the second hydraulic pump  174  to the first hydraulic actuator  154  to assist in raising the lift arm  132 . 
     The distinct arrangement the lift circuit  200  also facilitates energy recovery by the energy recovery system  230 . For example, the first hydraulic pump  172  raises the lift arm  132  by exclusively directing the first pressurized hydraulic charge to the cap end  168  of the first hydraulic actuator  154 , thereby extending the rod  164  to which the lift arm is operatively connected. To lower the lift arm, the first flow control valve  180  is repositioned to discharge the first hydraulic charge from the first hydraulic actuator  154  thereby allowing the lift arm to descend with respect to the frame under its own weight. Rather than direct the first hydraulic charge, still under relatively high pressure, to the hydraulic reservoir  170 , the charge valve  234  can be opened to direct the first pressurized hydraulic charge to the accumulator  232  where it can be temporarily maintained. In addition, the charge valve  234  can be configured to establish a pressure drop that impedes the first hydraulic charge from exiting the first hydraulic actuator  154  too quickly and therefore allows the lift arm  132  to lower at a suitable rate. When stored fluid pressure is needed, the discharge valve  240  can be selectively configured to direct the first pressurized hydraulic charge from the accumulator  232  to downstream of the outlets  178  of the first hydraulic pump  172  and/or second hydraulic pump  174 , hence recovering and recycling a portion of the energy already expended by the hydraulic system  152 . 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.