Patent Publication Number: US-11661723-B1

Title: Variable system pressure based on implement position

Description:
TECHNICAL FIELD 
     The present application relates generally to hydraulic systems such as those used on work machines including heavy equipment for construction, farm implements, and other machines adapted for performing work. More particularly, the present application relates to a variable system pressure in a hydraulic system for work machines. Still more particularly, the present application relates to a variable system pressure in a mining work machine allowing for variable payload machines to have same or similar linkage systems while avoiding undue wear and/or overloading conditions. 
     BACKGROUND 
     Commonality between components of work machines can create advantages for design and development, manufacturing, and resulting cost. However, user desired variability relating to size, speed, and function of the work machines has a tendency to cause or generate differences in work machines rather than commonalities. In the mining industry, various constraints on size can place limits on varying the size of work machines. That is, for example, in the case of underground loaders, constraints on height, width, turning radii, and other constraints may cause machine size to be limited and, in some circumstances, may tend toward forced commonality of physical components. This type of forced commonality can lead to other problems. For example, one solution may be to place larger hydraulic cylinders on machines designed for use with smaller hydraulic cylinders. This can result in overloading of particular components of the machine causing failure or premature failure. In addition, particular functions of the machine may have excessive power and, as a result, components related to that function may wear out prematurely due a lack of restrictions on use, for example. On the other hand, the machine could be designed to withstand the loads of larger hydraulic cylinders or higher pressures, but this may result in mass increases for the structure, which can decrease the performance of the machine. 
     U.S. Pat. No. 5,085,051 to Hirata discloses a load-sensing control responsive to a differential pressure. Hirata also discloses an instruction device that instructs a change in the differential pressure. U.S. Pat. No. 5,469,646 discloses an excavator adapted to be changed over simply to a fine operation mode so as to control the capacity of a hydraulic pump through load-sensing. 
     SUMMARY 
     In one or more embodiments, a hydraulic system for controlling an implement on a work machine may include a hydraulic reservoir, a hydraulic pump in fluid communication with the reservoir, a central valve in fluid communication with the pump and configured for controlling the implement, a pressure relief system arranged in fluid communication with the hydraulic pump and the central valve, and a controller. The controller may be configured for controlling the hydraulic pump, the central valve and the pressure relief system and selecting between operating the hydraulic system at a first pressure and a second pressure based on a factor relating to implement position. 
     In one or more embodiments, a method of operating a hydraulic system, may include operating a work machine and controlling an implement of the work machine with a hydraulic system. The method may also include monitoring a factor relating to implement position, operating the hydraulic system at a first of a plurality of available system pressures unless and until the implement position dictates switching to a second of the plurality of available system pressures. The method may also include switching to the second pressure and operating the hydraulic system at the second pressure unless and until the implement position dictates switching back to the first pressure. 
     In one or more embodiments, a work machine may include an implement and a hydraulic system for controlling the implement. The hydraulic system may include a hydraulic reservoir, a hydraulic pump in fluid communication with the reservoir, a central valve in fluid communication with the pump and configured for controlling the implement, a pressure relief system arranged in fluid communication with the hydraulic pump and the central valve, and a controller. The controller may be configured for controlling the hydraulic pump, the central valve and the pressure relief system and selecting between operating the hydraulic system at a first pressure and a second pressure based on a factor relating to implement position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front left perspective view of a work machine having a hydraulic system with a variable system pressure and having a raised bucket in a tipped position, according to one or more embodiments. 
         FIG.  2    is a front left perspective view of the work machine of  FIG.  1   , with a lowered bucket in dig or scoop position, according to one or more embodiments. 
         FIG.  3    is right side view thereof. 
         FIG.  4 A  is a hydraulic schematic of a hydraulic system providing for variable system pressure, according to one or more embodiments. 
         FIG.  4 B  is a hydraulic schematic of another hydraulic system providing for variable system pressure, according to one or more embodiments. 
         FIG.  4 C  is a control schematic for the system of  FIG.  4 B , according to one or more embodiments. 
         FIG.  5    is right side view of the implement portion of the work machine of  FIGS.  1 - 3    showing the lift arms and bucket in a dig or scoop position, according to one or more embodiments. 
         FIG.  6    is a right-side view of the implement portion of the work machine of  FIGS.  1 - 3    showing the lift arms and bucket in a poised to lift position, according to one or more embodiments. 
         FIG.  7    is a right-side view of the implement portion of the work machine of  FIGS.  1 - 3    showing the lift arms and bucket in a lifted position, according to one or more embodiments. 
         FIG.  8    is a graph showing lift height of the bucket pin relative to lift arm capacity for two different payload machines, according to one or more embodiments. 
         FIG.  9    is a graph showing lift height of the bucket pin relative to lift arm capacity for two different payload machines, according to one or more embodiments. 
         FIG.  10 A  is a graph showing lift height of the bucket pin relative to lift arm capacity for a machine having a varying system pressure, according to one or more embodiments. 
         FIG.  10 B  is a graph showing lift height of the bucket pin relative to lift arm capacity for a machine having a varying system pressure and including upper and lower limits, according to one or more embodiments. 
         FIG.  10 C  is a graph showing close up view of  FIG.  10 B  and depicting the relationship between a top limit and the varying system pressure curves, according to one or more embodiments. 
         FIG.  11    is a method diagram depicting a method of operation using a variable system pressure based on implement position. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a perspective view of a work machine  100 . As shown, the work machine  100  may be an underground loader used in mining operations and may include an implement  102  in the form of lift arms  104  and a tilting bucket  106 . That is, the lift arms  104  may be arranged and configured to lift the tilting bucket  106  and the tilting bucket  106  may be arranged and configured on the lift arms  104  to tilt relative to the lift arms  104 . The implement system may be moveable using a hydraulic system. The work machine may include a plurality of ground supporting traction elements  108  (e.g., wheels, tracks, skid feet, etc.) for translating the work machine relative to a supporting surface. The traction elements  108  may be coupled to a frame  110  of the work machine  100  with a suspension system. The work machine  100  may include an engine or motor  112  to generate power and to drive the traction system  108 , the hydraulic system, and other onboard equipment or systems. 
     In  FIG.  1   , the lift arms  104  are shown in a raised position and revealing a lift arm hydraulic system  114  including a pair of hydraulic cylinders for raising and lowering the lift arms  104 .  FIG.  2    is a perspective view of the work machine with the lift arms  104  in a lowered position and revealing a bucket hydraulic system  116  including a central hydraulic cylinder for controlling the tilt position of the bucket  106 .  FIG.  3    is a side view of the work machine  100 . 
     Referring now to  FIG.  4 A , a schematic drawing of an example of a hydraulic system  118  is provided. The present hydraulic system  118  may be configured for varying the maximum or available hydraulic system pressure based on one or more factors. In one or more embodiments, the factors may include implement position such as height, orientation, position, amount of travel, or other positional data points, for example. As mentioned, the work machine  100  may include a lift arm hydraulic system  114  and a bucket hydraulic system  116 . These systems may share one or more components and may be part of the same system  118  operating on a same hydraulic pressure. As shown, the hydraulic system  118  may include a shared hydraulic reservoir  120 , a shared hydraulic pump  122 , a series of hydraulic lines  124 , one or more hydraulic cylinders  126 A/B, one or more valves or solenoids  128 A/B/C/D, and a control system  130  for controlling the pump, the valves, the solenoids, and/or other aspects of the system. It is to be appreciated that while a shared reservoir and pump system is shown, other systems may include dedicated reservoirs or pumps. 
     The hydraulic reservoir  120  may be configured for holding a supply of hydraulic fluid for use by the system. The hydraulic reservoir  120  may include a holding tank, for example. The tank may be generally closed to prevent intrusion of contaminants, but may include valves or other ports allowing the tank to be maintained at or near atmospheric pressure or another baseline pressure. The holding tank may contain a supply of hydraulic fluid that may be drawn from the holding tank by a hydraulic pump  122  and delivered to a pressurized portion of the system. Various relief valves and/or return lines may deliver the hydraulic fluid back to the holding tank in particular conditions or circumstances. 
     The hydraulic pump  122  may be arranged in fluid communication with the hydraulic reservoir  120  and may operate to draw fluid from the hydraulic reservoir  120 , pressurize the fluid and deliver it to the operative side of the system. For example, the pump  122  may pressurize the fluid to extend one or more hydraulic cylinders. The pump may be sized to deliver a selected range of pressures suitable for the particular system being provided. In one or more embodiments, the pump may include a variable displacement load sense pump. 
     The hydraulic lines  124  may extend from the pump to the hydraulic cylinders  126 A/B and or from the pump to one or more valves  128 A/B/C/D and from the valves to the hydraulic cylinders  126 A/B. The hydraulic lines  124  may include pressure resisting lines capable of maintaining the hydraulic fluid at pressures created by the pump and delivering the hydraulic fluid. 
     The hydraulic cylinders  126 A/B may be configured for performing work by extending and/or retracting. The hydraulic cylinders  126 A/B may include a housing, a piston arranged within the housing and a rod coupled to the piston and extending out an end of the housing. The housing may include one or more ports for receiving and/or ejecting hydraulic fluid to fill or remove fluid from the housing on one or more sides of the piston causing the piston to articulate back and forth within the housing to extend or retract the piston rod. In the present case of an underground loader, the system may include a pair of lift arm cylinders for raising and lowering the lift arms and a single tilt cylinder for tilting the bucket. 
     The system may also include one or more valves  128 A/B/C/D. As shown in  FIG.  4 A , the system may include a central implement valve  128 A. In the present case, for example on an underground loader, the implement valve  128 A may include a lift valve section  132  and a tilt valve section  134 . The lift valve section  134  may control the hydraulic fluid, and the pressures thereof, that operate the lift arm hydraulic cylinders  126 A. The tilt valve section  134  may control the hydraulic fluid, and the pressures thereof, that operate the tilt cylinder  126 B. 
     The hydraulic system  118  may, as in the present example, have a variable displacement load sensing pump  122  that is controlled by the load sense signal circuit. When a control valve  132  or  134  is opened to provide flow to the load, or cylinders  126 A/B in this system, the load sense system may be supplied with pressure created by the work being done. The variable displacement load sensing pump  122  may provide sufficient flow for the pump discharge flow to maintain a margin over the load sense pressure unless the pump reaches maximum displacement without achieving this margin pressure. Margin may be a pressure somewhat greater than the load sense signal, for example. If the valve section  132  and  134  are not activated, the pump  122  will only supply enough flow to maintain the margin pressure and account for internal leakages in the system. In this embodiment maximum load sense pressure may be limited by the pressure relief system  136 . The amount of flow allowed into the load sense circuit may be a limited and very small percentage of the available pump flow. This may allow the pressure relief system to limit the pressure in the load sense circuit. 
     Additional valves in the present system may be provided and configured for varying the maximum hydraulic system pressure based on one or more factors as mentioned. That is, as shown, the hydraulic system may include a load sense pressure relief system  136  arranged between the hydraulic pump  122  and the lift valve section  132  and the tilt valve section  134  of the central implement valve  128 A. That is, the load sense pressure relief system  136  may be arranged to control pump  122  and the valve sections and may be arranged between the pump  122  and the cylinders  126 A/B. The load sense pressure relief system  136  may include a first load sense relief valve  128 B, such as a high pressure load sense relief valve, and a second load sense relief valve  128 C, such as a low pressure load sense relief valve. The relief valves  128 B/C may be configured to limit system pressures. For example, the relief valves may have a pressure setting, such as a threshold pressure. The load sense relief valves  128 B/C may remain closed unless or until the pressure in the system exceeds the threshold pressures. Upon reaching the threshold pressure of either relief valve, that respective relief valve may open allowing hydraulic fluid from the load sense circuit system to exit the circuit and return to the tank  120 . The relief of hydraulic fluid from the load sense circuit may provide for maintaining the hydraulic system at the threshold pressure without exceeding the threshold pressure. 
     The load sense pressure relief system  136  may include a pair of branches off of the hydraulic line connecting the pump  122  with the implement valve  128 A. The pair of branches may include a low pressure branch and a high pressure branch. The low pressure branch may include the low pressure relief valve  128 C and the high pressure branch may include the high pressure relief valve  128 B. The low and high pressure branches may be arranged in parallel and lead back to the hydraulic reservoir  120 . This arrangement may allow the system to operate at a system pressure dictated by whichever valve is open. For example, the system pressure may be limited to the threshold pressure of the low pressure relief valve and may not reach the threshold pressure of the high pressure relief valve. However, as shown, the system may also include a solenoid  128 D for varying the effect of the pressure relief valves on the system. 
     As shown in  FIG.  4 A , the low pressure branch of the load sense pressure relief system may include a solenoid valve  128 D arranged on the pump side of the low pressure relief valve  128 C. The solenoid valve  128 D may be an electrically actuated solenoid valve that is open in a non-energized condition and is closed in an energized condition. As such, the solenoid  128 D may be used to cut off flow of hydraulic fluid to the low pressure load sense relief valve  128 C. Cutting off the flow of hydraulic fluid to the low pressure load sense relief valve may cause the system to function as if the low pressure sense relief valve is not present. As such, when the solenoid valve  128 D is closed, the system pressure may rise above the threshold pressure of the low pressure load sense relief valve and the system pressure may be dictated by the threshold pressure of the high pressure load sense relief valve  128 B. 
     It is to be appreciated that while a system having a first low pressure load sense relief valve and a second high pressure load sense relief valve has been described, a system with a first high pressure load sense relief valve and a second low pressure sense relief valve may be provided. Moreover, while a system with two pressure sense relief valves has been described, additional load sense relief valves may be provided to allow for multiple system pressures including 2, 3, 4, or more system pressures based on one or more conditions. Still further, a system with a variable load sense relief valve may also be provided such that the available system pressures may be adjusted. For example, a controllable coil may be provided to allow the load sense relief setting to be varied and/or controlled periodically or continuously. 
     It is also to be appreciated that while a particular plumbing arrangement has been described, other plumbing arrangements may be provided that may provide a similar effect. For example, a two branch parallel load sense relief system may be provided where the branches are selectable by a valve, thereby allowing the hydraulic fluid flow to be directed passed a selected pressure sense relief valve. Still other hydraulic plumbing arrangements may be provided to establish the effect of selecting between multiple system pressures. 
     Further review of  FIG.  4 A  shows that the high-pressure load sense relief valve  128 B is shown within the boundary identified as the implement valve  128 A. That is, in one or more embodiments, the high-pressure sense relief valve  128 B may be a component of the central valve  128 A and the solenoid valve  128 D and the low pressure sense relief valve  128 C may be supplemental components that are provided to augment the implement valve  128 A and provide for multiple operation pressures. 
     The system may also include a control system  130 . The control system  130  may be configured to control the implement valve  128 A allowing the lift cylinders  126 A and the tilt cylinder  126 B to be controlled to operate the implement of the work machine  100 . That is, for example, opening the lift valve section  132  of the implement valve  128 A may deliver hydraulic fluid to the lift cylinders  126 A causing them to extend or to retract depending on the direction of the delivery of fluid. Similarly, opening the tilt valve section  134  of the implement valve  128 A may deliver hydraulic fluid to the tilt cylinder  126 B causing it to extend or retract depending on the direction of the delivery of fluid. Still further, based on one more factors, the control system  130  may be configured to operate the solenoid  128 D to close the solenoid  128 D and adjust the system pressure. 
     The control system  130  may be a standalone control system for the hydraulic system or the control system may be part and parcel with the control system for the work machine  100 . In either case, the control system may include a computing device having a processor and a computer readable storage medium. The computer readable storage medium may include computer implemented instructions stored thereon including method steps for controlling the equipment based on user input. That is, the work machine  100  may include one or more interfaces for controlling the equipment including, for example, joysticks, touch screens, levers, buttons, switches, throttles, etc. The control system  130  may be in electrical communication with the mentioned interfaces and may also be in electrical communication and/or signal communication with one or more aspects of the hydraulic system  118  such as the pump  122 , the implement valve  128 A, the solenoid  128 D and/or other aspects of the hydraulic system  118 . 
     Referring now to  FIG.  4 B , a schematic drawing of another example hydraulic system  218  is provided. Like the system  118 , the present hydraulic system  218  may be configured for varying the maximum or available hydraulic system pressure based on one or more factors. In one or more embodiments, the factors may include implement position such as height, orientation, position, amount of travel, or other positional data points, for example. As mentioned, the work machine  100  may include a lift arm hydraulic system  214  and a bucket hydraulic system  216 . These systems may share one or more components and may be part of the same system  218  operating on a same hydraulic pressure source. As shown, the hydraulic system  218  may include a shared hydraulic reservoir  220 , a shared hydraulic pump  222 , a series of hydraulic lines  224 , one or more hydraulic cylinders  226 A/B, one or more valves or solenoid valves  228 A/B/C, and a control system  230  for controlling the pump, the valves, the solenoids, and/or other aspects of the system. It is to be appreciated that while a shared reservoir and pump system is shown, other systems may include dedicated reservoirs or pumps. 
     The hydraulic reservoir  220 , the hydraulic lines  224 , the implement valve  228 A, and the hydraulic cylinders  226 A/B may be the same or similar to the respective hydraulic reservoir  120 , hydraulic lines  124 , implement valve  228 A, and cylinders  126 A/B described above. The pump  222  and the control  230 , on the other hand, may be somewhat different than the pump  122  and the control  130  described above. Regarding the pump, and like the pump  122 , the hydraulic pump  222  may be arranged in fluid communication with the hydraulic reservoir  220  and may operate to draw fluid from the hydraulic reservoir  220  and deliver a flow of fluid to the operative side of the system  218 . The pump may be sized to deliver a selected range of fluid volumes or flow rates for the particular system being provided. However, in the present embodiment, rather than a variable displacement load sense pump, a displacement control, also described as a positive flow control (PFC), pump may be provided. Regarding the control  230 , and as the pump name implies, the control  230  may be configured to manage both pressure and flow delivered by the pump  222 . A more detailed discussion of the control operation may be provided after several other aspects of the system  218  are introduced. 
     The system  218  may include a pressure relief valve  228 B in fluid communication with the pump  222  and with each of the lift and tilt valves  232 / 234 . The pressure setting of the pressure relief valve may be selectively adjusted via a solenoid based on the position of the implement as discussed in more detail below. More particularly, the pressure relief valve  228 B may be configured to compensate for and/or avoid pressure spikes in the system by relieving pressure in the primary circuit to tank when the pressure in the primary circuit exceeds a selected pressure setting of the pressure relief valve. In contrast to the system of  FIG.  4 A  where the pressure relief valve  128 B/C is in a load sense circuit, the pressure relief valve  228 B may be in the primary pressure generating circuit for the lift and tilt valves  232 / 234 . Moreover, in the present example, the pressure relief valve may be an adjustable or variable pressure relief valve  228 B that has multiple relief pressure settings that are selectable by the presence, or lack thereof, of a pilot pressure. As shown, a pilot line may extend from a pilot system to a solenoid valve  228 C. The solenoid valve may include a neutral position  262 A and a pilot pressure position  262 B. The valve  228 C is in the neutral position  262 A in  FIG.  4 B  and the neutral position  262 A may place the pressure relief valve  228 B in fluid communication with the tank  220 . However, when the solenoid is actuated by the control  230 , the solenoid valve  228 C may place the pressure relief valve  228 B in fluid communication with the pilot pressure by moving the valve to the pilot position  262 B. When the pressure relief valve  228 B is not subject to the pilot pressure, the pressure relief valve  228 B may have a first pressure setting and when the pressure relief valve  228 B is subject to the pilot pressure, the pressure relief valve  228 B may have a second pressure setting higher than the first pressure setting. As discussed in more detail below, the pressure controls provided by the adjustable pressure relief valve  228 B may be secondary to the pressure controls provided by pump  222  and control  230 . As such, the first pressure setting and the second pressure setting may be slightly higher than respective stored pressure settings in the controller that are used to control the pump. 
     As shown in  FIG.  4 B , the system may also include a pressure sensor  264  and the pump  222  may include a control or control system  230  that relies on the pressure sensor  264  as well as an implement position sensor to control the pressure in the system  218 . As with the control system  130 , the control system  230  may be a standalone control system for the hydraulic system or the control system may be part and parcel with the control system for the work machine  100 . In either case, the control system  230  may include a computing device having a processor and a computer readable storage medium. The computer readable storage medium may include computer implemented instructions stored thereon including method steps for controlling the equipment based on user input. That is, the work machine  100  may include one or more interfaces for controlling the equipment including, for example, joysticks, touch screens, levers, buttons, switches, throttles, etc. The control system  230  may be in electrical communication with the mentioned interfaces and may also be in electrical communication and/or signal communication with one or more aspects of the hydraulic system  218  such as the pump  222 , the implement valve  228 A, the solenoid valve  228 C and/or other aspects of the hydraulic system  218  such as the pressure sensor  264  and/or an implement position sensor. 
       FIG.  4 C  is a schematic drawing of the control  230  of the system  218 . As shown, the control  230  may be configured to control the pressure in the system by controlling the delivery of fluid to the system by the pump. For example, as shown, the control  230  may include a pressure sensor input  230 A in communication with the pressure sensor  264  and a linkage position input  230 B in communication with a linkage position sensor. In addition, the control may include a pump displacement solenoid output  230 C, a pump pressure cut off setting solenoid output  230 D, a main relief valve pressure setting output  230 E, and outputs for all other implement control solenoids  230 F, for example. 
     As discussed in more detail below, the control  230  may rely on a linkage position sensor or other position sensor to toggle between two pressure settings and may control one or more solenoids based on the linkage position. For example, and as mentioned above with respect to the hydraulic circuit, the control  230  may control the solenoid valve  228 C with the main relief valve pressure setting output  230 E. That is, based on the position of the linkage, the control  230  may leave the solenoid valve  228 C in the neutral position  262 A or actuate the solenoid valve to place the valve in the pilot position  262 B and, as such, select between at least two maximum pressures for the hydraulic system  218  based on implement position. In addition, however, and as a primary means of pressure control, the control  230  may also include a pump displacement solenoid output  230 C configured to control the displacement output of the pump for the purpose of controlling pressure in the circuit. Like the secondary means of pressure control, this aspect of the system may control system pressure based on implement position. That is, the control  230  may include two stored pressure settings (e.g., one for a first range of implement positions and a one for a second range of implement positions) and may function to control the pump to limit the output pressure of the pump based on the respective settings. As such, the control may have an internal toggle, for example, and may monitor the pump discharge pressure based on the pressure sensor input  230 A. The control  230  may continually compare the pressure sensor input  230 A to a selected stored pressure sensor setting and when the input  230 A exceeds the stored pressure setting, the control  230  may actuate the pump displacement control solenoid (e.g., to destroke the pump) to limit the output pressure of the pump. This primary means of pressure control may function to limit the pressure supplied to the system  218  based on implement position. However, due to delays associated with the electronic monitoring and control of the pump  222 , the secondary means of pressure control described above may assist in avoiding pressure spikes when maximum pressures are approached and/or reached. 
     Additionally or alternatively, yet another means of controlling pressure in the system may be provided. For example, as shown in  4 C, a pump pressure cut off setting solenoid output  230 D may be provided at the control  230 . This output  230 D may be actuated by the control in a manner similar to that of the main relief valve pressure setting output  230 E. That is, the control may monitor the linkage position and change output  230 D so that the pump is at a higher or lower high pressure cut off setting based on the linkage position. Powering  230 D changes the hydraulic capability of the pump so the pump reacts to pressure on its own vs. the  230  control adjusting the pump displacement. Once the pressure is under control (under the high pressure cut off setting), the pump hydraulic control will stop influencing pump displacement and the software control will regain authority over the pump displacement. 
     Turning now to  FIGS.  5 - 7   , a series of example loading conditions on the lift arms  104  of a work machine  100  are shown. As shown in  FIG.  5   , a breakout load  138  is shown. The load represents the force between the tip of the bucket and the material being lifted by the bucket. For example, when a loader is digging into a pile of dirt, rock, rubble, or other material, the force of the material on the bucket when lifting the collected material from a pile is reflected by the break out load  138 . It is to be appreciated that the actual load may be a varying distributed load throughout the depth of the bucket, but for purposes of the present explanation, a point load is depicted. This particular breakout load  138  may reflect a design criteria load and may be used in the design of the lift arm  104  and the bucket  106 . In particular, the breakout load  138  may be relevant for arranging the pin points of the overall linkage including the lift arms  104 , the bucket  106 , and the hydraulic cylinder connection points. In one or more embodiments, the breakout load may be targeted to a particular value such that hydraulic power available to the system does not overly affect the durability of the system, but is sufficient to achieve suitable or even optimized digging capabilities. 
     In addition to breakout load  138 , payload of the bucket may also be used as a design criteria for linkage and hydraulic system design. The payload may include the amount of material the bucket is capable of carrying. The payload may act on the bucket through the center of gravity of the material in bucket. 
       FIG.  6    shows a force  140  acting at the bucket pin where the bucket is pinned to the lift arms. That is, forces on the bucket may be depicted by a force  140  acting on the lift arms at the pinned pivot point  142  of the bucket. For example, the breakout force  138  may be modeled as a load  140  on the pin with the lift arm and bucket in a digging position. As shown in  FIG.  7   , the maximum payload may be modeled as a load  140  on the pin when the lift arms  104  are raised to their highest height where the lift arm hydraulic cylinders  126 A are extended to their fullest length. The payload acting on the system in this position may reflect an additional design criteria condition for the system. For example, given a particular arrangement of the pin points of the overall linkage, the system may be designed to be able to carry a particular payload in the condition shown. 
     As discussed above, commonality between work machines of varying capacities can be advantageous for purposes of design, manufacturing, and resulting equipment cost. Accordingly, it may be desirable to maintain linkage arrangements between work machines having varying load capacities. However, merely adjusting the hydraulic power (e.g., changing the cylinder size or cylinder pressure) of the machine may create opportunities for excessive loading, fatigue, or wear of particular components on higher capacity machines. For example, design criteria may call for limiting the breakout load  138  to avoid excessive loading or wear and manage durability of the equipment. The graphs in  FIGS.  8  and  9    depict the difficulties associated with maintaining a single linkage system and adjusting the capacity based on hydraulic power (e.g., changing the cylinder size or cylinder pressure). Both of these figures relate the percentage of lift capacity of the linkage of the work machine  100  to percentage of pin height where the pin height is the height of the pivot pin  142  connecting the bucket  106  to the lift arms  104 . As such, the breakout force  138  shown in  FIG.  5   , but modeled as a pin load  140  may define the bottom end of the curves and the payload force at max lift shown in  FIG.  7   , but also modeled as a pin load  140  may define the upper end of the curves. 
     In  FIG.  8   , the solid line  144  reflects a baseline machine with an acceptable breakout force (e.g., ˜100% of lift capacity) and an acceptable payload force (˜38% of lift capacity). The dashed line reflects a machine having a slightly higher hydraulic power, but with a power limited such that the breakout force  138  does not exceed durability limits of the equipment (e.g., the pin load  140  at the bottom end of curve has only slightly more lift capacity than the baseline). As shown, limiting the hydraulic power to control the breakout force  138  transcends the curve  146  of the higher capacity machine and results in a limit on the higher capacity machine&#39;s payload at max lift (e.g., the pin load  140  at the top end of curve is only slightly higher than the baseline). Accordingly, the design durability limitation prevents leveraging the potential additional hydraulic power. 
     In  FIG.  9   , the design durability limitation is ignored and a higher hydraulic power is provided to achieve the desired higher payload at max lift. As shown, the solid line  144 , again, reflects a baseline machine with an acceptable breakout force and an acceptable payload force. The dashed and dotted line  148  reflects a machine having a higher hydraulic power sufficient to achieve a desired payload at max lift. As can be seen from the graph, the higher payload machine may have a payload at max lift that is approximately 10% higher than the baseline machine. (e.g., pin load  140  at the top end of curve shows about 10% more lift capacity than baseline) However, and likewise, the breakout force (e.g., pin load  140  at the bottom end of curve) also exceeds the breakout force of the baseline machine and, as such, may exceed the design durability limitation for breakout force. 
     The presently described hydraulic system may allow for commonality of linkage designs, while also meeting the design durability limitations on the breakout force. As shown in  FIG.  10 A , the several lines (e.g., solid  144 , dashed  146 , and dashed and dotted  148 ) from  FIGS.  8  and  9    are included on a single graph. In addition, an operation curve  150  is overlayed on these lines. As shown, the load sense pressure relief system  118  or the PFC system  218  described above may be utilized to control the available hydraulic system pressure such that a selected curve may be followed based on one or more factors. For example, a higher capacity machine may include a higher-powered hydraulic pump to deliver higher pressures or larger hydraulic cylinders may be provided when compared to the baseline machine reflected by the solid line  144 . However, and in the case of the load sense system  118 , the low pressure load sense relief valve may be sized and selected to control the available hydraulic system pressure and approximate or only slightly exceed the baseline curve (e.g., the dashed line  146  slightly exceeds solid line  144 ). Based on one or more factors, the solenoid may be closed to remove the effect of the low pressure sense relief valve and, thus, allow the system to operate at a higher pressure based on the high pressure sense relief valve (e.g., the dashed and dotted line  148 ). In the case of the PFC system  218 , stored pressure settings in the control  230  may be selected to include a low pressure setting and to control the pump (e.g., via a pump displacement solenoid and/or a pump cut off solenoid) to limit the available hydraulic system pressure to approximate or only slightly exceed the baseline curve  144 . During this low-pressure operation, the solenoid valve  228 C may remain in its neutral position. Based on one or more factors (e.g., implement position), the control may include and may select a high pressure setting to allow the system to operate at a higher pressure. In this case, the control  230  may continue to control the pump (e.g., via a pump displacement solenoid and/or pump cut off solenoid), but may limit the available hydraulic system pressure to the higher pressure setting and, as such, allow the system to operate at a higher pressure (e.g., dashed and dotted line  148 ). In addition, the control  230  may actuate the solenoid valve  228 C when the one or more factors are met, thus, causing the hydraulic system to be poised for higher pressure and to accommodate spikes that may exceed the higher pressure until the pump controls take effect. 
     As shown in  FIG.  10 A , the operation curve  150  may follow the baseline curve  144  or the curve  146  that only slightly exceeds the baseline curve. The operation curve  150  may continue to follow this lower curve as the pin height is increased until the operation curve reaches a transition point  152 . At the transition point  152 , the operation curve  150  may shift laterally to the higher power curve  148  and may follow the higher power curve to the higher pin heights. As shown, the system may, thus, control the available pressure at lower pin heights and, as such, control the breakout force. However, at higher pin heights, the system may adjust the to a higher available pressure and, as such, provide for a higher payload. 
     Referring now to  FIG.  10 B , a graph that is the same or similar to the graph of  FIG.  10 A  is shown. However,  FIG.  10 B  includes horizontal lines that define a transition range  154 . For example, a bottom limit line  156  may be provided at or around a pin height of approximately 70% to 78% or approximately 71% to 75% or approximately 73% of an available pin height. In addition, a top limit line  158  may be provided at or around a pin height of approximately 80% to 95% or approximately 85% to 90% or approximately 87% of an available pin height. These range limits may be set to different values depending on the application and the needs of the design. These two lines may define a range  154  within which it is suitable, appropriate, or otherwise desirable to cause the transition between a lower system pressure and a higher system pressure. In the case of an underground loader, or other loaders for that matter, the bottom limit line  156  may be selected to avoid allowing use of the higher pressure in a digging situation. As such, the bottom limit line  156  may be selected to include pin heights that are above a height where an operator may dig with the machine. In this way, excessive breakout force may be avoided and yet, a higher payload at or near the max lift height may be provided. 
     The top limit line  158  may be determined by the low pressure load sense or the low pressure setting of the PFC, as the case may be. That is, where the maximum lift capacity of the higher pressure curve intersects with the lower pressure curve as shown by the vertical line  160  in  FIG.  10 C , the top limit is defined. This limit may help to avoid jolting. 
     While a work machine  100  in the form of an underground loader has been shown, work machines of all types may utilize the presently described hydraulic system having variable system pressure based on implement position. For example, wheel loaders, skid steers, farm equipment with one or more implements, trench digging equipment, and still other machines that perform work using an implement may include a hydraulic system as described herein. 
     Still further, it is to be appreciated that using a pin height for controlling the transition shown from a lower pressure curve to a higher pressure curve may be advantageous in the context of a loader because of the issues associated with breakout force and payload at maximum lift height. That is, the relationship between the problem areas is tied to pin height. In other contexts and with other machines and implements, problems associated with higher and lower pressure curves may be related to other factors and transitions between high pressure and lower pressure may be selected to depend on factors other than pin height. For example, a boom and stick on an excavator may have limits associated with a pin distance from the machine. That is, where the pin of the bucket is far from the machine, the lifting heavy payloads may be harder on the linkage and/or may affect machine stability. In this circumstance, limiting the machine hydraulic pressure and, thus, limiting the lift capacity of the implement at longer distances may be desirable. Still other implements and other positional and/or orientational conditions may be used to transition between one or more pressures in a hydraulic system. 
     INDUSTRIAL APPLICABILITY 
     In operation and use, the present hydraulic system having a variable system pressure based on implement position may provide for commonality of design between work machines having varying capacities, such as payload capacity. In one or more embodiments, as shown in  FIG.  11   , a method ( 200 ) of operation of such a machine may be provided. For example, the method may include operating a work machine and controlling an implement with a hydraulic system. ( 202 ) The method may also include operating the hydraulic system at a first of one or more available pressures. ( 204 ) The method may also include monitoring a factor relating to implement operation to determine which pressure to operate the hydraulic system. ( 206 ) In one or more embodiments, the factor may include a threshold bucket pin height, lift arm angle, or another measure of the distance of the bucket above the ground. In particular, the method may include operating the hydraulic system at the first (in some cases a low) system pressure unless/until the implement position dictates switching pressures. ( 208 ) That is, for example, when the bucket pin height exceeds a bottom limit defined by a threshold height above which an operator will dig with the work machine, the hydraulic pressure may be increased. In the case of a load sense system  118 , when the bucket pin height exceeds the threshold height, a control system may actuate a solenoid to close the solenoid. Closing the solenoid may close off flow of hydraulic fluid to a low pressure load sense relief valve and, as such, may allow the system pressure to increase up to a higher pressure defined by a high pressure load sense relief valve. In the case of a PFC system  218 , when the bucket pin height exceeds the threshold height, a control system may begin comparing the system pressure to a higher of two stored pressure settings to allow the system pressure to increase up to a higher pressure and may control pump flow to maintain systems pressures below the higher pressure setting. Additionally, or alternatively, when the bucket pin height exceeds a threshold height, the control may change cut off output so that the pump is at a higher or lower high pressure cut off setting based on the linkage position. Still further, when the bucket pin height exceeds the threshold height, the control of the PFC system  218  may actuate a solenoid to adjust the pressure setting on an adjustable pressure relief valve within the primary or main hydraulic system. That is, and in the case of the load sense system  118  or the PFC system  218 , the method may include switching pressures to a second or other pressure when the implement position dictates such. ( 210 ) The method may also include operating the hydraulic system at the second (in some cases high) pressure unless/until the implement position, such as the bucket pin height, dictates switching back to the first pressure such as when the bucket pin height falls below the threshold height. ( 212 ) Still further, the method may include monitoring the implement position or bucket pin height and maintaining the system  118 / 218  in a high pressure condition unless/until the implement position or bucket pin height is below a top limit defined by an intersection between the maximum lift capacity under the second pressure and that same lift capacity under the first pressure so as to avoid jolting. ( 214 ) 
     The above detailed description is intended to be illustrative, and not restrictive. The scope of the disclosure should, therefore, be determined with references to the appended claims, along with the full scope of equivalents to which such claims are entitled.