Abstract:
A pumping element for a hydraulic pump is provided. The pumping element includes a cylinder forming a compression chamber and having a discharge port. A piston having a pressure surface, a spill port, and a passageway connecting the pressure surface with the spill port is disposed in the cylinder for reciprocal movement between a first position and a second position. The pressure surface of the piston is adapted to increase the pressure of a fluid disposed in the compression chamber as the piston moves between the first position and the second position. The pressurized fluid flows through the discharge port of the cylinder. A metering sleeve is disposed around the piston and is configured to selectively cover the spill port as the piston reciprocates between the first and second positions. The metering sleeve has a groove that is adapted for fluid communication with the spill port as the piston reciprocates between the first and second positions.

Description:
TECHNICAL FIELD  
         [0001]    The present disclosure is directed towards hydraulic pumps and, more particularly, to a pumping element for a hydraulic pump.  
         BACKGROUND  
         [0002]    Hydraulic pumps are commonly used for many purposes and in many different applications. Vehicles, such as, for example, highway trucks and off-highway work machines, commonly include hydraulic pumps that are driven by an engine in the vehicle to generate a flow of pressurized fluid. The pressurized fluid may be used for any of a number of purposes during the operation of the vehicle. A highway truck, for example, may use pressurized fluid to operate a fuel injection system or a braking system. A work machine, for example, may use pressurized fluid to propel the machine around a work site or to move a work implement.  
           [0003]    A hydraulic pump typically includes a pumping element that applies work to an operating fluid to increase the pressure of the fluid. In one type of hydraulic pump, the pumping element includes a series of piston that are disposed in cylinders. The pistons are driven through a reciprocal movement within the cylinders to compress the operating fluid. The pumping element may be fixed displacement, where the stroke length of the pistons is constant. Alternatively, the pumping element may be variable displacement, where the stroke length of the pistons may be varied.  
           [0004]    As shown in U.S. Pat. No. 6,035,828 to Anderson et al., a fixed displacement pump may include a metering device that allows the output flow rate of the pump to be varied. In the described system, the metering device includes a series of metering sleeves that are disposed around a series of pistons.  
           [0005]    The metering sleeves are configured to selectively block a passageway that provides a fluid connection with a compression chamber in the cylinder. When the passageway is open, operating fluid may flow from the compression chamber through the passageway to thereby prevent pressurization of the operating fluid during the compression stroke of the piston. The rate at which the pump generates pressurized fluid may be controlled by varying the position of the metering sleeves. The rate of pressurized fluid generation may be increased by covering the passageway for a greater portion of the compression stroke. The rate of pressurized fluid generation may be decreased by leaving the passageway open for a greater portion of the compression stroke.  
           [0006]    The metering sleeves have a close tolerance relative to the outer surface of the pistons to minimize the amount of fluid that leaks from the passageway. It is expected that some operating fluid will leak from the passageway through the clearance between the metering sleeve and the piston surface. This fluid may be used to lubricate the surfaces of the metering sleeve and piston, which may facilitate movement between the metering sleeve and piston. Under some operating conditions, such as when the engine is cold, the viscosity of the operating fluid may be relatively high. The high viscosity of the fluid results in a greater drag between the metering sleeve and the piston. This increases the force required to move the metering sleeve relative to the piston. Accordingly, accurately controlling the position of the metering sleeve relative to the piston may be more difficult when the engine is cold.  
           [0007]    In addition, when the metering sleeves are covering the spill ports, an inner surface of the metering sleeves will be exposed to the pressurized fluid within the compression chamber. Particularly in high pressure systems, the pressurized fluid exerts a significant force on the inner surface of the metering sleeve. Over time, this force may cause the metering sleeve to swell or deform. The swelling or deformation of the metering sleeve may increase the clearance between the metering sleeve and the piston. The increased clearance may lead to an increase in the amount of fluid that leaks from the passageway, which may decrease the volumetric efficiency of the pump.  
           [0008]    The pumping element of the present disclosure solves one or more of the problems set forth above.  
         SUMMARY OF THE INVENTION  
         [0009]    According to one aspect, the present disclosure is directed to a pumping element for a hydraulic pump. The pumping element includes a cylinder that forms a compression chamber and has a discharge port. A piston having a pressure surface, a spill port, and a passageway connecting the pressure surface with the spill port is disposed in the cylinder for reciprocal movement between a first position and a second position. The pressure surface of the piston is adapted to increase the pressure of a fluid disposed in the compression chamber as the piston moves between the first position and the second position. The pressurized fluid flows through the discharge port of the cylinder. A metering sleeve is disposed around the piston and is configured to selectively cover the spill port as the piston reciprocates between the first and second positions. The metering sleeve has a groove that is adapted for fluid communication with the spill port as the piston reciprocates between the first and second positions.  
           [0010]    In another aspect, the present disclosure is directed to a method of operating a metering sleeve in a hydraulic pump. A piston is driven through a reciprocal movement in a cylinder to pressurize an operating fluid. The operating fluid is released from the cylinder through a discharge port when the pressure of the operating fluid reaches a predetermined limit. The position of a metering sleeve is adjusted to selectively cover a spill port to vary the amount of operating fluid pressurized by the piston. Pressurized operating fluid is allowed to flow from the spill port to a groove in the metering sleeve as the piston reciprocates within the cylinder.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic and diagrammatic representation of a hydraulic pump in accordance with an exemplary embodiment of the present invention;  
         [0012]    [0012]FIGS. 2 a  and  2   b  are schematic and diagrammatic representations of a metering sleeve and piston in accordance with an exemplary embodiment of the present invention; and  
         [0013]    [0013]FIG. 3 is a partial pictorial representation of a metering sleeve in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    An exemplary embodiment of a pump  20  is diagrammatically and schematically illustrated in FIG. 1. Pump  20  includes a housing  21  and an inlet  22 . Inlet  22  may be connected to a tank  12  that stores a supply of operating fluid. The operating fluid may be any hydraulic fluid, such as, for example, any lubricating oil commonly used to lubricate moving engine parts. In addition, tank  12  may be part of a vehicle lubrication system, such as, for example, an oil sump.  
         [0015]    A supply pump  14  may draw operating fluid from tank  12  and direct the operating fluid through an inlet line  16  to inlet  22  of pump  20 . Supply pump  14  may be a relatively low pressure pump, such as, for example, a sump pump as is commonly used in a vehicle lubrication system to distribute lubricating oil within an engine and/or vehicle. Supply pump  14  may increase the pressure of the fluid to a relatively low pressure, such as, for example, about 70 kPa (10.2 psi).  
         [0016]    As also illustrated in FIG. 1, pump  20  includes a pumping element  26 . Pumping element  26  is operable to increase the pressure of the operating fluid received through inlet  22 . Pumping element  26  includes a series of cylinders  46 , each of which has a compression chamber  48  and a discharge port  49 . Low pressure operating fluid may be directed from inlet  22  into each compression chamber  48 .  
         [0017]    Pumping element  26  also includes a series of pistons  32 . One piston  32  is slidably disposed within each cylinder  46 . As shown in FIGS. 2 a  and  2   b , each piston  32  includes an outer surface  76  and a pressure surface  70 .  
         [0018]    Pressure surface  70  is disposed adjacent compression chamber  48 . Each piston  32  is reciprocally moveable through a compression stroke, where each piston  32  is moved from a first position to a second position to increase the pressure of operating fluid contained in compression chamber  48 . The length of the compression stroke is indicated by distance  80 . The pressurized operating fluid may exit compression chamber  48  through discharge port  49 .  
         [0019]    As also shown in FIGS. 2 a  and  2   b , piston  32  includes a passageway  74  and a spill port  72 . Passageway  74  provides a fluid conduit between pressure surface  70  and spill port  72 . In the illustrated embodiment, spill port  72  provides two openings on either side of piston  32 . It should be understood that spill port  72  may provide a greater, or lesser, number of openings from passageway  74 .  
         [0020]    Referring to FIG. 1, a resilient member, such as, for example, spring  50 , may be operatively engaged with each piston  32 . Spring  50  may act on piston  32  to move piston  32  towards the first position. As shown, spring  50  may be disposed within cylinder  48 . Alternatively, spring  50  may be positioned in any other location readily apparent to one skilled in the art where spring  50  may act to move piston  32  towards the first position.  
         [0021]    As further shown in FIG. 1, pump  20  may also include an input shaft  52  that is operable to drive pumping element  26 . Input shaft  52  may include a spline or keyed end that is operatively engaged with the crankshaft or gear train of the engine. Input shaft  52  may be connected to the engine in any manner readily apparent to one skilled in the art that will result in a rotation of input shaft  52 .  
         [0022]    Pump  20  may further include a swashplate  28  that is rotatably disposed in housing  21 . Swashplate  28  may include an angled driving surface  29 . Input shaft  52  may be connected to swashplate  28  so that a rotation of input shaft  52  causes a corresponding rotation of swashplate  28  and driving surface  29 .  
         [0023]    Driving surface  29  of swashplate  28  is operatively engaged with each piston  32 . Driving surface  29  is angled so that rotation of swashplate  28  sequentially moves each piston  32  from the first position to the second position. After each piston  32  has reached the second position and as swashplate  28  continues to rotate, springs  50  will move each piston  32  from the second position towards the first position.  
         [0024]    A device, such as, for example, a pivoting shoe  30 , may be disposed between each piston  32  and driving surface  29 . Pivoting shoe  30  is configured to pivot relative to piston  32 . The pivoting motion ensures that the respective piston  32  will remain operatively engaged with driving surface  29  as swashplate  28  rotates.  
         [0025]    In the illustrated embodiment, driving surface  29  of swashplate  28  has a fixed angle. It should be noted, however, that pump  20  may include a mechanism configured to vary the angle of driving surface  29 . By varying the angle of driving surface  29 , the amount of motion, or the length of the compression stroke, of each piston  32  may be changed.  
         [0026]    As further illustrated in FIG. 1, a check valve  36  may be disposed proximate discharge port  49  of each cylinder  46 . Each check valve  36  may be configured to open when the fluid within compression chamber  48  bore reaches a predetermined level. When the operating fluid reaches the predetermined pressure, check valve  36  will open to allow the pressurized fluid to flow from compression chamber  48 .  
         [0027]    Hydraulic pump  20  may include a collector  38 . Pressurized operating fluid that is released from each compression chamber  48  through check valve  36  may be directed to collector  38 . Collector  38  may be configured to store a desired quantity of pressurized operating fluid.  
         [0028]    Collector  38  is connected to an outlet  24 , which may be further connected to an outlet line  18 . Outlet line  18  may be connected to a fluid rail  19 . Fluid rail  19  may be configured to distribute pressurized operating fluid to a system, such as, for example, a fuel injection system, associated with a vehicle and/or engine.  
         [0029]    As also schematically shown in FIG. 1, hydraulic pump  20  includes a series of metering sleeves  34 . One metering sleeve  34  is associated with each piston  32  and cylinder  46  combination. As described in greater detail below, each metering sleeve  34  is configured to control the rate at which pressurized fluid is generated by the respective piston  32 .  
         [0030]    As illustrated in FIGS. 2 a ,  2   b , and  3 , metering sleeve  34  includes a position notch  78  and an inner surface  84 . Inner surface  84  of metering sleeve  34  is configured to receive piston  32  and to cover spill port  72  to block passageway  74 . The width of metering sleeve  34  may be approximately equal to distance  80  of compression stroke  80  so that metering sleeve  34  may cover spill port  72  for the entire compression stroke  80 .  
         [0031]    As also shown in FIG. 3, inner surface  84  includes a series of grooves  82 . Grooves  82  enter into fluid communication with spill port  72  as piston  32  reciprocates between the first and second positions. In the illustrated embodiment, inner surface  84  includes a series of four grooves  82 . It should be understood, however, that inner surface  84  may include a greater, or lesser, number of grooves  82 .  
         [0032]    As shown in FIGS. 2 a  and  2   b , metering sleeve is disposed for sliding movement along outer surface  76  of piston  32 . Metering sleeve  34  may be moved between a first position, as illustrated in FIG. 2 a , and a second position, as illustrated in FIG. 2 b.    
         [0033]    The position of metering sleeve  34  relative to piston  32  determines the portion of the compression stroke  80  in which metering sleeve  34  covers spill port  74  in piston  32 . In the first position, metering sleeve  34  covers spill port  74  for the entire compression stroke  80  of piston  32 . In the second position, metering sleeve  34  leaves spill port  74  uncovered for the entire compression stroke  80  of piston  32 . Metering sleeve  34  may also be positioned between the first and second positions so that spill port  72  is covered for a portion of the compression stroke of piston  32 .  
         [0034]    With reference to FIG. 1, pump  20  may include a control device  44  that is operatively engaged with position notch  78  (referring to FIG. 3) to control the position of metering sleeve  34 . Control device  44  may be connected to pump outlet  24  through a control line  40 . Control device  44  may use pressurized fluid to create a pressure differential over metering sleeve  34  to move metering sleeve  34  in a first direction. A resilient member (not shown), such as, for example, a spring, may be engaged with metering sleeve  34  to move metering sleeve  34  in the opposite direction when the pressure differential is equalized. Thus, the position of metering sleeve  34  relative to piston  32  may be controlled to thereby control the portion of the compression stroke of piston  32  that spill port  72  is covered.  
       Industrial Applicability  
       [0035]    The operation of an exemplary embodiment of the described pumping element will now be described with reference to the figures. The described pump  20  may be included as part of a vehicle to provide pressurized fluid to a system in the vehicle. The vehicle may be, for example, a highway truck or an off-highway work machine.  
         [0036]    Operation of the engine of the vehicle results in a rotation of input shaft  52 . Rotation of input shaft  52  causes a corresponding rotation of swashplate  28  and driving surface  29 . Rotation of driving surface  29  acts to move each piston  32  through a compression stroke, i.e. from the first position towards the second position.  
         [0037]    When metering sleeve  34  is in the first position, spill port  72  is covered for the entire compression stroke of piston  32 . When piston  32  is moving towards the second position, pressure surface  70  of piston  32  will exert a force on operating fluid disposed in compression chamber  48 . The force exerted on the operating fluid will increase the pressure of the fluid. When the pressure of the operating fluid within compression chamber  48  reaches a predetermined limit, check valve  36  will open to allow the pressurized fluid to flow into collector  38 .  
         [0038]    To reduce the rate at which pressurized fluid is generated, metering sleeve  34  may be moved towards the second position, which will leave spill port  72  uncovered for a greater portion of the compression stroke of piston  32 . When spill port  72  is uncovered and piston  32  moves towards its second position, pressure surface  70  will force operating fluid from compression chamber  48  through passageway  74  and spill port  72 . Accordingly, when piston  32  is moving towards the second position, pressure surface  70  will not pressurize the operating fluid.  
         [0039]    If metering sleeve  34  is positioned between the first and second positions, spill port  72  will move under metering sleeve  34  at some point during the compression stroke of piston  32 . When metering sleeve  34  covers, or blocks, spill port  72 , operating fluid is not allowed to escape from compression chamber  48 . At this point, pressure surface  70  will act to pressurize the operating fluid remaining in compression chamber  48 . When the fluid reaches the predetermined pressure, check valve  36  will open to allow the pressurized fluid to flow to collector  38 . However, as some operating fluid escaped from compression chamber  48  when spill port  72  was uncovered, the quantity of pressurized fluid released to collector  38  will be less than would have been released had spill port  72  been covered for the entire compression stroke.  
         [0040]    As piston  32  slides within metering sleeve  34 , spill port  72  will move into fluid communication with grooves  82  in inner surface  84  of metering sleeve  34 . In certain situations, such as when the operating fluid in compression chamber  48  is approaching the predetermined limit, the operating fluid may exert a significant force on inner surface  84  of metering sleeve  34 . Grooves  82  allow the pressurized fluid to flow around metering sleeve  34 . This will distribute the force exerted by the pressurized fluid around the entire metering sleeve  34 .  
         [0041]    The distribution of the fluid force may reduce or prevent swelling or deformation of metering sleeve  34  that could result from repeated exposure to highly pressurized fluid. Reducing or preventing swelling and/or deformation of metering sleeve  34  may allow a close tolerance to be maintained between metering sleeve  34  and piston  32 . This will prevent or reduce an increase in leakage from compression chamber  48  as is typically experienced over an extended operation of pump  20 . By maintaining a constant amount of leakage, metering sleeve  34  may prevent a decrease in the volumetric efficiency of pump  20  over time.  
         [0042]    In addition, grooves  82  may reduce the force required to move metering sleeve  34  relative to piston  32  or to move piston  32  relative to metering sleeve  34 . The presence of grooves  82  in inner surface  84  will reduce the shear area between metering sleeve  34  and outer surface  76  of piston  32 . The reduction in shear area translates to a reduction in the drag force experienced when the surfaces of metering sleeve  34  and piston  32  are moved relative to each other. The reduction in force may be particularly apparent when the viscosity of the operating fluid is high, such as when pump  20  is operating in cold conditions. Thus, grooves  82  in metering sleeve  34  may effectively improve the lubrication characteristics between metering sleeve  34  and piston  32 .  
         [0043]    It will be apparent to those skilled in the art that various modifications and variations can be made in the described pump and pumping element without departing from the scope of the invention. Other embodiments may be apparent to those skilled in the art from consideration of the specification and practice of the pumping element disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the present disclosure being indicated by the following claims and their equivalents.