Abstract:
A member that is driven by a hydraulic actuator tends oscillate when fluid flow to the hydraulic actuator is terminated. A pressure relief device reduces that oscillation by connecting first and second first relief valves to the hydraulic actuator. The first relief valve provides a relatively large first relief passage as long as pressure in the hydraulic actuator remains above a threshold. A second relief valve opens a second relief passage at substantially the same threshold pressure and thereafter remains open as long as the pressure remains above another lower threshold. A timer causes the second relief valve to close a given interval of time, if the pressure does not drop below the other lower threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulically powered equipment, and more particularly to apparatus for reducing bounce of a hydraulically driven member that is stopped suddenly. 
     2. Description of the Related Art 
     With reference to  FIG. 1 , a backhoe  10  is a common type of earth moving equipment that has a bucket  12  attached to the end of an arm  14  which in turn is coupled by a boom  15  to a tractor  18 . A pivot joint  16  enables backhoe assembly  17  formed by the combination of the bucket, arm, and boom to pivot left and right with respect to the rear end of the tractor  18 . A pair of hydraulic cylinders  19  are attached to the boom  15  on opposite sides of the backhoe tractor  18  and provide the drive force for the pivotal action. Hydraulic fluid is supplied to the cylinders  19  through control valves that are manipulated by the backhoe operator. The pivotal movement of the boom  15  is referred to as “swing” or “slew”. 
     As the boom  15  slews, pressurized fluid is introduced into one chamber of each cylinder, referred to as the “driving chamber”, and fluid is drained from the other cylinder chamber, referred to as the “exhausting chamber”. Due to the mass of the boom and any load being carried, a significant amount of kinetic energy is associated with its motion. When an operator terminates slewing at a rapid pace by releasing the handle attached to the control valve, the energy associated with the boom&#39;s motion has to dissipate in order for the system to return to an “at-rest” state (the state of minimal energy). With a conventional control valve assembly, pressure in the former exhausting chambers of the swing cylinders  19  increases as the boom  15  continues to move in the driven direction, due to inertia. As this pressure continues increasing, a pressure relief valve typically is activated to prevent the cylinder pressures from reaching a dangerous level. This caused pressure in the driving cylinder chambers to decrease. 
     At this time point, there is a net pressure difference between the two chambers of each cylinder  19  which causes the direction of motion to reverse. As the motion reverses, the pressure relief valve closes trapping pressure in the former exhausting chambers and associated hydraulic lines. The trapped pressure begins to decay as the boom  15  now is being driven in the opposite direction which expands the former exhausting chambers and causes a rise in pressure in the former driving chambers of the cylinders. Eventually the pressure the former driving chambers becomes significantly greater than pressure in the former exhausting chambers resulting in another reversal of boom motion. The boom  15  oscillates, initially activating the pressure relief valves, but later just cycling back and forth, until the energy is dissipated to the environment through heat, sound, material hysteresis, etc. This phenomenon is known as “slew bounce” or “slew wag” and increases the time required to properly position the boom  15 . As a consequence, it adversely affects equipment productivity. 
     Various approaches have been devised to minimize the slew bounce. For example, U.S. Pat. No. 4,757,685 employs a separate relief valve for each hydraulic line connected to the swing cylinders, which valves vent fluid to a tank return conduit when excessive pressure occurs in those cylinders. Additional fluid is supplied from the tank return conduit through a make-up valve when a cylinder chamber cavitates. This system also incorporates a means for communicating pressurized fluid from the pump supply line to the tank return conduit when an operator slew control valve is in the neutral position. 
     U.S. Pat. No. 5,025,626 describes a cushioned swing circuit which also has relief and make-up valves connected to the hydraulic lines for the slew cylinders. This circuit also incorporates a cushion valve which in an open position provides a fluid path between the cylinder hydraulic lines. That path includes a flow restriction orifice. The cushion valve is biased into the closed position by a spring and a mechanism opens the cushion valve for a predetermined time period when the pressure differential between the cylinder chambers exceeds a given threshold. 
     SUMMARY OF THE INVENTION 
     A hydraulic system has a pump that supplies fluid under pressure from a tank. A control valve governs the flow of the fluid from the pump to a hydraulic actuator and back from the hydraulic actuator to the tank. A pressure relief apparatus is coupled to the hydraulic actuator to reduce bounce when the control valve closes. The pressure relief apparatus comprises two relief valves connected in parallel and operating in two stages, preferably with one valve having a significantly greater flow capacity than the other valve. 
     The pressure relief apparatus is attached to a valve block that has a first conduit to which the hydraulic actuator connects and a second conduit through which fluid flows to the tank. A flow control assembly provides a first passage between the first conduit and the second conduit to relieve pressure in the hydraulic actuator while that pressure exceeds a first threshold. The flow control assembly provides a second passage between the first and second conduits when pressure at the first conduit exceeds a second threshold level, and remains open even though pressure at the first conduit decreases below both the first and second threshold levels. 
     In the preferred embodiment, the pressure relief apparatus further includes a timer valve which causes the flow control assembly to close the second passage after a given time interval regardless of pressure at the first conduit. 
     Preferably the pressure relief apparatus comprises a housing with a bore and first and second relief valves within the bore. The first pressure relief valve has a primary poppet that selectively engages a first valve seat to open and close the first passage. The first relief valve opens and remains open as long as pressure at the first conduit exceeds the first threshold level. The second relief valve has a bleeder poppet which engages a second valve seat to control flow through the second passage. When the second passage is closed, pressure in the first conduit acts on a smaller area of the bleeder poppet than when the second passage is open. Thus a higher pressure is needed to open the second relief valve than is required to keep it open. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a backhoe incorporating the present invention; 
         FIG. 2  is a schematic diagram of a hydraulic circuit for operating a backhoe boom, wherein the hydraulic circuit includes novel bounce reduction valves; 
         FIG. 3  is a cross-section view through a bounce reduction valve in the closed state; and 
         FIG. 4  is a schematic diagram of the bounce reduction valve. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 2 , a hydraulic circuit  20  for the backhoe  10  has a pump  22  which forces fluid from a tank  24  into a supply conduit  26 . A conventional pressure relief valve  28  opens when the pump pressure exceeds a given safety threshold to relieve fluid from the supply conduit  26  to a tank return conduit  30  which conveys the fluid back to the tank  24 . The supply conduit  26  and the tank return conduit  30  are connected to hydraulic circuits for a plurality of functions on the backhoe  10 . 
     Of particular interest, the supply and tank return conduits  26  and  30  are connected to a standard three-position directional control valve  32  which is operated by the backhoe operator to swing the boom  15 . The directional control valve  32  selectively couples the supply conduit  26  and the tank return conduit  30  to a pair of actuator conduits  34  and  36  which in turn are connected to ports of hydraulic actuators, such as cylinders  38  and  40 , that control the swing of the boom  15 . The directional control valve  32  is illustrated centered in the neutral, closed position in which the actuator conduits  34  and  36  are disconnected from the pump and tank return conduits  26  and  30 . 
     In the exemplary hydraulic circuit, the first actuator conduit  34  is connected to the head chamber  41  of the first cylinder  38  and to the rod chamber  43  of the second cylinder  40 . Similarly, the second actuator conduit  36  is connected to the rod chamber  42  of the first cylinder  38  and to the head chamber  44  of the second cylinder  40 . Depending upon the position of the directional control valve  32 , hydraulic fluid from the pump  22  is sent to one of the actuator conduits  34  or  36  and the other actuator conduit  36  or  34  is connected through the directional control valve to the tank return conduit  30 . This action drives the cylinders  38  and  40  on opposite sides of the boom  15  to swing the boom in one direction or the other. Although the present invention is being described in terms of operating cylinders with pistons, it should be understood that the novel concepts can be used with other types of hydraulic actuators, such as a hydraulic motor with a rotating shaft. 
     A pair of bounce reduction valves  46  and  48 , serving as separate pressure relief apparatus, are connected to the two actuator conduits  34  and  36 . The first bounce reduction valve  46  has an inlet port  50  connected to the first actuator conduit  34  and an outlet port  52  directly coupled to the tank return conduit  30 . Similarly the second bounce reduction valve  48  has an inlet port connected to the second actuator conduit  36  and an outlet port coupled to the tank return conduit  30 . The first bounce reduction valve  46  opens when the pressure in the first actuator conduit  34  exceeds a threshold level and thereafter conducts fluid to tank return conduit  30 , as will be described in greater detail. Similarly, the second bounce reduction valve  48  opens when the pressure in the second actuator conduit  36  exceeds the given threshold and conveys hydraulic fluid to the tank return conduit  30 . 
     A first conventional anti-cavitation valve  54  is placed between the tank return conduit  30  and the first actuator conduit  34 . A second anti-cavitation valve  56  is located between the tank return conduit  30  and the second actuator conduit  36 . These anti-cavitation valves  54  and  56  open when the pressure in the respective actuator conduit  34  or  36  is less than the pressure in the tank return conduit  30 , as results from cavitation in a cylinder chamber connected to the respective actuator conduit  34  or  36 . 
       FIG. 3  illustrates a physical embodiment of the first bounce reduction valve  46  with the understanding that the second bounce reduction valve  48  has an identical construction.  FIG. 4  schematically depicts the components of that bounce reduction valve. The first bounce reduction valve  46  has a housing  60  which is threaded into an aperture  62  in a valve block  64  through which pass portions of the tank return conduit  30  and the two actuator conduits  34  and  36 . The aperture  62  for the first bounce reduction valve  46  communicates with the first actuator conduit  34  and the tank return conduit  30 . The corresponding aperture for the second bounce reduction valve  48  communicates with the second actuator conduit  36  instead of the first actuator conduit  34 . 
     The valve housing  60  has a bore  65  extending there through with an opening which forms a housing outlet port  67  into the tank return conduit  30 . A first relief valve  66  and a second relief valve  68  are located one behind the other within the housing bore  65 . A timer valve  69  is located within the bore  65  on a remote side of the second relief valve  68  from the first relief valve  66  and acts as a hydraulic timer, as will be described. As will be described, the first relief valve  66  opens and closes in response to pressure at the inlet port  50 , and the second relief valve  68  opens in response to that inlet pressure and closes due to either that pressure decreasing to the reset level ( FIG. 4 ) or the operation of the timer valve  69 . 
     The first relief valve  66  includes a nose member  74  that is secured within the housing bore  65  and engages the valve block  64  to close communication between the first actuator conduit  34  and the tank return conduit  30 . The nose member  74  has a central bore  78  with the inlet port  50  providing a passage between the central bore and the first actuator conduit  34 . Several nose outlet ports  84  extend laterally through a wall of the nose member  74 , forming paths from the central bore  78  to the tank return conduit  30 . The nose outlet ports  84  and the opening  65  of the housing bore  62  combine to form the outlet port  52  of the first bounce reduction valve  46  in  FIG. 2 . The interior end of the nose member  74  is closed by a cap  86  that is threaded therein to provide an extension of the central bore  78 . An internal chamber  85  is created in the central bore  78  on one side of the cap  86  and an intermediate chamber  92  is formed on the opposite side of the cap. The cap  86  has a lateral aperture  96  providing a fluid path between the central bore  78  and a feed passage  95  in the housing  60 . 
     An elongated tubular primary poppet  70  of the first relief valve  66  is slidably received in the central bore  78  of the nose member  74  and engages a first valve seat  72  when the first relief valve  66  is in the closed state. An aperture  80  extends longitudinally through the primary poppet  70  and forms a passageway between the inlet port  50  and the intermediate chamber  92  in the housing bore  65  between the two relief valves  66  and  68 . A valve spring  88  biases the primary poppet  70  away from the cap  86  and into engagement with the first valve seat  72  on the nose member  74  thereby closing the inlet port  50 . The tubular primary poppet  70  slideably projects through an aperture  87  in the cap  86  and a seal  90  prevents hydraulic fluid from flowing between the nose chamber  85  and the intermediate chamber  92 . 
     The second relief valve  68  has a body  100  which is threaded into the bore  65  of housing  60 . A spacer spring  94  within the intermediate chamber  92  biases the cap  86  away from the second relief valve body  100  thereby forcing the nose member  74  against the valve block  64 . The second relief valve body  100  has a secondary bore  102  from which a control aperture  104  opens through a second valve seat  106  into the intermediate chamber  92 . A bleeder poppet  110  has a conical surface  112  which moves with respect to the second valve seat  106  to open and close the control aperture  104  and thus the second relief valve  68 . The conical surface  112  is surrounded by a guide ring  115  having a circumferential surface that loosely engages the wall of the secondary bore  102 . The bleeder poppet  110  is biased into engagement with the second valve seat  106  by a bleeder spring  114 . The bleeder spring  114  engages a disk  116  secured in the open internal end of a plug  118  that closes the open end of the housing bore  65  with O-rings providing a seal there between. 
     A control chamber  120  is created between the second relief valve body  100  and the disk  116 , and a timer chamber  124  is formed on the opposite side of the disk within the plug  118 . A passage  122  extends through the disk  116  from the control chamber  120  to the timer chamber  124 . The timer chamber  124  and the control chamber  120  can be considered as a single control chamber because of their interconnection through the disk passage  122 . 
     The timer valve  69  comprises a timer spool  126  located within the timer chamber  124  and able to slide within the plug  118  against the force of a timing spring  128  that biases the timing spool away from the disk  116 . The timing spool  126  defines a dwell chamber  130  at the innermost portion of the plug  118 . A timer orifice  132  extends through the timing spool  126 , providing a restricted fluid path between the dwell chamber  130  and the timer chamber  124 . 
     The plug  118  has a transverse aperture  136  extending between the dwell chamber  130  and the feed passage  95  leading through the valve housing  60  to the lateral aperture  96  in the nose member  74 . The transverse aperture  136 , feed passage  95 , and the lateral aperture  96  form a passageway between the dwell chamber  130  and the nose chamber  85 . A check valve  134 , located in the feed passage  95 , allows fluid to flow in that passage only in a direction from the nose chamber  85  to the dwell chamber  130 . 
     A tank passage  140  also extends longitudinally through the valve housing  60 . The plug  118  has a second transverse aperture  142  which provides a path between the tank passage  140  and the timer chamber  124  which path is selectively opened and closed by movement of the timing spool  126 , as will be described. The other end of the tank passage  140  opens into a section of the valve housing bore  65  around the nose member  74  and thus communicates through the housing outlet port  67  with the tank return conduit  30 . 
     System Operation 
     This novel bounce reduction valve  46  is employed to reduce slew bounce in the backhoe  10 . With reference to  FIGS. 1 and 2 , assume that the backhoe boom  15  is being operated wherein pressurized hydraulic fluid from the pump  22  is flowing through the directional control valve  32  into the second actuator conduit  36 . That fluid continues to flow from the second actuator conduit  36  into cylinder chambers  42  and  44 . At the same time other fluid is exhausting from cylinder chambers  41  and  43  through the first actuator conduit  34  and control valve  32  into the tank return conduit  30 . 
     When the operator places the directional control valve  32  into the neutral, closed position, the inertial load of the backhoe assembly  17  exerts force on the cylinders  38  and  40 . This action increases the pressure in cylinder chambers  41  and  43  That increasing pressure is communicated through the first actuator conduit  34  to the inlet port  50  of the first bounce reduction valve  46 . 
     Referring to  FIGS. 3 and 4 , the increasing actuator pressure at the inlet port  50  is applied to the nose of the primary poppet  70  for the first relief valve  66 . When that pressure reaches a first threshold level, the primary poppet  70  cracks open allowing fluid to flow from the first actuator conduit  34  through the nose outlet ports  84  into the tank return conduit  30 . Movement of the primary poppet  70  away from the valve seat  72  raises pressure in the internal nose chamber  85  to an intermediate pressure level that is between the pressure levels in the first actuator conduit  34  and the tank return conduit  30 . The nose outlet ports  84  are sized to restrict fluid flow to create the intermediate pressure. That intermediate pressure level is communicated through the feed passage  95  where it causes the check valve  134  to open thereby introducing that pressure into the dwell chamber  130  behind the timing spool  126 . 
     Prior the first relief valve  66  opening, the timing spool  126  closed the second transverse aperture  142  in the plug  118 . As a consequence, fluid was essentially trapped in the control chamber  120  behind the bleeder poppet  110  of the second relief valve  68 , as only a small aperture  146  existed between that chamber and the tank passage  140 . However, now the pressure in the dwell chamber  130  increases to a level which causes the timing spool  126  to move away from the end wall of the plug  118  until it contacts the disk  116 . The initial motion of the timing spool  126  forces fluid into the timer chamber  124  through the bleed orifice  132 , thereby enabling the timing spool to move toward the disk  116 . Further movement of the timing spool  126  aligns its side passage  144  with the second transverse aperture  142  thereby opening that aperture that leads to the tank passage  140  and onward through the housing outlet port  67  to the tank return conduit  30 . This path exhausts the fluid flowing from the timer chamber  124  and the control chamber  120 . 
     The ongoing boom motion causes pressure in the first actuator conduit  34  to continue to rise until the set point of the second relief valve  68  is reached. That increased pressure is conveyed via the poppet&#39;s longitudinal aperture  80  into the intermediate chamber  92  and the control aperture  104  in the second relief valve body  100  where the pressure is applied to the tip of the bleeder poppet  110 . The increased pressure causes the bleeder poppet  110  to unseat. In the open state of the second relief valve  68 , an additional amount of fluid from the inlet port  50  flows through the control chamber  120 , disk passage  122  and the timer chamber  124 . That fluid is exhausted from the timer chamber  124  via second transverse aperture  142  and the tank passage  140  into the tank return conduit  30 . This further relieves the pressure in the actuator conduit  34  and the associated chambers of cylinders  38  and  40 . 
     Pressure in the intermediate chamber  92  acting on the relatively small tip area of the bleeder poppet in control aperture  104  must exceed a second threshold to open the second relief valve  68 , against the force of the bleeder spring  114 . The second threshold pressure level preferably is substantially equal to the first threshold pressure level so that the two relief valves  66  and  68  open at approximately the same time. However, once the bleeder poppet  110  cracks open, a larger combined area of the conical surface  112  and guide ring  115  is exposed to the pressure from the intermediate chamber  92 . Thus a significantly lower pressure (i.e. above a third pressure threshold) is required to maintain the second relief valve  68  open, than is required to force it open. The third pressure threshold level is less than both the first threshold level at which the first relief valve  66  opened and the second threshold level at which the second relief valve  68  opened. This characteristic is important to subsequent operation of the bounce reduction valve  46 , as will be described. The operation of the present bounce reduction valve is in contrast to conventional pressure relief valves which remain open only while the pressure differential exceeds the level required to open the valve. 
     As the boom  15  begins to slow, the fluid flow and pressure in the first bounce reduction valve  46  decreases. In due course, the flow decreases to an amount that can pass satisfactorily through only the second relief valve  68  at which time the pressure acting on the nose of the primary poppet  70  no longer overcomes the force of valve spring  88  and the first relief valve  66  closes. The now enlarged surface area of the bleeder poppet  110  in the second relief valve  68  enables that valve to remain open at this reduced pressure. Therefore all the flow through the first bounce reduction valve  46  passes through the second relief valve  68 . 
     While the bleeder poppet  110  is open, the pressure in the first actuator conduit  34  continues to decay until dropping below a level which enables the force of the bleeder spring  114  to reseat the bleeder poppet  110  thereby closing the second relief valve  68 . The second relief valve  68  closure is independent of the amount of flow there through. This terminates all flow of fluid through the first bounce reduction valve  46 . 
     Under normal operating conditions the pressure in the first actuator conduit  34  decreases sufficiently so that force of the bleeder spring  114  is able to close the second relief valve  68 . 
     However, in some situations, such as when the backhoe  10  is on an angle with a full bucket, the cylinder pressure remains relatively high and the bleeder poppet  110  remains partially open. As a result, the boom assembly  17  continues to move slowly, or drift, after motion damping has occurred. In the absence of a specific mechanism to constrain this drift movement, the boom assembly  17  continues to swing until striking mechanical stops at the extreme end of its travel. To address this situation, the present bounce reduction valves  46  and  48  integrate the timer valve  69  on the tank passage  140  to force the bleeder poppet  110  against seat  106  should drifting occur. 
     The timer valve  69  operates as follows. When the first relief valve  66  closes, pressure in its internal nose chamber  85  drops to the same level as in the tank return conduit  30 . That relatively low pressure level is communicated through the feed passage  95  and causes the check valve  134  to close. That closure traps fluid in the dwell chamber  130  behind the timer spool  126 . The force of the timer spring  128  causes the timer spool  126  to move farther into the plug  118  as the trapped fluid bleeds through the timer orifice  132 . That movement progressively decreases the opening between the timer chamber  124  and the second transverse aperture  142  which provides a path into the longitudinal passage  140  leading to the tank return conduit  30 . The amount of time required for the timer spool  126  to fully close the second transverse aperture  142  is a function of the volume of trapped oil, the timer spring force and the size of the timer orifice  132 . 
     During normal operation, as when the backhoe  10  is on flat ground, the relatively slow operation of the timer valve  69  does not affect reseating of the bleeder poppet  110 , which occurs solely in response to the inlet port pressure. That is the pressure at the inlet port  50  decreases to a relatively low level at which the bleeder poppet  110  reseats before the timer valve  69  closes the opening into the second transverse aperture  142 . 
     However, when the cylinder pressures prevent normal reseating of the bleeder poppet  110 , operation of the timer valve  69  produces closure of the second relief valve  66 . As fluid in the dwell chamber  130  is displaced through timer orifice  132 , the spring bias causes the timer spool  126  to continues moving farther into the plug  118 , thereby reducing the opening into the second transverse aperture  142 . Eventually, that opening decreases to a “critical orifice” area with a large pressure drop there across. As a result, pressure starts to increase in the timer chamber  124  and acts on the back side of the bleeder poppet  110  along with the force of the bleeder spring  114 . In due course, enough pressure builds in the timer chamber  124  to force the bleeder poppet  110  against the second valve seat  106 . This forced reseating effectively and consistently terminates any drifting that occurs. It should be noted that the distance that the boom  15  travels while drifting is controlled by the designed operating time of the timer valve  69 . 
     The timer spool  126  travels the remainder of its stroke until bottoming out in a rest position in which the second transverse aperture  142  is fully closed. 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.