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TECHNICAL FIELD  
         [0001]    This invention relates generally to a hydraulic circuit for controlling the elevational position of a bulldozer blade or the like, and more particularly, to the incorporation and control of a quick drop valve for improving the efficiency of the circuit.  
         BACKGROUND  
         [0002]    Quick drop valves are commonly used in hydraulic control circuits for bulldozer blades or the like in which the blade is allowed to free-fall to ground level under the force of gravity. Some of the fluid expelled from the hydraulic cylinders which control blade elevation is diverted by the quick drop valves to the expanding ends of the hydraulic cylinders to supplement the pump flow thereto. Without any type of quick drop valve, the expanding ends of the hydraulic cylinders may cavitate quite significantly. Since the cavitated ends of the cylinders have to be filled with fluid from the pump after the blade comes to rest on the ground, a considerable time lag occurs before sufficient downward force can be applied to the blade for penetrating the ground. The use of quick drop valves minimizes the cavitation and thus reduces the time lag.  
           [0003]    The duration of the time lag depends upon the efficiency of the quick drop valve, which is determined by the amount of expelled fluid that the quick drop valve diverts back to the expanding side of the cylinders. That amount is dependent upon how quickly the quick drop valve moves to the quick drop position in a free-fall situation and the percentage of the expelled fluid that the quick drop valve diverts back to the expanding ends once it is in the quick drop position.  
           [0004]    An example of a quick drop circuit is provided by U.S. Pat. No. 5,014,734 to Smith which provides a hydraulic circuit having a quick drop valve that is actuated based on the pressures created by the hydraulic fluid flow through the circuit. Actuation of the quick drop valve occurs somewhere within a range of movement of an operator controlled lever during a controlled lowering operation which may be non-intuitive to the operator. Further, the operation controlled lever lacks a position for a floating blade operation to allow the blade to freely move vertically when traveling along the surface of the ground.  
           [0005]    The present invention is directed to overcoming one or more of the problems as set forth above.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with one aspect of the invention, a fluid circuit for raising and lowering an implement includes a quick drop valve member movable between at least a first position and a second position, the first position corresponding to a non-quick drop hydraulic fluid flow path of the fluid circuit and the second position corresponding to a quick drop hydraulic fluid flow path of the fluid circuit, the quick drop valve member being movable between at least the first and second positions based on pressures in the fluid circuit produced by hydraulic fluid. The fluid circuit further including a control system configured to selectively apply a biasing force against the quick drop valve member biasing the quick drop valve in the first position, the control system providing the biasing force independent of pressures in the fluid circuit produced by the hydraulic fluid.  
           [0007]    According to another aspect of the present invention, a fluid circuit for raising and lowering an implement includes a hydraulic fluid pump, at least one hydraulic cylinder selectively hydraulically coupled to the hydraulic fluid pump, the at least one hydraulic cylinder including a lift side and a drop side and being coupled to a working implement, at least one control valve located between the hydraulic fluid pump and the at least one hydraulic cylinder, a hydraulic-fluid-actuated quick drop valve located between the control valve and the at least one hydraulic cylinder, the quick drop valve including a quick drop valve member movable between a first valve member position blocking hydraulic fluid communication between the lift side and drop side of the at least one hydraulic cylinder, and a second valve member position allowing hydraulic fluid communication between the lift side and the drop side of the at least one hydraulic cylinder, and a fluid lock selectively fluidly biasing the quick drop valve member in the first position.  
           [0008]    According to another aspect of the present invention, a fluid circuit for raising and lowering an implement includes a hydraulic fluid pump, a plurality of hydraulic cylinders selectively hydraulically coupled to the hydraulic fluid pump, the plurality of hydraulic cylinders each including a lift side and a drop side and being coupled to a working implement, at least one control valve located between the hydraulic fluid pump and the plurality of hydraulic cylinders, the control valve having four positions, the four positions corresponding to a rising implement operation of the fluid circuit, a controlled lowering of implement operation of the fluid circuit, a holding of implement operation of the fluid circuit and a floating of implement operation of the fluid circuit, a quick drop valve located between the control valve and the plurality of hydraulic cylinders, the quick drop valve including a quick drop valve member movable by hydraulic fluid within the fluid circuit between a first valve member position blocking hydraulic fluid communication between the lift sides and drop sides of the plurality of hydraulic cylinders and a second valve member position allowing hydraulic fluid communication between the lift sides and the drop sides of the plurality of hydraulic cylinders, and a solenoid valve having a flow-through position allowing pressurized pilot fluid to flow to the quick drop valve to bias the quick drop valve member in the first position, and a blocked position disconnecting the pressurized pilot fluid flow to the quick drop valve member, the solenoid valve being actuated to its blocked position by an electric switch activated by moving an operator controlled lever to a triggering position.  
           [0009]    According to yet another aspect of the present invention, a method for controlling movement of an implement includes positioning an operator controlled lever to at least a first position corresponding to a raising implement operation and the application of a biasing force against a quick drop valve member of a quick drop valve, positioning the operator controlled lever to at least a second position corresponding to a holding implement operation and the application of the biasing force against the quick drop valve member, positioning the operator controlled lever to at least a third position corresponding to a controlled lowering implement operation and the application of the biasing force against the quick drop valve member, and positioning the operator controlled lever to at least a fourth position corresponding to a releasing of said biasing force against the quick drop valve member to allow the quick drop valve member to move between a quick drop position and a non-quick drop position.  
           [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.  
           [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an exemplary embodiment of the invention and together with the description, serve to explain the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1A is a diagrammatic and sectional view of a hydraulic control circuit according to an exemplary embodiment of the present invention; and  
         [0013]    [0013]FIG. 1B is an enlarged view of the encircled portion of the quick drop valve of FIG. 1A.  
     
    
     DETAILED DESCRIPTION  
       [0014]    Reference will now be made in detail to the exemplary embodiments of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0015]    Referring to FIG. 1A, a quick drop valve  10  is shown incorporated within a hydraulic circuit  12  for controlling the elevation of a load, for example, an implement such as a bulldozer blade  14  of an earth moving machine. Hydraulic circuit  12  may include a pair of double acting hydraulic cylinders  16 , a pair of cylinder conduits  18 ,  20  connecting quick drop valve  10  to opposite ends of hydraulic cylinders  16 , a pump  22  and a tank  24  connected to a directional control valve  26 , and a pair of valve conduits  28 ,  30  connecting directional control valve  26  to quick drop valve  10 .  
         [0016]    Control valve  26  may be a four (4) position four (4) way valve of any conventional design. As will be described further below, control valve  26  may include a position for each of a raising blade operation, a holding blade operation, a controlled lowering blade operation, and a floating blade operation. Alternatively, control valve  26  may be formed of any other configuration, including a single valve (as shown) or multiple valves, and control valve  26  could be pilot actuated (as shown), electrically actuated, or mechanically actuated.  
         [0017]    An auxiliary control system for quick drop valve  10  may include a pilot circuit, generally indicated at  32 . Pilot circuit  32  may include a pilot pump source  34 , a tank  36 , a pressure relief valve  37  connected to a tank  38 , a first pilot fluid line  39  and a second pilot fluid line  40 . First pilot fluid line  39  extends from pilot pump source  34  to directional control valve  26  and may include a check valve  42 , an accumulator  44  and a pilot valve  46 . Pilot valve  46  may include a tank  48  and pilot fluid lines  50  to each side of directional control valve  26 , and may be controlled by a variable position, operator controlled lever  52 .  
         [0018]    Second pilot fluid line  40  may be coupled between pilot pump source  34  and quick drop valve  10  and may include a check valve  54 , a solenoid valve  56 , and a drainage line  58  with a restriction  60  and a tank  62 . Solenoid valve  56  may be a two (2) position two (2) way valve having a leakage line  63  connected to tank  62 . Alternatively, solenoid valve  56  could be a two (2) position three (3) way valve, or any other conventional valve configuration. Solenoid valve  56  may be electrically coupled via electric line  64  to an electric switch  65 . Switch  65  may be actuated or closed based on the position of operator controlled lever  52  to thereby provide selective actuation of solenoid valve  56 .  
         [0019]    Hydraulic cylinders  16  may be suitably connected to a work machine, not shown, in the usual manner with each hydraulic cylinder  16  having a head end or drop side  66  connected to cylinder conduit  18 , a rod end or lift side  68  connected to cylinder conduit  20 , a piston  70  slidably disposed therein, and a piston rod  72  connecting pistons  70  to blade  14 . Blade  14  may be acted on by gravity such that the weight thereof establishes a generally downwardly dropping direction tending to extend hydraulic cylinders  16 .  
         [0020]    Quick drop valve  10  may include a multi-piece housing  74  having a bore  76  therein and a plurality of annuluses  78 ,  80 ,  82  in open communication with, and axially spaced along bore  76 . Adjacent annuluses  78  and  80  may be separated by a control land  84  and adjacent annuluses  80  and  82  may be separated by another control land  86 . Housing  74  may also have a pair of communicating with the annuluses  78  and  80  respectively and a pair of valve ports  92 ,  94  communicating with annuluses  78  and  82  respectively. Cylinder conduits  18  and  20  may be connected to cylinder ports  88  and  90 , respectively, and valve conduits  28  and  30  may be connected to valve ports  92  and  94 , respectively. Alternatively, valve port  92  may be omitted and valve conduit  28  connected directly to cylinder conduit  18 . Another alternative would be to mount housing  74  directly to one of hydraulic cylinders  16  with the porting therein suitably changed.  
         [0021]    A cylindrical valve member  100  may be slidably disposed in bore  76  and have opposite ends  102 ,  104  and a reduced diameter portion  106  adjacent end  104 . A fluid control pocket  108  may be provided in valve member  100  intermediate ends  102 ,  104 . An axially extending stepped bore  110  may be formed in valve member  100  and have opposite ends  112 ,  114 . End  112  of stepped bore  110  may be sealingly closed with a threaded plug  116  and will hereinafter be referred to as the closed end while end  114  will be referred to as the open end. Valve member  100  may have a passageway  122  which continuously communicates the annulus  80  with an actuating chamber  124  at end  102  of valve member  100 . Valve member  100  is shown in FIG. 1A in a blocking or first position, in which annulus  78  is blocked from communication with annulus  80 . Valve member  100  may include a quick drop or second position at which annulus  80  is in communication with annulus  78  through fluid control pocket  108 .  
         [0022]    An elongate bias piston  126  may be slidably disposed in bore  110  of valve member  100  and may have opposite reduced diameter end portions  128 ,  130 . End portion  130  may project outwardly of open end  114  of valve member  100  and may normally be in contact with housing  74 . End portion  128  of piston  126  may define an actuating chamber  132  at closed end  112  of bore  110 . A radial passage  118  may communicate with actuating chamber  132  through a spring biased check valve  120  (FIG. 1B) arranged so as to only allow fluid flow into chamber  132  through passage  118 . Second pilot fluid line  40  may be in communication with actuation chamber  132  by way of a further radial passage  134  formed in housing  74  and a radial passage  136  formed in valve member  100 .  
         [0023]    A coil compression spring  138  may circumscribe the portion of piston  126  extending beyond valve member  100  and may be disposed between valve member  100  and the housing  74  for resiliently biasing valve member  100  to the first leftmost position. Spring  138  and the force exerted on the valve member by pressurized fluid in actuating chamber  132  may each provide a biasing force for biasing valve member  100  to the first position.  
         [0024]    A valve mechanism  140  may be provided for defining an annular orifice  142  between annuluses  80 ,  82 . Annular orifice  142  may allow substantially unrestricted flow between annuluses  80  and  82  when valve member  100  is in its first position. Valve mechanism  140  may define a more restrictive orifice between annuluses  80 ,  82  when valve member  100  is shifted to the right to its second position.  
         [0025]    Valve mechanism  140  may include a cylindrical sleeve  144  having a pair of axially spaced cylindrical lands  146 ,  148  with land  148  being cylindrically larger than land  146 . Sleeve  144  may be slidably disposed on the reduced diameter portion  106  of valve member  100  and may be retained thereon by a retaining ring  150 . With valve member  100  and sleeve  144  at the position shown in FIG. 1, annular land  146  may cooperate with land  86  of housing  74  to define the size of orifice  142 . Sleeve  144  may be moveable leftwardly relative to valve member  100  to a position at which sleeve land  146  is spaced from housing land  86  to provide substantially unrestricted fluid flow from the annulus  82  to annulus  80  when the valve member is at the first position. When valve member  100  is at the second position, the annular land  148  may cooperate with land  86  to define a more restrictive orifice  142 . Alternatively, sleeve  144  can be designed without lands and can be, for example, a conical or other shaped surface to provide a variable orifice  142 .  
         [0026]    Industrial Applicability  
         [0027]    As set forth above, control valve  26  may provide for four (4) distinct fluid circuit operations. These operations may include: (1) a raising blade operation; (2) a holding blade operation; (3) a controlled lowering blade operation; and (4) a floating blade operation. The floating blade operation may include both a substantially free vertical movement of blade  14  and a quick free-fall of blade  14  from a raised position, hereinafter referred to as a quick drop operation. The four (4) fluid circuit operations provided by control valve  26  may be independently actuated by shifting control valve  26  between its four (4) possible positions shown in FIG. 1A. Movement of control valve  26  between the four (4) possible positions may be achieved by regulating fluid pressure from pilot fluid lines  50  via pilot valve  46  based on an angular position of operator controlled lever  52 . For example, the position of operator controlled lever  52  shown in FIG. 1A may vent a fluid pressure through pilot fluid lines  50  such that spring  51  biases control valve  26  in it neutral position shown, which corresponds to the holding blade operation. The pilot pressure control of control valve  26  may be achieved in any conventional manner. Alternatively, pilot pressure control of control valve  26  may be replaced with an electrical control or with a mechanical control by way of a mechanical coupling between control valve  26  and operator controlled lever  52 .  
         [0028]    To initiate the raising blade operation, the operator may move operator controlled lever  52  to a position  152  (shown in dashed lines), which in turn provides the appropriate pilot pressure to shift control valve  26  leftwardly to connect pump  22  to valve conduit  30  and valve conduit  28  to tank  24 . The pressurized fluid from pump  22  passes through control valve  26 , valve conduit  30 , and into annulus  82 . Sleeve  144  functions similar to a check valve such that the fluid passing from annulus  82  to annulus  80  moves sleeve  144  leftwardly to provide substantially unrestricted fluid flow therebetween. The pressurized fluid in annulus  80  passes through port  90 , cylinder conduit  20 , and into lift sides  68  of both hydraulic cylinders  16  causing pistons  70  to retract and thereby raise blade  14 . The fluid expelled from drop side  66  passes through cylinder conduit  18 , port  88 , annulus  78 , port  92 , valve conduit  28 , and control valve  26  to tank  24 .  
         [0029]    To initiate the controlled lowering blade operation, the operator may move operator controlled lever  52  to a position  154  (shown in dashed lines), which in turn provides the appropriate pilot pressure to shift control valve  26  rightwardly to communicate pump  22  with valve conduit  28  and valve conduit  30  to tank  24 . The pressurized fluid from pump  22  passes through control valve  26 , valve conduit  28 , port  92 , annulus  78 , port  88 , cylinder conduit  18  and into drop sides  66  of hydraulic cylinders  16 . The fluid expelled from lift sides  68  passes through cylinder conduit  20 , port  90 , annulus  80 , annulus  82 , port  94 , valve conduit  30 , and control valve  26  to tank  24 . The flow forces acting on sleeve  144  bias it to the position shown in FIG. 1 to establish orifice  142 . Alternatively, a lightweight coil spring can be used to resiliently bias sleeve  144  to the position shown in FIG. 1A.  
         [0030]    With control valve  26  in a position corresponding to the controlled lowering blade operation, control valve  26  restricts the fluid being expelled from lift sides  68  to a flow rate less than a predetermined flow rate. When the fluid flow rate of fluid passing through orifice  142  is less than this predetermined flow rate, the differential pressure generated by orifice  142  is below a predetermined magnitude. Thus, the pressure in annulus  80  and passing through passageway  122  to actuating chamber  124  is insufficient to move valve member  100  rightwardly to its second, quick drop position against the biasing forces keeping valve member  100  in its first position.  
         [0031]    The biasing forces acting to keep valve member  100  in its leftmost, first position may include those of spring member  138  and biasing forces resulting from fluid pressure within actuation chamber  132 . As will be described further below, even if the fluid flow rate of fluid passing through orifice  142  were greater than the biasing force of spring member  138 , valve member  100  would still be unable to shift to its quick drop position because of the pilot pressure being supplied to actuation chamber  132  from pilot pump source  34  via second pilot fluid line  40 . The pilot pump fluid supplied to actuation chamber  132  may act to selectively bias valve member  100  in its first position because the right end of actuation chamber is not movable due to piston  126  abutting housing  74  and the left end of actuation chamber  132 , which is formed by valve member  100 , is movable to expand the actuation chamber  132  and force valve member  100  to its first position. The pressure of pilot pump fluid from pilot source pump  34  may be selected to achieve a pressure in chamber  132  that, when combined with the spring biasing force of spring member  138 , is greater than any biasing force that may be created in actuation chamber  124 , thus producing a fluid lock within chamber  132 .  
         [0032]    If blade  14  is positioned against the ground, the operator may want to initiate the floating blade operation. This operation allows blade  14  to freely move vertically as it travels along the ground. This operation is commonly used when the machine attached to blade  14  is moving in reverse. To initiate the floating blade operation, the operator may move operator controlled lever  52  to a position  158  (shown in dashed lines), which in turn provides the appropriate pilot pressure to shift control valve  26  rightwardly to block pump  22  and connect together valve conduit  28 , valve conduit  30 , and tank  24 . Connecting valve conduits  28  and  30  and tank  24  together allow hydraulic fluid to move freely between lift sides  68  and drop sides  66  of hydraulic cylinders  16 . This results in the desired free vertical movement of blade  14  as it moves across a varying contour of the ground.  
         [0033]    If the floating blade operation is initiated when blade  14  is above the ground, blade  14  will drop toward the ground. This dropping of blade  14  toward the ground will be slightly resisted by a restriction  156  formed within control valve  26  between tank  24  and the junction of valve conduits  28  and  30 . Restriction  156 , and an inherent delay associated with the flow of fluid between hydraulic cylinders  16  and control valve  26 , may result in a relatively slower drop of blade  14  than that provided by the quick drop operation when quick drop valve  10  is actuated. As in the controlled lowering operation, valve member  100  of quick drop valve  10  cannot be shifted to its quick drop position during the floating blade operation because of the pilot pressure being supplied to actuation chamber  132  from pilot pump source  34 .  
         [0034]    To allow a quick drop of blade  14 , the operator may move operator controlled lever  52  to a triggering position  160  (shown in dashed lines), which in turn provides the appropriate pilot pressure to keep control valve  26  in its rightmost position described above. Position  160  may be located in an over travel region of the movement of operator controlled lever  52 . The over travel region may include a biasing member, such as a spring, creating a biasing force to urge operator controlled lever  52  out of the over travel region. This biasing force may act to signal to the operator that operator controlled lever  52  is approaching or in position  160  corresponding to a quick drop operation.  
         [0035]    In addition to maintaining control valve  26  in its rightmost position, operator controlled lever  52  in triggering position  160  also acts to close switch  65 , which in turn actuates solenoid valve  56  to shift leftward to block the flow of pilot pump fluid being supplied to actuation chamber  132  by way of second pilot fluid line  40  and radial passages  134  and  136  of housing  74 . Cutting off the supply of pilot pump fluid to actuation chamber  132  acts to unlock quick drop valve  10  to allow it to shift under the pressure resulting from the flow of hydraulic fluid through hydraulic circuit  12 , as will be described below. Drainage line  58  and restriction  60  allow for controlled drainage to tank  62  of pilot pump fluid located in second pilot fluid line  40  and actuation chamber  132 . This connection to tank  62  allows for the depressurization of actuation chamber  132 .  
         [0036]    With valve member  100  of quick drop valve  10  unlocked by way of the actuation of solenoid valve  56 , fluid being expelled from lift sides  68  of cylinders  16  during a free-fall of blade  14  may provide fluid flow through orifice  142  that exceeds the predetermined flow rate, thereby generating a differential pressure sufficient to move valve member  100  rightwardly to its quick drop position. More specifically, when the differential pressure exceeds the predetermined magnitude, the higher pressure in annulus  80  is directed through passageway  122  into actuating chamber  124 . With the differential pressure exceeding the predetermined magnitude, the fluid generated force acting on valve end  102  is greater than the fluid generated force acting on opposite end  104  of valve member  100  by an amount greater than the biasing force of spring  138 . Thus, valve member  100  is moved rightwardly toward its quick drop position. As valve member  100  moves rightwardly, annular land  148  creates a more restrictive orifice  142  causing a much greater differential pressure, thereby causing valve member  100  to move more rapidly to the fully actuated quick drop position.  
         [0037]    With valve member  100  in its quick drop position, annulus  80  communicates with annulus  78  through pocket  108  thereby allowing the fluid expelled from lift sides  68  to pass therethrough and combine with the fluid passing through port  88  and cylinder conduit  18  to fill drop sides  66  of hydraulic cylinders  16 . The more restricted orifice  142  functions also to limit fluid flow therethrough so that a greater amount of fluid expelled from the lift sides is used to fill the expanding drop sides  66  of hydraulic cylinders  16 . The amount of fluid passing through orifice  142  is selected to maintain a differential pressure sufficient to keep valve member  100  in the quick drop position. The fluid passing through orifice  142  passes through control valve  26  and back to tank  24 .  
         [0038]    The operator can shift out of the quick drop operation by moving operator controlled lever  52  out of position  160  and thus causing solenoid valve  56  to shift rightward and communicate pilot pump source  34  to actuation chamber  132 . The pressure created in actuation chamber  132 , in addition to the biasing force of spring member  138 , causes valve member  100  to shift leftward to its first position. This shifting of valve member  100  to its first position will quickly cut off the flow of fluid between annulus  80  and annulus  78  through pocket  108  and result in shifting hydraulic circuit  12  to the floating blade operation detailed above. Alternatively, operator controlled lever  52  may be shifted from position  160  to the position corresponding to the holding blade operation to stop blade  14  from further downward movement. Either act of shifting operator controlled lever  52  out of position  160  will cause a shifting of quick drop valve  100  to its first position and result in a jolting of blade  14  out of its free-fall. This jolting of blade  14  is beneficial in shaking unwanted earth from blade  14 .  
         [0039]    When blade  14  contacts the ground after a quick drop operation, valve member  100  of quick drop valve  10  immediately shifts back to its first position automatically without any additional effort required by the operator. More specifically, when blade  14  contacts the ground, and extension of hydraulic cylinders  16  stops, fluid is no longer expelled from lift sides  68  of hydraulic cylinders  16 . With no fluid passing through orifice  142 , the pressure differential reduces thereby allowing spring  138  to move valve member  100  to the first position.  
         [0040]    Further ensuring that valve member  100  is in its first position during controlled lowering of blade  14 , radial passage  118  allows pressurized fluid from pump  22  to enter actuation chamber  132  to urge valve member to its first position. Spring biased check valve  120  provided in radial passage  118  prohibits pilot pump fluid from second pilot fluid line  40  from escaping actuation chamber  132  via radial passage  118 . Alternatively, spring biased check valve  120  may be omitted if an additional piston is located in bore  110  between radial passageway  136  and radial passageway  118  so as to form separate actuation chambers. The additional piston should be configured so not to be capable of completely blocking either of passageways  136  or  118 .  
         [0041]    In view of the foregoing it is readily apparent that the present invention provides an improved hydraulic quick drop circuit. For example, the present invention allows for the advantages of a quick drop valve that is triggered at a clearly identifiable position of the operator controlled lever. Further, location of the quick drop actuation at an extreme of the range of movement of operator controlled lever  52  provides for a greater modulation range of operator controlled lever  52  resulting in a greater control of the movement of blade  14 , especially in a controlled lowering operation.  
         [0042]    The present invention utilizes a fluidly controlled quick drop valve and thus avoids the drawbacks of a fully electrically controlled quick drop valve. Such fully electrically controlled quick drop valves require added components to take into account, for example, the need to deactivate the quick drop valve when the blade reaches the ground. Further, fully electrically controlled quick drop systems are less reliable than systems incorporating hydraulic circuits.  
         [0043]    Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. For example, pilot circuit  32  of the auxiliary control system could be replaced with an equivalent gas or electric circuit for biasing quick drop valve member  100  in its first position. The auxiliary control system could also be integrated with the hydraulic circuit  12  so that hydraulic fluid of hydraulic circuit  12  acts to bias quick drop valve member  100  in its first position. The auxiliary control system could also be configured so that blocking the flow of fluid, or other medium, to valve member  100  acts to bias valve member  100  in its first position. Finally, solenoid valve  56  and electric switch  65  may be replaced with a fluid or mechanical assembly on electronic control arrangement. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Summary:
A fluid circuit for raising and lowering an implement including a quick drop valve member movable between at least a first position and a second position, the first position corresponding to a non-quick drop hydraulic fluid flow path of the fluid circuit and the second position corresponding to a quick drop hydraulic fluid flow path of the fluid circuit, the quick drop valve member being movable between at least the first and second positions based on pressures in the fluid circuit produced by hydraulic fluid. The fluid circuit further including a control system configured to selectively apply a biasing force against the quick drop valve member biasing the quick drop valve in the first position, the control system providing the biasing force independent of pressures in the fluid circuit produced by the hydraulic fluid.