Patent Publication Number: US-11655130-B2

Title: Synchronized hybrid clamp force controller for lift truck attachment

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/041,014 filed on Jun. 18, 2020, and this application is a Continuation-in-Part of U.S. patent application Ser. No. 16/420,000 filed on May 22, 2019, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     The subject matter of this application generally relates to improved systems and methods for operating a lift truck attachment used to grasp and move loads. 
     Material handling vehicles such as lift trucks are used to pick up and move loads from one location to another. Because lift trucks must typically transport many different types of loads, lift trucks usually include a mast that supports a vertically extensible carriage, which can be selectively interconnected to any one of a variety of different hydraulically operated lift truck attachments, each intended to securely engage and move a specific type of load. For example, a particular lift truck attachment may include a pair of horizontally spaced forks intended to slide into respective slots of a pallet that supports a load to be moved. Another lift truck attachment may include a pair of opposed, vertically-oriented clamps intended to firmly grasp the lateral sides of a load so that the lift truck can raise the load and move it. 
     Examples of this latter type of attachment include carton clamp attachments intended to grasp boxes or other rectangular loads, paper roll clamps intended to grasp cylindrical loads, etc. Lift truck attachments such as carton or roll clamp attachments need a hydraulic control system designed to avoid damaging the load. As one example, hydraulic control systems for clamp-type attachments need to provide a sufficient lateral force to securely grasp the load so that it does not fall during transport, but at the same time not apply so much force on the load to damage it. Hydraulic control systems for clamp attachments therefore typically include some type of load-weight sensing mechanism along with a control system that regulates gripping force by gradually increasing gripping fluid pressure automatically from a relatively low initial pressure to a pressure just sufficient to allow the load to be raised, without slipping. 
     However, using a low initial pressure limits the speed with which the load-engaging surfaces can be closed into initial contact with the load, thereby limiting the productivity of the load-clamping system. This problem occurs because high-speed closure requires higher closing pressures than the desired low threshold pressure; such higher pressures become trapped in the system by fluid input check valves during initial closure, so that the desired lower threshold pressure is exceeded before automatic regulation of gripping pressure can begin. 
     Hydraulic control systems for clamp attachments will also typically coordinate the movement of the clamps towards the load, so that one clamp does not prematurely strike and damage the load, cause the load to skid towards the other clamp, etc. To this end, such control systems typically utilize flow dividers, such as spool and gear flow dividers to split hydraulic fluid evenly to each of the clamps. Spool-type flow dividers split flow through pressure-compensated fixed orifices, which ensures near-equal flow through the orifices, even when inlet and/or outlet pressures fluctuate. However, spool flow dividers must balance accuracy with the ability to tolerate oil contamination without failure. Spool flows dividers are designed to accurately divide flow only within a narrow range of flow rates; because spool dividers use fixed orifices, equal division of flow may not occur when used below the rated flow for a specific divider, and if flow exceeds the rating of the valve, the high pressure drop across the valve causes poor performance and fluid heating. Gear flow dividers, though able to perform over a wider range of operating flow rates than spool dividers, are generally very expensive and the hydraulic circuit must be properly designed to prevent intensification if one clamp is restricted from moving. 
     Use of flow dividers, such as spool flow dividers and gear flow dividers in hydraulic clamp control systems, also tends to limit the closing speed at which opposed clamps move towards a load. Specifically, as noted earlier, because increasing the inward speed of each clamp requires a higher pressure, and because each clamp is driven towards the load at the same pressure, the clamp force against that load can be quite high when the clamps simultaneously contact the load. Thus, limiting the force against the load, at the instant that two opposed clamps controlled with fluid provided though a flow divider, means limiting the closing pressure and hence the closing speed. To provide high-speed closure and a low initial clamp force, complicated hydraulic control systems may provide high and low relief settings selectable either manually, or automatically in response to clamp closure speed. 
     What is desired, therefore, is an improved hydraulic control circuit that enables high speed, synchronized closure of opposed clamps towards a load, and that prevents damage to the load upon contact by the clamps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG.  1    shows an exemplary hydraulic control circuit that uses fluid provided from a lift truck to operate respective hydraulic cylinders, which may each drive a respective clamp on a lift truck attachment. 
         FIG.  2    shows the pressures and forces applied by hydraulic cylinders controlled by the circuit of  FIG.  1   . 
         FIG.  3    shows the exemplary hydraulic control circuit of  FIG.  1    connected to a pair of hydraulic cylinders used to operate a pivot arm clamp. 
         FIG.  4    shows a first exemplary synchronizing plunger that may be used in the hydraulic control circuit of  FIG.  1   . 
         FIG.  5 A  shows the synchronizing plunger of  FIG.  3    in a mid-stroke position and pressurized from the rod side. 
         FIG.  5 B  shows the synchronizing plunger of  FIG.  3    in an end-of-stroke position and pressurized from the rod side. 
         FIG.  5 C  shows the synchronizing plunger of  FIG.  3    in a mid-stroke position and pressurized from the head side. 
         FIG.  6    shows a second exemplary synchronizing plunger that may be used in the hydraulic control circuit of  FIG.  1   . 
         FIG.  7 A  shows the synchronizing plunger of  FIG.  5    in a mid-stroke position and pressurized from the rod side. 
         FIG.  7 B  shows the synchronizing plunger of  FIG.  5    in an end-of-stroke position and pressurized from the rod side. 
         FIG.  7 C  shows the synchronizing plunger of  FIG.  5    in a mid-stroke position and pressurized from the head side. 
         FIG.  7 D  shows the synchronizing plunger of  FIG.  5    in an end-of-stroke position and pressurized from the head side. 
         FIG.  8    shows an alternate control circuit used to control respective hydraulically operated motors of a lift truck attachment. 
         FIG.  9    shows an alternate control circuit capable of coordinating the movement of hydraulic actuators while such actuators are either linked or not linked. 
         FIG.  10    shows an alternate control circuit using a bidirectional relief valve and a plurality of sequence valves that resynchronize hydraulic cylinders in an open position. 
         FIGS.  11 A and  11 B  show a Multi-Load Handler (MLH) attachment in a single pallet mode and a double pallet mode, respectively. 
         FIGS.  12 A and  12 B  show operations of an MLH, when in a double pallet mode, that move loads away from, and towards each other respectively. 
         FIG.  13    shows an exemplary hydraulic control circuit that may be used to control an MLH. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes novel systems and methods that enable hydraulic actuators on industrial equipment, such as a lift truck or a lift truck attachment, to alternate between a first configuration where the actuators are hydraulically linked and a second configuration where the actuators are not hydraulically linked. As used in this specification and in the claims, the term “hydraulic actuator” refers to any device that has first and second fluid line connections, where a difference in fluid pressure across the connections is used to impart motion to the actuator. Examples of hydraulic actuators include, but are not limited to, hydraulic cylinders and hydraulically operated motors. As used in this specification and the claims, when referring to a hydraulic control circuit used to control one or more such actuators, the term “input port” refers to a pair of connections that, in operation of the control circuit, are capable of receiving pressurized fluid from an external source such as a lift truck and thereby pressurizing at least one output port of the control circuit, as later defined, while simultaneously returning unpressurized fluid back to the external source, e.g. lift truck. Similarly, an “output port” as used in the specification and the claims, when referring to a hydraulic control circuit, refers to a pair of connections that, in operation of the control circuit and when both are connected to a hydraulic actuator as previously defined, are capable of delivering fluid pressurized by the input port of the control circuit to the hydraulic actuator, and simultaneously returning fluid from the hydraulic actuator to the control circuit. Also, as used in the specification and the claims, the terms “hydraulically linked,” “hydraulically linking,” and similar terms, when referring to two or more hydraulic actuators means that the fluid pressure at the discharge side of a first actuator is fluidly communicated to the input side of a second actuator, i.e. the hydraulically linked actuators are connected in series. Furthermore, as used in the specification and claims, the phrase “not hydraulically linked,” “not hydraulically linking,” and similar terms used with respect to two hydraulic actuators means that the fluid pressure at the discharge side of either actuator is not connected to the input side of the other actuator. Also, as used in this specification, the term “coordinated” when used with respect to two or more hydraulic actuators, hydraulic cylinders, clamps, etc. means that the movement of such elements must occur together, while the term “not coordinated” means that the movement of one hydraulic actuator, hydraulic cylinder, clamp, etc. may occur independently of the other such elements. For purposes of this disclosure, though the specification will refer specifically to hydraulic cylinders, those of ordinary skill in the art will recognize that any fluid power actuator that moves a device to which it is connected by expanding, contracting, rotating, or otherwise moving as a result of a change in fluid pressure through the fluid power actuator may be used in the disclosed systems and methods. 
     As noted previously, material handling vehicles that grasp and move loads typically alternate between different modes of operation. As one example, a paper roll clamp or a carton clamp will use hydraulic actuators not only to cause clamp arms to apply a force to a load so as to securely lift it, but also will position the clamp arms by either moving together to initially contact the load or moving apart to release the load. In such an application, efficiency is improved if clamp arms are positioned at a high speed and low force, but low speed and high force is desired to avoid damaging the load when clamping it. As another example, some material handling equipment allows a grasped load to be rotated about an axis, thus requiring that clamps rotate to first align with a load, then rotate after a load is grasped. Again, for efficient operation it may be desired to rotate at a high speed, low torque when no load is being grasped, but at a low speed, high torque when a load is being grasped to avoid damaging the load or imparting too much inertia to the vehicle. As yet another example, side-shifting forks often must move independently to provide a desired spacing between the forks, but also move in concert when side-shifting a load held upon the forks. 
     In each of these illustrative examples, the novel systems and methods disclosed by the present application beneficially allow material handling vehicles, attachments etc. to hydraulically link the actuators during one mode of operation and disengage that hydraulic linkage during another mode of operation. Referring for example to a clamp attachment as described in the preceding paragraph, when coordinating the movement of two clamps toward or away from a load, simultaneously operating hydraulically cylinders or other actuators that move the clamps can be performed at a high-speed of operation, but that high-speed operation risks damaging the load after contact. This risk can be reduced by operating the hydraulic cylinders in series, but this would make the clamps less efficient at grasping the load by reducing the effective cylinder area used to generate clamp force. Thus, one embodiment of the disclosed system and methods hydraulically links cylinders during clamp positioning, i.e. when the clamps are moved outwardly such as to release a load, and/or when the clamps are moved inwardly toward the load so as to clamp it, until a time proximate when the clamps grasp the load, at which point the hydraulic cylinders are no longer linked such that the effective cylinder area is increased and clamp force control can be adjusted more efficiently. Other alternative embodiments of the disclosed systems and methods may hydraulically link the cylinders that move the clamps during an opening movement, and bypass the hydraulic linkage during a closing movement, for example. Those of ordinary skill in the art will appreciate that similar advantages are attained in other types of material handling applications, e.g. side-shifting fork attachments, rotator clamps, etc. 
     Moreover, such benefits may preferably be attained without the use of flow dividers. As noted previously, existing material handling equipment that engages and moves a load are typically designed to coordinate the motion of clamps, forks, or other such members towards and away from each other using flow dividers. Each such clamp, fork, etc. is typically driven by a respective fluid power actuator, e.g. a hydraulic cylinder, and a flow divider is used to split pressurized flow equally towards each of the hydraulic actuators that move a respective clamp. The flow divider thus ensures that the opposed clamps move in a coordinated manner, toward or away from each other, under essentially identical pressures, but in doing so inhibits the speed at which the clamps move because a low initial pressure is desired when the clamps initially contact the load. The disclosed systems and methods may be used, however, to coordinate the movement of opposed clamps toward and away from each other without passing fluid through a flow divider, by hydraulically linking fluid power actuators that move the clamps. 
       FIG.  1    shows an exemplary system  10  that includes a hydraulic control circuit  12  that operates hydraulic actuators  20  and  22  using pressurized fluid provided from, e.g. a lift truck or other industrial equipment having a pump or motor  14  and reservoir  16 . Preferably, the hydraulic circuit  12  includes an input port having connections  19   a  and  19   b  thus permitting fluid connection to a lift truck or other industrial equipment so that fluid may be provided under pressure to one of the input connections  19   a ,  19   b  while depressurized fluid is returned to the lift truck via the other one of the input connections  19   a ,  19   b . Those of ordinary skill in the art will understand that during operation of the control circuit  12 , each of the connections  19   a  and  19   b  will alternately receive pressurized fluid and expel unpressurized fluid depending on which direction fluid is flowing through the circuit, e.g. whether the cylinders  20 ,  22  are retracting or extending. 
     The hydraulic circuit  12  preferably includes a first output port having connections  21   a ,  21   b  and a second output port having connections  23   a ,  23   b . Each output port is selectively connectable to a respective hydraulic actuator, such as one of the cylinders  20 ,  22  so that the actuators may be driven in a desired direction or other mode by selecting which connection of a respective output port to pressurize, while allowing fluid thereby expelled from the actuator to return to the circuit  12  from the other connection of the output port. For example, when connection  21   a  is connected to the rod side of cylinder  20  and connection  21   b  is connected to the head side of cylinder  20  as shown in  FIG.  1   , if output connection  21   a  is pressurized, fluid will flow into the rod-side of cylinder  20  which will then retract, causing fluid to be expelled from the head side of the cylinder  20  back into the circuit  12  through connection  21   b . Alternately, if output connection  21   b  is pressurized, fluid will flow into the head side of cylinder  20 , which will expand and cause fluid to flow from the cylinder  20  back into the circuit  21  through connection  21   a.    
     The hydraulic circuit  12  also preferably includes a selector, such as the sequence valve  28  of  FIG.  1   , which determines whether or not the first output port  21   a ,  21   b  and the second output port  23   a ,  23   b  are operated in series, as explained in detail later in this specification. Those of ordinary skill in the art will appreciate that the specific device or devices used as the selector may vary based on the type or types of hydraulic devices being controlled by the circuit, but broadly however, the selector is a device or arrangement of devices configured in the hydraulic circuit  12  capable of alternatingly selecting whether or not the control circuit  12  interconnects the output ports such that fluid returned from one hydraulic actuator into the control circuit  12  is used to pressurize a connection of the port of another hydraulic actuator. In some embodiments, as later described, the selector may alternatingly select whether connected hydraulic actuators are connected in series to an input port of the control circuit  12 , or whether connected hydraulic actuators are connected in parallel to an input port of the control circuit  12 . In other embodiments, the selector may select whether connected hydraulic actuators are connected in series to an input port of the control circuit  12 , or whether one hydraulic actuator is pressurized by the input port of the control circuit and exhausts fluid towards the input port while another hydraulic actuator is not pressurized by the input port and does not exhaust fluid towards the input port. Regardless of such variations, by selectively determining whether or not hydraulic actuators are linked in series, the control circuit  12  may be used in a variety of different hydraulically operated devices such as lift truck attachments to operate more efficiently. 
     For example, the embodiment of  FIG.  1    shows a circuit  12  used to provide pressurized fluid to a pair of hydraulic cylinders  20  and  22  typical of a carton clamp or roll clamp attachment where retraction of the rods of the cylinders  20  and  22  brings the clamps together and extension of the rods of the cylinders  20  and  22  moves the clamps apart. Opening and closing movement of the cylinders  20  and  22  is manually selectable by direction control valve  18 , which when moved to the left from the neutral position shown in  FIG.  1    will close the clamps towards the load by providing pressurized fluid to port  19   a  of the control circuit  12  and returning unpressurized fluid to the tank  16  through port  19   b  of the control circuit  12 , and when moved to the right from the neutral position shown in  FIG.  1    will open the clamps away from the load by providing pressurized fluid to port  19   b  of the control circuit  12  and returning unpressurized fluid to the tank  16  through port  19   a  of the control circuit  12 . Typically, the pump or motor  14 , the reservoir or tank  16 , and the directional control valve  18  are each located on a lift truck that supplies pressurized fluid to a lift truck attachment via fluid lines extending over the mast of the lift truck to the attachment, which in turn would typically include the hydraulic cylinders  20  and  22  along with their associated clamps and the control circuit  12  used to operate the attachment. 
     When an operator of a lift truck initially moves selector valve  18  to pressurize port  19   a  of control circuit  12 , pressurized fluid will flow through pilot operated check valve  24 , which is used to maintain the load-gripping force (pressure) in the primary cylinder  20 , through output port connection  21   a  and into the rod side of the primary cylinder  20  which will accordingly contract to move it&#39;s associated clamp inwardly, e.g. toward a load. Fluid will then be expelled from the head side of the primary cylinder  20  through output port connection  21   b  of the control circuit  12 . Because fluid sequence valve  28  (whose operation as the previously-described selector will be explained later) prevents the fluid from returning to the tank  16  through port  19   b , the fluid expelled from the primary cylinder  20  will flow through pilot-operated check valve  26 , through output port connection  23   a  of the control circuit  12 , and into the rod-side of secondary cylinder  22 , which will also contract to move its associated clamp inwardly, e.g. toward a load. Fluid is then expelled from the head side of secondary cylinder  22  and into output port connection  23   b  to return to the tank  16  via port  19   b  of the control circuit  12 . Thus, when sequence valve  28  is maintained in the closed position as shown in  FIG.  1   , cylinders  20  and  22  are connected in series, and movement of the clamps is coordinated while the clamps are moving inwardly toward a load prior to contacting the load, without using a flow divider, providing an improvement in clamp speed. 
     When the clamps contact the load, pressure rises in line  30  to which sequence valve  28  is connected. When the pressure reaches a threshold setting of the sequence valve  28 , indicating that the load is being clamped, that valve opens to allow fluid to flow from the head side of primary cylinder  20  and into the unpressurized tank  16 , and therefore prevents fluid from flowing into the rod side of cylinder  22 . As the load is clamped further by primary cylinder  20 , secondary cylinder  22  is locked in place; fluid cannot enter the rod side of secondary cylinder  22  to retract the rod since port  3  of pilot valve  26  is depressurized and port  1  is pressurized, while similarly secondary cylinder  22  cannot extend its rod since pilot valve  26  blocks flow out of the rod side of cylinder  22 . Thus, sequence valve  28  operates to alternate a mode of operation of the primary and secondary cylinders  20 ,  22 , during a closing movement, between a first mode of operation where the primary and secondary cylinders  20 ,  22  are hydraulically linked over a first range of motion of the primary cylinder, and a second mode of operation where the primary and secondary cylinders  20 ,  22  are not hydraulically linked over a second range of motion of the primary cylinder. Though  FIG.  1    shows that the sequence valve  28  is operated by a rise in pressure as a load is clamped, those of ordinary skill in the art will recognize that other means may be employed for actuating the sequence valve, or otherwise switching the cylinders  20  and  22  from a first, hydraulically linked mode to a second, non-hydraulically linked mode, such as using a valve actuated when a clamp arm or cylinder expands or retracts beyond a specific location, or using a sensor-operated solenoid valve, etc. In such a manner, for example, the primary and secondary cylinders may switch from being hydraulically linked as clamps reach a location proximate to a load, but not yet contacting it. 
     When an operator of a lift truck moves selector valve  18  to the right relative to the position shown in  FIG.  1   , to pressurize port  19   b  of control circuit  12 , pressurized fluid will flow to the head side of secondary cylinder  22  to extend its rod. Since port  3  of pilot operated check valve  24 , and port  3  of pilot operated check valve  26  are each connected to now-pressurized line  32 , which feeds the secondary cylinder  22 , each of check valve  24  and check valve  26  will now open, and pressure in line  32  added to the spring force of the sequence valve  28 , will cause the sequence valve  28  to close. Thus, as secondary cylinder  22  extends, fluid is expelled from its rod side and through pilot operated check valve  26  to enter the head side of primary cylinder  20 , which extends in concert with secondary cylinder  22  and thereby moves the clamps away from each other in a coordinated manner. As the primary cylinder  20  extends, fluid is expelled from its rod side, and through the pilot operated check valve  24  to return to the tank  16 . 
     In this manner, the hydraulic control circuit  12  operates to alternate a mode of operation of the primary and secondary cylinders  20 ,  22 , between a clamp-opening movement where the cylinders  20  and  22  are hydraulically linked, and a clamp closing movement where the cylinders  20  and  22  are not hydraulically linked over at least a portion of the closing movement. Those of ordinary skill in the art will recognize that alternate embodiments may include hydraulic control circuits that have cylinders  20  and  22  linked during the entirely of the opening movements and not linked during the entirety of the closing movement. 
       FIG.  2    generally illustrates how pressures and forces are transmitted through the primary and secondary cylinders  20  and  22 , and their associated clamps due to the operation of the hydraulic control circuit  12  as previously described. Preferably the rod-side area A 1  of the primary cylinder  20  is designed to yield the required load-gripping force at an expected input oil pressure. For example, if the required cylinder force is 4,180 lbs at an input pressure of 2000 psi, the required rod-side area A 1  is 2.09 in 2 . This area can be achieved by using a rod diameter of 1.10 inches (28 mm) and a bore of 1.97 inches (50 mm). The rod-side area A 3  of the secondary cylinder  22  is preferably designed to have equal, or near-equal, area to the head-side area A 2  of the primary cylinder. This matched area allows for equal movement of each cylinder, i.e. one inch of movement of the rod of the primary cylinder  20  will result in one inch of movement of the rod of the primary cylinder  22 . For example, using a primary cylinder  20  with dimensions of 1.10 inches (28 mm) rod diameter and 1.97 inches (50 mm bore diameter), the rod side Area A 1  of the primary cylinder is 2.09 in 2  and head area A 2  is 3.04 in 2 . The secondary cylinder  22  thus preferably has an equal rod said area A 3  of 3.04 in 2 . Such a cylinder might be constructed with a rod diameter of 1.26 inches (32 mm) and a bore diameter of 2.34 inches (59.4 mm). 
     As can be determined from  FIG.  2   , and assuming the rod-side area A 3  of the secondary cylinder  22  is equal to the head-side area of the primary cylinder  20 , activation of the sequence valve  28  will cause the clamp force against the load F P , F S  to double. Specifically, whether or not the cylinders are hydraulically linked, F P  and F S  must be equal since both forces act against the same immobile load, where in the hydraulically linked case, F P  is equal to P 1 A 1 −P 2 A 2  and F S  is simply equal to P 3 A 3 , since P 4  is equal to zero as it is connected to the tank pressure. Furthermore, since A 2  has been designed to be equal to A 3 , and given that P 2  must equal P 3  due to the hydraulic linkage, P 2 A 2  must be equal to P 3 A 3 . Given these relationships,
 
 F   P   =F   S   =P   3   A   3   =P   2   A   2  
 
and therefore
 
 F   P   =P   1   A   1   −P   2   A   2   =P   1   A   1   −F   P .
 
Rearranging gives
 
 F   P =½ P   1   A   1 .
 
     When activation of the sequence valve  28  disables the hydraulic linkage, however, both P 4  and P 2  become zero since they are connected to the tank, and
 
 P   3   A   3   =F   S   =F   P   =P   1   A   1  
 
Thus, when the cylinders  20  and  22  are not hydraulically linked, F P  is double the value that it is when the cylinders  20  and  22  are hydraulically linked. Accordingly, by hydraulically linking the cylinders during positioning, movement of clamp arms can be coordinated without the use of flow dividers (which would disadvantageously place restrictions on the inlet flow rate) and can occur at a high speed while minimizing the force on the load when it is initially clamped. Once clamping occurs, the hydraulic linkage of cylinders  20  and  22  can be bypassed, which allows clamp force to be applied more effectively.
 
       FIG.  3    shows an alternate embodiment where the control circuit  12  of  FIG.  1    may be used to control hydraulic actuators or cylinders  27 ,  29  typically found in a pivot arm clamp where the extension of the cylinders  27 ,  29  provides a gripping force on a load and the retraction of cylinders  27 ,  29  releases a load. Thus, unlike the embodiment of  FIG.  1   , the cylinders  27 ,  29  are connected to the control circuit so that, during clamp closing, pressurized fluid is provided to the head side of primary cylinder  27 , and is expelled from the rod-side of cylinder  27 , and when hydraulically linked, fluid expelled from the rod-side of cylinder  27  is provided to the head side of cylinder  29 , with the rod side of cylinder  29  connected to connection  23   b , and hence  19   b . In this embodiment, the head side area of cylinder  29  is preferably equal to the rod side area of cylinder  27  to ensure that, when hydraulically linked, equal movement of the cylinders  27 ,  29  occurs. 
     Referring to  FIGS.  1  and  3   , and as explained earlier, when the sequence valve  28  opens, thereby bypassing the hydraulic linkage between the primary and secondary hydraulic cylinders  20 ,  22  to further clamp a load, the secondary cylinder  22  in some embodiments may remain stationary while the primary cylinder  20  applies additional clamping force. Due to this asynchronous behavior of the primary and secondary cylinders, continued use of the hydraulic circuit  10  may cause one of the cylinders  20 ,  22  to reach their end-of-stroke before the other cylinder does, which can inhibit the ability of the system to either adequately clamp the load or retract the clamps to their fully retracted position. 
     Accordingly, in some embodiments the hydraulic circuit  10  may preferably include an optional resynchronizing valve  25  that allows fluid to bypass the hydraulic linkage when one cylinder has reached its end-of stroke before the other cylinder. When retracting the rods of the cylinders  20 ,  22 , the resynchronizing valve  25  allows oil to flow directly from the pressurized line  30  to the rod-side of the secondary cylinder  22  whenever the pressure difference between the rod-side of the primary cylinder  20  and the rod-side of the secondary cylinder  22  exceeds a threshold amount set by the spring setting of the resynchronizing valve  25 . If, for example, the rod of primary cylinder  20  is fully retracted while pressure is provided to clamping port  19   a , pressure will rise in line  30  until resynchronizing valve  25  opens to allow fluid to flow directly from pressurized line  30  into the rod-side of secondary cylinder  22  which can continue to move to the fully retracted position so as to resynchronize the cylinders  20 ,  22 . Conversely, if the secondary cylinder  22  reaches its end-of-stroke before the primary cylinder  20 , pressure will increase in line  30  until the pressure setting value of the sequencing valve  28  is reached, and oil is allowed to be exhausted from the head side of primary cylinder  20  until both cylinders are fully synchronized. 
     The spring setting of the resynchronizing valve  25  should be sufficiently high to both ensure that the sequence valve  28  opens before the resynchronizing valve  25  opens, and to otherwise prevent the valve  25  from opening when the cylinders  20 ,  22  are hydraulically linked while being positioned toward a load prior to clamping it. In that instance, since the head-side of the primary cylinder  20  is connected to the rod-side of the secondary cylinder  22 , the pressure setting of the spring of valve  25  should be set to a value higher than the highest anticipated pressure drop across the primary cylinder  20  during positioning, which in turn is related to the maximum intended positioning speed of the valve circuit  10 . When the primary cylinder  20  and the secondary cylinder  22  are clamping on a load, whether or not the cylinders  20  and  22  are hydraulically linked, and so long as the primary cylinder is not at the end-of-stroke, the pressure in the rod-sides of both cylinders will be the same, and any spring setting of the valve  25  that satisfies the above conditions would thus always keep the valve closed. In a preferred embodiment, the spring setting of the resynchronizing valve  25  may preferably be set to about 150 psi lower than the system pressure setting. 
     Those of ordinary skill in the art will appreciate that the resynchronizing valve  25 , configured to resynchronize cylinders  20  and  22  by moving the rods of both cylinders to the fully retracted position, may instead be configured to resynchronize cylinders  20  and  22  by moving the rods of both cylinders to the fully extended position, by e.g. connecting the input of the resynchronizing valve  25  to line  32  instead of line  30 , and connecting the output of the resynchronizing valve  25  to the head side of primary cylinder  20  instead of the rod side of secondary cylinder  22 . 
     As an alternative to using resynchronizing valve  25 , one or both of the primary and secondary cylinders  20 ,  22  may be configured to selectively operate as a valve that allows resynchronization by allowing oil to flow from the rod-side to the head side of the cylinder, or vice versa, when the cylinder has reached an end-of-stroke position. Referring to  FIG.  4    for example, either or both the primary or secondary cylinders  20  or  22  may comprise a synchronizing cylinder  40  having a cylinder shell  42  that encloses at least a portion of a sliding cylinder rod  44 , which is fixed in a threaded bore  48  of a sliding piston  46 . The piston  46  preferably includes a wear band  50  and a piston seal  52  to provide for sealed, sliding movement of the piston within the cylinder shell  42 . The cylinder rod  44  may define a conduit for pressurized oil to flow back and forth between the rod-side area of the cylinder  40  (i.e. area A 1  or A 3  of  FIG.  2   ) to the interior of the piston  46 . For example, the cylinder rod  44  may include a conduit  53  comprising a passage with a first portion that extends axially inwards from the end of the rod  44  embedded in the piston  46  to a second portion that includes a plurality of radial passages to the periphery of the cylinder rod  44 . The piston-side of the conduit  53  may be selectively sealed by a check ball  58  mounted on a spring  56  that pushes the check ball  58  toward the first, axial portion of the conduit  53 . The end of the spring  56  opposite the check ball  58  is in turn secured around a flange of a sliding plunger  54 . The flange of the plunger  54  fits within a seat of a retainer  59  such that oil within the interior of the piston  46  is sealed from entering the head side area of the cylinder  40  (i.e. Area A 2  or A 4  of  FIG.  2   ), or flowing in the opposite direction, when the flange of the plunger  54  rests in the seat of the retainer  59 . 
     Referring to  FIG.  5 A , when the cylinder  40  is pressurized from the rod-side so as to retract the rod, and is not at an end-of-stroke position, pressurized oil flows from the rod-side area of the cylinder  40 , through the radial portion and then the axial portion of passage  52  to push the check ball  58  inwards and allow oil to reach the interior cavity of the piston  46 . But the spring  56  pushes the plunger  54  against the seat of the retainer  59 , thus preventing oil from flowing into the head-side area of the cylinder  40 . When, however, the cylinder  40  retracts the rod a sufficient distance to reaches the rod&#39;s end of stroke position, as seen in  FIG.  5 B , the plunger  54  contacts cylinder head  57  which compresses the spring  56  between the flange of the plunger  54  and the unseated check ball  58 , such that the plunger  54  comes off of the seat of the retainer  59  and oil is allowed to flow from rod-side area of the cylinder  40 , to the interior of the piston  46 , and out to the head side area of the cylinder  40 , and ultimately to the other cylinder  20  or  22  (or the tank  16 ) via porting  55 , to allow resynchronization. As shown in  FIG.  5 C , when cylinder  40  is pressurized from the head side, in a mid-stroke position, pressurized oil pushes the plunger  54  off the seat of the retainer  59  and allows oil to flow into the interior of the piston  46 , but the plunger  54  causes the spring  56  to push the check ball  58  to seal the conduit  53  so that oil may not flow to the rod-side area of the cylinder  40 . 
       FIG.  6    shows an alternate synchronizing cylinder  60  capable of resynchronizing at either the fully retracted or fully extended end-of-stroke position of the rod of the cylinder  60 . Specifically, cylinder  60  may comprise a cylinder shell  62  within which a piston  66  is slidably and sealably secured via seal  74  and one or more wear bands  72 . Rigidly mounted within a first bore  65  of the piston  66 , by e.g., a heat shrink connection, is the end of a cylinder rod  64  that slides with the piston  66 . The piston  66  also defines a second bore  67  that houses a spool  68  that generally matches the shape of the second bore  67 , such that a gap is defined between the outer surface of the spool  68  and the inner surface of the second bore  67 . Both the second bore  67  and the spool  68  have a central region with a larger diameter/width than opposed peripheral regions of the second bore  67  and the spool  68 , respectively, where the central region of the spool  68  has a shorter length than that of the second bore  67 , and where the second bore  67  and the spool  68  are jointly shaped such that the central region of the spool  68  may slide back and forth within the central region of the second bore  67  between a first extreme where one peripheral region of the spool  68  extends out of the associated peripheral region of the second bore  67  and a second extreme where the opposed peripheral region of the spool  68  extends out of its associated peripheral region of the second bore  67 . In some embodiments, to facilitate the formation of a second bore  67  shaped to closely surround the spool  68 , the second bore  67  may be formed on one end using a retainer plug  70  secured within the piston  66  with a heat shrink connection, so as to surround one peripheral region of the spool  68 . 
     Referring to  FIG.  7 A , when the cylinder  60  is pressurized from the rod-side, spool  68  is pushed within the second bore  67  to allow oil to flow through the gap between the second bore  67  and the rod-side of the spool  68 , but oil is blocked from entry into the head side of the cylinder  60  because the spool  68  is pushed into, and closes, the head-side peripheral region of the second bore  67 . When, however, the retracting rod reaches the end-of-stroke position shown in  FIG.  7 B , cylinder head  76  pushes spool  67  inward such that pressurized oil can enter the head-side peripheral region of the second bore  67  and escape to the other cylinder  50  or  52 , or the tank  16  via porting  78 . 
     As can be seen in  FIGS.  7 C and  7 D , this operation reverses when the cylinder  60  is pressurized from the head side; during a mid-stroke position, the spool  68  slides so as to allow oil to flow from the head side of the cylinder  60  and into the area between the spool  68  and the second bore  67 , but blocks oil from entering the rod-side area of the cylinder  60 . When the extending rod  64  reaches the end-of-stroke position, cylinder retainer  80  pushes spool  67  inward such that pressurized oil can enter the rod-side peripheral region of the second bore  67  and escape to the other cylinder  50  or  52 , or the tank  16  via porting  82 . 
     The embodiments shown in  FIGS.  1  and  3    use a control circuit  12  intended to operate hydraulic actuators alternately in a first mode where the hydraulic actuators are connected in series so as to move in a coordinated manner, and a second mode where the movement of the hydraulic actuators is not coordinated, e.g. one hydraulic actuator is locked in place while the other moves.  FIG.  8    shows an alternate control circuit  84  for a rotator dual drive motor where the control circuit  84  includes a selector  88   a ,  88   b  capable of alternately driving two hydraulic motors  86   a ,  86   b  in series or in parallel where the movement of the motors is coordinated in both instances. Specifically, the control circuit  84  may include an input port  19   a ,  19   b  selectively connectable to a pump  14  and reservoir  16  on, for example, a lift truck having both a clamp selector valve  18  intended to alternately clamp and release a load as previously described, as well as a rotator selector valve  83  used to selectively rotate the clamps about an axis in a desired direction by moving the valve to the left or right of a centered position, or hold the angular orientation of the clamps fixed by moving the valve  83  to the centered position. 
     The control circuit  84  preferably has a first output port with connections  21   a ,  21   b  and a second output port with connections  23   a ,  23   b  each selectively connectable to a respective one of hydraulic motors  86   a ,  86   b . Thus, when connected as shown in  FIG.  8   , motor  86   a  may be driven in one direction by pressurizing connection  21   a  and allowing fluid to exhaust from the motor back into the control circuit  84  through connection  21   b , and may be driven in the opposite direction by pressurizing connection  21   b  and allowing fluid to exhaust from the motor back into the control circuit  84  through connection  21   a . Motor  86   b  may be similarly driven via connections  23   a  and  32   b.    
     The control circuit  84  preferably has a selector, shown in this example as comprising first and second solenoid valves  88   a ,  88   b , and used to determine whether pressurized fluid received through the input port  19   a ,  19   b  drives the motors  86   a ,  86   b  in series (useful, for example, to rotate clamps at a high speed when no load is grasped) or in parallel (useful, for example, to rotate clamps at a low speed but high torque when a load is grasped). Specifically, when the solenoids  88   a ,  88   b  are each in an un-energized state, pressurized fluid present at either of the input port connections will drive the motors  86   a ,  86   b  in parallel by routing fluid pressurized from the pump  14  to connections  21   a  and  23   a  when input connection  19   a  is pressurized and routing fluid pressurized from the pump  14  to connections  21   b  and  23   b  when input connection  19   b  is pressurized. In both circumstances, each of the non-pressurized output connections to the motors  86   a  and  86   b  are independently connected to the reservoir  16 , allowing the motors to exhaust fluid directly towards the reservoir  16 . 
     When both solenoids are energized, however, connection  23   b  of the control circuit&#39;s output port to the motor  86   b  is connected to connection  21   a  of the control circuit&#39;s output port to the motor  86   a , so as to rotate the motors  86   a ,  86   b  in series. In this configuration, when connection  19   a  is pressurized by the pump  14 , pressurized fluid flows out of connection  23   a  and into motor  86   b , which expels fluid back into connection  23   b  and through connection  21   a  to motor  86   a . Fluid from motor  86   a  flows back into the control circuit  84  through connection  21   b , and from the control circuit  84  out to the tank  16  through input connection  19   b . Pressurizing connection  19   b  while both solenoids are energized, conversely, maintains the serial connection of the motors  86   a ,  86   b  but rotates them in the other direction relative to the rotation that occurs when connection  19   a  is pressurized. Those of ordinary skill in the art will appreciate that, although  FIG.  8    shows two solenoids  88   a ,  88   b  as the selector that alternates the control circuit  84  between a parallel configuration and a serial configuration, other embodiments may use different selectors, e.g. pilot controlled valves that change configuration based on a detected clamping pressure. 
       FIG.  9   . shows yet another embodiment of a control circuit that coordinates the movement of hydraulic actuators in a selectively alternating one of a series configuration and a parallel configuration. Specifically, a hydraulic control circuit is used to coordinate the movement of hydraulic cylinders  92 ,  94  that for example, respectively move clamps towards and away from a load using pressurized fluid provided to connections  19   a ,  19   b  of an input port of the hydraulic control circuit. As can be seen from  FIG.  9   , the control circuit  90  includes all the elements of control circuit  12  shown in  FIGS.  1  and  3   , but also includes a flow divider  96  and a pressure-actuated valve  98  interposed between connection  19   a  of the input port to the control circuit  90 . 
     When pressurized fluid is provided to connection  19   b  of the input port of the control circuit  90 , the control circuit  90  operates in the same manner as control circuit  12  of  FIG.  1   ; cylinders  92  and  94  are connected in series so as to extend the rods of the cylinders in a coordinated manner, where fluid flows from the control circuit  90  into the head side of cylinder  94 , back from the rod side of cylinder  94  into the control circuit  90 , into the head side of cylinder  92  from the control circuit  90 , and out from the rod side of cylinder  92  back into the control circuit which in turn discharges fluid into the tank  16 . However, when pressurized fluid is provided to connection  19   a  of the input port of the control circuit  90 , that pressurized fluid is distributed by flow divider  96  in a manner determined by the position of pressure-actuated valve  98 . Specifically, the flow divider  96  splits fluid provided from connection  19   a  into a first path or line toward connection  21   a  connected to the rod-side of cylinder  92  and a second path or line toward the pressure-actuated valve  98 . The pressure-actuated valve  98  is spring-biased to a position that re-combines the flows split by the flow divider  96  so that the entire flow pressurizes port  21   a , which again causes the control circuit to behave exactly as does control circuit  12  of  FIG.  1   , i.e. cylinders  92  and  94  are connected in series so as to position clamps in a closing movement towards a load in a coordinated manner. When the clamps contact the load, pressure at port  19   a  increases to a level that moves pressure-actuated valve  98  so as to divert fluid from the second path, as just described, through a one-way check valve  99 , and to the rod-side of cylinder  94  so that pressure provided through input port connection  19   a  of the control circuit  90  operates cylinders  92  and  94  in parallel as a load is being clamped. 
     Because the coordinated operation of the cylinders  92  and  94 , when hydraulically linked in series with each other, requires that the head side area of cylinder  92  match the rod side area of cylinder  94 , the rod-side area of cylinder  92  would typically be smaller than the rod-side area of cylinder  94 . Thus, in order to equalize the force applied by the cylinders  92  and  94  and to coordinate the movement of the cylinders  92  and  94  when they are not hydraulically linked and controlled in parallel, the flow divider  96  preferably splits the flow from input connection  19   a  unevenly, in an amount proportional to the rod-side area of the cylinders driven by the respectively split fluid flow. Thus, in the illustrative example of  FIG.  9   , where cylinder  92  has a rod-side area of 2.09 in 2  and cylinder  94  has a rod-side area of 3.04 in 2  for a total area of 5.13 in 2 , the flow divider  96  preferably directs 41% of the flow into cylinder  92  (i.e. 2.09 in 2 /5.13 in 2 ) and 59% of the flow into cylinder  94  (i.e. 3.04 in 2 /5.13 in 2 ) when clamping on a load. This ensures that the flow into the cylinders  92  and  94  each causes the same linear retraction of the rod in each respective cylinder. 
     One advantage of the control circuit  90  in comparison to the control circuit  12 , when used to operate clamps on a load, is that the control circuit  90  may reduce or possibly eliminate the need for the re-synchronizing valve  25  or the use of valves in hydraulic cylinders such as those shown in  FIGS.  4  and  6   . Because the cylinders  92  and  94  move in concert both during positioning of the clamps and while the load is being clamped, each of cylinders  90  and  92  are much less likely to reach an end-of-stroke before the other cylinder does. 
       FIG.  10    shows a control circuit  100  that is an alternate embodiment of that shown in  FIG.  3   . The control circuit  100  may optionally include a bidirectional relief valve  102  to limit pressure during closing and opening of the  27  and  29 , to protect against structural damage to itself or surrounding objects. Furthermore, in the control circuit of  FIG.  3   , there is a possibility that the pilot-operated check valve  26  opens before check valve  24 , causing an intensification on port  1  of valve  24 , which could exceed the available pilot pressure to open valve  24 . To address this possibility, the control circuit  100  replaces the pilot-operated check valve  24  shown in  FIG.  3    with a counterbalance valve  104 . During closing operations, pressure through port  19   a  causes fluid to bypass the counterbalance valve  104  via check valve  105  and thereafter pressurize the rod-side of cylinder  27 . During closing operations, pressure through port  19   b  opens pilot operated control valve  26  and also opens the counterbalance valve  104  to thereby allow fluid to exhaust through port  19   a.    
       FIG.  10    also shows a relief valve  106   a  and a relief valve  106   b  that together allow resynchronization of the cylinders  27  and  29 . Specifically, during an opening operation, if the cylinder  29  reaches its end of stroke before cylinder  27 , relief valve  106   a  will open and allow fluid to enter the piston side of cylinder  27 . Conversely, if the cylinder  27  reaches its end of stroke before cylinder  29 , relief valve  106   b  will open and allow fluid to discharge from the rod side of cylinder  29 . 
     Referring to  FIGS.  11 A and  11 B , a Multiple Load Handler (MLH) is a type of lift truck attachment that includes four forks laterally slidable relative to each other to allow a lift truck to alternately engage one or two palletized loads. In a first configuration shown in  FIG.  11 A , the four forks may be divided into two pairs of adjacent forks such that each pair may slide into a respective aperture of a single pallet. The second configuration, shown in  FIG.  11 B , arranged the forks into two pairs of spaced apart forks, where each pair is arranged to engage and move a respective pallet. 
     Thus, an MLH has two different operations to laterally position forks. The first operation is to position the forks between “single” and “double” pallet modes as shown in  FIGS.  11 A and  11 B . This operation requires little actuator force, and preferably occurs at high speed with accurate synchronization between the two different pairs of forks. The second operation, which occurs in “double” mode, positioning each set of forks laterally relative to each other as shown in  FIGS.  12 A and  12 B . This is commonly referred to as “snapping” when closing and “spreading” when opening. This operation requires high actuator force and low speed, again preferably with accurate synchronization between the left hand and right-hand fork sets. 
     Because the MLH modes of operation operate between a first mode characterized by high speed and low force and a second mode characterized by low speed and high force, it is desirable to employ a hybrid clamp force control circuit, as previously described. However, unlike the systems previously described where the high force operation occurs when clamping around a single load, and synchronization of movement between hydraulic cylinders during clamping therefore occurs through the transfer of force through the load, in an MLH attachment each cylinder is moving an independent load, hence the control circuit must also provide for synchronization between the cylinders. This is particularly true when cylinders of different bores are used, since the same pressure would produce different forces in the cylinders, leading to different movement speeds. 
       FIG.  13    shows a control circuit  110  that receives and discharges fluid from inlet port  114   a ,  114   b  and receives and discharges fluid through a first outlet port  116   a ,  116   b  and a second output port  118   a ,  118   b .  FIG.  13    shows the first outlet port  116   a ,  116   b  connected to small bore cylinder  120  and the second outlet port  118   a ,  118   b  connected to large bore cylinder  122 , but those of ordinary skill in the art will understand that this configuration may be reversed. 
     During high speed, low force operation of an MLH attachment, such as when forks are being positioned between double and single pallet modes, the cylinders may be operated in either of a closing movement or an opening movement. In the opening movement, a selector valve  112  may be moved to pressurize connection  114   a , which provides fluid to a flow divider  124 . One side of the flow divider is directly connected to connection  116   a  which supplies the rod side of the small bore cylinder  120 , while the other side of the flow divider is connected to a pilot-operated directional control valve  126 , which has a spring bias that sets it to a default position in low force operation that also supplies fluid to connection  116   a  of the rod side of the small bore cylinder  120 , i.e. in low force operation, all the fluid in the flow divider exits connection  116   a  into the rod side of cylinder  120 , which contracts to expel fluid back into the control circuit  110  through connection  116   b . Pressurized fluid opens pilot-operated control valve  132  so that the pressurized fluid again exits the control circuit into the rod-side of large bore cylinder  122 , which contracts to expel fluid into the control circuit through connection  118   b , and then out of the control circuit  110  through inlet connection  114   a . In this manner, during high-speed low force operation the cylinders  120  and  122  are linked so that the output of one cylinder provides fluid to the input of another cylinder. 
     During closing movement of a high force, low speed operation however, such as when loaded pallets are snapped towards each other, this linkage is broken and the control circuit operated in non-linked mode. Specifically, when the selector valve  112  is again set to pressurize connection  114   a , but with loaded pallets being moved by the cylinders  120 ,  122 , sequence valve  134  opens, thus pressurizing the pilot line to port  1  of the pilot-operated directional control valve  126 . Valve  126  therefore moves to a position where a portion of the flow through flow divider  124 , instead of being directed to output connection  116   a  is instead directed to output connection  118   a  so that each cylinder  120 ,  122  is driven independently. Simultaneously, pilot line to port  1  of sequence valve  136  is also pressurized by actuation of valve  134 , which allows fluid to exhaust from cylinder  120 , into connection  116   b  and out connection  114   b . In some embodiments, the setting of sequence valve  114  may be approximately 2000 psi. 
     Flow divider  124  divides and recombines flow at the ratio equivalent to the difference in size between the cylinders  120  and  122 . For example, a primary (small) actuator with bore size of 40 mm and rod size of 25 mm has a rod side working area of 766 mm{circumflex over ( )}2, the corresponding secondary (large) actuator has a bore size of 50 mm and rod size of 30 mm has a rod side working area of 1257 mm{circumflex over ( )}2. The flow divider should therefore preferably divide 38% of the flow to the primary (small) actuator and 62% of the flow to the secondary (large) actuator to achieve synchronized movement per the equations below: 
     
       
         
           
             
               
                 Primary 
                 ⁢ 
                     
                 Actuator 
               
               = 
               
                 
                   Volume 
                   ⁢ 
                     
                   1 
                 
                 = 
                 
                   A 
                   ⁢ 
                   1 
                   * 
                   Stroke 
                 
               
             
             ⁢ 
             
 
             
               
                 Secondary 
                 ⁢ 
                     
                 Actuator 
               
               = 
               
                 
                   Volume 
                   ⁢ 
                   3 
                 
                 = 
                 
                   A 
                   ⁢ 
                   3 
                   * 
                   Stroke 
                 
               
             
             ⁢ 
             
 
             
               
                 Total 
                 ⁢ 
                     
                 Volume 
               
               = 
               
                 
                   Volume 
                   ⁢ 
                     
                   1 
                 
                 + 
                 
                   Volume 
                   ⁢ 
                   3 
                 
               
             
             ⁢ 
             
 
             
               
                 Flow 
                 ⁢ 
                     
                 Divider 
               
               = 
               Specifications 
             
             ⁢ 
             
 
             
               
                 Port 
                 ⁢ 
                     
                 2 
                 ⁢ 
                     
                 Division 
               
               = 
               
                 
                   
                     Volume 
                     ⁢ 
                       
                     1 
                   
                   
                     Total 
                     ⁢ 
                         
                     Volume 
                   
                 
                 = 
                 
                   
                     
                       766 
                       * 
                       stroke 
                     
                     
                       
                         ( 
                         
                           766 
                           + 
                           1257 
                         
                         ) 
                       
                       * 
                       stroke 
                     
                   
                   = 
                   
                     38 
                     ⁢ 
                     % 
                   
                 
               
             
             ⁢ 
             
 
             
               
                 Port 
                 ⁢ 
                     
                 4 
                 ⁢ 
                    
                 Division 
               
               = 
               
                 
                   
                     Volume 
                     ⁢ 
                       
                     3 
                   
                   
                     Total 
                     ⁢ 
                         
                     Volume 
                   
                 
                 = 
                 
                   
                     
                       1257 
                       * 
                       stroke 
                     
                     
                       
                         ( 
                         
                           766 
                           + 
                           1257 
                         
                         ) 
                       
                       * 
                       stroke 
                     
                   
                   = 
                   
                     62 
                     ⁢ 
                     % 
                   
                 
               
             
           
         
       
     
     As noted earlier, given that each cylinder is moving an independent load, and given that cylinders  120  and  122  have different bore sizes, the control circuit  110  preferably includes a synchronization mechanism that ensures that the cylinders  120  and  122  move at the same speed. Accordingly, the control circuit  110  preferably includes an intensifier relief valve  130  positioned between direction control valve  126  and output port  118   a . Intensifier relief valve  130  provides a pressure drop due to the work of the fluid against the spring of valve  130 , where the spring resistance is set so that the force applied by the large bore cylinder is the same as the small bore cylinder. In this manner, the two cylinders  120  and  122  move at the same speed. For example, assuming the cylinder  120  has a 40 mm bore with a 25 mm rod, and cylinder  122  has a bore of 50 mm and a 30 mm rod), and equal loads are carried on both pallets, a load that requires 2200 psi on the small bore would only require 1400 psi to achieve equal force. Thus valve  130  would be set to 800 psi to compensate for the difference and thereby allow the flow divider to operate more precisely. In some embodiments the intensifier relief valve may have a variable setting to accommodate different loads, different cylinders, and/or different configurations. Preferably, the spring force of valve  130  is set low enough so that whenever the system switches to non-linked mode, the valve  130  will open against the spring, i.e. sequence valve  134  has a higher spring resistance than the intensifier relief valve  130  such that any pressure in at port  114   a  large enough to actuate valve  126  will be large enough to actuate valve  130 . 
     During opening movement, selector valve may pressurize connection  114   b , which provides all pressurized fluid to port  118   b , which operates the control circuit in linked mode. Because port  114   b  is pressurized, the pilot line to port  3  of pilot operated control valve  132  causes each to open so that fluid from cylinder  122  can flow into cylinder  120 , and fluid from cylinder  120  can flow back to port  114   a  through flow divider  124 . 
     In some embodiments, the control circuit  110  may include a cross-over relief valve  128  across the output of the flow divider  124 . When in linked mode the cross-over relief valve has no effect on the control circuit  110 , but when in non-linked mode the cross-over relief will open when the pressure differential exceeds the setting of the valve  124 . This will allow flow to bypass the flow divider and resynchronize the when the forks are at the full closed position. 
     In some embodiments, the control circuit  110  may include a pilot drain orifice  138  that drains any trapped pressure in the pilot portion of the circuit, as well as normalizes the pressure between the pilot ports of sequence valve  136  and the direction control valve  126  to maintain the normal state of both. When the inlet pressure exceeds that of the setting of the sequence valve  134 , that valve will open and allow flow/pressure to pilot the sequence valve  136  and the direction control valve  126 . The orifice is sized such that is cannot drain the pressure faster than what sequence valve  134  can supply. 
     It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.