Abstract:
A hydraulic circuit that is capable of recovering even very small amounts of energy by magnifying the pressure of the return fluid such that an accumulator can be effectively charged to a suitable operating pressure. Pressurized fluid from the accumulator is then used to drive one or more hydraulic actuators.

Description:
RELATED APPLICATIONS 
       [0001]    This application hereby incorporates by reference and claims the benefit of U.S. Provisional Application No. 60/951,566 filed Jul. 24, 2007. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The present invention relates generally to hydraulic systems. More particularly, the invention relates to a system for recovering energy from a hydraulic lift. 
       BACKGROUND OF THE INVENTION  
       [0003]    Work equipment is widely used for performing tasks more efficiently than could otherwise be done by hand, and/or tasks that would be physically impossible to perform by hand. For example, such equipment can include back-hoes, front-end loaders, lifts, cranes etc. Typically, the equipment includes one or more work implements powered by a hydraulic circuit. In the case of front-end loaders, forklifts, cranes, etc., such hydraulic circuits are often used for raising and/or lowering loads. 
         [0004]    In a conventional hydraulic power circuit used for raising and/or lowering loads, a hydraulic pump supplies fluid to one or more hydraulic cylinders configured to raise or lower the work implement, such as a bucket of a front-end loader. In general, the bucket is raised or lowered and returns to its initial position with equal, less, or more mass depending on the particular application. As will be appreciated, however, the weight of a raised bucket, even if empty, exerts a force on the hydraulic cylinder. 
         [0005]    In the past, when the bucket was returned to its starting position (e.g., lowered), the pressurized fluid in the cylinder was vented to a reservoir. This resulted in lost power as the energy of the vented fluid was typically dissipated via a throttle valve as the fluid was returned to the reservoir. 
         [0006]    One solution for recovering this lost energy has been to direct the return fluid from the hydraulic actuator to an accumulator during the lowering of the implement. The pressurized fluid stored in the accumulator is then rerouted back to the hydraulic actuators during a subsequent load raising procedure. While such systems can work satisfactorily under certain operating conditions, in the case of a very light work implement the pressure of the return fluid may be too low to effectively charge the accumulator. Under such conditions, energy recovery can be difficult and, thus, often little or no energy is recovered. 
       SUMMARY OF THE INVENTION  
       [0007]    The present invention provides a hydraulic circuit that is capable of recovering even very small amounts of energy by magnifying the pressure of the return fluid such that an accumulator can be effectively charged to a suitable operating pressure. Pressurized fluid from the accumulator is then used to drive one or more hydraulic actuators. 
         [0008]    Accordingly, a hydraulic system comprises a plurality of hydraulic actuators configured to raise and lower a load together, a source of pressurized fluid connected to the actuators for supplying pressurized fluid to the actuators for raising the load, and an accumulator for storing pressurized fluid. During lowering, return fluid from less than all of the hydraulic actuators is diverted to the accumulator for energy recovery, and return fluid from at least one of the hydraulic actuators is drained to a reservoir separate from the accumulator thereby increasing the pressure of the return fluid that is diverted to the accumulator from the other hydraulic actuators. 
         [0009]    The accumulator can supply pressurized fluid to at least one of the hydraulic actuators during the raising of the load when the pressure of the fluid in the accumulator exceeds a prescribed level. The system can further comprise a fluid diverter operable during lowering to divert return fluid from less than all the actuators to the accumulator. A control valve can be provided that is movable between a closed position blocking flow of fluid from the source of pressurized fluid to the actuators and an open position permitting flow of fluid from the source of pressurized fluid to the actuators for raising the load. The accumulator can be connected to the supply of pressurized fluid via a check valve that permits flow of fluid from the accumulator to the supply when the pressure in the accumulator exceeds a prescribed level. At least one of the plurality of hydraulic actuators can be a hydraulic cylinder, and the source of pressurized hydraulic fluid can include a hydraulic pump. 
         [0010]    In accordance with another aspect, a work machine comprises a work implement configured to raise and lower a load, a plurality of hydraulic actuators operatively associated with the work implement, a source of pressurized fluid connected to the hydraulic actuators for supplying pressurized fluid to the actuators for raising the load, and an accumulator for storing pressurized fluid. During lowering, return fluid from less than all of the hydraulic actuators is diverted to the accumulator for energy recovery, and return fluid from at least one of the hydraulic actuators is drained to a reservoir separate from the accumulator thereby increasing the pressure of the return fluid that is diverted to the accumulator from the other hydraulic actuators. The accumulator can supply pressurized fluid to at least one of the hydraulic actuators during the raising of the load when the pressure of the fluid in the accumulator exceeds a prescribed level. The system can further comprise a fluid diverter operable during lowering to divert return fluid from less than all the actuators to the accumulator. A control valve can be provided that is movable between a closed position blocking flow of fluid from the source of pressurized fluid to the actuators and an open position permitting flow of fluid from the source of pressurized fluid to the actuators for raising the load. The accumulator can be connected to the supply of pressurized fluid via a check valve that permits flow of fluid from the accumulator to the supply when the pressure in the accumulator exceeds a prescribed level. At least one of the plurality of hydraulic actuators can be a hydraulic cylinder, and the source of pressurized hydraulic fluid can include a hydraulic pump. 
         [0011]    Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic illustration of an exemplary work machine in accordance the invention. 
           [0013]      FIG. 2  is a schematic illustration of an exemplary hydraulic circuit in accordance with the invention. 
           [0014]      FIG. 3  is the hydraulic circuit of  FIG. 2  in a raise (or extend) configuration. 
           [0015]      FIG. 4  is the hydraulic circuit of  FIG. 2  in a lower (or retract) configuration. 
       
    
    
     DETAILED DESCRIPTION  
       [0016]    Referring now to the drawings in detail, and initially to  FIG. 1 , an exemplary work machine  10  is shown. The work machine  10 , which can be any of a wide variety of work machines such as a back-hoe, a front-end loader, a crane, a lift, etc., includes a prime mover  12  for supplying power to a hydraulic circuit  14  for powering a work implement  16 . The prime mover  12  can be an internal combustion engine or an electric motor, for example. The prime mover may also supply power to other systems of the work machine  10 , such as a drive system or auxiliary power systems (not shown). As will be appreciated, the work implement  16  can be any of a wide variety of work implements but will typically be a work implement capable of raising and/or lowering a load such as a bucket, a back-hoe arm, a crane arm, a lift platform, etc. 
         [0017]    Turning to  FIGS. 2-4 , and initially to  FIG. 2 , the details of the exemplary hydraulic circuit  14  are illustrated. The hydraulic circuit  14  includes first and second hydraulic actuators  20   a  and  20   b  operatively associated with the work implement  16  for performing work thereon. The hydraulic actuators  20   a  and  20   b  are in the form of hydraulic cylinders having pistons  22   a  and  22   b  and respective piston rods  24   a  and  24   b  coupled to the work implement  16 . Each hydraulic actuator  20   a  and  20   b  receives pressurized fluid from a source of pressurized fluid  30  for extending the piston rods  24   a  and  24   b  to raise the work implement  16 . 
         [0018]    The pressurized fluid supply  30  includes a pump  34  that is driven by the prime mover  12  (not shown in  FIGS. 2-4 ). The pump  34  is connected to the hydraulic actuators  20   a  and  20   b  via a raise/hold valve  38  which controls flow to respective lines  42   a  and  42   b.  The raise/hold valve  38  is movable between a closed position blocking flow of fluid from the pump  34  to the hydraulic actuators  20   a  and  20   b,  and an open position permitting flow of fluid from the pump  34  to the hydraulic actuators  20   a  and  20   b  for raising the load. A pressure switch  46  is provided for cycling the pump  34  on/off to maintain a prescribed pressure for raising operations, while a pressure controlled relief valve  48  relieves fluid to reservoir  49  to prevent pressure from exceeding a prescribed level. 
         [0019]    An accumulator  50  is connected to the pressurized fluid supply  30  via a check valve  52  that permits flow of fluid from the accumulator  50  to the pressurized supply  30  when the pressure in the accumulator  50  exceeds a prescribed level, as will be described in greater detail below. The accumulator  50  is also connected via control valve  56  to hydraulic actuator  20   b.  The control valve  56  is movable from a first position whereat return flow from hydraulic actuator  20   b  is blocked, to a second position whereat return flow from hydraulic actuator  20   b  is directed to the accumulator  50  for recovery of energy. Control valve  56  also permits or blocks flow from hydraulic actuator  20   a  to reservoir  49  during lowering, as will be described below. 
         [0020]    Turning to  FIGS. 3 and 4 , the operation of the hydraulic circuit  14  will be described in the context of a lift, such as a man-lift. In  FIG. 3 , the hydraulic circuit is shown in a lifting configuration. When operation of the machine is initiated, the accumulator  50  may typically be nearly empty of hydraulic fluid. Accordingly, the pressure switch  46  will typically be closed (e.g., pump  34  is on) such that all hydraulic fluid needs are supplied by the hydraulic pump  34 . As would be typical, when a job first started, the work implement (e.g., lift platform) is in a lowered position and an operator would load thereon the tools and materials the operator needs for the task at hand. The operator might then climb on board and maneuver the machine to the work area. 
         [0021]    Once in position, a raise/hold lever associated with raise/hold valve  38  would be moved to open the raise/hold valve  38 . The pump  34  then supplies high pressure fluid to the hydraulic actuators  20   a  and  20   b  to raise the platform to a desired height. Meanwhile, check valve  52  prevents the accumulator  50  from being charged directly from the pump  34 . 
         [0022]    When the operator&#39;s task is complete, the operator lowers the platform by operating a lower/regen lever associated with to actuate the control valve  56 . As described above, the control valve  56  operates to direct return fluid from hydraulic actuator  20   b  to the accumulator  50 . Meanwhile, return fluid from hydraulic actuator  20   a  is drained to reservoir  49 . When hydraulic actuator  20   a  is drained to the reservoir  49 , hydraulic actuator  20   b  becomes the only support for the platform. Accordingly, the pressure of the fluid within hydraulic actuator  20   b  is doubled as compared to when both hydraulic cylinders  20   a  and  20   b  supported the platform. 
         [0023]    As will be appreciated, after a number of cycles the pressure and volume of fluid stored in the accumulator  50  will rise sufficiently to open the pressure switch  46  to turn the pump  34  off. Subsequent requirements for hydraulic fluid will then be met by the stored pressurized fluid in the accumulator  50  until the accumulator discharges enough volume to cause the pressure switch  46  to close. Once the pressure switch  46  closes, the hydraulic pump  34  is once again turned on and will supply any further demand until the accumulator  50  is once again charged. 
         [0024]    The invention can be particularly beneficial for use in equipment with electric motors powered by batteries. As will be appreciated, extending battery life is virtually always desirable and the invention may be capable of recovering up to 50% of the energy that would otherwise be lost in a conventional hydraulic circuit that discharges return flow directly to a reservoir. 
         [0025]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.