Abstract:
A stabilizer system and method that can provide high speed, relatively low force extension and retraction of a stabilizer leg during a first portion of a stroke, and low speed, relatively high force extension and retraction during a second portion of the stroke. Accordingly, the stabilizer system and method takes less time to deploy than conventional stabilizer systems but without sacrificing performance.

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/785,144 filed Mar. 23, 2006, which is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to hydraulic vehicle stabilizer systems and, more particularly, to such a system using a two-stage bi-rotational hydraulic pump. The invention also has more general applicability to bi-directional actuator systems using a two-stage bi-rotational hydraulic pump. 
   BACKGROUND OF THE INVENTION 
   A trend in the construction industry has been to utilize smaller, more versatile machinery on the job-site. For example, mini-excavators and skid-steer loaders are often used to perform a variety of tasks. In many cases, a skid-steer loader or mini-excavator is equipped with an attachment for performing a particular task. Such attachments are typically powered by an auxiliary hydraulic circuit on the skid-steer loader or mini-excavator. Numerous attachments exist for performing a variety of tasks. For example, attachments exist for allowing a skid-steer loader to be used as a backhoe, an earth auger, an angle broom, a drop hammer, a snowplow, a brush saw, etc. 
   When using some types of attachments, it is often desirable to use a stabilizer system to raise the vehicle&#39;s wheels off of the ground in order to provide a more stable operating foundation. Typical stabilizer systems include one or more stabilizer legs that are lowered and raised by hydraulic piston-cylinder assemblies respectively to lift and lower the vehicle relative to the ground. 
   When selecting a stabilizer system for a vehicle, design parameters include, among other things, the maximum load the stabilizer system must be able to accommodate, and the rate at which the stabilizer legs can be extended and/or retracted. As will be appreciated, for a stabilizer system having a pump of a given size, as the load rating of the system increases the rate at which the stabilizer legs can be extended and retracted typically decreases. Accordingly, as the load capacity increases the stabilizer system takes longer to deploy thereby making the system less convenient to use. 
   One way to increase the load rating and maintain the rate of extension and retraction is to use a more powerful pump. However, using a more powerful pump usually means increased costs and may also require more space. 
   Another problem that arises when using piston-cylinder assemblies having a relatively large diameter piston rod, as is often the case in a vehicle stabilizer system where heavy loads are encountered. In such assemblies, the amount of fluid displaced from the piston side chamber of the cylinder during retraction of the piston rod is considerably greater than the amount of fluid flowing into the rod side chamber of the cylinder. 
   SUMMARY OF THE INVENTION 
   The present invention provides a stabilizer system and method that can provide high speed, relatively low force extension and retraction of a stabilizer leg during a first portion of a stroke, and low speed, relatively high force extension and retraction during a second portion of the stroke. Accordingly, the invention provides a stabilizer system and method that takes less time to deploy than conventional stabilizer systems but without sacrificing performance. 
   According to one aspect of the invention, a hydraulic system particularly suited for operating a double-acting hydraulic actuator in such a stabilizer system comprises: 
   a first bi-directional pump and a second bi-directional pump that together can be coupled to and operatively driven bi-directionally by a prime mover, each pump having extend and retract ports that function as pressure and suction pumps depending on the direction of operation of the pumps; 
   a first extend circuit portion for fluidly connecting the extend port of the first pump to an extend chamber of the hydraulic actuator; 
   a first retract circuit portion for fluidly connecting the retract port of the first pump to a retract chamber of the hydraulic actuator; 
   a second extend circuit portion for fluidly connecting the extend port of the second pump to the first extend circuit portion; 
   a second retract circuit portion circuit for fluidly connecting the retract port of the second pump to the first retract circuit portion; 
   a non-return check valve connecting the second extend circuit portion to the first extend circuit portion for blocking reverse flow from the first extend circuit portion to the second extend circuit portion while permitting flow from the second extend circuit portion to the first extend circuit portion; 
   an unloader valve unloader valve connected to the second extend circuit portion between the non-return check valve and the extend port of the second pump, the unloader valve being responsive to fluid pressure in the first extend circuit portion such that unloader valve will open to a drain path when the fluid pressure in the first extend circuit portion increases above a predetermined value; 
   a pressure responsive return check valve connecting the second retract circuit portion to the first retract circuit portion for normally blocking flow reverse flow from the first retract circuit portion to the second retract circuit portion while permitting flow from the second retract circuit portion to the first retract circuit portion, the return check valve being responsive to pressure in the second extend circuit portion to allow reverse flow when the pumps are being operated to extend the actuator; and 
   an excess fluid dump valve connected to the first extend circuit portion for diverting to a drain path excess fluid exiting from the extend side of the actuator during actuator retraction, the excess fluid dump valve being normally closed during actuator extension and open in response to pressure in the first extend circuit portion during actuator retraction. 
   According to another aspect of the invention, a stabilizer system comprises: 
   a hydraulic cylinder assembly having a piston and rod supported therein; 
   a low pressure bi-rotational pump element connected via flow passages to a piston side and a rod side of the hydraulic cylinder assembly; 
   a high pressure bi-rotational pump element connected via flow passages to the piston side and the rod side of the hydraulic cylinder assembly; 
   wherein when the pump elements are rotated in a first direction fluid is supplied to the piston side of the cylinder for extending the rod by the high pressure bi-rotational pump element at least when a system pressure on the piston side exceeds a prescribed level, and by the low pressure bi-rotational pump element at least when the system pressure on the piston side is below the prescribed level; 
   wherein when the pump elements are rotated in a second direction fluid is supplied to the rod side of the cylinder for retracting the rod by the high pressure bi-rotational pump element at least when the system pressure on the rod side exceeds the prescribed level, an not by the low pressure bi-rotational pump; and 
   wherein the piston side of the hydraulic cylinder has a larger area than the rod side of the hydraulic cylinder such that during retraction of the hydraulic cylinder a greater amount of fluid exits the piston side of the hydraulic cylinder than enters the rod side of the hydraulic cylinder. 
   According to a further aspect of the invention, a hydraulic circuit connectable to a hydraulic cylinder for extending and retracting a rod of the hydraulic cylinder comprises: 
   a first flow passage connectable to a piston side of the hydraulic cylinder and second flow passage connectable to a rod side of the hydraulic cylinder; 
   a low pressure bi-rotational pump element connected to the flow passages for supplying fluid thereto; and 
   a high pressure bi-rotational pump element connected to the flow passages for supplying fluid thereto; 
   wherein when the pump elements are rotated in a first direction fluid is supplied to the first flow passage by the high pressure bi-rotational pump element being at least when a system pressure in the first passage exceeds a prescribed level, and the low pressure bi-rotational pump element at least when the system pressure in the first passage is below the prescribed level; 
   wherein when the pump elements are rotated in a second direction fluid is supplied to the second passage by the high pressure bi-rotational pump element at least when the system pressure in the second passage exceeds the prescribed level, and outflow from the low pressure bi-rotational pump is bypassed around the second passage. 
   The foregoing and other features of the invention are more particularly described in the following detailed description when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating an exemplary hydraulic vehicle stabilizer system according to the invention. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawing, an exemplary hydraulic system according to the invention is generally indicated at  10 . The system  10  generally comprises a pump assembly  12  that can be driven by a prime mover such as a reversible motor  13 , in particular a DC motor, in both directions, and hydraulic circuitry  14  for connecting the pump assembly to a hydraulic actuator  15 . As illustrated, the actuator  15  can be mounted to a body  16  of a vehicle  18  and connected to a stabilizer leg  20  for lowering and raising the leg upon extension and retraction of the actuator. The vehicle typically will be equipped with one or more additional stabilizer legs each serviced by a respective pump assembly (not shown) and associated hydraulic circuitry (not shown) that may be the same as that herein described. The stabilizer legs can be lowered to raise the vehicle body and support the vehicle body independently of its suspension. 
   As will be appreciated by those skilled in the art, a more powerful pump assembly can be used to increase the speed at which the vehicle can be raised and lowered. This however means increased costs and usually a larger package size and increased weight. 
   In accordance with the present invention, the pump assembly  12  includes a pair of bi-rotational pumps  22  and  23 , connected to the reversible motor  13  for conjoint operation. The pump  22  preferably is a high pressure, low volume pump whereas the pump  23  preferably is a low pressure, high volume pump. As explained in greater detail below, both pumps supply hydraulic fluid to the hydraulic actuator  15  for rapid lowering of the stabilizer leg  20  under low load conditions until the stabilizer leg engages the ground  26 . For the remainder of the stroke, the high pressure pump supplies high pressure fluid to the hydraulic actuator under high load conditions to lift the vehicle body off the wheels. To raise the stabilizer leg and lower the vehicle body back onto the wheels, reverse flow from the high pressure pump is used to retract the hydraulic actuator. 
   Although herein described as part of a vehicle stabilizer system, the hydraulic system  10  can be adapted for a variety of other applications where a two-stage actuator having extend and retract modes is necessary to move or lift an object. Such other applications include, for example, moving the boom on an aerial lift truck. In addition, while the present invention is particularly useful for a hydraulic cylinder with a relatively large diameter piston rod, principles so the invention may find application with other types of hydraulic actuators. 
   The pumps  22  and  23  may be gear pumps including intermeshing gears that are sized appropriately for the particular application. Other types of pumps could also be used. For conjoint operation, one gear component of one of the pumps can be fixedly connected (such as by a coupling) to a gear component of the other of the pumps, such that all the gear components are operational together. 
   The hydraulic circuitry  14  connects the pumps  22  and  23  to one another an to the double-acting hydraulic actuator  15  that includes a cylinder  28  and a piston  29  to which a piston rod  30  is connected, the piston separating a piston-side or extend chamber  32  of the cylinder from a rod-side or retract chamber  33  of the cylinder. The pumps each have extend and retract ports  36  and  37  that provide pressure or suction depending on the movement direction of the pumps. 
   The hydraulic circuitry  14  includes a first extend circuit portion  40  for fluidly connecting the extend port  36  of the first pump  22  to the piston-side chamber  32  of the hydraulic actuator  15 , a first retract circuit portion  41  for fluidly connecting the retract port  37  of the first pump to the rod-side chamber  33  of the hydraulic actuator, an extend relief valve  42  connected to the first extend circuit portion  40  between the first pump  22  and the hydraulic actuator  15 . The extend relief valve  42  is operationally responsive to fluid pressure in the first extend circuit portion  40  for allowing flow from the first extend circuit portion to a reservoir  43  (e.g. tank) when the fluid pressure in the first extend circuit portion increases above a prescribed amount, and a retract relief valve  44  connected to the first retract circuit portion  41  and operationally responsive to fluid pressure in the first retract circuit portion for allowing flow from the first retract circuit portion to the reservoir  43  when the fluid pressure in the first retract circuit portion increases above a prescribed amount. The extend and retract relief valves may be normally closed, adjustable, spring-biased ball valves. 
   The hydraulic circuitry  14  further includes a second extend circuit portion  48  for fluidly connecting the extend port  36  of the second pump  23  to the first extend circuit portion  40 , and a second retract circuit portion  49  for fluidly connecting the extend port  37  of the second pump  23  to the first retract circuit portion  41 . The second extend circuit is connected to the first retract circuit via non-return check valve  51  whereby reverse flow from the first extend circuit portion  40  to the second extend circuit portion  48  is normally blocked while permitting flow from the second extend circuit portion to the first extend circuit portion. 
   An unloader valve  53  is connected to the second extend circuit portion  48  between the non-return check valve  51  and the extend port  36  of the second pump  23 . The unloader valve is responsive via a pilot signal line  54  to fluid pressure in first extend circuit portion  40 . The unloader valve may comprise a normally closed, spring-biased ball valve with a spool valve portion responsive to pressure applied across the pilot signal line. The unloader valve fluidly connects the second pump  23  through a second drain path  55  to the reservoir  43  when the fluid pressure in the first extend circuit portion increases above a predetermined value when the actuator is under high load conditions, such as after the stabilizer leg  20  has contacted the ground  26  and is raising the vehicle body  16  off the wheels  57 . 
   Accordingly, both pumps  22  and  23  supply fluid to the extend side of the actuator  15  until the load on the actuator causes the pressure in the first extend circuit portion  40  to increase above the set value of the unloader valve  53 . Thereafter, the output of the second pump  23  is dumped to the reservoir  43  while the first pump continues alone to supply high pressure flow to the extend side of the actuator. This will result in rapid extension of the actuator and in turn rapid lowering of the stabilizer leg until the high load condition is encountered, i.e., the stabilizer leg contacting the ground. Then, high pressure fluid is supplied to the extend side of the actuator to lift the vehicle off its wheels, albeit at a slower rate but typically over a much shorter distance than the distance traversed by the stabilizer legs during initial lowering into contact with the ground. The second pump  23  is effectively isolated from the actuator during higher loads and pressures and the first pump  22  can be driven by the prime mover  13  to higher pressures. As will be appreciated, the extend relief valve  42  will be set at a value greater than the setting of the unloader valve  53  to allow for such higher pressures being supplied to the extend side of the actuator. 
   During extension of the actuator  15 , fluid from the retract side is returned to the retract ports  37  of the pumps  22  and  23  via the first and second retract circuit portions  41  and  49 . In addition, make-up fluid is supplied from the reservoir  43  to the retract ports  37  of the pumps via a filter  55  in a branch circuit portion  56 . The make-up fluid compensates for the rod volume differential of the hydraulic actuator. That is, during extension of the actuator, the fluid volume supplied to the extend side of the actuator exceeds the fluid volume being exhausted from the retract side of the actuator because of the volume of the rod extending through the retract chamber of the actuator. 
   To maintain the piston  29  of the actuator  15  in a set position when the motor  13  is shut down, a pair of check valves  58  and  59  are provided in the extend and retract circuit portions  40  and  41 , respectively, between the pumps  22  and  23  and the actuator  15 . The valves  58  and  59  are responsive to pressure in the opposite circuit portion via pilot signal lines  60  and  61  and are held open when the pumps are operating, and closed when they are not. The check valves may be simple spring-biased ball valves. 
   As thus far described, a problem arises because of the rod volume differential. During retraction of the actuator  15 , the fluid output of the actuator far exceeds the fluid input. If one were to rely on the retract relief valve  44  to dissipate the excess fluid without creating a lot of wasted energy, the system would have to be very large. It has been discovered that dissipating the rod volume excess over the retract relief valve  44  would require the system to retract at the retract relief valve pressure setting. This would cause the load on the motor  13 , e.g. the amps drawn by a DC motor, to be very high. If one were to minimize the load, this could create the possibility that the actuator would not be able to retract if the actuator encountered high resistance, such as may occur if the stabilizer leg is stuck in mud or caught by an obstruction. 
   In accordance with the present invention, a return check valve  64  is provided between the first retract circuit portion  41  and the second retract circuit portion  49 . As a result, only the first pump  22  will generate flow to the rod (retract) side of the actuator during retraction of the actuator. Because of the small differential between the diameters of the rod  30  and piston  29  (relatively small cross-sectional area of the rod-side chamber of the actuator, the actuator will retract at a rate that can be similar to the high speed extend rate of the actuator when both pumps are supplying fluid to the extend side of the actuator. 
   In order to allow the return fluid from the retract side of the actuator to flow to the retract port of the second pump  23  during extension of the actuator, the return check valve  64  is responsive to pressure in the second extend circuit portion  48  at the extend port  36  of the second pump via a pilot signal line  66  (in an alternative arrangement the pilot signal line may supply pressure from the first extend circuit to keep the valve  64  open at all times during extension of the actuator). In this manner, the check valve  64  will be held open when the pumps are operating to extend the actuator. Otherwise, the check valve would block flow to the second pump while allowing flow in the reverse direction, although allowing make-up fluid to be drawn therethrough from the reservoir when the actuator is being extended. 
   Further in accordance with the invention, another pilot-operated valve  70  is provided to divert to the reservoir  43  the excess fluid exiting the extend side of the actuator during actuator retraction. The pilot-operated valve is closed during actuator extension. During actuator retraction, the valve will be held open by pressure in the first extend circuit portion supplied via pilot signal line  72 . This enables the rod side of the piston to see the maximum pressure permitted by the retract relief valve as may be needed if resistance is encountered during retraction, such as might occur if the stabilizer leg is stuck in the mud or encounters a tree trunk. 
   The reservoir connections  43  described above can be to a common reservoir or to different reservoirs that may be fluidly interconnected. The reservoir connections each represent a drain path for hydraulic fluid. 
   The operation of the hydraulic system  10  should be apparent from the above, but will also be briefly discussed. During operation in the extend mode of the actuator, the pumps  22  and  23  are operated conjointly to provide fluid through the extend circuit portion into the extend chamber of the actuator. Since the flow from the retract chamber of the actuator is less than the flow being provided to the extend chamber, the pressure from the actuator usually will be insufficient to activate the retract relief valve  44 , and as such, this valve  44  stays shut, with the entire flow being provided to the two pumps. As above described, the check valve  64  will be held open by pressure at the extend sides of the pumps and in particular the second pump. This allows flow from the retract chamber of the actuator to flow to the retract ports of both pumps for rapid withdrawal of the fluid from the retract chamber. Makeup flow is provided from the reservoir through a filter  43  to compensate for the smaller flow being provided from the retract chamber. 
   In the low-load, extend mode of operation, the unloader valve  53  remains closed, and flow from both pumps is provided through first and second extend circuit portions to the extend chamber of the actuator, and both pumps suck fluid from the retract chamber of the actuator. 
   In the high-load, extend mode of operation, that is, when pressure in the extend circuit portion at the extend port  36  of the first pump  22  increases above the set point of the unloader valve  53 , the unloader valve opens, and directs flow from the second pump  23  through the drain path  55  to the reservoir  43 . As a result, the second pump is effectively isolated, and only the flow from the first pump is applied to the extend chamber of the actuator. The first pump  22  can thereby be driven to higher pressures to move the actuator. 
   During the retract mode of operation, the pumps  22  and  23  are operated in the reverse mode and fluid is provided to the retract chamber of the actuator by the first pump  22 . Pressure in the first retract circuit portion will open the excess fluid bypass valve  70  thereby allowing excess fluid being exhausted from the extend side of the actuator to be dumped to the reservoir. The balance of the flow will be supplied to the extend port of the first pump  22 . The second pump is again effectively isolated, the pump drawing fluid from the reservoir  43  via a filter  76  and check valve  77 , and discharging the fluid back to the reservoir via the filter  55 . 
   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.