Abstract:
A tandem fixed displacement pump circuit with torque control includes first and second pumps that draw fluid from a reservoir to supply respective load circuits, pressure relief valves for each of the load circuits, and a pilot operated sequence valve that monitors the pressure for each of the circuits and reduces the pressure of one of the circuits as the pressure in the other circuit rises. The sequence valve modulates to control the pressure in the secondary priority circuit so that a predetermined input torque value is not exceeded.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of hydraulics. More particularly, this invention relates to a tandem fixed displacement pump with a torque control circuit associated therewith. 
     Tandem fixed displacement pumps in general are well known in the hydraulic art. Tandem fixed displacement pumps can be useful in an open circuit arrangement where they supply pressurized fluid to two different load circuits while being driven by a single source of rotary power or driveline. For example, one of the pumps of a tandem fixed displacement pump can be used to operate the loader bucket (boom and tilt functions) on a small utility tractor or skid steer loader and the other pump can be used to operate various “high flow” attachments, including but not limited to road planers, brush cutters, or post hole augers. 
     Due to a variety of potentially constraining factors, including but not limited to engine size, couplings, and input shaft strength, there is a limited torque capacity available to the tandem fixed displacement pump. Problems can result when high pressure is demanded by both of the load circuits at the same time. Some means and method of coordinating the pressure requirements in each of the load circuits would be desirable to reduce the input torque required so that it does not exceed the maximum allowable torque. 
     Therefore, a primary objective of the present invention is the provision of a tandem fixed displacement pump with a torque control circuit that keeps the available torque capacity from being exceeded. 
     Another objective of this invention is the provision of a tandem fixed displacement pump system that provides priority pressure to a primary (high flow) load circuit and a secondary flow to a second load circuit when the available torque of the driveline permits. 
     Another objective of this invention is the provision of a tandem fixed displacement pump with torque control that is economical to manufacture, simple to adjust, reliable, and durable in use. 
     These and other objectives will be apparent from the drawings, as well as from the description and claims that follow. 
     SUMMARY OF THE INVENTION 
     The present invention relates to hydraulics, more particularly, a hydraulic circuit that includes a tandem fixed displacement pump used to supply fluid to a primary priority load circuit and a secondary priority load circuit. The tandem fixed displacement pump circuit with torque control includes first and second fixed displacement pumps that draw fluid from a reservoir to supply respective load circuits, pressure relief valves for each of the load circuits, and a pilot operated sequence valve that monitors the pressure for each of the circuits and reduces the pressure of one of the circuits as the pressure in the other circuit rises. The sequence valve modulates to control the pressure in the secondary priority circuit so that a predetermined input torque value is not exceeded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a hydraulic schematic depicting the present invention. 
     FIG. 2 is a graph showing loader pressure and input torque required by the fixed displacement pump as a function of primary or high flow pressure, both with and without this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a tandem fixed displacement pump circuit  10  according to this invention. An input shaft  12  powered by a conventional engine (not shown) drives a tandem fixed displacement pump  14 . The tandem fixed displacement pump  14  includes a first gear pump  14 A and a second gear pump  14 B. The first and second pumps  14 A,  14 B have fixed volumetric displacements per revolution of the input shaft  12 . 
     In the example shown in FIG. 1, the first pump  14 A has a greater displacement than the second pump  14 B. The first pump  14 A connects to and draws hydraulic fluid from a tank or reservoir  16 A. The pump  14 A pressurizes the fluid and delivers it through a fluid passageway or hydraulic line  18  to a primary priority or high flow circuit control valve  20 . In a small utility tractor or skid steer loader, the control valve  20  controls the attachments including but not limited to road planers, brush cutters, or post hole augers. An adjustable variable pressure relief valve  22  connects to the line  18  so as to adjustably limit the pressure of the fluid supplied to the control valve  20 . As the attachments are operated, the pressure in the line  18  tends to rise. 
     The second pump  14 B connects to and draws hydraulic fluid from a tank or reservoir  16 B. The pump  14 B pressurizes the fluid and delivers it through a fluid passageway or hydraulic line  24  to at least one secondary priority or low flow circuit control valve  26 . In a skid steer loader or small utility tractor, the low flow circuit control valve(s)  26  operate the boom and tilt functions of the loader bucket. An adjustable variable pressure relief valve  28  connects to the line  24  so as to adjustably limit the pressure of the fluid supplied to the control valve  26 . As the bucket is moved and loaded, the pressure in line  24  tends to increase. As is well known in the art, it is desirable to have check valves (not shown) in the secondary control valve  26  or the circuitry downstream of the valve  26  so that the bucket does not drop at low pressures. 
     A pilot line  30  connects to the line  18  upstream of the relief valve  22 . A similar pilot line  32  connects to the line  24  upstream of the relief valve  28 . The pilot lines  30  and  32 , illustrated by dashed lines in FIG. 1, join at a line  34  (also dashed) that is operatively connected to one end of a sequence valve  36 . 
     The sequence valve  36  is also connected to line  24  upstream of both the relief valve  28  and pilot line  32  by a fluid passageway or line  38 . The valve  36  is a normally closed, two-position, two-port valve that is urged closed by a spring  39  whose force is adjustable in a conventional manner. The valve  36  has a first or closed position in which the flow of fluid from the second pump  14 B through the line  38  to a tank or reservoir  40  is blocked. In the closed position (on the left in FIG.  1 ), fluid also cannot flow to the tank  40  from the pilot lines  30 ,  32 ,  34 . The valve  36  has a second position (on the right in FIG. 1) in which fluid can flow through the valve  36  to the tank  40  when the pressure in the pilot line  34  acting on the valve  36  exceeds the opposing spring force exerted by the spring  39 . 
     Orifices  42  and  44  are provided in the lines  30 ,  32  respectively to ensure pressure drops in the lines. Preferably the diameters of the orifices  42 ,  44  are equal, thus the pressure seen in line  34  is an average of the pressures in lines  30  and  32 . An optional damping orifice  46  is also provided in the third pilot line  34  between the sequence valve  36  and the junction of the first and second pilot lines  30 ,  32 . Preferably the orifices  42 ,  44 ,  46  are fixed orifices. The particular diameter of the orifices  42 ,  44 ,  46  can be selected to give the desired response characteristics. 
     In one example found to work well on a rubber tracked loader, 0.025 inch was selected as the diameter of orifices  42 ,  44 , relief valves  22 ,  28  were set at 3400 psig, and the sequence valve was designed to open at 2200 psig at 1500 rpm engine speed and 2375 at 2500 rpm. A 0.030-inch diameter damping orifice  46  was used. 
     In operation, FIG. 2 shows the results both with and without the invention on the rubber tracked loader discussed above. Refer to the legend on FIG.  2 . The maximum torque available under any condition is 2400 in-lb. With the invention, the pressure in the secondary priority or low flow loader circuit is reduced when the primary priority or high flow circuit pressure increases, so that the total torque to drive both pumps  14 A,  14 B is limited to the predetermined level. When the high flow circuit is at low pressure, the loader pressure is allowed to go to the higher, normal setting of the relief valve  28 . The pilot pressure operated sequence valve  36  accomplishes the pressure reduction. Because the orifices  42 ,  44  are of equal size, the sequence valve  36  is piloted by the average of the pressures in the outlet lines  28 ,  24  of the two fixed displacement pumps  14 A,  14 B, respectively. 
     The invention gives a lower loader pressure as the high flow pressure increases, thus limiting the total input torque required. The slope of the resulting input torque curve is low, making the curve relatively flat or slowly rising. The input torque stays below the predetermined level for the entire operating range of high flow pressure. This represents a significant improvement over the typical results achieved in a conventional rubber tracked loader without the invention. See FIG.  2 . Without the pilot operated sequence valve  36  and the associated lines and orifices, the torque demanded quickly exceeds the maximum torque available. The input torque exceeds 2400 in-lbs. after only about 2100 psig of high flow pressure in the graph of FIG.  2 . Excess torque beyond this point can break input shafts or couplings and could conceivably stall smaller engines. 
     Thus, it can be seen that the present invention at least accomplishes its stated objectives. 
     One skilled in the art will appreciate that a common tank or reservoir can be used for the tanks or reservoirs  16 A,  16 B, and  40  without detracting from the invention. 
     Although a gear pump has been discussed in the above description of the preferred embodiment, other types of fixed displacement pumps, including those with gerotors or vanes, would suffice for this invention. 
     In the drawings and specification, there has been set forth a preferred embodiment of the invention, and although specific terms are employed, these are used in a generic and descriptive sense only and not for purposes of limitation. Changes in the form and the proportion of parts as well as in the substitution of equivalents are contemplated as circumstances may suggest or render expedient without departing from the spirit or scope of the invention as further defined in the following claims.