Abstract:
A hydraulic control system has a pilot pressure supply circuit that applies a pressure differential dependent on engine speed to opposite sides of an anti-stall valve to turn off the pilot flow when the pressure differential falls below a certain value, resulting in holding the positions of the hydraulic devices being controlled until engine speed recovers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This claims the benefit of U.S. Provisional Patent Application No. 60/518,956 filed Nov. 10, 2003. 
     
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       FIELD OF THE INVENTION  
       [0003]     This invention relates to hydraulic control systems, and in particular to such systems in which a single prime mover is used to power one or more hydraulic pumps to supply multiple hydraulic loads.  
       BACKGROUND OF THE INVENTION  
       [0004]     Many types of modern hydraulic machines, including forestry machines, excavators, earth moving machinery, and material handlers feature a variety of hydraulic functions. Each function may be controlled separately, but all may receive their energy from a single prime mover, such as a diesel or gasoline engine. Because it is very unlikely that all these functions will all demand full power simultaneously, it is common for the machine designer to select a prime mover of less power than the total possible hydraulic power consumption.  
         [0005]     The designer is faced with a dilemma in selecting the relationship between engine power and installed hydraulic power consumption. If the engine is large enough to cover even very unlikely combinations of multiple hydraulic functions, the machine will be excessively expensive to build and to operate. If the engine is too small, some combinations of multiple functions will exceed the available power. This will cause the engine to slow, or even stall. This leads to a decrease in production, increased wear on the engine, and operator frustration. This dilemma is made worse by the fact that individual machines will operate in differing conditions, with differing loads, and with operators of differing skills and expectations.  
         [0006]     Decreasing hydraulic load when engine capacity is exceeded has been tried in various forms. Most systems decrease pilot flow, destroke a variable output pump or otherwise reduce the hydraulic load when pilot pressure drops due to a decrease in engine speed and thus a decrease in the speed of the pilot pump. Other systems decrease hydraulic power demand of a main pump when secondary controls are activated, which is intended to prevent the total hydraulic load from becoming large enough to stall the engine.  
         [0007]     Typical systems decrease, without completely eliminating, the hydraulic power demanded by the pilot valves. If the manual input is held constant or increased when engine speed drops, the engine may continue to operate at a lower speed. This results in decreased fuel efficiency, increased engine wear, continued decreased hydraulic power, and may eventually lead to engine stall.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention provides a hydraulic control system that automatically reduces or eliminates the hydraulic load on the prime mover in response to the capacity of the prime mover being overcome by the hydraulic load. The invention accomplishes this in the preferred embodiment by hydraulically sensing a certain reduction in the speed of the prime mover, and in response thereto metering by relieving the pilot pressure that controls the main hydraulic functions.  
         [0009]     The invention thereby provides quick, smooth, and potentially total removal of hydraulic load on the prime mover while holding the positions of the hydraulic functions when engine speed drops, plus quick, smooth, and total return of hydraulic load and functions when the engine speed recovers. This enables a machine designer to select an engine size that will be efficient for most operations, without concerns for occasional different or combined operations to produce engine speed decreases and stalls.  
         [0010]     The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is schematic view of a hydraulic control system of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring to  FIG. 1 , a hydraulic control circuit  10  of the invention is illustrated. An engine driven pump  12  is a fixed displacement pilot pressure pump and an engine driven pump  14  is a main pump that supplies the various functions operated by the circuit  10 . As illustrated, the main pump  14  is driven by the same prime mover  16  as the pump  12 .  
         [0013]     In the position illustrated, a pilot operated three way two position anti-stall valve  20  is in its at-rest position, wherein pilot flow to the main hydraulic circuit is stopped and pilot flow from the main hydraulic circuit (from the joysticks  22 ,  24 ) is vented to tank (with valve  54  in its normal actuated position). When valve  20  is shifted leftwardly from the position illustrated, into its normal operating position, it supplies pilot flow to the joysticks  22 , 24  that are used to control the multiple hydraulic functions, one function being a hydraulic motor  26  and another being a hydraulic actuator  28 . Any number of hydraulic work devices  26 ,  28  could have been illustrated, two being adequate to describe the invention. The stall valve  20  is biased, and in the absence of pressure at port  36  sufficient to overcome the pressure at port  34  is held, in this position by the balance of spring force, which is preferably adjustable as illustrated by adjustable spring  30 . The difference in pressures applied to the two pilot pressure ports  34 ,  36  at opposite ends of the valve  20  controls the position of and opening through the valve  20 . The ports  34 ,  36  are connected to opposite sides of an orifice  38  through which passes the pilot flow from pump  12 , which is normally directed to the joysticks  22 ,  24  and over relief valve  42 . Thus, the differential pressure across orifice  38  is directly proportional to the pilot flow to the main circuit, and therefore to the speed of engine  16 .  
         [0014]     If the demanded hydraulic power (load) exceeds the available engine power (capacity), the torque demands of the main pump  14  will slow the engine  16 . This decreases the pilot flow produced by pump  12 , and thus decreases the pressure drop across the orifice  38 . When this differential pressure is no longer large enough to overcome the bias of spring  30 , the anti-stall valve  20  will switch to its at-rest position. In this position, all pilot pump  12  flow is directed to the tank  40  through relief valve  42 , and the pressure in the downstream pilot control circuits is also dumped to the tank  40 , through the damping orifice  44 . This forces all the open center multi-section valves  46 ,  48  to gradually return to the mid-position, in which the hydraulic lines from the hydraulic devices being operated are blocked and therefore held in position. This smoothly stops all hydraulic motion of the motor  26  and actuator  28 , and decreases the load on the engine  16  produced by the main pump  14 .  
         [0015]     When the engine speed recovers sufficiently, the increased pilot flow through the orifice  38  allows the pressure differential on the anti-stall valve  20  to overpower the spring  30 . The valve  30  returns to the normal position, and pilot control flow, controlled by joysticks  22 ,  24 , returns smoothly to the control valves  46 ,  48 , through the damping orifice  44 . This will modulate in this manner to signal the operator to adjust the joysticks to reduce the hydraulic demands on the prime mover and therefore return to continuous operation at near capacity.  
         [0016]     The force of the spring  30  in the anti-stall valve  20  can preferably be adjusted so as to vary the engine speed at which the valve  20  actuates from the operating to the at-rest position. An electrically operated override valve  50  is provided to allow the operator to disable the anti-stall feature, when desired by actuating the valve  50 , for example when the operator desires to run the engine at less than full speed. Electrically operated safety valves  52 ,  54  may also be provided that when in operation are on, but that relieve pilot pressure when they are off.  
         [0017]     A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiment described.