Abstract:
A fluid system for plural motor driven pumps is disclosed. The fluid system includes a hydraulic motor, a fluid reservoir, and a plurality of fixed displacement pumps. A control valve selectively directs flow of fluid to either a reservoir or the hydraulic motor. A control system responsive to an external condition and an internal condition generates a signal for each condition. The control valve is operated by the signals. The internal signal may be generated by an internal pressure monitoring device.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of hydraulics on refuse trucks and more particularly to a hydraulic pump unloading or control system on refuse trucks. The system may have a plurality of fixed displacement pumps driven by a single engine and a valve incorporating electrical control circuitry for selectively activating diverse combinations of pumps in response to drive conditions such as engine speed, the circuit being activated, and the pump pressure during activation. 
     BACKGROUND OF THE INVENTION 
     Many hydraulic circuit configurations and combinations of valves have been devised with the purpose of modifying the hydraulic pressure or flow supply based on the power requirements of the system or based on the availability of power to drive the system. A majority of such systems use variable displacement pumps that are typically more expensive and complex than standard fixed displacement pumps. However, there are some systems that use fixed displacement pumps in combination with hydraulic control circuitry to accommodate variability in hydraulic pressure and flow supply. For example, U.S. Pat. No. 4,164,119 provides an unloading system with fixed displacement pumps that prevents stalling of the drive engine in response to flow or pressure conditions in the hydraulic line. The multiple fixed displacement pump system of U.S. Pat. No. 4,002,027 modifies flow supply by combining two pump outputs when necessary based on pressure or flow conditions in the hydraulic lines with the aim of eliminating the need for high engine and pump speed solely to supply flow requirements. Yet another system in U.S. Pat. No. 4,381,904 provides a circuit with numerous fixed displacement pumps selectively activated in response to pressure and flow conditions in the hydraulic line as a means of providing variable pressure and flow requirements. 
     The prior art in hydraulic pump unloading or control systems with a plurality of fixed displacement pumps has heretofore used pressure and/or flow response means to drive the logic of the variable flow and pressure supply. Pressure and flow responsive mechanisms in the hydraulic circuits provided a means of indirectly measuring and reacting to the power supply of the driving means of the hydraulic system. However, the addition of complex and/or numerous valving and hydraulic mechanisms to the circuits not only increases the cost of the system, but also can make precise and accurate control of the pressure and flow supply more difficult to manage and predict. 
     Some of the pump control systems have been adapted specifically to tractors or refuse equipment where fluctuating hydraulic needs are common and are further complicated because the drive speed of the pump(s) varies with the speed of the tractor or refuse truck. Often the demands on the hydraulic system are greatest when the engine speed is at its lowest because the tractor or refuse truck is at a standstill. In refuse trucks for example, it has been common to have a single fixed displacement pump to provide for the needs of the hydraulically operated packer. The packer requires a certain level of flow to function adequately and the pump must be run at high speeds to provide that flow. This requires the operator to speed up the engine of the refuse truck to drive the pump at the required speed even if the truck is at a standstill and the horsepower requirements are low. This is normally the case on a front or side loading truck where the refuse is pushed into an empty body. Pressures are low so required horsepower is also low. Some refuse trucks are equipped with variable displacement pumps to handle the changing needs of the hydraulic system and adapt to varying engine speed, but typically there are only a few operating modes and the capabilities of nearly infinite adjustment is deemed too expensive and unnecessary. Further, these types of systems require a more sophisticated mechanic to be able to troubleshoot and repair them. For example, packers and loaders are used on a refuse truck when the truck is stopped and the packer is used when the truck is moving between stops, but there are not commonly many other distinctive modes of operation. A few different operating modes of the hydraulic system would address all of the requirements for variability. 
     Another variable flow requirement typical of refuse packers is introduced with the inclusion of the telescopic cylinders that are used to drive the packer on front and side loader type refuse trucks. As the packer compresses the refuse, the telescopic cylinder extends and the demand for fluid flow is high due to the relatively large bore and considerable length of the telescopic cylinders used in this application. In addition, the pressure demand is at a high particularly at the end of the packing cycle when the refuse body is almost full. In this condition, the telescopic cylinders extend so as to sufficiently compress or pack the refuse. Even when the body is full, this only happens at the very end of the packing cycle. The first part of the cycle is used to sweep the material toward the body. This uses very little pressure. As the packer returns to its starting position the telescopic cylinder retracts and only a small fraction of the extension flow rate is needed to provide acceptable retracting rates given that the hydraulic fluid now acts on the rod end of the cylinder where most of the volume is occupied by the telescopic rods. The volume ratio for equal extension and retracting speeds can be 4:1 or higher in typical telescopic cylinders. In a typical refuse truck a single fixed displacement pump is usually selected that meets the flow rate requirements of the packer cylinder(s) in the extension cycle. As the same high flow rate is applied to retract the telescopic cylinder, the system wants the retracting speed to be 4 times (for a 4:1 volume ratio) the extension speed. This creates a problem in that it is difficult to evacuate all the fluid from the base end of the cylinder fast enough to allow such high-speed cylinder retraction. The large flows cannot be accommodated by standard lines and valving. What typically happens is that the flow out of the base end of the cylinder is therefore limited by these components. The flow going into the rod end side of the telescopic cylinder is therefore also limited. The excess flow must go over the relief valve. This flow goes over the relief valve at system pressure. Often the volume of fluid that passes over the relief valve is considerable. This generates a significant amount of heat. Common attempts to solve the problem include the inclusion of large and expensive dump valves at the base end of the cylinder, but even with those additions it is frequently not practical to allow such high-speed retraction. Many refuse trucks also include automated loading systems and constantly running packers. These also increase the overheating problem in a hydraulic circuit. The constant heat generation as the packer cylinders retract becomes an even more significant problem. 
     The present invention is directed to overcoming one or more of the problems set forth above. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention there is provided an improved pump control system for use on refuse trucks, wherein a plurality of fixed displacement pumps are selectively turned “on” or “off” in response to an external signal. 
     Another aspect provides an improved pump control system that uses an engine speed measuring device. 
     Still another aspect provides an improved pump control system, wherein a plurality of fixed displacement pumps are selectively turned “on” or “off” in response to the combination of an external signal and an internal signal. 
     Yet another aspect provides an improved pump control system that uses an engine speed measuring device and a device to measure system pressure. 
     In accordance with the present invention there is provided a fluid system for use on a refuse truck and including a hydraulic motor which may be a hydraulic ram; a fluid reservoir; a plurality of fixed displacement pumps; drive means operatively connected to the fixed displacement pumps for driving the same; a control valve for selectively directing a flow of fluid to either the reservoir or the hydraulic motor; and a control system having means for determining an external condition and generating a corresponding signal, and means responsive to the signal for switching the control valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made more particularly to the drawings, which illustrate the best presently known mode of carrying out the invention and wherein similar reference characters indicate the same parts throughout the views. 
     FIG. 1 is a hydraulic schematic of the preferred embodiment of the invention. 
     FIG. 2 is an electrical schematic of the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     As herein described, when fluid flow is being dumped, a plurality of fixed displacement pumps  12 ,  13 , and  14  are referred to as “off.” When pressure is supplied to a hydraulic motor, the fixed displacement pumps  12 - 14  are referred to as “on”. 
     Briefly, a control valve  15 ,  16  or  17  and a function valve  54  or  64  are controlled by signals external to a fluid system  1 . They may also be controlled by signals within the fluid system. The signals are responsive to operating conditions of an engine  10  which drives the fixed displacement pumps  12 - 14 , the mode of operation of the hydraulic motor in the form of a hydraulic cylinder  20  or  21 , or the pressure in the fluid system  1 . The signals external to the fluid system  1  are preferably electrical signals that shift the control valves  15 - 17  in a specific combination to achieve the desired fluid flows based on the operating speed of the engine  10 , the hydraulic cylinder  20  or  21  being actuated, and/or the pressure being generated, for example, in a packer cylinder (such as  20 ) during packing of refuse. The present embodiment measures the speed of the engine and sends an electrical signal in response to the attainment of a specific speed. In addition, pressure switches associated with a packer panel are utilized to provide an electrical signal to control which pumps  12 - 14  are “on.” The position of the hydraulic cylinder  20  or  21  is also sensed. Signals relating to engine speed, the mode of the hydraulic cylinder  20  or  21 , and the packing pressure are used to determine whether one or more of the will be turned “on” or “off” by shifting the control valve  15 ,  16  or  17 . In other words, the flow is selectively directed to either a reservoir  11  or a fluid-driven mechanism, i.e. hydraulic cylinder  20  or  21 . This dumping is done at very low pressure so as not to generate much heat. 
     Prior arrangements relied on pressure and flow responsive means integral to the fluid system  1 . In the present embodiment, with the exception of the pressure switches, the logic control of the fluid system  1  is external to the hydraulic lines. A transmission electronic control unit (not shown) is utilized to monitor engine speed and provide a corresponding signal. 
     Referring now to FIG. 1, the fluid system  1  includes the engine  10  operatively connected to the fixed displacement pumps  12 - 14 . The fluid system  1  also includes the first control valve  15  operatively connected to pump  12 , the second control valve  16  operatively connected to pump  13 , and the third control valve  17  operatively connected to pump  14 . The first control valve  15  and the second control valve  16  are operatively connected to a first hydraulic cylinder  20 , having a base end  23  and a rod end  24 , and the third control valve  17  is operatively connected to a second hydraulic cylinder  21 . For example, the first hydraulic cylinder  20  may be a telescopic cylinder, such as a packing cylinder, and the second hydraulic cylinder  21  may be a conventional hydraulic cylinder, such as a lifting cylinder. 
     The first control valve  15  includes a first solenoid-operated valve  25  which has a first solenoid  26 , a first valve position  27 , a second valve position  28 , and a first bias spring  29 . The first control valve  15  also includes a pilot operated two-way valve  30  with an open fluid passageway  32  and a closed fluid passageway  31  and a second bias spring  33 . A first check valve  35 , a first small orifice  36 , and a first control orifice  37  are also incorporated into the first control valve  15 . In the fluid system  1 , the control valve functions in like manner to turn “on” the fixed displacement pumps. By example, as the first pump  12  supplies fluid to the first control valve  15 , fluid flows through the first control valve  15  and passes through the open fluid passageway  32  of the two-way valve  30  back to the fluid reservoir  11 . Effectively, in this position the first pump  12  is “off.” Fluid also flows through the first small orifice  36  and through the second valve position  28  in the first solenoid-operated valve  25  back to the fluid reservoir  11 . The pressure drop across the first small orifice  36  is sufficiently high to hold the open fluid passageway  32  of the two-way valve  30  in position against the second bias spring  33 . When the first solenoid  26  is energized, the first solenoid-operated valve  25  shifts such that the first valve position  27  is active and the fluid flow through the first solenoid-operated valve  25  is blocked. With the flow through the first solenoid-operated valve  25  blocked, there is no flow through the first small orifice  36  and therefore no longer a pressure drop across the first small orifice  36  and therefore no pressure to hold the open fluid passageway  32  of the two-way valve  30  in position against the second bias spring  33 . The second bias spring  33  then shifts the two-way valve  30  to the closed fluid passageway  31 , blocking the flow to the fluid reservoir  11 . In this state, fluid from the first pump  12  has sufficient pressure to open a second check valve  40  and enter a first pressure line  41 . Energizing the first solenoid  26  sends pressurized fluid from the first pump  12  to the first hydraulic cylinder  20 . However, if the flow through the first control orifice  37  exceeds a predetermined level, the pressure drop across the first control orifice  37  will allow the first check valve  35  to open against a third bias spring  38 . With the first check valve  35  open, flow is restored through the first small orifice  36  and the accompanying pressure drop across the first small orifice  36  causes the two-way valve  30  to shift back so that the open fluid passageway  32  is again active and a portion of the fluid supplied by the first pump  12  returns to the fluid reservoir  11  instead of being supplied to the first pressure line  41 . The two-way valve  30  remains open with the open fluid passageway  32  active until the flow through the first control orifice  37  drops to a prescribed level and reduces the pressure drop so that the first check valve  35  is again forced closed by the third bias spring  38 . When the first check valve  35  closes the flow through the first small orifice  36  stops and the pressure drop ceases thereby allowing the second bias spring  33  to shift to the closed fluid passageway  31  of the two-way valve  30  to the active position. The two-way valve  30  will modulate in this fashion to maintain the flow supplied by the first pump  12  at or below a desired level. The second control valve  16  and the third control valve  17  operate in an identical manner to the first control valve  15 . The second control valve  16  includes a second solenoid  50  and a second solenoid-operated valve  51 . The third control valve  17  includes a third solenoid  60  and a third solenoid-operated valve  61 . Turning the pumps “on” and “off” can also be accomplished by other means well known to those skilled in the art, such as by using a “dry valve” that starves the pump of much of the hydraulic fluid. 
     Flow from the second pump  13  enters the first pressure line  41  through a third check valve  52  and the third pump  14  supplies a second pressure line  62 . The first pump  12  and the second pump  13  supply fluid flow to the first hydraulic cylinder  20  while the third pump  14  supplies fluid flow to the second hydraulic cylinder  21 . In the fluid system  1 , the fixed displacement pumps may all have the same volume capacity or various volume capacities. In the preferred embodiment, the first pump  12  is smaller than the second pump  13 , and the third pump  14  is sized to meet the needs of the second hydraulic cylinder  21 . In the preferred embodiment, the first pump  12  has a capacity of 22 gallon a minute per pump, the second pump  13  has a capacity of 35 gallon a minute per pump, and the third pump  14  has a capacity of 31 gallon a minute per pump. The first pressure line  41  feeds a first function valve  54  that includes a third fluid passageway  55 , a fourth fluid passageway  56 , and a fifth fluid passageway  57 . A first relief valve  53  may allow for fluid to return to the fluid reservoir  11  in the event that pressure levels in the first pressure line  41  exceed certain levels. When the first function valve  54  shifts so that fifth fluid passageway  57  is active, the first hydraulic cylinder  20  extends; when the third fluid passageway  55  is active the first hydraulic cylinder  20  retracts. With the fourth fluid passageway  56  active, the first function valve  54  is in the neutral position and fluid flow returns to the fluid reservoir  11 . 
     The second pressure line  62  feeds a second function valve  64  that includes a sixth fluid passageway  65 , a seventh fluid passageway  66 , and a eighth fluid passageway  67 . A second relief valve  63  may allow for fluid to return to the fluid reservoir  11  in the event that pressure levels in the second pressure line  62  exceed certain levels. When the second function valve  64  shifts so that the eighth fluid passageway  67  is active, the second hydraulic cylinder  21  retracts; when the sixth fluid passageway  65  is active, the second hydraulic cylinder  21  extends. With the seventh fluid passageway  66  active, the second function valve  64  is in the neutral position and fluid flow returns to the fluid reservoir  11 . Control valves  15 - 17 , and function valves  54  and  64  may be combined into a single valve block in any combination. 
     Referring now to FIG. 2, the fluid system  1  includes an electrical circuit  100  that accompanies and controls the fluid system  1 . In the preferred embodiment the electrical circuit  100  includes an extend switch  102 , a retract switch  103 , a power switch  125 , a first relay coil  105 , a second relay coil  106 , a third relay coil  107 , a fourth relay coil  108 , a fifth relay coil  109 , a sixth relay coil  110 , a high speed switch  136 , a mid-speed switch  135 , a normally closed low pressure switch  137 , and a normally closed high pressure switch  138 . The extend switch  102  and the retract switch  103  may be combined in the form of a single double pole, double throw switch to keep the operator from actuating both of them at the same time. Corresponding respectively to each relay coil are first relay contacts  115 , second relay contacts  116 , third relay contacts  117 , fourth relay contacts  118 , fifth relay contacts  119 , and sixth relay contacts  120 . All relay contacts are normally open contacts in the preferred embodiment. A fourth solenoid  122  shifts the first function valve  54  to extend the first hydraulic cylinder  20 . A fifth solenoid  123  shifts the first function valve  54  to retract the first hydraulic cylinder  20 . A first diode  130  and a second diode  131  are included in the electrical circuit  100  to maintain proper function of the preferred embodiment. Energizing the first solenoid  26  effectively turns “on” the first pump  12 , whereas energizing the second solenoid  50  effectively turns “on” the second pump  13 . Energizing the third solenoid  60  effectively turn “on” the third pump  14 . When the normally closed high pressure switch  138  is activated, the first solenoid  26  is de-energized effectively turning “off” the first pump  12 . When the normally closed low pressure switch  137  is actuated, the second solenoid  50  is de-energized shutting “off” the second pump  13 . The normally closed low pressure switch  137  also causes the first solenoid  26  to be energized even if the normally closed high pressure switch  138  is activated. 
     The preferred embodiment of the invention has low, middle, and high-speed conditions, as well as low, medium and high pressure conditions. It is to be understood that the electrical circuit  100  can be simplified by reducing the number of inputs or that better matching of the output horsepower to the available horsepower could be accomplished by increasing the number of pressure and speed inputs. Any number of operation modes can be created by the addition of inputs and the addition of more pumps and the appropriate modifications to the electrical circuit  100 . 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
                 H 
                 I 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Condition: 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Under 1200 RPM 
                 X 
                 X 
                 0 
                 0 
                 0 
                 X 
                 X 
                 0 
                 0 
               
               
                 1200-1800 RPM 
                 0 
                 0 
                 X 
                 X 
                 0 
                 0 
                 0 
                 X 
                 X 
               
               
                 Over 1800 RPM 
                 0 
                 0 
                 0 
                 0 
                 X 
                 0 
                 0 
                 0 
                 0 
               
               
                 Extend on 
                 X 
                 0 
                 X 
                 0 
                 0 
                 X 
                 X 
                 X 
                 X 
               
               
                 Retract on 
                 0 
                 X 
                 0 
                 X 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Under 1500 psi 
                 X 
                 X 
                 X 
                 X 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 1500-2300 psi 
                 0 
                 0 
                 0 
                 0 
                 0 
                 X 
                 0 
                 X 
                 0 
               
               
                 Over 2300 psi 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 X 
                 0 
                 X 
               
               
                 Pump Conditions: 
               
               
                 Pump 12 
                 X 
                 X 
                 0 
                 X 
                 0 
                 0 
                 X 
                 0 
                 X 
               
               
                 Pump 13 
                 X 
                 0 
                 X 
                 0 
                 0 
                 X 
                 0 
                 X 
                 0 
               
               
                 Pump 14 
                 X 
                 X 
                 0 
                 0 
                 0 
                 X 
                 X 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     Table 1 shows nine of the conditions of the present invention and the corresponding condition of the first pump  12 , the second pump  13 , and the third pump  14 . An “X” in the chart means the function is active or that the pump is “on.” An “O” in the chart means that the function is not active or that the pump is “off.” The following describes the electrical circuit  100  in condition A of Table 1. In this condition, the first pump  12  and the second pump  13  pressurize the first pressure line  41  and the third pump  14  pressurizes the second pressure line  62 . The pressurization of the first pressure line  41  allows the first hydraulic cylinder  20 , such as a telescopic cylinder, to be actuated. The telescopic cylinder may be used to push a packer panel to compact refuse into a refuse body. The pressurization of the second pressure line  62  allows the second hydraulic cylinder  21 , such as an automated lift, to be actuated. 
     Closing the power switch  125  activates the electrical circuit  100  of the preferred embodiment. With the system active and the engine  10  in the low speed range, the mid-speed switch  135  is closed, the sixth relay  110  is energized and the sixth relay contacts  120  close to energize the third solenoid  60  in the third control valve  17  turning “on” the third pump  14  to the second pressure line  62 . With pressurized flow available at the second pressure line  62 , the second hydraulic cylinder  21  can be operated. When the extend switch  102  is closed with the engine  10  still at low speed, the first relay coil  105 , the third relay coil  107 , and the fourth relay coil  108  are energized and the first relay contacts  115 , the third relay contacts  117 , and the fourth relay contacts  118  are closed. Closing the first relay contacts  115  also energizes the fifth relay coil  109  and closes the fifth relay contacts  119 . With the first relay contacts  115  and the third relay contacts  117  closed, the second solenoid  50  becomes energized and turns “on” the second pump  13 . When the first relay contacts  115  and the fifth relay contacts  119  are closed, electrical power reaches the first solenoid  26  which turns “on” the first pump  12 . Closing the first relay contacts  115  provides power to the fourth solenoid  122 , which shifts the first function valve  54  so that the fifth fluid passageway  57  is active and the first pressure line  41  is directed to the base end  23  of the first hydraulic cylinder  20  and the first hydraulic cylinder  20  extends. For example, fluid would enter the fifth fluid passageway  57  go through the first pressure line  41  enter the base end of a telescopic cylinder, extending the packer panel. Flow from both the first pump  12  and the second pump  13  are combined in the first pressure line  41  to maximize fluid flow while the engine  10  is at low speed. At low speed the fixed displacement pumps deliver minimal flow and it is advantageous to combine the flows of all pumps available. 
     Condition B of Table 1 allows for the retraction of the first hydraulic cylinder  20 . The following describes Condition B after extending the first hydraulic cylinder  20  as provided in Condition A. The retract switch  103  is closed and the extend switch  102  opens. The sixth coil relay  110  is still energized at low engine speed and the sixth relay contacts  120  are closed to provide power to the third solenoid  60  turning “on” the third pump  14  so that the second hydraulic cylinder  21  can be used. Closing the retract switch  103  energizes the second relay coil  106 , the third relay coil  107 , and the fourth relay coil  108  which closes the second relay contacts  116 , the third relay contacts  117 , and the fourth relay contacts  118 . With the second relay contacts  116  and the fourth relay contacts  118  closed, the first solenoid  26  is energized turning “on” the first pump  12  to pressurize the first pressure line  41 . The second pump  13  is not “on” in this mode of operation as the electrical circuit  100  does not energize the second solenoid  50 . When the second relay contacts  116  are closed, the fifth solenoid  123  is energized and the first function valve  54  shifts the third fluid passageway  55  to the active position such that from the first pressure line  41  is directed to the rod end  24  of the first hydraulic cylinder  20  and the first hydraulic cylinder  20  retracts. For example, fluid would enter the third fluid passageway  55  go through the first pressure line  41  enter the rod end  24  of a telescopic cylinder, retracting the packer panel. Only flow from the first pump  12  is used to retract the first hydraulic cylinder  20 . For example if the first hydraulic cylinder  20  were a telescopic cylinder, the volume needed at the rod end  24  of the telescopic cylinder is much smaller than that at the base end  23  to achieve an adequate rate of travel, thus only a single small volume pump is required. 
     Condition C of Table 1 is when the speed of the engine  10  increases to a middle speed, such as over 1200 RPM. The mid-speed switch  135  opens in response to a signal from the transmission electronic control unit (not shown) that directly monitors engine speed, and the sixth relay coil  110  is de-energized. When the sixth relay coil  110  is de-energized, the sixth relay contacts  120  open so that power is no longer supplied to the third solenoid  60 , turning “off” the third pump  14 . When the third pump  14  is turned “off,” the second hydraulic cylinder  21  is inactive. This prevents the second hydraulic cylinder  21  from inadvertently operating when going above a low speed. In alternate embodiments of the invention, various functions or series of functions could be turned on or off at certain speed ranges as desired. With the engine  10  at middle speed, the extend switch  102  is closed and the first relay coil  105 , the third relay coil  107 , and the fourth relay coil  108  are energized to close the first relay contacts  115 , the third relay contacts  117 , and the fourth relay contacts  118 . With the first relay contacts  115  and the third relay contacts  117  closed, power is available to the second solenoid  50  which turns “on” the second pump  13  to pressurize the first pressure line  41 . In this mode power is not supplied to the first solenoid  26  and, therefore, the first pump  12  remains inactive. The fourth solenoid  122  is also energized and shifts the first function valve  54  so that the fifth fluid passageway  57  is active and fluid flows to the first hydraulic cylinder  20  causing it to extend. 
     Condition D of Table 1 is retracting the first hydraulic cylinder  20  at mid-speed. The sixth relay coil  110  is not energized because the mid-speed switch  135  is open. Therefore the sixth relay contacts  120  are open and the third solenoid  60  is not energized so the third pump  14  remains “off.” Closing the retract switch  103  energizes the second relay coil  106 , the third relay coil  107 , and the fourth relay coil  108  which closes the second relay contacts  116 , the third relay contacts  117 , and the fourth relay contacts  118 . With the second relay contacts  116  and the fourth relay contacts  118  closed, the first solenoid  26  is energized turning “on” the first pump  12  to pressurize the first pressure line  41 . The second pump  13  is not “on” in this mode of operation as the electrical circuit  100  does not energize the second solenoid  50 . Closed second relay contacts  116  energize the fifth solenoid  123  and the first function valve  54  shifts the third fluid passageway  55  to the active position such that flow from the first pressure line  41  is directed to the rod end  24  of the first hydraulic cylinder  20  and the hydraulic cylinder  20  retracts. 
     Condition E is the high speed condition. In this condition both mid-speed switch  135  and high-speed switch  136  are open. In this condition the first relay coil  105 , the second relay coil  106 , the third relay coil  107 , the fourth relay coil  108 , the fifth relay coil  109 , and the sixth relay coil  110  are not energized and their corresponding relay contacts are open, therefore the first pump  12 , the second pump  13 , and the third pump  14  are “off.” This is important for going down the road as the valving and piping for handling the flows at low and medium speeds cannot handle the high flows generated at high speed. 
     Condition F of Table 1 is similar to Condition A except that the load pressure is now over 1500 psi. In Condition A, the first solenoid  26 , the second solenoid  50 , and third solenoid  60  were all actuated turning “on” their respective pumps. For Condition F, the normally closed low pressure switch  137  opens and de-energizes the first solenoid  26 , turning “off” the first pump  12 . Because the required power is a function of pressure and flow, in this condition and as the pressure is increased, the flow is decreased to maintain a workable output power, without adding heat to the system by sending fluid to the fluid reservoir at high pressure. 
     Condition G of Table 1 is when the load pressure in the first hydraulic function is over 2300 psi. The power switch  125  is closed and the mid-speed switch  135  is closed. This powers the sixth relay  110  which closes the sixth relay contacts  120  and energizes the third solenoid  60  which turns “on” the third pump  14 . When the operator actuates the extend switch  102 , the first relay coil  105 , the third relay coil  107 , the fourth relay coil  108 , and the fifth relay coil  109  are energized closing the first relay contacts  115 , the third relay contacts  117 , the fourth relay contacts  118 , and the fifth relay contacts  119 . The fourth solenoid  122  is energized as current flows through the first relay contacts  115 . The normally closed low pressure switch  137  and the normally closed high pressure switch  138  are both in the actuated position. Current flows through the third relay contacts  117  to the open contact of the normally closed low pressure switch  137  to the first solenoid  26 , which turns “on” the first pump  12 . With the normally closed low pressure switch  137  actuated the second solenoid  50  is not energized and the second pump  13  remains “off.” 
     Conditions H and I are the same as Conditions F and G respectively, except that the speed is in the midrange. This opens the mid-speed switch  135  de-energizing the third solenoid  60  turning “off” the third pump  14 . 
     It is understood that hydraulic and electrical circuits can be configured in numerous ways and that the logic of the preferred embodiment can take many specific forms without departing from the scope and general principles of the present invention. The scope of the invention should be derived from the following claims rather than the foregoing description.