【正文】 vide hydraulic balance in the same way as the pump. Both these pumps are constantdisplacement units.The delivery or capacity of a vanetype pump in gallons per minute cannot be changed without changing the speed of rotation unless a special design id used. Figure shoes a variablecapacity slidingvane pump. It does not use dual suction and discharge ports. The rotor runs in the pressurechamber ring, which can be adjusted so that it is offcenter to the rotor. As the degree of offcenter or eccentricity is changed, a variable volume of fluid is discharged. Figure shows that the vanes create a vacuum so that oil enters through 180 of shaft rotation. Discharge also takes place through 180 of rotation . There is a slight overlapping of the beginning of the fluid intake function and the beginning of the fluid discharge.Figure shows how maximum flow is available at minimum working pressure. As the flow decreases to aminimum valve, the pressure increases to the maximum. The pump delivers only that fluid neended to replace clearance floes resulting from the usual slide fit in circuit ponents.A relief valve is not essential with a variabledisplacementtype pump of this design to protect pumping mechanism. Other conditions within the circuit may dictate the use of a safety or relief valve to prevent localized pressure build up beyond the usual working levels.For automatic control of the discharge, an adjustable springloaded governor is design to protect pumping mechanism. Other conditions within the circuit may dictate the use of a safety or relief valve to prevent localized pressure build up beyond the usual working levels.For automatic control of the discharge, an adjustable springloaded governor is used. This governor is arranged so that the pump discharge acts on a piston or inner surface of the ring whose movement is opposed by the spring. If the pump discharge pressure rises above that for which the by governor spring is set, the spring is pressed. This allows the pressurechamber ring to move and take a position that is less off center with respect to the roter. The pump then delivers less fluid, and the pressure is established at the desired leval. The discharge pressure for units of this design varies between 100 and 2500psi.The characteristics of a variabledisplacementpump pensator are shown in Figure. Horsepower input values also shown so that the power input requirements can be accurately puted. Variablevolume vane pumps are capacity of multiplepressure levels in apredetermined pattern. Twopressure pump controls can provide an efficient method of unloading a circuit and still hold sufficient pressure available for pilot circuits.The black area of the graph of Figure shoes a variablevolume pump maintaining a pressure of 100psi against a closed circuit. Wasted power is the result of pumping oil at 100psi through an unloading or relief valve to maintain a source of positive pilot pressure. Twopressuretype controls include hydraulic, pilotoperated types and solenoidcontrolled, pilotoperated types. The minus of Figure shoes the solenoid energized so that the pilot oil is diverted to the tank. Thus, the pilot oil obtained from the pump discharge cannot assist the governor spring. Minimum pressure will result. The plus Figure shows the solenoid energized so that oil assists pensator spring. The amount of assistance is determined by the small ball and spring, acting as a simple relief valve. This provides the predetermined maximum operating pressure.Another type of twopressure system employs what is termed a differential unloading governor. It is applied in a highlow or twopump to a minimum deadhead pressure setting. Deadhead pressure refers to a specific pressure level established ad a resulting action and the resulting flow at deadhead condition are equal to the leakage in the system and pilotcontrol flow requirements. No major power movement occurs at this time, even though the hydraulic system may be providing a clamping or holding action while the pump is in deadhead position.The governor is basically a hydraulically operated, twopressure control with a differential piston that allows plete unloading when sufficient external pilot pressure is applied to pilot unload port.The minimum deadhead pressure setting is controlled by the main governor spring A. the maximum pressure is controlled by the reliefvalve adjustment B. the operating pressure for the governor is generated by the largevolume pump and enters through orifice C.To use this device let us assume that the circuit require a maximum pressure of 1000psi, which will be supplied by a 5gpm pump. It also needs s large flow (4gpm) at pressure up to 500psi。 it continues to 1000psi at the reduced flow rate. A twopumped system with an unloading governor on the 40gpm pump will provide the needs.We can unload the 40gpm pump at 500psi to a minimum pressure setting of 200psi (or another desired value), which the 5gpm pump takes the circuit up to 1000psi or more.Note in Figure that two sources of pilot pressure are required. One, the 40gpm pump, provides pressure within the housing so that maximum pressure setting can be obtained. The setting of the spring, plus the pressure within the governor housing, determines the maximum pressure capacity of the 40gpm pump. The second pilot source is the circuit proper, which will go to 1000psi. This pilot line enters the governor through orifice D and acts on the unloading piston E. The area of piston E is 15 percent greater than the effective area of the relief poppet F. The unloading differential built into this governor control is, therefore, approximately 15 percent. The governor will unload at 500psi and be activated at 15 percent below 500psi, or 425psi. By unloading, we mean zero flow output of the 40gpm pump.As pressure in the circuit increase from zero to 500psi, the pressure within the governor housing also increases until the reliefvalv