STRUCTURE AND FUNCTION HYDRAULIC PUMP FUNCTION 1. LS VALVE The LS valve detects the load and controls the discharge amount. This valve controls main pump discharge amount Q according to differential pressure PLS (= PP-PLS ) (the difference between main pump pressure PP and control valve out- let port pressure PLS ) [called the LS differential pressure]. Main pump pressure PP , pressure PLS [called the LS pressure] coming from the control valve output, and pressure PSIG [called the LS selec- tor pressure] from the LS-EPC valve enter this valve.
The relationship between discharge amount Q and differential pressure PLS , (the difference between main pump pressure PP and LS pressure PLS ) (= PP- PLS ) changes as shown in the diagram on the right according to LS selector current isig of the LS-EPC valve. When isig changes between 0 and 1A , the set pressure of the spring changes according to this, and the switching point for the pump dis- charge amount changes at the rated central value between 0.64 2.1 MPa {6.5 21.5 kg/cm 2 ). 2.
PC VALVE When the pump discharge pressure PP is high, the PC valve controls the pump so that no more oil than the constant flow (in accordance with the discharge pressure) flows even if the stroke of the control valve becomes larger. In this way, it carries out equal horsepower control so that the horsepower absorbed by the pump does not exceed the engine horsepower. In other words, if the load during the operation becomes larger and the pump discharge pressure rises, it reduces the discharge amount from the pump; and if the pump discharge pressure drops, it in- creases the discharge amount from the pump.
The relationship between the pump discharge pressure PP and pump discharge amount Q is shown in the diagram on the right, with the cur- rent given to the PC-EPC valve solenoid shown as a parameter. However, in the heavy-duty op- eration mode, there are cases where it is given the function of sensing the actual speed of the engine, and if the speed drops because of an increase in the load, it reduces the pump dis- charge amount to allow the speed to recover.
In other words, when the load increases and the engine speed drops below the set value, the command current to the PC-EPC valve sole- noid from the controller increases according to the drop in the engine speed to reduce the pump swash plate angle. 10-44