STRUCTURE, FUNCTION, & MAINTENANCE STANDARD HYDRAULIC PUMP 12 Function (1) LS valve The LS valve detects the load and controls the dis- charge amount. This valve controls main pump discharge amount Q according to differential pressure PLS (= PP - PLS ) [called the LS differential pressure] (the difference between main pump pressure PP and control valve outlet port pressure PLS ). Main pump pressure PP , pressure PLS {called the LS pressure} coming from the control valve output, and pressure Psig {called the LS selector pressure} from the proportional solenoid valve enter this valve.
The relationship between discharge amount Q and differ- ential pressure PLS , (the difference between main pump pressure PP and LS pressure PLS ) (= PP - PLS ) changes as shown in the diagram at the right according to LS pressure 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 selec- tor point for the pump discharge amount changes at the rated central valve between 0.69 2.2 MPa {7 22 kg/cmІ}.
(2) PC valve When the pump discharge pressure PP1 (self-pres- sure) and PP2 (other pump pressure) are high, the PC valve controls the pump so that no more oil than the constant flow (in accordance with the discharge pres- sure) flows even if the stroke of the control valve becomes larger. In this way, it carries out equal horse- power 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 increases the discharge amount from the pump.
The relationship between the average of the front and rear pump dis- charge pressures (average discharge amount of F, R pumps ( PP1 + PP2)/2) and pump discharge amount Q is shown on the right, with the current given to the PC-EPC valve solenoid shown as a parameter. The controller senses the actual speed of the engine, and if the speed drops because of an increase in the load, it reduces the pump discharge 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 solenoid from the controller increases according to the drop in the engine speed to reduce the pump swash plate angle.
10-36 PC210/240-7K