(1) Operation overview The negative control signal pressure of the central bypass oil channel works under the following conditions, and controls the discharge of the pump.
① There is no neutral state.
② The pilot joystick is not fully operated.
As shown in Figure 1-33, the hydraulic oil of the pump Pl flows into the oil return pipe 9 through the central bypass passage 3, the passage 4 and the hole 7 (in the thimble). Because of the throttling of the hole, the channel is loaded. When the negative control signal pressure F1 passes through the negative control circuit to the pump Pl regulator, the pressure is transferred. The negative control signal controls the pump regulator to reduce the discharge of the pump P1.
As shown in Figure 1-34, the negative control relief valve includes a thimble 2, a spring 3 and a housing. If the pressure of the channel reaches the force of the spring, the hydraulic oil level of the central bypass pipe increases, and the hydraulic oil flow of the channel pushes the thimble into the oil return channel.
Without load, hydraulic oil flows into the tank through the channel, hole and return channel. In this way, the pressure FL of the channel becomes the maximum (FLu), because all the discharged oil volume reduced through the oil hole will reduce the stroke of the pump Pl to minimize the inclination angle, so the discharge volume of the pump Pl also reaches the minimum.
(2) Negative control of modern excavator (with fine adjustment measurement)
As shown in Figure 1-35, in the case of negative control with fine-tuning measurement, if the pilot control lever of the boom is operated with a slight amount, the pilot pressure XAb1 moves the boom spool to the left part. In this way, the channel is partially opened, the central bypass pipe is slightly closed, and the hydraulic oil is thus diverted.
One part flows through the oil hole through the channel, and the other part flows into the parallel channel, the channel and the gate Ab1. The flow rate of the channel decreases through the oil hole, and the pressure F of the channel also decreases slightly. When the pressure F decreases, the discharge flow of the pump P2 increases. As the pilot control lever moves, more flow of the channel is closed due to the movement of the spool, and then the flow of the central bypass pipe is zero. The pressure in the channel is zero, and the discharge volume of the pump P2 becomes maximum.
Precise movement of the brake is achieved by adjusting the pump discharge amount by micro-operation of the pilot control lever.
The negative control working principle of the pump Pl is also applied to the oil hole.
