Created on:2021-09-13 10:42

Construction machinery_ Excavator hydraulic pump failure_ Control principle

Construction machinery_ Excavator hydraulic pump failure_ Control principle

The hydraulic pump used in the large excavator (dh220, 280 and other models) is K3V series swashplate axial piston pump. Its structure is a through shaft double piston pump, followed by an auxiliary pump to provide power for remote control. The main pump consists of a variable piston pump and a regulator that controls the displacement. The front and rear main pumps are equipped with a regulator to control the displacement of the front and rear pumps respectively. Constant power control, changing power setting value and flow control can be realized through the regulator.

Hydraulic pump control principle

As shown in Figure C, the displacement adjustment schematic diagram of the main (front) hydraulic pump, and the control principle of the rear pump is the same as that of the front pump. The differential cylinder 1 is used to push the inclination change of the pump swashplate, the servo valve 2 is used to control the movement direction of the variable piston, and the reversing of the servo valve is controlled by the flow control piston and the power control piston. P1 is the hydraulic oil pressure output by the pump itself, P2 is the output pressure of another pump, PF is the oil pressure output by the auxiliary pump, Pf1 is the secondary pressure output through the electromagnetic proportional reducing valve, and PI is the back pressure of the central oil return channel of the multi-way directional control valve. The displacement adjustment process of the main hydraulic pump is as follows.

C.jpg

① The small chamber of the flow control differential cylinder always outputs pressure oil through the auxiliary pump or the main pump (the higher pressure of the two), and the large chamber of the differential cylinder can be connected with the pressure oil output by the main pump or with the oil tank through the servo valve. In the non operation state, the multi-channel directional valve is in the middle position, the oil output by the main pump is returned to the oil tank through the back pressure valve through the central oil return channel, and a certain back pressure P1 is generated in front of the back pressure valve. The pressure generated by the oil with household pressure overcomes the spring force at the right end of the flow control piston, the flow control piston moves to the right and pushes the servo valve to reverse, and the servo valve works in the left position, The oil output from the main pump (pressure about P1) enters the large cavity of the differential cylinder, the differential cylinder moves to the right, the inclination of the swashplate is reduced to the minimum (but not zero, set by the limit screw), and the feedback mechanism also turns the servo valve back to the middle position. At this time, the output pressure of the main pump is low (P1) and the displacement is small, the pump is in the unloading state, and the engine can enter the automatic idle state, Fuel consumption is saved; When the multi-way directional valve reverses (the engine will automatically change from idle speed to set running speed), the central oil return channel of the multi-way valve is cut off, no oil passes through the back pressure valve, the pressure PI in front of the back pressure valve becomes zero, the flow control piston will move to the left under the action of spring force, the belt driven servo valve works in the right position, the oil in the large cavity of the differential cylinder flows through the oil tank, and the differential cylinder moves to the right, The inclination angle of the swashplate becomes larger. When the swashplate angle is the largest, the servo valve is returned to the middle position through the feedback mechanism, and then the power can be adjusted by the power control mechanism according to the load.

② The right hydraulic pressure on the constant power control power control piston is the sum of the hydraulic pressures acting on areas A1, A2 and A3, while the left force is the sum of the spring force and the hydraulic pressure of the power setting piston (area A4). Then the force balance equation of the power control piston is:

pf1 A1 +p2A2 +p1A3 =kx0 +pf1A4 (7-1)

Where k - compression coefficient of spring;

X0 -- pre compression shrinkage of spring.

The action area of hydraulic pressure and spring compression coefficient are constant. After the power is set, Pf1 and x0 are also fixed values. When the discharge pressure P1 of the pump itself or the discharge pressure P2 of another pump rises, the right force on the power control piston increases, and the balance is broken. The power control piston moves to the right, and a new balance is established after the compression of the spring increases. The higher the pressure of the main hydraulic pump, the greater the displacement of the control piston. When the control piston moves to the right, it will drive the servo valve core to move to the right. The large cavity of the differential cylinder is connected with the pressure oil output by the main pump. When the differential cylinder moves to the right, the inclination angle of the swashplate of the pump will be automatically reduced, and the displacement of the pump will also be reduced. Control the input torque of the pump below the specified value to match the power required by the pump under this pressure state with the output power of the engine without overloading the engine. When the differential cylinder moves, the feedback mechanism returns the servo valve to the middle position. In this way, different P1 and P2 pressure values correspond to different displacement. Similarly, when P1 and P2 are reduced, the displacement of the pump is adjusted in a large direction to increase the operation speed. In short, the constant power control can realize the control of light load, high speed, heavy load and low speed of the working mechanism, make full use of the output power of the engine and improve the operation efficiency.

③ An electromagnetic proportional reducing valve 5 is set on the power setting regulator. Changing the input current value command of the proportional reducing valve can change the set value of power. The secondary pressure oil of the electromagnetic proportional reducing valve is led to the power control mechanism of the regulator. Since A4 > A1, if the secondary pressure value Pf1 of the proportional reducing valve is changed, the balance point of the balance equation can be changed, so that the main pump pressures P1 and P2 when the power control piston starts to move, that is, the power setting value of the pump is changed according to the operating conditions The operation command enables the engine to obtain the best operating state (sets the power output value of the engine under different operating conditions), and the working mechanism to obtain the best operating speed.

Through the above analysis, it can be seen that the sum of the hydraulic power of the excavator dual hydraulic pump should match the engine output power. When the load changes, the displacement of the two pumps should be adjusted at the same time, otherwise, the excavator will not work normally.

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