Created on:2021-04-13 09:34

Constant power variable and power matching variable

Constant power variable and power matching variable

d. The control mode of constant power variable and power matching variable power matching (also known as power adaptation) can make the working pressure and output flow of the variable pump adapt to the demand of the system at the same time, so as to improve the efficiency of the system.

Figure R (a) shows the variable mechanism of a variable displacement pump with constant power (with servo amplification). The servo slide valve 2 is installed in the variable piston 1, so the servo slide valve and the variable piston form a direct position feedback relationship. The high pressure oil from the pump outlet enters the lower chamber a of the variable piston through the check valve 9, and is led to chamber D and chamber C of the slide valve through the hole B in the variable piston [Fig. R (c)]. The pressure bearing area of the control piston in chamber D is (π / 4) (d 2-D 2), and the hydraulic pressure on the pressure bearing area is balanced with the spring force on the servo slide valve. If the hydraulic pressure is greater than the spring force, the slide valve moves up, and the oil in the upper chamber of the variable piston flows through the port F and then through the opened control valve port and the central hole h to the oil return chamber, so the pressure in the upper chamber of the piston decreases. When the pressure in the upper chamber of the variable piston drops to pl ≤ P (A2 / A1) (A1 and A2 are the pressure bearing areas of the upper and lower surfaces of the variable piston respectively), the variable piston moves up and pushes the variable mechanism to make the pump variable.

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Two springs 4 and 5 are arranged on the control slide valve, in which the inner spring 4 is not compressed in the initial position. At the beginning of the upward movement of the control spool, only the outer spring 5 functions. The hydraulic pressure acting on the slide valve is balanced with the elastic force of spring 5. When the slide valve moves over a certain distance, the spring 4 begins to be compressed, and the sum of the two spring forces is balanced with the hydraulic pressure. Because of the above two springs, the flow pressure characteristic of the pump is the broken line shown in Figure s. By properly selecting the slope and intercept of lines AB and 6C in figure s (i.e. the stiffness and pre compression of springs 4 and 5), the flow pressure characteristic curve of the pump can be approximated to hyperbola, that is, PQ = const (constant), which means that the output power of the pump is approximately unchanged, so it is called constant power mechanism.

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Figure t shows the principle of a power matching control mode. It consists of hydraulic power matching valve 1, electro-hydraulic proportional valve 2, shuttle valve 3 and variable control piston 4. The function of the shuttle valve is to make the control oil pressure to the power matching valve always be the load pressure PL when the variable control piston moves to the left or right. If the pressure loss of the proportional valve is set to △ P0, when the power matching valve is in balance, the working pressure Ps of the pump is equal to the sum of the load pressure pl of the hydraulic cylinder or the hydraulic motor (hydraulic cylinder in the figure) and the pressure loss △ P0 of the proportional valve. When the load pressure changes, the working pressure of the pump also changes accordingly, but it always matches the demand of the load and does not produce a great pressure loss. The pressure loss of the proportional valve is set by the compression force of the power matching valve spring. If the pressure loss of the proportional valve is greater than the set △ P0 value, it means that the flow through the proportional valve is too large, then the power matching spool is unbalanced and moves to the left, and the control variable controls the piston to reduce the displacement of the pump. Finally, the pressure loss of the proportional valve returns to △ P0, and the flow through the proportional valve returns to the set value. By continuously changing the working current of the proportional valve to control the opening of the valve port, the output flow of the pump can be continuously and proportionally controlled to meet the needs of the system. This control mode does not produce overflow, and the output pressure and flow of the pump always match the requirements of the hydraulic system, so the system has high efficiency, small energy loss and system heating, and remarkable energy-saving effect, which is especially suitable for high-power hydraulic system.

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