The control valve has two types of relief valves to limit the pressure in the oil circuit.
(1) Main safety valve
The main safety valve is used to limit the pressure of the main hydraulic system.
As shown in Figure 1-45, the hydraulic oil of pump Pl and pump P2 enters the control valve through ports P1 and P2, respectively. Directly enter the main relief valve through the check valve CMR1 or CMR2 and the channel. The pressure setting of the channel is limited by the main relief valve.
The working principle of the main relief valve is as follows.
As shown in Figure 1-46, when the pressure of the channel is less than the pressure set by the main relief valve, the ejector pin 10 is fixed, and the hydraulic oil of the channel will not flow to the oil return port. When the pressure of the channel approaches the set pressure, the ejector pin 10 is pushed away and presses the spring 11. The amount of oil in the oil chamber leaks to the oil return port through the channel, and the pressure decreases. At the same time, because the pressure drops when passing through the hole 7, the hydraulic oil of the channel flows into the channel, and then, because the pressure is discharged when passing through the channel, the pressure of the spring chamber 3 decreases. The pressure of the channel pushes the plunger 8 to the right against the spring 9. Then, the plunger 8 opens, and the hydraulic oil of the channel enters the oil return port and maintains the set pressure value. The pressure setting is adjusted by adjusting screw 12.
(2) Modern excavator oil port relief valve
Used to limit the impact pressure of the cylinder oil circuit.
As shown in Figure 1-47, the oil port relief valve and supplementary valve are installed between the cylinder port of the working equipment (boom, arm and bucket) and their spool. Because the spool is in the neutral position, external force Acting on the cylinder rod, the cylinder pressure may be too large, so the port relief valve limits the pressure setting of the valve.
Oil port relief valve also has the function of supplementary valve. In the case where an external force acts on the cylinder, it may happen that the amount of fuel supplied is insufficient to match the speed of the cylinder. If this happens, there may be vacuum and cavitation. To prevent this from happening, the make-up valve works to cut off the vacuum created by the return to the cylinder. Add oil to the cylinder from the hydraulic tank.
The hydraulic oil between the cylinder and the valve core flows into the channel to pressurize the port relief valve. The hydraulic oil of the channel flows into the spring chamber 3 through the channel. If the pressure is lower than the set pressure, the ejector pin 2 is closed because the force of the spring is greater than the pressure. Therefore, the channel and the spring chamber maintain the same pressure. Because the pressure added by the spring chamber to the base and plunger is greater than the pressure of the passage, the valve seat and plunger are pushed to the left to fix, and then the hydraulic oil of the passage will not leak into the oil return passage.
① Oil port overflow function. As shown in Figure 1-48, when the pressure of the channel is increased to the set pressure, the thimble is pushed against the spring. Due to the reduced pressure, the hydraulic oil in the oil port chamber flows into the oil return channel through the channel. Because of the pressure of the channel, the plunger moves to the left and stops at the end of the plug.
The hydraulic oil of the channel flows into the oil chamber and the spring chamber through the channel 6. The hole between the outer diameter chamber of the plunger end and the inner diameter chamber of the plunger produces different pressures acting on the channel, so the pressure of the spring becomes lower, so that the plunger is pushed to the right due to the opening of the channel, so the hydraulic oil of the channel Flow into the oil return channel.
② Supplementary function. The hydraulic oil at the cylinder rod (top) end leaks from the port relief valve because the vacuum occurs at the cylinder rod (top) end and the hydraulic oil needs to be supplied. When there is a vacuum in the channel, a vacuum also occurs through the spring chamber of the channel. The pressure in the oil return channel acts on the machine base. Because the spring chamber and plunger are under vacuum, the machine base moves to the right through the oil return press. The hydraulic oil in the oil return channel flows into the channel to cut off the vacuum of the channel.
