There are four main functions of the relief valve for hydraulic excavators:
1) It is used in the quantitative pump (gear pump) hydraulic system, and it is used to overflow the excess oil to the tank to maintain the hydraulic system pressure constant (approximately constant).
2) Used in the variable pump hydraulic system, only when the pressure exceeds a certain preset pressure value, the overflow is opened, so that the system pressure no longer rises, and plays a role in safety protection. .
3) In the system, it is used to protect the actuator (such as oil cylinder) or structural parts from being damaged under the action of strong external forces. In this case, the overflow valve is called an overload valve.
4) Act as a pressure limiting valve to buffer and limit the pressure of the rotary mechanism.
Commonly used relief valves are direct-acting and pilot-operated. Direct-acting is used for low pressure and pilot-operated for medium and high pressure.
3. Reducing valve Relief valve relief valve is to use outlet oil pressure and spring force balance to achieve pressure control. The outlet oil pressure of the valve is lower than the inlet pressure, so that there are several branches with different pressures in the same hydraulic system. The pilot control valve in the hydraulic pilot circuit of the hydraulic excavator is a pressure relief type pilot control valve.
4. Directional Control Valve 3.4.1 Check Valve Check Valve Symbol: Oil can only flow from A to BA B
4, 2 directional valve The directional valve is a valve that changes the direction of oil flow by means of the relative movement between the valve core (also known as the valve stem) and the valve body. According to the movement mode of the valve core, the directional valve can be divided into rotary valve type and slide valve type. According to different operation methods, the directional valve can be divided into manual, motorized, electric, hydraulic and other forms. The operation mode of the directional valve (multi-way valve) of the excavator is manual and hydraulic (pilot operated). According to the number of working positions and the number of channels, the directional valve has two positions, three positions and multiple positions, two-way The difference between multi-pass and multi-pass.
Commonly used directional valve symbols:
As shown in the figure, the thick line box represents the working position of the directional valve, and one box represents one position. For example, the two-position valve has two working positions, and the three-position valve has three working positions; the thin line in the thick line box represents the pipeline Connection mode, the arrow indicates the flow direction, and the horizontal line indicates the cutoff. A working position with several external interfaces is called a several-way valve.
Figure 3.4.2-1 (a) is the working principle of a directional valve of a multi-way valve: this is a three-position six-way, pilot control valve, when the spool is in the neutral position, P, T, A, B ports They are not connected to each other. The function of this reversing valve is "0" type. The boom, arm and bucket reversing valve of the excavator adopt this "0" type function. Its function is to cut off the connection with the oil pump and the oil tank. The passage of the oil and close the oil passage of the cylinder. When the pilot control oil enters from port b and pushes the spool to move to the left, the connection of the reversing valve oil path is shown in Figure 3.4.2-1 (b). Port P is connected to port B, and port T is connected to port A. When the pilot control oil enters from port b and pushes the spool to the right, port P and port A communicate with port T and port B.
Figure 3.4.2-1 (c) When the spool is in the neutral position, the A, B and T ports are connected. The function of this type of directional valve is "Y" type. "Type function.
4.3 Reversing performance of reversing valve
(1) Working reliability: reliable commutation, reliable reset (spring reset type). When the reversing thrust is insufficient, the reversing cannot be performed normally.
(2) Reversal resistance:
Hydrodynamics: It is an important part of commutation resistance.
Spring force: (for reset).
Hydraulic clamping force: An effective measure to eliminate the hydraulic clamping force is to open several equalizing grooves on the surface of the valve core.
Friction.
(3) Pressure loss: related to opening amount, flow rate, flow channel shape, roughness and turning.
(4) Internal leakage: related to fit clearance, seal length, working pressure, oil viscosity, etc.
(5) Others: commutation impact, commutation frequency, commutation time, service life.
