The overflow valve is used for hydraulic excavator with four main functions:
1) It is used in the hydraulic system of the constant displacement pump (gear pump) to keep the pressure of the hydraulic system constant (approximately constant) by overflowing excess oil to the oil tank.
2) It is used in the hydraulic system of variable displacement pump. Only when the pressure exceeds a preset pressure value, the overflow valve will be opened, so that the system pressure will no longer rise and play a role in safety protection. At this time, the overflow valve is often called a safety valve.
3) In the system, it is used to protect the actuator (such as oil cylinder) or structural parts from damage under strong external force. In this case, the overflow valve is called overload valve.
4) It is used as a pressure limiting valve to buffer and limit the pressure of the rotary mechanism.
There are two kinds of commonly used overflow valves: direct acting valve and pilot operated valve. The direct acting valve is used for low pressure and the pilot operated valve is used for medium and high pressure.
3. Relief valve relief valve uses the balance of outlet oil pressure and spring force to realize 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 reducing valve type pilot control valve.
4. Directional control valve 3.4 1 check valve check valve symbol: oil can only flow from a to Ba and B
4. 2 reversing valve reversing valve is a kind of valve that changes the flow direction of oil by means of the relative movement between the valve core (also known as the valve rod) and the valve body. According to the movement mode of the spool, the directional valve can be divided into rotary valve type and slide valve type. According to different operation modes, the reversing valve can be divided into manual, motorized, electric, hydraulic and other forms. The directional valve (multi-way valve) of Yuchai excavator is operated manually and hydraulically (pilot operated). According to the number of working positions and channels, the directional valve can be divided into two position, three position and multi position, two-way, three-way and multi-way.
Common reversing valve symbols:
The thick wireframe indicates the working position of the reversing valve, and one box indicates 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 indicates the connection mode of the pipeline, the arrow indicates the flow direction, the horizontal line indicates the cut-off, and several external interfaces in a working position are called several way valves.
(a) The working principle of a directional valve for a multi-channel valve: This is a three position six way pilot control valve. When the valve core is in the middle position, ports P, t, a and B are not connected to each other. The function of this directional valve is "0". The directional valve for the boom, stick and bucket of the excavator adopts this "0" function. Its function is to cut off the channel connected with the oil pump and oil tank and close the oil circuit of the oil cylinder. When the pilot control oil enters from port B to push the valve core to move to the left, the oil circuit of the reversing valve is connected as shown in figure 3.4.2-1 (b), port P is connected with port B, and port t is connected with port a; When the pilot control oil enters from port B and pushes the valve core to move to the right, port P is connected with port a and port t is connected with port B.
(c) When the valve core is in the middle position, ports a, B and T are connected. The function of this directional valve is "Y" type, and this "Y" type function is mostly used for the travel directional valve of excavator.
4.3 reversing performance of reversing valve
(1) Working reliability: reliable commutation and reliable return (spring return type). When the reversing thrust is insufficient, the normal reversing is not possible.
(2) Reversing resistance:
Hydraulic power: it is an important part of reversing resistance.
Spring force: (for resetting).
Hydraulic clamping force: the 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 volume, flow velocity, channel shape, roughness and turning.
(4) Internal leakage: it is related to fitting clearance, sealing length, working pressure, oil viscosity, etc.
(5) Others: commutation impact, commutation frequency, commutation time and service life.
