Design Hydraulic Systems
The hydraulic transmission system is an integral part of the hydraulic machinery. The design of the hydraulic transmission system must be carried out at the same time as the overall design of the host machine. When starting the design, we must proceed from the actual situation, organically combine various transmission forms, give full play to the advantages of hydraulic transmission, and strive to design a hydraulic transmission system with simple structure, reliable operation, low cost, high efficiency, simple opertion and convenient maintenance.
Design Steps
There is no strict sequence in the design steps of the hydraulic system, and each step is often interspersed with each other. Generally speaking, after clarifying the design requirements, proceed roughly as follows.
1) Determine the form of hydraulic actuator;
2) Carry out working condition analysis and determine the main parameters of the system;
3) Develop a basic plan and draw up a schematic diagram of the hydraulic system
4) Select hydraulic components
5) Performance calculation of hydraulic system
6) Draw working drawings and prepare technical documents
Clarify design requirements
There is no strict sequence in the design steps of the hydraulic system, and each step is often interspersed with each other. Generally speaking, after clarifying the design requirements, proceed roughly as follows.
1) Determine the form of hydraulic actuator;
2) Carry out working condition analysis and determine the main parameters of the system;
3) Develop a basic plan and draw up a schematic diagram of the hydraulic system
4) Select hydraulic components
5) Performance calculation of hydraulic system
6) Draw working drawings and prepare technical documents
7) Requirements for dust protection, explosion protection, cold protection, noise, safety and reliability
8) Requirements for efficiency, cost, etc.
Develop a Basic Plan
3.1 Develop a speed adjustment plan
After the hydraulic actuator is determined, the control of its movement direction and speed is the core issue in formulating the hydraulic circuit.
Directional control is achieved using directional valves or logic control units. For general hydraulic systems with small and medium flow rates, the required actions are mostly achieved through an organic combination of directional valves. For hydraulic systems with high pressure and large flow, the logical combination of cartridge valve and pilot control valve is often used to achieve this.
Speed control is achieved by changing the input or output flow of the hydraulic actuator or utilizing the volume change of the sealed space. The corresponding adjustment methods include throttling and speed regulation, volumetric speed regulation and the combination of the two - volumetric throttling and speed regulation.
Throttle speed regulation generally uses a quantitative pump to supply oil, and a flow control valve is used to change the flow rate of the input or output hydraulic actuator to adjust the speed. This speed regulation method has a simple structure. Since this system must use a flash valve, it has low efficiency and high heat generation. It is mostly used in situations with low power.
Volumetric speed regulation achieves the purpose of speed regulation by changing the displacement of a hydraulic pump or hydraulic motor. The advantage is that there is no overflow loss and throttling loss, and the efficiency is high. But in order to dissipate heat and compensate for leakage, an auxiliary pump is required. This speed regulation method is suitable for hydraulic systems with high power and high movement speed.
Volumetric throttling speed regulation generally uses a variable pump to supply oil, and a flow control valve to adjust the flow rate of the input or output hydraulic actuator, and make the oil supply amount adapt to the oil demand. This type of speed control loop is also more efficient and has better speed stability, but its structure is more complex.
Throttle speed regulation has three forms: oil inlet throttling, return oil throttling and bypass throttling. The inlet throttling has a small starting impact, the return throttling is often used in situations with negative loads, and the bypass throttling is mostly used in high-speed speed control circuits. Once the speed regulation plan is determined, the circulation form of the loop will also be determined.
Throttle speed regulation generally adopts an open cycle form. In an open system, the hydraulic pump draws oil from the tank, and the pressurized oil flows through the system to release energy and then is discharged back to the tank. The open circuit has a simple structure and good heat dissipation, but the fuel tank is large and air is easily mixed in.
Volumetric speed regulation mostly adopts closed cycle form. In a closed system, the suction port of the hydraulic pump is directly connected to the oil discharge port of the actuator, forming a closed circulation loop. Its structure is compact, but its heat dissipation conditions are poor.
3.2 Develop a pressure control plan
When the hydraulic actuator is working, the system is required to maintain a certain working pressure or work within a certain pressure range, and some require multi-stage or stepless continuous pressure adjustment. Generally, in a throttling and speed regulating system, oil is usually supplied by a quantitative pump. Use the relief valve to adjust the required pressure and keep it constant. In the volumetric speed control system, a variable pump is used to supply oil, and a safety valve is used for safety protection.
In some hydraulic systems, high-pressure oil with a small flow rate is sometimes required. In this case, a booster circuit can be used to obtain high pressure instead of a separate high-pressure pump. When the hydraulic actuator does not need oil supply for a certain period of time during the working cycle, and it is inconvenient to stop the pump, it is necessary to consider selecting an unloading circuit.
When the working pressure in a certain part of the system needs to be lower than the main oil source pressure, a pressure reducing circuit should be considered to obtain the required working pressure.
3.3 Develop sequential action process
The sequential actions of each actuator of the host machine vary according to the type of equipment. Some operate according to fixed procedures, while others are random or artificial. The control mechanisms of construction machinery are mostly manual, and are generally controlled by manual multi-way reversing valves. The sequential actions of each actuator of processing machinery are mostly controlled by stroke. When the working part moves to a certain position, an electrical signal is sent through the electric stroke switch to the electromagnet to push the solenoid valve or directly press the stroke valve to control the subsequent actions. The travel switch is more convenient to install, while the travel valve needs to be connected to the corresponding oil circuit, so it is only suitable for occasions where pipeline connection is more convenient.
There are also time control, pressure control, etc. For example, a hydraulic pump starts without load. After a period of time, when the pump operates normally, the delay relay sends an electrical signal to close the unloading valve and establish normal working pressure. Pressure control is mostly used in machine tools with hydraulic clamps, extruder presses, etc. When an actuator completes a predetermined action, the pressure in the circuit reaches a certain value, and an electrical signal is sent through the pressure relay or the sequence valve is opened to allow pressure oil to pass through to start the next action.
3.4 Select hydraulic power source
The working medium of the hydraulic system is completely provided by the hydraulic source, and the core of the hydraulic source is the hydraulic pump. The throttling and speed regulating system generally uses a quantitative pump to supply oil. In the absence of other auxiliary oil sources, the oil supply volume of the hydraulic pump must be greater than the oil demand of the system. The excess oil flows back to the oil tank through the overflow valve. The overflow valve At the same time, it plays the role of controlling and stabilizing the oil source pressure. Most volumetric speed control systems use a variable pump to supply oil, and a safety valve to limit the maximum pressure of the system.
In order to save energy and improve efficiency, the oil supply volume of the hydraulic pump should try to match the flow required by the system. For situations where the amount of oil required by the system varies greatly in each stage of the working cycle, multi-pump oil supply or variable pump oil supply is generally used. For situations where the required flow rate is small for a long time, an accumulator can be added as an auxiliary oil source.
The oil purification device is indispensable in the hydraulic source. Generally, a coarse filter is installed at the inlet of the pump, and the oil entering the system is filtered again through the corresponding fine filter according to the requirements of the protected components. In order to prevent impurities in the system from flowing back to the oil tank, a magnetic filter or other types of filters can be installed on the oil return line. According to the environment in which the hydraulic equipment is located and the temperature rise requirements, heating, cooling and other measures should also be considered.
3.5. Draw hydraulic system diagram
The hydraulic system diagram of the whole machine is composed of the planned control circuit and hydraulic source. When combining each circuit, redundant components should be removed and the system structure should be simple. Pay attention to the interlocking relationship between various components to avoid malfunctions. Energy loss links should be minimized. To improve the working efficiency of the system, in order to facilitate the maintenance and monitoring of the hydraulic system, necessary detection components (such as pressure gauges, thermometers, etc.) should be installed in the main sections of the system.
Key parts of large equipment should be equipped with equipment parts so that they can be quickly replaced when an accident occurs to ensure continuous operation of the main equipment. Each hydraulic component should use domestic standard parts as much as possible, and the diagram should be drawn according to the normal position of the functional symbols of hydraulic components stipulated in national standards. For self-designed non-standard components, structural schematic diagrams can be used to draw them.
