The hydraulic transmission system is an integral part of hydraulic machinery, and the design of the hydraulic transmission system should be carried out simultaneously with the overall design of the main engine. When starting the design, we must start 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 work, low cost, high efficiency, simple operation and convenient maintenance.
1.1 Design steps
There is no strict sequence for the design steps of the hydraulic system, and the steps are often interleaved with each other. Generally speaking, after clarifying the design requirements, proceed roughly as follows.
1) Determine the form of hydraulic actuators;
2) Carry out working condition analysis to determine the main parameters of the system;
3) Develop basic schemes and draw up hydraulic system schematic diagrams;
4) Select hydraulic components;
5) Performance check calculations for hydraulic systems;
6) Draw work diagrams and prepare technical documents.
1.2 Clear design requirements
Design requirements are the basis for each project design. Before formulating the basic plan and further embarking on the design of each part of the hydraulic system, the design requirements and other aspects related to the design must be clearly understood.
1) Overview of the main engine: use, performance, process flow, working environment, overall layout, etc.;
2) What actions are to be completed by the hydraulic system, the sequence of actions and their interlocking relationship;
3) The motion form and speed of the hydraulic drive mechanism;
4) The size and nature of the load of each action mechanism;
5) The performance requirements for speed range, smoothness of motion, conversion accuracy, etc.;
6) Requirements for automation procedures and operation control methods;
7) For dustproof, explosion-proof, Requirements for cold resistance, noise, safety and reliability;
8) Requirements for efficiency and cost.
3.1 Formulate the basic plan
(1) Formulate the speed control plan After the
hydraulic actuator is determined, the control of its movement direction and movement speed is the core issue of drafting the hydraulic circuit.
The direction control is realized by reversing valve or logic control unit. For general hydraulic systems with small and medium flow, most of the required actions are achieved through the organic combination of reversing valves. For hydraulic systems with high pressure and large flow, the logic combination of cartridge valve and pilot control valve is now used to achieve.
Speed control is achieved by changing the input or output flow of hydraulic actuators or using the volume change of the sealed space. The corresponding adjustment methods include throttling speed regulation, volumetric speed regulation and a combination of the two-volumetric throttle speed regulation.
Throttle speed regulation generally uses a quantitative pump to supply oil, and a flow control valve is used to change the flow of input or output hydraulic actuators to adjust the speed. This kind of speed control method has a simple structure. Because this system must use a flash valve, it has low efficiency and large heat generation, and is mostly used in occasions with low power.
Volume speed regulation is to achieve the purpose of speed regulation by changing the displacement of the hydraulic pump or hydraulic motor. The advantage is that there is no overflow loss and throttling loss, and the efficiency is higher. But in order to dissipate heat and supplement leakage, an auxiliary pump is required. This speed regulation method is suitable for hydraulic systems with high power and high movement speed.
Volume throttling speed regulation generally uses a variable pump to supply oil, and a flow control valve is used to adjust the flow of input or output hydraulic actuators, and make the amount of oil supply compatible with the amount of oil required. This kind of speed control loop has higher efficiency and better speed stability, but its structure is more complicated.
Throttle speed regulation has three forms: oil inlet throttling, oil return throttling and bypass throttling. The start-up impact of the oil inlet throttle is small, the oil return throttle is often used in occasions with load, and the bypass throttle is mostly used for high speed.
Once the speed control loop is determined, the loop form is also determined.
Throttle speed regulation generally adopts an open loop form. In an open system, the hydraulic pump sucks oil from the oil tank. After the pressure oil flows through the system to release energy, it is discharged back to the oil tank. The open circuit has a simple structure and good heat dissipation, but the oil tank is large and easy to mix in air.
Volume speed control mostly adopts closed loop form. In a closed system, the suction port of the hydraulic pump directly communicates with the oil discharge port of the actuator, forming a closed loop. Its structure is compact, but its heat dissipation conditions are poor.
(2) When formulating 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 the throttle speed control system, The oil is usually supplied by a quantitative pump, and the required pressure is adjusted with an overflow valve and kept 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, sometimes high-pressure oil with a small flow is required. At this time, you can consider using a booster circuit to obtain high pressure instead of a single high-pressure pump. When hydraulic actuators do not need to supply oil for a certain period of time in the working cycle, and it is inconvenient to stop the pump, consider choosing an unloading circuit.
When the working pressure needs to be lower than the main oil source pressure in a certain part of the system, consider using a pressure reducing circuit to obtain the required working pressure.
(3) Develop a sequential action plan. The sequential actions of each actuator of the host are based on different types of equipment. Some operate according to a fixed program, while others are random or artificial. The operating mechanism of construction machinery is mostly manual, which is generally controlled by manual multi-way reversing valve. The sequential action of each actuator of the processing machinery mostly adopts stroke control. When the working part moves to a certain position, the electric stroke switch sends an electric signal to the electromagnet to push the solenoid valve or directly depress the stroke valve to control the continuous action. The stroke switch is more convenient to install, and the stroke valve needs to be connected to the corresponding oil circuit, so it is only suitable for occasions where the pipeline connection is more convenient.
There are also time control, pressure control and so on. For example, when a hydraulic pump starts without load, after a period of time, when the pump is running normally, the delay relay sends an electric signal to close the unloading valve and establish a normal working pressure. Pressure control is mostly used in machine tools with hydraulic fixtures, extruder presses and other occasions. When a certain actuator completes a predetermined action, the pressure in the circuit reaches a certain value, and an electrical signal is sent through a pressure relay or a sequence valve is opened to allow pressure oil to pass to start the next action.
(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 throttle speed control system generally uses a fixed pump for oil supply. In the absence of other auxiliary oil sources, the oil supply of the hydraulic pump should be greater than the oil demand of the system. The excess oil flows back to the tank through the overflow valve, and the overflow valve At the same time, it controls and stabilizes the oil source pressure. Most of the volumetric speed control system uses a variable pump to supply oil, and a safety valve is used to limit the maximum pressure of the system.
In order to save energy and improve efficiency, the oil supply of the hydraulic pump should match the flow required by the system as much as possible. In the case where the oil volume required by the system differs greatly at each stage of the working cycle, multi-pump oil supply or variable pump oil supply is generally used. In the case of small flow required 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, the inlet of the pump should be equipped with a coarse filter, and the oil entering the system will be filtered again through the corresponding fine filter according to the requirements of the protected component. In order to prevent the impurities in the system from flowing back to the fuel tank, a magnetic filter or other types of filters can be installed on the oil return path. According to the environment of the hydraulic equipment and the requirements for temperature rise, measures such as heating and cooling must also be considered.
3.2 Drawing the hydraulic system diagram
The hydraulic system diagram of the
whole machine is composed of the planned control circuit and hydraulic source. When each circuit is combined with each other, the redundant components should be removed, and the system structure should be simple. Pay attention to the interlocking relationship between the components to avoid misoperation. To minimize the energy loss link. Improve the 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.
The key parts of large-scale equipment should be attached with equipment parts so that it can be quickly replaced in the event of an accident to ensure continuous work.
Each hydraulic component should use domestic standard components as far as possible, and the figure should be drawn according to the normal position of the hydraulic component function symbol specified by the national standard. For self-designed non-standard components, structural schematic diagrams can be used to draw.
The system diagram should indicate the name and action of each hydraulic actuator, indicate the serial number of each hydraulic component and the code of each electromagnet, and attach the action table of the electromagnet, stroke valve and other control components.