Single Double Acting Cylinder Hydraulic Design
2023-12-11
First of all, we must realize the importance of single/double acting cylinder sealing to hydraulic technology. It can be said that there is no hydraulic technology without sealing. If the sealing is not good, the hydraulic technology is not good. The guiding principle of hydraulic seal design is to look at the seal with a line instead of a point, but you can't ignore the point, and don't underestimate the point seal. The so-called line is the sealing system. The so-called point is specific to a certain sealing position. The design principle must be a combination of lines and points; secondly, the basis for hydraulic seal design must be understood exactly. Only with an accurate understanding of the basis, comprehensive consideration, weighing the relationship between these basis and the seal design, and the relationship between these basis, can the selection of the seal or sealing device structure and the material selection of these seals and sealing devices be carried out; Should also carefully select the size of the seal, the sealing device related parts, the geometric position accuracy, the surface roughness and the gap between the parts; there should be no negligence in the above aspects, otherwise it cannot be called elaborate design, and the hydraulic seal will be given. The design leaves hidden dangers, and there will be the possibility of leakage.
(1) Principles of hydraulic seal design for single/double acting cylinder
The principle of hydraulic seal design is based on the line (sealing system) design. The point (specific seal position) design is included in the line design and should be accurate. The line and point design are inseparable and form a whole.
The line design has two meanings: for a full hydraulic device, it should be the sealing design of the entire hydraulic system. If there is a leak in any part of the system, the entire sealing system is imperfect; second, it is possible to consider sealing for a certain component It is a point, such as the seal at a certain joint in the pipeline, but if the seal in the hydraulic cylinder is considered, it must be a wired concept. Even with the consideration of the sealing system, it may be that a sealing system is formed by the sealing of several positions, the hydraulic cylinder sealing is effective, and there will be no leakage exceeding the regulation. It may not be successful if you choose the seal or the form and structure of the sealing device only according to the rated pressure of the hydraulic cylinder. If the rated pressure of the hydraulic cylinder is PN=50MPa, and the actual working pressure is higher or very high or very low, such as its working pressure PN≥50MPa or. When PN≤50MPa, the above design may fail, and leakage will occur during the whole process of the hydraulic cylinder. Of course, at PN=50MPa, the hydraulic cylinder will not leak. Each type of structure of the seal or sealing device has its own optimal pressure sealing effective area, and different types of structure of the seal or sealing device have different optimal pressure sealing effective areas. If it is not based on its rated pressure value to select a certain form of seal or sealing device, but based on its rated pressure value, and divide it from zero pressure to rated pressure pN=50MPa into several pressure areas: low pressure, Medium and low pressure, medium high pressure, high pressure, relative to its pressure section, corresponding to the selection of different forms of seals or sealing devices. It is used to select a number of different structural seals (sealing devices) to form the sealing system of the hydraulic cylinder. At this time, the hydraulic cylinder will not leak during the whole working process-regardless of low pressure, medium pressure or high pressure. Figure 1, especially the synthetic sealing characteristic curve 3 in this figure, fully illustrates: the hydraulic seal design must follow the principle of line design as the leading, accurate point design, and the combination of line and point. In other words, the design of hydraulic seal should be based on the sealing system thinking Consider the sealing performance, and design with the sealing position requirements as the starting point; that is, rationally select different types of seals or sealing devices, and optimize their combination, make full use of the sealing mechanism and characteristics of each type of seal, and adopt a series connection method to achieve a better composition The sealing system, plus the reasonable groove size, surface roughness, and matching clearance of the relevant parts. Coupled with a reasonable assembly process, the expected sealing performance and no leakage effect can be obtained.
1. Sealing curve of O-ring 2. Sealing curve of Stem Seal 3. Synthetic sealing characteristic curve
Fig.1 Schematic diagram of superposition of sealing curves of Stem seal and O-ring
(2) The basis of hydraulic seal design of single/double acting cylinder
The basis of hydraulic seal design is firstly the working conditions. The seal must meet the requirements of the working conditions. The form and structure of the seal or the sealing device and the materials made of them must correspond to the working conditions. Only in this way can it be possible to achieve the purpose of the hydraulic seal design: prevent airtightness The fluid in the cavity flows from the high pressure side to the low pressure side across the boundary of the closed cavity, which is called leakage.
Common working conditions include pressure, speed, medium, and temperature; the environmental location refers to the ocean, land, sky, tropical, temperate, and frigid zone; outdoor, indoor, clean and dusty, full of corrosive gas or only air Places, etc.
Here are a few points to be emphasized. The above items are all factors that must be considered in the design of hydraulic seals. Neglecting any of them may lead to design failure. Secondly, pressure, temperature and speed should never be understood from a static point of view. It must be dealt with from a dynamic point of view, which is sufficient for their changes as a basis for design. Therefore, the hydraulic seal designed must meet the requirements of its entire change process. For example, pressure is the main parameter value of hydraulic seal design. If the rated pressure of a hydraulic system is 40Mpa, it is sufficient as a static seal design. However, as a dynamic seal design, a certain form of structural seal is selected. Although it can ensure that the executive body in the system, such as a hydraulic cylinder, does not leak at a pressure of p=40MPa, the pressure of the system is often processed by zero pressure (sometimes this process is very short) , But always exist) is possible to reach 40MPa, then this actuator before 40MPa, it has zero pressure, low pressure, low pressure, etc., and the hydraulic cylinder may leak at low pressure, and when the hydraulic seal is designed, it is particularly dynamic For sealing, the process of changing the pressure of the liquid medium from zero pressure to rated pressure (especially pressure above medium and high pressure) should be considered. In order to ensure a good seal without leakage, it is possible to choose several different types of seals or several sets of seals Or the sealing device meets the design of hydraulic seal. For example, three-component or five-component seals are used for hydraulic cylinders internationally. The speed of work must also be treated with a changing perspective. Changes in speed affect the friction between relative moving parts, frictional resistance, wear of seals, and even start-up and many other influences. Temperature is also a variable amount, a variable parameter. It has a great influence on the sealing material in the hydraulic seal. Some hydraulic equipment will work at tens of degrees below zero Celsius and then work to tens of degrees above zero Celsius or even hundreds of degrees. Some seal materials can withstand such a large amount. Temperature change range. Although some sealing materials have many excellent properties, such as oil resistance, wear resistance, and high mechanical strength, they have low temperature resistance. For example, polyurethane seals, when the temperature exceeds 80, it will not adapt. The sealing performance Deteriorated and cannot be used. Therefore, the temperature range is also a variable parameter that must be considered in the design of hydraulic seals. In addition, although the load can be reflected by pressure, the pressure cannot fully reflect the impact of the load on the seal. Therefore, the actual load must be understood and analyzed when designing the seal. Such as the change of the value of the load, the change of the direction, their respective change amplitude and frequency, and the time occupied by the change phase, such consideration will definitely improve and perfect the reliability of the designed hydraulic seal.
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