Special lecture on mechanical equipment: “Pumps” – Learn a little every day (6) O-rings “Improve yourself””

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This post was last edited by A’Dai A’Mu on 2013-12-12 at 12:36. In order to encourage more active participation in discussions and a more diligent approach to learning, the top five users each time will receive a “Diligent Learning Reward.” There are also series of lectures on mechanical equipment, with one focusing on “pumps”; these posts summarize the knowledge learned each day and are updated regularly: http://bbs.hcbbs.com/thread-1256318-1-1.html. Some people might think that it’s terrifying to spend their working days doing nothing useful, performing menial tasks without knowing what to do or what to learn, day after day, year after year, with no vision for their future We must start working hard from now on; even if we learn a little knowledge every day, it will add up over time and help us enrich ourselves. We need to set clear goals – only by filling our minds first can we enrich other aspects of our lives as well. . . . . . It only takes a short ten-odd minutes per day, and you will see the future! ! ! (Anyway, I have learned it; whether you learn it or not is up to you. Besides those experts like Hai Chuan, there are still others who know it.) . . ) Enough talk – let’s start with the basics today!! Today we learn about *O-rings (Part 1): An overview of O-rings and their sealing principle. An O-ring, also known as an O-shaped rubber seal, is a rubber ring with a circular cross-section. The O-ring is the most widely used type of seal in hydraulic and pneumatic systems. O-rings have excellent sealing properties and can be used for both static and dynamic sealing ； It can be used alone, and it is also a fundamental component in many combined sealing devices. It has a wide range of applications; with the right material selection, it can meet the requirements of various media and different operating conditions. An O-ring is a type of extrusion seal. The basic working principle of extrusion seals is that the seal element undergoes elastic deformation, thereby creating a contact pressure at the sealing surface. If this contact pressure is greater than the internal pressure of the medium to be sealed, no leakage occurs; otherwise, leakage takes place. (II) Compression ratio and current stretching amount The O-ring is a typical extrusion-type seal. The compression ratio and extension amount of the O-ring’s cross-sectional diameter are key aspects of seal design, and they are of great significance for the sealing performance and service life. The excellent sealing performance of O-rings depends to a large extent on the proper matching between the size of the O-ring and the size of the groove, thereby achieving an appropriate degree of compression and stretching of the O-ring. 　　2.1. Compression Ratio The compression ratio W is generally expressed by the following formula: W = (d0 – h) / d0 × 100% Where d0 is the cross-sectional diameter of the O-ring in its free state (in mm); h is the distance between the bottom of the O-ring groove and the surface to be sealed (i.e., the groove depth), which corresponds to the cross-sectional height of the O-ring after compression (in mm). When selecting the compression ratio for an O-ring, the following three factors should be considered: 1. There must be a sufficient sealing contact area ； 　　2. Minimize friction as much as possible ； 　　3. Try to avoid permanent deformation as much as possible. 　　It is not difficult to see from these factors that there are contradictions among them. A high compression ratio allows for a high contact pressure, but an excessively high compression ratio will undoubtedly increase sliding friction and permanent deformation. On the other hand, if the compression ratio is too low, it may lead to leakage as some of the compression is lost due to errors in the coaxiality of the sealing groove and issues with the O-rings. Therefore, when selecting the compression ratio of the O-ring, various factors need to be taken into consideration. Generally, the compression ratio of static seals is higher than that of dynamic seals, but this value should be less than 25%; otherwise, the compressive stress is significantly reduced, leading to excessive permanent deformation, which is particularly severe under high-temperature conditions. 　　The selection of the compression ratio W for O-ring seals should take into account the operating conditions, whether it is a static seal or a dynamic seal ； Static seals can be further divided into radial seals and axial seals ； The leakage gap of a radial seal (also known as a cylindrical static seal) is a radial gap, while the leakage gap of an axial seal (also known as a planar static seal) is an axial gap. Axial seals are divided into two types based on whether the pressure medium acts on the inner or outer diameter of the O-ring: those subjected to internal pressure and those subjected to external pressure. An increase in internal pressure causes stretching, while a decrease in external pressure reduces the initial stretching of the O-ring. For the various forms of static seals mentioned above, the direction in which the sealing medium acts on the O-ring differs, which is why the pre-pressure design also varies. For dynamic seals, it is necessary to distinguish between reciprocating motion seals and rotating motion seals. 　　1. Static seal: Similar to reciprocating motion seals, for cylindrical static seal devices, W is generally set at 10%–15% ； For planar static sealing devices, W is set at 15%–30%. 　　2. For dynamic seals, there are three possible situations ； For reciprocating motion, W is generally set at 10%–15%. When selecting the compression ratio for rotary motion seals, the Joule heating effect must be taken into account. Generally, the inner diameter of an O-ring used for rotary motion should be 3%-5% larger than the shaft diameter, while the compression ratio W of the outer diameter is 3%-8%. O-rings for low-friction motion are generally chosen to have a low compression ratio in order to reduce frictional resistance, with W ranging from 5% to 8%. In addition, the expansion of the rubber material caused by the medium and temperature must also be taken into account. Typically, beyond the specified compressive deformation, the maximum allowable expansion rate is 15%; exceeding this limit indicates that the selected material is inappropriate, and an O-ring made of another material should be used, or the specified compressive deformation rate should be adjusted. 　　2.2 Stretch amount Generally, an O-ring experiences a certain degree of stretch after being inserted into the sealing groove. Similar to the compression ratio, the extent of stretching also has a significant impact on the sealing performance and service life of O-rings. A large degree of stretching not only makes it difficult to install the O-ring, but also reduces the compression ratio due to changes in the cross-sectional diameter d0, thereby causing leakage. The stretching amount a can be expressed by the following formula: α=(d+d0)/(d1+d0), where d is the shaft diameter (mm) ； d1----Inner diameter of the O-ring (mm). 　　The range for the stretching amount is 1%-5%. The table shows the recommended values for the elongation of O-rings; depending on the shaft diameter, the appropriate elongation value for the O-ring can be selected from the table. Preferred range for compression ratio and extension amount of O-ring; sealing type; sealing medium; extension amount α (%); compression ratio w (%) – Static sealing, hydraulic oil: 1.03–1.04, 15–25; air