【Mechanical Design】It may look simple, but the design is not – The design of O-rings and their grooves

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O-ring sealing is a typical extrusion-type seal. The compression and stretching rates of the O-ring’s cross-sectional diameter are key aspects of sealing design, and they are of great significance for sealing performance and service life. O-rings are generally installed in sealing grooves to provide sealing. The excellent sealing performance of an O-ring 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. When designing and manufacturing sealing devices, if the compression amount of the O-ring is too small, leakage will occur ； Excessive compression can lead to rubber stress relaxation in the O-ring, resulting in leakage. Similarly, excessive stretching of the O-ring during operation can also accelerate aging and lead to leakage. Countries around the world have relatively strict regulations on this. 1. Design principles for O-ring sealing 1) Compression ratio The compression ratio W is generally expressed by the following formula: W = (do – h)/do% Where do is the cross-sectional diameter of the O-ring in its free state (mm), and h is the distance between the bottom of the O-ring groove and the surface to be sealed, that is, the cross-sectional height of the O-ring after compression (mm). When selecting the compression ratio of the O-ring, three factors should be considered: a. there must be a sufficient sealing contact area, b. friction should be kept as low as possible, and c. permanent deformation should be avoided as much as possible. It is not difficult to see from these factors that there are contradictions among them. A high compression ratio can result in 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, the required amount of compression may not be achieved due to errors in the coaxiality of the sealing groove and issues with the O-rings, which can lead to leakage. 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 30% (depending on the rubber material); otherwise, the compressive stress decreases significantly, leading to excessive permanent deformation, a problem that is particularly severe under high-temperature conditions. The selection of the compression ratio W for the O-ring seal 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. Internal pressure causes stretching, while external pressure reduces the initial stretching of the O-ring. For the various forms of static seals mentioned above, the direction of the force exerted by the sealing medium on the O-ring differs, which results in different designs for the preload. For dynamic seals, it is necessary to distinguish between seals for reciprocating motion and those for rotational motion. 1. Static seal: Similar to reciprocating motion seals, for cylindrical static seal devices, W is generally set at 10%~15% ； For the planar sealing device, W is set at 15%–30%. 2. For dynamic seals, three situations can be distinguished: a. For reciprocating motion seals, W is generally set at 10%–15%. b. For rotary motion seals, the Joule heating effect must be taken into account when selecting the compression ratio. Generally, the inner diameter of the O-ring used for rotary motion should be 3% to 5% larger than the shaft diameter, while the compression ratio W for the outer diameter is 3% to 8%. c. O-rings for low-friction motion: To reduce frictional resistance, a lower compression ratio is generally chosen, namely W=5%~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 range indicates that the chosen material is inappropriate, and an O-ring made of another material should be used, or the specified compressive deformation rate should be adjusted. Regarding the specific values of compressive deformation, generally, various countries establish standards or provide recommended values based on their own practical experience. 2) Stretch amount: After being inserted into the sealing groove, O-rings generally experience a certain degree of stretching. Unlike compression ratio, the amount of stretching also has a significant impact on the sealing performance and service life of O-rings. A large amount 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 do, thereby causing leakage. The stretch amount α can be expressed by the following formula: α = (d + do) / (d1 + do), where d is the shaft diameter (mm) ； d1——Inner diameter of the O-ring (mm) ； do——Cross-sectional diameter of the O-ring (mm). 3) Contact width: After the O-ring is inserted into the sealing groove, its cross-section undergoes compressive deformation. Both the width after deformation and its contact width with the shaft are related to the sealing performance and service life of the O-ring; values that are too low can affect the sealing quality ； If it is too large, friction increases, generating heat due to friction, which affects the lifespan of the O-ring. The width BO (mm) of the O-ring after deformation is related to the compression ratio W of the O-ring and its cross-sectional diameter dO; it can be calculated using the formula BO={1/(1-W)-0.6W}dO, with W taking values between 10% and 40%. The width b (mm) of the contact surface between the O-ring and the shaft also depends on W and dO: b=(4W2+0.34W+0.31)dO, again with W ranging from 10% to 40%. This formula can be used to estimate the friction force in O-ring seals where high friction limitations are required, such as in pneumatic seals or hydraulic servo control components. (To be continued.) . . . . . ）