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The function of an O-ring


O-ring The role of
 
What is an O-ring? An O-ring is a sealing element with an O-shaped (circular) cross-section that is installed in a groove and subjected to moderate compression. It can be used to seal a wide variety of fluids, including oils, water, air, and gases. Applications include both static and dynamic sealing.
Characteristics of O-rings

1. Requires a relatively small installation space. The shape and structure of the O-ring’s mounting location can be designed to be extremely simple, enabling miniaturization and weight reduction of the machine.
 
2. The use of a simple O-ring allows for significant simplification of both its shape and installation location, making it easy to install and remove.
 
3. It can be used over a wide range of applications. Depending on the selected elastomeric material, it exhibits stable sealing performance in water, oils, and air, as well as in various gases and chemical media, and is suitable for service within a temperature range of –60 to +220°C.
 
4. A wide variety of cost-effective, standardized materials that are fully capable of meeting diverse application requirements, offered in an extensive range of sizes, easy to select and procure, and economically priced. Materials for O-rings: The performance characteristics required of O-ring materials stem from the fact that O-rings generate compressive stress during installation, thereby fulfilling their sealing function. The fundamental requirement is to maintain an appropriate level of stress without causing excessive deformation.
 
This fundamental performance must remain intact even under service conditions. Synthetic rubber is the material that best exhibits such properties. However, no single material can simultaneously address all challenging performance requirements. Therefore, the appropriate material should be selected on a case-by-case basis, depending on the specific application.
Proper Usage of O-Rings
 
1. The relationship between the cross-sectional diameter (wire diameter) of an O-ring and its compression set. At a given compression ratio, a larger cross-sectional diameter results in lower permanent deformation; using O-rings with a larger cross-section can achieve stable sealing performance. Particularly when used for dynamic seals, a larger cross-sectional diameter helps prevent torsional distortion.
 
2. At high pressures, O-rings achieve their sealing function primarily through the compressive stress of the material. Additionally, the pressure of the sealed fluid contributes to a secure seal. Therefore, the pressure limits must be considered from a theoretical standpoint. In practical applications, the presence of the sealing fluid inevitably generates pressure, which helps prevent the O-ring from being extruded or pinched into the mating surfaces.
 
Therefore, the pressure resistance of an actual O‑ring is determined by the relationship between the material’s mechanical strength—represented by its hardness—and the clearance. When the retaining ring is subjected to pressure from two directions, it can be installed on both sides of the O‑ring; when the pressure is applied from only one direction, a retaining ring should be fitted on the side opposite to the pressure. Retaining rings come in two forms: circular and slanted‑cut; in terms of performance, the circular type delivers the best results.
 
From the standpoint of ease of installation, a beveled joint is more convenient. 3. For cylindrical static seals used in different applications, even when O‑rings and groove dimensions are selected in accordance with JIS standards, variations in dimensional tolerances may lead to sagging at the inner diameter of the groove; therefore, care should be taken during installation to prevent the O‑ring from being pinched or damaged. Plane static seals are classified into two types: those subjected to internal pressure (a) and those subjected to external pressure (b). For internal‑pressure applications (a), the O‑ring is typically matched to its outer diameter D. For external‑pressure applications (b), the O‑ring is generally matched to its inner diameter. Since the risk of extrusion is usually negligible during installation, it is important to account for bolt elongation under load and flange‑surface deformation, which tend to be more pronounced than in cylindrical static seals.
 
In case (C), since there is no wall adhesion, the seal may wear or detach due to pressure fluctuations; therefore, do not use it. When the inner diameter is relatively large (150 or greater) and the cross‑sectional diameter is very small (3 or less), an O‑ring like that shown in Figure 5 may fly out, and part of the O‑ring may be sheared off during clamping. To prevent this, a larger cross‑sectional diameter can be employed. For vacuum flanges, O‑rings used in vacuum equipment require attention to the following points compared with those used in general hydraulic applications. In vacuum equipment, because the sealing medium is a gas rather than a liquid, leakage through gaps at the contact interface is more likely; thus, extreme care must be taken to ensure that the surface roughness of the mating surfaces is properly controlled.
 
Failure to select rubber materials that are compatible with the vacuum level allows gases to permeate the rubber, causing additives to migrate to the surface and resulting in an inability to maintain the desired vacuum. For the sake of simplifying the machine, triangular grooves—also known as end-face chamfered grooves—are sometimes employed.
For dimensions D and d, please refer to the D and d values specified for the groove in the dimension table. Additionally, select dimension G within the range of 1.3 to 1.4W (where W is the cross-sectional diameter of the O-ring). When used under internal pressure, if dimension D falls within a relatively small range—approximately 30 or less—installation may sometimes be problematic; in such cases, it is advisable to increase dimension D slightly by about 0.2 to 0.3 mm for easier installation. For pneumatic seals: when using an oil‑lubrication method, a single O‑ring may suffice for the piston seal; however, considering service life and sealing performance, we recommend using two O‑rings.
 
For piston rod seals, it is recommended to use one O-ring together with a dust seal, and apply lubricant between the O-ring and the dust seal. 2. When operating without oil lubrication, two O-rings may be used for the piston seal, with grease applied to the O-rings as the lubricant. For piston rod seals, always follow the oil‑lubricated installation procedure. 4. Design considerations: Clearance and extrusion—O‑rings can extrude from the groove due to fluid pressure, clearance, and the hardness of the rubber material.
 
(These values are based on the pressure when the cylinder bore is not yet expanded; at high pressures, the radial clearance can be designed to approximately 75% of this value.) With regard to compression set, the minimum allowable compression set is about 8%, while the maximum should be kept below 30% of the rubber’s permanent compressive deformation. When using O‑rings, please refer to the groove dimensions and surface roughness specified in the dimension table.

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