Machinery's Handbook, 31st Edition
O-RINGS 2681 O-Ring Squeeze.—The installed squeeze of an O-ring has a great effect on its sealing ability and effective life. Squeeze is the difference, often expressed as a percent, between the stretched O-ring cross-sectional diameter and the gland depth F (see Fig. 2). Any stretch applied to the O-ring by a radial gland will reduce the O-ring’s cross-sectional diameter and affect squeeze. High squeeze values are used with very soft O-rings or with high pressures, while low squeeze values should be favored in dynamic applications where friction (heat and drag) is a concern. Sealing against vacuum also usually requires higher squeeze, with values between 10 and 30 percent not unusual. Target squeeze will depend on O-ring size, material, and application parameters. For most elastomers and applications, the O-ring gland should ensure a minimum squeeze of 0.005 in. (0.13 mm) regardless of O-ring size to overcome the elastomers’ tendency to permanently deform a very small amount. Standard general squeeze target ranges can be found in the sections specific to each gland type. O-rings with smaller cross sections tolerate higher squeeze better than larger cross section O-rings, but compression set or rupture can become a problem for most materials if squeeze is above 30 percent. (Relevant squeeze equations can be found for each gland type on page 2682.) For inch-system static seals, many vendors recommend gland dimensions that adhere to the SAE AS5857 standard. This standard targets greater squeeze values than the general- use standards and applies to low-temperature, low-swell, and vacuum applications. It falls to the designer to decide if the greater squeeze values are necessary for a given appli- cation. Many O-ring suppliers have developed nonstandard glands; designers also may customize gland dimensions to optimize a particular application. Gland Fill.—When installed, an O-ring compresses slightly and distorts into free space within the gland. Additional expansion or swelling also may occur due to contact of the O-ring with fluid or heat. The gland must be large enough to accommodate maximum expansion of the O-ring, to prevent it from extruding into the clearance gap or rupturing the assembly. In a dynamic application, any extruded O-ring material will quickly wear and fray, severely limiting seal life. When designing a gland for a squeeze value that is higher than standard, gland fill must be carefully evaluated. Groove width can be increased to accommodate the squeezed vol- ume of the O-ring, plus 15 percent to accommodate swell. This corresponds to an 85 per- cent gland fill target, assuming straight groove sides, no O-ring swell, and no internal or external radii. When significant swell is expected, further examination of gland fill must be undertaken. (Relevant gland fill equations can be found for each gland type below.) Axial Face Seal Glands.— When an O-ring is compressed between two flat parts, this forms an axial face seal. This type of gland is always static, and it normally has no clear- ance gap between parts. A groove is cut into the face of one of the parts to house the O-ring, with gland fill considered when customizing gland dimensions. Groove diameter will de - pend on which direction the pressure or vacuum is coming from. If a face seal is expected to experience reversals in the direction of pressure, an O-ring is not the best solution; a flat gasket or other type of seal should be investigated. Internal Pressure Axial Face Seals: If the pressure differential tends to expand the O-ring away from the center, this is considered internal pressure. When the gland is closed, the O-ring deforms as discussed below, with the outer diameter interfering (called cramping or cramming ) in this case. Once squeezed, the O-ring will be in good contact with the outer wall of the gland, and the internal pressure should not cause O-ring move- ment. For SAE and ISO standard glands, the groove outer diameter maximum is often set equal to the O-ring nominal outer diameter; then the groove is adjusted to fulfill squeeze and gland fill targets. Using nominal dimensions, the SAE system allows up to 1 percent cramping, while the ISO system allows up to 5 percent cramping (reduces to 3 percent for inner diameters > 250 mm). Larger cramping values can work in some situations. External Pressure Axial Face Seals: If the pressure differential tends to collapse the O-ring inward toward the center, this is considered external pressure. The inner diam- eter of the face groove should be specified to stretch the squeezed O-ring. SAE and ISO
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