Machinery's Handbook, 31st Edition
2668 O-RINGS O-Ring Size Selection.— O-ring sizing involves determining the best inside and cross- sectional diameter for an application. Choice of inner diameter usually is determined by the basic needs of the assembly being sealed: the groove for the seal must fit in one of the assembled parts, leave enough material on the part to provide sufficient stiffness, and allow for installation. Deciding which part will bear the groove and choosing the O-ring is heavily influenced by cost and assembly considerations. There are more variables involved in determining cross-sectional diameter that can substantially impact sealing effectiveness. Smaller cross section O-rings have the advan- tages of lighter weight, compact footprint, better resistance to explosive decompression, lower friction in dynamic sealing applications, and better physical performance for many materials. Disadvantages of smaller cross sections include higher likeliness of leaks due to surface defects in the gland, less stability in reciprocating applications, higher vulner- ability to compression set, higher percentage swell (expansion due to fluid exposure or heat), and the need for tighter gland tolerances. When working with an assembly containing thin or fragile parts, compression force of the O-ring also must be taken into account. Compression force depends on O-ring material properties, cross section, length (mean circumference), and amount of squeeze induced in the O-ring cross section by the gland. SAE and ISO standards indicate appropriate sizes for various applications. A greater range of sizes are suitable for static applications than for dynamic applications. There also are differences in size ranges for internal pressure (pushing out from the O-ring’s center) and external pressure (pushing inward from outside the O-ring) axial face seal applica- tions. Fig. 2, Fig. 3, and the tables on the following pages show size ranges and relevant gland dimensions for SAE and ISO standard size codes.
Fig. 2. Gland and O-Ring Details
Fig. 3. Piston and Rod Gland Details
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