Machinery's Handbook, 31st Edition
2684 O-RINGS If stretch is required, a PTFE O-ring can be heated up to 212 °F (100 °C) with boiling water or a dry heat source. If the gland is reopened after the O-ring is squeezed, the O-ring should be replaced, as PTFE will not rebound from compression and may not seal a second time. Axial Face Gland Dimensions: Axial face seal grooves should have edge breaks or radii on the external edges to protect the O-ring from sharp edges during installation. Rec- ommended edge breaks are 0.005–0.01 in. or 0.13–0.25 mm. Flatness must be tightly controlled for both mating parts, with 0.0004 in. per 4 in. (0.01 mm per 100 mm) a good target. Parallelism of the groove floor to the mating face also should be tightly controlled. A target parallelism of 0.0008 in. (0.02 mm) is normally sufficient. Ideally, groove walls should be vertical but may slope from vertical by up to 5 degrees to aid manufacturing. Table 2 , Table 3, Table 4, and Table 5 contain a sampling of gland data for standard size ranges for axial face sealing. Many O-ring suppliers recommend axial gland dimensions according to the SAE AS5857 standard, which recommends greater squeeze and is suit- able for low-temperature, low-swell, and vacuum applications.
Fig. 6. Dovetail and Half-Dovetail Grooves Dovetail Grooves.—It is sometimes necessary to retain an O-ring in a static face groove more securely than by using the O-ring stretch alone. In these cases, if sticky O-ring grease is not a practical solution, a dovetail or half-dovetail groove (see Fig. 6) can be used. This is an expensive option, and it requires tight tolerances. Entry radii are critical for preventing installation damage and extrusion. A half-dovetail groove is appropriate in cases where the applied pressure does not change direction. Be sure to orient a half-dovetail so that the pressure differential forces the O-ring against the vertical side of the groove. Full dovetail grooves can be used in ap- plications with pressure reversals. The nominal mean diameter of these grooves should be the same as the nominal mean diameter of the O-ring (see Fig. 6 and Table 12). Dovetail and half-dovetail grooves are not recommended for O-ring cross sections smaller than 0.139 in. (3.53 mm). These groove shapes also should not be used with rigid O-rings, because installation would cause permanent deformation. These glands can eas- ily fail due to O-ring swell, and can cause problems in vacuum applications where the groove can release the trapped volume of air contained in its corner(s). Radial (Piston or Rod) Glands.— Piston and rod seals can be static or dynamic, pneu- matic or hydraulic. Different gland dimensions and O-ring size ranges may be required for different applications. Radial O-ring grooves for piston or rod seals should have edge breaks on the external edges to protect the O-ring during installation. Recommended edge breaks are 0.005–0.01 in. or 0.1–0.3 mm. Rod seals also will require a lead-in cham- fer on the rod to facilitate assembly; piston seals will need an equivalent lead-in to the housing bore. Pneumatic radial seals are typically limited to pressures less than 150 psi (1 MPa), while hydraulic seals can perform at much higher pressures. Rod Glands: Rod gland designs tend to begin with a known rod diameter. Rod glands con- tain the O-ring in a groove cut into the housing bore, and the groove diameter should be sized to cramp the O-ring outer diameter. The O-ring is often simultaneously stretched by the rod. Here the groove maximum outer diameter often is set equal to the nominal O-ring diam- eter as a starting point. Alternatively, using the minimum O-ring outside diameter and the
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