Machinery's Handbook, 31st Edition
O-RINGS 2683 Example: Calculate stretch/cramping, squeeze, and gland fill for a -116 size O-ring (-116A in metric, shown in parentheses) in a standard axial face gland with external liquid pressure. Given: Use gland standard SAE AS6235A-2015 (ISO 3601-2-2016): ID max = 0.746 ID min = 0.728 ( ID max = 18.95) ( ID min = 18.49) W max = 0.106 W min = 0.1 ( W max = 2.71) ( W min = 2.53) WR min = W min = 0.1 WR max = W max = 0.106 ( WR min = W min = 2.53) ( WR max = W max = 2.71) IDG max = 0.742 IDG min = 0.737 ( IDG max = 18.9) ( IDG min = 18.84) AF max = 0.09 AF min = 0.086 ( AF max = 2.1) ( AF min = 2) AG min = 0.141 AG max = 0.151 ( AG min = 4) ( AG max = 4.2) Calculate: L max = 0.115 L min = 0.112 ( L max = 3.125) ( L min = 3.018) SID max = 0.728 SID min = 0.725 ( SID max = 18.322) ( SID min = 18.215) Stretch % max = 2.4% Stretch % min = 1.2% ( Stretch % max = 3.8%) ( Stretch % min = 2.9%) ODG max = 1.044 ODG min = 1.019 ( ODG max = 27.3) ( ODG min = 26.85) Sq % max = 19% Sq % min = 10% ( Sq % max = 26%) ( Sq % min = 17%) OVol max = 0.024 OVol min = 0.02 ( OVol max = 392.5) ( OVol min = 331.98) GVol max = 0.039 GVol min = 0.033 ( GVol max = 643.31) ( GVol min = 571.15) Fill % max = 72% Fill % min = 53% ( Fill % max = 69%) ( Fill % min = 52%) Validate for the -116 SAE O-Ring: Stretch % max = 2.4% Stretch % min = 1.2% Well within SAE limit of 5%. Sq % min = 10% Sq % max = 19% Meets SAE recommendation of 10–20% for this cross section. Fill % max = 72% Fill % min = 53% Within SAE target maximum of 85%. Validate for the -116A ISO O-Ring: ( Stretch % max = 3.8%) ( Stretch % min = 2.9%) Well within ISO limit of 8%. ( Sq % min = 17%) ( Sq % max = 26%) ( Fill % max = 69%) Within ISO target maximum of 85%. PTFE O-Rings in Axial Face Seals: Axial face seals can work with very stiff O-ring materials. Solid PTFE seals are often chosen because they are highly chemically inert and less costly than perfluorinated elastomer seals with similar chemical resistance. Glands for solid PTFE O-rings often target a squeeze of 10 to 20 percent for O-rings with cross- sectional diameters up to 0.210 in. (5.33 mm), and 10 to 15 percent for cross sections 0.210 to 0.275 in. (5.33 to 6.99 mm). ISO recommends 19–30% for this cross section. This slight lower limit violation invites caution in critical applications but is not uncommon. ( Fill % min = 52%) It is common practice to dimension the groove so that the groove’s mean diameter (inner diameter plus one cross-sectional diameter) is equal to the O-ring’s free-state mean diam- eter. Deliberate contact with the inner or outer groove walls also can be targeted as with elastomeric O-rings, but stretch and cramping should be zero or nearly zero. A groove with a full-radius floor is best to support the O-ring and control deformation. The groove width is normally 10 percent greater than the O-ring cross-sectional diameter. If a clear- ance gap is present, it must not exceed 0.005 in. (0.13 mm). PTFE O-rings and their glands are extremely vulnerable to nicks, scratches, and particle contamination. These seals must be handled with care and installed in a clean environment.
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