ParFab Design Guide

and splice surface contact area can have a synergistic effecton the required stretch force. Industrial seal recovery time should beallowed into the process when high installation stretchpercentages occur.It is also suggested that an installed ID (IP) stretch percentageof 0.5% to 3% be used for all traditional gland configurations.Beyond 3%, the life of the seal may be reduced, and exces-sive strain can occur at the spliced joint. Designing in a smallamount of installation stretch can assist the assembly process,holding the seal in the proper position until the mating compo-nents are in place. This can also reduce the possibility of sealpinching due to the presence of excess material. can cause the polymer chains within the seal to fractureundesirably, reducing the long term sealing effectiveness.Flange surfaces usually can not be held perfectly flat after thebolts are tightened during assembly. As a result, the seal maybecome over-compressed in the areas of the bolts. Proper groove design can prevent this from happening. Use of agroove allows for metal-to-metal or plastic-to-plastic contact ofthe mating parts of the assembly, preventing over-compres- sion of the seal element. A single groove is usually all that isneeded. Flange Surfaces The most frequently used profiles are “O” or “D”-shapedprofiles, with either solid or hollow cross-sections. Flange fasteners should be located in positions that create uniformpressure distribution at the corners, and minimal clearancegaps around the component periphery. For these profiles, a high bolt torque is usually not needed to seal perfectly. Corner Radius Gland Fill (Volume-to-Void Ratio) In order for a spliced, solid round cross-section seal (o-ring) toseat properly in an application, the inner radius of the grooveat the corners must be equal to or greater than the cross- sectional width of the seal. Other profiles, especially hollowones, require a larger inside radius to prevent kinking orpinching. Typically a design allowance of 21/2 to 3 times the cross-section is used for the inside corner radius. Gland fill is defined as the cross-sectional amount of material(volume) found in a gland cross-sectional area (void). This value is typically referred to in terms of a percentage. Gland Surface Finish 95% Gland Fill - MAXIMUM! The surface finish of the gland is a very important part of thesealing solution. The surface should be free of nicks, burrs,scratches or dents. As illustrated in the diagram below, a surface finish not to exceed 64 microinches on the glandsides and a 32 microfinish on the sealing compressionsurfaces is typically recommended. Figure 3 - Maximum Gland Fill Percentage For static applications a 95% maximum gland fill is recom-mended. It is extremely important that the gland fill percent-age be established in terms of a range, using minimum andmaximum values, which has taken into account the tolerancestack-ups of the assembly and the seal cross-sectionstogether. If a clearance gap is present in the assembly, theassociated dimensions must be included in the calculations,as they were when the cross-section squeeze dimensionswere established. Figure 4 - Typical Gland Surface FinishSolid Cross-Section Profiles Minimum Gland Fill Percentage Formula (Minimum Seal C/S Area) (Maximum Gland C/S Area) Various types of solid cross-section seal profiles can be usedto generate effective static environmental sealing results in an application. Typical standard configurations include, but arenot limited to solid O-, D-, U-, P-, rectangular or squareprofiles.Solid round cross-section seals are typically termed O-Ringsin the sealing industry. Use of a solid round cross-section is preferred in many applications because the seal can be (Maximum Seal S/C Area) (Minimum Gland C/S Area) Maximum Gland Fill Percentage Formula Seal Material Selection Optimizing the balance of physical properties and theenvironment will allow the establishment of the best sealingsolution for each application. Please refer to pages 8through 10 for additional information on physical andchemical properties. X100 Gland Groove Considerations All elastomer materials are subject to compression set, or a loss in return force, over time. Over-compression (squeeze) X100 6
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