the recommended O-ring entering sleeve (a soft,thin wall, metallic sleeve) is not available, papersleeves and covers may be fabricated by using theseal package (glossy side out) or lint-free bondpaper (see views B and C of fig. 7-10).After you place the O-ring in the cavityprovided, gently roll the O-ring with your fingersto remove any twist that might have occurredduring the installation. After installation, anO-ring should seat snugly but freely in its groove.If backup rings are installed in the groove, becertain the backup rings are installed on thecorrect side of the ring.BACKUP RINGSBackup rings, also referred to as retainer rings,antiextrusion devices, and nonextrusion rings, arewasher-like devices that are installed on thelow-pressure side of packing to prevent extrusionof the packing material. Backup rings in dynamicseals minimize erosion of the packing materialsand subsequent failure of the seal. At lowerpressures, backup rings will prolong the normalwear life of the packing. At higher pressures,backup rings permit greater clearances betweenthe moving parts. Normally, backup rings arerequired for operating pressures over 1500 psi.Backup rings can be made of polytetra-fluoroethylene, hard rubber, leather, and othermaterials. The most common material currentlyused is tetrafluoroethylene (TFE). Backup ringsare available as single-turn continuous (uncut orsolid), single-turn (bias) cut, and spiral cut. Seefigure 7-11. Leather rings are always furnished insolid ring form (unsplit). Rings of TFE areavailable in all three types.Packaging and StoringBackup rings are not color-coded or otherwisemarked and must be identified from the packaginglabels. The dash number following the militarystandard number found on the package indicatesthe size, and usually relates directly to the dashnumber of the O-rings for which the backup ringis dimensionally suited. Backup rings made ofTFE do not deteriorate with age and do not haveshelf life limitations. TFE backup rings areprovided by manufacturer either in individuallysealed packages or on mandrels. If unpackagedrings are stored for a long time without the useof mandrels, a condition of overlap may develop.Overlap occurs when the ID of the backup ringbecomes smaller and its ends overlap each other.To correct this overlap condition, stack TFE ringson a mandrel of the correct diameter, and clampthe rings with their coils flat and parallel. Placethe rings in an oven at a maximum temperatureof 1770C (3500F) for approximately 10 minutes.Do not overheat them because fumes fromdecomposing TFE are toxic. Remove andwater-quench the rings. Store the rings at roomtemperature before you use them (preferably for48 hours).InstallationCare must be taken in handling and installingbackup rings. Do not insert them with sharptools. Backup rings must be inspected priorto using them for evidence of compressiondamage, scratches, cuts, nicks, or frayed con-ditions. If O-rings are to be replaced wherebackup rings are installed in the same groove,never replace the O-ring without replacingthe backup rings, or vice versa. Many sealsuse two backup rings, one on either side of theO-ring (fig. 7-12). Two backup rings are usedprimarily in situations (such as a reciprocatingpiston seal) where alternating pressure directioncan cause packing to be extruded on both sidesof the gland.Figure 7-11.—Types of backup rings.7-12
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