SizesA standardized dash number system forO-ring sizes is used in many military and industrialspecifications. The O-ring size is identified by adash number rather than the actual dimensionsfor convenience. The basis for the dash numbersis contained in Aerospace Standard AS568. Fornongasket O-rings (packing), the dash numbersare divided into groups of one hundred. Eachhundred group identifies the cross section size ofthe O-rings within the group (table 7-2).The 900 series dash numbers contained inAS568 identify all the presently standardizedstraight thread tube fitting boss gaskets. With theexception of -901, the last two digits of the dashdesignate the tube size in 16ths of an inch. Forexample, the -904 size is for a 1/4-inch tube.DimensionsThe critical dimensions of an O-ring are its ID,its cross sectional diameter (W), and the heightand width of the residual molding flash (seefig. 7-7).Nominal dimensions have been used todescribe O-ring sizes, although this practice israpidly being replaced by the use of dash numbers.The actual inside diameter of a seal will be slightlyless than the nominal ID, but the actual OD willTable 7-2.—O-Ring Dash Numbers Versus Cross SectionSizesbe slightly larger than the nominal OD. Forexample, an AS568-429 O-ring is described innominal dimensions as 5 inches ID by 5-1/2 inchesOD by 1/4-inch W. Actual dimensions are 4.975inches ID by 5.525 inches OD by 0.275 inches W.SpecificationsMaterial and performance requirementsfor O-rings are often identified in militaryspecifications. The dimensions of these O-ringswill usually be found in accompanying slash sheets(which bear the specification number and are apart of the specification) or will be identified byvarious drawings and standards that relate to thespecification. Included among the specificationsare Air Force-Navy Standards (AN), Mili-tary Standards (MS), and National AerospaceStandards (NAS). If the specification does notidentify sizes, the sizes should be identified by theAS568 dash number. Usually, you can usedrawings, technical manuals, and allowance partslists (APLs) to identify replacement O-rings.(Notes 2 and 3 of table 7-1 list some of thefrequently used military specifications).Cure DateA cure date is as applicable to natural orsynthetic O-rings as it is to rubber hoses. This dateis the basis for determining the age of O-rings.It is extremely important that the cure date benoted on all packages.Shelf Life and Expiration DateAll elastomers change gradually with age;some change more rapidly than others. Theshelf life for rubber products is contained inMIL-HDBK-695.Check the age of natural or synthetic rubberpreformed packings before installation todetermine whether they are acceptable for use.Make a positive identification, indicating thesource, cure date, and expiration date. Ensure thatthis information is available for all packing used.Shelf life requirements do not apply once thepacking is installed in a component.The expiration date is the date after whichpacking should not be installed. The expirationdate of all packings can be determined by addingthe shelf life to the cure date.7-8
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