8.9.10.11.Low distortion of structureSmall space requirementReliabilityEffectiveness over wide pressure andtemperature rangesAs stated previously, O-rings are used in bothstatic (as gaskets) and dynamic (as packing)applications. An O-ring will almost always be themost satisfactory choice of seals in staticapplications if the fluids, temperatures, pressure,and geometry permit.Standard O-ring packings are not specificallydesigned to be used as rotary seals. Wheninfrequent rotary motion or low peripheralvelocity is involved standard O-ring packings maybe used, provided consistent surface finishes overthe entire gland are used and eccentricities areaccurately controlled. O-rings cannot compensatefor out-of-round or eccentrically rotating shafts.As rotary seals, O-rings perform satisfactorilyin two application areas:1. In low-speed applications where the surfacespeed of the shaft does not exceed 200 ft/min2. In high-speed moderate-pressure appli-cations, between 50 and 800 psiThe use of low-friction extrusion-resistantdevices is helpful in prolonging the life andimproving the performance of O-rings used asrotary seals.O-rings are often used as reciprocating sealsin hydraulic and pneumatic systems. While bestsuited for short-stroke, relatively small diameterapplications, O-rings have been used successfullyin long-stroke, large diameter applications.Glands for O-rings used as reciprocating seals areusually designed according to MIL-G-5514 toprovide a squeeze that varies from 8 to 10 percentminimum and 13.5 to 16 percent maximum. Asqueeze of 20 percent is allowed on O-rings witha cross section of 0.070-inch or less. In somereciprocating pneumatic applications, a floatingO-ring design may simultaneously reduce frictionand wear by maintaining no squeeze by the glandon the O-ring. When air pressure enters thecylinder, the air pressure flattens the O-ring,causing sufficient squeeze to seal during thestroke. If the return stroke does not use pneumaticpower, the O-ring returns to its round crosssection, minimizing drag and wear on the returnstroke.IdentificationAs a maintenance person or supervisorworking with fluid power systems, you must beable to positively identify, inspect, and install thecorrect size and type of O-ring to ensure the bestpossible service. These tasks can be difficult sincepart numbers cannot be put directly on the sealsand because of the continual introduction of newtypes of seals and obsolescence of others. (NavalShips’ Technical Manual, chapter 078, containsa table that cross-references obsolete and currentO-ring specifications for ship applications.)O-rings are packaged in individually sealedenvelopes. O-ring seals manufactured to govern-ment specifications are marked according to therequirements of the specific military specificationand standard. The required marking for eachpackage is as follows:1.2.3.4.5.6.7.8.9.National stock number (NSN)NomenclatureMilitary part numberMaterial specificationManufacturer’sManufacturer’sManufacturer’snamecompound numberbatch numberContract numberCure dateNOTE: Keep preformed packings in theiroriginal envelopes, which provide preservation,protection, identification, and cure date.When you select an O-ring for installation,carefully observe the information on the package.If you cannot positively identify an O-ring,discard it. The part number on the sealedpackage provides the most reliable and completeidentification.7-7
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