Figure 2-42.—Hydraulic seals.trimmed to extremely close tolerances. In somelanding gear struts, an elliptical seal is used. Theelliptical seal is similar to the O-ring seal except forits cross-sectional shape. As its name implies, itscross section is elliptical in shape. Both the O-ringand elliptical seals are shown in figure 2-42.Advances in aircraft design have made newO-ring composition necessary to meet changingconditions.Hydraulic O-rings were originallyestablished under AN (Air Force-Navy) specificationnumbers (6227, 6230, and 6290) for use in fluid atoperating temperatures ranging from -65°F to +160°F.When new designs raised operating temperatures to apossible +275°F, more compounds were developedand perfected.Recently, newer compounds were developedunder MS (Military Standard) specifications thatoffered improved low-temperature performancewithout sacrificing high-temperature performance.These superior materials were adopted in theMS28775 O-ring, which is replacing AN6227 andAN6230 O-rings, and the MS28778 O-ring, which isreplacing the AN6290 O-ring. These O-rings are nowstandard for systems where the operatingtemperatures may vary from -65°F to +275°F.Packings used in naval aircraft hydraulicinstallations are manufactured from synthetic rubber.They are used in units that contain moving parts, suchas actuating cylinders, selector valves, etc. Althoughpackings are made in many forms, the O-ring type ismost widely used. The U-rings, V-rings, and othervarious types are obsolete in most cases and are notdiscussed in this training manual.The O-ring packing seals effectively in bothdirections. This sealing is done by distortion of itselastic compound.Views A and C of figure 2-43show O-rings of the proper size and installed ingrooved seats.Notice that the clearance for theO-rings is less than their free outer diameter. Thecross sections of the O-rings are squeezed out ofround prior to the application of pressure. In thismanner, contact is ensured with the inner and outerwalls of the passage under static (no pressure)conditions. Views B and D of figure 2-43 show theaction of the O-rings when pressure is applied. Youshould also observe, in views C and D of figure 2-43,that backup rings are installed. In hydraulic systemsof 1,500 psi pressure or less, AN6227B, AN6230B,and MS28775 packings are used. In such installa-tions, backup rings are not required, although they aredesirable. In most modern aircraft with hydraulicsystem pressures up to 3,000 psi, backup rings areused in conjunction with the MS28775 packings.Gaskets are used in the sealing of boss fittings,end caps of actuators, piston accumulators, and otherFigure 2-43.—Action of O-rings.2-31
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