Often the selection of seals is limited to sealscovered by military specifications. However, thereare occasions when nonstandard or proprietaryseals reflecting the advancing state of the art maybe approved. Thus, it is important to follow themanufacturer’s instructions when you replaceseals. If the proper seal is not available, youshould give careful consideration in the selectionof a suitable substitute. Consult the Naval Ships’Technical Manual, military standards, militarystandardization handbooks, and other applicabletechnical manuals if you have any doubts inselecting the proper seal.Seals are made of materials that havebeen carefully chosen or developed for spe-cific applications.These materials includetetrafluoroethylene (TFE), commonly calledTeflon; synthetic rubber (elastomers); cork;leather; metal; and asbestos. Some of the mostcommon materials used to make seals for fluidpower systems are discussed in the followingparagraphs.CORKCork has several of the required properties,which makes it ideally suited as a sealing materialin certain applications. The compressibility ofcork seals makes them well suited for confinedapplications in which little or no spread of thematerial is allowed. The compressibility of corkalso makes a good seal that can be cut to anydesired thickness and shape to fit any surface andstill provide an excellent seal.One of the undesirable characteristics of corkis its tendency to crumble. If cork is used aspacking or in areas where there is a high fluidpressure and/or high flow velocity, small particleswill be cast off into the system. Cork use in fluidpower systems is therefore limited. It is sometimesused as gasket materials for inspection plates ofhydraulic reservoirs.Cork is generally recommended for use wheresustained temperatures do not exceed 2750F.CORK AND RUBBERCork and rubber seals are made by combiningsynthetic rubber and cork. This combination hasthe properties of both of the two materials.This means that seals can be made with thecompressibility of cork, but with a resistance tofluid comparable to the synthetic rubber on whichthey are based. Cork and rubber composition issometimes used to make gaskets for applicationssimilar to those described for cork gaskets.LEATHERLeather is a closely knit material that isgenerally tough, pliable, and relatively resistantto abrasion, wear, stress, and the effects oftemperature changes. Because it is porous, it isable to absorb lubricating fluids. This porositymakes it necessary to impregnate leather for mostuses. In general, leather must be tanned andtreated in order to make it useful as a gasketmaterial. The tanning processes are thosenormally used in the leather industry.Leather is generally resistant to abrasionregardless of whether the grain side or the fleshside is exposed to abrasive action. Leather remainsflexible at low temperatures and can be forcedwith comparative ease into contact with metalflanges. When properly impregnated, it isimpermeable to most liquids and some gases,and capable of withstanding the effects oftemperatures ranging from –700F to +2200F.Leather has four basic limitations. First, thesize of the typical hide limits the size of the sealsthat can be made from leather. A secondlimitation is the number of seals that areacceptable. Another limitation is that under heavymechanical pressures leather tends to extrude.Finally, many of the properties (such asimpermeability, tensile strength, high- andlow-temperature resistance, pliability, andcompatibility with environment) depend upon thetype of leather and impregnation. Leathers nottanned and impregnated for specific conditionsand properties will become brittle, dry, andcompletely degreased by exposure to particularchemicals. Leather is never used with steampressure of any type, nor with acid or alkalisolutions.Leather may be used as packing. Whenmolded into V’s and U’s, and cups, and othershapes, it can be applied as dynamic packing,while in its flat form it can be used as straightcompression packing.METALOne of the most common metal seals used inNavy equipment is copper. Flat copper rings aresometimes used as gaskets under adjusting screwsto provide a fluid seal. Molded copper rings aresometimes used as packing with speed gearsoperating under high pressures. Either type is7-2
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