Figure 7-1.—Spiral-wouna metallic-asbestos gasket.easily bent and requires careful handling. Inaddition, copper becomes hard when used overlong periods and when subjected to compression.Whenever a unit or component is disassembled,the copper sealing rings should be replaced.However, if new rings are not available and thepart must be repaired, the old ring should besoftened by annealing. (Annealing is the processof heating a metal, then cooling it, to make itmore pliable and less brittle.)Metallic piston rings are used as packing insome fluid power actuating cylinders. These ringsare similar in design to the piston rings inautomobile engines.Metal is also used with asbestos to formspiral-wound metallic-asbestos gaskets (fig. 7-1).These gaskets are composed of interlocked pliesof preformed corrugated metal and asbestosstrips, called a filler.The filler may or may not be encased in a solidmetal outer ring. These gaskets are used in flangedconnections and for connecting the body to thebonnet in some valves, and are usually requiredin specific high-pressure, high-temperatureapplications.RUBBERThe term rubberand synthetic rubbers,covers many naturaleach of which can becompounded into numerous varieties. Thecharacteristics of these varieties have a wide range,as shown in table 7-1. The table shows, with theexception of a few basic similarities, that rubbershave diverse properties and limitations; therefore,specific applications require careful study beforethe sealing material is selected.Natural rubbers have many of the charac-teristics required in an effective seal. However,their very poor resistance to petroleum fluids andrapid aging when exposed to oxygen or ozone limittheir use. Today their use has almost ceased.There are two general classes of syntheticrubber seals. One class is made entirely of a certainsynthetic rubber. The term homogeneous, whichmeans having uniform structure or compositionthroughout, is frequently used to describe thisclass of seal. The other class of seal is made byimpregnating woven cotton duck or fine-weaveasbestos with synthetic rubber. This class issometimes referred to as fabricated seals.Additional information on sealing materialsis provided in the Military Handbook, GasketMaterials (Nonmetalic), MIL-HDBK-212; andthe Naval Ships’ Technical Manual, chapter 078.TYPES OF SEALSFluid power seals are usually typed accordingto their shape or design. These types includeT-seals, V-rings, O-rings, U-cups and so on. Someof the most commonly used seals are discussedin the remainder of this chapter.T-SEALSThe T-seal has an elastomeric bidirectionalsealing element resembling an inverted letter T.This sealing element is always paired with twospecial extrusion-resisting backup rings, one oneach side of the T. The basic T-seal configurationis shown in figure 7-2, view A. The backup ringsFigure7 - 2 . – T - seals .7-3
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business