Figure 7-20.–Cup packing.seals. The flangemade of leather,(sometimes calledfabric-reinforcedthe hat) isrubber, orhomogeneous rubber. Lip sealing occurs only onthe packing ID (fig. 7-21). Flange packings aregenerally used only for rod seals when otherpackings such as V-rings or U-seals cannot beused.DIRT EXCLUSION SEALS(WIPERS AND SCRAPERS)Dirt exclusion devices are essential if asatisfactory life is to be obtained from most rodseals. The smooth finished moving rod surface,if not enclosed or protected by some sort ofcovering, will accumulate a coating of dust orabrasive material that will be dragged or carriedinto the packing assembly area on the return rodstroke. Exclusion devices called wipers or scrapersare designed to remove this coating. While theterms wiper and scraper are often usedinterchangeably, it is useful to reserve scraperFigure 7-21.—Typical flange packing cross section.for metal lip-type devices that remove heavilyencrusted deposits of dirt or other abrasivematerial that would merely deflect a softer lip andbe carried into the cylinder. Sometimes a rod willhave both a scraper and a wiper, the former toremove heavy deposits and the latter to excludeany dust particles that remain. Whenever metallicscrapers are used with felt wipers in the samegroove, the felt wiper must not be compressed norrestricted in any way that affects its function asa lubricator. A wiper installed in a seal assemblyin a pneumatic application may remove too muchoil from the rod, requiring some method ofreplacing the oil. A common remedy is to providea periodically oiled felt ring between the wiperand the seal. Felt wipers provide lubricationto extended operating rods, thus increasingcomponent wear life. These wipers are only usedto provide lubrication to parts.Much longer life could be obtained from mostseals if proper attention were given to wipers andscrapers. Often, wiper or scraper failure is notnoticed when a seal packing fails. As a result, onlythe packing is replaced, and the same worn wiperor scraper is reinstalled to destroy anotherpacking. Check the wiper or scraper conditionupon its removal. If the wiper is worn, dirty, orembedded with metallic particles, replace it witha new one. It is usually good practice to replacethe wiper every time you replace the seal and evenmore frequently if the wiper is readily accessiblewithout component disassembly. If replacementsare not available, wash dirty wipers that are stillin good condition with suitable solvent andreinstall them. Remember that a wiper or scraperis deliberately installed as a sacrificial partto protect and preserve the sealing packing.Therefore, from a user’s standpoint, wipers andscrapers should be inspected and replaced asnecessary.STORAGE OF SEALSProper storage practices must be observed toprevent deformation and deterioration of seals.Most synthetic rubbers are not damaged bystorage under ideal conditions. However, mostsynthetic rubbers will deteriorate when exposedto heat, light, oil, grease, fuels, solvents, thinners,moisture, strong drafts, or ozone (form of oxygenformed from an electrical discharge). Damage byexposure is magnified when rubber is undertension, compression, or stress. There are several7-17
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