U-CUPS AND U-PACKINGSThe distinction between U-cups andU-packings results from the difference in materialsused in their fabrication. The U-cup is usuallymade of homogeneous synthetic rubber;U-packings are usually made of leather or fabric-reinforced rubber. Special aspects of each type willbe discussed separately. However, all U-cups andU-packings have cross sections resembling theletter U. Both types are balanced packings, bothseal on the ID and the OD, and both are appliedindividually, not in stacks like V-rings. Sizedifferences between U-cups and U-packings areusually substantial enough to prevent inter-changeability. There are a few sizes with smallerdiameters and cross sections that may appearto be dimensionally equivalent but are not.Therefore, U-packings should not be substitutedfor U-cups (or vice versa) in any installation.U-CUPSThe U-cup (fig. 7-18) has been a popularpacking in the past because of installation easeand low friction. U-cups are used primarily forpressures below 1500 psi, but higher pressures arepossible with the use of antiextrusion rings. Fordouble-acting pistons, two U-cups are installedin separate grooves, back-to-back or heel-to-heel.Two U-cups are never used in the same groove.This heel-to-heel type of installation is commonfor single-acting (monodirectional) seals, such asU-cups and V-rings, and is necessary to preventa pressure trap (hydraulic lock) between twopackings. Installation of two U-cups with sealinglips facing each other can result in hydraulic lockand must be avoided.Leather U-PackingsAs a rule, leather U-packings are made withstraight side walls (no flared sealing lips). Seefigure 7-19. The leather may be chemically treatedor otherwise impregnated to improve its per-formance. Leather U-packings are available instandard sizes conforming to industrial specifica-tions. For support, the cavity of the U-packingshould contain a metal pedestal ring or should befilled with a suitable material. Leather U-packingswith an integral pedestal support have beeninstalled in some submarine steering and divingram piston seals.CUP PACKINGSCup packings resemble a cup or deep dish witha hole in the center for mounting (fig. 7-20). Cupseals are used exclusively to seal pistons in bothlow- and high-pressure hydraulic and pneu-matic service. They are produced in leather,homogeneous synthetic rubber, and fabric-reinforced synthetic rubber. Although the cuppacking lip flares outward, the rubbing contactis made at the lip only when the fluid pressure islow. As the fluid pressure increases, the cup heelexpands outward until it contacts the cylinderwall, at which point high-pressure sealing is ineffect. As the pressure loading shifts the sealingline to the cup heel, the lip is actually pulled intothe cup and away from the cylinder wall. On thereturn stroke when the pressure is relaxed, the heelwill shrink slightly, leaving only the lip in contactwith the wall, avoiding unnecessary wear at theheel.For reciprocating pistons, two cups installedback-to-back in separate glands are required.FLANGE PACKINGSFlange packings are used exclusively in low-pressure, outside-packed installations, such as rodFigure 7-18.—Typical U-cup seal.Figure 7-19.—U-packing.7-16
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