U-CUPS  AND  U-PACKINGS The   distinction   between   U-cups   and U-packings results from the difference in materials used  in  their  fabrication.  The  U-cup  is  usually made   of   homogeneous   synthetic   rubber; U-packings are usually made of leather or fabric- reinforced  rubber.  Special  aspects  of  each  type  will be discussed separately. However, all U-cups and U-packings  have  cross  sections  resembling  the letter U. Both types are balanced packings, both seal on the ID and the OD, and both are applied individually,   not   in   stacks   like   V-rings.   Size differences  between  U-cups  and  U-packings  are usually  substantial  enough  to  prevent  inter- changeability. There are a few sizes with smaller diameters  and  cross  sections  that  may  appear to  be  dimensionally  equivalent  but  are  not. Therefore, U-packings should not be substituted for  U-cups  (or  vice  versa)  in  any  installation. U-CUPS The  U-cup  (fig.  7-18)  has  been  a  popular packing  in  the  past  because  of  installation  ease and  low  friction.  U-cups  are  used  primarily  for pressures below 1500 psi, but higher pressures are possible with the use of antiextrusion rings. For double-acting  pistons,  two  U-cups  are  installed in 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  common for single-acting (monodirectional) seals, such as U-cups and V-rings, and is necessary to prevent a  pressure  trap  (hydraulic  lock)  between  two packings. Installation of two U-cups with sealing lips facing each other can result in hydraulic lock and  must  be  avoided. Leather U-Packings As a rule, leather U-packings are made with straight  side  walls  (no  flared  sealing  lips).  See figure 7-19. The leather may be chemically treated or  otherwise  impregnated  to  improve  its  per- formance.  Leather  U-packings  are  available  in standard sizes conforming to industrial specifica- tions.  For  support,  the  cavity  of  the  U-packing should contain a metal pedestal ring or should be filled with a suitable material. Leather U-packings with   an   integral   pedestal   support   have   been installed in some submarine steering and diving ram piston seals. CUP  PACKINGS Cup packings resemble a cup or deep dish with a hole in the center for mounting (fig. 7-20). Cup seals are used exclusively to seal pistons in both low-  and  high-pressure  hydraulic  and  pneu- matic   service.   They   are   produced   in   leather, homogeneous  synthetic  rubber,  and  fabric- reinforced  synthetic  rubber.  Although  the  cup packing  lip  flares  outward,  the  rubbing  contact is made at the lip only when the fluid pressure is low. As the fluid pressure increases, the cup heel expands  outward  until  it  contacts  the  cylinder wall,  at  which  point  high-pressure  sealing  is  in effect. As the pressure loading shifts the sealing line to the cup heel, the lip is actually pulled into the cup and away from the cylinder wall. On the return stroke when the pressure is relaxed, the heel will  shrink  slightly,  leaving  only  the  lip  in  contact with the wall, avoiding unnecessary wear at the heel. For reciprocating pistons, two cups installed back-to-back  in  separate  glands  are  required. FLANGE   PACKINGS Flange packings are used exclusively in low- pressure,  outside-packed  installations,  such  as  rod Figure  7-18.—Typical  U-cup  seal. Figure  7-19.—U-packing. 7-16