CHAPTER  7 SEALING DEVICES AND MATERIALS Recall from chapter 1 that Pascal’s theorem, from which the fundamental law for the science of   hydraulics   evolved,   was   proposed   in   the seventeenth century. One stipulation to make the law   effective   for   practical   applications   was   a piston that would “fit” the opening in the vessel “exactly.”   However,  it  was  not  until  the  late eighteenth  century  that  Joseph  Brahmah  invented an effective piston seal, the cup packing. This led to  Brahmah's  development  of  the  hydraulic  press. The packing was probably the most important invention in the development of hydraulics as a leading   method   of   transmitting   power.   The development of machines to cut and shape closely fitted  parts  was  also  very  important  in  the development  of  hydraulics.  However,  regardless of how precise the machining process is, some type of packing is usually required to make the piston, and  many  other  parts  of  hydraulic  components, “fit exactly.” This also applies to the components of  pneumatic  systems. Through  years  of  research  and  experiments, many different materials and designs have been created  in  attempts  to  develop  suitable  packing devices.  Suitable  materials  must  be  durable,  must provide effective sealing, and must be compatible with the fluid used in the system. The packing materials are commonly referred to as seals or sealing devices. The seals used in fluid power systems and components are divided into two general classes-static seals and dynamic seals. The static seal is usually referred to as a gasket. The function of a gasket is to provide a material that  can  flow  into  the  surface  irregularities  of mating areas that require sealing. To do this, the gasket  material  must  be  under  pressure.  This requires   that   the   joint   be   tightly   bolted   or otherwise  held  together. The  dynamic  seal,  commonly  referred  to  as a packing, is used to provide a seal between two parts  that  move  in  relation  to  each  other. These   two   classifications   of   seals—gaskets and  packing—apply  in  most  cases;  however, deviations  are  found  in  some  technical  publi- cations.  Certain  types  of  seals  (for  example,  the O-ring,  which  is  discussed  later)  may  be  used either  as  a  gasket  or  a  packing. Many  of  the  seals  in  fluid  power  systems prevent  external  leakage.  These  seals  serve  two purposes—to seal the fluid in the system and to keep  foreign  matter  out  of  the  system.  Other  seals simply prevent internal leakage within a system. NOTE: Although leakage of any kind results in  a  loss  of  efficiency,  some  leakage,  especially internal leakage, is desired in hydraulic systems to provide lubrication of moving parts. This also applies  to  some  pneumatic  systems  in  which  drops of  oil  are  introduced  into  the  flow  of  air  in  the system. The first part of this chapter deals primarily with the different types of materials used in the construction of seals. The next section is devoted to the different shapes and designs of seals and their  application  as  gaskets  and/or  packings  in fluid  power  systems.  Also  included  in  this  chapter are  sections  concerning  the  functions  of  wipers and backup washers in fluid power systems and the  selection,  storage,  and  handling  of  sealing devices. SEAL  MATERIALS As  mentioned  previously,  many  different materials have been used in the development of sealing  devices.  The  material  used  for  a  particular application  depends  on  several  factors:  fluid compatibility, resistance to heat, pressure, wear resistance,  hardness,  and  type  of  motion. The   selection   of   the   correct   packings   and gaskets and their proper installation are important factors  in  maintaining  an  efficient  fluid  power system.  The  types  of  seals  to  be  used  in  a particular piece of equipment is specified by the equipment  manufacturer. 7-1