Once  flexible  hose  assemblies  are  installed, there  are  no  servicing  or  maintenance  require- ments   other   than   periodic   inspections.   These inspections  are  conducted  according  to  mainte- nance instruction manuals (MIMs), maintenance requirement   cards   (MRCs),   and   depot-level specifications. TYPES OF FITTINGS AND  CONNECTORS Some  type  of  connector  or  fitting  must  be provided to attach the lines to the components of the  system  and  to  connect  sections  of  line  to each  other.  There  are  many  different  types  of connectors and fittings provided for this purpose. The  type  of  connector  or  fitting  required  for  a specific  system  depends  on  several  factors.  One determining factor, of course, is the type of fluid line  (pipe,  tubing,  or  flexible  hose)  used  in the  system.  Other  determining  factors  are  the type  of  fluid  medium  and  the  maximum  operating pressure  of  the  system.  Some  of  the  most  common types of fittings and connectors are described in the  following  paragraphs. THREADED  CONNECTORS There are several different types of threaded connectors. In the type discussed in this section, both the connector and the end of the fluid line (pipe) are threaded. These connectors are used in some  low-pressure  fluid  power  systems  and  are usually made of steel, copper, or brass, and are available  in  a  variety  of  designs. Threaded connectors are made with standard pipe  threads  cut  on  the  inside  surface.  The  end of  the  pipe  is  threaded  with  outside  threads. Standard  pipe  threads  are  tapered  slightly  to ensure tight connections. The amount of taper is approximately  3/4  inch  in  diameter  per  foot  of thread. Metal  is  removed  when  a  pipe  is  threaded, thinning  the  pipe  and  exposing  new  and  rough surfaces. Corrosion agents work more quickly at such points than elsewhere. If pipes are assembled with  no  protective  compound  on  the  threads, corrosion  sets  in  at  once  and  the  two  sections stick  together  so  that  the  threads  seize  when disassembly is attempted. The result is damaged threads and pipes. To  prevent  seizing,  a  suitable  pipe  thread compound  is  sometimes  applied  to  the  threads. The  two  end  threads  must  be  kept  free  of 5-12 compound  so  that  it  will  not  contaminate  the fluid. Pipe compound, when improperly applied, may  get  inside  the  lines  and  components  and damage  pumps  and  control  equipment. Another  material  used  on  pipe  threads  is sealant tape. This tape, which is made of PTFE, provides  an  effective  means  of  sealing  pipe connections   and   eliminates   the   necessity   of torquing  connections  to  excessively  high  values in order to prevent pressure leaks. It also provides for ease of maintenance whenever it is necessary to disconnect pipe joints. The tape is applied over the male threads, leaving the first thread exposed. After   the   tape   is   pressed   firmly   against   the threads,  the  joint  is  connected. FLANGE  CONNECTORS Bolted   flange   connectors   (fig.   5-14)   are suitable  for  most  pressures  now  in  use.  The flanges  are  attached  to  the  piping  by  welding, brazing, tapered threads (for some low-pressure systems),  or  rolling  and  bending  into  recesses. Those illustrated are the most common types of flange  joints  used.  The  same  types  of  standard fitting shapes (tee, cross, elbow, and so forth) are manufactured  for  flange  joints.  Suitable  gasket material must be used between the flanges. WELDED  CONNECTORS The   subassemblies   of   some   fluid   power systems  are  connected  by  welded  joints,  especially in high-pressure systems which use pipe for fluid lines. The welding is done according to standard Figure 5-14.—Four types of bolted flange connectors.