the required state of cleanliness of all the units and their fluids.  Regular  operation  of  hydraulic  equipment prevents the accumulation of sludge and the freezing of adjacent  parts.  Regular  use  also  aids  in  preventing corrosion.  The  necessity  for  proper  lubrication  and cleanliness cannot be too strongly emphasized Detailed  instructions  on  the  maintenance  of  a specific unit should be obtained from the appropriate manufacturer’s  technical  manual,  but  the  following general information will also be useful. The Fluid System If  an  inspection  of  an  oil  sample  drawn  from  a hydraulic system reveals evidence of water, sludge, or acidity, the system must be drained, then cleaned with prescribed  acid-free  cleaning  fluid  (flushing  oil),  and filled with clean hydraulic oil. A hydraulic system may be drained and cleaned as follows: 1.   Remove the permanent filters and wash them in flushing   oil.   Then   use   low-pressure   air   for   drying purposes.  If  the  filters  have  replaceable  elements,  install new  elements. 2.  Drain  the  system  of  old  hydraulic  oils  as completely  as  possible. 3. Close all connections, and fill the system with acid-free  cleaning  fluid. 4.   Start andoperate the unit under idling conditions to  fill  the  system  thoroughly  with  cleaning  fluid. 5.   Secure the unit and allow it to stand idle for the prescribed period (usually about an hour). This period of idleness permits the cleaning fluid to dissolve any sludge. 6.   Start and operate the unit with a light load for 3 to  5  minutes,  unless  otherwise  specified.  Allow  the equipment to stand idle for about 15 minutes, then repeat the whole cleaning process. Do this two or three times. Never operate a hydraulic unit with a full load when it  is  filled  with  cleaning  fluid.  Keep  the  operating pressure as low as possible. 7. If time permits, allow the system to stand idle for  an  additional  hour  following  the  series  of  short operating  periods. 8.  Drain  the  system  of  cleaning  fluid.  Reclean permanent   filters   or,   if   necessary,   install   new replaceable filters. Close the system, and fill it with the proper  hydraulic  oil. As  the  system  is  filled,  strain  the  hydraulic  oil through a fine wire screen of 180 or 200 mesh. If the oil is  not  clean,  run  it  through  a  centrifuge.  You  should provide adequate protection against dust and moisture entering the system. Moisture should be expelled from the  oil  before  it  is  poured  into  a  system.  Oil  with noticeable  water  content  should  be  rejected  or centrifuged When a hydraulic system is being filled, sufficient hydraulic  fluid  should  be  used  to  completely  fill  the active parts of the mechanism, leaving no air pockets. Air valves should he opened during the filling process, so that air can escape to the oil expansion box. Be sure the valves are closed tightly after the system has been filled. For more information on hydraulic fluid filtration, read   NSTM,   Chapter   556,   “Hydraulic   Equipment (Power  Transmission  and  Control).”  For  additional information on hydraulic fluids, refer to NSTM,  Chapter 262, “Lubricating Oils, Greases, and Hydraulic Fluids and  Lubricating  Systems.” Pumps and Motors Whether   the   pumps   and   motors   of   hydraulic transmission are of the axial or radial piston type, the maintenance  procedures,  as  well  as  the  operating principles,   are   relatively   the   same.   In   general, maintenance information on other types of pumps also applies  to  hydraulic  pumps  and  motors.  For  more information concerning hydraulic pumps and motors, read  section  2  of  NSTM,   Chapter  556,  “Hydraulic Equipment (Power Transmission and Control).” Neoprene is the most commonly used seal around the shafts of most modern hydraulic pumps and motors, but other types of shaft packing are also used. On  some  modern  hydraulic  transmissions,  shaft stuffing  box  packing  is  of  the  square-braided  pure asbestos type. This packing is easily removed, but you must take care to be sure that it is not replaced too tightly. If properly installed, this packing makes a tight joint when you apply light pressure. If packing wears quickly, the shaft should be inspected for roughness. If a lathe is available, you may be able to eliminate the roughness from the shaft by a finishing cut to smooth the surface. If a lathe is not available, it may be necessary to replace the  shaft.  Packing  should  be  renewed  at  prescribed intervals  to  eliminate  the  possibility  of  the  packing becoming hard and scoring the shaft. When packing is being   replaced,   make   certain   there   is   a   uniform thickness around the shaft. An excess of packing on one side  of  the  shaft  will  cause  breakage.  Stuffing  boxes 8-8