ENGINEMAN  1  &  C pins or keys; or breakage of gearing, shafting, or linkage. Elimination of troubles resulting from any of these causes should be accomplished ac- cording to the manufacturer’s instructions for the specific  piece  of  equipment. MAINTENANCE The principal requirements necessary to keep a hydraulic transmission in satisfactory operating condition are regular operation, proper lubrica- tion, and the maintenance of all the units and the fluid in the required state of cleanliness. Regular operation of hydraulic equipment prevents the accumulation of sludge and the freezing of adja- cent  parts,  and  aids  in  preventing  corrosion.  The necessity of 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 the 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  permanent  filters  and  wash  them in flushing oil. Then use low pressure air for dry- ing purposes. If filters have replaceable elements, install  new  elements. 2. Drain the system of old hydraulic oil as completely  as  possible. 3. Close all connections and fill the system with  acid-free  cleaning  fluid. 4. Start and operate the unit under idling con- ditions in order to fill the system thoroughly with the 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 a short interval of time (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. After  each  short  operating  period,  turn  the cleaning handles of edge type filters (if installed) and drain from the filter an amount of cleaning fluid equal to its volume. 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,  if  necessary;  install  new replaceable filters. Close the system, and fill it with the proper hydraulic oil. As the system is filled, the hydraulic oil should be strained through a fine wire screen of 180 or 200 mesh. If oil is not clean, it should be run through a centrifuge. Adequate protection should be  provided  against  dust  and  moisture.  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, suf- ficient hydraulic fluid should be used to com- pletely  fill  the  active  parts  of  the  mechanism, leaving  no  air  pockets.  Air  valves  should  be 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. Pumps and Motors Whether the pumps and motors of hydraulic transmissions 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. Neoprene is utilized as a seal around the shafts of most modern hydraulic pumps and motors, but other types of shaft packing are also used. 7-18