the required state of cleanliness of all the units and theirfluids. Regular operation of hydraulic equipmentprevents the accumulation of sludge and the freezing ofadjacent parts. Regular use also aids in preventingcorrosion. The necessity for proper lubrication andcleanliness cannot be too strongly emphasizedDetailed instructions on the maintenance of aspecific unit should be obtained from the appropriatemanufacturer’s technical manual, but the followinggeneral information will also be useful.The Fluid SystemIf an inspection of an oil sample drawn from ahydraulic system reveals evidence of water, sludge, oracidity, the system must be drained, then cleaned withprescribed acid-free cleaning fluid (flushing oil), andfilled with clean hydraulic oil. A hydraulic system maybe drained and cleaned as follows:1. Remove the permanent filters and wash them influshing oil. Then use low-pressure air for dryingpurposes. If the filters have replaceable elements, installnew elements.2. Drain the system of old hydraulic oils ascompletely as possible.3. Close all connections, and fill the system withacid-free cleaning fluid.4. Start andoperate the unit under idling conditionsto fill the system thoroughly with cleaning fluid.5. Secure the unit and allow it to stand idle for theprescribed period (usually about an hour). This periodof idleness permits the cleaning fluid to dissolve anysludge.6. Start and operate the unit with a light load for 3to 5 minutes, unless otherwise specified. Allow theequipment to stand idle for about 15 minutes, then repeatthe whole cleaning process. Do this two or three times.Never operate a hydraulic unit with a full load whenit is filled with cleaning fluid. Keep the operatingpressure as low as possible.7. If time permits, allow the system to stand idlefor an additional hour following the series of shortoperating periods.8. Drain the system of cleaning fluid. Recleanpermanent filters or, if necessary, install newreplaceable filters. Close the system, and fill it with theproper hydraulic oil.As the system is filled, strain the hydraulic oilthrough a fine wire screen of 180 or 200 mesh. If the oilis not clean, run it through a centrifuge. You shouldprovide adequate protection against dust and moistureentering the system. Moisture should be expelled fromthe oil before it is poured into a system. Oil withnoticeable water content should be rejected orcentrifugedWhen a hydraulic system is being filled, sufficienthydraulic fluid should be used to completely fill theactive 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 surethe valves are closed tightly after the system has beenfilled. For more information on hydraulic fluid filtration,read NSTM, Chapter 556, “Hydraulic Equipment(Power Transmission and Control).” For additionalinformation on hydraulic fluids, refer to NSTM, Chapter262, “Lubricating Oils, Greases, and Hydraulic Fluidsand Lubricating Systems.”Pumps and MotorsWhether the pumps and motors of hydraulictransmission are of the axial or radial piston type, themaintenance procedures, as well as the operatingprinciples, are relatively the same. In general,maintenance information on other types of pumps alsoapplies to hydraulic pumps and motors. For moreinformation concerning hydraulic pumps and motors,read section 2 of NSTM, Chapter 556, “HydraulicEquipment (Power Transmission and Control).”Neoprene is the most commonly used seal aroundthe shafts of most modern hydraulic pumps and motors,but other types of shaft packing are also used.On some modern hydraulic transmissions, shaftstuffing box packing is of the square-braided pureasbestos type. This packing is easily removed, but youmust take care to be sure that it is not replaced too tightly.If properly installed, this packing makes a tight jointwhen you apply light pressure. If packing wears quickly,the shaft should be inspected for roughness. If a lathe isavailable, you may be able to eliminate the roughnessfrom the shaft by a finishing cut to smooth the surface.If a lathe is not available, it may be necessary to replacethe shaft. Packing should be renewed at prescribedintervals to eliminate the possibility of the packingbecoming hard and scoring the shaft. When packing isbeing replaced, make certain there is a uniformthickness around the shaft. An excess of packing on oneside of the shaft will cause breakage. Stuffing boxes8-8
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