should be packed loosely and the packing gland set uplightly to allow adequate leakage for cooling andlubrication. See NSTM, Chapter 078, “Gaskets,Packings, and Seals,” for more detailed discussion ofO-rings and other types of seals for hydraulic systemequipment.There is very little likelihood of poor alignmentbetween the driving and driven members of a hydraulictransmission if the wedges, shims, jacking screws, oradjusting setscrews were properly set and secured whenthe connecting units were installed. However, when acasualty occurs or a unit is replaced, it is possible for theunit to become misaligned enough to cause severe stressand strain on the coupling and connected parts.Excessive misalignment should be eliminated as soonas possible by replacing any defective parts and byreadjusting the aligning devices. If this is not done, pins,bushings, and bearings will wear out too fast and willhave to be replaced frequently.Since there is no end play to either the pump shaftor the motor shaft, flexible couplings are generally usedin hydraulic transmissions. Such couplings permitsatisfactory operation with a slight misalignment,without requiring frequent renewal of parts.Pipings and FittingsIf properly installed, the piping and valves of ahydraulic system are seldom a source of a trouble,except for leakage. Some leaks, however, can be seriousenough to cause a reduction in the efficiency of the unit.You should make frequent inspections for leakage andtake steps to eliminate any leakage found. Guidance andrequirements for the installation, inspection, andmaintenance of piping and associated fittings arecontained in NSTM, Chapter 505, “Piping Systems,”If leaks occur at a flanged joint in the line of ahydraulic system, tighten the flange bolts evenly, but notexcessively. If the leaks persist, use the auxiliary gearwhile the leaking flange is being refitted with copperasbestos or O-ring packing. Be sure the flange surfacesare cleaned carefully before the packing is applied.CAUTIONExposure to asbestos fibers is a recognizedhealth hazard. Refer to NSTM, Chapter 635,“Thermal, Fire, and Acoustic Insulation,” forsafety requirements applicable to handlingasbestos packing and gaskets.If certain measures are taken, operation of hydraulicequipment may be continued while leakage repairs arcbeing made in some parts of the system. When the linesin an auxiliary system leak, they should be valved offfrom the main line connection to prevent leakagebetween the two systems. If leaks occur in the pumpingconnections to the three-way valves of a steering gearinstallation, the pump can be cut out with the valve, andanother pump cut in. If the three-way valves fail to cutout the leaking unit, and it becomes necessary to cut outboth pumps of a steering gear installation, the valvesmay be closed at the ram cylinder. Hydraulic systemswill work without pressure control. So by closing thevalves in the lines where they join the main piping,leaking pressure control pipes or cylinders can be cutout of the system for repairs.Expansion lines and replenishment lines inhydraulic systems of older ships are seldom a source ofleakage or breakage, since they are not under anyappreciable pressure. However, all hydraulic lineconnections must be maintained intact. in more recentinstallations, however, replenishing lines are underpressure as much as 300 psi. In these moderninstallations, the hydraulic systems should not beoperated during the repair of these lines.Relief valves and shuttle valves of a hydraulicsystem may also be a source of trouble. The seats ofrelief valves that are leaking should be reground. Lossof power is a symptom of a leaking relief valve. Shuttlevalves may stick and fail to cut off. This condition isevidenced either by the escape of oil from thehigh-pressure side of the line into the expansion tank orby the failure of the pressure control. When a shuttlevalve fails to operate, the stop valves should be closedand the defective valve removed and repaired.Incorrectly adjusted needle valves can be anothersource of trouble. Needle valves that are adjusted toofine may cause the device operated by the valve to stopshort of its intended stopping point. This may happenbecause the valve adjustment allows more fluid to passthrough leakage points in the system than through thevalve. NSTM, chapter 556, provides a good source ofgeneral information concerning different types of valvesused in hydraulic systems and their maintenance.HYDRAULIC CARGO HATCH COVERSCargo hatch cover opening and closing operationsare supplied by an electrohydraulic power unit. Thesystem consists of an electric motor-driven hydraulicpump mounted on a hydraulic fluid reservoir tank and8-9
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