STANDBY PUMP.—The standby pump is ofthe same type as the main hydraulic pump, butit is mounted at the forward end of the mainreduction gear housing and is driven through adisconnect coupling. The suction strainer and suc-tion gauge for this pump are mounted separately.The primary function of the standby pump is toassist the main pump in effecting pitch changes.When the control pitch (C/P) unit is in the holdingpitch position, the standby pump discharge oil isunloaded back to the sump through the hydraulicblock. But, whenever a pitch change is ordered,the pump discharge oil is directed to the hydraulicblock high pressure passage.LOWER OIL TANK.—The lower (sump) oiltank is usually located aft and below the OD boxassembly. The oil capacity of the sump variesdepending on the type and class of ship. Twopumps, the main and standby hydraulic pumps,take suction on the lower oil tank through a footvalve, which permits the oil to flow from the tankbut does not allow it to return through the suc-tion line.UPPER GRAVITY OIL TANK.—This tankis located above the maximum draft line. Its mainpurpose is to maintain hub oil pressure above thatof the surrounding seawater when the C/P unitis secured. In the Kamewa installation, theupper gravity oil tank serves an additional pur-pose. During C/P unit operation the tank assistsin maintaining the sliding ring chamber pressure.PRINCIPLES OF OPERATIONThe CRP propeller provides the ahead andastern propulsion thrust for a vessel by a changein the pitch of the propeller blades. Such changescan be obtained even when the main propulsionmachinery, including the propeller shaft, areturning at a high rate of speed. Blade pitch con-trol permits a full range of ahead and asternthrusts. Maximum ahead thrust is provided withthe blades in the full ahead pitch position, andmaximum astern thrust is provided with the bladesin the full astern pitch position. When the pro-peller blades are set at zero thrust, the propellershaft may be turning at any speed without im-parting thrust to the vessel.When a change of propeller pitch position isordered, a pitch position command from the pro-pulsion control system is fed to the controls. Thiscommand signal activates the electrohydraulic ser-vocontrol valve which, in turn, activates the flowof control oil to and from the OD box to changethe position of the valve rod actuator. Thehydraulic power oil flows to the OD box and isadmitted to the valve rod via the annular chamberin the OD box and the ports in the valve rod. Theoil flows within the bore of the valve rod to thehub servomotor, and returns from the hub via apassage formed between the valve rod and thepropulsion shaft bore. ‘The oil leaves the OD boxvia ports in the OD box shaft and the annularchamber to return to the sump tank. Control oilis regulated by a set of sequencing and reducingvalves in the hydraulic system which maintain therequired pressure level. Control oil is supplied tothe electrohydraulic servocontrol valve. From theservocontrol valve, the control oil flows to oneside of the low pressure (LP) chamber of the ODbox to drive the valve rod actuator. Control oilreturns to the sump through the OD box manifoldfrom the other side of the LP chamber.When the propeller is operating at the desiredblade pitch position, the OD box valve rodactuator is hydraulically locked and the hub ser-vomotor is hydraulically held in a stationary posi-tion. The configuration of the regulating valve pinin the hub servomotor allows hydraulic power oilto circulate continuously through the hub servo.The oil pressure developed on each side of the hubservomotor piston is balanced and established atthe level necessary to counteract blade loadingwhich would tend to change pitch position. Ahydraulic pitch change signal from the elec-trohydraulic servo control valve moves the valverod actuator and the valve rod. This movementchanges the size of the oil passages to each faceof the hub servomotor piston, thereby creatinga differential pressure in the circulating oil to eachface of the piston. The regulating valve pin thensupplies high pressure oil to one face of the pistonand connects the other face to the return oilpassage. The high pressure oil develops thenecessary pressure on the piston face to overcomeblade loading and move the turning mechanismand the blades to the desired pitch position. Bladepitch will continue to change until the oil portopenings equalize and the oil pressure developedENGINEMAN 1 & C4-14
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