rise to a predetermined pressure, the valveautomatically returns to the neutral position and,consequently, to open-center flow.One of the advantages of the open-centersystem is that the continuous pressurization of thesystem is eliminated. Since the pressure isgradually built up after the directional controlvalve is moved to an operating position, there isvery little shock from pressure surges. Thisprovides a smooth operation of the actuatingmechanisms; however, the operation is slowerthan the closed-center system in which the pressureis available the moment the directional controlvalve is positioned. Since most applicationsrequire instantaneous operation, closed-centersystems are the most widely used.HYDRAULIC POWER DRIVE SYSTEMThe hydraulic power drive has been usedin the Navy for many years. Proof of itseffectiveness is that it has been used to train andelevate nearly all caliber guns, from the 40-mmgun mount to the 16-inch turret. In addition togun mounts and turrets, hydraulic power drivesare used to position rocket launchers andmissile launchers, and to drive and control suchequipment as windlasses, capstans, and winches.In its simplest form, the hydraulic power driveconsists of the following:1. The prime mover, which is the outsidesource of power used to drive the hydraulic pump2. A variable-displacement hydraulic pump3. A hydraulic motor4. A means of introducing a signal to thehydraulic pump to control its output5. Mechanical shafting and gearing thattransmits the output of the hydraulic motor to theequipment being operatedHydraulic power drives differ in somerespects, such as size, method of control, and soforth. However, the fundamental operatingprinciples are similar. The unit used in thefollowing discussion of fundamental operatingprinciples is representative of the hydraulic powerdrives used to operate the 5"/38 twin mounts.Figure 12-5 shows the basic components ofthe train power drive. The electric motor isconstructed with drive shafts at both ends. Theforward shaft drives the A-end pump throughreduction gears, and the after shaft drives theauxiliary pumps through the auxiliary reductiongears. The reduction gears are installed becauseFigure 12-5.-Train power drive—components.
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