The CRP propeller provides the ahead and astern propulsion thrust for a vessel by a change in the pitch of the propeller blades. Such changes can be obtained even when the main propulsion machinery, including the propeller shaft, are turning at a high rate of speed. Blade pitch control permits a full range of ahead and astern thrusts. Maximum ahead thrust is provided with the blades in the full ahead pitch position, and maximum astern thrust is provided with the blades in the full astern pitch position. When the propeller blades are set at zero thrust, the propeller shaft may be turning at any speed without imparting thrust to the vessel.

When a change of propeller pitch position is ordered, a pitch position command from the propulsion control system is fed to the controls. This command signal activates the electrohydraulic servocontrol valve which, in turn, activates the flow of control oil to and from the OD box to change the position of the valve rod actuator. The hydraulic power oil flows to the OD box and is admitted to the valve rod via the annular chamber in the OD box and the ports in the valve rod. The oil flows within the bore of the valve rod to the hub servomotor, and returns from the hub via a passage formed between the valve rod and the propulsion shaft bore. The oil leaves the OD box via ports in the OD box shaft and the annular chamber to return to the sump tank. Control oil is regulated by a set of sequencing and reducing valves in the hydraulic system which maintain the required pressure level. Control oil is supplied to the electrohydraulic servocontrol valve. From the servocontrol valve, the control oil flows to one side of the low pressure (LP) chamber of the OD box to drive the valve rod actuator. Control oil returns to the sump through the OD box manifold from the other side of the LP chamber. When the propeller is operating at the desired blade pitch position, the OD box valve rod actuator is hydraulically locked and the hub servomotor is hydraulically held in a stationary position.

The configuration of the regulating valve pin in the hub servomotor allows hydraulic power oil to circulate continuously through the hub servo. The oil pressure developed on each side of the hub servomotor piston is balanced and established at the level necessary to counteract blade loading which would tend to change pitch position. A hydraulic pitch change signal from the electrohydraulic servo control valve moves the valve rod actuator and the valve rod. This movement changes the size of the oil passages to each face of the hub servomotor piston, thereby creating a differential pressure in the circulating oil to each face of the piston. The regulating valve pin then supplies high pressure oil to one face of the piston and connects the other face to the return oil passage. The high pressure oil develops the necessary pressure on the piston face to overcome blade loading and move the turning mechanism and the blades to the desired pitch position. Blade pitch will continue to change until the oil port openings equalize and the oil pressure developed on each face of the piston is balanced.

Removing the oil signal from the valve rod actuator stops motion in the valve rod and the hub servomotor. The self-centering feature of the servomotor over the regulating valve pin provides the restoring force to counteract any hydrodynamic tendency to change pitch from that set by the command signal.

INSPECTIONS

The inspections mentioned here are the minimum requirements for reduction gears. Where defects are suspected, or operating conditions so indicate, inspections should be made at more frequent intervals.

No inspection plates or other fittings of the main reduction gear may be opened without the permission of the engineer officer. Before replacing of an inspection plate, connection, fitting, or cover which permits access to the gear casing, make a careful inspection to ensure that no foreign matter has entered or remains in the casing or oil lines. An entry of the inspections, and the name of the CPO or officer who witnesses the closing of the inspection plate, should be made in the Engineering Log.

PMS INSPECTIONS

The PMS requirements discussed in this section are general in scope. Inspection requirements for your ship are listed in the ships PMS Manual and should be referred to for all maintenance action.

Gears should be jacked DAILYAT ANCHORso that the main gear shaft is moved 1 1/4 revolutions. This jacking should be done with lubricating oil circulating in the system. You should take the following actions QUARTERLY:

1. Sound with a hammer the holding down bolts, ties, and chocks to detect signs of loosening of casing fastenings.

2. Open inspection plates, inspect gears, and oil-spray nozzles. Wipe off oil at different points and note whether the surface is bright or if already corroded, and whether or not new areas are affected.

3. Inspect the strainers for the oil-spray nozzles to see that dirt or sediment has not accumulated in them.

4. Take and record all main thrust bearing readings.

When conditions warrant or if trouble is suspected, a work request should be submitted to a naval shipyard to perform a 7-YEAR INSPEC-TION of the main reduction gears. This inspection includes clearance readings of bearings and journals; alignment checks and readings; and any other inspections, tests, or maintenance work that may be considered necessary.

If the ships propeller strikes ground or a submerged object, a careful inspection should be made of the main reduction gear immediately following the OCCURRENCE of the casualty. In this inspection, the possible misalignment of the bull gear and its shaft should be considered. Where practicable, a naval shipyard should be re-quested to check the alignment and concentricity of the bull gear.