Bird-Johnson Building World's Most Powerful CP Propellers For DDG-51
In March this year, Bath Iron Works Corporation awarded to Bird-Johnson Company of Walpole, Mass., a contract to supply the controllable- pitch main propulsion equipment for the DDG-51, the lead ship of the Arleigh Burke Class of guided-missile destroyers. Delivery of these CP propellers, at 50,000 shp each, will mark a world record CPP power output.
Extensive research and development enabled Bird-Johnson's engineers to develop this CPP system to conform to the unique requirements of the DDG-51, a surface combatant designed to reach speeds in excess of 30 knots. The CP propellers for the DDG-51 will provide a full range of ahead and astern thrust by changing the propeller pitch without changing the directional rotation of the propulsion machinery. The CPP system will impart a high degree of maneuverability to the ship as well as providing improved power utilization, faster acceleration, quicker stopping time, better slow-speed control, and reduced maintenance of engines and gear boxes.
Five major subassemblies will make up the CPP system: the propeller blades, hub assembly, oil distribution box, Prairie air emission system, and hydraulic system.
Propeller Blades. Each nickelaluminum- bronze propeller blade will be finish-machined overall by numerically controlled machining, and skewed in form to minimize vibration and noise. The complete propeller assembly will consist of five such blades and measure 17 feet in diameter.
Hub Assembly. Pitch changes are effected by the hub servomotor, which consists of a crosshead, piston, closed cylinder, piston rod, and main regulating valve. Sliding blocks fit in machined slots in the crosshead, and an eccentrically located pin on the underside of each crank ring fits in a machined hole in each sliding block. The crank ring, which is a closed die steel forging, fits over and rotates on a centerpost integral to the hub body. This crossbar and centerpost construction insures hub rigidity. Each propeller blade is attached to a crank ring with non-corroding K-Monel blade bolts. A bearing ring retains each crank ring in the hub body. When the piston, crosshead, and piston rod assembly moves axially, the sliding block exerts force on the pin of the crank ring. Axial movement of the piston rod thus causes rotation of the crank ring and blade assembly.
All moving parts of the hub work in oil. Blade seals preclude the possible admission of seawater into the hub internals and prevent oil leakage.
The hub body is centrifugally cast of nickel-aluminum-bronze and mounted on the propeller shaft flange with O-ring-sealed bolts.
Dowels transmit all torque between the hub and body shafting and between the blades and crank rings.
Oil Distribution Box. The OD box is mounted on the forward end of the main reduction gear casing. It supplies the passages for pressure oil and return oil, provides a mechanical pitch indication, and also contains the auxiliary servomotor that actuates the valve rod. The OD box includes provisions for mechanically locking the propeller in the full ahead position for safe emergency take-home operation in the unlikely event of failure of the redundant hydraulic system.
Prairie System. A rotary seal at the OD box admits air from the ship's air supply to the rotating Prairie tubing located within the valve rod. Air flows through the tubing, enters drilled passages in the hub, and is directed to the base of each blade. Air then passes through a machined channel running along the blade's leading edge from the root to the tip. It is discharged into the water over the blade's surface through holes near the leading edge of the blade. Check valves prevent seawater from entering the system when it is secured.
Hydraulic System. The hydraulic system is of modular design, employing components selected to meet naval operating, shock, noise, vibration, and safety requirements.
The hydraulic oil power module (factory-assembled to simplify installation) delivers oil to the OD box and thereby to the hub mechanism, with the main pump providing oil flow and pressure under normal operating conditions. The pump discharges oil through filters to the pressure control assembly that regulates control oil and pressure oil. A second pump is provided for standby operation. Each pump alone provides sufficient oil flow to meet the specified pitch-changing time.
The Bird-Johnson DDG-51 propellers are being designed and manufactured for quiet operation. Due to its unidirectional rotation, the CP propeller always maintains the same leading edge, regardless of direction of thrust. The hydrodynamic blade form is never compromised by strength considerations for astern rotation. The result is that highly skewed blades can be used to effectively reduce pressure pulses and the resultant noise and vibration.
In addition to the CPP main propulsion system, on-board and shorebased spares have also been contracted for the DDG-51. Bird- Johnson operates a round-the-clock Parts Department for the prompt processing of all spare parts requests.
A Service Department is staffed with representatives experienced in marine propulsion products and with demonstrated proficiency in mechanics, hydraulics, and electrical systems. Service representatives undergo intensive training in the design, operation, installation, maintenance, and repair of Bird- Johnson products to provide superior service—anytime and anywhere.
These representatives are stationed strategically throughout the U.S. to assure timely support of customer requirements.
For further information on Bird- Johnson's products, service, facilities, and capabilities, Circle 59 on Reader Service Card
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