Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGcomparatively great amount of time for maintenance,ELECTROHYDRAULIC ELEVATORSMany naval ships are equipped with electrohydraulicelevators which are used to handleairplanes, bombs, freight, mines, torpedoes,ammunition, and other material. Electrohydraulicelevators may be divided into two generaltypes: the direct plunger lift and the plungeractuatedwire rope lift.DIRECT PLUNGERLIFT ELEVATORS.-The platform of the direct plunger lift typeelevator is raised and lowered by direct connectionunder the platform, with one or morevertical hydraulic rams. Oil from a high pressuretank is directed into the ram during thehoisting operation. Lowering is accomplishedby the oil being discharged from the rams intoa low pressure tank. Pressure is maintainedin the high pressure tank by means of twoelectrical variable displacement pumps, whichtake suction from the low pressure tank. Oneof the pumps is capable of maintaining elevatoroperation atreduced speed. Two electric sumppumps keep the volume of oil in the pressuresystem within specified limits.Special control valves (operated by pilotvalves or a motor) in the pressure and exhaustlines regulate elevator speeds by varying theamount of oil admitted to or discharged fromthe rams. Positive stops and mechanical locks,interlocked with the elevator control system,enable the platform to be stopped, locked, andheld in position at deck level. An equalizersystem maintains the platform at uniform levelunder conditions of unequal loading. Automaticquick-closing valves in the oil line prevent anunrestricted fall of the elevator.PLUNGER-ACTUATED WIRE ROPE LIFTELEVATORS.— The primary difference betweenthe direct plunger lift elevator and the plungeractuatedwire rope lift elevator is that thelatter type is raised by wire rope fastened tothe platform at two or four symmetricallylocated points. Most hydraulic airplane elevatorsare of the plunger-actuated wire rope lift type.The wire ropes in an airplane elevator, througha series of sheaves, are actuated by a horizontalhydraulic ram located beneath the hangar deck.Hydraulic bomb elevators differ fromplunger-actuated wire rope lift elevators in thatthe hoisting wire ropes are wound on drumsdriven through reduction gears by the hydraulicmotor. Raising, lowering, or speed changes areaccomplished by varying the stroke of thevariable delivery pump through differentialgearing. Hydraulic accumulators are not usedwith hydraulic bomb elevators,WINCHESA winch is a deck machine used for hoistingor hauling loads. The main components of awinch are a wire rope drum (or drums), a reductiongear train, and a power unit. Somewinches are provided with one or two gypsyheads for handling manila or other fiber lines.Most ships constructed before World War IIwere provided with steam-powered winches, afew of which remain in naval service. DuringWorld War II, Auxiliary ships were providedwith winches powered by either alternating ordirect-current electric motors. On modernships, a-c electric drive winches are used.Figure 21-8 shows an a-c electric motor drivewinch.Where stepless speed control between zeroand design maximum is required, a variablespeed hydraulic transmission is included betweenthe electric motor and the gear train onthe same bed frame. The variable speed hydraulictransmission consists of a variablevolume pump connected by high pressure tubingto a hydraulic motor, which is usually of thefixed-displacement type. The fluid output fromthe pump passes through the motor and returnsto the pump in a closed circuit, and the speed ofthe motor varies as the volume of fluid from thepump varies. The speed and direction of rotationof the motors are obtained through a manuallyoperated lever control at the pump or at a remotestation. Figure 21-9 shows a typical electrohydraulicwinch.One type of steam-driven winch is illustratedin figure 21-10. The winch illustrated is equippedwith two gypsy heads (1), one mounted at eachend of the main drive shaft, and a single hoistingdrum (10), The drum is provided with astandard type of brake band (3) with a foot-operatedcontrol and ratchet lock. The winch isdriven by a two-cylinder, single-expansion,double-acting reciprocating engine. The driveis by means of a train of spur gears, A gearshift is provided to give two drive speeds.The clutch mechanism consists of a sleeve(11) which is keyed to the crankshaft (12) and528