Chapter 7 DECK MACHINERIES 7.1 General introduce The range of deck machinery is extensive and varied. It can be broadly divided into: + Anchor handling (windlasses and capstans) and mooring (winches and capstans). + Hatch cover handling. + Cargo handling (winches and cranes). - The basic requirement of all these above is to control load associated with chain cable or wire rope Each type of equipment has its own operational requirements, but design and operation are common. -Most deck machinery is idle during much of its life and due to this intermittent duty requirement. But in severe weather conditions, the machinery must operate immediately, when required. Gears and drives are normally designed to a limited rating of one haft to one hour. -It is essential that deck machinery should require minimum maintenance (Totally enclosed equipment with oil bath lubrication for gears & bearings is now standard but maintenance can not be completely eliminated and routine checking & greasing should be carried out on a planned schedule). -In deck machinery, prime movers may be used to perform more than one basic duty. For example, mooring winches are often combined with windlass units so that one prime mover drives both. The port mooring winch motor can thus be used to drive the starboard windlass and vice versa. The three most common forms of main drive used on deck machinery are steam, electric and hydraulic. Steam drives Steam drive is often applied in ships with main steam propulsion plant, especially in large tankers. The machines to drive deck machinery can be steam reciprocating engines or steam turbine. However, this type is not commonly used at present due to the restricted application of steam propulsion Electric drives Electric drives are most commonly used for deck machinery. The electric motors are generally totally enclosed, watertight and in most cases embody a spring applied, magnetically released, fail safe disc brake. -The direct current (D.C) motor can be used with good advantage of flexible control and is normally applied on ships with D.C generators. - Alternative current (A.C) motors are also in common use. The speed of A.C motor can be changed by means of pole changing connections or changing the value of the resistance connected to the rotor Normally, the electric motor drives load directly or through a mechanical reduction gear. Hydraulic drives Hydraulic drives can be broadly sub-divided into constant pressure, constant volume and variable displacement systems. + Constant pressure systems in which the pump always created the maximum pressure, and the speed control of the actuator (hydraulic motors) is gained by throttling the required amount of hydraulic oil to the actuators (hydraulic motors). Due to a large amount of heat is produced by friction losses and due to problems of control, this system is used in small power system such as steering gears and hydraulic remote control system. + Constant volume systems are more common in which the pump has constant capacity and a relief valve is fitted to maintain oil pressure under preset limit. The high torque vane type system which operates between 25bar - 45 bar is a example. As the pump (vane pump) delivers a constant volume of oil, speed control of the hydraulic motor is obtained by throttling the required amount of oil to the motor through a control valve, the remainder being by-passed to the pump suction. The pump discharge pressure is determined by the load, speed and direction of rotation being controlled at the hydraulic motor by the lever of the directional control valve (balanced spool valve) combining check valves and pressure control valves etc.

Constant volume system (vane type system) +A third form of hydraulic drive(variable pressure and volume) consists of a variable displacement axial/radial piston pump which is driven at constant speed, normally by an A.C motor . This pump supplies oil to a fixed displacement hydraulic motor coupled to the mechanical portion of the machinery. The piston stroke and hence the pump delivery is controlled by servo motor. Hydraulic motor in this drive can be axial/radial piston motor. This type of system incorporates a constant horsepower device which limits the maximum amount of power absorbed from the pump driving motor under loaded conditions. Nowadays, the variable displacement system (variable displacement pump) with constant pressure can be applied to anchor and mooring equipment, and hatch cover handling. Many of the hydraulic systems, fitted to deck machinery are of the "unit" type-one pump driving one motor. The hydraulic pump maybe fixed displacement or variable displacement type.

"Unit" type system There is also a "ring main" system in which one centrally located hydraulic pump is able to cater for the needs of a number of auxiliaries working simultaneously at varying loads. For instance, the hydraulic pump of the cargo handling system can be changed over to drive the motor of windlass and mooring winches. Using this system, numbers of components could be minimized and offers considerable savings on capital cost.

Ring-main system 7. 2 Cargo Handling (crane, winches) The duty of a deck winch is to lift and lower a load by means of a fixed wire rope on a barrel, or by means of whipping the load on the warp ends; to top or to luff the derricks. It is essential that the winch should be capable of carrying out the following requirements: +Lift the load at suitable speeds + Hold the load from running back +Lower the load under control +Take up the slack on the slings without undue stress; +Drop the load smartly on the skids by answering the operator's application without delay; +Allow the winch to be stalled when overloaded, and to start up again automatically when the stress is reduced; + Have good acceleration and retardation. * In addition when the winch is electrically driven the requirements are: + prevent the load being lowered at a speed which will damage the motor armature; + stop the load running back should the power supply fail; + prevent the winch starting up again when the power is restored Cargo handling equipment consists of derricks and deck cranes ● Derricks consist of: - Union Purchase rig was used on most older ship. - Slewing derricks.

Union purchase rig

Slewing Derrick On Union Purchase rig, a fixed outreach system uses two derricks, one is topped to a position over the ship's side and the other to a position over the cargo hold. The main advantages of this system are that only two winches are required for each pair of derricks and speed of lifting the load is faster than the slewing derrick system (It has a faster cycle time). The disadvantages of this system are that firstly if the outreach requires adjustment, cargo work must be interrupted, and secondly the load that can be lifted is less than the safe working load of the slewing derrick since an indirect lift is used. More over considerable time and man power is required to prepare a ship for cargo working.

The slewing derrick has the advantages: • There is no interruption in cargo work for adjustments and • Cargo can be more accurately placed in the hold Disadvantage of the slewing derrick system: Three winches are required for each derrick to hoist, to luff and to slew ● Deck cranes: • Deck crane can rotate on roller bearings. A toothed rack is formed on the periphery of the supporting seat and a motor driven pinion meshes with the rack to provide drive. • Spring loaded disc or band brakes are fitted on all the drive motors. These are arranged to fail safe in the event of a power or hydraulic failure. The brakes are also arranged to operate in conjunction with motor cut-out when the crane has reached its hoisting and luffing limits or if slack turns occur on the hoist barrel (drum). • One deck crane can be used for loading and discharging cargo of two cargo holds. • Three hydraulic motor must be used : one for lifting cargo, one for luffing and one for slewing, however some deck cranes, the jib is luffed by hydraulic rams (there are two motor, one for hoisting and one for slewing). One hydraulic pump is used for these three actuators. • Deck crane requires less time to prepare for working cargo than derricks • This system is able to accurately place cargo in the hold • In deck crane system, hydraulic pumps and hydraulic motor can be axial piston type or vane type .

Hydraulic system for deck crane 7. 3. Windlass and Mooring system (winches and capstans) Requirements for anchor windlasses The efficient working of the anchor windlass is essential to the safety of the ship. An anchor windlass can expect to fulfill the followings: - The windlass cable-lifter brakes must be able to control the running anchor and cable when the cable-lifter is disconnected from the gearing during 'letting go'. Average cable speeds vary between 5-7m/sec during this operation. - The windlass must be able to heave a certain weight of cable at a specified speed. This 'full load' duty of the windlass varies (it may be as high as 70 tone) but is commonly between 4 and 6 times the weight of one anchor, the speed of haul being at least 9m/min and up to 15m/min. - The braking effort obtained at the cable-lifter must be at least equal to 40 percent of the braking strength of the cable. -Most anchor handling equipment incorporates warping ends for mooring purposes. The design of anchor windlasses may vary depending on specific purposes.

Anchor windlass with two cable lifters and two warping ends Windlass and mooring winches • Due to the low speed of rotation required of the cablelifter while heaving anchor (3-5 rpm), a high gear reduction is needed when the windlass is driven by a hydraulic motor or high speed electric motor. • Windlasses are normally controlled from a local position, the operator manually applying the cablelifter brake as required to control the speed of running cable, and • While heaving and dropping anchor, the operator is positioned at the windlass or at ship side so that he can monitor anchor handling. • Due to the large size of the reduction gears and the clutching arrangements required, these gears are often of the open type and are lubricated with grease of open gear compounds. Mooring winches • The duty of the mooring winch is to warp the ship • Mooring winches provide the facility for tensioning the rope up to the stalling capacity of the winch, usually 1.5 times full load, thereafter the load is held by brake when the power is shut off. • The winch can not pay out rope unless the brake is released (manually operated) • Mooring winches are usually controlled at the local position, i.e. at the winch • As mooring techniques vary widely, the position and type of control must be engineered to suit the application.

Hydraulic system for anchor windlasses Capstans/anchor capstans Capstans and anchor capstans can be designed for electric or hydraulic drive. A capstan normally consists of an electric or a hydraulic motor driving a shaft situated vertically, a warping end rigidly fitted on the free end of the shaft. The driving motor can be designed to fit below or above deck. -For anchor capstan, the arrangement is as for a capstan, but with a cable-lifter between the warping end and deck. The cablelifters are mounted horizontally being driven by vertical shaft. The cable-lifter can be fixed by a shaft key, or carried in plain bearings and a thrust bearing. In the latter case the cable-lifter is disengaged by a coupling.

Capstan

Hydraulic system for windlass and mooring winch

Windlass and mooring winch

Mooring winch

7.4. Hatch cover handling • Although not strictly a 'machinery' item, the mechanical complexity of present day cargo hatch cover-whose periodic maintenance may fall in the domain of the ship's engine department. • Many types of mechanically operated hatch covers can now be found at sea. • Hydraulic system is used nowadays for hatch cover handling. Pressurized hydraulic oil of this system can be from a 'hatch cover' hydraulic pump or from pumps of the windlass and mooring winches.

Hydraulic system for handling hatch cover

Questions. 1. State the function of mooring winch and windlass. 2. State the function of cargo handling equipment. 3. What is "Unit" type and "Ring main" type hydraulic system. 4. State function of parts of the hydraulic system for mooring winch and windlass. Explain operating principle of this hydraulic system for mooring winch and windlass. 5. State function of parts of the hydraulic system for a slewing derrick and explain operating principle of this hydraulic system for a slewing derrick. 6. Explain operating principle of the hydraulic system for handling hatch cover.