Marine Auxiliary Machinery Chapter 9 Lesson 1 Deck Machinery General By Professor Zhao Zai Li 05.2006
Deck machinery (1) • The range of deck machinery is extensive and varied, it can be divided broadly into: • Anchor handling (windlasses and capstans). • Mooring (winches and capstans). • Cargo handling (winches and cranes). • The basic requirement of all the above is to control loads associated with chain cable or wire rope and whilst each type of equipment has its own operational requirements, certain aspects of design and operation are common.
Deck machinery (2) • Most deck machinery is idle during much of its life and due to this intermittent duty requirement, gears and drives are normally designed to a limited rating of one half to one hour. • Despite long periods of idleness, often in severe weather conditions, the machinery must operate immediately, when required.
Deck machinery (3) • It is essential that deck machinery should require minimum maintenance. • Totally enclosed equipment with oil bath lubrication for gears and bearings is now standard but maintenance cannot be completely eliminated and routine checking and greasing should be carried out on a planned basis. • Prime movers may be used to perform more than one basic duty.
Deck machinery (4) • 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. • This applies also to the power supply where generators or hydraulic pumps are also cross-connected.
Deck machinery (5) • There are many instances where remote or centralised control is of great advantage, for example, the facility for letting go anchor from the bridge under emergency conditions, the use of shipside controllers with mooring winches or the central control position required for the multiwinch slewing derrick system. • The three most common forms of main drive used on deck auxiliaries are steam, electric and hydraulic.
Deck machinery(6) • When fitted, steam auxiliaries are frequently of the totally enclosed type using forced or splash lubrication or a combination of both. • A typical arrangement has an oil pump driven from the crankshaft supplying pressure oil to the crankshaft and connecting rod bearings whilst crossheads, eccentrics and gearing are splash lubricated from the sump or drip trays.
Deck machinery (7) • The steam cylinders and valves are not normally lubricated as, due to the low working pressure and condensation, steam temperatures encountered on deck rarely exceed 180º C. • A superheated steam supply creates problems with cylinder lubrication and in any event has little effect on non-expansive working. • Inlet steam temperatures are limited for use with cast iron cylinders by Classification Society rules. • Full load crankshaft speeds are normally between 90-160 rev/min increasing to twice this figure for light line duties．
Deck machinery (8) • An alternative form of steam drive is the reversing steam turbine which is illustrated in Figure 12.1. • This machine requires less maintenance and it is able to accept higher steam pressures and temperatures, (up to 24 bar,290º C) directly through a reducing valve from the main boilers, thus increasing turbine efficiency and minimising its size and weight. • On large equipment, even though the turbine has to be geared down from a normal full load speed of 2000-2500 rev/min there is a saving in total weight when compared with engine driven equipment．
Deck machinery (9) • As illustrated in Figure 12.1, the turbine shaft and rotor are supported in anti-friction bearings; the coupling end bearing is grease lubricated and the governor mechanism and location bearing are splash oil lubricated. • Although the bearings warm up rapidly due to conduction of heat along the shaft from the exhaust casing, normal lubricating oils and greases have proved satisfactory．
Deck machinery(10) • Both reciprocators and turbines, as used on supertankers，drive the 1argest deck machinery in service. • This equipment is normally locally controlled at the machinery position, however, it is of great advantage to have shipside control for both windlass and mooring winch operation and this can be simply achieved by the use of push pull hydraulic mechanisms which are effective up to approximately 45m．
Figure 9.1 Sectional arrangement of reversing steam turbine
Electric drives (1) • Electric drives are most commonly used for deck machinery. • The motors are generally totally enclosed, watertight and in most cases embody a spring applied, magnetically released, fail safe disc brake. • The direct current motor, although it is relatively costly and requires regular brushgear maintenance, is frequently used where the characteristic flexibility of control may be used to good advantage.
Electric drives (2) • The control of d.c. motors by contactor-switched armature resistances, common in the days when ships’ supplies were mainly d. c., has now largely been superseded by a variety of Ward Leonard type control systems which give a better, more positive control particularly for controlled lowering of loads. • The Ward Leonard generator may be driven by either a d. c. or an a. c. motor.
Electric drives (3) • The most important feature of d. c. drive is its efficiency, particularly in comparison with a. c. drives, when operating at speeds in the lower portion of its working range. • The d. c. motor is the only electric drive at present in production which can be designed to operate in a stalled condition continuously against its full rate torque and this feature is used extensively for automatic mooring winches of the ‘live motor’ type.
Electric drives (4) • The majority of d.c. winch motors develop full output at speeds of the order of 500 rev/min and wherever necessary are arranged to run up to two to four times this speed for light line duties. • Windlass motors on the other hand do not normally operate with a run up in excess of 2:1 and usually have a full load working speed of the order of 1000 rev/min．
Electric drives (5) • D.C. motors may also be controlled by static ‘thyristor’ converters which convert a.c. supply into a variable d. c. voltage of the required magnitude and polarity for any required armature speed. • These converters must be of a type capable of controlled rectification and inversion with bi-directional current flow if full control is to be obtained (Figure9.2) .
Figure 9.2 Load/speed characteristic of Ward Leaoard thyristor controlled winch.
Electric drives (6) • A.C. motors, either ‘wound rotor’ or ‘cage’, are also in common use. • With these the speed may be changed by means of pole changing connections or, in the case of the wound rotor induction motor, by changing the value of the resistance connected to the rotor. • Both methods involve the switching of high currents at medium voltage in several lines simultaneously and the use of multi pole contactors is common. • These drives offer a very limited choice of only two or three discrete speeds such as 0.65,0.325 and 0.1025 m/sec corresponding to 4.8 and 24 pole operation.
Electric drives (7) • The wound rotor type is slightly more flexible in ‘hoist’ control but, as with the resistance controlled d.c. motor, difficulty is experienced in providing effective control of an overhauling load e.g lowering a suspended load. • These disadvantages are often outweighed by lower cost, particularly of the cage induction motor. in comparison with the more flexible d.c. • Typical performance curves are shown in Figure 9.3.
Figure 9.3 Performance curves of a 3 tonne winch. A C Pole-changing cage motor.
Electric drives (8) • Another form of induction motor control system is based on the relationship between output torque and applied voltage, the torque being proportional to the voltage squared. • The controller takes the form of a three-phase series regulator with an arm in each supply line to the motor. • A stable drive system can only be achieved by this means if a closed loop servo control system is used in conjunction with a very fast acting regulator which automatically adjusts the output torque to suit the load demand at the set speed.
Electric drives (9) • Control of an overhauling load is possibly by means of injection braking techniques. • A combined system employing both these control principles can provide full control requirements for all deck machinery. • The a. c. drives described operate at the supply frequency and consequently rapid heating of the motor will occur if the drive is stalled when energised . • The majority of a.c. motors on deck machinery run at a maximum speed corresponding to the 4 pole synchronous speed of 1800 rev/min on a 60 Hz supply.
Electric drives (10) • These speeds are similar to the maximum speeds used with d.c. drives and the bearings and shaft details tend to be much the same. • The motor bearings are normally grease lubricated; however, in some cases where the motor is flange mounted on an oil bath gearcase, the driving end bearing is open to the gearcase oil and grease lubrication is not required.