Lesson 19 • Cargo handling equipment • (货物装卸设备)
The various items of machinery and equipment can be found outside of the machinery space.
These include deck machinery such as mooring equipment(系缆设备),anchor handling equipment(抛锚设备),cargo handling equipment and hatch covers(舱口盖).
Other items include lifeboats(救生艇) and liferafts(救生筏),emergency equipment, watertight doors, valve actuator, stabilizers(减摇装置) and bow thrusters(船头推进器).
Cargo winches are used with various derrick(吊杆) systems arranged for cargo handling.
The unit is rated according to the safe working load to be lifted and usually has a double speed provision(装置) when working at half load.
In the cargo winch, spur reduction gearing transfers the motor drive to the barrelshaft.
A warpend(卷绕端) may be fitted for operating the derricktopping lift (顶牵索) (the wire which adjusts the derrick height).
Manually operated band brakes may be fitted and the drive motor will have a brake arranged to fail-safe(故障保护), i.e. it will hold the load if power fails or the machine is stopped.
A derrick rig(装置) is known as ‘union purchase’(双杆联吊装置).
One derrick is positioned over the quayside and the other almost vertically over the hold(货舱).
Topping wires fix the height of the derricksand stays(稳索)to the deck may be used to prevent fore and aft movement.
Topping wires Derricks Cargo handling wires Winches
A combination of movements from two winches enables lifting, transferring and lowering of the cargo.
Positioned between the holds, often on a plateform which can be rotated through 360°, the deck crane provides an immediately operational unit requiring only one man to operate it.
In a general cargo crane, three separate drives provide the principal movements:
a hoisting(起升) motor for lifting the load, a luffing(变幅) motor for raising or lowering the jib(boom), and a slewing(旋转) motor for rotating the crane.
The crane is usually mounted on a pedestal(底座) to offer adequate visibility to the operator.
For occasional heavy loads, two cranes can be arranged to work together.
The use of hydraulic drives in cranes, winches and similar equipment is now accepted as a conventionalmethod of drive in many case.
Many reasons can be suggested for the use of hydraulic systems in marine engineering:
a) A convenient method of transferring power over relatively long distances from, say, a central pump room to remote operating sites.
b) Fully variable speed control of both linear and rotary motion, with good ‘inching’ capability and smooth take up of load.
c) High static forces or torques can be achieved and maintained indefinitely(无限期地).
d) Complete safety and reliability is assured under the most difficult environmental conditions;
overload conditions are safeguarded by using a relief valve to limit maximum output torques or forces.
The wide range of high torque, low speed motors are available for driving winch drums directly so dispensingwith(免除)the need to fit reduction gearing.
Motors of this type are more common on larger cranes where reduction gearboxes would be expensive.
All fixed capacity motors, whether high or low speed, require a high flow at low pressure to obtain high light hook speeds and consequent short cycle times.
With large cranes this can become an embarrassmentand one solution with low speed motors is to use the two speed type to double the speed and half the output torque at light hook load.
High speed motors with reduction gearing have the advantage that the brake can be mounted on the high speed shaft and so be much smaller and cheaper.
Because of the availability of standard low cost gearboxes this arrangement is popular on light cranes.
Variable capacity axial piston motors have the important advantage that light hook speeds can be increased up to four times from a given oil flow by reducing the motor capacity to 25 per cent of the maximum.
Hydraulic Crane • The hoisting of the cargo is effected by a hoisting winch driven by a hydraulic motor that
that provides a constant torque in both directions of rotation and at all speeds within the regulation range of the motor.
The winch is equipped with a mechanical belt brake that acts on the outside of the rotating motor housing.
During operation, the brake is released by means of oil pressure which compresses a spring.
The crane jib is raised and lowered by means of a luffing winch which is constructed in a similar manner to the hoisting winch.
The pumps are axial piston pumps and are driven by a common 3-phased synchronous motor(三相同步电动机) at a constant speed of rotation.
The crane is turned by a hydraulic motor of the same type as that described above, in that the rotor turns a pinion which engages with a toothed ring on the crane housing.
An oil cooler is used to remove the heat generated in the oil during the operation of the crane.
The system is also provided with a preheater for heating the oil in cold weather.
If the temperature of the oil reaches 85oC, a thermostat(温度调节装置)in one of the oil lines switches off the electric motor.
Pressure relief valves are built into the system to protect the system and the crane against excessive loads.