Crane design
Shipboard cranes are usually of electro-hydraulic design. A crane is normally required to perform three functions—namely, to hoist, to luff, and to slew. Hoisting is the raising of the crane wire whilst the crane jib remains in a constant position.
Luffing is the raising or lowering of the crane jib, and slewing is the swinging round (or rotating) of the crane. Cranes on a few ships are also able to travel along the deck on rails, but this is unusual.
It is common for shipboard cranes to be level luffing. This means that if the crane is topped from maximum to minimum radius, or vice versa, the crane hook will maintain a level path, allowing the load to move horizontally72. The power which enables the crane to hoist, luff and slew is provided by electric motors which drive hydraulic pumps. The hydraulic pumps drive the winches required for the hoisting,and drive the machinery which enables the crane to luff and slew.
Twin cranes have been fitted to some conbulk and forest product ships. The basic idea has been described as extremely simple, and consists of two independent cranes of equal capacity mounted on a common platform. The common platform can be rotated independently and the cranes can be slewed relative to the platform. Each crane can be used by itself, with each serving an adjoining hold, but when a heavy lift is required the jibs are slewed parallel to one another and a lifting beam is attached between the crane hooks. This arrangement allows the lifting of loads of up to twice the safe working load of one crane.
The speed of the hoisting and luffing motions of the cranes is synchronised to ensure smooth operation.
When the cranes are operated in the twin mode the individual slewing motions are inoperable and only the platform slewing motion can be used. Operation of the platform slewing motion causes the platform to rotate and with it the two cranes, with their jibs parallel, thus enabling large loads to be safely slewed.
If shipboard cranes are intended to be used with grabs, as grabbing cranes they are likely to be fitted with rope-operated grabs, for which the crane will be provided with two rope drums and two wire ropes, one to hold the grab and the other to open and close it. Alternatively, electro-hydraulic grabs are easy to fit to existing cranes and grabs can also be operated by remote radio control. These systems replace the simple but inefficient self-dumping grabs which are used on a single fall of wire rope.
Crane maintenance:
The detailed maintenance required by cranes depends upon their construction, and must be learnt from their instruction manuals, which normally provide detailed guidance as to frequency of service, materials to use and items to inspect. The main areas which require attention arethe following:
Daily, when cranes are being used to work cargo
>>Inspect runner wires for wear or damage. Check the rig for damage and for fastenings becoming slack.
>>Check the limit and cutout switches. Carry out oiling and greasing as necessary. Ensure the ventilation for the crane motors is open, to prevent overheating.
Before arrival in port
>> Follow the procedures recommended by the crane manufacturers. They should include all theinspections and tests listed below for regular maintenance.
Regular maintenance
>>The filters of the hydraulic system must be regularly cleaned or renewed and the hydraulic oil must be changed as required.
>>The oil in the gearboxes must be kept topped up to the correct level and must be changed at the required intervals.
>>Suitable grease must be applied to all moving parts of the system, such as the bearings of winches and sheaves, the pivot points or hinges for jib and the roller slewing ring. When the crane is required to operate regularly in temperatures below freezing greasing must be done with low temperature grease.
>>Wire ropes should be regreased when they are visibly dry or dry to the touch, when ‘bald spots’ form in the grease on the outer surface, or when corrosion exists.
>>Wire ropes must be inspected for flattening and kinking, and for broken, worn or corroded wires, and must be condemned when more than 5 per cent of the wires in any length often rope diameters are damaged. For example, if the wire rope has a diameter of 24 mm and is of 6 x 37 construction the total number of wires in the rope is 222, of which 5 per cent is 11. If more than 11 broken strands are found in a length of (24 x 10) mm—i.e., 240 mm—the rope must be condemned.
>>The brakes for the hoisting winch and for the slewing and luffing machinery must be inspectedfor contamination with oil, for damage to the adjustments, and for wear. Where necessary they must be degreased—for example, with trichlorethylene—or the linings must be renewed.
>>All parts of the crane must be inspected for damage and for fastenings or fittings which have become loose, worn or fractured as a consequence of blows, vibration or working in a seaway.
Loose or damaged slewing ring securing bolts, for example, would seriously reduce the safe lifting capacity of the crane.
>>Crane winch drums must be inspected to ensure that the wire ropes remain firmly attached to them. Wire must be correctly spooled on to the drum without crossing turns and the reliable operation of spooling and full drum devices and slack wire detectors must be confirmed.
>>Deck cranes are provided with limit switches to prevent over-hoisting, over-lowering, and luffing, slewing or travelling beyond permitted limits. When a limit switch operates the crane cannot pass the limit, but still retains power to move away from the limit.
>>Limit switches must be tested frequently to ensure that they work and that they are properly adjusted to operate in the correct position. The over-ride keys for the limit switches must be tested frequently to ensure that they work and that they are properly adjusted to operate in the correct position. The over-ride keys for the limit switches should be kept safely in the care of a responsible officer.
>>The emergency cutout stops all power to all controls on the crane, by tripping the main electrical breaker, as soon as it is activated. The brakes on the hoisting and luffing winches are spring-loaded, and will close tight if power is lost, holding the crane and its load fixed.
>>The emergency cutout can be activated by the crane operator and will operate automatically if there is a power failure or if the crane is overloaded. The emergency cutout is tested by hitting the cutout button, and by overloading under controlled conditions when the cranes are retested.
>>High temperature trips and low oil level trips, if fitted, must also be checked regularly and maintained in good condition.
>>The crane cab and machinery must be kept clean and light bulbs must be renewed. Cracked or broken windows, and plastic windows which have become dull or opaque must be renewed. The sun visor is essential to prevent the driver from being dazzled and must be maintained in good condition.
>>Heaters for the crane are normally on a separate power circuit to that for driving the crane.
They should remain switched on throughout the voyage to keep the electrical equipment for the crane warm and dry. The correct operation of cooling fans should be confirmed.
>>The crane maintenance programme is likely to depend upon the crane operating hours, so a record of these should be kept.
>>Electrical maintenance should include the occasional checking of all terminal screws in the switchgear cabinets, terminal boxes and on the equipment itself, and tightening them if slack.
>>Sheaves should be inspected for damage from time to time, particularly when the crane is being used with a self dumping grab. The operation of such grabs can cause a ‘ripple’ in the wire rope fall, which may jump out of its own sheave and finish up in the next sheave, or jammed betweentwo sheaves.
Annual thorough survey
>>The annual thorough survey of a crane is carried out by a surveyor representing a statutoryauthority or a classification society, and the nature and extent of the survey is described in classification society publications.
>>Besides covering all the areas listed above under ‘maintenance’, the structure of the crane will be carefully examined for strength with hammer testing and drilling of the material when found necessary. Sheaves may be unshipped for inspection.
>>The cranes must be retested at four-yearly intervals, or more often if found necessary.
Operation of cranes
>>The ship’s officers can keep the limit switch override keys in their possession, thus ensuring that the limits cannot be overriden without their knowledge. Keys to the cabinet containing the main power breaker should also be kept by the duty officer who can then be certain that the crane cannot be restarted by an unauthorized person after it has been immobilised. It is good practice to immobilise the cranes at the end of the working day and any time that they are being maintained.
>>It may be necessary to override a limit switch to stow or unstow the crane jib, or to move it to other exceptional positions whilst it is carrying no load. This must only be done under the supervision of competent ship’s personnel.
>>When shore drivers are employed to drive the cranes they will require careful supervision from ships’ officers to ensure that they use the ship’s equipment safely, and do not damage it.
>>Cranes should be driven smoothly.
Maintenance and inspection of grabs:
If grab parts such as bushes, sheaves, wire ropes or chains are to be renewed it will be necessary to unrig and dismantle the grab, partly or completely. Before doing so, full notes or plans should be made to assist with the reassembly and rerigging. Alternatively, another fully rigged grab can be used as a model if necessary.
>>All the grease nipples should be found and grease should be applied frequently when the equipment is in use, and regularly at other times, following the procedures described in the general section on greasing and oiling.
>>The grab may be fitted with a feature such as a dashpot, used to control the speed of opening.
This reduces the violence of the grab’s action and so reduces damage. A dashpot, or any similar arrangement of pistons, must have its oil level maintained with the correct clean oil according to the manufacturers’ advice.
>>When chains are used for opening and closing of grabs they should be lubricated with a light grade SA 30 Oil.
