2. Jib Cranes
Cranes have replaced derricks on many modern ships. Positioned between the holds on platforms which can be rotated through 360 degrees, deck cranes provide an immediately operational unit requiring only one man to operate it. Double gearing is a feature of most designs, providing a higher speed at lighter loads. Various types of cranes exist for particular duties, for example, a general duties crane uses a hook and a grabbing crane is for use with bulk cargoes.
A general cargo crane has three separate drives that provide the principal movements:
- A hoisting motor for lifting the load.
- A luffing motor for raising or lowering the jib.
- A slewing motor for rotating the crane.
In the movement of jib up and down, the limit switch works when:
- Jib is limited to certain angle of inclination.
- The movement of the jib above this limit is prevented by luffing limit switch; this stops the motor and luffing drum.
The up and down movement of the hook from the jib by means of running wire; the hoisting limit switch works when:
- It is mainly provided to avoid the damage occurred to the base plate fitted on the cargo hold where the hook lands during a heaving up operation.
- The hoisting motor will stop at a distance before the hook block touches the base plate.
The rotation movement of whole crane through 360o with all the machinery; the aft most crane near to the accommodation can be rotated only 180o and it is provided with a slewing limit switch.
Slack wire limit
If the operators by mistake pay more wire rope, which is lowering, the wire gets slack. Then the slack wire limit switches activate and stop the hoisting operation by stopping the hoisting motor and drum.
This is to avoid winding of the wire irregular in the drum while heaving and causes damage to the drum, supporting brackets, and other parts related to the hoisting. The operator’s cab is designed to provide a clear view of all cargo working areas so that the crane operator can function alone.
Using Two Cranes Together
The crane is usually mounted on the pedestal to offer adequate visibility to the operators. For occasional heavy loads arrangements, two cranes are made to work together, i.e. twinning. This can be done with a single operator using a master and slave control system in the two cranes. A common revolving platform will be necessary for this arrangement. The operating medium for deck crane motors may be hydraulic or electrically operated.