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TOWER CRANE


12/14/2009        

There are two main types of tower crane in general use;

Basic Parts of Tower Crane


Self Erect Tower Crane
This is a tower crane that is designed to be easily and rapidly transported and erected. Some of them carry their own generator and are therefore completely self contained.


Assisted Erect Tower Crane
This type of tower crane is usually larger with a greater lifting capacity than the Self Erect. Its design calls for it to be builton site, piece by piece, often with the assistance of a mobile crane, hence its name  assisted Erect


Both yypes of tower crane can be found in operation with a variety of jib configurations, all designed to provide a specific type of lifting service to sites where the lifting requirements and the environment produce different demands.


The two types of basic jib design are:

top slewing crane top slewing cranetop slewing crane

Horizontal Jib
This jib takes the form of a simple structure extending from the tower, along which a trolley can travel, carrying the hoist rope and hook assembly to vary radii.


Luffing Jib
The luffing jib has no trolley, the variation of hook radii is achieved by altering the jib angle, the same as with a mobile crane.


More detailed information on these crane and jib types can be found by reading the following pages.



Horizontal Jib Tower Crane Self Erect


This configuration of crane is not found as frequently in this country as on the continent, where it is common use as a small builders crane, employed in lifting activities more usually carried out by telescopic forklifts in the UK .


The true self erect tower crane is smaller and of lower capacity than its assisted erect counterpart, but has the great advantage of being quick and easy to erect. The erection procedure is well within the capability of a trained fitter or competent driver and the newly erected crane does not have to undergo the sort of comprehensive commissioning procedures required by an assisted erect crane.


In recent years there has been a move toward tower cranes that are quick to erect, but the small size and capacity of the self erect crane has always proved a barrier to its usefulness on most construction sites. Tower crane manufacturers are now producing a design of crane which embodies the best features of both the self erect and assisted erect tower cranes. Called the  city Crane  these machines can be erected quickly and have a size and capacity which makes them an attractive proposition for use on general construction sites.


Some self erect tower cranes are mounted on a crawler track base and with the provision of an on-board generator and independent of an external power supply. These cranes have the abilty to be extremely mobile around a site, travelling fully erected without a load. Whatever their base type, self erect cranes usually require the use of outriggers which are used to both support and level the crane during lifting operations. Because of their importance to crane stability, outriggers should be checked by the driver frequently for security and level.


Horizontal jib tower crane assisted erect


This configuration of crane is widely used in the construction industry. It requires assistance in erection and this is usually provided by a suitable sized mobile crane, erection times vary but on average a time of two to three days can be expected for a crane of thirty metres square.


Erection of this type of crane is normally carried out by a crew and an engineer, trained in the specific erection procedures required. At the conclusion of the erection the crane will be subjected to a thorough examination and an application of a 25% overload followed by the setting of the safety devices.


Jibs of 50m and longer are available for applications where greater area coverage is required, and tower heights of up to 180m can be readily found for the project that needs this sort of lift height. A range of hoist and trolley winches are offered by the crane manufacturers, providing the crane user with a wide choice of hook speeds and pull combinations. Rail-mounted bases are often available for this type of crane.


Cranes of this type are excellent in the role of rapid light weight material handling and when correctly sited and driven will provide comprehensive and efficient lifting services for a large area of a site. Though not usually of very large maximum lifting capacity, average construction industry capacities being between 5000kg and 10000kg, the horizontal jib tower crane can lift its maximum capacity out to a radius in excess of 25% of its total, and at maximum radius a healthy capacity still remains.


Lifting jib tower crane assisted erect


This configuration of crane tends to be used where special circumstances exist that would restrict the use of a horizontal jib tower crane. The main advantage of a luffing jib is that both jib and suspended load can be kept from over areas where there intrusion is not wanted.


As far as its erection is concerned it follows the same lines as the assisted erect horizontal jib tower crane discussed previously, each erection being followed by a thorough examination and test.


Jib lengths, tower heights and winch types, as with horizontal jib cranes, are available for selection by the user, so as to be able to customise the crane to the lifting environment and requirements. This configuration is not as quick as the horizontal jib crane, particularly when the lifting operation requires frequent radius alterations, the luffing winch having to move the weight of both the load and the jib; also alteration of the load radius is accompanied by a change in load height which must be compensated for by the operator if level luffing is required. Some winch units can do this automatically but of course they are more expensive.
Unlike the horizontal jib crane the capacity of a luffing jib drops immediately radius is increased, therefore confining maximum capacity to minimum radius.
The asset with this type of tower crane is that it can travel over any length of straight or curved track to cover greater areas:


Its limitations however are quite severe:


It can travel freely only when its working height is within certain limits specified by the manufacturer. (Maximum free standing height.)


In addition the rail track must be kept clear at all times, and this requires a certain amount of responsible supervision.


The area occupied by the rail track cannot be utilised as a site storage area, and on the some sites this alone can be a serious handicap.


This is comparatively expensive form of crane mounting.


This type of Tower Crane is generally self erecting, and usually also generates its own power.


The crew of two men, working as a team, are usually competent in both crane operating and banksman (slinger) duties.


This crane offers greater mobility, i.e. it can be easily transported from one site to another when folded, but when it has been erected to its full operating height this extreme mobility is lost, it can be moved from one part of the site to another only after careful levelling of the ground.


Its working height is restricted to within limits specified by the manufacturer.
This is similar in principle to the lorry mounted tower crane, but with less restriction to working height and freedom of movement.


The crawler mounted tower crane usually has some ability to travel unloaded around the site. Great care should be taken to provide a suitable surface to travel on in terms of support and level.


Transportation by low loader is required for moving from site to site.

Static-mounted



This is the most popular type in general use.


Its lack of mobility is offset by an almost limitless working weight.


When the crane exceeds its free-standing height it must be secured to the building at regular intervals by ties, in accordance with the manufacturers instructions and specifications. (Tied in.)


The cost o f the ties and the time involved in their installation would have to be taken into consideration during the planning stage.


A static tower crane usually creates very few problems regarding erection and dismantling.





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