Cranes are commonly used in the construction industry and in the manufacturing of heavy equipment. Construction cranes are either fixed to the ground or mounted on a purpose-built vehicle. On the basis of their structure and function cranes are divided into a tower or a derrick which is equipped with cables and pulleys that are used to lift or lower materials.
Tower cranes are fixed to the ground, which helps them to attain a height and enhance their lifting capacity. These are used in the construction of tall buildings. All the tower cranes basically have similar parts, which consist of a base, mast (or tower), and the slewing unit.
The base is bolted to a large concrete pad that supports the crane, these are poured several weeks before the crane arrives, large anchor bolts embedded deep into this pad support the crane and hence prevent its falling over. The base is then connected to the mast which gives the tower crane its height. The top of the mast is attached to the slewing unit, which has the gear and the motor which helps the crane to rotate.
The slewing unit in turn has three parts all on top of it, namely the long horizontal jib or the working arm, the shorter horizontal machinery arm, and the operator’s cab. The long horizontal jib is the portion of the crane which carries the load in and out from the cranes center by means of a trolley, which runs along the jib. The shorter horizontal machinery arm contains the crane’ motors and electronics as well as the large concrete counterweights. The Operator’s cabin is at the top of the tower.
To save space, the vertical part of the crane is often built in a well in the center of the building, which is then converted into a lift shaft when the crane is dismantled. Assembling of the tower crane is usually done by a telescopic crane of smaller lifting capacity but greater height, and in the case of tower cranes that have risen while constructing very tall buildings, a smaller crane will sometime be lifted to the roof of the completed tower to dismantle the tower crane afterward.
A typical tower crane can lift a maximum load of 18 metric tons, which is approximately 39700 pounds; however, the crane cannot lift that much of weight if the load is at the end of the job. The closer the load is positioned to the mast, the more weight the crane can lift safely.
To know as to how much load is safe for the crane to lift the tower crane uses two switches namely, maximum load and load moment. The maximum load switch monitors the pull on the cable and makes sure that the load does not exceed 18 tons, whereas the load moment switch makes sure that the operator does not exceed the ton-meter rating of the crane as the load moves out on the jib.
Now it would be interesting to know as to how the tower cranes work at the construction site: They arrive at the site in tractor-trailer rigs, depending on the size of the crane about 10-12 tractor-trailer rigs are used. After this, the crew uses a mobile crane to assemble the jib and the machinery section and places these horizontal members on an approximately 40-foot mast that consists of two mast sections. The mobile crane then adds the counterweights.
The mast which is a large triangulated lattice structure typically 10 feet square then rises from this firm foundation. This triangulated structure gives the mast the strength to remain upright. The tower crane rises to its maximum height by using one mast section at a time, the crew uses a top climber that fits between the slewing unit and the top of the mast. This is done by hanging a weight on the jib to balance the counter weight.
The crew then detaches the slewing unit from the top of the mast, this is done by using large hydraulic rams in the top climber to push the slewing unit up to 20 feet. The crane is then used to lift another 20-foot mast section into the gap opened by the climbing frame. Once bolted in place the crane gains a height of 20 feet.
This process is reversed once the building is finished and its time to dismantle the crane and as stated earlier a smaller crane is used to dismantle the tower crane except for the mast which is dismantled by the crane itself.