There are several industrial and commercial buildings which now exceed 60 stories or more. These buildings all require tall cranes to be able to help move the materials to the higher floors. There are cranes which are operated from the back of trucks or other types that have their own vehicle attached. Tower cranes are the largest kinds on the market.
Tower cranes are the stand-alone structures which are usually seen on high-rise building projects. Often, they are part of a major city's downtown skyline. Wherever new construction like skyscrapers or apartment buildings and commercial facilities like shopping center are being constructed, chances are a crane would be on site.
Kinds
The two major kinds of cranes could be distinguished by the manner in which their jib or boom lifts materials. The jib is the metal frame that extends from the main section. On a flat tower crane, the jib remains horizontal as it lifts things. On a luffing kind of tower crane, the jib can ratchet to upward or downward angles. The lifting capacity for both kinds could vary from 30,000 lbs. to 10,000 lbs.
Body
The body of the crane is composed of a mast. This is a vertical steel frame which is a combination of individual parts. In order to increase the overall height of the machine, sections are added. The mast extends upward to wherever the desired height is, to the control module, that is a small room that has glass windows on all four sides or to the tower as it is also referred to. The operator of the crane works from inside of the tower.
Lift
To raise supplies, the crane utilizes a braided metal cord. The cord extends all the way to the end of the jib or boom from a motor located near the control module. There is a pulley system situated at the end of the jib, through which the cord is positioned and lowered down. The jib that holds the cord becomes balanced by a counter jib located on the opposite side of the tower. The counter jib holds weights. These weights help to prevent the crane from tipping over when lifting heavy supplies.