hoist and move materials. There are many types of tower cranes. Although they are different in type, the main parts are the same, as follows: Mast: the main supporting tower of the crane. It is made of steel trussed sections that are connected together during installation. Slewing unit: the slewing unit sits at the top of the mast. This is the engine that enables the crane to rotate. Operating cabin: on most tower cranes the operating cabin sits just above the slewing unit. It contains the operating controls, load-movement indicator system (LMI), scale, anemometer, etc. Jib: the jib, or operating arm, extends horizontally from the crane. A "luffing" jib is able to move up and down; a fixed jib has a rolling trolley that runs along the underside to move goods horizontally. Counter jib: holds counterweights, hoist motor, hoist drum and the electronics. Hoist winch: the hoist winch assembly consists of the hoist winch (motor, gearbox, hoist drum, hoist rope, and brakes), the hoist motor controller, and supporting components, such as the platform. Many tower cranes have transmissions with two or more speeds. Hook: the hook (or hooks) is used to connect the material to the crane. It is suspended from the hoist rope either at the tip, for luffing jib cranes, or in the hoist rope belly underneath the trolley for hammerhead cranes. Weights: Large, moveable concrete counterweights are mounted toward the rear of the counterdeck, to compensate for the weight of the goods lifted and keep the center of gravity over the supporting tower. Assembly A tower crane is usually assembled by a telescopic jib (mobile) crane of greater reach (also see "self-erecting crane" below) and in the case of tower cranes that have risen while constructing very tall skyscrapers, a smaller crane (or derrick) will often be lifted to the roof of the completed tower to dismantle the tower crane afterwards, which may be more difficult than the installation. Tower cranes can be operated by remote control, removing the need for the crane operator to sit in a cab atop the crane. Operation Each model and distinctive style of tower crane has a predetermined lifting chart that can be applied to any radii available, depending on its configuration. Similar to a mobile crane, a tower crane may lift an object of far greater mass closer to its center of rotation than at its maximum radius. An operator