TOWER CRANE

How does a tower crane work?

 

Tower cranes are seen on almost every major construction site throughout the world. These machines are built to lift heavy loads efficiently and safely. A modern tower crane consists of three major sections: the base, the tower mast, and the slewing unit

The jib extends horizontally at various lengths depending on the model and configuration of the crane. A trolley attached to the underside of the jib moves loads in and out from the cranes center axis. Hoist and trolley lines run throughout the jib enabling vertical and horizontal movement of the load. Rotation of the jib is achieved by employing the "slewing" action

To move a load in and out from the cranes center axis a trolley is employed along the underside of the horizontal jib. Powering the trolley is the trolley drive mounted on the horizontal jib. A cable from the trolley drive runs through the jib and is attached to the trolley. A trolley then controls the loads radial movement and helps stabilize the load

A hook or hoist is attached to the hook block assembly. Hook block assemblies house a series of sheaves and pulleys that carry the rope. The block provides greater flexibility and facilitates the lifting of a load. The primary function being for the free rotation and positioning of a load.

A cranes drives, drums, gears, electronics, and counter-weights are found on the counter-jib platform. Located behind the operator cab, the counter-jib provides a balancing force to the load on the horizontal jib. Drives, drums and electronics are located to the rear of the counter-jib to maximize torque and for ease of maintenance.

Counterweight

Balancing the forces on the crane caused by the load is a primary function of the counterweight. Counterweights are fabricated from concrete and come in modular units. The number of counterweights depends on the expected max load capacity, jib length and tower height.

Hoist Unit

The hoist unit houses the hoist drum, gear box, gear shift, brake, and supporting components. Typical placement is located behind the counterweights - toward the extreme rear of the counter-jib. Placing all hoist and trolley components in one location makes for efficient servicing of the crane.

Hoist Drum

A component of the hoist unit that lifts and lowers a load by means of the hoist line. Power to the drum is supplied by the hoist drive. The hoist drum winds the rope or cable in a specific cross-pattern to avoid entanglement of the line.

Hammerhead or "high-top" cranes utilize a tower top section for greater torque and stability. Pendant bars are attached from the tower top to the jib and counter-jib that provides further stability. Anemometers and aircraft-warning lamps are placed on the top of the tower.


 

Pendant Bars

Pendant bars provide additional stability to the crane. The bars are attached from the tower top to the jib and counter-jib.

Trolley and Hoist Lines

Loads are moved up and down by the hoist line and in and out by the trolley line. These lines run throughout the horizontal jib, counter-jib and tower top through a series of pulleys and guides. The trolley line is connected to the trolley unit and the hoist line is connected to the hook block.

Slewing Assembly

The jib, counter-jib, tower top, and slewing assembly collectively make up the slewing unit. This section allows the crane to rotate and perform lifting.

Operator Cab

Movements of the crane are controlled from the operator's cabin. Within the cabin sits the operators chair with joystick controls, electronic monitoring devices and communication systems. Many cabins come with climate control to ensure a comfortable work environment. The operator cab is part of the slewing assembly.

Slewing Ring

The slewing ring drives the rotational movement of the slewing unit (jib, counter-jib, cab, and tower top). Powered by the slewing gear drive the slewing ring is fixed to a ball track and is made up of several hundred precisely machined gear teeth. The slewing ring is capable of enormous torque, yet precise movement.
 

Top Climbing Unit

How does a crane "grow" taller? The top climbing unit is an essential piece of any tower crane. This specialized section enables tower crane sections to be inserted and connected to the tower. A large hydraulic jack then lifts the Slewing Assembly, Jib, and Counterjib one tower section higher. Another section can then be inserted.

Bottom Climbing Unit

Climbing inside buildings is made possible by the bottom climbing unit. Climbing collars are used with a hydraulic power pack to lift the tower sections upward. The crane leaves Its foundation at the ground, and is completely supported by the structure of the building it is constructing. This leaves the lower levels of a building vacant for finish work, and at times occupancy before the building is completed.

 

Tower Sections

A crane’s tower mast is constructed of a series of tower crane sections. The structure of a section is reinforced by a braced square or truss. Sections are bolted together as the tower crane is erected. Access ladders are incorporated in the interior of the sections to enable workers to move up and down the crane.

Tie-In

A tie-in secures the tower mast to a structure or framework and provides stability when the crane is under load or experiencing wind forces. The number of tie anchors are calculated by taking into account such factors as the type of crane, jib length, height under hook, and maximum out-of-service wind speed.

 

Foundation Anchors

Foundation anchors are steel posts embedded in re-enforced concrete that are bolted to the tower. These anchors provide a secure connection to prevent the crane from swaying or experiencing structural failure due to overload and wind forces.