How Do Construction Cranes Work?
Metropolitan horizons are dotted with cranes, constant fixtures in downtowns with buildings rising higher and higher. Without construction cranes, building skyscrapers would be impossible, but with cranes, we’re able to build towering structures with relative ease. Cranes are capable of moving extremely heavy objects into place, enabling methods of construction that were unthinkable just a couple hundred years ago.
Although most of us frequently see cranes, many people don’t know exactly how cranes work. How do tower cranes reach such incredible heights? How do they assist in constructing a building? What does the operator do while controlling a crane?
In this article, we’ll answer all of those questions. Although there are many types of cranes, we’ll focus specifically on tower cranes, which are the ones you see most often on large commercial construction sites. These cranes are extremely sophisticated, and they can also be expensive, sometimes costing more than $15,000 a month to rent.
You can jump straight to our infographic, or read more about:
- The various parts of a crane
- How tower cranes are stabilized and assembled
- How tower cranes perform lifts
- How an operator controls a crane
Parts of a Crane
To understand a crane’s movement, it’s helpful to have a good grasp of the various parts that make up a construction crane. All of these parts work together to make the incredible lifting power of a crane possible.
When looking at a crane, you can identify all of the parts easily by starting at the base of the crane and working your way up, then running your eyes along the length of the crane from the long end to the short end.
- Concrete foundation: A tower crane always sits on a concrete foundation, where anchors are placed to secure the crane to the ground.
- Tower or mast: From its base, the crane rises up with the tower, also called the mast, which consists of lattice sections stacked on top of one another.
- Turntable: At the top of the tower, the crane has a turntable that enables it to rotate 360 degrees.
- Operator’s cab: Near the turntable, the operator’s cab gives the crane operator a place to control the crane with an unobstructed view.
- Jib: Stretching out forward from the cab is the jib, the long horizontal section of a crane.
- Trolley and hook block: Along the jib, a trolley with a hook block runs back and forth, enabling loads to be moved along the length of the crane.
- Counterjib and counterweights: Behind the cab is the counterjib, where counterweights are placed to stabilize the crane at rest and during movement.
- Main winch and motors: At the back of the counterjib sits the main winch and motors, which enables the long rope to be lowered or raised to hoist heavy loads.
- Tower peak or apex: Rising above the cab is the tower peak, also known as the apex, where the pendants extend out to support the jib and counterjib.
Remember that there are many varieties of tower cranes, and some will have parts that are not mentioned here. For example, a luffing jib crane has an arm that is able to raise up and down, and a flat top crane does not have the apex and pendant system shown in the crane above.
Some tower cranes also have self-climbing hydraulic systems, which makes it possible for them to climb internally as a skyscraper is constructed around them.
Overall, though, the basic parts of tower cranes are fairly similar, and this list will give you a good understanding of the parts that make up the cranes that you see in your everyday life.
How Cranes Are Built
Before cranes can perform the essential lifting operations that make them valuable on job sites, they must be stabilized and assembled.
A tower crane is always built in three distinct phases:
- A concrete foundation is prepared so that the crane will be safely anchored to the ground and stable during lifting.
- A mobile crane assembles the tower crane, lifting several segments of tower into place along with core components.
- The tower crane builds the rest of its own structure as it rises to its ultimate height.
The concrete foundation is essential for the safe operation of the crane. Complex calculations take into account all of the forces that will act on the crane as it performs lifts, rotations and trolley movements. When the foundation is prepared, special consideration is taken to ensure that underground utilities are not affected by the placement of the crane. Once the foundation is ready, the base of the crane is anchored on and the rest of the crane is ready for assembly.
A mobile crane is used to lift the initial tower crane components into place. The mobile crane attaches several sections of the tower until it reaches a certain height. After that, the mobile crane loads key components into place, like the jib, counterjib, cab, turntable and motor. At this point, the tower crane is able to operate on its own.
For the rest of the job, the tower crane will be able to build its own tower using a unique mechanism:
- A climbing unit is assembled at the base of the tower and lifted up to the top.
- A new tower section is raised up by the hook, then travels by trolley to attach to the climbing unit.
- A hydraulic jack raises the tower up, creating a space to add the new tower section, which is then bolted to the tower.
This process is repeated to add new sections to the tower, raising the crane higher with each additional tower section. While adding tower sections in this way, counterweights are used carefully so that the crane acts as a perfectly balanced scale to prevent tipping.
With a firm understanding of crane assembly, you’ll be able to appreciate the incredible work a tower crane does.
How a Tower Crane Performs Lifts
A tower crane will perform hundreds of lifts over the course of a construction project, moving heavy materials like steel and concrete into place for construction workers assembling a building.
Each lift involves the same basic moves:
- The rope is lowered down along with the hook so that the load can be attached.
- A crew of riggers securely attach the load to the hook, following basic principles of physics to ensure a stable lift.
- The crane operator performs a series of moves, including hoists, which raise the load with the rope, rotations, which spin the crane, and trolley travel, which moves the load along the jib.
With these maneuvers, a crane operator is able to move a load within a very large area. In fact, the largest freestanding tower crane, the Kroll K10000, covers the area of six football fields with its reach of 330 feet. Because of their enormous power, construction cranes move massive amounts of weight — thousands of pounds — with the simplicity of a joystick.
What a Crane Operator Does
Crane operators sit inside the cab and use controls to maneuver the crane. Each day, a crane operator must climb a ladder inside the tower — sometimes hundreds of feet — to get to the cab and start work for the day. Importantly, a crane operator must always perform daily safety checks before starting operations.
Once in the cab, the crane operator has several key responsibilities:
- Communicate with radio and hand signals to ensure a safe lift.
- Monitor computer safety systems that keep track of wind speed and weight capacity limits.
- Use joysticks to perform crane maneuvers like swings, hoists and trolley travel.
A crane operator always relies on a signal person, who communicates with radio commands and visual hand signals, to keep lifting operations safe. Proper communication prevents accidents and collisions with potential hazards, like power lines.
Keeping an eye on computer safety systems is vital to a safe lift. A crane operator must always stay mindful of wind speed, as high winds makes crane maneuvers more complicated and potentially risky. Additionally, a crane has different weight capacities depending on how far away the load is from the tower, so a crane operator uses a load moment indicator to make sure the lift stays within safe parameters at all times.
To operate a crane, an operator uses two joysticks. The left joystick controls swing and trolley travel while the right joystick controls the hoist. With careful movements, the operator can use just these two joysticks to safely move a load anywhere within the crane’s operating radius.
Cranes Make Tall Buildings Possible
Iconic skylines simply would not exist without tower cranes, which have made it safe and efficient to move the steel and concrete that are essential to skyscraper construction. With a deep understanding of construction cranes, it’s possible to drive around and see cranes with a new appreciation for their impressive engineering.
Next time you see a crane, feel free to impress your friends with your knowledge of self-climbing tower cranes and impressive components like jibs, winches and hook blocks.