What are cranes?
Cranes are defined as machines used to lift, lower and move heavy loads. It is said that cranes were invented by the Greeks who began to use them for construction to make loading and unloading more convenient and less labour intensive for people, also cutting down the time that construction took.
Modern-day cranes are huge, carrying massive weights to incredible heights, with some tower cranes reaching up to 1 000m in height.
Types of cranes
The various types of cranes have different abilities for different circumstances.
Mobile cranes are designed to be able to move or travel on-site fairly easily. They are mounted on wheels or crawlers and do not need a permanent foundation to lift from.
The size between the largest and smallest mobile cranes differs widely and is generally proportional to mobility.
Mobile cranes can also be classified by boom type and carrier type. Booms are either conventional lattice truss boom or telescopic boom. Most telescopic boom cranes today use hydraulic cylinders to extend and retract the boom. They are commonly known as hydraulic cranes. Each of these types of booms carries the load weight differently.
Lattice Boom Crane
The lattice boom forms a triangle with its pendants and backstays, with the boom being a compression member, and the pendants are tension members. The structural system thus formed is a light, strong and rigid system. Boom length can be increased significantly with little penalty in capacity for the added dead weight.
The lattice boom is modular, and the length can be increased between the butt section and the tip section by manually inserting compact boom segments known as inserts. These range in length from 10 to 40 feet and are quickly installed by means of adding pins. The boom needs to be lying horizontally on the ground in order to do this and will take ironworkers and a small assist crane.
Telescopic Boom Crane
Telescopic booms carry their load as flexible, cantilevered box beams, a lot like fishing rods. They are extremely strong and have a high lift capacity when close to vertical. (For high boom angles, the internal loading is mostly compression.)
The main advantages of a telescopic boom crane are transferability and quick setup time. Telescopic cranes are most commonly mounted on wheeled carriers. The boom segments fit inside one another and are easily retracted to roadable lengths. Once at the site, the boom can be extended fully in a short time. To extend the reach height, even more, lattice boom extensions are frequently added manually to the tip of the telescopic boom.
A carrier is what makes a mobile crane moveable and consists of a special truck chassis, turntable and wheels. Crawler “carriers” are not usually referred to as carriers but rather as crawler bases, and consist of a structural frame called:
- a carbody,
- a turntable,
- and crawler treads.
Crawler carriers, or bases, are ideally suited for rugged project site situations. The carriers have a large footprint that provides a sizeable ground-bearing area which is ideal for travelling along rough site roads or paths, especially with sandy soil conditions. Crawlers need to be trucked in and assembled on site. Smaller crawler-mounted cranes can be loaded on a truck fully assembled (excluding boom).
The largest land-based mobile cranes available currently are crawler mounted. These behemoths (1 000 tons and up) are mounted on crawlers because there is no other carrier type that would provide enough or economical dead load weight distribution to the ground. These large cranes are not very agile and take extensive time to assemble and transport.
Wheeled carriers come in three basic types.
- Truck carrier – These can travel long distances on public highways. Its heavy-duty suspension and power train are mainly designed for highway travel and for graded project site roads. This carrier type has proven to be multifarious. It provides a base for both lattice boom and telescopic boom cranes, ranging in capacities from 5 tons to well over 500 tons.
- Rough terrain (RT) carrier – This type of carrier has four oversized wheels and is intended for off-road usage because it does not have a separate driver’s cab. The rear axle of a rough terrain carrier has an oscillating hydraulic suspension that gives it superior off-road travel capabilities. During pick and carry operations, the rear axle must be in a locked position.
- All-terrain (AT) carrier – This carrier is suitable for travelling on both highways and ungraded roads. This is handled by a fully hydraulic, computer-controlled suspension and all-wheel steering for its multiple axles. It offers high manoeuvrability in restricted urban settings.
FIXED STATIC-BASED CRANES
Tower crane manufacturers have made the tower erection process simpler. The lattice towers are made to break down into roadable lengths and widths. On-site, a medium-sized assist crane is needed to assemble the crane and machinery deck near the ground on a section of short tower.
The tower crane is then designed to jack itself up to make room for the insertion of another segment of tower. This process repeats as construction proceeds upward. Dismantling does this in the reverse order. The tower needs to be braced every 100 to 150 feet by using guys or, preferably, by bracing back to the building structure. The tower can even be incorporated inside the building structure.
Another type of fixed crane is a derrick. Its boom is typically a conventional lattice boom and the mast backstays or legs are rigid members (stifflegs) as opposed to wire rope guys. The boom and mast butts are mounted on a turntable and the stifflegs pin into fixed foundations.
- Stiffleg derricks range in size from 30 tons for rooftop-mounted models to more than 800 tons for ground-based models. The ground-mounted models most often will require a steel support tower and large concrete mat or piling foundations. Some stiffleg derricks can be mounted on rails for limited site mobility but will then need ballasting.
- A guy derrick uses wire rope guys to tie back the top of the mast. The guys are anchored to large concrete foundations. Erecting the mast and guys can be difficult at existing facilities because guy laydown space will be needed. Guy derricks are not a good model to be used for sites where space is restricted. Properly tensioning the guys can be a time-consuming operation.
- Small rooftop derricks offer inexpensive lifting capability in congested areas. With good planning, they are quick to install, move and dismantle. The large stiffleg derricks and guy derricks have the advantage of large lift capacity at a very long radius. Due to being fixed to their foundation, there is no chance of tipping.
If you need rigging and cranework on your project, get in touch with us on +27 (0) 82 659 3315 or send us a query here.