On 30 June 1859 Jean François Gravelet Blondin, a French acrobat known as ‘The Great Blondin’ crossed over the Niagara Falls walking on a rope tensioned across the top of the waterfall. His name lives on in the crane industry for in France and Italy, in particular, blondin is the name given to cable cranes.

The first known cable cranes, or blondins, were built around 1860, mainly for wood transportation. Today they are mostly used for the specific application of transporting buckets of concrete on dam construction projects.

Cable cranes consist essentially of a single or twin track rope pulled between the two sides of a valley where a concrete dam is to be built. On the track rope a carriage goes to and fro, moved by a ring of rope (the translation rope). The winch moving this rope usually has a driving sheave. A hook transporting the bucket is suspended from the carriage with a hoisting rope, usually moved by a drum winch.

Pier Giorgio Graziano is technical director of cable crane manufacturer Poma Italia. He has nearly 30 years’ experience in the ropeways field since joining Breco, the British Ropeway Company, in 1971. “I was immediately fascinated by the ropeway field and especially by this strange type of equipment transporting huge buckets full of concrete from one side to the other of steep valleys,” he says. “So as soon I had an occasion I joined Agudio.” In 1991 Agudio was taken over by the French group Poma, the world leader in ropeway transportation. The name Poma will be familiar to anyone who skis in the Alps, though fortunately its dreaded button lifts seem to be increasingly replaced by more civilised chair lifts and cable cars.

The main competitor of Poma Italia, as Agudio became, in the field of blondins is PWH (now part of Krupp of Germany), which has China’s Three Gorges Dam among its credits.

Graziano delivered a paper with Poma Italia general manager Gianfranco Marten-Perolino, ‘Cable cranes are still up to date’, at the 1999 Congress of OITAF – Organizzazione Internationale Trasporti a Fune (the International Organisation for Transportation by Rope) – in San Fransisco.

Types of cable cranes

Graziano explained the different layouts of cable cranes, depending on the system used to move the track ropes in order to cover the required dam area. The layouts can be radial, parallel or oscillating.

Radial cable cranes have one side of the track rope connected to a fixed point, and the other fixed to a mobile tower or carriage moving on a circular runway. With this type of layout a circular area is covered. The fixed point can be at ground level or on top of a high tower (towers up to 120m have been built). The mobile carriage can be at ground level, or on a tower 30m high or more.

Parallel cable cranes have two sides of the track rope fixed to mobile towers, each moving on a runway. With this layout a rectangular area is covered. Again, the mobile towers can be at ground level or high up, depending on the terrain and the need to keep the bucket clear of the dam crest.

Oscillating cable cranes have two sides of the track rope fixed to the top of oscillating towers. The synchronous oscillation of the two towers allows the track rope to cover a rectangular area. Again the height of the towers can change but a certain height of the tower is necessary to cover the required area since the head of an oscillating tower can usually move each side up to approximately 30% of its height.

Main advantages of cable cranes over tower or derrick cranes in dam construction, according to Graziano, are that they are safe and economical. Dams are always built where there is a river, which can cause flooding, with the possibility of the dam’s building yard being inundated. Cable cranes mean that the most important equipment is out of the way of possible flood water. Furthermore, it has been known for people and equipment to be rescued from floods by using the cable crane. Even bulldozers weighing many tonnes have been rescued by two cable cranes coupled by lifting beams, Graziano says.

Economy benefits stem from improved logistics and reduced handling. Concrete is transferred from the batching and mixing plant into the buckets of the cable crane which carry it directly to the casting block, eliminating the transfers and transshipments necessary with other means. And other machines, of which several would be required, often have to be moved around the site during construction.

The cable crane is controlled by a single operator from a cabin positioned in a suitable place, giving the best view of the bucket loading area and possibly also of the unloading area. Indicators show the position of the bucket, enabling blind manoeuvres and operation when visibility is poor.

Since the anchorage position of the track ropes are usually placed well outside the dam’s area and the position of the ropes is usually quite high above the dam site, the cable cranes are well clear of any mine explosion zones.

Even if the main duty of a cable crane is concrete transportation, they can be used for auxiliary functions like transporting formwork, rebar and equipment from one yard to an other.

Graziano also says that the use of cable cranes allows a better organisation of the building yards. “The crushing and screening plant, the batching and mixing plant, the platform of the runway for loading the buckets of the cable cranes, can be arranged according to organic criteria and in the most appropriate position, to rationalise the flow from one to the other plant without caring too much of their distance from the place where the concrete has to be placed. It also becomes possible to keep the dam and adjacent areas clear of obstructive machinery that might hinder the remaining operations.” An alterative to cable cranes for dam construction are belt conveyors which have the advantage of high capacity, says Graziano, but the disadvantage of being subject to all the problems connected with impinging on the site of the dam. “As far as I know, the use of belt conveyors is limited to gravity dams with a maximum height of about 100m,” he says, and adds that they are mainly used by American companies. An example of belt conveyors in use is the Three Gorges Dam in China which uses conveying equipment from Rotec of the USA as well as Potain of France (Mass movement Aug99, p13). That site is big enough to feature blondins as well, however, and has two supplied by Krupp which can each carry 25t loads across a 1,406m span at a speed of 7.5m/s.

Developments in cable cranes The systems used today are not very different from the cable cranes used 20 or 30 years ago but there have been some developments. Most notably, capacities have increased from 4m3 buckets of concrete to 9m3, or 28t. This has been made possible by advances in wire rope technology and the use of two twin track ropes instead of a single track rope.

Speeds of carriage translation and bucket lift have also increased. A few years ago translation speed was limited to about 3m/s. Now speeds of 7m/s for the translation and 3m/s for lifting are usual. The increase was possible mainly due to the use of better systems of carrying the slack rope, such as the fixed ones used by Agudio/Poma, or the ‘opening’ slack carriers used by PWH/Krupp.

This type of slack carrier is fixed on the track rope or ropes and allows the carriage to pass through at high speed. One of the main technical problems with a cable crane is how to support the operation ropes (especially the lift rope which is subject to a high variation of tension, depending on whether the bucket is full or empty). These systems are a big improvement on the first slack carriers which used a knot rope system, each carrier having a hole of different dimension, being released from the carriage when the appropriate knot of the knot rope met the appropriate slack carrier. The ‘automatic’ slack carriers of the past had a mechanical gear on each carrier, actuated by a rope which moved each carrier at a different speed (slower near the track rope ends and faster near the carriage) to position the different carriers. “The system was quite clever but it was never possible to make it work well due to slippage of the operating cable and other practical problems,” says Graziano.

Cable cranes have also adopted all the main safety developments introduced in passenger transportation, such as electronic overspeed devices, the control of integrity of mechanical transmission of the winch, and maximum torque and maximum torque gradient control.

Another improvement is in the use of electronics to monitor the position of the bucket.