Double level luffing jib cranes are a common sight in ports as ship unloaders, but seldom has this design featured on tower cranes even though there are several apparent benefits such as:
* System integrated horizontal movement of the load.
* Fast slewing operation, especially when the jib is retracted.
* High-speed extension or reduction of the working radius in comparison to single jib luffers.
On top of these, in the out of service position, the jibs of standard luffers have to be stored at a certain angle. Generally, the longer the jib, the more outreach must be accepted for weathering. An articulated jib crane design allows you to store the jib in a folded position at minimum radius, a real benefit in cramped sites.
And, because of its limited height, an articulated luffing jib crane can work under a saddle jib umbrella crane, saving extra tower sections of the umbrella crane in comparison to a singe boom luffing jib crane.
In most cases a moving counterweight supports the luffing prime mover. Compared with single jib luffing cranes, the double-lever types provide a shorter minimum reach. In folded position the hook block can reach equal positions next to the crane tower like saddle jib crane versions.
On the other hand, the articulated crane also has disadvantages, for example:
* The double lever jib design requires heavier crane uppers than single jib luffers, adding to manufacturing costs and limiting free standing ability.
* A tower crane with single luffing boom jib can extend the hook height dramatically by luffing the boom, whereas double lever jib designs depend on the outer jib length, and the maximum under hook height is limited to the level of the connection point of the jib with the machinery deck.
In the early 1960s, Swedish company Tornborgs Maskinfabrik AS was the most successful articulated jib tower crane manufacturer up to 46tm. A small city class crane, invented in 1961, the Magni S-40 “jackknife” crane provided a maximum capacity of 2.2t at 18m and 1.6t at 25m radius.
Capacity was slightly improved by the later Magni S-46 with 1.25t up to 30m outreach. A short counterjib of only 5.1m or 4.6m, depending on whether the counterweight was concrete or steel plates, added to its versatility.
Today both crane types can be still found working in the London inner city district. Besides the traditional Tornborgs tower system the light weight crane upper is fitted on towers of different manufactures like BPR and Peiner.
At Bauma in 1995, BKT tried to encroach on Tornborgs’ territory by presenting the BM 45 (today marketed under the name MRC45-B3 in the Potain range).
Although the crane improved on the design of the Tornborgs veterans, it has had only limited market success. One machine has been delivered to Hong Kong, and another was sold in the US. The main reason for its relatively poor sales may be the more expensive design compared with standard luffing jib cranes, and its low capacity.
On top of this, the system’s integrated rigging procedure (above all the time consuming rope reeving) will add to installation costs in comparison to standard luffers. Generally, the complete crane upper is pre assembled on the ground before being lifted as a single 9.5t unit on the crane tower. No counterweight is required for the upper crane. Up to 30m free standing height can be obtained on 3m or 6m long tower sections with only a size of 1.28m square making the crane suitable for installation inside the lift shafts of apartment buildings. Alternatively, a 3.4m by 3.4m wide city cross undercarriage can be used.
Like the Tornborgs cranes, the maximum capacity is limited to 2t while 1.5t can be lifted at 30m radius. In contrast to the Tornborg models, the jib can be used in articulating mode or as a standard single boom luffer to extend the maximum under hook height. Also, the very short counterjib radius of only 3.7m makes the crane suitable for cramped site conditions.
In the 1970s, Kroll tested a 42tm knuckle boom type crane based on the successful K-40 saddle jib city crane, but using another jib. The crane was developed with the UK crane market in mind, but no orders were received.
More successful were the double level jib cranes K-103V and K-202V, also manufactured by Kroll. The K-102V provides a maximum lifting capacity of 8t, and 2t at a 35m radius, while the larger K-202v can lift 10t up to a 21.5m radius, and 8t at 25m maximum outreach. Just two K-202V models were built, and delivered to Novatome in France 1974 to work on a nuclear power site. The K-202V is a real heavyweight – the upper crane above the slewing ring weights 79.5t (while the smaller K-103V upper weights 49.72t). Both cranes follow the double luffer jib crane design more commonly used on harbour cranes. In both cases, the base boom section is designed as tube sections connected by a number of screws instead of pins, adding to the required rigging time.
The luffing system is driven by two winches, which are connected by a rack system rather than by ropes. The power required to move the jib is relatively moderate, and is further reduced by means of movable steel weights. Besides the moving counterweight, fixed concrete ballast is used. The counter jib length is so short that the tail swing radius and the outermost point of the jib, when parked in the innermost position, are identical.
Originally designed as multifunctional cranes, they can be installed on portal basis for harbour work or used as climbing tower cranes by putting the crane upper on standard tower systems. Due to the high weight, only limited free standing under hook heights could be realised (about 32m while using a 1.6m square sized tower system).
Tower crane rental company Vertical Transportation still runs three Kroll K-103V veterans in its fleet, coping with special narrow site conditions in London.
For the mega dam project Three Gorges, meanwhile, Shanghai Port Machinery Plant built five giant MQ2000 “high frame portal cranes”. The bottom climbing cranes are equipped with double lever jib to deliver 20t concrete buckets at a radius up to 71m. Design integrated horizontal movement of the hook during luffing and speedy luffing are essential for this heavy-duty job. At 35m radius, 63t can be lifted. Although 100m under hook height can be achieved, the crane is based on a rail going portal. The required dimensions of the machine lead to a rigging time and size of the crane components only acceptable for long running industrial construction projects.
When looking on the history of articulated luffing jib cranes, one could assume that this special tower crane design is a passing fad of the last century. However, there are a growing number of urban or industrial construction sites that have to be served economically by a versatile crane concept.
Until now, Swiss tower crane service company Yerly J.M.SA has specialised in rigging and renting of self-erecting cranes. However, when the company’s head, Jean-Marc Yerly, was on holiday in Marseille he noticed harbour cranes with double lever jibs. He realised that their jib design could be the solution for some of the main problems of cramped construction sites. Foe example:
* Safe multiple crane operation is guaranteed by the retractable jibs.
* No extra tower segments have to been used just for oversailing lower cranes, leading to a cost reduction in rigging time and rental rate.
* Productivity is raised since no nearby cranes have to delay performing a lift while another crane is crossing its working area.
* Nearby obstacles such as high buildings or natural obstacles like trees can be passed by.
* The low overall height means the crane is more resistant to storms.
Normal luffers cannot work on narrow building sites with head restrictions (for example, at airports). However, articulated jib cranes, have only one half of the complete jib length up in the air when working at minimum radius. The question for Yearly was: How could the benefit of the jib design been transformed in a lightweight, rigging and transport friendly design?
In 1997, he met Professor Jacques Bersier from the “University of Applied Sciences Western Switzerland, Fribourg” a Swiss technical institute with an excellent reputation. In 1998, a patent pending study was finished. With the financial help of a governmental institution to support innovation the first prototype crane was manufactured until March 2005. Last summer the crane, named Cobra 263 C, successfully passed all tests on the Yerly test ground in Domdidier, Switzerland.
The Cobra 263 C surpasses European veterans with an articulated jib. Up to 20m radius 12t can be lifted, and at the crane’s maximum radius of 47m, the capacity is 5.6t. This performance is possible even thought only 1.6m wide BPR tower sections based on a 5m by 5m undercarriage are used for a maximum free standing height of 31m.
The secret of this extraordinary capacity together with the compact tower dimension lies in the movable counterweight, following a unique radial shaped 12.8m long counterjib. Via rope connection with the jib, the counterweight moves up in correspondence to the unfolding jib when booming out. Side rollers guide the counterweight carrier, leading to a smooth, shock free movement. Thanks to the moving counterweight in connection with the articulated jib design Jean-Marc Yerly claims the capacity of his crane to be as twice as high than a standard crane of similar size. Furthermore the moving counterweight together with the articulated jib design reduces the required energy for luffing in comparison to single boom luffers.
The system integrated design feature of the articulated jib leads to a variation in the outreach that is twice as fast as standard single boom crane types, hence avoiding one of the main disadvantages of luffers compared with saddle jib cranes. Maximum luffing speed is 60m/min. In the feasibility study, a hydraulic cylinder was intended to unfold the jib. But for maintenance reasons, a sophisticated rope system is used in the prototype. This consists of two 7.5kW luffing winches located in the base boom section together with the hoisting winch.
One luffing winch is responsible for moving out the outer jib while the other retracts it. It is therefore possible to retract the outer jib closely to the jib foot, to obtain just 2m minimum working radius. Furthermore, in a proposed alternative operation mode, the outer jib can be fully stretched and connected to the base jib by a remote controlled bolting system in order to work with the same crane as a conventional luffer where a extra under hook height of up to 46m is required. All motions of the crane can be controlled via the touch screen in the cabin, where the electrical cabinet is also integrated. While the machinery deck of the prototype was fabricated in Italy, jib and cabin fabrication and fitting of the crane components were carried out at Yerly’s plant.
Jean-Marc Yerly plans to manufacture most of the Cobra crane’s components at his plant. Well-known companies will supply the main components to ensure quality and reliability. The slewing ring, for example, is delivered by Rothe Erde, the stepless frequency regulated drives are from SEW, while the electrical control system is from Schneider Electric Car.
All operation data can be displayed on the cabin monitor and online. In contrast to the Kroll veterans, probably the most significant development is in the transportation and rigging solutions for the Cobra cranes. The complete 260tm crane with 31m tower can be transported in only five truck loads. The machinery platform with folded A-frame and cabin form one transport unit. In order to reduce transport dimensions the luffing winch on the one-piece machinery deck is located alongside the middle axis. Hence, the A frame can be folded around the winch during transportation.
Many features facilitate the rigging procedure. For example, the roller block of the luffing ropes is fixed to the A-fame head during transportation avoiding time-consuming reeving of the luffing rope during erection. Mechanical devices at the foot bolt connection points speed up jib connection to the machinery deck. The four concrete ballast plates are safely fixed to the counterweight carrier by using the counter jib platform. Base jib section and outer jib make up a folded unit – with the aid of the two luffing winches in the base section, they can be installed as one package horizontally, reducing the hook height needed from the assistance crane. On the drawing board is already a larger Cobra crane, providing the same design features. The Cobra 638 will lift 4.4t at 60m radius.
The strange looking prototype Cobra 263c towering over the industrial area of Domdidier already attracts a great deal of attention from local contractors and from abroad. With the substantial improvement in capacity in contrast to former European cranes following the articulated jib crane design, and thanks to the sophisticated transportation and rigging devices, the Cobra crane will find a ready niche market. First inquiries are already lying on the desk of Mr Yerly.
Although, at a first glance, marketing is concentrated on the Swiss home market, the company also intends to market the crane internationally.
For more details, go to: www.cobra-crane.com
