Saddle jib tower cranes follow very similar design principles wherever the cranes originate. It is a different story, however, for luffing jib tower cranes where manufacturers follow their own rules. Heinz-Gert Kessel reports
Design features of flat top and saddle jib cranes have become streamlined all over the world but it is a different story for luffing jib tower cranes. For this crane type manufacturers follow their own design rules. Most opt for a fixed ballast arrangement while some prefer moving ballast systems to compensate jib moment when booming up or down. A classic rope operated luffing device is most common, while others use hydraulic cylinders for small and medium cranes up to 250 tonne-metres.
Even focusing on fixed ballast cranes there is a surprising variation in design of the machinery deck, A-frame and luffing jib reeving method. In addition, ballast can be either concrete or steel. Even within one manufacturer’s luffer range different design roots are apparent. A common design trend worldwide remains to be developed. It may be that luffers are still considered as special custom-built equipment. Their application, however, is increasing so manufacturers should reduce manufacturing cost by streamlining design features and, at the same time, make their designs more modular and easier to transport and erect.
Unlike the old days, rope-activated counterweight systems are now rare. German manufacturer Wilbert’s “Heavy-Lifter” product line is notable for its crane size and the patented pendulum ballast system. An idea behind it is to use the same 3.30 x 3.30 metre tower system for 900 to 2,400 tonne-metre class cranes. Most other movable counterweight systems rely on designs created by Franc Jost. In one case the swinging mechanism is on the top of the machinery deck, as represented in the original BKT design, the Manitowoc Potain MR Series and Everdigm cranes above 300 tonne-metres.
A second approach places the swinging ballast device under the machinery deck as demonstrated by Terex cranes above the 260 tonne-metre class, and all rope operated Jost luffing jib crane types as well as Chinese middle class luffers following a Jost design. Examples of the latter include Useter and SCJC (Sichuan Jincheng Construction Machinery Co.). In all cases the ballast is stored inside a basket or hung in a frame which moves behind the machinery deck in low angle boom positions.
In contrast to comparable fixed ballast versions the requested overall tail radius is generally larger. Only some higher capacity models, for example, the new Terex CTL1600-66 show a combination of fixed and moving ballast to compensate the moment created by the moving jib on the tower system. Most crane designers, however, prefer a fixed ballast arrangement on cranes above 1,000 tonne-metres.
Sting in the tail
In the early days a unique design feature of a luffing jib tower crane was its ability to boom up to add extra height and to cope with cramped construction sites. Today the tail radius of a luffing jib crane is also important. Strict prohibition on oversailing neighbouring properties means that the whole tower crane structure needs to be kept within the construction site boundary. The jib must be parked in a steep out of service position and the machinery deck should be as compact as possible.
Wolff’s 320B generation updates show a reduction in the tail radius. On the original version it was 10.3 m and it has been reduced to 7.2 m on the BE-G7. Reduction of the tail radius was made possible by adding ballast, in this case using steel instead of concrete for the slabs. Using steel also reduces transport volume but cost should be considered. Where the simpler mechanism of a saddle jib type crane already makes it cheaper than a luffer, a movable steel ballast system further raises the cost in the same capacity class. Concrete ballast versions are preferred by crane manufacturers in the medium capacity class.
In addition, installation, dismantling and transport costs must be considered. Luffers with short tail radius have heavier machinery decks connected to the tower with a large slewing ring support structure. To reduce this central crane component weight, Favelle Favco in the late 1980s developed the split deck made up of a rear frame section attached by four large pins to the front part of the machinery deck. Together with a two leg folding A-frame this design was adapted by many larger capacity cranes from China, for example, SEPMW (Shanghai Electric Power Machinery Works), XCMG, Yongmao and ZS cranes from Zhong Sheng Construction Machinery (Nanjing) Heavy Industry.
Now more widespread in larger luffers, in the 700 to 3,500 tonne-metre range, this design principle has become well-known. Inside the rear frame counterweight is stored. In most cases steel ballast is used due to restricted available space. While originally the steel plates are inserted in a hanging position into the ballast carriage, Favelle Favco recently changed to laying ballast blocks, to speed up handling. On one hand it is easier to handle slim ballast plates secured with four chains in windy conditions. On the other hand inserting the first ballast block into the outlet of the rear machinery deck will close the gap, leading to safe placement of the rest of the ballast blocks.
It is typical for hoist and luffing winches as well as drive units to be arranged on top of the split machinery deck. When short radius is required, for example, 9 m in the 1,250 tonne-metre class, space under the foldable A-frame is limited, especially when a large prime mover (diesel hydraulic drive) is used. On a high capacity crane it might mean not leave space for two strong diagonals leading from the boom stop point of the A-frame to the machinery deck. Even when the A-frame strut in this case is reinforced, it is obvious that in cases of high wind pressure a long jib in steep out of service position will not be held by the boom stops, but will destroy the whole A-frame if it moves backwards.
To prevent this Chinese crane manufacturers Yongmao and ZS relocate the space-consuming diesel engine inside the machinery deck framework so that just the winch units have to fit under the A-frame. It avoids space problems and reduces noise but potential for overheating must be considered. At least the oil cooler should remain above the machinery deck. Wolff gained experience placing the hoisting winch on top of the jib base section on medium capacity luffers. In the author’s opinion this could also be considered for larger cranes, moving the hoisting winch and runner winch inside the wide jib foot section as found on Liebherr and NOV lattice boom offshore pedestal cranes.
In this way space under the A-frame could be given to a walk in electric cabinet or sound-proofed diesel hydraulic container. Lack of space puts the third winch for a light load runner hoist on traditional Favelle Favco cranes on top of the diesel hydraulic unit. Yongmao relocated this winch into the A-frame near the part reinforced for the boom stops.
At first glance self contained diesel hydraulic drives appear suitable for cranes working with high line pull on the tallest buildings. Problems remain with oil spoiling during operation, less acceptance from a more environmentally sensitive construction industry and lower energy efficiency. Favelle Favco in co-operation with sister company Krøll, plus most Chinese manufacturers using the original Favco design, offer electrically driven or diesel hydraulic versions.
Favelle Favco is a pioneer of high capacity winches for lifts of 25 or 50 tonnes of steelwork in single fall operation. Lifting the 100 tonne columns at the base of the tallest high-rises requires just two falls of rope. Fewer turning points in the rope run lengthens rope life and reduces friction. The best way to do this is to feed just the boom head sheaves directly from the hoisting winch. At a low boom angle the heavy hoist rope drags over the top surface of the boom unless it is placed higher. Liebherr’s new 710HC-L has a double deck arrangement with the winch and two hoisting drives on the top and the electrical cabinet underneath.
In 1982 Peiner introduced a kind of second level on the SN500, SN630 and SN1000 models with only 7.50 m tail radius. Tricky rigging of the A-frame, however, together with the hoisting winch platform was time consuming. SYM experimented in 2001 with another two-level design. On the S480LH24 the electrical cabinet and resistor switchboards were inside a lattice frame work. All the winches were on top of the A-frame.
While the common Favelle Favco split deck design gives a rigid and versatile low cost machinery deck the question remains how to connect the slewing frame to the machinery deck. Component size for transport and weight for erection means that on large cranes the slewing ring has to be disconnected from the front machinery deck section. It involves removal of a large number of screws. On its new FHTD product line manufacturer Finehope avoids this time consuming work by connecting the machinery deck with four large pins to the slewing unit.
Another way to reduce the weight of the individual machinery deck components was first developed by Wolff for the 355B to 1250B range. Now this design is found on a growing number of Chinese luffers. Examples include Zoomlion, SCJC and Useter. In this case a central triangular connection framework above the slewing ring joins the foldable A-frame, machinery deck and jib foot. Special benefits are the light weight lay out of the machinery deck and the forces generated by the jib entering the slewing ring in a central line. Terex, Jaso, Yongmao and SYM use a similar design.
Most luffing jib crane manufacturers prefer a folding two leg A-frame structure but some opt for a massive lattice tower head section towering over the centre of the crane while at the back the machinery platform is connected by pendants. In this way Wilbert found the necessary flexibility to enhance the A-frame height to be able to raise the crane’s maximum capacity just by inserting an additional section into the tower head. In most cases such a massive tower head generates a lager transport volume in contrast to a folding A-frame arrangement.
With a short tail radius it must be considered that the assist crane hook finds a place behind the tower head when adding the machinery deck. When competing against fast rigging hammerhead cranes the luffing rope installation must be considered. Missing pre-reeving arrangements and the long way between the A-frame top and the sheave block being parked on the jib, cause erection delays.
Rigging time is extended by at least half a day over saddle jib versions in the same capacity class. Devices have being developed to keep the luffing rope in reeved position during transport. Comansa and Wilbert followed modern truck crane design by storing the luffing rope in pre-reeved position on the hoisting winch. Wolff avoids reeving the luffing rope during crane installation by integrating the luffing winch into the A-frame or tower head up to the 350 tonne-metre class.
Liebherr reduces the time for luffing rope reeving by minimising the number of sheaves and raising the line pull. Favelle Favco and Manitowoc Potain opt for a small auxiliary assist winch which could speed up reeving by having a lightweight erection rope pulling the luffing rope.
In 2014 the Japanese crane manufacturer Ogawa developed a patented method of avoiding reeving of the luffing rope, for its OTS-140ES. Pre-reeving at the back of the A-frame and only two rope pendants holding the jib are connected during crane rigging at the front. When booming up the pendant lines are moved over the A-frame head sheaves and these pendant ropes are stored by a traverse moving up and down at the back of the A-frame operated by the luffing rope.
The trend for reducing tail radius is more obvious on Japanese luffers. In Europe where tail radius is 7 to 8 m, Japanese manufacturers reduce it to 5 or 6 m, in the same capacity class. For this purpose wide machinery platforms are accepted where sometimes even luffing and hoisting winches form a line and the back of the platform, with handrails, follows the rear slewing circle.
Jost, Useter, SCJC, Krøll and Favelle Favco use an enclosed electrical cabinet behind the driver’s cabin. Other manufacturers, including Potain, Yongmao, ZS, XCMG and Liebherr still use separate cabinets close to the drive units. On Liebherr’s latest model, however, the 710HC-L centralises the cabinet in the lattice framework forming the base of the machinery deck. Wilbert uses a suspended air-conditioned container unit underneath the machinery deck to centralise electronic controls. While space saving this arrangement means extra erection weight and needs additional access platforms.
Wilbert is considering a side mounted E-container in which there is space left at the front to store the operator cabin for transport. In 1987 Favelle Favco was a pioneer of in giving a choice of where to place the side mounted crane cabin to give the best view and to reduce space climbing the crane down alongside a building. Anchored close to the facade of a building an essential benefit can be that the cabin be installed inside the A-frame, like the Wolff 900B, for climbing down, or at the opposite side of the machinery deck.
Ease of movement
To reduce the dead weight and transport volume of the jib many manufacturers prefer a triangular cross-section. Adequate side loading capacity, however, especially when long jib combinations are rigged, must be respected. A square section jib can be considered when heavy lift capability is required. Even for heavy lift cranes, however, jib sections wider than 3.30 m should be avoided because of transport costs and permits for oversized loads are becoming stricter.
With worldwide rental fleet management in mind, machinery deck components and slewing units, in addition to jib and tower sections, should be designed to fit standard shipping containers wherever possible. This already happens with most hammerhead cranes. At the same time crane component weights should be able to be adapted to the capacity of roof top dismantling cranes to avoid several dismantling steps with different crane sizes.
With its new 700 tonne-metre class 710HC-L Liebherr has taken on this design challenge and reduced the component weight to 10 tonnes. The lightweight Liebherr 200DR 5/10 derrick can be used for dismantling. In addition, all components of the 710HC-L are built to fit standard containers. To achieve this the slewing ring support was designed to be as small as possible compared with the standard form of this part in the same capacity class. To reduce erection weight, the hoisting winch can be separated by four pins from the hoisting winch platform which, in turn, is fixed by another four pins on top of the machinery platform framework.
This winch mounting design could also perhaps allow different winch types and sizes to be placed on top of the same basic machinery platform on customer request. Such design flexibility could also lead the way for modularisation in the luffing jib crane line. It could extend from tower sections to machinery deck components, identical counterweight blocks and jib modules shared across some capacity classes, as is already typical in flat top crane design.