Luff the load

The ability to lift and slew a load is common to all cranes. Increasingly the need to lift a load up and over obstacles or up and on to the top of structures is in demand. People are probably familiar with the concept of luffing jibs on larger all terrain cranes, 200 tonnes plus. On smaller cranes operators have had to use manually fixed offsets to allow them to handle 'up and over' jobs. What is relatively new is the use of luffing jibs on smaller ATs and very recently on new RTs.

Why a luffing jib?

Firstly, it is worth looking at some definitions. To luff means to move the jib of a crane vertically to lift a load. A luffing jib moves under power (hydraulic or cable) with a load or into a working angle to lift a load. Manually fixed offsets are set at ground level. The boom is lowered and the pins retaining the vertical movement of the offset are removed. The boom is then slowly raised until the desired angle and associated pinning can be achieved. To change the angle of offset this process needs to be repeated. These are time consuming and physical operations, and at acute offset angles and with long extensions reaching the boom head, re-pinning can involve substantial work from the top of a ladder.

With the hydraulic jib there is no such work. Once fitted the jib can be luffed with load by the operator from the cab, for example on certain Grove telescopic cranes, from 5° through to 40°. The variety of lifts available, the flexibility that allows changes mid-lift, and the lack of set up time means the operator can get on with what is important. Grove product specialist Neil Hollingshead says that this is ideally suited to working in confined spaces and is also an advantage when a miscalculation has been made in setting the jib, allowing it to be adjusted without de-rigging the crane.

Extended use

The hydraulic luffing jib concept on mobile cranes has its roots in East Asia. Asia has always been the place where hydraulic luffing jibs have been seen since the introduction of the mini rough terrain, or city crane, concept. It can be difficult and heavy work for operators to fit large manual swingaway jibs so this generated the need for the swingaway set up action and for the luffing jib to be powered. First examples were seen on Kobelco city cranes in the late 1980s and then on larger Kato and Tadano city cranes in the early 1990s.

At the same time Krupp first looked at the concept with the KMK 4080 all terrain and Grove (which took over Krupp in 1995) says that it was one of the first German manufacturers to feature a hydraulically powered telescoping and luffing jib. It was introduced in response to a requirement from customers in Japan. At that time this feature was only sold into the Asian market as other markets did not seem to appreciate the benefits, Grove says. An analogy, from the motor industry, could be the option of power steering. At first nobody really understood why they needed it, but once they had driven a car with it fitted, they would never go without it. Outside Asia the perception was that the luffing jib was a real benefit but that the hydraulic swingaway action was 'nice to have' but not that necessary. After all there was another alternative that relied on gravity, (described in the Technology section). This perception combined with other design pressures, such as the need to cut down weight to meet axle weights for roading regulations, contributed to a lack of interest in the concept.

Acceptance of the idea in the rest of the world began when the hydraulically powered swingaway set up action was removed while retaining the hydraulic luffing jib. Originally introduced on Krupp's KMK 3050, in 1994, and the KMK 2035, in 1995, the hydraulic luffing jib feature is now common across the Grove GMK range of all terrains. While it took a little while for it to become a major selling point, the use of the hydraulic luffer on five and six axle machines, in particular the GMK 5180 and GMK 6220, showed real performance advantages according to Grove. As operators got used to the benefits on the larger cranes they began to look for this feature on smaller all terrains. Demand has increased to the extent that Grove estimates that 85% of its machines now sold feature the hydraulic system as opposed to the manual.

Latest development at Grove is the use of a hydraulic luffing jib on the new US-built RT 9000E (First sight Nov01, p17). The 115t-rated rough terrain has the Megaform boom cross section design and the hydraulic luffer from the GMK 5100 that was developed in the German Wilhelmshaven factory.

Kato offers hydraulic luffing jibs on many of its cranes, such as the newly upgraded MR-100 Lsp-V and the KA-3000 (Kato's new cranes Sep01, p21). Most of these models are not sold outside Asia but the KA-900 is an exception. The KA-900, first shown at Bauma in 1998, is a 90t capacity all-terrain designed for the Scandinavian market. As standard it has a hydraulic luffing jib that can also telescope under load. The so-called Super Luffing jib is a three-section full power extension and retraction type with a maximum load of 4.3t, variable in length between 7.7m and 19.1m and offsettable between 2° and 60°.

Tadano says that it has been a pioneer in the development of fully hydraulic, telescopic luffing fly jibs, particularly for use in built-up urban areas of Japan where set-up areas are small, buildings are tall and getting close to the lift is difficult. Tadano says that because crane designers are constantly trying to go as high as possible, the only solution on city cranes and smaller ATs is the compact telescopic jib.

Technology

In terms of technology, luffing falls into two categories: a swingaway, with or without inserts, that can be luffed hydraulically; and a larger jib extension which is luffed using cables. Grove's system in the former category uses a single hydraulic reeling drum that feeds oil to a single hydraulic cylinder, which moves the jib extension from 5° to 40°. It is possible to luff under load and the largest hydraulic luffing jib plus extension combination fitted to GMK ATs is 38m (GMK 5200 and GMK 6220). The luffing jib can be fitted either to the boom head or to lattice extensions, which in turn are fitted to the boom head. The boom is held in a level position and the jib is manually swung away from its stowed position. The operator then uses controls on the side of the jib to hydraulically alter its angle. As the angle is lowered the momentum of the swingaway means it is easily offered up to the boom head. The swingaway is then manually pinned and hydraulic connections secured. In addition, the lowered position of the boom/swingaway combination means it is easily acessible, so that the operator can reeve the jib head quicker. Once reeved, the main boom can be raised or lowered or, at the flick of a switch the luffing jib can be raised or lowered.

Whether using the main boom or the luffing jib the lifting operation is logged and protected by the EKS system. On larger GMKs the Electronic Crane Operating System (ECOS) automatically slows down the slewing motion when jib extensions are fitted.

On larger Grove cranes, 200t and above, the luffing jib is moved by cables. This needs two hoist drums and a mast to keep the luffing cable above the jib. Jib extensions of more than 90m can be luffed like this. The GMK 6300 can have a luffing jib up to 61m long. Set up takes longer with cable luffed jibs than hydraulic ones. The sections are put together on the ground and then hoisted. Luffing is done with the auxillary hoist control.