High-strength water-quenched structural steels with yield limits of at least 690N/mm2 have been used for approximately 25 years in the construction of mobile cranes. In this time, it has been possible to rapidly advance the use of these steels and the associated design methods. Almost all structural plate construction in modern mobile cranes is made from S960QL. The fundamental demands on all-terrain cranes can be basically summarised as:

• high lifting capacity

• high manoeuvrability

• optimised price/performance ratio.

As all terrain cranes are part of the traffic on public roads they are subject to the motor vehicle construction and use regulations applicable in the countries where they operate and consequently are usually strictly limited with respect to axle loads (commonly 12t) and the permissible total weight. In spite of those weight and dimension limits, there is demand for continuously higher AT-crane performance. Those basically contradictory demands can only be met by the use of high-strength structural steels with yield limits of up to 1,100N/mm2.

Demand is for the telescopic boom, carried by the crane within the admissible weight, to cover an increasingly large working area (both lifting height and radius) and so its length must be increased while maintaining or even reducing the construction weight. Boom lengths have been increased to the extent that the typical 60m of outgoing models of six axle cranes have been replaced by a six section 72m long design on the latest model.

To satisfy the above conditions at minimal cost the design engineer must create and apply optimised designs that take into account modern production methods. In addition to a safe design for the high static and dynamic stresses, the operating suitability and durable dimensioning of the bearing steel structures make them an ideal application for high-strength structural steels.

The stress-strain diagram of the S960QL in comparison with the S355 shows the relevant essential dimensioning mechanical characteristics. Besides the high yield strength, equally the high elongation of more than10% and the high impact value (> 27 joule) are of particular importance, also at low temperatures of -40°C

or -50°C.

The essential structural weldments of a mobile crane are divided into the following three groups:

• carrier frame

• superstructure

• telescopic boom.

High-strength structural steels are used in all these main groups.

Carrier frame

Figure 14 shows the structural weldment of a carrier frame which has two essential functions. On the one hand it serves as a chassis for the carrier and must resist the stresses of all effects resulting from the dynamics of road travel. And it is also a base for the crane. Forces are transmitted, via the slewing ring, into the torsion-resistant box type construction and subsequently, via supports (outriggers and supporting rams) into the ground of the jobsite.

In Figure 14 the structural welds are marked by arrows. Due to the notch effect, these areas are particularly important – especially with regard to fatigue strength – during design and production as well as during regularly performed inspections.

Shown in Figure 15 is the comprehensive weldment of a carrier frame. Due to the required light-gauge design hardly any plate thicker than 8mm is used, even on the highest capacity mobile cranes. The box-type design must undergo extensive finite element calculation for its entire bearing capacity under the full range of stress conditions and be stiffened accordingly.

Superstructure

The slewing ring gives the crane a 360° working radius with constantly high lifting capacities. The load bearing weldment of the superstructure is shown in Figure 16 and critical welding areas are marked by arrows. The superstructure functions as a complicated and highly stressed load absorbing structure for the luffing ram and the telescopic boom. At its rear, the counterweight is pinned for crane operation and the hoisting gear is also mounted in that area.

Lateral forces resulting from the mass moment inertia of the hoist load and telescopic boom particularly influence superstructure dimensions. The great elasticity of high-strength steels where tension is highest requires computational consideration and also the superposition of elastic deformation, both on the carrier frame and the superstructure. Geometric and material technology non-linear finite element calculations become necessary. Deformations under operating conditions must be minimised by light weight but firm designs.

Boom

Due to marginal geometric conditions (limited width, height and vehicle length) on one hand, and the quest for ever greater maximum boom length, but with low weight and high stability against deformation, the design of the boom has been subject to the highest demands. Consequently, the most important advances have been made in the area of boom design.

The steel telescopic boom, shown in figure 18, consists of a base section, pulley head and individual telescopic sections with collars, inserted one into another. Inside the boom is the extension mechanism which adjusts the hydraulically operated and electronically controlled boom to variable and pre-selectable lengths. To each telescoped condition, dependent on the crane configuration, a maximum load capacity is allocated and protected by the safe load indicator (SLI).

The technology of modern telescopic booms is not solely characterised by the use of high-strength structural steels with a minimum yield strength of up to 1,100MPa but without the use of such steels the overall technological solution would not be practicable.

The modern telescopic boom concept, for example, as followed by Liebherr, is based on the following technological focal points:

• a single ram extension system

• automated telescoping operation

• an ovoid boom profile of S960QL and S1100QL steel.

These technologically matched and coordinated points allow the realisation of modern, progressive boom designs. This boom technology has been awarded the Innovation Prize of the German Steel Industry.

Development of the ovoid boom profile made it possible to satisfy the widest range of demands. Old and new Liebherr boom profile shapes are shown in Figure 19. Essential characteristics for the new boom profile are the following:

• optimised adjustment to the static and dynamic stresses

• reduction of the construction weight compared with the former cross- sectional profiles of comparable load capacity

• adjustment to the single ram telescoping system

• optimised design with regard to production engineering.

The ovoid cross-sectional shape of telescopic booms (now up to 84m long, on Liebherr’s LTM 1500), meets the load capacity demands and simultaneously simplifies design, production and final assembly. As buckling stiffeners or cross-sectional doublings are not required within the main pressure area the full extent of each section can be utilised due to the reduction of the distances between the individual boom sections.

Flectional bearing capacity, total stability and local cross-sectional performance are precisely matched with one another by comprehensive finite element calculations which are also decisive for the dimensioning. The high load capacity demands on the boom extended at short radii (stability) and at large radii (flexural strength), and the coordination of the individual boom sections, depend on the selection of the structural steel, the cross-sectional profile, and the plate thickness. Deformation performance of the boom is improved by an ovoid profile. In spite of the capacity utilisation of the high-strength structural steels (S960QL in the top shell and S1100QL in the bottom shell), the high deformation stability for the various stress configurations of the crane is attained.

The shell-shaped boom profiles must be produced to very close shape tolerances, achieved with the new cross-sectional shapes. The weld seam volume of previous designs has been drastically reduced which means less heat is applied so residual stress development and deformations due to welding are reduced which improve the production process. Also reduced is the need for weld seam testing and time consuming straightening work. Increased, however, is the number of folding processes to produce the ovoid boom profile. The semi-circular bottom shell is produced by a multitude of cold-formed foldings (polygonally) but the overall production and processing times have been reduced.

Stay rods for lattice boom systems

The boom systems of large lattice boom cranes are secured by highly stressed tension rods. These tension rods are subjected to high alternating stresses, swinging due to wind action and their own weight. Transportation and assembly at the construction site should be performed safely and easily. So as not to reduce the load capacity of a lattice boom crane those tension elements must be designed to weigh as little as possible. These flexible tension rods, connected by fork-finger couplings, are made of high-strength quenched and tempered structural steels of up to S960QL. Figure 21 shows a comparison of the shaft cross-sections of different steels at the same load and the corresponding weight advantages.

The design examples explained above, for components using high-strength structural steels, contribute significantly to the success of modern mobile cranes. Ultimately the use of high strength steels benefits the end user who is lifting and positioning heavy loads, and the material has advantages over alternatives, if one is even available.