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Structural mechanics simulation at the world market leader for mobile cranes

FEM simulations in crane development at Liebherr

The second part of the series on the importance of simulation at Liebherr-Werk Ehingen (LWE) is dedicated to structural mechanics or FE analysis for determining strength and deformation. This topic is extremely important when it comes to the efficient development of high-performance cranes. In parts 1 and 3, the simulation itself and the topic of CFD are examined in more detail.

Decrease in weight, increase in performance

A constant task of developers is weight reduction. A low dead weight is advantageous during transport and also benefits the length of the booms, since only the dead weight and not the payload is relevant  when erecting.

In addition, the reduction of the dead weight component, which is a burden on the crane structure, increases the load-bearing capacity. The goal of lightweight design is supported by innovative materials. Steel manufacturers developed weldable fine-grained structural steels with increasingly higher strengths.

An interesting trend for LWE. After all, doubling the steel strength means halving the weight of the welded structure! As a result, steel structures have become increasingly thinner, despite the required high load bearing capacity.

Load bearing capacity achieved by an 8-axle mobile all-terrain crane in 1982.
Load bearing capacity in 2021 is equal to its heavier successor. Simulation makes it possible.

Stability issues

The predominantly pressure-loaded, rather thin-walled steel structures are very susceptible to stability. Structural analysis calculations are therefore essential. Typical physical phenomena are bend buckling, plate buckling and shell buckling. In most cases, closed analytical calculations can only be carried out in individual standard cases.

When it comes to the calculation and verification of steel assemblies that are at risk of stability, numerical simulation using FEM is an enormous relief. Here, LWE relies on professional software from Ansys in combination with the consulting services of CADFEM.

The picture shows a high-precision simulation of buckling behavior on a crane boom, carried out with Ansys Mechanical.

 Joachim Henkel Computational Engineer and Head of Structural Engineering, Liebherr-Werk Ehingen GmbH
Joachim Henkel
Computational Engineer and Head of Structural Engineering, Liebherr-Werk Ehingen GmbH

The results of the calculation show that by using this approach, you get very close to the real structural mechanical behavior of the overall crane.

Simulation is indispensable

Joachim Henkel heads the structural engineering department at Liebherr-Werk Ehingen:

“The telescopic boom of the all-terrain crane shows just how indispensable Ansys FEM calculations are for us when optimizing the weight and stiffness of assemblies. The ‘oval profile’ patented by Liebherr has proved to be the preferred cross-sectional shape in telescopic boom construction. It is relatively easy to manufacture and has an extremely favorable weight-to-load ratio.”

“However, a prerequisite for using of the novel profile shape in telescopic crane construction was a reliable concept for calculating the thin-walled shells based on FEM. In addition, the high imperfection sensitivity required a high quality and manufacturing standard. The optimum of this patented cross-sectional shape could only be achieved through countless variant calculations using FEM in Ansys.”

Challenges in structural mechanics

At LWE, 80% of the challenges for which simulation software is used are of a structural-mechanical nature. Joachim Henkel describes the requirements that he and his team at LWE have for modern simulation software: 

“First and foremost is the ease of use and learnability, followed by the speed and realism of the calculations. The trend is to simulate several adjacent main assemblies in one FE model. These are connected to each other via contact boundary conditions. This requires performance, which Ansys provides. The results of the calculation show that this approach comes very close to the real structural-mechanical behavior of the overall crane. 

For the precise and reliable design of the slewing bearing between the undercarriage and uppercarriage on the crane, it is necessary, for example, to take into account the stiffness behavior of the respective adjacent main assemblies, namely the undercarriage and slewing platform. This is done by realistically modeling the slewing bearing with all supporting, retaining and radial tracks (including track rollers) as well as the adjacent steel structure of the undercarriage and the revolving platform. Ansys masters this in an exemplary manner.”

Ansys at LWE

Geometry preparation is usually performed with Ansys SpaceClaim or the DesignModeler. With this, a reduced external geometry is worked out from the existing CAD data. If necessary, detailed points (e.g., at load application areas) are retained as a solid model in order to be able to carry out more precise evaluations there.

The calculation is then carried out in Ansys Mechanical. In order to be able to calculate as many load cases as quickly as possible, you start with a linear calculation. Subsequently, selected load cases are examined geometrically and materially non-linearly in greater depth.

00:04 minAnalyzing the buckling behavior of thin-walled structures is an important engineering task.

In order to determine the need for further buckling tests in advance, LWE uses a tool that provides a linear buckling test according to the standard (EC 1993-1-5) as an extension within the Workbench. If additional buckling tests are necessary, an ACT extension (developed with CADFEM) is used to apply the imperfections. Furthermore, special analyses, such as the calculation of the slewing bearing of the undercarriage and uppercarriage, the floor plates or the load on the hinges are also possible.

00:17 minCalculating the movement of the crawler crane chain using the Ansys Motion multibody simulation module.
Ansys Motion: Analysis of the rolling behavior of the chain

Ansys Motion helps to calculate the rolling behavior of the chain over the return rollers or over the drive wheels of a crawler crane. The same applies to a crawler track to determine the load on the track rollers or the behavior of a freely suspended chain.

Assessment required

The LWE simulation team headed by Joachim Henkel repeatedly receives requests from customers and from internal quality assurance to assess damage with regard to stability behavior, especially on the diagonal tubes of the boom of lattice boom cranes. The previous approach was either an exact 3D remodeling of the defective tube or replacement of the defective rod in order to eliminate any doubts. Both are very time-consuming and therefore cost-intensive. 

The solution was found in an adapted extension in Ansys.For this purpose, a geometrically and materially nonlinear calculation model was created, which is particularly characterized by a speedy calculation process. The stability behavior was verified using the European buckling stress lines according to the standards DIN 18800-2 and EN 1993-1-1, which were developed from real tests.

Combination Ansys / Excel

On a model verified in this way, a simplified local buckling was generated using the quasi-collapse affine imperfection forms, which can also vary parametrically in shape and position on the tube. Studies show that the resulting reductions in the ultimate load are particularly influenced by the position and size of the buckling as well as by the thinness of the overall system.

In order to provide LWE with a faster assessment of the individual cases, a separate program was developed, which enables the assessment of defective tubes on the lattice tubes in question across the entire spectrum. The basis of the data is a database of 7,452 calculated individual results. The variation of the parameters is selected in such a way that a linear interpolation between the results can be tolerated.

The generation, evaluation and verification of the generated data sets was largely automated through an interaction between Ansys and the Excel program. The quick assessment of the stability behavior of lattice tubes with local plastic deformations is thus ensured within the scope of the spectrum required for LWE.

Joachim Henkel
Computational Engineer and Head of Structural Engineering, Liebherr-Werk Ehingen GmbH

Without numerical simulation, predictions about the reliability and durability of a crane component subjected to greater mechanical stresses during its service life would be pure speculation.

Ansys fundamentally necessary

Joachim Henkel assesses the benefit of Ansys as a simulation tool in the field of structural engineering as fundamental:

 “Ansys enables us to operate lightweight construction on the crane. There can be no competitive product without lightweight construction, and no lightweight construction without simulation. The challenges arising from the ever lighter and thus thinner-walled steel structures can only be mastered efficiently with numerical simulation. Without numerical simulation, predictions about the reliability and durability of a crane component subjected to greater mechanical stresses during its service life would be pure speculation.”

“Due to the increasingly powerful Ansys simulation programs and the increasing computing power of the systems, we are also able to create ever more detailed calculation models that are very close to the actual mechanical behavior. This makes predictions about the suitability for use absolutely reliable.”

The first part of the series gives an overall view of the simulation at Liebherr-Werk Ehingen.

In the third part of our LWE series, read about where and how flow simulations significantly shorten the time to market.

Liebherr Werk Ehingen GmbH

Author: Thomas Löffler
Images: © Liebherr
Published: June 2022

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