Redesign of the Slewing Group of a Truck Loader Crane at F.lli Ferrari S.p.A.

Article written in June 1998 with S. Morino and F. Martinelli for 31st ISATA

Abstract

Today market competition forces the optimization and reengineering even of traditional and consolidated mechanical components. The paper describes the experience we performed at the F.lli Ferrari Gru – a small company leader in the truck loader cranes – in the redesign of the cast steel-slewing group. We started from a traditional 2D drawing ready for production, generated by means of a simple 2D CAD system. Using a PC based CAD system, we generated a complete 3D parametric model of the crane’s rotating unit. We discussed such model with the designers, in order to optimize the component from the structural, manufacturing and costs points of view. Using the new generation parametric and features based CAD system, we have been able to quickly apply the changes to the three-dimensional model (i.e. to reorganize the design process and re-parametrize the basic components) and to verify and quantify the corresponding advantages obtained. This experience demonstrates how this approach, widely adopted by large mechanical companies, can be practically and successfully used even in a small company.

Introduction

The recent evolution of the CAD market is proposing to the attention of technical managers and designers a set of new software tools: the mid-range CAD products. These new systems run on low cost personal computers, based on the Microsoft Windows operating system, and powered by Intel or Digital Alpha processors. They offer a functionality level competitive with the high level area, at a price usually under the 5,000 US$. As imposed by the high sales volumes and by the low selling prices, the software quality, usually, is very high. In general, these systems are easy to install, to learn and to use, friendly in the user interaction, well integrated and compliant with the rules and the “traditions” of Microsoft Window. Furthermore, they are based on robust and well-tested geometric engines, such as ACIS, from Spatial Technologies, or Parasolid, from EDS. Obviously they aren’t as powerful and rich in functionality as the systems in the high level area; in spite of these limits they provide a valuable solution for many application contexts. In particular, these systems can constitute a real opportunity for many small and medium companies to approach new and consolidated CAD technologies, previously inaccessible due to costs and complexity problems.

On the other hand, large companies are only partially interested in these new systems; many of them use the latest technologies in the CAD area since long time. Furthermore, large companies need a wider set of functionality and require a different type of services and interaction with the CAD supplier, characteristics the mid-range systems doesn’t provide.

Generally, small companies haven’t the financial resources required acquiring and managing a high-end CAD workstation and the corresponding software modules. In particular, companies from the mechanical field lack also the human resources for the long training typical of high-end systems. Moreover, these companies can’t afford the transition costs for the integration of a complex CAD system in the design and production processes.

Starting from these observations, we decided to directly investigate the use of a mid-range CAD system in the design and production processes of a small company, in order to verify the advantages and the costs of such choice. The target was not limited to test the use of new geometric modeling capabilities in such technical environment. Our aims were also to verify the capabilities of the CAD system in using and interacting with legacy data, in cooperating with traditional design tools and processes, in supporting a preliminary structural analysis and in fully managing the parametric changes of the geometry as they are suggested by the experimental and simulation tests.

The study case

In order to evaluate a mid-range CAD system within the design and production processes of a medium or small company, firstly we needed to look for and select a mechanical company in our geographical area. Then, a significant part, or component, among the ones designed and produced by the company should be chosen. We selected the F.lli Ferrari, a specialized small company, leader in the design and production of truck loader cranes. Since some years, all the design process and all the technical communications with subordinate supplier are based on technical drawings, on paper, produced by means of a well-known brand CAD system. Therefore, all parts libraries and the entire drawings archive are constituted by 2D CAD drawings. The company uses 2D representations even for the more complicated parts; that requires the use of drawings with many cross sections and views.

When we started the project, the technical management at the F.lli Ferrari was already investigating the mid-range products as a concrete alternative to their current non-parametric 2D CAD system. In order to evaluate the use of such systems on a significant part, with designers and engineers at the F.lli Ferrari, we analyze and select two components of the crane: the crane’s rotating unit and the corresponding support.

The support of a new crane’s slewing group is a component usually designed with reference to previous cast iron basements from F.lli Ferrari experience. For example, the starting point for designing the model-728 crane basement was the cast iron support of model-722 crane. Because the shape of this component is very peculiar, for the designers it is rather difficult to employ some of the consolidated computational structural methods.

Typically, in order to define the basement component for a new crane model, the designer alters the dimensions of the old design by scaling them up or down as required by the new loads and performances. The result is a preliminary prototype design whose being discussed by the design group allows finding the best solution from a structural, manufacturing and economic point of view. This discussion and calculations, which have been taken into account, lead to the design drawings of the basement; usually after a period of tree-four months, it is possible to test the prototype unit that during this time has been manufactured.

However, the F.lli Ferrari’s prototype design procedure requires that a simulation of the overall crane’s life be carried out. This, obviously, makes the designer sure that his assumptions and calculations are correct and ensures the safety of the basement. The cast iron support of the slewing group plays a strategic role for the birth of a new crane project, both for saving money and for getting tighter schedules of fabrication.

The design group cannot afford any safety risk about this component; consequently, the strength of the design is often achieved at the expense of the weight, which weight means more costs.

The shape of the basement is so complicated that the weight penalty may be significant, and can’t be easily reduced due to problems in using standard formulas of science of construction. It may happen that a lot of cast iron material is used at points that are not very stressed and these situations can’t be easily highlighted.

Fig. 1: The rotating unit.

The modeling and analysis activities

First of all, we start modeling the crane slewing group geometry with a modeling system. Based on our previous experiences, and on simple availability criteria, we choose to utilize SolidWorks, from SolidWorks Corp., as the reference CAD system. This system is generally considered one of the best representatives of the mid-range CAD system category.

Using the parametric and feature-based approach, typical of the mid-range category, we was able to quickly define a fully parametrized model of the rotating unit and the support. Due to the complex shape of the support and to some limit of the CAD system, we need to explore some alternative construction procedure before to be able to complete the geometric model. In this phase, we get the main part of data from some preliminary drawings and directly from the F.lli Ferrari design team.

Once the CAD model was available, we discuss it with the design team, we collect and apply improvement suggestions, and consolidated it. By means of a FEA (Finite Elements Analysis) module, integrated within SolidWorks, we were able to obtain a map of stress values.

Fig. 2: The crane’s support.

The redesign activities

After the modeling activity was completed, we evaluated the possibility to perform a deep redesign of the selected parts. The main target was to redesign the support having as input data the actual constraints and the loads (both already verified on the crane prototype unit loaded at the maximum lifting capacity).

The first aim was to look at the thickness of the body of the basement. As a matter of fact, if it would be possible to reduce the wall thickness, even by one millimeter, a reduction of weight and costs of 5% on the production runs could be achieved. The analysis of the 3D model brought us to the conclusion that the reduction of the wall thickness was possible, but not all over the body in a uniform way. We obtained some advantages especially in the upper and in the lower part of the cast iron basement so that we could save some eight Kilos of material.

Then we have paid attention to the stiffening ribs. The internal one, connecting the lower bearing support to the forward pin of the basement was over-dimensioned and so we could reduce its thickness getting also a less encumbering shape. This solution let us, not only save material, but also getting a less complicated (and thus safer) casting procedure, obtained minimizing the length of the stiffening rib joining the foreword fairing wall.

These two steps were a significant upgrading of the existing cast iron support.

The following, and really innovative step, was to modify the shape of the fairing wall, generating a structural frame connecting the bearing supports (the upper and the lower one) to the forward pin (obviously again in order to save both weight and money).

This was obtained with the analysis of the inertial data of the different sections of the fairing wall. Starting from a continuous wall, we took away material all around the neutral fibers, optimizing shapes and profiles. We could generate a kind of fairing structural frame, easy to manufacture with casting technology, ten Kilos less heavy and with a good and innovative look.

Fig. 3: The assembly of the two parts.

The performances of this new solution have been calculated by placing maximum loads in twelve different angular configurations of the crane. For each of them the 3D parametric model showed the stress distribution in the basement. This work required many iterative changes and calculations to obtain the maximum economic saving for the production runs. In this phase, the integration between parametric-modeling and finite elements analysis modules allow us to quickly implement our optimization hypothesis as changes to the geometry and directly obtain updated results from the analysis model.

Of course, safety considerations have always had the maximum priority. The computer aided modeling and finite elements stress analysis allowed us to reach these promising results just in a few days. This new solution, obtained modifying the original design, demonstrates how the 3D-stress analysis approach, if well integrated in a powerful CAD system, makes it possible reengineering even traditional and consolidated mechanical components.

Conclusions

The new mid-range CAD system, with their rich set of integrated engineering tools, can really enhance the design and production processes of small and medium companies. They constitute a real occasion for these companies to approach the newest CAD technologies and to start the long and expensive integration and reengineering of their internal processes.

The described experience demonstrate that, in medium or small company, these new systems can successfully and gradually replace the more traditional 2D CAD systems and provide a valuable support to all the design and engineering activities.

Acknowledgements

We wish to tank the designers, the production engineers and the managers from F.lli Ferrari involved in this project, for their collaboration and the support provided.

References

[1] U. Cugini, G. Bocchi, F. Folini and M. Galloni. Rapid prototyping and parametric CAD systems. In: Proceedings of 27th ISATA International Symposium on Automotive Technology and Automation, Dedicated Conference on Rapid Prototyping for the Automotive Industries and Laser Applications for the Transportation Industries, (Aachen Germany, October 31–November 4 1994), pp. 51–57, ISATA, Automotive Automation Limited, Croydon UK, 1994.

[2] R. Wysack. Designing parts with SolidWorks. CAD/CAM Publishing, San Diego CA, USA, 1997.[3] I.Bozza and F.Folini. Proposal for a meta-model of product-development processes.To appear on: 2nd International Conference on Planned Maintenance, Reliability and Quality, (Oxford, England 2nd-3rd April 1998).