Author: Wouter van den Bos, Faculty of Mechanical Engineering, Section Transport Technology and Logistics at Delft University of Technology in the Netherlands / founder of SDC Verifier.
Ensuring a crane's integrity and stiffness is crucial for safe and efficient operation. Traditionally, Finite Element Analysis (FEA) models have relied on simplified beam elements to check a crane's overall strength.
However, these elements can miss crucial details in complex geometries, such as small plates of various shapes, welds, and larger components like gantry systems, leading to inaccurate stiffness predictions. FEA allows us to determine a crane's stiffness, natural frequency, and vibration modes, which are essential for smooth operation and safety. Understanding these vibrations helps predict the crane's response to movements, ensuring optimal performance.
Impact of Crane Stiffness on Control
A crane with insufficient stiffness will experience excessive swaying and flexing during operation. This makes precise positioning of the trolley and spreader difficult, leading to inefficiencies and potential safety risks. Therefore, proper crane stiffness is crucial for smooth, accurate control.
Terminal operators often specify a minimum natural frequency, essentially the inherent vibration rate of the crane, to ensure efficient and safe control. While design codes guarantee structural integrity, accurate prediction of this frequency is crucial to avoid overly "wobbly" cranes.
Measuring the natural frequency of an existing crane is relatively straightforward with specialized equipment that excites the crane and measures its vibration response. The challenge lies in predicting natural frequency at the design stage. Traditional FEA models using 1D beam elements provide a general idea of crane behavior, but often overestimate stiffness and, consequently, natural frequency. This can lead to the issues described above.
A More Detailed Approach for Accurate Predictions
Our structural design and analysis software SDC Verifier has tackled the challenges of overestimated stiffness and structural integrity with a hybrid approach. The common beam modeling approach can still be used for overall crane calculations, including stiffness. However, to capture the details and improve prediction accuracy, we incorporate detailed plate models of parts in critical areas. These areas include: welded connections where complex geometry can impact stiffness as the forces between the members are transferred through the joints; bolted connections; corners and sharp transitions; and gantry systems.
In addition to improving stiffness predictions the detailed connections modeled with 2D plate elements also provide valuable insights into fatigue life and local stress concentrations. Analyzing stress in welded components can predict where fatigue cranes are more likely to start and develop, allowing for better preventative maintenance to extend the life of the crane. Detailed plate models help identify areas with the highest stress levels. Addressing these concentrations early in the design phase helps prevent potential failures and ensures the crane's long-term structural integrity.
Our hybrid modeling approach does not require modeling the full crane in detail, giving a chance to model the large part with the well-known beam elements. At the same time, it provides a more comprehensive understanding of the crane's behavior, leading to not only accurate stiffness predictions but also improved overall strength design and performance.
Understanding the Achilles' Heel: The Gantry System
Beam elements provide a basic understanding of gantry stiffness but miss important aspects like axial stiffness. They overestimate stiffness by not accounting for the bending of individual gantry components. Additionally, these models struggle to capture torsional effects, leading to inaccurate predictions of the gantry's ability to resist lateral forces.
In essence, the gantry system behaves more like a complex lattice than a simple beam. By incorporating detailed plate models for the gantry, we can account for the bending and torsion of individual components, providing a far more accurate representation of its actual stiffness characteristics. This allows us to predict the crane's overall behavior with greater accuracy and avoid potential operational issues down the line.
Based on the received FEA results and real measurements of the natural frequencies (see table above), several key findings were revealed:
This successful collaboration with Kalmar Netherlands showcases the power of structural design and analysis software SDC Verifier has tackled the challenges of overestimated stiffness and structural integrity with a hybrid approach. SDC Verifier's advanced FEA modeling techniques. Our software simplifies incorporating detailed models, especially for critical areas like the gantry system, leading to unmatched accuracy and improved crane design.
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