Weight optimization of axle-to-body star casting fitting used in multi-joint buses

From past to present, people have lived by forming communities. With each passing day and the advancement of technology, the distances between these communities have increased. Alongside this, the growth and density of populations have made the development of transportation systems—intended to transfer people from one point to another—inevitable. Technology, infrastructure, and population density have become significant factors in facilitating human mobility. In addition to regional differences, climatic conditions, population density, and levels of development form the foundation of human transportation, also known as mass transit, where many people are carried together. These factors also play a decisive role in determining the preferred type of public transportation. With the progressing world, human communities have begun to cluster into villages, towns, and cities, living under conditions of dense population. This situation has, in turn, created the demand for different forms of public transportation as a necessity. Vehicles operating on the BRT route, especially multi-articulated buses with the highest passenger transport capacity, inevitably exhibit higher exhaust gas emissions and greater fuel consumption compared to other vehicles. However, considering environmental impacts, reducing the carbon footprint has become an inevitable reality in today's developing world. With technological innovations, newly designed vehicles aim primarily to minimize the carbon footprint by reducing emission levels. To achieve these objectives, it has become essential to conduct weight optimization studies on prototype vehicles already produced as well as on components of vehicles still at the design stage. Ensuring safety as the top priority at every stage of the design process is of utmost importance. In the category of multi-articulated buses, which possess higher passenger transport capacity than other public transportation vehicles, BRT buses are improved through weight optimization of their structural components within the framework of safety standards, in order to reduce the carbon footprint and emission levels released into the environment. In this way, lighter vehicles are produced, contributing both to the reduction of greenhouse gas emissions from diesel engines and to national economic savings due to cleaner environments and lower operational costs. For components used in vehicles, achieving the ideal distribution of material to enhance durability and efficiency is of great importance. To reach this goal, structural optimization methods are employed. Through structural optimization, the dimensions, shape, and overall geometry of a part are optimized to enhance durability while avoiding unnecessary weight. The main objectives of this process can be summarized as reducing component weight, increasing rigidity, decreasing stress levels, improving manufacturability, and lowering costs. For components used in vehicles, achieving the ideal distribution of material to enhance durability and efficiency is of great importance. To reach this goal, structural optimization methods are employed. Through structural optimization, the dimensions, shape, and overall geometry of a part are optimized to enhance durability while avoiding unnecessary weight. The main objectives of this process can be summarized as reducing component weight, increasing rigidity, decreasing stress levels, improving manufacturability, and lowering costs. For the design, the complete axle was positioned under the bus body with average height and length parameters, ensuring a standard driving level height of 340 mm (step height). The bogie arms on the axle were designed considering the assembly requirements for their connections to the body and their angles relative to the axle. Subsequently, the star-shaped cast connection bracket, located on the axle-facing side, was modeled using solid modeling methods. AISI 4140 steel was selected for the design. The star-shaped part is mounted onto the axle with eight main bolted connections. On the underside of the cast component, a Ø60 mm diameter and 14 mm deep recess was created to fit into the set region on the axle for centering purposes. The final weight of the designed product was 17.5 kg. In this thesis study, considering Istanbul and other highly populated metropolitan cities where passenger density in public transportation is significant, static and dynamic analyses of the star-shaped cast connection bracket between the body and the fourth axle of the AKIA LF25 multi-articulated BRT bus, which has five axles with the first and fifth axles steerable, were conducted using the ANSYS software. Random vibration analyses were carried out on ANSYS as part of the weight optimization process. To ensure an optimal mesh structure, mesh independence studies were performed, and a finite element model with 1,131,821 elements was adopted. Examination of the mesh quality revealed a maximum skewness of 0.58 and an average element quality of 0.89. Within the optimization studies conducted on the bracket, static analyses were performed first to determine its load-carrying capacity and stress distribution. Then, natural frequency analyses were conducted to evaluate potential resonance risks, thereby aiming to prevent vibration-induced damage throughout the service life of the component. To replicate real-world conditions, field tests were designed, and the accelerometers and strain gauges used in these tests were introduced in detail. Before conducting road tests, the strain gauges and accelerometers were mounted onto predetermined points. The cleanliness and smoothness of the bonding surfaces were considered crucial for accurate measurements. Therefore, dust, residues, and contaminants on the cast component were first removed using compressed air. Then, surface roughness was minimized by sequentially polishing with sandpapers of 80, 120, 240, 320, 600, and 800 microns. After polishing, surfaces were cleaned with isopropyl alcohol to prevent interference with the sensors. Strain gauges and accelerometers were mounted with C33A adhesive, and the gauges were further protected with AK22 coating to safeguard them from potential impacts and foreign objects during vehicle operation. Through these sensors, measurements were collected under both normal driving and sudden braking conditions, enabling the identification of critical regions on the bracket. Road tests were carried out on traffic routes, where both normal driving and sudden braking data were recorded. The obtained real road signals were converted into Power Spectral Density (PSD) format in ANSYS, and random vibration analyses were conducted using the 3-sigma approach. Based on strain-life (S-N) curves from the literature, topology optimization was applied to reduce weight. Design modifications were iteratively made to the initial model in CATIA, followed by re-analysis in ANSYS. Each design revision served as an improvement over the previous iteration, with the final design achieved after the fifth revision. In this study, not only the behavior of the existing bracket was investigated but also significant weight reduction was targeted. Following the repeated analyses and revisions, the weight of the star-shaped cast connection bracket was reduced from its initial 17.5 kg to 10.4 kg, corresponding to a 41% weight reduction. In the initial design, under the reference static load of 49.5 kN, the maximum stress on the bracket was recorded as 200 MPa, with a safety factor of 3.25. In the final design, under the same reference load, the maximum stress reached 400 MPa, with a safety factor of 1.63. Since the yield strength was not exceeded, the final design was deemed safe. Analysis of triaxial accelerometer data using the 3-sigma rule yielded a stress level of 11 MPa with 99.73% accuracy, and no resonance conditions were observed in either of the designs.