Department of Mechanical and Mechatronic Engineering

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Browsing Department of Mechanical and Mechatronic Engineering by browse.metadata.advisor "Bredell, JR"

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    Dynamic wind load effects in a photovoltaic single-axis tracker mounting rail.
    (Stellenbosch : Stellenbosch University, 2024-02) Koekemoer, JH; Bredell, JR; Venter, G; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Single-axis trackers are actuated structures often used in utility-scale photovoltaic (PV) installations. These installations are sensitive to dynamic wind load effects due to their lightweight, flexible support structures and large PV module area. Significant damage to single-axis trackers have been reported in literature, despite the use of modern design methods. Design codes prescribe wind loads for a representative geometry but exclude potentially aeroelastic sensitive structures. Additionally, boundary layer wind tunnel testing and computational fluid dynamics studies are often associated with significant uncertainties. This study aimed to determine wind load effects on an existing 32 m × 25 m single-axis tracking PV array using field measurements. In this way, the uncertainties associated with wind tunnel testing and computational studies could be avoided. The project focuses on a critical component of the support structure, namely the mounting rail that attaches the PV modules to the rotating torque tube. Representative mounting rails were instrumented with strain gauges to capture dynamic wind load effects over periods of up to 109 days. The strain gauge locations were determined using results from calibrated finite element analyses of the structure. Equivalent static normal forces and moments acting on the mounting rail could be calculated using strain data and load calibration. These equivalent static loads would produce similar deformations and stresses compared to the dynamic wind loads while assuming simplified load distributions. The experimentally determined wind loads were correlated with wind speed, wind direction, and tracker tilt angle. The most critical wind directions (east and west) have a larger PV module area projected normal to the oncoming wind and subsequently showed high wind loads. Higher equivalent wind loads were also observed when the PV modules were more inclined relative to the oncoming wind. An interior located rail showed lower peak loads compared to an exterior rail, likely due to shielding from the surrounding structure. Wind load coefficients were lower compared to design codes for the range of wind conditions and tracker positions seen during the measurement period. This was expected, since design codes represent a critical combination of geometrical parameters to provide conservative estimates of wind loads. Analysis of dynamic load effects revealed contribution of torsional and bending modes of the torque tube to the normal forces and moments experienced by the mounting rails. A fatigue assessment found an insignificant fatigue damage for wind speeds below 21 m/s. Measurements suggest wind speeds above 21 m/s may be expected to cause fatigue damage. Design trends in utility-scale trackers show a decrease in mounting rail length to reduce the capital cost of new installations. The calibrated finite element model was expanded to assess the impact of a reduction in mounting rail length. Significant increases in the stress in the mounting rail and PV module were seen with a reduction in rail length, assuming all other parameters remain unchanged. Increases in stress may be non-linear, depending on the position and component.
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    Experimental testing and simulation of a nutating grinding mill.
    (Stellenbosch : Stellenbosch University, 2024-02) Van Tonder, JJ; Bredell, JR; Coetzee, CJ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Mined materials are comminuted for valuable mineral extraction, often using a nutating grinding machine. The HICOM mill is known for efficient grinding but faces operational challenges, primarily due to fatigue at kinematic joints. This study investigates nutating mill dynamics, focusing on force responses at key kinematic joints. The NuMILL, an experimental model representing the HICOM, was designed for data collection. In addition to the experimental investigation, two simulation methods were used: Multi-Body Dynamics (MBD) and Discrete Element Method (DEM), where MBD deals with internal mechanical loads and DEM with loads acting on the chamber as a result of material contact. The critical load path was identified as the crankpin joint of the torque arm, which experiences high cyclic loads. The MBD and DEM simulations had limitations when used independently. The combined DEM and MBD model, accounting for both structural and charge material loads, was evaluated against experimental measurements. Its accuracy in predicting crankpin resultant forces varied with rotational speed, showing errors of 19 %, 6 %, and 1% at 100 RPM, 400 RPM, and 700 RPM, respectively. This combined simulation method demonstrated its potential for real-world application in predicting kinematic joint forces, as illustrated through its application to the HICOM mill.
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    The Influence of High Strength Steel on the Fatigue Life of Welded Joints in the Automotive Industry.
    (Stellenbosch : Stellenbosch University, 2022-04) Ramsay, Gareth Allan; Venter, Gerhard; Bredell, JR; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: The ability to predict and protect against fatigue failure of structurally critical components, while still being able to produce high performance and costefficient designs is of great importance to the automotive industry. This study investigates the influence of high strength steels on the fatigue life of welded joints commonly used in the automotive industry, and compares the experimental fatigue data to commonly used fatigue design approaches, namely the BS 7608 approach and Shigley’s approach. Two joint details of interest are considered, namely a non-load bearing fillet welded T-joint and a load bearing fillet welded cruciform joint. Each joint geometry has three different base and filler material combinations, with varying material strengths, i.e. a total of six different specimen configurations. Two material combinations have a high strength steel (Strenx® 700 MC D) for the base material, with one combination having a matched filler material and the other having an undermatched filler material. The third material combination has a lower strength steel (S 355 JR AR) for the base material, with a matched filler material. Tensile tests were performed to confirm the base material mechanical properties and weld quality of the manufactured specimens. The welded specimens of both joint geometries were fatigue tested and the obtained data was used to generate experimental S-N curves. The experimental S-N curves for each joint geometry and material combination were compared with each other, as well as with the two fatigue design approaches. The investigation showed that there is no significant benefit to using high strength steel as the base material for fatigue loaded welded joints. Moreover, in some cases the use of high strength steel actually proved to be detrimental to the fatigue performance of the joint, compared to the use of a lower strength steel (e.g. T-joint with non-load bearing weld attachment). The results suggest that the fatigue performance of a non-load bearing weld attachment (T-joint) is highly dependent on material combination and strength, with lower strength base materials offering better fatigue performance than higher strength base materials. In this case the material combination and strength seems to be the dominating factors in fatigue performance over joint geometry. In contrast, the load bearing welded joint (cruciform joint) was shown to be less dependent on material combination and strength, with the joint geometry potentially being the more dominating factor on fatigue performance, as shown by the similarity in fatigue performance with the varying material combinations. The BS 7608 design curves generally tended to be quite conservative depending on the joint geometry and material combination considered. The BS 7608 does not account for material strengths and this could be why its fatigue performance predictions are quite conservative in certain cases. In general, Shigley’s material strength dependent approach overestimated the fatigue performance of the investigated welded joint details and is therefore not recommended for use in the fatigue design of these joints.