Browsing by Author "Van der Merwe, Andre Francois"

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    An air suspension cushion to reduce human exposure to vibration
    (Stellenbosch : University of Stellenbosch, 2007-03) Van der Merwe, Andre Francois; Van Niekerk, J. L.; University of Stellenbosch. Faculty of Engineering. Dept. of Industrial Engineering.
    Off-road working vehicles are subjected to high levels of vibration input on the rough terrain and irregular roads they work. The human operators are therefore exposed to high levels of whole body vibration (WBV) and at risk of developing health problems. A number of international standards address the matter of whole body vibration, and the European Union issued a directive which limits the exposure of workers in the EU to WBV. Unfortunately, to date there is no law in South Africa requiring compliance with any of these EU standards nor guidelines. There are vehicles which are not fitted with suspension and/or suspension seats. The three wheeled logger used in forestry is a highly manoeuvrable and effective bulk handler, but without any form of suspension and no space under the operator’s seat to install a suspension seat. However, a suspension cushion can be retrofitted to existing vehicles largely alleviating the problem. To isolate low frequency vibration large suspension travel is required which makes an air suspension cushion attractive, as it can fully collapse. Additionally, a Helmholtz resonator if added to the cushion in the form of a pipe and tank, provides anti-resonance at a specific frequency. The resonator can be tuned by adjusting the pipe’s length and diameter as well as the volume of the tank. Larger diameter pipes have less friction and give better reduction of the transmissibility curve at the anti-resonance frequency. The SEAT value is a single number used to compare suspension seats for a specific input vibration. It is calculated from the weighted input acceleration power spectral density curve and the suspension seat transmissibility curve. The former is obtained from the vehicle and is vehicle, path and speed dependent. The latter is the only variable that can be improved by using a better suspension seat/cushion. The input power spectral density often contains significant energy at frequencies where the human operator is most sensitive. The cushion resonator could be tuned to position the anti-resonance in the transmissibility curve at these frequencies. The resultant output vibration would thus be lower than the input vibration at that frequency. In this dissertation an analytical model describes the state variables in the cushion, pipe and tank. A Simulink model predicts the transmissibility curve with a solid mass as well as with a two degree of freedom seated human model. Initially the prototype was tested with a solid mass to compare the transmissibility curve produced by the simulation with the experimental results. It was required to evaluate the contribution of the resonator without the complexity of the human impedance. Subsequent tests were carried out with human subjects. Test results showed high inter subject similarity at the anti-resonance frequencies. Design guidelines are formulated that can be used by the suspension cushion designer to specify the pipe diameter and length and the volume of the tank to determine the optimal transmissibility. Input psd from ISO7096 class EM3 vehicles is used as an example during the design process. A prototype air suspension cushion was designed to reduce output vibration on the three wheeled logger. Laboratory tests with human subjects showed a significant improvement at the problematic frequencies through the tuning of the resonator. Using a Helmholtz resonator with the air suspension cushion the overall SEAT value improved by 25% compared with a 100mm foam cushion. However, the current tank and pipe need to be reduced in size for practical implementation to the vehicle. Future work would include finding an alternative mass to replace the air in the pipe. This should reduce the size of the tank and the pipe required. Additionally the simultaneous effect of multiple resonators at different frequencies should be investigated. This is required for vehicles having an input psd with significant energy at more than one frequency band.
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    Analysis of a time based and corrective maintenance system for a sugar producing company
    (South African Institute for Industrial Engineering, 2014) Chindondondo, Brian; Nyanga, Lungile; Van der Merwe, Andre Francois; Mupinga, Tapiwa; Mhlanga, Samson
    ENGLISH ABSTRACT: The Southern Africa sugar producers’ market is under threat from Brazilian sugar, with the latter’s sugar landing in Southern African countries - Zimbabwe in particular – at 75% of the selling price of local producers. This paper outlines an investigation of the current maintenance systems of a Zimbabwean sugar producer and its shortcomings, with a view of optimization. Research has shown that maintenance can contribute between 15 to 75% of production costs, thus playing a critical role in product pricing and consequently competition. Maintenance records and interviews to maintenance personnel from the company were used to extract data relevant to the study. A bottleneck asset in the production line was chosen as a subject to measure the impact of the current maintenance system in cost terms. The study shows that the company is using a hybrid of time based and corrective maintenance. The paper also shows that the current maintenance philosophy resulted in downtime of eighty (80) hours on the subject asset over a four (4) year period, that translated to an equivalent of USD1.9 million in potential revenue lost; which downtime could have been avoided with better maintenance methods. The paper concludes by recommending the introduction of Condition Based Maintenance on the subject asset as an alternative to optimize the maintenance function of the company.
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    A decision support system framework for machine selection
    (South African Institute for Industrial Engineering, 2014) Nyanga, Lungile; Van der Merwe, Andre Francois; Matope, Stephen; Dewa, Mncedisi Trinity
    ENGLISH ABSTRACT: Selecting the right machinery to machine a part is a multi criteria decision making problem which is crucial in production planning. The process becomes more complex and tedious when one has to choose from a variety of machines available in an online registry. This paper investigates this decision making process with the objective to increase machineutilisation for small, medium and micro enterprises. A real time information based decision support system is necessary to assist the decision maker. A decision support system framework for machine selection for a manufacturing agent is proposed based on the Analytical Hierarchy Process (AHP). A human expect uploads parameters for a part that has to be machines. Based on these parameters, suitable machines are sought for from an online machine registry and ranked according to their capability to produce the desired part.
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    Design of cotton ginning dryer control system
    (South African Institute for Industrial Engineering, 2014) Maradzano, Isabellah; Nyanga, Lungile; Van der Merwe, Andre Francois; Dlodlo, Zwelibanzi B.; Chikowore, Takawira R.; Mhlanga, Samson
    ENGLISH ABSTRACT: The most important factor in preserving the quality of cotton during ginning is the fibre moisture content. At higher moisture content, cotton fibres are stronger but trash is harder to remove. Selecting ginning moisture content is a compromise between good trash removal and quality preservation. In the paper, inlet and outlet moisture content of cotton being fed into and out of the dryer are monitored at temperatures given in the dryer manual and literature. A mathematical model for drying cotton is then formulated by analysing the experimental results, cotton dryer historical records and dryer manual. The results show that there is a linear relationship between the initial moisture content, final moisture content and drying temperature. A control system integrating a Barionet controller to regulate and supply of heat to the system based on the initial moisture content is then proposed. The objective of the control system is to enable online monitoring of the dryer as well as giving early warning signs when the system is about to get out of control hencesafeguarding from overheating and avoid under-drying of cotton.
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    Development of a condition based maintenance system for a sugar producing company
    (South African Institute for Industrial Engineering, 2014-07) Chindondondo, Brian; Nyanga, Lungile; Van der Merwe, Andre Francois; Mupinga, Tapiwa
    ENGLISH ABSTRACT: With globalisation taking centre stage in almost every market; traditionally monopolistic manufacturing companies in Zimbabwe are struggling to compete with international companies whose operations are optimised. This paper focuses on maintenance practises of a Zimbabwean sugar manufacturer with an aim to improve the company’s asset care plans. The company’s current maintenance philosophy and its shortcomings on certain equipment in the plant were identified together with the associated annual maintenance costs. A pareto analysis on the equipment’s breakdown history was used to determine the conditions that can be monitored using Condition Based Maintenance (CBM) and experiments to establish the feasibility of monitoring these conditions were done. A cost benefit analysis of the company’s customary time based and corrective maintenance to a proposed CBM system on the subject equipment was used to show a probable significant reduction in annual maintenance costs should the CBM be implemented. The paper concludes by recommending an implementation matrix for the proposed CBM system with its three main elements of data acquisition, data processing and maintenance decision to introduce an optimised maintenance system that will reduce overall production cost thus bringing an opportunity to compete globally for the local sugar manufacturer.
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    Effect of mould temperature on the filling behavior of molten resin in plastic injection moulding of high density polyethylene (HDPE)
    (South African Institute for Industrial Engineering, 2014) Gwebu, Andile M.; Nyanga, Lungile; Van der Merwe, Andre Francois; Nyadongo, Sipiwe T.; Mhlanga, Samson
    ENGLISH ABSTRACT: The growth of injection moulding as a manufacturing process has led to the development of more products resulting in an increase in the focus on product quality, as manufacturers try to lower production costs, reduce product weight and eliminate defects. Of particular concern is the flow behavior of molten resin in the mould cavities during the production process. Inconsistencies in the flow of the melt tend to result in uneven or incomplete filling of the mould, resulting in production of defects (short shots). In the paper the Taguchi method is used to determine the optimum values of the injection moulding process parameters for High Density Polyethylene (HDPE.) plastic parts. A moulding processing window in which the process will achieve maximum quality, with major focus being on mould filling is developed.
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    An empirical analysis of operational disturbances and their impact on business performance in manufacturing firms : case tooling industry South Africa
    (South African Institute for Industrial Engineering, 2014) Dewa, Mncedisi Trinity; Matope, Stephen; Van der Merwe, Andre Francois; Nyanga, L.
    ENGLISH ABSTRACT: Globalization has managed to break trade barriers and the manufacturing environment has become more competitive. Market share is now determined by quality of goods and services irrespective of location. Today’s business environment for manufacturers requires flexible, responsive and robust systems, which produce a variety of products at competitive prices. To gain a competitive edge, the paradigms of e-manufacturing and distributed manufacturing have been recently advocated by researchers as potential solutions. However, irrespective of these technological advancements, manufacturing firms in the tool and die sector are still struggling to perform efficiently in the face of recurring operational disturbances. The paper identifies the most prevalent operational disturbances which occur in South Africa’s manufacturing firms in the tooling industry and their impact on business performance. A field study was conducted on a number of organizations which form an industrial cluster in the Western Cape manufacturing sector and seven typical disturbances were evaluated together with their root causes. The results gathered portrayed the correlation between identified disturbances and their corresponding consequences. The findings of the study were recommended to be used to develop models and computerized systems to solve the pending pandemic.