Browsing by Author "Durandt, Casper"
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- ItemThe productive use of free time : the utilisation of deterministic maintenance opportunity windows due to excess capacity in large coupled production lines with finite buffers(Stellenbosch : Stellenbosch University, 2015-12) Durandt, Casper; Smit, Eon van der Merwe; Du Preez, Nicolaas Deetlefs; Stellenbosch University. Faculty of Economic and Management Sciences. Graduate School of Business.ENGLISH ABSTRACT: Engineering design rules for large serial coupled production processes result in excess capacity and redundancy being built into production lines. The V-profile engineering design rule is applied to protect machines before and after the slowest machine from starving and blocking. As a result, machines that are feeding and drawing from the constraint machine, have locked-in free time, also called maintenance opportunity windows (MOWs), which eventually causes idle time if not used timeously. During a typical production run, the point of constraint moves randomly around the serial production process due to stochastic (due to the unpredictability of breakdowns) and deterministic (due to stoppages resulting from known replenishment rates for raw material and refuelling) behaviour and requirements of production machines. Operators are typically not able to assess a large production process as a whole. An operator might be the current bottleneck and not be aware of this. Similarly, an operator might be unaware that he has free time at his disposal. Free time can become useful knowledge if the operator knows of its presence and magnitude. The purpose of this study was to calculate, in real time, the size of the non-constraint so that this time can be used productively to improve overall performance of the production process. The Theory of Constraints, Just in Time and Kanban do not address conveyor travel times and the issue of excess capacity in modern coupled manufacturing. Being made aware of free time allows the operator real-time control to perform opportunistic preventative maintenance activities while the production process is running. It also indicates to an operator that he is the current bottleneck and that he/she should make every effort to keep his/her machine going to the benefit of the complete production process. Downtime at the bottleneck can be communicated and added to free time at other production stations in real time; this additional free time can then also be used to perform opportunity-based preventative maintenance. Recent advancements in high-speed wireless communication methods make it possible to keep a real-time status of all production material in the process and of the throughput rates of machines. Knowing the status of accumulation quantities, throughput rates and levels are the building blocks of the relevant algorithms that calculate free time on the production line in real time. Three test criteria were defined to solve the problem defined in this research. The tests are: (i) a test for free-time accuracy; (ii) a start-up test; and (iii) a judicious use test. These tests proved that free time can be calculated accurately; operators can improve the start-up performance and judiciously use free time during the normal running state. The outcome of an experiment at the Valpré bottling facility in Heidelberg also suggested an overall improvement in operational efficiency and labour productivity. The free-time concept was presented and debated at eight international and numerous local conferences and meetings where the concept was shaped into the practical tool it has proven to be.