The productive use of free time : the utilisation of deterministic maintenance opportunity windows due to excess capacity in large coupled production lines with finite buffers

Durandt, Casper (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

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.

AFRIKAANSE OPSOMMING: Ingenieursontwerpsreëls vir groot serie-gekoppelde produksieprosesse het tot gevolg dat oortollige kapasiteit ingebou word. Die V-profiel ingenieursontwerpsreël word toegepas om masjiene voor en na die middelste masjiene te beskerm teen materiaaltekorte of stroom-af ophoping. As gevolg hiervan, het produksiemasjiene stroom-af en stroom-op ingeboude ‘vrye tyd’ (ook bekend as kritiese aftyd of onderhoudsgeleendheidsvensters), wat verander in ledige tyd indien dit nie betyds aangewend word nie. Tydens produksie beweeg die punt van die beperking lukraak rond as gevolg van stogastiese en deterministiese gedrag en die behoeftes van die produksiemasjiene – stogasties as gevolg van die onvoorspelbare masjienstoppe en deterministies as gevolg van bekende aanvullingstempo’s vir rou materiaal en brandstof. Operateurs is tipies nie in staat om die produksieproses as ’n geheel te evalueer nie. ’n Operateur kan die huidige bottelnek wees en onbewus wees daarvan. Net so kan ’n operateur vrye tyd tot sy beskikking hê, maar eweneens nie bewus wees daarvan nie. Hierdie ingeboude vrye tyd is bruikbare kennis indien die operateur bewus is daarvan. Die uitdaging in hierdie studie was om die grootte van die nie-beperking in reële tyd te bereken, sodat hierdie tyd produktief gebruik kan word om die algehele prestasie te verbeter. Die Teorie van Beperkings, Knapbetyds en Kanban spreek nie die vervoerbandreistye en die kwessie van oortollige kapasiteit in ‘n moderne gekoppelde vervaardigingslyn aan nie. Kennis van die hoeveelheid vrye tyd gee die operateur reële tydbeheer om opportunistiese voorkomende onderhoudsaktiwiteite uit te voer, terwyl die produksieproses ononderbroke loop. Dit wys ook vir ’n operateur wanneer hy die huidige bottelnek is en dat hy dan alles in sy vermoë moet doen om sy produksiemasjien tot voordeel van die hele produksieproses aan die werk te hou. Onlangse vooruitgang in hoë-spoed kommunikasie-metodes maak hierdie geleentheid nou 'n werklikheid. Dit is moontlik om 'n huidige status van alle produksiemateriaal in die proses en die huidige deurvloeikoers van masjiene te hou. Die kennis van die status van die hoeveelhede en deurvloeikoerse van materiaal in die proses is die boustene van die betrokke algoritmes wat vrye tyd op die produksieproses bereken in huidige tyd. Drie toetskriteria is in die studie gedefinieer om die probleem op te los. Die toetse is: (i) 'n toets vir vrye tyd akkuraatheid; (ii) 'n aanvang-van-produksie toets; en (iii) 'n oordeelkundige gebruikstoets. Hierdie toetse het getoon dat vrye tyd akkuraat bereken kan word, dat operateurs die aanvang-van-produksie prestasie kan verbeter en dat vrye tyd gedurende normale produksie oordeelkundig gebruik kan word. Die uitkoms van 'n eksperiment by die Valpré botteleringsfasiliteit in Heidelberg het gedui op 'n algehele verbetering in operasionele doeltreffendheid en produktiwiteit van arbeid. Die vrye-tyd konsep is aangebied en bespreek by agt internasionale en verskeie plaaslike konferensies en vergaderings waartydens die konsep gevorm is tot die praktiese instrument wat dit vandag is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98099
This item appears in the following collections: