Designing unbalanced lines - understanding protective capacity and protective inventory

Abstract

The purposes of this paper are, first, to introduce several concepts and definitions related to Theory of Constraints design and management of unbalanced lines and, second, to illustrate the concepts of productive and protective capacity and inventory in a constrained line. Drum-buffer-rope is the Theory-of-Constraints based scheduling mechanism used to manage throughput at constraint work stations and flow at non-constraint work stations. A small simulation model is given to illustrate the importance of protective capacity and protective inventory at non-constraint stations. The line consists of several stations with the centre station being the constraint station. The capacity of (and inventory at) non-constraint stations is varied during the simulation. Line output increases as inventory at non-constraint stations increases. This result is contrary to traditional teaching about line design which says that line output is a function solely of the capacity of the slowest station.