Improvability properties of closed transfer lines

In many practical applications, serial production lines are closed with respect to the fixed number of pallets on which the workpieces are transported from one operation to another. Such closed lines are found, for instance, in automotive assembly operations. Unlike the open lines, where the first and last machines are never starved and blocked, respectively, the closed lines may have the first machine starved and the last machine blocked due to the lack or the excess of empty pallets. The question arises, how should the number of pallets be chosen so that the production rate is maximized? It is shown in this paper that a closed transfer line is unimprovable with respect to the number of pallets if the difference between the probabilities of the first machine starvation and the last machine blockage is minimized. Being easily observable on the factory floor, this improvability indicator could be used as a guide for continuous improvement processes