On the natural cycle time of assembly lines

Heavy products such as airplanes are assembled in an assembly line fashion. These assembly lines are designed to complete a unit every few hours and up to a few days. The size of the product and the limited space available for work stations motivate the operation of such lines in two or three shifts. The objective of this research is to analyze the effects of multiple shifts on production scheduling of assembly lines in heavy industry. Two effects are: (1) on learning and improvement of performance as a function of experience, and (2) on the production control system. The use of shifts causes a synchronization problem when the cycle time and the length of each shift do not have some special attributes; this problem can be solved by using parallel assembly line operating at their ‘natural cycle time.’ A procedure for calculating the natural cycle time is suggested. The effect of parallel assembly lines on the expected labor cost of the line was checked by designing a set of randomly generated problems. The simulation study suggests that the parallel line configuration does not change significantly the total number of workers in the production system when when facility layout cost is kept constant. The major conclusion of this study is that management should consider parallel line configuration whenever a synchronization problem exists and causes difficulties in production control as well as reduction in the learning effect.