Assembly lines are a special case of product layout. In a general sense, the term assembly line refers to a progressive assembly linked by some type of material handling device. The usual assumption is that some form of pacing is present, and the allowable processing time is equivalent for all workstations. Within this broad definition, there are important differences among line types. A few of these are material handling devices (belt or roller conveyor, overhead crane), line configuration (U-shape, straight, branching), pacing (machine, human), product mix (one product or multiple products), workstation characteristics (workers may sit, stand, walk with the line, or ride the line), and length of the line (few or many stations).
The range of products partially or completely assembled on lines includes toys appliances, autos, garden equipment, perfumes and cosmetics, and a wide variety of electronic components. In fact, It is probably safe to say that virtually any product with multiple parts and produced in large volume uses assembly lines to some degree. Clearly, assembly lines are an important technology to really understand their managerial requirements one must have some familiarity with how a line is balanced. An important consideration that should not be overlooked in designing assembly lines is the human factor Early assembly lines were machine paced; that is, they moved at a predetermined pace, regardless of whether or not the work was completed at a station. Under this structure, workers who fell behind had to rush to complete their assigned tasks, with the result often being faulty workmanship. In recent years, worker-paced assembly lines, which were advocated initially by Japanese manufacturers have replaced machine-paced lines in many facilities. With the worker-paced line the operator continues to work on the product until the work assigned is satisfactorily completed. Only then is the product allowed to move on to the next station. The quality of the products made on a worker-paced line is significantly higher than that of
products made on a comparable machine-paced line. When a Japanese manufacturer took over the production of televisions from a U.S. company the number of defects dropped from 160 defects per 100 TV s to 4 defects per 100 TV s, even though the output per day and the workforce remained virtually unchanged. This dramatic increase in quality was attributed, in large part, to the installation of a worker-paced assembly line that replaced the previously existing machine-paced line.
Definitions. Before we begin our analysis of assembly lines, there are two terms that need to be defined, and that are illustrated in Exhibit 8.9. • Product interval time
The product interval time is the actual time between products being completed at a station. This is often referred to as cycle time or more recently talk time (from the Swedish word meaning "cycle or cycle time")." As we shall see shortly the talk time for an assembly line determines the capacity of that line. We will use talk time to describe the product interval time in our analyses.
Product duration time. The overall time it takes to complete an individual product, from start to finish, is known as the product's throughput time and is also referred to as cycle time, especially when looking at it from the customer's perspective. Throughput time is important when you are looking at the delivery time for customized products. We will use throughput time to describe the product duration time.