Paced Lines

In paced synchronous lines, the synchronization is achieved by transferring the jobs between stations at pre-determined and fixed time intervals, namely the cycle time of the line.  This job transfer takes place irrespective of whether or not the individual stations complete their task. This means that the conveyor is the bottleneck, since it dictates the speed of the line.  As a result, the rate of a paced line is fixed, but the quality (i.e., the fraction of outputs with all tasks correctly completed) may vary.  Specifically, a job with a task that did not get completed during the cycle time is termed defective (e.g., must be fixed offline).  Ideally, the throughput would be the rate of the line, given by the inverse of the cycle time.  However, since lines do go down, the actual throughput is given by the ratio of the line availability and the cycle time of the line.  Click here for a formula.  Furthermore, it is important to note that because a paced line cycles whether or not an individual task is completed, the throughput may contain defective products (i.e., products with one or more incomplete tasks).  Hence, the defect rate is an important performance measure in a paced assembly line.

Principles for Paced Lines

The following principles are applied to all paced lines, irrespective of their failure mode.

• Principle (Process Times): Defect rate is increasing in the mean and standard deviation of the individual process times at all stations.

 
• Principle (Line Length): For a line where the processing times are identical at all stations, the defect rate is increasing in the number of the stations in the line.

Depending on how machine failures in the stations affect one another, paced synchronous lines can be further classified as:

• Coupled: if when any of the individual stations fail, all other stations in the line stop working immediately. Thus, the entire line comes to a standstill whenever any machine fails.

 
• Uncoupled: if when an individual station fails, the other stations in the line may continue until they finish their current job. As the stations complete their jobs, they become idled and eventually, when all the stations stop, the line becomes entirely stopped.