Computation Section
Process Flow Models

- Production Lines


When portions of the process are arranged into time-paced production lines, the operation time does not entirely determine the amount of resource time used by the operations in the line. A line is characterized by it's cycle time, the time interval between the departure time of units leaving the line. The operations in a line are paced to this cycle time. When the operation time is less than the cycle time, the resource performing the operation remains idle for a time equal to the difference between cycle time and operation time.

A line can accommodate operations with times greater than the cycle time by providing parallel stations. For example, if an operation time is 9 minutes and the cycle time is 5 minutes, two parallel stations must be required. Each station requires two line cycles for a unit of product.

For this case we use the time_f (time function) to compute the extra resource time required for an operation that is part of a production line.


As illustrated previously, we must choose the Time Function feature when building the process definition.

To illustrate the use of the time function feature, we use the example previously to show defects, but here we assume that the processing operations and inspection operation are arranged in a paced line. Although the operations have different processing times, the line is operated so that every unit of product remains at each station for the same amount of time.

The line function is invoked by including the word "line" as part of the Type definition in column B below. The time function is computed in column L. For a line operation, the function depends on the cycle time of the line included as the P1 entry in column J. It also depends on the number of parallel stations provided for the operation placed in column K. For an operation in a line with a time longer than the cycle time, more than one station must be provided. Multiple stations process units simultaneously. The delay in time caused by the line operation is computed with the formula below and presented in column L.


For the example, we use a cycle time of 0.5 hours. Because operation 3 has a processing time of 0.9 hours we use two parallel stations for this operation. When a unit passes through this operation, two cycles of the line must pass (1 hour) before the unit is released. The function computed in column L shows that 0.1 hour of resource time is lost. For operations with only one station, the lost time is simply the difference between the operation time and the cycle time.

With a demand for 50 units per week, the operation flow is 1.9052 per hour. The WIP indicated in column T indicates that the line has sufficient capacity to carry this load.



Return to Top

tree roots

Operations Management / Industrial Engineering
by Paul A. Jensen
Copyright 2004 - All rights reserved

Next Page