Going with the flow time

While cycle time represents the minimum possible time between the completion of two successive jobs, flow time is the time it takes one unit to get completely through a process from beginning to end, including any wait time in the process. Many organizations confusingly use the term cycle time to represent flow time, cycle time, and takt time. We know this is confusing. Understanding how your organization defines and distinguishes among these concepts is critical because the terms have no standard definition used in all sources. For clarity, we’ve separated the three concepts. Another terminology trap is that some companies use thruput time as a synonym for flow time but never use it for takt time. For clarity, we avoid the use of thruput time in this book. The best way to illustrate the difference between flow time and cycle time is to look at an assembly line producing automobiles.

? Cycle time: If production is occurring continuously, the cycle time is the period between the completion of one car and the next if the process is working at capacity. A typical automotive plant has a cycle time somewhere around 60 seconds.

? Flow time: The flow time is the amount of time that anyone car spends on the assembly line from start to finish. The flow time for a car is around 8 to 16 hours. This is the sum of each operation’s cycle time and any time spent waiting in queues along the way (delays).

The rush order flow time represents the time it takes for one flow unit (a car, a customer, a dozen cookies) to go through the system from beginning to end without any waiting. It’s simply the flow time minus the wait, and you can often calculate it as the sum of all the operation cycle times. However, if some of the operations can occur simultaneously, the picture is more complex. The process cycle efficiency is the ratio of rush order flow time to the flow time. Process cycle efficiency is always less than or equal to 1; this metric reveals how much of the flow time is wasted in waiting