Calculating Cycle Time and Takt Time
It’s a fact that people get more work done if there are fewer interruptions. In a production environment, the same principle applies. A production process that’s free from interruptions, delays, and backlogs — from beginning to end — is said to have a good “flow.”
The goal is to create continuous flow — ensuring that everyone receives the right work, in the correct quantity, at the right time.
The Lean method for achieving continuous flow is known as line balancing. In a balanced line, work is distributed evenly across the line. This means each employee completes the same amount of work in roughly the same amount of time, which establishes a continuous workflow.
Line balancing is a process consisting of several steps.
You start the line balancing process by calculating cycle time. Cycle time is the time it takes an operator to complete a task and move a product or component to the next operator in line. When calculating cycle time, explain to operators what you’ll be doing and why, then observe and time their actions.
One way you can capture this data is with a cycle time worksheet. You enter operator names in one column, task descriptions in another column, and sample times in a third column. Consider capturing video footage to record what each operator does and how long it takes them.
From the sample times you observe, you can extract the key data you’ll use next.
You use the most frequent time as a baseline from which to improve the time to low repeatable, if possible. The most frequent time is also used to calculate the total cycle time for the tasks in the process.
You use the high time to isolate potentially recurring problems in the process.
You use the low repeatable times to set realistic target times for each task.
Next, you calculate the total cycle time by adding up the most frequent time values for each task.
Once you’ve calculated the cycle time, you’re ready to calculate the takt time — a measure of how fast operators must work to meet customer demand. To calculate takt time, you divide the net available time per period by the customer demand per period.
Consider an example of a precision tool manufacturing plant. Say each shift at the plant lasts eight hours, or 480 minutes. Customer demand is 120 products per shift. To determine takt time, you divide 480 by 120. This gives you 4 minutes per unit product. In practice, you need to adjust the net operating time to include time operators spend on breaks, meetings, and other activities.
Calculating cycle time and takt time are vital first steps in the line balancing process.
Balancing the Work
As with any process, you’re not done until you’ve completed all the steps! The first two steps of the line balancing process, which is used to ensure continuous flow, involves calculating the cycle time and takt time. Once you’ve calculated both, a few more steps remain.
First, you need to create an operator balance chart. This chart is typically a bar graph that depicts the current state of work.
When you create the chart, ensure you represent the cycle times on the vertical axis. You use the horizontal axis to represent the operators — remember to add a line to depict takt time. Each bar in the chart should show an operator’s most frequent time for completing a task.
If all tasks are completed in less time than the takt time, this tells you tasks are unbalanced and overproduction is likely an issue.
Consider an example consisting of three operators — operator A, B, and C. Operator A has a cycle time of 3 minutes, operator B has a cycle time of 2 minutes, and operator C has a cycle time of 2.6 minutes. The takt time is 4 minutes, and there’s a total cycle time of 7.6 minutes.
To create the operator balance chart, you need to determine the ideal number of operators. The formula for this is: the total cycle time divided by takt time.
In our example, the takt time for manufacturing is 4 minutes and the total cycle time is 7.6 minutes.
The ideal number of operators for the process is calculated as 7.6 minutes divided by 4, which equals 1.9. And because having 0.9 of a person doesn’t make sense, we round that up. So ideally, you only need 2 operators for the process in this instance.
You can then use this information to perform the final step in balancing the work. You use the ideal number of operators, in this case 2, to balance the work according to the two bars. Both operators need to complete their tasks within 4 minutes to meet takt time. So you divide the work as evenly as possible between operators A and B.
It’s not always possible to divide labor in a way that lets you achieve perfect takt time, especially when you’re working with uneven numbers. In that case, it’s best to give the final worker in the process slightly less work than they would need to meet takt time, to allow extra time for moving a product to the next process.
You need to use good judgment when distributing work to ensure you do so in the best possible way — everyone should work as close to takt time as possible. No workers should be overloaded.
To balance a line, you can use cycle time and takt time as basis for determining the optimal number of operators needed per process in order to divide work evenly.