Yamazumi and Line Balancing: Distributing Work Across Operators

A flow cell stands or falls with how the work is distributed. The yamazumi chart (Japanese for “stacked up”) shows the work elements of each operator as a stack and places the planned cycle time next to it. You see at a glance how many operators you need, who is overloaded and where waste sits in the stack.

What the yamazumi chart shows

Each bar stands for one operator. It is made up of the work elements that operator performs in one cycle: loading, welding, assembling, inspecting, and walking too. The height of the bar is the operator’s work content in seconds. A horizontal line marks the upper limit that every bar has to stay below.

The operator balance chart of the value stream map compares cycle time with takt time for each process. The yamazumi chart goes one level deeper: it breaks the work content into elements and distributes them across people. It is the tool for designing a cell in the future state.

Calculating the number of operators

The basis is the total work content of the cell and the planned cycle time. The planned cycle time is slightly below takt time so that downtime and small fluctuations do not break takt.

Why balance to planned cycle time and not to takt time

If you load every operator up to exactly takt time, you plan without a reserve. Every breakdown, every jammed part, every question then costs output that cannot be made up. The planned cycle time anticipates the expected losses: at 95% uptime, only 57 s of the 60 s are available on average.

Calculating against takt time in the example gives 147 s ÷ 60 s = 2.45, also 3 operators. The difference shows when you balance: a bar at 58 s would be below takt time but above the planned cycle time, and would become the bottleneck with normal downtime.

Capturing work elements: value, necessary, waste

A work element is the smallest step that can sensibly be transferred to another operator, timed with a stopwatch over several cycles. Assign each element to one of three groups. The simplest way is through the name, for example “VA: assemble”, “NVA-N: clamp”, “W: walk”.

• Value-added (VA): changes the part in a way the customer pays for, such as welding or assembling.
• Necessary non-value-added (NVA-N): unavoidable today, such as loading, clamping, unloading, inspecting.
• Waste (W): walking, searching, waiting for the machine, rework. It still belongs in the stack, because it uses up time.
• Separate manual work and machine time: only the time during which the operator is tied up goes into the bar. If a machine runs on its own, the operator works elsewhere in parallel; the machine itself has to stay below the planned cycle time with its own cycle time.

Balancing step by step

Distribute the elements in process sequence so that every bar sits just below the planned cycle time. The process boundaries from the value stream map no longer apply: one operator can take the end of welding 2 and the start of assembly.

• List the elements in process sequence, with time and group (VA, NVA-N, W).
• Fill the first operator with elements until the next element would exceed the planned cycle time.
• Continue with the next operator until all elements are assigned.
• Remove waste from the stack, above all walking and waiting, and redistribute.
• Check the balance rate and try out the result at the workstation.

Common mistakes

• Balancing to takt time instead of planned cycle time: the cell has no reserve and misses the volume at the first breakdown.
• Forgetting walking: in the example, with 8 s of walking per cycle, Op 1 and Op 2 would reach 57 and 58 s and be above the planned cycle time. Walking belongs in the stack as its own element.
• Distributing by process boundaries instead of by elements: one operator per station rarely gives bars of equal height.
• Rounding up generously: if the calculation gives 2.1 operators, first cut waste by the missing seconds rather than planning a third operator who waits a third of the time.
• A single measurement instead of several cycles: element times vary; take a representative value from several measurements and note outliers separately.
• Putting machine time into the operator’s bar: the operator then looks fully loaded while actually just waiting.

Yamazumi in the editor

In Pro mode, enter the work steps of a process with duration and operator number under “Work elements”. The “Operator balance” tab then shows, per process, one stacked bar per operator with the takt time line, the total work content, the number of operators needed and the balance rate. Bars above takt time are labeled in red.

The tool calculates “needed” against takt time (rounded up). Also check for yourself that every bar stays below your planned cycle time. For an overview across all processes, use the “Takt balance” tab with cycle time and effective cycle time per process.

Frequently asked questions

Keep learning