Lean tools

Yamazumi chart

In French: Yamazumi (diagramme d’équilibrage)

The Yamazumi chart stacks the work content of each station into bars and compares it with the takt time, to see at a glance who is overloaded and who has spare capacity.

What it is

Yamazumi means "stacking" in Japanese. The chart has one station per column; each column stacks the tasks that make it up, often split into value-added and non-value-added work. A horizontal line marks the takt time.

The reading is immediate: a bar above the line signals a station that cannot keep up with demand — it is the bottleneck, and the whole line produces at its pace. A bar clearly below signals spare capacity, hence waiting or, worse, overproduction if the station keeps on producing.

The point of the Yamazumi is not to note the imbalance but to reallocate it: tasks are moved from one station to another, non-value-added work is removed from the loaded station, the takt is not touched. The takt is a customer datum, not an adjustment variable.

Rebalancing without levelling blindly

The natural reflex, faced with unequal bars, is to spread evenly: everyone at ninety per cent of the takt, nobody overloaded. That is the worst of outcomes. The available time is then spread in thin slices over all the stations, where it serves no purpose and becomes diffuse waiting, impossible to recover.

Good practice is the opposite: load the stations as close as possible to the takt and concentrate the freed time on a single one. That one becomes visibly under-loaded, and its content can be redistributed or the headcount redeployed. That is what turns a balancing exercise into a real gain rather than a prettier chart.

Two precautions go with the exercise. You move work content, not people — the task changes station, the operator is not moved by surprise. And you check that the moved task can be done at the receiving station: tooling, access, skill. A balance that is perfect on paper but assumes a tool absent from that station will not last a day.

The two calculations that go with the chart

Minimum number of operators = Σ cycle times / takt time

Line efficiency = Σ cycle times / (number of operators × takt time)

The minimum number is a theoretical floor: it is rounded up to the next whole number and allows for no disruption. An efficiency of 100 % would assume every station exactly at takt, which no real line achieves — the aim is to get close to it, not to display it.

The common mistake

Timing once, one operator, one cycle. The chart obtained is a snapshot: it captures the variability of the gesture as much as the real content of the station, and the balancing derived from it comes undone the following week. Several cycles are needed and, if possible, several operators — and if the gap between them is large, the problem is not the balancing but the work standard.

On the shop floor — Electronics manufacturing services

On a six-station line, the Yamazumi showed that the bottleneck was not the station reputed to be the hardest but the one before it, loaded with two checks added over the years and never removed. Neither of them was in the standard.

The other tools illustrated by this trade: electronics and aerospace.

Frequently asked questions

What is a Yamazumi chart?

It is a stacked bar chart that shows, station by station, the working time needed for the tasks assigned to it. A horizontal line on it marks the pace imposed by demand. Comparing each bar with that line shows in a second which stations cannot keep up and which have margin.

How is a Yamazumi chart built?

Each station is broken down into elementary tasks, the duration of each is measured over several repetitions, then those durations are stacked into one column per station before the pace line is drawn. The number of repetitions decides the reliability of the result: a duration recorded only once reflects the day’s randomness as much as the real content of the station.

What is the difference between a Yamazumi and a VSM?

Both look at a flow, at two different scales. Value stream mapping follows a product across all the steps and reveals the dead time between them. The balancing chart focuses on a single line and details the split of work between its stations. The first is used to choose where to act, the second to act.