The Line Where One Operator Was Always Behind
A manual assembly line had five workstations. The tasks were divided by part (Station 1: left side, Station 2: right side, and so on). After two weeks, Station 3 was always behind — the operator there had the most work. The line’s output was set by Station 3; the other four operators stood around waiting. The supervisor added a sixth person to Station 3. Output went up, but the new person was idle 40% of the time. The problem was not understaffing — it was that the tasks were not balanced. The work was split by part, not by time. Station 3 had 90 seconds of tasks; the others had 45. We re-balanced: moved a 20-second task from Station 3 to Station 2 (which had 45 s of slack). Now every station took 65 seconds. The line output rose by 30%, and the sixth operator was moved to another line. No new hire — just a re-division of the work.
Line balancing is the discipline of dividing work so every station takes the same time. It is the cheapest output improvement in a manual line. Here is how to do it.
The Bottleneck Is the Slowest Station
On a manual line, throughput is set by the slowest station — the one that takes the longest. The other operators wait for it. Adding a person to the slowest station helps only if that person actually reduces the time; otherwise, you have two people standing around at Station 3 while the others are idle. The goal is to make every station take the same time — the cycle time of the line.
The station that was always behind (90 s) was the bottleneck. The line ran at 90 s per unit. The other stations had 45 s of work — they were idle half the time. Re-balancing brought Station 3 down to 65 s and raised Station 2 to 65 s. The line now runs at 65 s per unit — a 38% output increase.
The line balancing rule: List every task and its time. Assign tasks to stations so each station’s total time is as close as possible to the line cycle time. The slowest station sets the output. Move tasks off it until all stations are equal.
How to Balance: The Steps
- List every task: Write down each assembly step (pick part, fasten, check, place).
- Time each task: Measure (stopwatch) each task over 20 cycles; use the average.
- Set the target cycle time: The required output (e.g., 60 units/hour = 60 s per unit).
- Assign tasks to stations: Group tasks so each station totals near the cycle time. Respect task precedence (you cannot fasten before you pick).
- Identify the overloaded station: The one with the most time. Move a task to a station with slack.
- Repeat until all stations are balanced (within 10% of each other).
Precedence: You Cannot Reorder Everything
Some tasks must happen in order (you cannot tighten the bolt before you place the part). The precedence constraints limit how much you can rebalance. If a task is “after pick, before tighten,” it must sit in the middle. The goal is to balance within those constraints — not to create an arbitrary order. Draw a precedence diagram if the order is complex.
The Idle Time Tax
When a station finishes early, the operator waits. That idle time is lost capacity. A station that takes 45 s on a 65 s cycle is idle 20 s per unit — 30% of the shift. Over a 480-minute shift, that is 144 minutes of waiting. Balancing those 20 s across the line is a 30% output increase. The line that added a sixth person did not need a person — it needed to give that 20 s to Station 3.
A Balancing Worksheet
| Task | Time (s) | Station (before) | Station (after) |
|---|---|---|---|
| Pick left part | 10 | 1 | 1 |
| Fasten left | 20 | 1 | 1 |
| Pick right part | 15 | 2 | 2 |
| Fasten right | 30 | 2 | 2 |
| Install bracket | 25 | 3 | 3 |
| Tight bracket | 40 | 3 | 3 |
| Quality check | 25 | 3 | 4 |
| Pack | 20 | 4 | 4 |
| Station total | 45 / 45 / 90 / 20 | 30 / 45 / 65 / 45 |
Before: Station 3 at 90 s, others idle. After: moved quality check (25 s) from Station 3 to Station 4. Now 30/45/65/45 — closer to balanced. The line cycle drops from 90 s to 65 s.
A Balancing Checklist
- List every task with its measured time.
- Draw the precedence order (what must happen before what).
- Set the target cycle time (required output).
- Assign tasks to stations to hit the cycle time.
- Find the overloaded station (the bottleneck).
- Move a task with slack to the overloaded station.
- Repeat until all stations are within 10% of each other.
- Verify the new assignment with a timed trial run.
- Watch for new bottlenecks (the next slowest station).
- Rebalance after any product or task change.
The Bottom Line
The line where one operator was always behind was a work division by part, not by time. List every task, time it, and reassign so every station takes the same. The 30% output increase came from moving one 25-second task — not from hiring. The line that runs smoothly is the one where nobody is waiting, and nobody is overloaded.
