An assembly line runs at the speed of its slowest station. How to calculate takt time, measure work content, rebalance stations and decide the right number of people, with the pitfalls that undo good balancing.
An eight-station sub-assembly line had a target of 480 units per shift and rarely made 400. Every manager had a theory: absenteeism, parts shortages, the new operators. When the team timed every element of work at every station, the answer was plainer. One station carried 58 seconds of work against a takt time of 52 seconds, while three others carried less than 35. The line could never beat its slowest station, and the operators at the light stations spent a third of their time waiting.
Line balancing is the discipline of matching the work at each station to the rate customers need.
Start with takt time
Takt time is the rate of customer demand:
takt time = available working time ÷ customer demand
If a shift has 7.5 hours of available time (27,000 seconds) and the customer needs 480 units, takt time is 56.25 seconds. In practice, planners usually design to a slightly faster planned cycle time to allow for losses, for example 52 seconds.
Takt time is not how fast the line can run. It is how fast it needs to run.
Measure the real work content
- Break the job into elements: small, repeatable steps such as "pick housing, place in fixture".
- Time each element over several cycles with several operators, separating work from waiting and walking.
- Note which elements must happen in a set order (precedence) and which can move between stations.
- Separate value-adding work from waste, such as reaching, searching and re-orienting, using the eight wastes as a guide.
Build a balance chart
A stacked bar chart, one bar per station showing the time of each work element, with a line at takt time, shows the problem instantly. Stations over takt are bottlenecks. Stations well under takt hide waiting time.
Rebalancing
- Remove waste first, especially at the bottleneck station.
- Move elements from overloaded to underloaded stations, respecting precedence.
- Combine stations where total work allows.
- The theoretical minimum number of operators is total work content divided by planned cycle time, rounded up. If total work is 340 seconds and the planned cycle time is 52 seconds, you need at least seven people, not eight.
- Where work cannot be divided evenly, consider U-shaped cells where operators can cover more than one station, or a flexible operator who helps where needed.
Pitfalls
- Balancing to averages. If a station's time varies widely, it will still block the line. Reduce the variation, not just the average.
- Ignoring walking and handling between stations.
- Rebalancing on paper without trying it on the line.
- Forgetting that demand changes. A line balanced for 480 units a shift needs a different balance at 360 or 600. Prepare standard balances for several demand levels.
Make it stick
Document the new balance as standard work: the sequence, time and standard work-in-progress at each station. Display takt time and actual output by hour at the line, so everyone can see whether the line is on pace. Review the balance whenever demand, product mix or the process changes.
A well-balanced line does not make people work harder. It removes the waiting and overload that make work frustrating, and it lets the line deliver what the customer needs, every hour.
