A sewing line is only as fast as its slowest station. Line balancing is the work of dividing a garment's operations among operators so that each station has roughly the same amount of work, matched to the output the line needs. A well balanced line has steady flow, low WIP and few people waiting. A poorly balanced line has piles of bundles in front of some operators and others with nothing to do.
What you need before you start#
- Operation breakdown: every sewing and manual operation in sequence, with its SMV and machine type. See SMV calculation.
- Garment SMV: the total of all operation SMVs.
- Target output: pieces per hour or per day.
- Available manpower and the machines on the line.
- Operator skills: who can do which operation and how well. A skill matrix makes this quick.
Step 1: Calculate pitch time#
Pitch time is the average work content each operator should carry:
If you start from a required output instead of a fixed number of people, use the takt time (available time divided by required output) as the upper limit for each station.
Step 2: Theoretical manpower per operation#
For each operation:
An operation with SMV 1.20 and a pitch time of 0.60 needs 2.0 operators. One with SMV 0.25 needs 0.42 operators, which means it should be combined with another short operation.
Step 3: Allocate operators#
Round the theoretical values into whole people by:
- Combining short operations on the same machine type, in sequence where possible.
- Splitting long operations between two or more operators.
- Sharing operators between two operations on one machine or nearby machines.
- Matching skill: give the operations with the highest work content to the fastest, most consistent operators.
Keep machine type in mind. An operator cannot share work between a lockstitch and an overlock machine unless both are at the station.
Step 4: Check the station loads#
For each station, add the SMV of the operations allocated, divided by the number of operators at the station. That is the station cycle time. The highest station cycle time is the line's real pace: the bottleneck.
Step 5: Calculate balancing efficiency#
Count each operator as a station here, with the station time being that operator's work load.
A higher number means the work is spread more evenly. Factories often aim for 85% or better on the plan, then improve it on the floor.
Worked example#
A simple pull-on skirt has these operations (SMV in minutes):
| # | Operation | Machine | SMV |
|---|---|---|---|
| 1 | Overlock side seams | Overlock | 0.80 |
| 2 | Join waistband ends | Lockstitch | 0.30 |
| 3 | Attach waistband with elastic | Overlock | 1.10 |
| 4 | Topstitch waistband | Coverstitch | 0.70 |
| 5 | Hem | Coverstitch | 0.60 |
| 6 | Attach label | Lockstitch | 0.30 |
| 7 | Trim and check | Manual | 0.40 |
| Total | 4.20 |
With 7 operators: pitch time = 4.20 / 7 = 0.60 min.
| Operation | SMV | Theoretical operators |
|---|---|---|
| 1 Side seams | 0.80 | 1.33 |
| 2 + 6 Lockstitch work | 0.60 | 1.00 |
| 3 Waistband attach | 1.10 | 1.83 |
| 4 Topstitch waistband | 0.70 | 1.17 |
| 5 Hem | 0.60 | 1.00 |
| 7 Trim and check | 0.40 | 0.67 |
First allocation, one operation group per operator where possible:
| Operator | Work | Load (min) |
|---|---|---|
| A (overlock) | Side seams | 0.80 |
| B, C (overlock) | Waistband attach, shared | 0.55 each |
| D (lockstitch) | Waistband ends and label | 0.60 |
| E (coverstitch) | Topstitch | 0.70 |
| F (coverstitch) | Hem | 0.60 |
| G (manual) | Trim and check | 0.40 |
Operator A is the bottleneck at 0.80 minutes. Treating each operator as a station:
Balancing efficiency = 4.20 / (7 × 0.80) × 100 = 75%
Rebalanced: the three overlock operators share side seams and waistband attach (0.80 + 1.10 = 1.90 minutes, or 0.63 each), and the two coverstitch operators share topstitch and hem (0.70 + 0.60 = 1.30 minutes, or 0.65 each).
| Operator | Load (min) |
|---|---|
| A, B, C (overlock) | 0.63 each |
| D (lockstitch) | 0.60 |
| E, F (coverstitch) | 0.65 each |
| G (manual) | 0.40 |
The bottleneck drops to 0.65 minutes. Balancing efficiency = 4.20 / (7 × 0.65) × 100 = 92.3%, and the line's pace at standard rises from 75 to about 92 pieces per hour (60 / 0.65). Operator G still has spare time and can take bundle handling or thread trimming off the machine operators.
The pitch time calculator and the line balancing worksheet do this arithmetic for you.
Balancing on the floor#
A balance on paper is a starting point. After the line starts:
- Watch the WIP. Bundles piling up in front of a station mean that station is slower than planned.
- Time the bottleneck with a short time study. The real cycle time is often different from the SMV plan.
- Rebalance by moving small elements, adding a helper, or improving the method at the slow station.
- Check again after each change. Moving work can create a new bottleneck somewhere else.
See bottleneck analysis for a structured way to find and fix the constraint.
Common mistakes#
- Balancing with SMVs only and ignoring the actual performance of the operators assigned.
- Ignoring machine type, so an operator is given operations on machines that are not at their station.
- Balancing once for the whole style, even though the line's performance changes during the learning curve.
- Treating helpers as free capacity. Helpers add to manpower and must be counted in efficiency.
- Hiding imbalance with WIP. A big buffer keeps everyone busy but adds lead time and hides quality problems. See WIP control.
Checklist#
- Operation breakdown complete with SMV and machine type
- Pitch time or takt time calculated
- Theoretical manpower per operation calculated
- Allocation matches machine availability and operator skills
- Bottleneck identified and balancing efficiency calculated
- Floor check after the first hours of production