A car seat is awkward: 15–25 kg, bulky, and covered in material that must not be marked. At a 45-second takt, the operator has time to lift, locate, torque and inspect, but not to fight the fixture. The fixture and the lifting aid decide whether the line reaches rate.
Key takeaways
- Balance the line around the heaviest and longest operations, not the average.
- Use a lifting aid or a tilt fixture to remove the weight from the operator.
- Torque and trim operations should be separated if they compete for the same hands.
- Mixed-model sequencing must be visible to every station.
The operations and their constraints
A typical seat line includes frame build, foam and cover, trim, electrical harness, switch pack, test and final inspection. Frame build and trim are the long operations; electrical and switch pack are short but delicate. Test may be automatic or manual.
At 45 seconds, a station can hold about 38 seconds of work at 85% loading. That is enough for frame build or trim, but not both. The line usually needs 12–18 stations depending on the seat complexity.
Fixture and lifting
The seat should be presented at a height and angle that lets the operator work without bending. A tilt fixture that rotates the seat from horizontal to vertical allows access to the underside without lifting. A lifting aid should take the weight during transfer from the conveyor to the fixture.
The fixture should locate the seat by the frame, not the upholstery. Locating on the upholstery compresses the foam and leaves marks. Use soft pads on the fixture surfaces that contact the trim.
| Station group | Typical content | Handling |
|---|---|---|
| Frame build | 32–38 s | Fixed fixture, torque tools |
| Foam and cover | 34–40 s | Tilt fixture, soft pads |
| Trim and clips | 30–36 s | Rotating fixture, part bins |
| Electrical | 25–32 s | Harness board, test point |
| Test | 40–45 s | Automatic or manual |
| Inspection | 20–30 s | Lighting, checklist |
Torque and traceability
Seat frames have several critical fasteners. Each torque tool should be programmed for the joint and should send the result to the line controller. The controller should interlock the next station if a torque is out of limit.
The tool should be mounted on a balancer or a fixture arm so that the operator does not carry its weight. A heavy torque tool used 400 times per shift causes fatigue that shows up as missed fasteners.
Mixed-model sequencing
A seat line often builds several models in sequence. The sequence must be visible at every station, usually on a display or a travelling tag. The station should be able to confirm the model before picking parts, and the line should reject a wrong-model part before it is fitted.
Changeover between models should be designed into the fixture: adjustable width, quick-change nests and a small number of model-specific tools. A changeover that takes more than one cycle is a changeover that will be done while the line runs, which creates errors.
Ergonomics and quality
Seat assembly has a high risk of musculoskeletal injury because of the weight and the posture. Provide lifting aids, anti-fatigue mats, adjustable height and job rotation. A line that meets rate but injures operators is not a successful line.
Trim damage is the most common quality defect. Protect the seat with covers during transport, use soft-faced fixtures and keep tools away from the trim. Inspect at the station, not only at the end of the line.
Frequently asked questions
How many stations does a seat line need?
It depends on the seat complexity and the takt. At 45 seconds, a typical line has 12–18 stations, including test and inspection.
Can one operator handle two stations?
Only if the combined content is below 85% of takt and the stations are adjacent. Seat assembly usually has too much handling for two stations per operator.
Should test be manual or automatic?
Automatic test is more repeatable and faster, but it requires a fixture and a test program. Manual test is flexible but depends on the operator.
Send the seat drawing, takt and model mix to our engineers through the contact page; we will return a station balance and a fixture concept.
