This project was the redesign of the torsion springs in a seat recliner mechanism for an automotive customer. The aim was simple to state: make the backrest easier and smoother to adjust for every passenger, and keep it that way for the life of the seat.
Requirements
- Temperature: the springs had to work from -30°C to 75°C, the range a car interior can see between a winter night and a summer day in the sun.
- Chemical exposure: contact with cleaning agents and automotive fluids.
- Torque tolerance: ±2% on torque values, so that every seat feels the same.
- Ease of use: the torque had to allow easy adjustment by occupants of different physical strength.
- Consistency: the same performance through the whole adjustment range of the seat.
Our Solution
Material
We chose AISI 316 stainless steel. Type 316 is not the strongest of the stainless spring wires, so the spring has to be sized with that in mind, but it has the best resistance to cleaning chemicals and fluids among the common grades. Its fatigue behavior also suits the repeated motion of seat adjustment.
Design
A recliner spring does a balancing job. It has to be strong enough to bring the backrest forward when the lever is released, but light enough that a smaller passenger can push the seat back without effort. We matched the torque curve to that balance and shaped the coils so that stress is spread evenly along the spring, with no single point taking more than its share. The leg ends were formed to locate positively in the recliner, so the load is applied the same way on every cycle.
Surface Treatment
The springs were passivated. Passivation removes the free iron left on the surface by forming tools and restores the protective layer of the stainless steel, which improves its corrosion resistance further.
Results
- The recliner mechanism operated noticeably more smoothly, and adjustment took less effort.
- Feedback from test users and from the customer was positive.
- In longevity testing, the springs kept their performance after 100,000 cycles.
What This Project Shows
Comfort features depend on small parts behaving the same way every time. Getting there took the right stainless grade for the environment, a torque curve matched to how people actually use the seat, and a tolerance tight enough that one seat feels like the next.
If you are developing a seat or interior mechanism that needs a torsion spring, send us your requirements through the contact page, or see our custom torsion springs.
