A luxury car manufacturer came to us with a reliability problem on its convertible models. The torsion springs in the convertible top mechanism were not delivering consistent torque, the top no longer moved smoothly, and owners were complaining. The reported failure rate was 18% within the first year of use.
The Problem
The original springs lasted around 20,000 open-and-close cycles before their torque started to drop. Once that happens, the top stops moving evenly through its travel. Our goal was to double the life to 40,000 cycles and keep the torque consistent for the whole of it.
Requirements
- Service life: more than 40,000 cycles without significant loss of torque.
- Motion: uniform, smooth movement of the top from fully open to fully closed.
- Environment: high summer temperatures, rain, and wet, corrosive conditions on an exterior part.
- Space: the spring had to fit the compact space inside the existing mechanism, with dimensions held within ±0.3 mm.
Our Solution
Material
We chose 17-7 PH stainless steel. It is a precipitation-hardening grade: the spring is coiled first and then heat treated, which brings it to a strength well above ordinary stainless spring wire. It also resists fatigue and corrosion, which is what an exterior automotive part that moves thousands of times needs.
Design
Early torque loss in a torsion spring usually means the working stress is too high for the material, so the spring slowly takes a permanent set. We reworked the wire size, the body diameter and the number of coils to bring the stress at full travel down to a level the material can sustain for the required life, while still fitting the space available. The coils are spaced so they do not bind against each other, which keeps the movement smooth across the whole range.
Surface Treatment
The springs were passivated to restore the corrosion resistance of the stainless surface after forming, and then given a clear coat for extra protection and a clean appearance.
Results
- The first-year failure rate fell from 18% to under 5%.
- The customer reported that the top operated more smoothly and consistently.
What This Project Shows
When a spring loses force early, a stronger material alone rarely solves it. The fix here came from two changes made together: a higher-strength stainless grade, and a geometry that lowers the stress the spring sees on every cycle.
Working on a mechanism where spring life is the weak point? Send us your drawing through the contact page, or see what we can do with custom torsion springs.
