Torsion Spring Torque Relaxation - Grade Choice Above 150C
torsion spring torque relaxation
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Torsion Spring Torque Relaxation: Choosing the Grade Above 150 C
A torsion spring that works perfectly at room temperature can lose a fifth of its torque after a few months on a hot assembly. This is not fatigue and not corrosion. It is stress relaxation: under a sustained load at elevated temperature, the material slowly converts elastic strain into permanent deformation, and the torque that holds the part in place quietly disappears.
This article explains where relaxation becomes significant in stainless torsion springs, how grade choice changes the rate, and how to size the spring so the torque is still there at end of life.
Relaxation Is Time at Temperature, Not Load Cycles
Fatigue is driven by cycles. Relaxation is driven by time and temperature under a held deflection. A spring sitting compressed on a shelf does not relax; a spring held at working angle at 180 C for 2,000 hours does. For appliance and automotive parts that stay deflected for years, relaxation is usually the limiting design case, not cycle life.
The rate depends on three variables: absolute temperature, initial stress level as a fraction of yield strength, and time. Doubling the absolute temperature can increase relaxation rate by an order of magnitude, which is why the practical threshold for cold drawn stainless springs sits around 150 C.
How Much Torque Is Lost
Order of magnitude for cold drawn 302 and 304 at moderate stress, held for 1,000 hours:
- Up to 100 C: roughly 3-5% torque loss, often inside normal measurement scatter
- 150 C: roughly 6-10%
- 200 C: roughly 10-15%
- 250-300 C: roughly 15-25%, and geometry may start to creep
Above 300 C, cold drawn austenitic wire is no longer the right choice for a spring that must hold torque. Either the design moves to a heat resisting grade, or the spring is replaced by a different mechanism.
Three Levers That Reduce Relaxation
1. Lower the initial stress
The single most effective lever. A spring designed at 40% of yield relaxes far less than the same spring at 70%. If the assembly has room for one more coil or 0.1 mm more wire diameter, use it: the extra material buys torque stability rather than peak torque.
2. Stabilising treatment after coiling
A stress relief at 200-300 C removes the coiling stress, and a stabilising treatment at higher temperature under pre-set deflection goes further by relaxing the spring before it enters service. The spring is preset to a slightly higher torque, treated, and then delivers a stable torque from the first cycle.
3. Grade selection
302 and 304 behave similarly up to about 250 C. 316 adds molybdenum for corrosion resistance in hot humid service but is not stronger at temperature. 430 resists scaling to higher temperature but has lower elevated temperature strength, so it suits non-critical positioning springs rather than load holding. Cold drawn wire retains more of its cold work than annealed wire and therefore relaxes more slowly, but it also loses that advantage if the service temperature approaches the annealing range.
Specification Window
| Parameter | Range |
|---|---|
| Wire diameter | 0.15 - 12 mm |
| Grades | 201, 302, 304, 316, 430 |
| Condition | Cold drawn, or annealed and re-drawn |
| Recommended continuous service | Up to 150 C for load holding springs |
| Short term excursion | Up to 250 C with reduced torque guarantee |
| Heat treatment | Stress relief 200-300 C, stabilising on request |
| Torque verification | Measured after treatment at declared working angle |
Applications Where This Matters
- Oven door hinges and damper springs
- Cooker and grill latch mechanisms
- Automotive interior and under-hood clips
- Industrial damper and flap return springs
- Heater and HVAC actuator springs
How to Specify a Hot Service Spring
State the continuous service temperature, the peak temperature and how long the peak lasts, the working deflection angle, the torque required at end of life rather than at delivery, and the expected service hours. With those five figures the design can be checked against relaxation instead of being validated by trial and error in the field.
TOPONE has drawn stainless steel wire since 1993 and supplies coiled torsion springs from 0.15 mm to 12 mm, so grade, wire condition and spring heat treatment are specified together and the torque guarantee holds at the temperature the part actually sees.
