Understanding Torsion Spring Fatigue Life
Torsion springs work by twisting wire around a coil axis. Each cycle changes torque and bending stress. Fatigue life estimates how many cycles the spring may survive before cracking. This calculator turns geometry, torque, material strength, and correction factors into a practical design check.
Why Fatigue Matters
A torsion spring can look safe under one static load, yet fail after many repeated moves. Door hinges, clips, return levers, and latch systems often move thousands or millions of times. Small stress changes can greatly change life. That is why alternating stress and mean stress are reviewed together.
Important Inputs
Wire diameter controls stress strongly because it is cubed in the stress formula. Mean coil diameter sets the spring index. A poor index raises local stress. Torque range defines the fatigue cycle. Material strength sets the upper stress limit. Surface, size, reliability, and temperature factors reduce endurance strength for real conditions.
Design Interpretation
The fatigue safety factor is not a guarantee. It is a screening value. A factor above one means the selected fatigue criterion is satisfied. A value below one means the cycle is too severe. The predicted cycles use a simplified S-N curve between low cycle and endurance regions. Laboratory data should replace estimates for critical parts.
Improving Life
Increase wire diameter when space allows. Reduce maximum torque or preload. Use smoother forming and careful shot peening when available. Avoid sharp hooks and scratches near bends. Select a material with better tensile strength and clean surface quality. Keep spring index within a practical range. Reduce heat exposure if temperature weakens the material.
Using Results Responsibly
Treat the output as an engineering estimate, not certification. Actual life depends on manufacturing quality, residual stress, corrosion, lubrication, shot peening, duty speed, and test environment. Prototype testing is recommended for safety related assemblies. Recheck the design whenever torque, wire size, material, or environment changes. Save the CSV and PDF records for design reviews.
Common Limits
Watch for coil clash, leg interference, and permanent set. Fatigue calculations do not replace checks for yield, angular travel, or installation clearance. When stress is near the limit, choose conservative factors and test several samples under real duty conditions before release for production approval.