A valve spring that lost 3 mm of free length after two weeks. The valve was a pneumatic spool valve cycling 12 times a minute. The spring was a standard music-wire compression spring, 2 mm wire, 20 mm free length, designed for 2 million cycles per the catalog. It sagged at 10,000 cycles. The customer thought the spring was counterfeit. It was the right spring — run at the wrong stress.
Spring fatigue life isn’t a fixed number. It’s a curve against stress. A music-wire spring designed for 2 million cycles at its catalog load (say 30 N) will fail early if it runs at 45 N. The catalog number assumes the stress stays below the fatigue limit for that wire material. Music wire (ASTM A228) has an endurance limit around 45% of its tensile strength — but only when the surface is clean and the stress is steady.
Why this spring died early
Three things stacked up:
- Over-travel. The valve was assembled with a spacer that compressed the spring an extra 2 mm beyond the designed stroke. That pushed the stress from 700 MPa to 850 MPa — above the endurance limit for music wire of this diameter. The spring was living past its fatigue limit from day one.
- Preset. A properly made spring is preset (compressed solid during manufacturing) to remove residual stress and set the free length. This spring wasn’t preset. The first few thousand cycles did the presetting on the machine — which showed up as sag.
- Surge. At 12 cycles/min the valve is slow, but the spring’s natural frequency (about 20 Hz for this size) was excited by the valve’s rapid opening edge. The spring surged — it bounced internally at its natural frequency, adding dynamic stress on top of the static load.
Fix one: remove the spacer. The spring returned to its designed stroke. The free-length loss stopped after the first few thousand cycles (the residual presetting finished). Fix two: for a permanent solution, we changed to a spring preset at the factory and shot-peened (shot peening adds compressive residual stress to the surface, which raises the fatigue limit by 20-30%). The peened spring cost 30% more and lasted the machine’s life.
How to check if a spring is overloaded
Measure the installed length and the solid length. If the spring is compressed more than 85% of its stroke to solid, the stress is too high. Look at the spring rate — if the free length dropped more than 3% after the first 100 cycles, the spring wasn’t preset and will keep sagging. And listen for surge: a spring that makes a “ting” sound when released is surging. Add a small spacer at the spring end (to change the natural frequency) or increase the wire diameter.
The selection table that works
| Material | Max stress for 1M cycles (% of tensile) | Notes |
|---|---|---|
| Music wire (A228) | 45% | Cheap, standard, needs presetting |
| Oil-tempered (A229) | 40% | Good for dynamic loads |
| Chrome-silicon (A401) | 50% | High temp, high fatigue |
| Stainless 302 | 35% | Corrosion resistance, lower fatigue |
Size the spring so its maximum working stress stays under 40% of tensile strength for a 1-million-cycle life. If the application runs over 10 million cycles, drop to 30%. These margins look conservative until you remember that catalog cycle ratings are measured on perfect test springs, not on springs assembled against spacers in a vibrating valve.
A spring that sags early is almost always running past its fatigue limit, not a counterfeit. Check the installed stroke against the design, buy preset springs, and consider shot peening for dynamic loads. The catalog’s 2-million-cycle rating assumes the spring runs at the catalog stress. It wasn’t.
