At Murphy & Read, we understand that a spring is often the heartbeat of a mechanical system. Extending the life of a spring—especially in high-cycle or high-stress applications—requires a combination of precise design mathematics and specialized secondary processes. This guide outlines the core practices used to maximize fatigue life and prevent premature failure.
1. Designing for the Right Stresses
The most fundamental factor in spring life is the operating stress level relative to the material’s elastic limit. A spring designed to operate near its maximum theoretical stress will have a significantly shorter life than one designed at a lower stress level.
- Safe Stress Ranges: For the greatest number of cycles, the maximum stress should generally not exceed 35-45%, of the minimum tensile strength of the material. The maximum % of MTS is dependent on the material.
- Material Selection: High-tensile materials like Music Wire or Chrome Silicon are preferred for high-stress applications due to their superior fatigue resistance.
2. Shot Peening
Shot peening is a cold-working process used to produce a compressive residual stress layer and modify the mechanical properties of the metal. It involves impacting the surface of the spring with round metallic, glass, or ceramic particles (shot) at high velocities.
| Benefit | Description |
|---|---|
| Inhibits Cracks | Compressive stress prevents the initiation of fatigue cracks on the surface. |
| Increases Fatigue Life | Can increase the fatigue life of a spring by 200% to 500% in many applications. |
| Surface Cleanup | Removes minor surface imperfections and scale from heat treatment. |
3. Setting Operations (Removing Set)
When a spring is compressed to a point where the stresses exceed the elastic limit of the material, it will not return to its original length—this is called “taking a set.” At Murphy & Read, we can perform a “Preset” or “Remove Set” operation during manufacturing.
By intentionally compressing the spring to its solid height during the production process, we introduce beneficial residual stresses. This makes the spring more stable in the field and prevents further length loss during actual service.
4. Cycle Frequency and Fatigue
The rate at which a spring is compressed and released (Cycle Frequency) directly affects its internal temperature and fatigue limit. High-frequency cycling can cause heat buildup due to internal friction within the molecular structure of the steel, leading to a reduction in tensile strength and faster failure.
5. Resonance Frequency (Spring Surge)
Every spring has a natural frequency. If the operating frequency of the system matches the spring’s natural frequency, “resonance” occurs. This is also known as Spring Surge.
- The Danger: During resonance, the coils of the spring can vibrate independently, creating stress waves that travel through the wire. This can cause the spring to experience stresses much higher than calculated for the static load.
- The Solution: Engineers should design the spring’s natural frequency to be significantly higher than the operating frequency to avoid destructive harmonic interference.
By combining these practices—proper stress calculation, shot peening, setting, and frequency analysis, Murphy & Read ensures your custom springs provide the longest possible service life in even the most demanding environments. Please contact us if you have any questions, or submit your RFQ and we will be in touch soon.