op residual tensile stresses in the surface area of parts. These residual tensile stresses make parts susceptible to premature fracture and failure when repeatedly stressed. High-energy mass finishing processes can be implemented to modify this surface stress condition and replace it with uniform residual compressive stresses.
Many manufacturers have discovered that as mass finish- ing processes have been adopted, put into service, and the parts involved have developed a working track record, an unanticipated development has taken place—their parts are better. In the case of automotive valve springs (Fig 5.): they last longer in service, are less prone to metal fatigue failure and, from a quality assurance perspective, are much more predictably consistent and uniform.
Application: Springs of any type have a predictable life based on the material, shape, movement range, load and interference/resonance with a mating spring in multiple spring applications. High-performance automotive valve springs are some of the most stressed movement control applications. The spring is con- trolling the vertical movement of a mass (the valve) over a distance (over 0.5" or 12.7 mm) at very high frequencies (5000–10,000 openings a minute). The valve spring is typically a drawn wire wound into the coil shape and “shot peened” for “stress relief”.
Developing an overall edge and surface finish continuity and equilibrium can have a significant effect on
performance and service life of critical components.
The peening process consists of steel balls pounded into the spring’s surface during an aggressive tumbling or wheel blast action. This does impart some compressive stress but in a macro form. Even with this level of compres- sive residual stress the spring will fracture unpredictably in many high-
Figure 5
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