Two-Stage Shot Peening for Coil Spring Fatigue Strength
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Solution Overview
Problem
Conventional shot peening methods for coil springs face challenges such as difficulty in applying shots to the inside of the coil due to compression requirements and temperature control issues in warm peening, leading to suboptimal residual stress distribution and increased costs with expensive alloy components.
Innovation Solution
A two-stage shot peening process is employed, where the first stage uses high kinetic energy shots for deep compressive residual stress and the second stage uses low kinetic energy shots to enhance surface stress, reducing noise, vibration, and power consumption, without the need for coil compression equipment and with controlled temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If stress peening is used to improve fatigue strength, then compressive residual stress can be produced, but equipment for compressing the coil spring is required and shots cannot be easily applied to the inside of the coil spring
Solution Approach 1:
The shot peening process is divided into two distinct stages: first shot peening with high kinetic energy shots for deep compressive residual stress, and second shot peening with low kinetic energy shots for enhanced surface stress. This segmentation allows each stage to target specific depth zones without requiring coil compression equipment
Solution Approach 2:
The patent changes the kinetic energy parameter of shots between two stages. First shot peening uses high kinetic energy shots (steel shots or steel shot and glass bead mixture) to achieve deep compressive residual stress, while second shot peening uses low kinetic energy shots (glass beads) to enhance surface compressive residual stress, eliminating the need for compression equipment
2Strength
If warm peening is used to produce compressive residual stress in deep regions, then stress distribution can be improved, but temperature control is difficult
Solution Approach 1:
The patent replaces the thermal field of warm peening with a mechanical field approach using two-stage shot peening. By controlling shot kinetic energy and material properties instead of temperature, the process achieves deep compressive residual stress without the complexity of temperature control systems
Solution Approach 2:
Instead of changing temperature parameters as in warm peening, the patent changes the kinetic energy parameter of shots. First shot peening uses high kinetic energy to penetrate deep into the spring wire, while second shot peening uses low kinetic energy for surface enhancement, achieving the same goal without temperature control
3Strength
If alloy components are added to spring steel to improve fatigue strength, then fatigue strength can be improved, but the cost of the coil spring increases
Solution Approach 1:
The patent converts the potentially harmful effect of high shot kinetic energy (which could cause surface damage) into a benefit by using it first to create deep compressive residual stress, then using low kinetic energy shots to refine the surface. This approach improves fatigue strength through process control rather than expensive alloying
Solution Approach 2:
The patent uses conventional spring steel without expensive alloy components, relying instead on the two-stage shot peening process to achieve enhanced fatigue strength. The process uses disposable shots (steel shots, glass beads) that are inexpensive compared to alloying elements
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach achieves a more effective compressive residual stress distribution, improving fatigue strength while reducing manufacturing costs and maintaining high stress levels from the surface to a deep position, thus extending the fatigue life of coil springs.
Implementation Method 1
a first shot peening process with a first shot having high kinetic energy produced by high-speed impingement
Implementation Method 2
the fatigue strength of a coil spring can be improved by applying compressive residual stress to the vicinity of the surface of the spring by shot peening
Implementation Method 3
a second shot peening process with a second shot having low kinetic energy produced by low-speed impingement
Implementation Method 4
the fatigue strength of a coil spring can be improved by applying compressive residual stress to the vicinity of the surface of the spring by shot peening
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
A spring wire (20) is subjected to a first shot peening process (S6) and a second shot peening process (S7). In the first shot peening process (S6), a first shot is projected on the spring wire (20) at a first projectile speed. High kinetic energy of the first shot produces compressive residual stress in a region ranging from the surface of the spring wire (20) to a deep position. In the second spring wire process (S7), a second shot is projected at a second projectile speed lower than the speed of the first shot. The kinetic energy of the second shot is lower than that of the first shot. The low kinetic energy of the second shot increases the compressive residual stress in a region near the surface of the spring wire (20).