Steering Rack Residual Stress Machining for Fatigue Strength
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Solution Overview
Problem
Current methods for manufacturing steering racks fail to achieve a uniform stress distribution, leading to 'chaotic' internal stress curves with tensile and compressive stresses, which complicates weight reduction and fatigue strength optimization without increasing weight or structural changes.
Innovation Solution
A method integrating X-ray residual stress measurements and blasting techniques into the processing train to convert the chaotic stress curve into a permissible residual stress curve, introducing defined compressive residual stresses without tensile stresses, thereby optimizing the rack's diameter and stress distribution without structural changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If conventional machining methods (milling, forming) are used to manufacture steering racks, then the rack can be produced with basic structural integrity, but the weight is high and manufacturing cost is high
Solution Approach 1:
The patent applies induction hardening to change the material parameters of the rack, specifically hardening the toothing section to achieve a uniform hardening zone. This allows the rack to be made from thinner-walled tubes while maintaining strength requirements, thereby reducing weight without significantly increasing manufacturing complexity
Solution Approach 2:
The patent performs preliminary straightening and stress distribution optimization before final assembly. By addressing stress distribution issues early in the manufacturing process through controlled deformation and heat treatment, the rack can achieve required strength with less material, reducing weight while keeping manufacturing feasible
2Weight of moving object
If thinner-walled tubes are used to reduce rack weight, then weight decreases, but strength and fatigue resistance deteriorate
Solution Approach 1:
The patent changes the material parameters through induction hardening, creating a uniform hardening zone in the toothing section. This local parameter change increases surface hardness and fatigue resistance, allowing thinner-walled tubes to achieve required strength levels that would otherwise require thicker walls
Solution Approach 2:
The patent applies selective hardening only to the toothing section where strength is most critical, rather than hardening the entire rack. This local quality enhancement optimizes strength-to-weight ratio by concentrating material properties where needed most, allowing weight reduction in non-critical areas
3Strength
If induction hardening is applied to achieve uniform hardening zone, then local strength improves, but the chaotic stress pattern persists and cannot be fully eliminated
Solution Approach 1:
The patent performs preliminary straightening with controlled deformation before induction hardening. This preliminary action partially addresses the chaotic stress pattern by introducing controlled stress distribution, which when combined with subsequent uniform induction hardening, creates a more favorable overall stress state than either process alone
Solution Approach 2:
The patent merges multiple processes (forming, straightening, induction hardening) into an integrated manufacturing sequence. By combining these processes with carefully selected parameters, the patent achieves a synergistic effect where the cumulative stress distribution from multiple processes creates a more uniform overall pattern than any single process could achieve alone
4Strength
If complex machining processes (extrusion, forging) are used to optimize stress distribution, then strength improves, but device complexity and manufacturing cost increase significantly
Solution Approach 1:
The patent uses induction hardening with specifically selected parameters (power, frequency, scanning speed) to achieve uniform hardening zone and improved stress distribution. This parameter optimization allows conventional simple processes to achieve results that would otherwise require complex extrusion or forging operations, thereby reducing device complexity while maintaining strength
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 enables a uniform stress distribution in the gearing, enhancing fatigue strength and allowing for weight savings while minimizing the rack diameter, thus meeting increased vehicle axle load requirements without increasing costs or structural complexity.
Implementation Method 1
at least the area of the toothing (2) receives pre-stressing with defined, introduced compressive residual stresses (σ-E) without tensile stresses (σ+)
Implementation Method 2
The tensile stresses (σ+) in an axial direction (x) and the compressive stresses (σ-) of the gearing (2) in the transverse direction (y) are determined by X-ray measurements
Data Source
Figure 1
Figure 2.1
Figure 2.2
AI summary
The invention relates to a method for machining a rack and to a rack (1) machined according to said method, for example a steering rack. In said method, the stress pattern that is present after hardening and/or straightening the rack and that has a chaotic internal stress distribution of tensile and compressive stresses is converted into a stress pattern that optimizes the strength and the use of the material and also the diameter of the rack, such that, without altering the structure, at least the region of the gear teeth (2) is pre-stressed, in a functionally combined series of steps of a machining pass, with a deliberately introduced internal compressive stress without tensile stress and with a predominantly uniform stress distribution or stress plane.