Steering Rack Bar Hardening Layout for Strength Without Embrittlement
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Solution Overview
Problem
Forming a deep hardened layer on rack bars for increased strength can embrittle the material, compromising its toughness and axial and bending strength, especially when the rack diameter is reduced for weight optimization.
Innovation Solution
A hardened layer is formed continuously over the entire circumference of the rack bar, with varying depths from the bottom land, side, and back, using induction heating to ensure increased axial and bending strength while minimizing embrittlement, and to maintain the center portion's toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a deep hardened layer is formed on the rack bar to increase strength, then axial strength and bending strength are improved, but the rack bar becomes embrittled and toughness decreases
Solution Approach 1:
The patent applies different hardened layer depths at different locations on the rack bar. The hardened layer depth is greatest at the rack teeth (contact surface), moderate at the side surface, and smallest at the back surface. This localized variation in hardness provides maximum strength where loads are applied while preserving toughness in the center portion, thereby resolving the contradiction between strength and embrittlement.
2Weight of moving object
If the rack diameter is reduced for weight optimization, then weight is decreased, but strength requirements become more difficult to meet
Solution Approach 1:
The patent changes the physical and chemical parameters of the rack bar material by forming a hardened layer through heat treatment. This creates a martensitic structure with significantly increased hardness and strength at critical locations, allowing the rack bar to maintain required strength levels even with reduced diameter for weight optimization.
Solution Approach 2:
The rack bar effectively becomes a composite structure with a hardened outer layer (martensite) and a softer center portion. This composite arrangement provides high strength at the surface for load-bearing while maintaining toughness in the center, enabling weight reduction without sacrificing strength.
3Strength
If a hardened layer is formed continuously over the entire circumference including the back, then strength is increased, but the center portion decreases and embrittlement increases
Solution Approach 1:
The patent specifically avoids forming a hardened layer at the back surface of the rack teeth, maintaining it at a minimum depth. This localized exclusion of hardening preserves the center portion's toughness and prevents embrittlement while still providing adequate strength at the critical contact surfaces and side surfaces where loads are applied.
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 enhances the axial and bending strength of the rack bar while reducing embrittlement, ensuring the rack bar's structural integrity and weight reduction objectives are met without compromising its durability.
Implementation Method 1
using induction heating to ensure increased axial and bending strength while minimizing embrittlement
Data Source
Figure 1~2A
Figure 2B~2D
Figure 3~4
AI summary
A rack bar (11) includes: a rack tooth row (112) including a plurality of rack teeth (111) meshing with pinion teeth (9a); a hardened layer (K) provided continuously over an entire circumference of the rack tooth row (112); and a center portion (S) provided inside the hardened layer (K) and having lower hardness than the hardened layer (K). When the rack bar (11) is viewed in an axial direction of the rack bar (11), a depth of the hardened layer (K) from the following positions i), ii), and iii) increases in this order: i) a bottom land (111v) of the rack teeth (111); ii) a side (Ills) of the rack bar (11) relative to the bottom land (111v); and iii) a back (111b) of the rack bar (11) relative to the bottom land (111v).