Stepped Rack Bar Geometry for Steering Mesh Durability
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Solution Overview
Problem
Steering systems with ball screws experience high localized contact stresses at the rack and pinion mesh, leading to wear and reduced durability due to increased assist levels.
Innovation Solution
A rack bar blank with a stepped diameter configuration, featuring a smaller diameter ball screw region and a larger diameter pinion mesh region, with the latter having increased effective face width to distribute torque reaction forces more evenly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If assist levels (ball-nut torque) are increased to improve steering assist capability, then steering assist capability is improved, but contact stresses at the rack and pinion mesh exceed material capacity, resulting in high wear and reduced durability
Solution Approach 1:
The rack bar is designed with a stepped diameter configuration where the pinion mesh region has a larger diameter than the ball screw region. This creates local structural differentiation that increases the moment of inertia and section modulus at the pinion mesh location, thereby distributing contact stresses more effectively without increasing the overall size of the rack bar or affecting the ball screw assist capability.
2Reliability
If a larger diameter rack bar is used throughout to reduce contact stresses, then durability at rack and pinion mesh is improved, but weight and material usage increase
Solution Approach 1:
Instead of uniformly increasing the rack bar diameter, the invention applies a localized diameter increase only at the pinion mesh region where contact stresses occur. The ball screw region maintains its original smaller diameter, optimizing the weight-strength ratio by placing material only where structurally necessary.
Solution Approach 2:
The rack bar is segmented into two distinct diameter regions: a smaller diameter ball screw region and a larger diameter pinion mesh region. This segmentation allows each region to be optimized independently for its specific functional requirements, reducing overall material usage while maintaining durability at the critical mesh location.
3Force
If a non-cylindrical section with flats is used to provide partial ball-screw torque reaction, then torque reaction capability is improved, but device complexity increases
Solution Approach 1:
The invention maintains a cylindrical rack bar geometry for simplicity while creating local structural differentiation through the stepped diameter configuration. The larger diameter pinion mesh region inherently provides increased torque reaction capability through its greater section modulus, eliminating the need for complex flat sections or non-cylindrical geometries.
Data Source
AI summary
A rack bar blank for a steering system includes a first region having a first diameter. The rack bar blank also includes a second region having a second diameter that is greater than the first diameter prior to formation of teeth on the second region and a ball screw thread on the first region.