Steering Shaft Hard Stop Geometry for Low Breakaway Retention
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Solution Overview
Problem
Current stake processes for retaining intermediate shafts in vehicle steering systems face challenges in maintaining a secure fit without high breakaway or running loads, leading to potential detachment during shipping, handling, and assembly, especially with increased ergonomic requirements.
Innovation Solution
The proposed steering shaft assembly features a male shaft with radially extending teeth that axially move within a female shaft, utilizing a hard stop mechanism with a thinner end portion and counter-bore design to prevent locking and deformation, allowing for a consistent press load and reduced tool wear, and incorporating a metal ring or non-overmolded metal spline for metal-to-metal contact to ensure a positive hard stop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If stakes are used to retain the male shaft inside the female tube, then the shafts are prevented from becoming detached during shipping and handling, but the stakes require high pull apart forces (2.5 times the mass of the column) and create high breakaway loads that exceed ergonomic requirements
Solution Approach 1:
The patent changes the geometric parameters of the female shaft by creating a localized thin-walled section (reduced wall thickness) that forms a hard stop. This geometric modification allows the male shaft to be retained axially without requiring high breakaway forces, as the thin-walled section deforms elastically to accommodate the male shaft while maintaining retention.
Solution Approach 2:
The patent applies local quality by creating a specific region in the female shaft with different wall thickness characteristics. The thin-walled section is localized at the end portion of the female shaft, while the rest of the shaft maintains its original wall thickness. This localized modification provides the hard stop function without compromising the overall structural integrity.
2Length of moving object
If the male shaft is pushed in from full extension, then axial displacement is achieved, but the slip joint locks up and exceeds the breakaway collapse requirement
Solution Approach 1:
The patent implements beforehand cushioning by designing the thin-walled section to deform elastically before the male shaft reaches its full retracted position. This elastic deformation acts as a cushion that prevents the slip joint from locking up, allowing smooth axial movement while maintaining retention throughout the displacement range.
3Power
If a line-to-line tight fit is used for high torsional rate torque transfer, then torque transfer efficiency is improved, but the stroke load increases which creates high breakaway requirements
Solution Approach 1:
The patent changes the fit parameters by using a loose fit instead of a line-to-line tight fit, combined with the thin-walled hard stop section. This parameter change allows torque to be transferred through the spline engagement while the thin-walled section accommodates stroke movement without generating high breakaway loads, thus decoupling torque transfer efficiency from stroke load requirements.
Data Source
AI summary
A steering shaft assembly includes a male shaft having a plurality of teeth extending radially outwardly from a portion of the male shaft. The steering shaft assembly also includes a female shaft receiving a portion of the male shaft and fixed to the male shaft in a rotational direction, the male shaft axially moveable relative to the female shaft, the female shaft having an end portion and a body portion, wherein the end portion of the female shaft has an end wall thickness that is less than a body wall thickness of the body portion, the end portion curved radially inwardly to define a hard stop position during axial movement of the male shaft. An end of the plurality of teeth of the male shaft contact the curved end portion of the female shaft at the hard stop position.