Ball Screw Retainer Stiffness Uniformity
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Solution Overview
Problem
Conventional ball screw devices in steering systems face issues with non-uniform stiffness in retainer separating portions, leading to difficulties in ball insertion and retention, which can result in increased friction and potential ball loss due to varying stiffness across the retainer's central and end portions.
Innovation Solution
The design incorporates a retainer with modified retainer grooves that reduce the stiffness difference between central and base portions by adjusting the geometry, such as expanding the central portion's width or thickness, and forming grooves in the base portions to decrease the second moment of area, thereby achieving more uniform stiffness across the retainer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the stiffness of end portions of separating portions is increased to prevent ball loss, then ball retention is improved, but ball insertion becomes difficult due to high stiffness
Solution Approach 1:
The patent applies local quality by creating different stiffness characteristics in different regions of the separating portion. The end portions are designed with higher stiffness to prevent ball loss, while the central portion is designed with lower stiffness to facilitate ball insertion. This is achieved through geometric modifications such as varying the thickness or cross-sectional area along the length of the separating portion, allowing each region to fulfill its specific functional requirement without compromising the other.
2Reliability
If the stiffness of central portion of separating portions is increased to properly retain balls, then ball retention is improved, but ball insertion becomes more difficult due to high stiffness throughout
Solution Approach 1:
The patent resolves this contradiction by implementing local quality differentiation within the separating portion. Instead of making the entire separating portion uniformly stiff, the design creates a gradient or zoned stiffness distribution where the central portion has reduced stiffness (through geometric modifications like reduced thickness or added flexures) compared to the end portions. This allows the central region to yield during ball insertion while the stiffer end regions maintain ball retention.
3Reliability
If clearance between balls and retainer is reduced to prevent ball loss, then ball retention is improved, but friction increases due to dimensional variation of separating portions
Solution Approach 1:
The patent applies parameter changes by carefully controlling the dimensional parameters of the separating portions, particularly their stiffness characteristics and clearance dimensions. By optimizing the stiffness of the separating portions through geometric modifications, the retainer can maintain larger clearances without compromising ball retention. This reduces the likelihood of balls being caught on dimensional variations, thereby minimizing friction while still preventing ball loss through proper structural design.
Data Source
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AI summary
A ball screw device and a steering system that include a retainer having more uniform stiffness are provided. Between a rack shaft and a ball screw nut, a cylindrical retainer (50) having a plurality of retainer grooves (51) that retain balls in a rollable manner is provided. Each retainer groove (51) has two short inner surfaces (53) that intersect the extending direction of the retainer groove (51) and two long inner surfaces (54) that intersect these short inner surfaces (53) and extend along the extending direction of the retainer groove (51). In each of interior-angle portions where the short inner surfaces (53) and the long inner surfaces (54) intersect, a corner round surface (55) is formed. The radius of the corner round surface (55) is set to be smaller than one half of the width of the retainer groove (51).