Worm Reducer Tip Support Structure for Backlash Noise Suppression
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Solution Overview
Problem
The existing worm reducers in electric power steering devices suffer from unwanted tooth rattling noise due to backlash at the meshing portion between the worm teeth and wheel teeth, which is exacerbated by displacement of the worm tip end portion when changing the rotation direction, leading to durability issues.
Innovation Solution
The worm reducer incorporates a holder and elastic member to bias the worm tip end portion towards the worm wheel, with holder engagement portions and pad engagement portions providing preload and grease retention features to stabilize the worm tip end, reducing displacement and suppressing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the worm tip end portion is made swingable and displaceable to suppress backlash, then tooth rattling noise is reduced, but the worm tip end portion may be vigorously displaced in the third direction when rotation direction changes, causing abnormal noise
Solution Approach 1:
The holder includes engagement portions at specific positions (sandwiching the pad from both sides in the third direction) to provide localized constraints. This creates different degrees of freedom in different directions: the worm tip can swing in the first direction to suppress backlash, but is constrained in the third direction to prevent abnormal noise.
Solution Approach 2:
The solution addresses the problem by introducing a third direction (orthogonal to both the biasing direction and axial direction) and providing constraints specifically in this dimension through the holder engagement portions. This allows the worm to have the needed freedom in the first direction while being constrained in the third direction.
2Reliability
If an annular gap is present between the worm and bush to allow elastic biasing, then backlash is suppressed, but the worm tip end portion lacks stability in the third direction
Solution Approach 1:
The holder provides localized constraints at specific positions (engagement portions) rather than complete fixation. This allows the worm to maintain the annular gap for elastic biasing in the first direction while being stabilized in the third direction through the engagement portions.
Solution Approach 2:
The constraint system is segmented into different functional parts: the annular gap provides freedom in the first direction for backlash suppression, while the holder engagement portions provide constraints in the third direction for stability. This segmentation allows simultaneous achievement of both requirements.
3Object-generated harmful factors
If the worm is fully constrained to prevent displacement, then abnormal noise is suppressed, but backlash increases and tooth rattling noise occurs
Solution Approach 1:
The holder engagement portions are positioned to constrain only in the third direction while leaving the first direction free. This local constraint approach prevents abnormal noise without interfering with the elastic biasing mechanism that suppresses backlash.
Solution Approach 2:
The solution differentiates constraints by direction: full freedom in the first direction (for backlash suppression) versus constrained in the third direction (for abnormal noise prevention). This dimensional differentiation resolves the contradiction between the two harmful factors.
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
The solution effectively minimizes displacement of the worm tip end portion during rotation direction changes, thereby reducing tooth rattling noise and enhancing the durability of the worm reducer.
Implementation Method 1
an elastic member assembled to the holder and elastically biasing the tip end portion of the worm toward a worm wheel side via the pad
Implementation Method 2
a pad elastic pressing portion that elastically presses a part of the holder in the second direction to apply a preload to a contact portion between the holder engagement portion and the pad engagement portion
Data Source
Figure 1
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AI summary
A pad (20) has, at a plurality of locations in a circumferential direction, a pad oil retention recess (80) for retaining grease. The pad oil retention recess (80) opens on an inner circumferential surface of an end portion, which is on one side in a second direction, of a pad fitting hole (61) externally fitted to a tip portion of a worm (18), and on a surface of the pad (20), the surface facing the one side in the second direction.