Worm-Type Power Steering Backlash Reduction

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Electric power steering devices with worm-type speed reducers often generate rattling noise due to backlash in the meshing portion of the worm teeth and worm wheel, which is exacerbated by rotational vibrations applied from the wheel side.

Innovation Solution

An electric power steering device with a biasing mechanism that oscillates the worm shaft around a base edge bearing, using a guide piece, wedge piece, and elastic member to bias the worm teeth toward the worm wheel, thereby reducing backlash and perpendicular displacement, and utilizing a guide surface and wedge surface to bear components of the meshing reaction force orthogonal to the worm shaft.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Volume of moving object

If a worm-type speed reducer is used to transfer auxiliary power from the electric motor to the steering shaft, then the power steering device becomes more compact and lighter, but backlash in the meshing portion generates rattling noise under rotational vibration

Engineering Contradiction:

Improvesize of power steering deviceVSAvoidrattling noise

Core Design Contradiction:

Volume of moving objectVSObject-generated harmful factors

Solution Approach 1:

The biasing mechanism applies a preliminary force to the worm shaft, causing the worm teeth to be biased toward the worm wheel and eliminating backlash before rotational vibration occurs. This preliminary action prevents the harmful rattling noise from generating in the first place, rather than merely mitigating it after occurrence.

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The elastic member (spring) is pre-loaded to continuously push the worm shaft in a direction that maintains contact between the worm teeth and worm wheel. This preliminary positioning ensures that the meshing portion remains engaged without gaps, preventing rattling noise during steering operations.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If the worm shaft is allowed to oscillate to remove backlash, then meshing contact is improved, but perpendicular displacement of worm teeth occurs under rotational vibration

Engineering Contradiction:

Improvemeshing contact stabilityVSAvoidperpendicular displacement of worm teeth

Core Design Contradiction:

ReliabilityVSStability of the object's composition

Solution Approach 1:

The support structure for the worm shaft is segmented into two functional zones: the biasing mechanism that allows oscillation in the meshing direction to eliminate backlash, and the guide surfaces that constrain movement in the perpendicular direction. This segmentation enables independent control of different movement components.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different portions of the worm shaft support system have different mechanical properties: the biasing mechanism provides compliance in the axial direction to allow backlash elimination, while the guide surfaces provide rigidity in the radial direction to prevent perpendicular displacement. This local differentiation of mechanical properties resolves the contradiction.

Inventive Principle:

Principle #3Local quality

3Manufacturing precision

If the biasing mechanism uses elastic members to oscillate the worm shaft, then backlash is reduced, but the structure becomes more complex

Engineering Contradiction:

Improvebacklash reductionVSAvoidbiasing mechanism structure

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The elastic member serves as an intermediary element between the fixed housing and the oscillating worm shaft. It transmits the biasing force while allowing controlled oscillation, simplifying the overall mechanism compared to direct mechanical spring systems or hydraulic actuators.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The elastic member automatically adjusts the worm shaft position based on operating conditions, eliminating backlash without requiring external control systems. The mechanism self-regulates through the elastic deformation of the spring, reducing the need for additional control components.

Inventive Principle:

Principle #25Self-service

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 suppresses the generation of rattling noise by minimizing perpendicular displacement of the worm teeth, ensuring stable operation even under rotational vibrations.

Implementation Method 1

an elastic member for a wedge piece applying elastic force toward one circumferential side of the annular space to the wedge piece

Methodology Applied

Scientific EffectElastic force: Elasticity

Implementation Method 2

the guide surface and the wedge surface are in contact with two circumferential parts of the outer circumferential surface of the tip side bearing

Methodology Applied

Scientific EffectFriction: Friction

Data Source

PatentUS10059366B2Electrically driven power steering device

Publication Date: 2018.08.28 NSK LTD

AI summary

An electric power steering device includes a housing, a rotating shaft for steering, a worm wheel, a worm shaft, a tip side bearing, an electric motor, and biasing mechanism. The biasing mechanism includes a wedge piece and an elastic member for a wedge piece. The wedge piece is disposed between an outer circumferential surface of the tip side bearing and an inner circumferential surface of the housing in a state where the wedge piece in a circumferential direction is displaceable. The elastic member for a wedge piece applies elastic force toward one circumferential side to the wedge piece.