Worm Speed Reducer Asymmetric Offset for Steering Consistency
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Solution Overview
Problem
Existing electric power steering systems using worm speed reducers experience varying steering feel due to differences in frictional resistance torque between clockwise and counterclockwise rotations, which is costly to address with specialized cutting tools for different pressure angles on tooth flanks.
Innovation Solution
A worm speed reducer design with a worm shaft and worm wheel where the worm shaft's center axis is offset relative to the worm wheel's center axis, utilizing a bias member to preload the worm shaft, thereby reducing frictional resistance torque differences between rotation directions without the need for specialized cutting tools.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the worm shaft is configured like a right-hand thread with elastic biasing, then backlash is eliminated, but frictional resistance torque varies with steering direction
Solution Approach 1:
The patent applies asymmetry by offsetting the worm shaft center axis from the worm wheel center axis in a specific direction. This asymmetric positioning creates different meshing conditions for clockwise and counterclockwise rotations, balancing the frictional resistance torque in both directions while maintaining the elastic biasing configuration that eliminates backlash.
2Ease of operation
If different pressure angles are used on tooth flanks for different rotation directions, then frictional resistance torque difference is suppressed, but manufacturing cost increases due to specialized cutting tools
Solution Approach 1:
Instead of using asymmetric tooth flanks with different pressure angles that require specialized cutting tools, the patent achieves frictional resistance torque balance through asymmetric positioning of the worm shaft center axis. This approach maintains symmetric tooth geometry that can be manufactured with standard cutting tools, thereby suppressing frictional resistance torque difference without increasing manufacturing cost.
Solution Approach 2:
The patent changes the positional parameter of the worm shaft center axis by offsetting it from the worm wheel center axis. This parameter change modifies the meshing geometry and contact conditions, achieving suppression of frictional resistance torque difference through geometric adjustment rather than through complex tooth profile design requiring specialized tools.
3Manufacturing precision
If multiple cutting tools with different shapes are used for different specifications, then tooth flanks with different pressure angles are achieved, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent makes the offset configuration serve multiple functions: it simultaneously achieves frictional resistance torque balance, maintains standard symmetric tooth geometry, and allows use of universal cutting tools for all worm shaft specifications. This eliminates the need for specialized cutting tools while maintaining manufacturing precision through geometric parameter optimization.
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
This design effectively suppresses the difference in frictional resistance torque between clockwise and counterclockwise rotations, improving steering consistency while reducing manufacturing costs by eliminating the need for multiple cutting tools.
Implementation Method 1
a bias member that is supported by the housing and that elastically biases the second end of the worm shaft in a preloading direction
Implementation Method 2
meshing frictional resistance generated when the worm shaft rotates clockwise is larger than meshing frictional resistance generated when the worm shaft rotates counterclockwise
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
A center axis (C1) of a worm shaft (18) is offset in an offset direction with respect to a plane that is orthogonal to a center axis (C2) of a worm wheel (19) and that passes through a central position of the worm wheel in a tooth width direction. A bias member (60) biases a second end (18b) of the worm shaft in a preloading direction in which a center distance between the worm shaft and the worm wheel decreases. When the worm shaft is configured like a right-hand thread, in a left-handed coordinate system, a direction toward a first end of the worm shaft in an axial direction (a direction toward an electric motor) corresponds to a thumb direction. The preloading direction corresponds to an index finger direction. The offset direction corresponds to a middle finger direction.