Offset-Hob Worm Wheel Geometry for Consistent Steering Torque
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Solution Overview
Problem
The existing electric power steering systems with worm speed reducers experience varying frictional resistance torque due to differences in rotating direction, leading to inconsistent steering feel and high manufacturing costs due to the need for specialized cutting tools to vary pressure angles between tooth flanks.
Innovation Solution
A worm speed reducer design featuring a worm shaft and worm wheel with different pressure angles at the central position of the tooth width direction, achieved by using a hob with an offset center during the cutting process, which reduces the difference in frictional resistance torque between rotating directions and allows for cost-effective manufacturing of worm wheels with varying specifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the worm shaft is supported such that the first end pivots around the second end with the second end elastically biased toward the worm wheel side, then backlash is eliminated, but frictional resistance torque varies according to steering direction
Solution Approach 1:
The patent applies asymmetry by making the pressure angles of the tooth flanks asymmetric. Specifically, the first tooth flank has a different pressure angle than the second tooth flank, creating an asymmetric tooth profile that compensates for the elastic deformation of the worm shaft during operation. This asymmetric design ensures that the meshing characteristics remain consistent regardless of the direction of rotation, thereby eliminating the variation in frictional resistance torque while maintaining backlash elimination.
Solution Approach 2:
The patent applies local quality by varying the pressure angle at different locations of the tooth flanks. The pressure angle is not uniform across the tooth width but is specifically optimized at the central position of the tooth width direction. This localized optimization of the pressure angle distribution allows the gear meshing to compensate for the elastic deformation of the worm shaft, ensuring consistent steering feel without requiring multiple cutting tools.
2Ease of operation
If the pressure angle between tooth flanks is varied to suppress frictional resistance torque difference, then steering consistency is improved, but manufacturing cost increases due to need for multiple specialized cutting tools
Solution Approach 1:
The patent applies universality by designing a single hob that can manufacture worm wheels with different pressure angle specifications. The hob is designed with a specific geometry that allows it to cut tooth flanks with different pressure angles by simply changing the cutting position or orientation, rather than requiring multiple specialized hobs. This multi-functional hob reduces manufacturing costs while still enabling the production of worm wheels with the required asymmetric pressure angles for consistent steering performance.
Solution Approach 2:
The patent applies parameter changes by varying the pressure angle parameter at different locations of the tooth flanks during the cutting process. Instead of requiring different cutting tools for different pressure angles, the invention changes the pressure angle parameter through the cutting process itself by adjusting the hob position or cutting conditions. This allows a single hob to produce worm wheels with the optimized asymmetric pressure angle distribution, reducing tooling costs while maintaining steering consistency.
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 difference in frictional resistance torque between rotating directions, enhancing steering consistency and reducing manufacturing costs by using a common hob to produce worm wheels with different specifications.
Implementation Method 1
The worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position in the tooth width direction
Implementation Method 2
a bias member supported by the housing to elastically bias the second end of the worm shaft toward the worm wheel side
Implementation Method 3
a worm wheel with pairs of tooth flanks, each pair defining a tooth space... The rotation output from the electric motor is transmitted to a worm shaft in a worm speed reducer. The rotation output from the electric motor is decelerated via a worm wheel that meshes with the worm shaft
Implementation Method 4
meshing frictional resistance resulting from rotation of the worm wheel in the first rotating direction is larger than meshing frictional resistance resulting from rotation of the worm wheel in a second rotating direction
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
A plane that contains a central axis of a worm shaft (18) and that is orthogonal to a central axis of a worm wheel (19) is arranged at a central position of the worm wheel in a tooth width direction (W) thereof when no load is applied to a worm speed reducer (15). The worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position toward an offset direction corresponding to a first direction along the tooth width direction. A pair of tooth flanks has different pressure angles at the central portion in the tooth width direction.