Worm Gear Helix Angle Optimization for Thrust Force Management
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Solution Overview
Problem
Conventional motor retractors for seat belt webbing take-up devices require strengthened structural components to withstand thrust forces, hindering the realization of a lightweight and cost-effective design due to the transmission of large reaction forces through gear mechanisms.
Innovation Solution
A webbing take-up device with a worm gear and worm wheel mechanism, where the helix angles of the teeth are set to direct thrust forces towards the frame, eliminating the need for enhanced structural strength in the case and cover, allowing for a lightweight and compact design by ensuring stable transmission of rotating forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a gear mechanism is used to transmit rotating force from the motor to the take-up shaft, then the take-up shaft can be rotated effectively, but large thrust force acts on the case and cover requiring extra strength improvement
Solution Approach 1:
The patent changes the geometric parameters of the worm gear teeth by optimizing the helix angle. By setting the helix angle within a specific range (15° to 25°), the thrust force direction is modified to act toward the leg plate fixed side rather than radially outward on the case, thereby reducing the required strength of the case and cover while maintaining effective power transmission.
2Reliability
If the case and cover are strengthened to resist large thrust force, then structural reliability is improved, but device weight increases and cost rises
Solution Approach 1:
By optimizing the helix angle parameter of the worm gear teeth, the patent redirects the thrust force toward the structurally stronger leg plate fixed side. This parameter change maintains structural reliability under load while allowing the use of lighter materials and thinner walls for the case and cover, thereby reducing overall device weight and manufacturing cost.
3Strength
If the helix angle of worm gear teeth is optimized, then thrust force is directed toward the frame reducing structural requirements, but gear manufacturing precision requirements increase
Solution Approach 1:
The patent selects a moderate helix angle range (15° to 25°) that balances two competing requirements: directing thrust force toward the frame to reduce structural component requirements, and maintaining manufacturability with reasonable precision tolerances. This parameter optimization avoids extreme angles that would either fail to redirect force effectively or require excessively precise manufacturing.
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 enables a stable and efficient webbing take-up operation without the need for increased structural strength, resulting in a lightweight, compact, and cost-reduced device that effectively manages thrust forces and maintains performance.
Implementation Method 1
a worm gear accommodated in the case and connected to the motor; a worm wheel accommodated in the case coaxially with the take-up shaft and meshing with the worm gear, the worm wheel being rotated by rotation of the worm gear
Data Source
AI summary
In a webbing take-up device 10, a worm gear 34, which is connected to a motor 44, engages with a gear wheel 32, and when the worm gear 34 rotates, driving force is transmitted via the gear wheel 32 and a clutch 26, whereby a take-up shaft 20 is rotated in a take-up direction. Helix angles of teeth of the worm gear 34 and the gear wheel 32 are set so that thrust force generated at this time along the axial direction of the gear wheel 32 faces a leg plate 16 fixed side of a case 30. Accordingly, because the thrust force acts on the leg plate 16 fixed side of the case 30, i.e., on a frame 12, there is no need to provide the case 30 or a cover clutch 31 with high strength and rigidity.