Strain Wave External Gear Teeth for Easier Guided Assembly
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Solution Overview
Problem
Manufacturing externally toothed gears with molded articles faces challenges such as misalignment and burrs due to parting-line traces, making it difficult to assemble with internally toothed gears, especially when the tooth profile is three-dimensional and changes along the trace.
Innovation Solution
The design incorporates external tooth extension portions with a tapered tooth profile that gradually decreases in size along the tooth trace direction, using draft angles to guide insertion and prevent misalignment, and includes a cylindrical barrel part that can flex radially to facilitate assembly with internally toothed gears.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a molded article is used for the externally toothed gear, then manufacturing efficiency is improved, but parting-line traces and misalignment occur making assembly difficult
Solution Approach 1:
The externally toothed gear is divided into two distinct portions: external tooth portions that mesh with internal teeth, and external tooth extension portions that serve as insertion guides. This segmentation allows the parting line to be positioned at the boundary between these portions, isolating the parting-line traces from the meshing area and preventing interference with assembly operations.
Solution Approach 2:
The external tooth extension portions act as an intermediary element between the operator and the main external tooth portions. These extension portions with tapered profiles guide the insertion process and prevent misalignment, mediating the assembly operation to achieve proper alignment without direct manipulation of the critical meshing teeth.
2Reliability
If a three-dimensional tooth profile is used, then tooth contact is enhanced, but insertion alignment becomes difficult due to varying tooth size along the trace
Solution Approach 1:
The gear is segmented into external tooth portions with three-dimensional profiles for enhanced contact and external tooth extension portions with tapered profiles for alignment guidance. This segmentation allows each portion to fulfill its specific function without compromising the other.
Solution Approach 2:
Instead of making the main tooth portions tapered for alignment (which would reduce meshing quality), the invention inverts the approach by adding separate extension portions with tapered profiles that serve the alignment function, thereby preserving the three-dimensional profiles of the main tooth portions for optimal tooth contact.
3Manufacturing precision
If the parting line is positioned at the boundary between external tooth portions and extension portions, then misalignment is prevented, but draft angle application becomes complex
Solution Approach 1:
Different draft angles are applied to different portions of the gear: a first draft angle is applied to the external tooth portions and a second draft angle is applied to the external tooth extension portions. This local differentiation of molding parameters allows precise control over each portion's geometry while accommodating the parting line position at the boundary.
Solution Approach 2:
The molding process parameters are changed locally across different portions of the gear. By varying the draft angle parameter between the external tooth portions and extension portions, the invention achieves both precise alignment (through the parting line position) and proper ejection characteristics (through localized draft angles).
Data Source
Figure 1(A)~1(B)
Figure 2
Figure 3(A)~3(C)
AI summary
An externally toothed gear (3) of a strain wave gearing (1) is provided with external teeth (34), the tooth profile of which gradually changes along the tooth trace direction. The external teeth (34) are each provided with an external tooth portion (341) that is capable of meshing with internal teeth (24) of an internally toothed gear (2), and an external tooth extension portion (342) that does not mesh with the internal teeth (24). The external tooth extension portions (342) have a tapered tooth profile that tapers off, and serves as a guide for instances when the external teeth (34) are inserted between the internal teeth (24). The tooth profile (342c) of the external tooth extension portions (342) is slightly smaller than the tooth profile (341c) at first ends (341a) of the external tooth portions (341), to which the external tooth extension portions (342) are linked. The tooth profile (342c) does not protrude in the tooth depth direction or the tooth thickness direction from the tooth profile (341c) due to misalignment of a molding die. It is easy to carry out fitting work for inserting the externally toothed gear (3) into the internally toothed gear (2) using the external tooth extension portions (342) as a guide.