Mainspring Neck Geometry for Low-Core-Radius Durability
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Solution Overview
Problem
Mainsprings with low core radius suffer from significant plastic deformation and premature breakage due to pronounced curvature differences between the wound and manufactured states, leading to reduced durability and efficiency.
Innovation Solution
The neck portion preceding the calendered portion is lengthened to 1.5 to 10 times the external radius of the calendered portion, reducing curvature differences and plastic deformation during winding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the core radius is reduced to increase the number of coils, then the productivity and energy storage are improved, but the spring is subjected to significant curvature variations causing plastic deformation and premature breakage
Solution Approach 1:
The patent applies preliminary action by pre-forming the mainspring with a specific geometry during manufacturing that anticipates the stresses it will encounter during winding. The spring is given a predetermined curvature profile with a transition zone that prepares it for the upcoming operational stresses, reducing plastic deformation when first wound. This pre-conditioning of the spring geometry before use allows the spring to withstand the curvature variations inherent in low core radius applications.
2Use of energy by moving object
If the core radius is reduced to increase the number of coils, then the energy storage capacity is improved, but the stress at the beginning of the calendered area approaches the elastic limit causing breakage
Solution Approach 1:
The patent applies local quality by creating a non-uniform curvature profile along the length of the mainspring. Specifically, it introduces a transition zone with a specific curvature radius (Rc) that is 0.6 to 1.5 times the outer radius of the coils (R). This localized geometric modification at the critical transition area between the eye and the calendered portion distributes stress more favorably, preventing stress concentration at the beginning of the calendered area while maintaining high energy storage capacity in the coil portion.
3Reliability
If the neck portion length is increased to reduce plastic deformation, then the reliability is improved, but the device complexity increases
Solution Approach 1:
The patent applies parameter changes by optimizing the curvature radius (Rc) of the transition zone to be within a specific range (0.6 to 1.5 times the outer radius R of the coils). This quantitative parameter specification provides a clear design criterion that balances reliability improvement with manufacturing simplicity. By defining a numerical range rather than an arbitrary geometric feature, the patent maintains reliability while avoiding excessive complexity in the spring geometry.
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 enhances the mainspring's durability and efficiency, maintaining performance with an efficiency greater than or equal to 80% between winding and unwinding, while reducing the risk of premature breakage.
Implementation Method 1
the spring can provide all the available energy... significant plastic deformation... reduce its plastic deformation
Data Source
AI summary
A timepiece mainspring including, in the manufactured state, an eye and a portion formed of coils with an outer coil of radius R, the eye and the portion formed of coils being connected by a neck portion having substantially zero curvature, the timepiece mainspring wherein the neck portion has a length LC comprised between 1.5 and 10 times, and preferably between 2 and 8 times, the radius R. The mainspring having this specific geometry reduces the risk of premature breakage during use, typically for an application with a k factor lower than 10.