Variable-Stress Spiral Spring for Tape Measure Retraction Control
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Solution Overview
Problem
Conventional tape measures with spiral springs often experience high retraction speeds and forces at the end of tape retraction, leading to issues like whip and excessive force on the tape hook, due to constant stress levels throughout the spring.
Innovation Solution
A variably stressed spiral spring design where the stress decreases towards the outer end, resulting in a lower maximum torque and flatter torque profile, achieved by varying the free coil diameter along the length of the spring, which reduces retraction speed and force.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a constant stress spiral spring is used, then the spring structure is simple and easy to manufacture, but the retraction speed and force become excessively high at the end of tape retraction
Solution Approach 1:
The spring is designed with varying stress characteristics along its length. Specifically, the outer end section of the spring has a larger free coil diameter compared to the inner end section, creating a gradient stress distribution. This local variation in geometry allows the spring to provide higher torque when needed (at the beginning of retraction) while reducing torque and speed near the end of retraction, thereby eliminating whip and excessive force without complicating the overall spring structure.
2Device complexity
If a constant stress spiral spring is used, then the spring structure is simple, but excessive force is applied to the tape hook and housing at the end of retraction
Solution Approach 1:
The spring incorporates a section with varying stress characteristics, where the free coil diameter increases from the inner end toward the outer end. This creates a torque profile that decreases toward the end of retraction, reducing the force applied to the tape hook and housing when the tape is nearly retracted. The rest of the spring maintains constant stress characteristics, preserving structural simplicity while addressing the force issue only where necessary.
3Speed
If a variably stressed spiral spring is used, then retraction speed and force are reduced, but the spring geometry becomes more complex
Solution Approach 1:
Instead of making the entire spring geometry complex, the invention applies geometric variation only to a specific section of the spring. The outer end section has a gradually increasing free coil diameter, while the majority of the spring maintains a constant, simple geometry. This localized approach achieves the desired speed and force control while minimizing the increase in overall geometric complexity.
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 decreases the maximum retraction speed and force of the tape blade, minimizing whip and reducing the impact on the tape housing, while maintaining sufficient retraction energy and torque.
Implementation Method 1
a spiral spring coupled between a tape blade and tape measure housing such that the spring stores energy when the tape blade is extended from the housing and releases energy driving retraction of the tape blade
Data Source
AI summary
A tool, such as a tape measure, including a spring-based retraction system is shown. The spring-based retraction system is driven by a spiral spring, that has a variable preformed stress profile along the length of the spring.