Alternator Pulley Coil Spring Geometry for Torsional Durability
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Solution Overview
Problem
The durability of pulley structures for alternators is compromised due to excessive torsional deformation of coil springs, leading to potential cracks and wear, especially during engine startup and shutdown cycles, and existing solutions either increase the pulley size or reduce durability.
Innovation Solution
A pulley structure with a trapezoidal-shaped coil spring wire that reduces bending stress and wear by bringing the neutral axis closer to the inner surface during diameter expansion, maintaining a compact size and enhancing torsional strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the number of windings and wire diameter of the coil spring are increased to improve durability, then the durability against torsion improves, but the size of the pulley structure increases
Solution Approach 1:
The spring wire cross-section is designed with an asymmetric trapezoidal shape where the rotation axis direction length at the inner diameter side (Ti) is longer than at the outer diameter side (To). This asymmetric geometry shifts the neutral axis toward the inner circumferential surface, reducing the distance to the tensile stress zone and thereby reducing bending stress during diameter expansion deformation, achieving improved durability without increasing size
Solution Approach 2:
The invention changes the geometric parameters of the spring wire cross-section from conventional shapes (circular, square, rectangular) to a specific trapezoidal shape with Ti > To. This parameter change optimizes the stress distribution during torsional deformation, allowing the spring to withstand repeated diameter expansion cycles without increasing the number of windings or wire diameter, thus maintaining compact pulley structure dimensions
2Adaptability or versatility
If the diameter expansion deformation of the coil spring is excessively repeated during engine operation, then the operational flexibility improves, but cracks and breakages occur on the inner circumferential surface reducing durability
Solution Approach 1:
The asymmetric trapezoidal cross-section with Ti > To positions the neutral axis closer to the inner circumferential surface, which is the location experiencing tensile stress during diameter expansion. This reduces the bending stress amplitude on the inner surface, preventing crack initiation and propagation even when diameter expansion deformation is excessively repeated during engine startup and shutdown cycles
Solution Approach 2:
The invention converts the potentially harmful concentrated tensile stress on the inner circumferential surface during diameter expansion into a more favorable stress distribution. By using the asymmetric trapezoidal cross-section, the neutral axis relocation reduces the maximum bending stress in the tensile zone, transforming the harmful stress concentration into a more distributed and manageable stress state that withstands repeated operational cycles
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 trapezoidal coil spring design enhances durability and reduces wear without increasing the pulley size, effectively managing torsional stress and maintaining performance during repeated engine cycles.
Implementation Method 1
the torque is transmitted or blocked between the outer rotating body and the inner rotating body due to diameter expansion or reduction deformation of the coil spring
Implementation Method 2
the durability against torsion (torsional deformation in the diameter expansion direction and in the diameter reduction direction) of the coil spring
Data Source
AI summary
A pulley structure may be equipped with an outer rotating body, an inner rotating body, and a coil spring. The pulley structure has a cross section of the spring wire of the coil spring along a direction running along the rotational axis and parallel to the rotational axis being a trapezoidal shape, and the length of an inner-diameter-side portion in the rotational axis direction in the cross section being greater than the length of an outer-diameter-side portion in the rotational axis direction in the cross section.