Sculptured Throat Spool Reduces Stress Risers in Helicopter Straps
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Solution Overview
Problem
Tension-torsion strap assemblies in rotary aircraft, such as helicopters, experience stress risers due to abrupt bending of the strap over the spool edge, leading to potential damage from tensile and bearing stresses during articulation and twisting.
Innovation Solution
A sculptured throat spool design is implemented to minimize alternating tensile and bearing stresses by modifying the spool's geometry, featuring a combination of semi-circular and linear edge portions with a sloped contour, reducing stress concentrations in the strap fibers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional spool with a sharp edge is used, then the structure is simple and easy to manufacture, but stress risers occur at the edge causing potential damage to the strap
Solution Approach 1:
The spool edge is modified from a sharp corner to a curved transition surface. The sculptured throat features a rounded profile that smoothly guides the strap around the spool, eliminating the abrupt angle that causes stress concentration. This curvature distributes the stress along the strap fibers, preventing the stress risers that lead to strap damage while maintaining structural integrity.
Solution Approach 2:
The spool geometry is modified locally at the throat region where the strap makes contact, while the rest of the spool structure remains conventional. The sculptured throat introduces a localized curved surface with specific radius of curvature optimized to reduce stress, without requiring complete redesign of the entire spool assembly.
2Adaptability or versatility
If the strap is articulated during pitch changes, then the blade pitch can be adjusted, but stress risers occur in both tensile and bearing stresses at the spool edge
Solution Approach 1:
The curved transition surface of the sculptured throat provides a gradual path for the strap during articulation movements. This curvature eliminates the abrupt bending that occurs at sharp edges, distributing both tensile stress from strap tension and bearing stress from the bending action across a larger area of the strap, thereby reducing peak stress values during pitch changes.
Solution Approach 2:
The spool geometry parameter at the throat is changed from a sharp angle to a curved profile with optimized radius. This parameter modification alters the stress distribution characteristics, transforming the stress concentration at the edge into a distributed stress pattern that accommodates the articulation motion without excessive stress peaks.
Data Source
AI summary
A sculpted throat spool of a tension-torsion strap assembly is disclosed. Using the sculpted throat spool minimizes the stress riser typically present at the edge of the spool when a tension-torsion strap is articulated.