Pitch Diameter Displaced Screw Thread Asymmetry
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Solution Overview
Problem
Conventional screws used in accumulators have a weaker internal screw due to its susceptibility to tensile forces, leading to premature breakage under equal compressive and tensile loads, as the internal screw has lower fracture strength compared to the external screw.
Innovation Solution
The pitch diameter is displaced by making the thread ridge width greater than the thread groove width, specifically using a ratio of 1.25:0.75, to enhance the fracture strength of the internal screw, thereby reducing the tensile stress concentration at the thread bottom.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If the pitch diameter is located at the center of meshing heights with equal thread ridge width and thread groove width, then the compressive force on the external screw equals the tensile force on the internal screw, but the internal screw is weaker and more susceptible to breakage under tensile force
Solution Approach 1:
The patent applies asymmetry by making the thread ridge width greater than the thread groove width, creating an asymmetric thread profile. This asymmetry displaces the pitch diameter and redistributes the stress, reducing tensile stress concentration on the internal screw while maintaining force balance in the meshing connection.
Solution Approach 2:
The patent applies local quality by specifically modifying the thread ridge width at the pitch diameter location rather than changing the entire thread geometry. This localized modification targets the stress concentration area, reducing tensile stress where it most affects the internal screw's fracture strength.
2Ease of manufacture
If the thread ridge width equals the thread groove width, then the meshing geometry is symmetric and standard, but the internal screw experiences high tensile stress concentration at the thread bottom
Solution Approach 1:
The patent applies parameter changes by modifying the thread ridge width parameter to be greater than the thread groove width. This parameter change displaces the pitch diameter and alters the stress distribution, reducing tensile stress concentration at the thread bottom of the internal screw while maintaining manufacturability.
3Device complexity
If the internal screw has the same dimensions as the external screw, then the screw pair is symmetric and easy to manufacture, but the internal screw is weaker due to tensile force susceptibility
Solution Approach 1:
The patent applies asymmetry by creating an asymmetric thread profile where the thread ridge width exceeds the thread groove width. This asymmetry is specifically applied to the internal screw to enhance its fracture strength under tensile load while maintaining overall screw pair simplicity and manufacturability.
Solution Approach 2:
The patent applies local quality by modifying only the critical thread ridge width parameter of the internal screw rather than changing the entire screw design. This localized modification targets the weakness of the internal screw under tensile force while preserving the overall symmetry and simplicity of the screw pair configuration.
Data Source
AI summary
A screw comprising a first member having a plurality of alternating first thread ridges and first thread grooves, a ridge width of the first thread ridges being larger than a groove width of the first thread grooves at a measurement diameter of the first member. The measurement diameter is determined by calculating a pitch diameter of a hypothetical member having a plurality of hypothetical thread grooves and hypothetical thread ridges, with the hypothetical thread grooves being identical to the first thread grooves, and with the hypothetical thread ridges and the hypothetical thread grooves having identical, but opposite, profiles, and with the measurement diameter being identical to the pitch diameter of the hypothetical member.