Tensioner Base Thread Design to Prevent Corner Cracking
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Solution Overview
Problem
The existing tensioner assemblies for vehicle engine accessories are prone to cracking at the corner between the outer periphery and the end surface of the pulley arm's base portion due to excessive tensile stress when the support bolt is tightened, especially when made of materials like aluminum alloy with a thin wall thickness.
Innovation Solution
The tensioner assembly design features a pulley arm with an internal thread only in the axial rear portion of the through hole, avoiding threaded engagement at the corner, which reduces tensile stress and minimizes crack formation by locating the internal thread's axial front end radially aligned with or axially rearward of the end surface of the shoulder, and optionally incorporating a chamfer or curved surface to distribute stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the support bolt is tightened to secure the rolling bearing and tension pulley, then the assembly is firmly fixed, but excessive tensile stress concentrates at the corner of the base portion causing cracks
Solution Approach 1:
The patent applies local quality by creating a chamfered surface at the corner of the base portion where tensile stress concentrates. This local geometric modification changes the stress distribution characteristics specifically at the vulnerable corner area, reducing stress concentration without affecting the overall assembly fixation strength. The chamfered surface provides a gradual transition that prevents crack initiation while maintaining firm fixation elsewhere.
2Strength
If the internal thread is formed over the entire length of the through hole, then the support bolt is securely anchored, but tensile stress is applied to the thin-walled corner portion causing cracks
Solution Approach 1:
The patent applies segmentation by dividing the through hole into two distinct zones: an axially front portion without internal thread and an axially rear portion with internal thread. This segmentation prevents tensile stress from being applied to the thin-walled corner portion while still providing secure bolt anchoring in the rear portion. The unsupported front portion acts as a stress-relief zone that protects the vulnerable corner area.
3Quantity of substance
If the wall thickness of the base portion is reduced to minimize material usage, then manufacturing cost decreases, but the corner becomes more susceptible to cracking under tensile stress
Solution Approach 1:
The patent applies local quality by introducing a chamfered surface specifically at the vulnerable corner of the base portion. This local geometric feature redistributes tensile stress away from the thin-walled corner area, enabling the use of reduced wall thickness without compromising crack resistance. The chamfer creates a stress-flow path that bypasses the thin-walled section, maintaining reliability while minimizing material usage.
Data Source
AI summary
A tensioner assembly includes a pulley arm pivotable about a pivot shaft, and a tension pulley mounted on the pulley arm at its end remote from the pivot shaft and pressed against a belt to keep the tension of the belt. The tension pulley is rotatably mounted around a base portion of the pulley arm through a rolling bearing with the inner surface and axial rear end surface of the bearing inner race in contact with the outer peripheral surface of the base portion and an end surface of a shoulder of the base portion, respectively. The base portion has an axial through hole formed with an internal thread on its inner surface. A support bolt is threaded into the through hole from the axial front end to press the rolling bearing against the end surface of the shoulder. The internal thread has an axial front end located in the vicinity of, or axially rearwardly of, the end surface of the shoulder.