Work Arm Boss Structure for Weld Fatigue Resistance
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Solution Overview
Problem
Existing work machine arm designs face fatigue issues due to stress concentration at welds between bosses and side plates, leading to premature fatigue cracks, especially under bending deformations.
Innovation Solution
A work arm design featuring a box-shaped structure with bosses having a cylindrical body section and a flange section, where the outer diameter of the flange section on the inner surface side is smaller than the outer diameter on the outer surface side, reducing flexural rigidity and bending stress on the welds, and using a cylindrical boss coupling member to couple the bosses, thereby enhancing the fatigue life of the welds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the thickness of the weld between the flange portion of the boss and the side plate is made equal to the plate thickness of the side plate, then the magnitude of stress exerted on the weld is made substantially equal to the magnitude of stress exerted on the side plate, but a fatigue crack initiates from the weld because the welding material has lower fatigue strength than the base material
Solution Approach 1:
The invention applies local quality by creating a groove-shaped thick adjustment portion at the specific location where the flange portion joins the side plate. This localized thickening concentrates the stress away from the weld line and into the boss body, allowing the weld to be thinner while maintaining overall structural integrity and reducing fatigue stress at the critical weld interface.
Solution Approach 2:
The invention changes the geometric parameter of the boss by introducing a groove-shaped thick adjustment portion that alters the stress distribution. This parameter change transforms the uniform thickness design into a variable thickness design, where the thick adjustment portion creates a stress bridge that reduces fatigue stress concentration at the weld between the flange and side plate.
2Reliability
If groove-shaped thick adjustment portions are formed on the flange portions to make the thickness equal to the side plate thickness at the welding position, then stress concentration at welds is prevented, but the device complexity increases
Solution Approach 1:
The invention merges the thick adjustment portion with the flange portion of the boss, creating an integrated structure rather than a separate component. This integration reduces device complexity by eliminating the need for additional parts while still achieving the stress distribution benefits of having adjusted thickness at the weld location.
Solution Approach 2:
The groove-shaped thick adjustment portion is applied locally only where needed at the flange-side plate interface, rather than throughout the entire boss structure. This localized approach minimizes the increase in device complexity while targeting the specific area where stress concentration occurs.
3Reliability
If the outer diameter of the flange section on the inner surface side is made smaller than the outer diameter on the outer surface side, then flexural rigidity is reduced and bending stress on welds is reduced, but the boss structure becomes more complex
Solution Approach 1:
The invention applies asymmetry by making the outer diameter of the flange section on the inner surface side smaller than the outer diameter on the outer surface side. This asymmetric geometry reduces flexural rigidity in the out-of-plane direction, allowing the flange to flex more easily and reduce bending stress transmitted to the weld during out-of-plane bending deformations of the side plates.
Data Source
AI summary
A work arm of a work machine includes a box-shaped structure including multiple plate members, a pair of bosses attached to opposing plate members, and a cylindrical boss coupling member coupling the pair of bosses together. Each boss includes a boss body section having a pin insertion hole through which a coupling pin is inserted and extending in an arrangement direction of the plate members, and a flange section extending outward from an outer circumferential portion of the boss body section and joined to a corresponding one of the plate members via a first weld. The boss body section includes a cylindrical outer body portion on an outer surface side of the box-shaped structure and a cylindrical inner body portion on an inner surface side of the box-shaped structure. The boss coupling member is joined, at both axial ends thereof, to respective ends of the inner body portions via second welds. The inner body portion is configured such that an outer diameter at a flange section-side part thereof is smaller than an outer diameter at an axial end of the outer body portion. This allows improvement of fatigue life of the weld joining the boss and the plate member.