Lower Link Stress Relief for Piston Crank Mechanism
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Solution Overview
Problem
The existing multi-link type piston crank mechanism for internal combustion engines experiences stress concentration at internal threads due to large loads from multiple pins, leading to durability issues and potential fatigue at the boundary between the case-hardened layer and base metal.
Innovation Solution
The lower link is designed with a two-part structure where the upper and lower sections are coupled by bolts, with one section featuring a bolt inserting hole and the other an internal thread, and a recessed portion is formed to divert the stress transmitting path, reducing stress concentration at the internal thread roots and improving durability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the lower link uses a two-part structure with internal threads for bolt connection, then the facility of assembly operation is improved, but stress concentration occurs at the internal thread roots leading to durability deterioration
Solution Approach 1:
The invention extracts the harmful stress concentration zone by forming a recessed portion at the root of the internal thread. This recessed portion removes material from the high-stress area, creating a stress relief zone that prevents stress concentration while maintaining the threaded connection functionality. The recessed portion is specifically positioned at the root of the internal thread where stress concentration occurs most severely.
Solution Approach 2:
The invention applies local quality modification by creating a recessed portion only at the critical stress concentration zone (root of internal thread) rather than modifying the entire structure. This localized modification allows the rest of the internal thread to maintain its full strength while only the problematic area is relieved of stress concentration. The recessed portion changes the local stress distribution without affecting the overall structural integrity.
2Strength
If the lower link receives large loads from multiple pins, then the strength and rigidity requirements are increased, but stress concentration at internal threads leads to fatigue at the case-hardened layer boundary
Solution Approach 1:
The recessed portion extracts the stress concentration hazard from the structure by removing material at the critical location. This creates a stress relief zone that prevents the concentration of stresses that would otherwise lead to fatigue initiation at the case-hardened layer boundary. The extraction of material at this specific location eliminates the stress concentration without compromising the overall load-bearing capacity.
Solution Approach 2:
The recessed portion acts as a beforehand cushioning measure by pre-emptively relieving stress concentration before fatigue can initiate. By creating this stress relief zone in advance, the structure is protected against the development of fatigue cracks that would normally occur at the case-hardened layer boundary under repeated loading conditions.
Data Source
AI summary
A lower link for a piston crank mechanism of an internal combustion engine includes an upper section, a lower section, and a crank pin bearing section disposed between the upper section and the lower section, and mounted on a crank pin of a crank shaft. One of the upper section and the lower section is formed with a bolt inserting hole. The other of the upper section and the lower section is formed with an internal thread portion including an open end. One of the bolts passes through the bolt inserting hole, is screwed into the internal thread portion, and includes an end bared from the open end which is formed in a surface perpendicular to a bolt center axis. The other of the upper section and the lower section includes a recessed portion formed in the surface to divert a stress transmitting path.