# Integrated Design of Hydraulic Coupling Bolts for Large Shaft Systems
## 6 Conclusions  
In this study, a design method considering the entire operation process of an M72 specification hydraulic coupling bolt for a large shaft system was proposed. First, the tightening load conditions, which guarantee both secure tightening and structural integrity after the first and second tightening steps, were established by combining theoretical formulas with FE analysis. Next, extensive FE analysis was conducted to optimize the configurations of the adapter and the oil passage with a focus on structural strength during tightening  
Figure 15: Experimental procedures and results for coupling bolt operational test: (**a**) 1 st tightening process; (**b**) 2 nd tightening process; (**c**) untightening process. and untightening operations. Finally, based on the design and analysis results, a prototype of a hydraulic coupling bolt was manufactured, and actual operational experiments were performed to validate the proposed design method. The main conclusions are summarized as follows:  
The second tightening load was calculated considering the yield stress of the material, stress concentration and safety factors, yielding a value of 825 kN, which is equivalent to 730 bar of tensioner hydraulic pressure. While keeping the second tightening load constant, the contact pressure and von-Mises stress distribution on the sleeve evaluated by FE analysis were primarily influenced by the first tightening load. Within an acceptable stress range, the first tightening load suitable for ensuring both secure clamping and structural integrity during the entire tightening operation was determined to be 452 kN, corresponding to 400 bar of tensioner hydraulic pressure. The pitch and length of the thread of the adapter were selected as design variables. FE analysis demonstrated that increasing the thread length and decreasing the pitch enhanced the structural stability of the adapter. Consequently, the optimal combination of variables was identified as a 2 mm pitch and a 91 mm thread length. In the design of the oil passage shape, two design variables, oil passage length and pitch, were chosen. FE analysis of the untightening process was conducted to assess the influence of each variable on the maximum von-Mises stress experienced by the sleeve. It was revealed that decreasing the pitch and increasing the passage length led to a larger total area for the passage, resulting in a lower hydraulic pressure for the untightening. The optimal oil passage shape was determined to be a pitch of 10 mm and an oil passage length of 158 mm, considering both structural integrity and workability. Operational experiments using an actual hydraulic coupling bolt with M72 specification were performed to verify the effectiveness of the suggested design method. It was confirmed that both the structural integrity and sound operation of the bolt with the optimized shape were secured under the load conditions determined through theoretical calculations and computational analysis. Therefore, it is expected that the findings of this study will be valuable for designing various hydraulic coupling bolts of large shaft systems. The present study holds significant value as it proposes an integrated approach to hydraulic coupling bolt design, considering both tightening and untightening processes through theoretical and numerical analyses, thereby distinguishing it from existing methods. However, it is important to note that the proposed design was developed based on an existing bolt configuration, and the design variables were determined through practical case studies without explicit decision-making criteria. To achieve a genuine \"integrated design\", a more systematic approach, involving a reconsideration of each critical aspect of the coupling bolt configuration, must be undertaken without being constrained by the existing bolt structure. This represents a crucial direction for our ongoing research efforts. Conceptualization, K.-H.L. ; formal analysis, S.-C.P., I.-K.L., S.-K.L. and K.-H.L. ; investigation, S.-C.P. ; methodology, S.-C.P.