# Integrated Design of Hydraulic Coupling Bolts for Large Shaft Systems
## 1 Introduction
In addition, the effect of lubricating conditions on the tightening force and structural integrity of the torque tightening bolts was experimentally examined. [PERSON] et al. [16] proposed a method to evaluate the fatigue durability of the connecting rod bolts in a high-speed engine using FE analysis. The reliability of the method was validated by comparing the results with those obtained from a fatigue endurance limit test. [PERSON] et al. [17] performed FE analysis on the hydraulic coupling bolt fastening and examined stress variation at the interface between the bolt and the sleeve as a function of the insertion displacement of the bolt. [PERSON] [18] designed the oil passage shape of hydraulic coupling bolts using FE analysis to facilitate the disassembly of the bolt and sleeve, ultimately enhancing the convenience of maintenance work. [PERSON] et al. [19] conducted an FE analysis on the structural integrity of a two-stroke ship engine system coupling bolt under maximum torque. [PERSON] et al. [20] proposed a novel flat-face coupling system in hydraulic power transmission equipment. This system significantly reduces the resistance resulting from high hydraulic fluid pressure during coupling by incorporating an instantaneous pressure relief module. Through a series of computational fluid dynamics numerical investigations, they achieved a design with optimal flow characteristics and hydraulic power transmission efficiency. Existing studies primarily concentrated on assessing the fastening force and structural strength of coupling bolts through FE analysis and often overlooked the complete operation of the bolt, which restricts their applicability to the design process. Furthermore, there is a lack of research to verify the reliability of design and analysis results through actual operational experiments, especially when it comes to coupling bolts for large shaft systems. Therefore, in this study, an integrated design approach that encompasses both the tightening and untightening process was established for hydraulic coupling bolts with an M72 specification, which were designed for use in large shaft systems. In Section 2, the initial step involved determining the second tightening load by considering material strength and conducting a theoretical analysis that involved stress concentration and safety factors. Following this, the upper and lower limits of the first tightening load were calculated based on the thick-walled cylinder theory. FE analysis was then performed under these load conditions for the tightening process to determine the suitable first tightening force, ensuring secure fastening both axially and radially. In addition, design variables related to the shapes of the adapter (Section 3) and oil flow passage (Section 4) were selected, and a series of FE analyses was performed on the tightening and untightening process to identify the optimal variable combination from the viewpoint of structural integrity. Finally, as described in Section 5, prototypes of hydraulic coupling bolts were produced, and operational tests were performed to assess the validity of the design method proposed in this study. The entire design process is schematically illustrated in Figure 3.