Enhancing the Sealing Performance of Bolted Ball Joints by Gaskets: Numerical Simulation and Experiment

Abstract

With the increasing utilization of bolted ball joint steel mesh structures in offshore floating platforms and deep-sea fish cages, the issue of seawater infiltrating the joints and members through the installation gaps of the bolted ball joint, leading to subsequent corrosion, has become increasingly prominent. This article presents an innovative method to improve the sealing performance of bolted ball joints. The approach involves creating sealed surfaces within the contact gaps between the sleeve and connecting components by adding circular grooves and sealing washers to both ends of the sleeve. Subsequently, a two-dimensional finite element analysis model of the bolted ball joint with the sealing structure was created using SOLIDWORKS 2021 and ANSYS Workbench 2022 R1. The study analyzes the sealing gasket’s contact pressure at various compression levels and evaluates its performance with bubble tests for air tightness. Research results show a linear relationship between the contact pressure and compression rate, achieving sealing pressures of 2.91 MPa, 4.22 MPa, and 5.95 MPa at compression levels of 8%, 11%, and 14%, respectively. Experimental testing demonstrates that the improved bolted ball joint exhibits excellent sealing performance.

1. Introduction

The bolted ball joint steel grid structure is a spatial grid system comprised of members assembled through bolted ball joints based on specific geometric principles. This architectural framework offers advantages such as well-distributed stress, dependable connections, and convenient construction processes. Its applicability extends beyond terrestrial spatial frameworks, encompassing diverse settings like exhibition centers, sports venues, and industrial factories, while also finding progressive integration within the realm of marine engineering [1,2,3]. Prominent applications include floating platforms, deep-sea fish cages, and more, as depicted in Figure 1.

In the marine environment, the corrosion of steel materials can rapidly lead to a reduction in the cross-sectional area and load-bearing capacity of steel components. This vulnerability represents a critical weakness that undermines the safe operation of steel grid structures with bolted ball joints [4,5]. To address this issue, several corrosion protection measures for steel components in oceanic settings are employed, including weathering steel anti-corrosion, hot-dip galvanizing anti-corrosion, and coating anti-corrosion techniques [6,7,8]. Weathering-resistant steel anti-corrosion involves incorporating trace amounts of alloying elements into conventional steel. This process facilitates the formation of a dense oxide coating on the steel’s surface, effectively preventing further corrosion and oxidation. In contrast, the approach of hot-dip galvanizing anti-corrosion entails immersing steel components into molten zinc, resulting in the formation of an iron-zinc alloy coating on the steel’s surface. This layer is a barrier against corrosive agents, significantly enhancing the steel’s anti-corrosion capabilities. Another strategy, coating anti-corrosion, centers around applying specialized anti-corrosion coatings onto the surfaces of steel structures. These coatings establish a protective shield, effectively isolating the steel from direct contact with the external environment. Among the aforementioned corrosion protection measures, coating anti-corrosion is extensively employed in numerous practical projects owing to its benefits of low cost, ease of repair, and straightforward construction procedures. This approach has yielded a favorable effect on the external corrosion protection of members and joints, effectively controlling corrosion on the structural exterior.

However, installation gaps between the components of the bolt ball joint and the rods, as shown in Figure 2, allow seawater to infiltrate the interior of the joints and rods, causing the corrosion of the bolt threads and the steel pipe’s interior. This has emerged as the primary challenge in implementing the bolt ball joint steel grid structure in marine engineering. Given the limited internal space within the joint and the fine nature of components like bolt threads, achieving complete coverage of all surfaces with coating anti-corrosion and hot-dip galvanizing anti-corrosion is challenging. Moreover, it would compromise installation precision, rendering coating anti-corrosion and hot-dip galvanizing anti-corrosion unsuitable for addressing this issue.

Therefore, how to improve the joint structure and enhance the sealing performance of the joint, thereby eliminating the problem of internal corrosion caused by installation gaps, has essential research value. Huang [9] proposed a method for achieving the sealing of bolt ball joints by adding an outer layer on the surface of the bolt ball and a sealing sleeve on the periphery of the sleeve to achieve sealing protection of the bolted ball joint. However, this sealing method is complex in construction and becomes challenging to apply when there are numerous connecting rods at the joint. Zhang [10] proposed a sealed connection rod for the bolted ball joint, which seals the inside of the member by adding sealing partitions at both ends of the rod. However, this sealing method focuses solely on safeguarding the member’s internal corrosion, overlooking the corrosion susceptibility of the sleeve and bolt ball components. Furthermore, the mentioned proposed sealing methods are currently in their initial stages, lacking the crucial analysis of sealing performance and experimental validation. Consequently, there exists a necessity for in-depth research to develop a pragmatic and viable sealing approach that not only enhances the sealing performance of bolted ball joints but also effectively addresses internal corrosion challenges.

To address the aforementioned challenges, this article presents a novel approach aimed at enhancing the sealing performance of bolted ball joints. The primary objectives of this approach are to effectively mitigate the risk of interstitial corrosion in bolted ball joints within the realm of offshore engineering, improve the safety and durability of bolted ball joint steel mesh structures in marine applications, and further promote their use in offshore engineering projects. This study is organized as follows: Section 1, ‘offers an overview of this study. Section 2, offers a detailed explanation of the concept and construction of the sealing method. Section 3, conducts an analysis of the sealing performance of bolted ball joints with the proposed sealing structure using numerical simulation. Section 4, presents the results of tests on the sealing performance of the bolted ball joint. Section 5, draws the main conclusions of this study based on the analyses.

2. Ideas and Structural Design

To tackle the issue of seawater intrusion and corrosion in bolted ball joints arising from installation gaps, this paper explores the utilization of sealing washers to fill these installation gaps in bolted ball joints to seal the joint. Sealing washers are commonly used components in sealing applications, and their function is to occupy the spaces between two contacting surfaces, thus forming a sealing interface to prevent the permeation or leakage of gases or liquids between these surfaces.

For achieving effective joint sealing, sealing washers must be crafted from materials exhibiting commendable compressibility and resilience. Additionally, in consonance with the operational environment and objectives of the bolted ball joint, these sealing washers should also possess attributes like corrosion resistance and longevity. Common sealing materials encompass rubber, silicone, and polytetrafluoroethylene. Among these options, Nitrile Butadiene Rubber (NBR), a prominent synthetic rubber in the sealing realm, emerges as a prime contender. Noteworthy for its exceptional qualities including heightened wear resistance, robust adhesion, impressive heat resistance, resilience against aging, and superior airtightness [11,12], NBR squarely meets the sealing requirements of the bolted ball joint. Consequently, this paper selects NBR as the designated material for the sealing gasket.

The enhanced bolted ball joint featuring a sealing structure is depicted in Figure 3. This configuration primarily comprises a bolt ball, high-strength bolt, sleeves, pin, cone head, and sealing washer. Diverging from conventional bolted ball joints, the innovation introduced in bolted ball joints with sealed structures centers around the incorporation of circular grooves and sealing washers at both ends of the sleeve. These sealing washers are securely embedded within the circular grooves to ensure precise alignment and steadfastness during installation. Upon the tightening of the high-strength bolt, the sealing washer undergoes compression. This ingenious design effectively engenders a sealing interface within the contact gap between the sleeve and bolt ball and the cone head, thereby thwarting the infiltration of seawater into the joint’s interior. During the installation process, the prescribed procedure entails initially positioning the sealing washer within the circular groove at each end of the sleeve. Subsequently, the sleeve and sealing washer are collectively inserted into the high-strength bolt. The subsequent step involves threading the fastening screw into the pinhole to facilitate the bolt’s insertion into the threaded hole until the predefined depth is attained.

3. Numerical Simulation

3.1. Guidelines for Sealing Evaluation

Based on the inherent nature and sealing principles of NBR materials, the contact pressure between the sealing cushion and the corresponding component’s contact surface plays a pivotal role in determining the sealing efficacy of the overall structure. To establish a dependable seal, the contact pressure must surpass the pressure exerted by the sealing medium while also maintaining a specific contact length. Consequently, the criteria used to assess the structural sealing are selected as the guiding standards for evaluating the effectiveness of the sealing structure [13,14]. The criteria can be expressed as a formula:

$${\mathrm{P}}_{\mathrm{c}}\ge {\mathrm{P}}_{\mathrm{m}}$$

(1)

where P_c is the effective contact pressure on the sealing contact surface, and P_m is the pressure of the gas filled inside the joint.

3.2. The Numerical Model

3.2.1. Geometric Modeling

Firstly, the joint model undergoes simplification by transforming the outer section of the sleeve from a hexagonal shape to a circular one, and the resultant simplified joint model is depicted in Figure 4a. The focal point of this study is the contact region of the seal. The analysis assumes the sleeve’s centerline as the axis of symmetry, rendering the geometry and boundary conditions of the simplified nodal model axisymmetric. Consequently, a two-dimensional axisymmetric model is employed for analysis. Due to symmetry considerations, one of the seals at the sleeve’s ends is selected for detailed analysis. The specific process for creating the model is as follows: Firstly, an assembly model of the bolted ball joint with the sealing structure is constructed using SOLIDWORKS. Subsequently, the stretch excision command is employed to derive a 1/4 model of the component under analysis. With the use of the isometric surface command, cross-sections of each component within the joint model are replicated. Subsequently, the joint entity is removed, yielding a two-dimensional axisymmetric geometric model of the joint, as depicted in Figure 4d. Finally, the completed joint model file is imported into ANSYS for further processing. The finite element model (FEM), post-processing, is shown in Figure 4e, and the processing procedure is detailed in subsequent Section 3.2.2, Section 3.2.3, Section 3.2.4 and Section 3.2.5

3.2.2. Material Constitutive Relationship

During the finite element analysis process, the components including high-strength bolts, sleeves, and cone heads are constructed using Q355 steel. Due to their minimal deformations, these components are treated as either rigid or elastic, possessing an elastic modulus of 206 GPa and a Poisson’s rate of 0.3. On the other hand, the sealing ring is fashioned from NBR material, with its constitutive relationship defined through the utilization of the Mooney Rivlin 2 parameter model [15,16]. The corresponding strain energy density function is provided below:

$$\mathrm{W}={\mathrm{C}}_{10}\left({\mathrm{I}}_1-3\right)+{\mathrm{C}}_{01}\left({\mathrm{I}}_2-3\right)$$

(2)

where W is the strain energy, C₁₀ and C₀₁ are material-related constants, and I is the principal strain in the principal direction. The constants for C₁₀ and C₀₁ in this article are 1.87 and 0.47, respectively [11].

3.2.3. Meshing

During the finite element analysis process, the PLANE182 planar element is employed for simulation. The PLANE182 element, a quadrilateral 4-node element, possesses the capability for plasticity, hyperelasticity, stress stiffness, large deformation, and extensive strain. This enables it to provide more accurate simulations of the deformation of incompressible hyperelastic materials. Figure 4e illustrates the mesh division of the finite element model. In this diagram, the rubber ring is assigned a mesh size of 0.2 mm, while the remaining components are given a mesh size of 0.5 mm. The division results in a total of 13,456 nodes and 4292 elements. Increasing the mesh count by a factor of 1 beyond this configuration yields calculation results with an error margin below 0.45%. However, this enhancement comes at the expense of significantly increased computation time. Consequently, the mesh configuration employed in this study strikes a balance between computational efficiency and accuracy, rendering it reasonable and capable of yielding satisfactory calculation precision.

3.2.4. Contact Setting

Considering the interaction between the components of the sealing structure, the contact of the bolt with the cone head and the sleeve is set as frictional contact, and the coefficient of friction is set as 0.15. The contact of the sealing washer with the sleeve and cone head is set as frictional contact, and the coefficient of friction is set as 0.2 [15].

3.2.5. Loads and Boundary Conditions

The sleeve’s left and upper boundaries are constrained in both X and Y directions, and constraints are imposed in the Y direction on the bolts and cone heads. With the use of the displacement loading mode, a displacement is applied in the X direction to the right boundary of the bolt, as visually illustrated in Figure 4e.

3.3. Numerical Simulation Results

To ensure the attainment of effective sealing performance, the typical compression rate for rectangular seals falls between 8% and 14% [17]. For the scope of this study, the model’s compression spans from 0.16 mm to 0.28 mm. Leftward displacements of 0.16 mm, 0.22 mm, and 0.28 mm are then applied to the bolt’s tail end to simulate corresponding seal compressions during the tightening process of the bolted ball joint. Subsequently, the deformation cloud diagrams of the sealing washer, the equivalent force cloud diagram, and the contact pressure cloud diagram are depicted in Figure 5, Figure 6 and Figure 7.

Based on the analysis of the deformation of the cloud map, it becomes evident that the deformation distribution of the sealing ring maintains considerable uniformity. With the gradual increase in the compression rate, the extent of the sealing ring’s deformation also amplifies. It is noteworthy that the most pronounced deformation regions are localized at the upper and lower extremities of the contact surface between the sealing ring and the cone head. This phenomenon is attributed to the progressive filling of the contact gap by the sealing ring during the compression, leading to a more notable deformation in the upper and lower portions of the contact surface. This phenomenon plays a pivotal role in the sealing process, ensuring the efficacy and stability of the seal’s performance.

The stress cloud diagrams reveal that the equivalent stress level of the seal escalates as the compression increases. Notably, the stress values at the upper and lower extremities of the contact surface between the seal and the cone head are notably higher, exhibiting a concentration of stress. This observation aligns with the findings presented in the deformation diagrams.

Based on the analysis of the contact pressure cloud diagram of the sealing ring, it becomes evident that the principal sealing effect manifests on the contact surfaces located on the left and right sides of the sealing ring. Notably, the sealing performance is dictated by the smaller of these two contact surfaces. Upon data extraction, the correlation between the contact pressure on the left and right sides of the sealing surface and the actual path distance under varying compression levels is established. This relationship is visually depicted in Figure 8.

As depicted in Figure 8, it becomes evident that the seal’s contact pressure is most pronounced within the central region. This central section, spanning a length of approximately 3 mm and constituting 75% of the contact area, assumes the primary role in effecting sealing. With an increase in the compression of the sealing ring, the contact pressure on the sealing surface correspondingly escalates, and their relationship exhibits an approximate linearity. At 14% compression, the sealing gasket attains a sealing pressure of 5.95 MPa. This outcome substantiates the efficacy of the bolted ball joint’s sealing structure within high-pressure contexts. Moreover, it underscores the sealing structure’s commendable sealing performance, thereby affirming its viability in marine environments for safeguarding bolted ball joint mesh structures against corrosion.

4. Experimental Testing

4.1. Experimental Design

The chosen experimental approach employs the bubble leakage detection method. This involves introducing a specific pressure of leakage-indicating gas into the test specimen immersed in liquid. This process generates a pressure differential between the interior and exterior of the airtight cavity. If there exists a gap causing leakage within the sealing cavity, the leakage-indicating gas will flow from the higher-pressure side to the lower-pressure side through the gap. This action results in the formation of bubbles around the leakage gap. This visual occurrence aids in ascertaining the presence of a leakage phenomenon within the airtight cavity [18]. This method offers a swift and intuitive evaluation of the specimen’s sealing quality with heightened sensitivity. In this test, air serves as the leakage-indicating gas, while water functions as the displaying liquid.

The experiment utilizes bolted ball joint specimens with the specifications and dimensions depicted in Figure 3. The components of the bolted ball joint are made from Q235 steel, and the sealing ring is crafted from NBR material. To facilitate the inflation of the bolted ball joint’s internal pressure, a sealing plate is affixed to the end of the member, accompanied by the installation of an inflatable valve, as depicted in Figure 9a. Subsequently, an air compressor pump is engaged, connected to the member’s interior to instate inflation and pressurization. This process serves the purpose of assessing the sealing pressure and detecting any potential leaks within the sealing structure. To ensure heightened reliability in the test outcomes, simultaneous pressure experiments are conducted on the four members. The essential instruments and equipment for this experiment include an air compressor, a pressure gauge, a vernier caliper, and a torque spanner. Specifically, the air compressor delivers high-pressure air inside the sealing rod, utilizing the XSZG-30A model screw air compressor by the XINLEI brand. This model consistently generates high-pressure air, reaching up to 0.8 MPa. The pressure gauge displays the air pressure output, with the experiment utilizing a pressure gauge with a range of 1.6 MPa and an accuracy rating of 1.6, ensuring precise measurement of the air pressure value. The vernier caliper measures the distance between the cone head and the bolt ball, enabling the calculation of the compression rate of the seal. In this experiment, digital display vernier calipers with a range of 0–150 mm guarantee accurate measurements. Lastly, the torque spanner plays a crucial role in tightening the bolted ball joint in accordance with specified values. The experiment employs a torque spanner with a range of 50–1000 N·m, a divisional value of 0.1, and a precision level of ±2% as indicated by the numerical display on the torque spanner. The experimental test principle and necessary equipment are elaborated in Figure 9b and

Table 1. Instrument and equipment for testing the sealing performance.

Device Name	Model	Amount	Performance Parameters
Air Compressor	XSZG-30A	1	Maximum output pressure 0.8 MPa (8 atmospheric pressure)
Pressure gauge	Y-100	1	Range 1.6 MPa, accuracy 1.6 grade
Dial calipers	LJ800-001	1	Range 0–150 mm
Torque spanner	WT8-30	1	Torque range 50–1000 N-m, graduation value 0.1, accuracy ±2%

, respectively.

4.2. Experiment Process

The initial step involves the assembly of the components of the bolted ball joint, with the application of a bolt torque during assembly adhering to the specified standard value of 137 N·m [19]. Subsequently, the distance between the cone head and the bolt ball is measured using vernier calipers, yielding a measurement of 36.50 mm, and the average compression rate of the two seals is determined to be 12.57%. Following this, the inflatable device is connected according to the test scheme and principles illustrated in Figure 9. After the connection is completed, the specimen is fully immersed in water. The air compressor is then activated, initiating a step-by-step increase in high-pressure air output, with 0.1 MPa increments. Each pressure level is maintained for 5 min, with an inspection for any signs of leakage at the seal. If no abnormalities are observed, the pressure is continuously raised until a leakage is detected or the maximum output pressure of the air compressor pump is reached.

Furthermore, upon immersing the test piece in water, bubbles might emerge at the sealing point. If these bubbles do not reappear after wiping or poking, it can be inferred that the initial bubble formation site is devoid of leakage apertures. Conversely, a consistent and persistent emission of bubbles would indicate the presence of a leakage hole at the point of bubble origination [18].

4.3. Experimental Results and Analysis

Two sets of nodal models were subjected to experimentation: the conventional bolted ball joint and the modified bolted ball joint. During the initial inflation stage of the conventional bolted ball joint, air bubbles emerged, and their quantity increased with rising inflation pressure, as depicted in Figure 10a. Notably, the conventional bolted ball joint exhibited no sealing properties whatsoever. In contrast, the modified bolted ball joint displayed impeccable performance, exhibiting no leakage and maintaining effective sealing throughout the initial inflation and pressurization stages until it reached the maximum inflation pressure of 0.8 MPa from the air compressor, as illustrated in Figure 10b.

Owing to power limitations in the employed inflation equipment, the experimental gas output pressure was constrained to 0.8 MPa—a water pressure value equivalent to a depth of 81.63 m in seawater. In practical applications of deep-sea fish cages, a typical depth of around 20 m is employed. This result significantly bolsters the groundwork for implementing bolted ball joints in offshore environments, ensuring their dependable adaptation to the challenges presented by seawater conditions.

Additionally, the sealing method lacks experimental validation under higher gas pressure due to the limitations of the bubble leak detection method and the specific inflation equipment used. Future research should focus on conducting refined experiments to confirm the sealing method’s efficacy and validate its compatibility with the numerical simulation approach. It is worth noting that this study was conducted as a preliminary phase of a larger project focusing on deep-sea fish-farming nets utilizing the bolted ball joint.

5. Conclusions

This paper introduces a methodology to enhance the sealing performance of bolted ball joints, offering a solution for the corrosion protection of grid structures with bolted ball joints in marine environments. Numerical simulation analyses and sealing performance tests were conducted on the bolted ball joint equipped with the sealing structure. The primary conclusions are outlined below:

(1)

According to the numerical simulation results, it is evident that the sealing pressure rises proportionally with the augmentation of the compression rate, illustrating a clear linear correlation between the two factors. The sealing gasket achieves impressive sealing pressures of 2.91 MPa, 4.22 MPa, and 5.95 MPa at compression rates of 8%, 11%, and 14%, respectively.

(2)

Concerning the NBR seals of the specifications employed in this study, the higher contact pressure is predominantly concentrated in the intermediate region. The primary sealing function is effectively performed by the central area of the contact surface, encompassing a length of approximately 3 mm and accounting for 75% of the total contact area.

(3)

Airtightness testing experiments were conducted on both the conventional and modified bolted ball joints using the bubble leakage detection method. The outcomes revealed that the conventional bolted ball joint exhibited no sealing whatsoever, while the bolted ball joint incorporating the sealing structure demonstrated impeccable sealing performance.

(4)

The sealing method for bolted ball joints provides a cost-effective solution with simple installation and maintenance. Retaining the traditional appearance of the joint, it guarantees impeccable internal sealing. This prevents seawater intrusion and corrosion, aligning perfectly with the application demands of the bolted ball joint in the realm of marine engineering.

(5)

The sealing method facilitates the successful adaptation of steel grid structures, extensively employed in terrestrial settings, for utilization in marine engineering applications, encompassing offshore floating platforms and deep-sea fish cages. Analytical and computational findings underscore that this approach effectively ensures optimal sealing performance even at depths of up to 607.14 m in seawater.