A Thesis for the Degree of Doctor of Engineering
Study on Design and Optimization of
a Novel Flat-Face Quick Multi Coupling System for
Hydraulic Power Equipment
By
Yuting Wu
Department of Mechanical and Aerospace Engineering
Graduate School
GYEONGSANG NATIONAL UNIVERSITY
August, 2021
Study on Design and Optimization of
a Novel Flat-Face Quick Multi Coupling System for
Hydraulic Power Equipment
A Thesis Submitted to the Faculty of the Graduate School
of the Gyeongsang National University
By Yuting Wu
In partial fulfillment of the requirements
for the degree
of Ph.D. of Engineering
August, 2021
Dr. Sung-Ki Lyu, Thesis Supervisor
Approved by the Committees of the Graduate School
of Gyeongsang National University in partial fulfillment
of the requirements for the degree of Ph.D. of Engineering
Thesis Committee:
Jungwon Yoon
Chairperson
Su-Jin Kim
Do-Yeong Lee
Long Lu
Sung-Ki Lyu
(Name
Date:
and
2021.
08.
signature)
25
School of Mechanical and Aerospace Engineering
GRADUATE SCHOOL
GYEONGSANG NATIONAL UNIVERSITY
TABLE OF CONTENTS
List of tables............................................................................................................................. i
List of figures .......................................................................................................................... ii
Nomenclature ......................................................................................................................... v
ABSTRACT IN KOREAN ................................................................................................... vi
ABSTRACT IN ENGLISH ................................................................................................ viii
CHAPTER 1 Introduction .................................................................................................... 1
1.1 Background .............................................................................................................. 1
1.2 Research method ...................................................................................................... 6
1.3 Research objective ................................................................................................... 8
CHAPTER 2 Mechanism Design of Flat-Face Coupling Systems ................................... 10
2.1 Mechanism and kinematics of CFFCS .................................................................. 10
2.2 Mechanism design of NFFCS ................................................................................ 14
2.3 Chapter summary ................................................................................................... 20
CHAPTER 3 Numerical Investigation of FFCS ................................................................ 21
3.1 Characteristics of valve opening ............................................................................ 21
3.1.1 Investigation methods and boundary conditions ............................................ 21
3.1.2 Results and discussion.................................................................................... 24
3.2 Flow characteristic of FFCSs ................................................................................. 34
3.2.1 Investigation methods and boundary conditions ............................................ 34
3.2.2 Results and discussion.................................................................................... 37
3.3 Chapter summary ................................................................................................... 39
CHAPTER 4 Performance Improvement and Experiment on NFFCS .......................... 40
4.1 Performance improvement of NFFCS ................................................................... 40
4.2 Structural stability investigation of NFFCS........................................................... 45
4.2.1 Numerical method .......................................................................................... 45
4.2.2 Numerical results ........................................................................................... 49
4.3 High pressure burst experiment ............................................................................. 63
4.4 Chapter summary ................................................................................................... 69
CHAPTER 5 Design and Structure Analysis on Multi-coupling Drive Part .................. 71
5.1 Design and structure analysis of 4-circuit hydraulic quick action multi-coupling 71
5.1.1 Mechanism of 4-circuit hydraulic quick action multi-coupling ..................... 72
5.1.2 Analysis conditions of 4-circuit hydraulic quick action multi-coupling ........ 77
5.1.3 Structural analysis of the 4-circuit hydraulic quick action multi-coupling .... 79
5.2 Design and structure analysis of 6-circuit hydraulic quick action multi-coupling 82
5.2.1 Mechanism of 6-circuit hydraulic quick action multi-coupling ..................... 82
5.2.2 Analysis conditions of 6-circuit hydraulic quick action multi-coupling ........ 90
5.2.3 Structural analysis of the 6-circuit hydraulic quick action multi-coupling .... 93
5.3 Chapter summary ................................................................................................... 98
Conclusions ......................................................................................................................... 100
References ........................................................................................................................... 103
Acknowledgement .............................................................................................................. 107
LIST OF TABLES
Table 4.1 The geometrical design modifications made in the initial design to obtain the
modified design .................................................................................................................... 42
Table 4.2 Comparison of the turbulent kinetic energy distribution results for the initial and
modified designs ................................................................................................................... 43
i
LIST OF FIGURES
Fig. 1.1 The applications of hydraulic power transmission technology .................................. 2
Fig. 1.2 The section 3D of the quick disconnect coupling assembly ...................................... 2
Fig. 1.3 The kinds of hydraulic quick coupling ...................................................................... 4
Fig. 1.4 The multi-function quick connector .......................................................................... 5
Fig. 1.5 Comparisons of CFD results with experiments ......................................................... 7
Fig. 2.1 Internal mechanism of the CFFCS........................................................................... 12
Fig. 2.2 Kinematics of the CFFCS ........................................................................................ 12
Fig. 2.3 Theoretical modeling of the different processes of the CFFCS ............................... 13
Fig. 2.4 Internal mechanism of the NFFCS .......................................................................... 14
Fig. 2.5 Kinematics of the NFFCS ....................................................................................... 17
Fig. 2.6 Kinematics of the NFFCS ........................................................................................ 19
Fig. 3.1 Meshing of the two models ...................................................................................... 23
Fig. 3.2 Instantaneous pressure distributions at different times during the coupling process of
the CFFCS ............................................................................................................................. 25
Fig. 3.3 Instantaneous velocity distributions at different times during the coupling process of
the CFFCS ............................................................................................................................. 26
Fig. 3.4 Instantaneous pressure distributions at different times during the coupling process of
the NFFCS............................................................................................................................. 29
Fig. 3.5 Instantaneous velocity distributions at different times during the coupling process of
the NFFCS............................................................................................................................. 31
Fig. 3.6 Comparison of the valve opening forces required by the CFFCS and NFFCS ....... 33
Fig. 3.7 Meshing and inlet boundary conditions ................................................................... 36
Fig. 3.8 Pressure distribution results for the two flat-face coupling systems ........................ 37
ii
Fig. 3.9 Velocity and streamline distributions for the two flat-face coupling systems ......... 38
Fig. 4.1 The turbulent kinetic energy distribution of the initial NFFCS design.................... 40
Fig. 4.2 Flow characteristics of the modified NFFCS design ............................................... 44
Fig. 4.3 Classification of fluid structure as a combination of fluid and solid domain .......... 46
Fig. 4.4 The mesh of the couplings in structural analysis ..................................................... 47
Fig. 4.5 The boundary conditions of the couplings in structural analysis ............................. 48
Fig. 4.6 The section view of the total deformation of the couplings..................................... 50
Fig. 4.7 The internal structure of the total deformation of the couplings.............................. 51
Fig. 4.8 The pressure distribution of the couplings ............................................................... 52
Fig. 4.9 The velocity distribution of the couplings ............................................................... 53
Fig. 4.10 The section view of the stress of the couplings ..................................................... 55
Fig. 4.11 The internal structure of the stress of the couplings .............................................. 56
Fig. 4.12 The equivalent elastic strain of the couplings ........................................................ 58
Fig. 4.13 The position of the maximum equivalent elastic strain in the couplings ............... 59
Fig. 4.14 The internal structure of the safety factor of the couplings ................................... 61
Fig. 4.15 The position of the minimum safety factor of the couplings ................................. 62
Fig. 4.16 The prototype of the modified NFFCS .................................................................. 64
Fig. 4.17 The experimental device: experimental ................................................................. 67
Fig. 4.18 The burst pressure of 5 NFFCS samples in experiment ........................................ 68
Fig. 5.1 The modeling of 4-circuit hydraulic quick coupling equipment.............................. 72
Fig. 5.2 The Schematic diagram of Pythagorean Theorem ................................................... 74
Fig. 5.3 The connection process of prototype ....................................................................... 76
Fig. 5.4 The 3D modeling of the 4-circuit hydraulic quick coupling equipment .................. 77
Fig. 5.5 The mesh of the 4-circuit hydraulic quick coupling equipment .............................. 78
Fig. 5.6 The boundary conditions of the 4-circuit hydraulic quick coupling equipment ...... 78
Fig. 5.7 The structural analysis results of the 4-circuit hydraulic quick coupling equipment ......... 81
Fig. 5.8 The modeling of 6-circuit hydraulic quick coupling equipment.............................. 82
iii
Fig. 5.9 The design of the locked function............................................................................ 83
Fig. 5.10 The coupling process of guide pin ......................................................................... 84
Fig. 5.11 The principle of the locking module ...................................................................... 87
Fig. 5.12 The prototype of 6-circuit hydraulic quick coupling equipment ........................... 88
Fig. 5.13 The connecting process of prototype ..................................................................... 89
Fig. 5.14 The mesh of the 6-circuit hydraulic quick coupling equipment ............................ 91
Fig. 5.15 The boundary conditions of the 6-circuit hydraulic quick coupling equipment .... 92
Fig. 5.16 The structural results of the 6-circuit hydraulic quick coupling equipment .......... 97
iv
NOMENCLATURE
V
Arbitrary control volume
Φ
General scalar
⃗
v
Speed vector, m/s
vg
⃗⃗⃗
Grid velocity of the moving mesh
ρ
Fluid density, kg/m3
Γ
Diffusion coefficient
S∅
Source term of ∅
u
Velocity, m/s
p
Pressure, Pa
μ
Viscosity, Pa·s
f
Force density
τ
Force, N
d
Displacement, m
q
Heat flow, J
T
Temperature, K
R
Radius of the center circle, m
a
The distance from point A to the lever rotating center, m
b
The distance from the lever rotating center to the guide pin 2 located in B, m
NFFCS
Novel flat-face coupling system
CFFCS
Conventional flat-face coupling system
v
ABSTRACT IN KOREAN
고유압장비용 노벨 평면접촉식 퀵 멀티 커플링 시스템의 설계 및
최적화에 관한 연구
우위팅
기계항공공학부 기계설계학전공
경상대학교 대학원
지도교수: 류 성 기
유압 커플링 시스템은 유압 동력 전달 장비에서 중요한 역할을 하고 있다. 평면
커플링 시스템은 친환경 특성으로 최근 몇 년 동안 광범위하게 연구되었다. 유압
장비의 필요한 작동 압력이 증가함에 따라 연결 프로세스가 점점 어려워진다. 고압에서
편리한 연결을 위해 본 논문에서 노벨 평면접촉식 커플링 시스템을 제안하게 된다.
vi
기존의 평면 커플링 시스템을 기반으로 노벨 설계에서는 순간 압력 제거 모듈을
추가하여 커플링 시 높은 유압으로 인한 저항을 대폭 감소 시키게 된다.
본 연구에서는 노벨 설계의 작동 메커니즘을 설명하기 위해 시스템 운동학의 이론적
모델을 설정하고, 노벨 동적 메쉬 기술과 ANSYS Mosaic 메쉬 기술을 기반으로 한
일련의 전산 유체 역학 수치 조사를 구현하여 설계의 합리성을 확인하게 되었다. 유압
에너지 전달 과정 중 에너지 손실이 최소화 시키기 위해 노벨 평면접촉식 커플링
시스템의 최적 설계가 탐구하게 되었다.
노벨 편면접촉시 커플링 최적된 설계를 바탕으로 유체와 구조 결합된 수치 연구
방법과 실험적 검증 방법을 활용하여 CFFCS 와 NFFCS 시스템의 작업 안전성 및
구조적인 안정성을 연구하고 비교하게 되었다. 또한 NFFCS 기술을 기반으로 한 4
회로 및 6 회로 유압 퀵 액션 멀티 커플링 구동부 설계를 제안하게 됬어다. 빠르고
안전한 연결의 기구학 원리를 설명하게 되었다. 유한요소법을 기반으로 한 일련의 구조
안정성 시뮬레이션을 수행하고 안전성과 신뢰성에 대해 평가하였다.
vii
ABSTRACT IN ENGLISH
Study on Design and Optimization of a Novel Flat-Face Quick Multi
Coupling System for Hydraulic Power Equipment
Yuting Wu
Dep. of Mechanical Design and Production Engineering
Graduate School of Gyeongsang National University
Supervised by Professor Sung-Ki Lyu
The coupling system plays a vital role in hydraulic power transmission equipment. Flat-face
coupling systems have been extensively studied in recent years due to their environmental
friendly features. As the working pressure of hydraulic equipment is increased, their
connection process becomes difficult. To improve the convenience of making high-pressure
connections, a novel flat-face coupling system is proposed in this article. In the proposed
design, which is based on the conventional flat-face coupling system, the resistance caused by
high hydraulic fluid pressure during coupling is drastically reduced by adding an instantaneous
pressure relief module.
In this study, the theoretical model of the system kinetics is established to illustrate the
operational mechanism of the novel design, and a series of computational fluid dynamics
viii
numerical investigations based on the novel dynamic mesh technology and ANSYS Mosaic
meshing technology are implemented to verify the rationality of the proposed design.
Additionally, an optimal design of the novel flat-face coupling system is proposed to reduce
the energy loss during hydraulic power transmission.
On the basis of this design, based on the fluid-structure coupling numerical research method
and experimental verification method, the construction safety and stability of the NFFCS
system are studied, and the safety level of construction stability is compared with that of
CFFCS. In addition, the mechanical design of 4-circuit and 6-circuit hydraulic quick action
multi-coupling drive part based on NFFCS technology is also proposed, and the kinematics
principle of its rapid and safe connection is explained. A series of structural stability
simulations based on finite element method were implement, and the safety and reliability
were discussed.
ix
CHAPTER 1
INTRODUCTION
1.1 Background
Hydraulic power transmission technology has seen continuous development since the
construction of the first hydraulic press in 1795. Relying on their reliability, high precision,
rapid response and large driving force, hydraulic equipment are widely used in aerospace,
automobiles, agricultural machinery, petroleum industry, military machinery, manufacturing
and other fields[1-7], as shown in Fig.1.1.
Due to the work requirements of hydraulic power systems, the existence of hydraulic oil
pollution, leakage, corrosion and other hidden dangers is inevitable [8]. Therefore, a numerous
scholars have been working to improve the work performance, safety, environmental
protection, etc. of hydraulic systems. Sakaino et al.
[9]
proposed a novel EHA (electro-
hydrostatic actuators) design to monitor the oil leakage and static friction, and empirically
verified their proposed model. Rinneberg et al
[10]
proposed an accessorial identification
system for intelligent equipment based on the passive radio frequency sensing technology.
This research provides a reliable alternative for equipment operators. Wang et al. [11] proposed
a new quick coupling system with a “points to points” mechanism as shown in Fig. 1.2. This
design improves the reliability of the quick coupling system under blind operation.
1
Fig. 1.1 The applications of hydraulic power transmission technology
Fig.1.2 The section 3D of the quick disconnect coupling assembly
2
In the hydraulic power system, the hydraulic energy is transmitted and controlled by the
pressure difference of the fluid. Hydraulic quick couplings are an essential element in
hydraulic power systems as they can connect or disconnect the high-pressure hydraulic circuits
flexibly and quickly without the need for extra tools or special equipment [12].
According to the shape of the connector, hydraulic quick couplings can be divided into three
main categories as shown in Fig.1.3: (a) poppet type, (b) spherical type, (c) flat-face type [1315]
. Compared with poppet type and spherical type couplings, flat-face type is a recent
development that can minimize the leakage of hydraulic fluid during connection and
disconnection to reduce the environmental pollution. On the other hand, the complicated
geometry of the flat-face connection system leads to a higher energy consumption during
hydraulic transmission. In order to improve this pressure loss problem in the hydraulic power
system, a series of studies and optimization schemes have been proposed [16-20].
(a) Poppet type
(b) Spherical type
3
(c) Flat-face type
Fig.1.3 The kinds of hydraulic quick coupling
Hydraulic quick coupling is a kind of equipment that can realize quick assembly and
disassembly of pipelines without the use of tools, which is one of the important equipment for
mechanical equipment to save time and effort
[12]
. In the manufacturing process, both socket
and plug must be hardened to make them have the characteristics of pressure resistance and
fatigue resistance. For a single hydraulic quick coupling, the finite element method is generally
used to study its mechanical characteristics under working conditions
[21-23]
. The results of
numerical simulation can analyze the mechanical characteristics of a single hydraulic quick
coupling such as stress, strain, deformation and safety factor. In the industry, this analysis
method is often used to evaluate the safety of hydraulic quick couplings, and it also provides
an important basis for the optimization of hydraulic quick couplings [24]. Wang et al. used the
finite element method to analyze the performance of underwater multi-functional quick
couplings which shows in Fig.1.4, and proved that the maximum stress of the elastic chuck
was less than the yield strength of the material, and analyzed the thrust and tension required
for connection and disconnection
[25]
. Zhang et al. used the finite element method to analyze
the strength of the quick coupling connection of platform conductors, and concluded that the
restraining effect of the conductor is mainly provided by the conductor on the side under
tension
[26]
. However, in a real working environment, a single hydraulic quick coupling will
be impacted by the instability of high-speed hydraulic oil. Therefore, the hydraulic coupling
4
is inevitably affected by the flow load. The finite element method has limitations in analyzing
hydraulic quick couplings under static working conditions. Although many hydraulic quick
couplings have been widely used in industry and have a certain understanding of their fluid
properties [27-30], the understanding of mechanical properties is still insufficient.
With the development of advanced technologies such as smart factories and multifunctional
machinery, the design of hydraulic machinery has also become diversified. It is often
necessary to use the power transmission of multiple hydraulic circuits to build a
multifunctional hydraulic machine. Therefore, a single hydraulic circuit can no longer meet
the needs of modern hydraulic machinery design. In multi-circuit hydraulic machinery,
individual hydraulic quick couplings need to be connected one by one, which is timeconsuming and laborious, and cannot meet the requirements of simultaneous connection.
Thence, hydraulic multi-circuit quick coupling drive part is widely used by virtue of its timesaving, labor-saving, synchronization, and non-dislocation connection.
Fig.1.4 The multi-function quick connector
5
1.2 Research method
In the previous section, an overview of the categories and characteristics of traditional
hydraulic quick couplings and the different research methods for design stage were carried out.
It can be found that with the development of computer technology, the calculation accuracy
and time-consuming of simulation method have been greatly optimized. With its rapid
development, simulation method has gradually replaced the cumbersome and cost-consuming
experimental methods in the design stage of hydraulic products. Among the many methods to
solve the problems of hydraulic machinery, CFD (Computational Fluid Dynamics) method
based on the FVM (Finite Volume Method) is widely used in the development of various fluid
machinery. In this research, CFD method was applied to observe and analyze the flow
characteristics of CFFCS and NFFCS. To verify the correctness of the numerical investigation,
an in-depth study on the research of Kim et al. who used the same calculation method was
conducted in this section[31].
In the study of Kim et al., a series of flow simulations based on commercial software
ANSYS fluent were implemented to verify and optimize the hydraulic power transmission
performance of a 1/2 inch poppet type hydraulic quick coupling[31]. Modified Bernoulli
Equation is used to estimate and verify the correctness and feasibility of the simulation results.
All practical fluids are viscous and offer resistance to fluid flow. So that there is some losses
in fluid flow between two section. Bernoulli’s equation was derives on the assumption that
fluid is non-viscous i.e. frictionless, which is not applicable of practical fluid and hence
Bernoulli’s equation is modified by considering losses. Modified Bernoulli’s equation for
hydraulic fluid can be organized into the following forms.
𝑃1⁄
𝑉1
𝑃2
𝑉2
𝛾 + ⁄2𝑔 = ⁄𝛾 + ⁄2𝑔 + ℎ𝐿
where ℎ𝐿 = loss of energy between inlet and outlet of the objective coupling.
6
(1-1)
In Kim’s research, a numerical simulation of poppet coupling is carried out by using Fluent.
2,300,000 grid nodes were constructed using the traditional grid division method. In the
simulation, VG32 is used as a flowing medium, and its density and dynamic viscosity are 860
𝑘𝑔/𝑚3 and 3.698 𝑁 ∙ 𝑆/𝑚2 , respectively. In the setting of boundary conditions, the inlet flow
is set as 3gpm, 6gpm, 9gpm, 12gpm, 15gpm and 18gpm, and the outlet is set to atmospheric
pressure. To verify the correctness of the numerical investigation, in this research,
experimental method is also carried out. Five groups of comparative verification were carried
out on the inlet flow rates same as the inlet setting value of CFD simulation.
The comparison of the pressure drop results obtained by CFD simulation and experimental
methods is shown in Fig 1.5. The results show that the cross appears at a flow rate of 15gpm
and the graphics remain basically similar. The reason for the nuance can be attributed to
simplification of the coupling geometry such as removing gaps or flattening the part with
springs in the actual model. The simulation results are basically consistent with the
experimental results, which shows that the accuracy and feasibility of the simulation meet the
requirements. Based on this research, in this work, the same simulation method as Kim et al.
will be utilized to ensure the correctness of the research conclusions.
Fig.1.5 Comparisons of CFD results with experiments
7
1.3 Research objective
With the development of industry, the working pressure required in hydraulic power
systems has been increased
[32]
. This increase in the working pressure of the system also
increases the difficulty of the coupling process of hydraulic circuits. A variety of methods have
been proposed to improve the convenience of the coupling process under high operating
pressures. However, it has been difficult to greatly improve the conventional flat-face coupling
system (CFFCS) due to the size limitations of hydraulic circuits.
In this research, a novel flat-face coupling system (NFFCS) design with an instantaneous
pressure relief based on Pascal's law is proposed. And through a series of numerical studies,
the flow characteristics are compared by the 10mm CFFCS and NFFCS.
The instantaneous pressure relief effect of the proposed NFFCS design is verified through
transient simulation results based on the novel dynamic mesh analysis method. In order to
reduce the energy loss during hydraulic power transmission, the optimization of the hydraulic
circuit flow characteristics of NFFCS is implemented based on ANSYS Mosaic meshing
technology and CFD simulation technology. Finally, the optimal geometric design is
determined through comparison of the investigation results.
In addition, the numerical investigation on the structural safety and stability of the optimal
NFFCS will be carried out by means of the fluid-structure interaction method. The mechanical
characteristics such as deformation, internal stress, strain, safety factor and other mechanical
characteristics of the CFFCS and NFFCS are analyzed and compared when subjected to high
pressure. It shows the rationality of the design of NFFCS.
Moreover, through experimental methods, multiple sets of high-pressure blasting
experiments were carried out on NFFCS samples, which verified the correctness of the
simulation results. After verifying the structural safety and stability of a single NFFCS, two
different designs of hydraulic multi-circuit hydraulic quick coupling drive part with 4-circuit
and 6-circuit are proposed, and the kinematic characteristics of the mechanism are analyzed in
8
detail. Furthermore, the finite element method is used to study the structural stability
characteristics of the design.
In summary, the goal in this study is to propose a novel flat-face coupling system based on
the principle of mechanical design. Combined with numerical simulation and experimental
methods, the proposed design is optimized and verified. On the basis of satisfying various
safety performances, the NFFCS is applied to the 4-circuit and 6-circuit hydraulic quick action
couplings.
9
CHAPTER 2
MECHANISM DESIGN OF FLAT-FACE COUPLING
SYSTEMS
2.1 Mechanism and kinematics of CFFCS
There are many types of quick connectors for hydraulic power transmission systems. The
traditional poppet type and spherical type quick connectors make the hydraulic oil converge
in the gap between the valve core and the coupling glove due to the irregular geometry and
uneven contact surface. During the disconnection process of the coupling system, this
hydraulic oil flows out into the environment and causes pollution. In order to overcome this
problem, environmentally friendly flat-face type coupling systems have been proposed, which
have been widely used and recognized. In recent years, with the increase of operating pressures
in hydraulic power transmission systems, the connecting characteristics of CFFCS are no
longer sufficient to meet the requirements due to high coupling resistance. In order to make
the coupling process more convenient, a NFFCS with an instantaneous pressure relief function
is proposed in this research. The design of the NFFCS and its comparison with the CFFCS are
presented below.
Fig. 2.1 shows the mechanism design of the CFFCS in the separated state. The plug
connector on the left side and the socket connector on the right side of Fig. 2.1 both have oneway self-sealing valves (1 and 4) that ensure the blockage of hydraulic fluid in the
disconnected state. During the connecting process, the pressure of the sealed high-pressure
10
hydraulic fluid acts on the one-way self-sealing valve to generate a high amount of force
(coupling resistance) that must be overcome to complete the connecting process.
Fig. 2.2 shows the kinematics of the connecting process of the plug and socket connectors
of the CFFCS. Status A is the state where the plug and the socket have just made contact. Over
the course of pushing the socket to the left while the plug is fixed, Process 1 and Process 2
take place. Upon completion of these processes, the fluid path is fully opened. In order to
facilitate the understanding, the theoretical models of the two processes are established, as
shown in Fig. 2.3, where only the influence of the mechanical spring element on the thrust is
considered.
It can be seen that during process 1, in order to move to the left, the pin (5) in the socket
that is rigidly connected to the shell of the entire socket pushes the valve (1) of the plug to
overcome the elastic resistance of the spring K2. At the same time, due to the reaction force
given by the glove (2) in the plug, the glove (3) in the socket moves to the right against the
elastic force of the spring K1. The theoretical mechanical model of this process is shown in
Fig. 2.3(a). d1 represents the distance between the glove (3) and valve (4) in the socket. When
d1 reduces to zero, the entire system reaches status B. As shown in Fig. 2.3(b), during Process
2, the valve (4) in the socket moves to the right along the guide (6). During this process, the
springs K1, K2, and K3 work simultaneously. When valve (4) starts to move, the hydraulic
fluid starts to flow through the entire coupling system. Finally, the fluid path is fully opened
when the valve (4) makes contact with the guide (6) (i.e. d2=0).
It can be seen that in the connection process of the CFFCS, in addition to overcoming the
resistance of the spring, it is also necessary to overcome the resistance due to the pressure of
the hydraulic fluid acting on the single self-sealing valve (1).
In actual operation, the resistance due to fluid pressure is usually very large and requires
the use of additional tools to assist in establishing the connection. Therefore, the CFFCS
cannot meet the requirements for quick coupling under higher-pressure conditions. To solve
this problem, we propose the NFFCS design that is presented below.
11
Fig. 2.1 Internal mechanism of the CFFCS
Fig. 2.2 Kinematics of the CFFCS
12
(a) Process 1
(b) Process 2
Fig. 2.3 Theoretical modeling of the different processes of the CFFCS
13
2.2 Mechanism design of NFFCS
Fig. 2.4 shows the internal mechanism of the NFFCS. Compared with the CFFCS, the main
difference lies in the design of the plug connector. Valve (4) and pin valve (5) divide the plug
into the L and R chambers, where the high pressure hydraulic oil is completely sealed in the L
chamber due to the pressure acting on the valves. The valve (1) in chamber R has no fluid
pressure acting on it, which means that opening this valve (1) requires us to only overcome
the spring resistance.
Fig. 2.5 shows the kinematics of the coupling process of the plug and socket connectors.
The state of the plug and socket connectors just as they make contact is shown as status A.
Then the socket connector is pushed to the left until it is fully connected. According to the
movement of the mechanism, the whole operation can be divided into four processes. For the
convenience of observation, the theoretical models for these four processes are established
(similar to the CFFCS, i.e. only considering the effect of the mechanical elements) as shown
in Fig. 2.6.
Fig. 2.4 Internal mechanism of the NFFCS
14
Similar to the CFFCS, during Process 1, the pin (8) in the socket pushes the valve (1) in the
plug to overcome the elastic resistance of the spring K2 while moving to the left. At the same
time, the glove (2) in the plug pushes the glove (6) in the socket to overcome the elastic
resistance of K1 while moving to the right. The theoretical model of this process is shown in
Fig. 2.6(a). d3 represents the distance between the pin sleeve (3) and the pin valve (5). Under
the action of thrust, d3 gradually decreases to zero, at which point the entire system reaches
status B. Then, during Process 2 (shown in Fig. 2.6(b)), the pin valve (5) begins to overcome
the resistance of spring K4 to move to the left due to the thrust. As shown in status C, a gap A
is generated between pin valve (5) and valve (4), connecting the L and R chambers. It is worth
mentioning that, unlike the valve of the CFFCS, the newly designed pin valve (5) has a small
cross-sectional area and therefore, according to Pascal's law, experiences a very small force
due to the hydraulic fluid pressure. Thus, the resistance from high-pressure hydraulic fluid that
needs to be overcome during process 2 is very small and the pressure relief effect can be
achieved with a small thrust force. d1 represents the distance between the glove(6) and valve(7)
position blocks in the socket. With further pushing to the left, when d1 becomes equal to zero,
the entire system reaches status C. During Process 3 (shown in Fig. 2.6(c)), springs K1, K2,
K3, and K4 work simultaneously. The movement of valve (7) means that the hydraulic fluid
now begins to flow through the entire coupling system. d4 represents the distance between
valve(4) and pin sleeve(3). When d4 becomes equal to zero, the entire system reaches status
D. It is worth noting here that although Process 2 and Process 3 occur in a set order, Process
3 is extremely short and can be ignored. Thus, the system quickly transits to the next process,
i.e. Process 4. That is to say that the opening of pin valve (5) during Process 2 does not exert
any pressure on the opening of valve (1). This feature has been designed to prevent the
overflow of hydraulic fluid when the coupling is not fully connected. As shown in Fig. 2.6(d),
during Process 4, the valve (4) in the plug is pushed to the left. Since the pin valve (5) is turned
on, the pressure difference between the L and R chambers is almost zero, which means that
the pressure acting on the left side of the valve (4) disappears and so it can be easily pushed
15
away. During this process, springs K1, K3, and K4 continue to work together. When valve (7)
and guide (9) make contact (i.e. d2=0), it means that the circuit is fully connected and the
coupling process has been completed.
To sum up, in the NFFCS, due to the pin valve (5) of the instantaneous pressure relief device,
at first, only some of the high-pressure hydraulic fluid is introduced into the socket during the
connection process to achieve the effect of pressure relief. At the same time, the entire
hydraulic circuit is opened, which greatly reduces the resistance due to pressure that needs to
be overcome during the coupling process. In this chapter, we have only qualitatively analyzed
the pressure that we need to overcome during the coupling process. In the following section,
numerical simulation is used to quantitatively observe and analyze the magnitude of the
pressure that is needed to be overcome.
16
Fig. 2.5 Kinematics of the NFFCS
17
(a) Process 1
(b) Process 2
18
(c) Process 3
(d) Process 4
Fig. 2.6 Kinematics of the NFFCS
19
2.3 Chapter summary
In this chapter, the working principles of CFFCS and NFFCS are analyzed and compared
from the perspective of design mechanism. In the CFFCS, the liquid is blocked by the oneway self-sealing valve to prevent leakage. This also causes the sealed high-pressure hydraulic
oil to generate back pressure on the back of the one-way self-sealing valve, which requires a
large assembly force to be overcome when connecting. During the coupling process, it is not
only necessary to overcome the resistance generated by the back pressure, but also the spring
resistance generated by the spring. In the whole process, springs 1 and 2 have been working
until the hydraulic circuit is turned on. In NFFCS, the pin valve is designed based on Pascal’s
laws and the plug is divided into two chambers L and R. Due to the existence of the sealing
valve, the high-pressure hydraulic oil is completely blocked in the L chamber. Therefore,
during the opening process, there is no back pressure resistance in the R chamber, and the
valve only needs to overcome the spring resistance, which greatly reduces the connection force
required during the connection process. During the process of turning on Land R, due to the
pin valve has a small force-receiving area, the resistance to be overcome from the highpressure hydraulic oil is also small. At the same time, part of the hydraulic oil will also be
introduced into the socket to achieve the effect of pressure relief and greatly reduce the back
pressure resistance that needs to be overcome during the coupling process.
20
CHAPTER 3
NUMERICAL INVESTIGATION OF FFCS
3.1 Characteristics of valve opening
3.1.1 Investigation methods and boundary conditions
The opening and closing of the valve is typically an almost instantaneous process. In actual
operation, it is difficult to observe the change of the internal flow field during the opening of
the high-pressure valve. With the development of information technology, various simulation
tools based on numerical investigation methods are being widely used in various industries to
overcome the deficiencies of experimental methods [33-35]. For valve problems, computational
fluid dynamics (CFD) methods are often used in the industry to study the dynamic changes in
the flow field [36-38].
In this numerical investigation, the global unstructured tetrahedral mesh is used for the
discretization of the high-pressure liquid basin, and the grid at the valve gap is locally refined.
The Reynolds Average Navier-Stokes (RANS) equation governing the fluid flow is solved in
the commercial CFD software ANSYS Fluent using a workstation with an Intel Xeon E5 2690
v4 processer operating at 2.6GHz with 128GB of RAM. In the simulation, the fluid is assumed
to be incompressible and conforming to the RANS equation. The turbulence model is set as
Standard k-epsilon, which is a stable, economical and highly accurate turbulence model. The
SIMPLE solver is used to solve momentum and continuous equations. This method uses a
second-order upwind style to discretize the equations in parallel grid nodes to ensure the
accuracy of the simulation results. The boundary conditions are set as pressure inlet and
21
pressure outlet for this calculation domain. The initial pressure value of the valve is set as
630kPa, and the outlet pressure is set as 530kPa. In this simulation, ISO VG32 hydraulic oil
with a density of 844.4 kg/ 𝑚3 and a dynamic viscosity of 0.0135 kg/ms is used as the
transportation medium.
In order to more intuitively observe the NFFCS’ effects of instantaneous pressure relief and
easy coupling, same sized (ISO 16082 size 10) CFFCS and NFFCS were pre-processed for
modeling and simulation separately. In this study, the entire valve opening process of the two
hydraulic couplings was simulated using novel dynamic mesh technology. The dynamic mesh
algorithms mainly include layering, smoothing and remeshing. Considering the complexity of
the internal geometric structure of the hydraulic coupling, it is difficult to use a layering
method that is highly dependent on prismatic meshes. Therefore, in this study, smoothing and
remeshing methods were adopted for dynamic meshing.
The general conservation equation of a dynamic mesh [39, 40] for a general scalar (Φ) on an
arbitrary control volume (V), whose boundary is moving, can be written as
∂
∫ 𝜌∅𝑑𝑉 + ∫ 𝜌∅(𝑣 − ⃗⃗⃗⃗
𝑣𝑔 ) ∙ 𝑑𝐴 = ∫ (𝛤∇∅) ∙ d𝐴 + ∫ 𝑆∅ 𝑑𝑉
∂t 𝑉
𝐴
𝐴
𝑉
(3-1)
Where 𝑣 is the speed vector; ⃗⃗⃗⃗
𝑣𝑔 represents the grid velocity of the moving mesh, 𝜌 is the
liquid density, 𝛤 is the diffusion coefficient, and 𝑆∅ represents the source term of ∅.
Fig. 3.1(a) and Fig. 3.1(b) show the cross-sections of meshing on the CFFCS and NFFCS,
respectively. According to the rules of the dynamic meshing method, the volume of the grid
cannot be zero. Therefore, a gap must be left between the valve and the glove during modeling.
In order to simulate the true opening process to the greatest extent, the gap between the valve
and the glove must be very small. Thus, the initial gap in this study is defined as 0.2mm.
22
For the CFFCS, the watershed wall contacting the valve is set as a moving rigid body, and
its velocity is defined by the profile programming. In the NFFCS, the instantaneous pressure
relief pin valve will be opened before the larger valve is opened. For this process, the velocity
at the flow area of the pin valve and the larger valve were defined separately. The total coupling
duration of the two valves is set as 2.0s.
(a) CFFCS
(b) NFFCS
Fig. 3.1 Meshing of the two models
23
3.1.2 Results and discussion
Fig. 3.2 and Fig. 3.3 respectively show the pressure and velocity distribution contours at
three moments during the valve opening process of the CFFCS: (a) the beginning of coupling
(t=0.5s, the valve is not opened); (b) the moment the valve has just started opening (t=1s); (c)
the valve is fully opened (t=2s). It can be seen from Fig. 3.2(a) and Fig. 3.3(a) that the two
chambers are not connected and the pressure is evenly distributed in the high-pressure chamber
on the left side of the valve and in the low-pressure area on the right. At this time, the flow
velocity in the two chambers is almost zero. Since at least one layer of mesh is required in the
dynamic mesh setting, it is inevitable that a large flow velocity will be generated in the gap in
the simulation results. In the actual opening process, due to the existence of the seal, such a
large flow velocity does not exist in the gap. This study only observes the characteristics of
the flow in the fluid domain during the coupling process, thus the influence of such
unavoidable limitations of the simulation methodology on the research results is ignored.
The left and right chambers start to connect during the leftward movement of the valve (as
shown in Fig. 3.2(b) and Fig. 3.3(b)). At this time, the gap between the valve and glove reaches
its smallest value. In this state, the fluid is pushed by the high pressure in the left chamber to
pass through the gap at a great velocity, and form an obvious velocity gradient in the lowpressure area. At this time, the pressure distribution of the two chambers still does not change
much.
At t=2.0s, CFCCS completes the coupling process, and the entire hydraulic circuit is fully
connected (as shown in Fig. 3.2(c) and Fig. 3.3(c)). In this state, the pressure distribution in
the entire watershed tends to be uniform, the inlet maintains a high pressure, and there is a
high and even pressure gradient. The overall flow velocity distribution corresponds to the
pressure distribution.
24
(a) t=0.5s
(b) t=1.0s
(c) t=2.0s
Fig. 3.2 Instantaneous pressure distributions at different times during the coupling process of
the CFFCS
25
(a) t=0.5s
(b) t=1.0s
(c) t=2.0s
Fig. 3.3 Instantaneous velocity distributions at different times during the coupling process of
the CFFCS
26
Fig. 3.4 and Fig. 3.5 respectively show the pressure and velocity distributions of the NFFCS
at different moments during the coupling process. The five most representative time points
were selected for observation and analysis. As shown in Fig. 3.4(a), at the initial time (t=0s)
the valve in the coupler is closed. In this state, from the pressure distribution, it can be observed
that the pressure accumulation phenomenon occurs at the inlet and forms an obvious highpressure zone. The pressure in the outlet chamber is low and there is no obvious pressure
gradient. Similar to CFFCS, it can be observed that the entire watershed is divided into two
chambers by the pin and the valve, as shown in Fig. 3.5(a), and the velocity in both the
chambers is zero.
As the gap of the pin valve becomes larger (t=0.2s), the fluid in the high-pressure side
begins to flow into the outlet chamber through the pin valve gap and the pressure difference
between the inlet and the outlet is reduced. Since the gap is small, the flow velocity in the gap
is large (as shown in Fig. 3.5(b)), which results in a very small pressure in the gap (as shown
in Fig. 3.4(b)). Due to high-speed flow at the end of the gap, the fluid accumulates at the
corners and is squeezed by the following high-speed fluid, which forms a local high pressure
region.
After the pin valve is fully opened (t=0.4s), the pressure at the inlet and outlet becomes
almost equal (as shown in Fig. 3.4(c)). It can be seen that the opening of the pin valve connects
the two chambers to achieve the effect of instantaneous pressure relief. At this time, the big
valve in the plug starts to open. As the size of the valve opening increases, the extreme low
pressure in the valve gap caused by simulation limitations disappears, and the maximum flow
velocity reduces accordingly, as shown in Fig. 3.4(d) and Fig. 3.5(d).
At t=2.0s, when the valve is fully opened (Fig. 3.4(e) and Fig. 3.5(e)), the pressure
distribution in the watershed tends to be uniform. A high pressure can be found at the inlet
side, and a high-pressure gradient can also be found in the overall watershed. In this state, the
velocity distribution corresponds to the pressure distribution.
27
During the coupling process of the NFFCS, the opening of the pin valve connects the highpressure cavity with the low-pressure cavity to achieve the effect of pressure relief so that the
big valve can be easily opened.
(a) t=0s
(b) t=0.2s
28
(c) t=0.4s
(d) t=1s
(e) t=2s
Fig. 3.4 Instantaneous pressure distributions at different times during the coupling process of
the NFFCS
29
(a) t=0s
(b) t=0.2s
(c) t=0.4s
30
(d) t=1s
(e) t=2s
Fig. 3.5 Instantaneous velocity distributions at different times during the coupling process
of the NFFCS
31
In order to observe the impact of hydraulic power on the coupling system, the axial
resistance caused by the hydraulic oil to the valve opening during the coupling processes of
CFFCS and NFFCS were monitored separately. The results of this study are summarized in
Fig. 3.6. The solid line represents the opening force results for the CFFCS and the dashed line
represents the opening force results for the NFFCS. It can be observed that the maximum
resistance for the CFCCS is about 122.3N, which occurs when the valve starts to be pushed
(t=0s). This force then quickly drops to an approximately stable value (about 5N). When the
valve is opened (t=1.0s), the thrust has a small increase and then stabilizes again. On the other
hand, the NFCCS releases the pressure by opening the pin valve before the big valve is opened
to achieve full conduction.
When the pin valve is opened, the maximum axial thrust of 80.7N appears at the first
moment of pushing (t=0s), which then gradually decreases and stabilizes. It can be seen that
the maximum axial resistance occurred at the same time (the beginning of the movement) in
both the mechanisms. However, the presented NFCCS showed a resistance value that is 34%
less than that showed by the CFFCS. Although the grid size at the gap has been divided to be
an extremely small value, it still is difficult to obtain the data that is completely consistent with
the actual due to the limitations of the dynamic mesh algorithm. However, the overall trends
shown in the simulation can be considered as a reasonable depiction of the actual. Thus, we
can safely infer that the design of the NFCCS can relieve the high pressure instantly with a
small opening force so that the big valve can be easily opened to connect the hydraulic circuit.
32
Fig. 3.6 Comparison of the valve opening forces required by the CFFCS and NFFCS
33
3.2 Flow characteristic of FFCSs
When hydraulic oil flows in the coupling system, frictional resistance and turbulent energy
loss occur due to the complicated geometric structure inside the coupler, which causes a
pressure drop between the inlet and the outlet
[41]
. In order to improve the efficiency of
hydraulic power transmission while ensuring the best flow characteristics in the NFFCS design,
a series of numerical simulations, comparisons, discussions and design optimizations were
carried out, which are presented in this chapter. Firstly, the internal flow characteristics of the
CFFCS and the initial design of the NFFCS were compared to ensure that the pressure drops
of the two coupling systems are at the same level. Then, for the initial design of the NFFCS, a
series of geometric optimizations were proposed and the flow characteristics were discussed,
leading to the best final design.
3.2.1
Investigation methods and boundary conditions
In this chapter, CFD analysis is performed on the CFFCS and the initial and modified
models of the NFFCS to observe their pressure, flow velocity and turbulent kinetic energy
distribution. Among these outcomes, TKE (Turbulence Kinetic Energy), which is one of the
most common physical quantities in the turbulence model, is often used to observe the energy
loss of liquid flow.
Fig. 3.7(a), Fig. 3.7(b) and Fig. 3.7(c) respectively show the mesh on the major internal
components and the inlet boundary conditions for the CFFCS model, the initial NFFCS model
and the modified NFFCS model. In this simulation, ANSYS Mosaic Meshing Technology was
used to generate mesh in a unique poly-hexcore form shown in Fig. 3.7(d). The poly-hexcore
mesh provides higher accuracy and faster calculation speed for complex CFD problems [42].
34
(a) The CFFCS
(b) Initial design of the NFFCS
35
(c) Optimal design of the NFFCS
(d) Internal view of the Poly-Hexcore mesh
Fig. 3.7 Meshing and inlet boundary conditions
The international standards ISO 16028 and ISO 4399
[15, 43]
give the pressure drop
requirements and measurement methods for various sizes of flat-face type coupling systems.
In this study, the coupling system size of 10mm was modelled and observed as an example.
The influence of the tiny geometry of internal components and springs was considered in this
investigation. The boundary conditions for simulations based on the ISO standard are set as;
mass-flow inlet = 0.3237kg/s and pressure outlet = reference to atmospheric pressure. The
hydraulic oil and turbulence models used are the same as those mentioned in Chapter 3.1.
36
3.2.2
Results and discussion
In order to more intuitively evaluate the flow characteristics of the NFFCS, they are
compared with the simulation results of the CFFCS. The pressure drops of the CFFCS (50.76
kPa) and the NFFCS (49.49 kPa) were obtained through calculation of the average pressure at
both sides of the inlet and outlet. It can be seen that although the internal geometry of the
NFFCS is more complicated due to the inclusion of the instantaneous pressure relief device,
its pressure drop is about 2.5% less than that of the CFFCS. Fig. 3.8 shows the pressure
distribution of two flat-face type coupling systems. It can be seen that the pressure gradient
distribution in the two systems in the steady state is uniform, and the pressure remains high at
the inlet. The pressure drop around the guide pin of the valve at the inlet side of the CFFCS is
more obvious, which is caused by the sudden decrease in the flow cross-sectional area and the
increase in velocity. This is the main reason why the pressure drop of the CFFCS is higher
than that of the NFFCS.
(a) The CFFCS
(b) Initial design of the NFFCS
Fig. 3.8 Pressure distribution results for the two flat-face coupling systems
37
Fig. 3.9 shows the velocity and streamline distribution results of the two flat-face coupling
systems. By comparing the velocity distributions, it can be seen that there is a sharp increase
in the velocity at the guide pin position near the inlet of the CFFCS. As mentioned before, due
to the sudden reduction of the geometrical cross-sectional area, the surge in the flow rate of
the hydraulic oil flowing through it leads to greater energy consumption, which results in the
greater pressure drop in the CFFCS. The initial model of the NFFCS was designed with
changes included mainly inside the plug while the socket was the same as that used in the
CFFCS. It can be seen from Fig.3.9 that the streamlines in the sockets of the two coupling
systems show obvious vortex phenomena near the outlet. This is due to the sudden increase in
the flow cross-sectional area, which causes the flow rate to reduce and the pressure to increase
sharply.
(a) The CFFCS
(b) Initial design of the NFFCS
Fig. 3.9 Velocity and streamline distributions for the two flat-face coupling systems
38
3.3 Chapter summary
In this chapter, the dynamic mesh technology in Ansys Fluent is used to analyze and
compare the flow characteristics and opening force of CFFCS and NFFCS during valve
opening. When the CFFCS started to open, the flow rate will pass through the gap between
the valve and the glove at a relatively high speed until the coupling ends. When the hydraulic
circuit is fully connected, the entire flow area has a higher-pressure gradient, and the inlet
pressure is higher than the outlet. During the coupling process of NFFCS, when the pin valve
is opened, the high-pressure liquid in the inlet section flows out at a greater speed through the
gap opened by the pin valve. After the pin valve is fully opened, the L and R chambers are
connected, and the pressure at the inlet and outlet is almost the same, achieving the effect of
instantaneous pressure relief. When the NFFCS is fully coupled, the pressure presents a
gradient distribution, and the pressure at the inlet is relatively high. In addition, the axial
resistance caused by the hydraulic oil to the valve opening during the coupling processes of
CFFCS and NFFCS were also monitored. It can be found that the maximum resistance during
the coupling process of CFFCS is about 122.3N, while the maximum axial thrust of NFFCS
when the pin valve is opened is 80.7N. And then gradually decreased and stabilized. It can be
seen from the results that the resistance overcome by NFFCS in the coupling process is reduced
by 34% compared with CFFCS, which is enough to show that the design of NFFCS has the
function of instantaneous pressure relief.
39
CHAPTER 4
PERFORMANCE IMPROVEMENT AND
EXPERIMENT ON NFFCS
4.1 Performance improvement of NFFCS
The pressure drop (49.49kPa) of the initial NFFCS design was similar to that of the CFFCS
and met the standard (100kPa) specified in ISO 16028. However, in order to obtain better flow
characteristics, a series of design optimizations were carried out on the parts that caused large
energy loss, judged based on the turbulent kinetic energy observations.
Fig. 4.1 shows the turbulent kinetic energy distribution of the initial NFFCS design. It can
be seen that the turbulent kinetic energy has a large value at positions 1 and 2 marked by the
red dashed box, while a large area of energy loss occurs at position 3.
Fig. 4.1 The turbulent kinetic energy distribution of the initial NFFCS design
40
In order to obtain the optimal design, extensive sampling data of modifications and
simulations were made at these three positions. The specific geometric design and turbulent
kinetic energy distribution before and after optimization are shown in Table 4.1 and Table 4.2,
respectively.
It can be observed that there is a large unevenness in position 1 where the plug valve and
the socket pin make contact, which can be assumed to be the reason of the large energy loss.
The unevenness of position 1 was smoothed as shown in Table 4.1 without changing the
structural reliability of the parts. From the result of turbulent kinetic energy distribution at
position 1 shown in Table 4.2, it can be seen that the maximum value for the modified design
is 4.27 m2 /𝑠 2 , which is 28.9% less than the 6.01 m2 /𝑠 2 value observed for the initial design.
Similar to position 1, position 2 also has micro-geometric unevenness due to the improper
chamfering of the component, which causes considerable energy loss at this position. By
modifying the improper chamfer, the maximum value of the turbulent kinetic energy is
reduced from 10.67 m2 /𝑠 2 to 7.50 m2 /𝑠 2 , which is a reduction of 29.7%. A comparison of
the results at position 2 is shown in Table 4.2.
By observing the turbulent kinetic energy distribution at position 3 in the initial design, it is
apparent that the vortex phenomenon is occurring at the end of the socket pin mentioned above.
Thus, it can be inferred that the wide range of turbulent kinetic energy distribution here is
caused by the interaction between the vortices. As shown in Table 4.1, at position 3, the pin
fixing part of the initial design has eight small holes to allow passage of the hydraulic fluid.
This design reduces the flow cross-sectional area, which leads to the vortex generation and
energy loss problems. In the modified design, the flow cross-sectional area on the pin fixing
component is increased as much as possible while ensuring the stability of the rigid structure.
From position 3 in Table 4.2, it can be seen that near the outlet, the large-area distribution of
41
turbulent kinetic energy has been greatly improved in the modified design, and the maximum
value of turbulent kinetic energy has been reduced from 10.5 m2 /𝑠 2 to 9.46 m2 /𝑠 2 .
Table 4.1 The geometrical design modifications made in the initial design to obtain the
modified design
Position
Initial design
Modified design
1
2
3
42
Table 4.2 Comparison of the turbulent kinetic energy distribution results for the initial and
modified designs
Position
Initial design
Modified design
1
2
3
43
Similarly, the flow characteristics of the modified NFFCS were also analyzed. The
pressure distribution and the velocity and streamline distribution of the NFFCS are shown in
Fig. 4.2(a) and Fig. 4.2(b), respectively. By calculating the average pressure difference
between the inlet and the outlet, the pressure drop was found to be 42.79 kPa, which is 13.5%
less than the pressure drop of the initial design (49.49 kPa). It can be seen from the pressure
distribution that the overall pressure drop gradient distribution from inlet to outlet is uniform.
By comparing Fig. 4.2(b) with Fig. 3.9(b) (initial design), it can be seen that the vortex
phenomenon occurring at the outlet has also been significantly improved.
(a) Pressure distribution
(b) Velocity and streamline distribution
Fig. 4.2 Flow characteristics of the modified NFFCS design
44
4.2 Structural stability investigation of NFFCS
In this chapter, the numerical investigation on the structural safety and stability of the NFFCS
will be carried out by means of the fluid-structure interaction method. The mechanical
characteristics such as deformation, internal stress, strain, safety factor and other mechanical
characteristics of the CFFCS and NFFCS are analyzed and compared when subjected to high
pressure. It shows the rationality of the design of NFFCS. In addition, through experimental
methods, multiple sets of high-pressure blasting experiments were carried out on NFFCS
samples, which verified the correctness of the simulation results.
4.2.1 Numerical method
This physical phenomenon when one or more solid structures interact with the internal or
surrounding fluid is defined as fluid-structure interaction (FSI) as shown in Fig.4.3. A typical
FSI is combined computational fluid dynamics and structural mechanics [44]. The fluid solver
and the solid solver are used to calculate fluid-solid interaction problems. The fluid solver is
mainly responsible for the calculation of physical quantities such as flow field pressure,
velocity, temperature, and composition, while the solid structure solver is responsible for the
calculation of displacement, stress, and strain
[45, 46]
. Among these solution variables, the
physical quantities that exist in both fluid and solid solutions are pressure and displacement.
The fluid control equation is shown in Equation 4-1 [47].
ρ(𝑢𝑡 + (u ∙ ∇)u) = −∇p + μ∇2 𝑢 + 𝑓
(4-1)
∇u = 0
where ρ is the fluid density, u is the velocity, p is the pressure, μ is the viscosity, and f is
the force density. The fluid is subject to Dirichlet boundary conditions. For the solid governing
equation, as shown in Equation 4-2:
45
𝜌𝑠 𝑑𝑠 =̈ ∇ ∙ 𝜌𝑠 + 𝑓𝑠
(4-2)
Fluid-solid coupling follows the most basic principles of conservation. At the interface of
fluid-solid coupling, the force (τ), displacement (d), heat flow (q), temperature (T), and other
variables of the fluid and solid should be equal or conservation, that is, satisfy the following
Equation 4-3.
𝜏𝑓 ∙ 𝑛𝑓 = 𝜏𝑠 ∙ 𝑛𝑠
𝑢𝑓 = 𝑢𝑠
𝑞𝑓 = 𝑞𝑠
(4-3)
𝑇𝑓 = 𝑇𝑠
Fluid-structure interaction is generally divided into one-way interaction and two-way
interaction. One-way fluid-structure interaction refers to the one-way transmission of data at
the coupling interface, that is, the results of CFD simulation of temperature, pressure load and
others are transferred to the solid structure, but the analysis results of the solid structure are
not transferred to the fluid domain. In this article, a one-way fluid-structure interaction method
is used, and the FSI is carried out with the help of structural statics module in ANSYS
Workbench.
Fig.4.3 Classification of fluid structure as a combination of fluid and solid domain
46
In the structural simulation, S45C is used for casting of both CFFCS and NFFCS. The
Young's modulus of this material is 205GPa, the Poisson's ratio is 0.29, the yield strength is
490MPa, and the tensile strength is 686MPa. The grids of CFFCS and NFFCS are shown in
Fig. 4.4. Due to the complexity of their structure, unstructured grids are used. The grid
numbers are 0.4 million and 0.17 million respectively. In the structural analysis of CFFCS and
NFFCS, the pressure load calculated by Fluent is applied, as shown in the red area in Fig. 4.5.
The blue area is the fixed part. A fixed constraint is added at the inlet and outlet of the coupling,
the critical burst pressure 25.1MPa is imposed at the inlet. According to the pressure drop
requirement of ISO16028, the outlet pressure is 25MPa.
(a) CFFCS
(b) NFFCS
Fig. 4.4 The mesh of the couplings in structural analysis
47
(a) CFFCS
(b) NFFCS
Fig. 4.5 The boundary conditions of the couplings in structural analysis
48
4.2.2 Numerical results
In this study, high-pressure hydraulic oil flows through the coupling, so high requirements
on the safety of its components are proposed. In this paper, the force and strength of the internal
parts of the coupling are analyzed based on the flow load. Fig. 4.6 shows the maximum
deformation of the entire coupling analyzed by structural simulation, and Fig. 4.7 shows its
internal deformation. The pressure and velocity distribution are shown in Fig. 4.8 and Fig. 4.9
which are analyzed by Fluent. It can be seen from Fig. 4.6 that the maximum deformation of
CFFCS is 0.0045mm, while the maximum deformation of NFFCS is 0.0049mm. The
maximum deformation of both is far in line with the safety standards specified by the
components. However, due to the more complex internal components of the NFFCS, its
maximum deformation is larger than that of the CFFCS, but the gap is only 0.0004mm. In
CFFCS, the maximum deformation occurs at the connection between plug and socket, which
is caused by the thin wall here. In NFFCS, the maximum deformation at the connection
between plug and socket was improved by thickening. As can be seen from Fig. 4.6 and Fig.
4.7, the largest deformation of NFFCS appears at the export side, and the smallest deformation
occurs at the valve inside the plug. The largest deformation occurs due to the larger flow
velocity at the export side that is shown in Fig. 4.9, which leads to the large impact on the wall
and then results the large deformation. It can be seen in Fig. 4.9 that the distribution of the
highest velocity is in area 1 in the socket. This is due to the sudden increase in the flow rate
caused by the sudden contraction of the area of the flow pipe. At the same time, the sudden
pressure drop is happened as shown in Fig. 4.8. Although the flow rate here is the largest, the
mechanical parts here have received the same impact in all directions, so the amount of
deformation is small. The area 2 with the smallest deformation is because not only the impact
caused by the small flow rate is small, but also the same and small impact is received in all
directions of its mechanical parts, which results in the small deformation.
49
(a) CFFCS
(b) NFFCS
Fig. 4.6 The section view of the total deformation of the couplings
50
(a) CFFCS
(b) NFFCS
Fig. 4.7 The internal structure of the total deformation of the couplings
51
(a) CFFCS
(b) NFFCS
Fig. 4.8 The pressure distribution of the couplings
52
(a) CFFCS
(b) NFFCS
Fig. 4.9 The velocity distribution of the couplings
53
Fig. 4.10 shows the equivalent stress distribution of coupling in the structural analysis, and
Fig. 4.11 shows the equivalent stress distribution of the internal components. In Fig. 4.11, the
position where the maximum stress appears is enlarged. In Fig. 4.10, the maximum stresses of
CFFCS and NFFCS are 114.46 and 114.45 MPa respectively, which are almost the same and
both are less than the maximum allowable stress of the material. However, it can be seen from
the minimum stress value that the minimum stress value of NFFCS is much smaller than that
of CFFCS, and the distribution range is wide, which indicates that the performance of NFCCS
is stronger than CFFCS in stress analysis. In CFFCS, the maximum stress appears at the
connecting part of the plug and the socket, which is almost the same position where the
maximum deformation occurs. This is caused by the thin wall here. After improvement, the
stress of NFFCS here is greatly reduced. It can be seen from the equivalent stress distribution
in Fig. 4.11 that the equivalent stress of the internal valve components of the NFFCS is less
than 0.003MPa. This is because the internal components are subjected to equal forces in all
directions. After they cancel each other out, the final total force exerted on the internal
components is almost 0. Due to the pipeline has a certain thickness, the equivalent stress on
the outer wall of the pipeline is relatively small, the maximum value is 9.08MPa and the
minimum value is less than 0.003MPa. The equivalent stress of the inner wall of the pipeline
is relatively large, and the maximum stress at the inlet is 72.46 MPa, which is due to the large
pressure at the inlet (as shown in Fig. 4.8). The maximum equivalent stress at the outlet is
52.87MPa, which is caused by the high flow velocity (Fig. 4.9). It can be seen from Fig. 4.10
and Fig. 4.11 that the maximum equivalent stress appears inside the socket, and the enlarged
view is shown in Fig. 4.11. At this place, the flow direction of the high-speed hydraulic oil is
changed, moreover the cross-sectional area of the pipeline gets bigger which leads to the
increase of pressure. Therefore, due to the high velocity and high pressure, the equivalent
stress is the largest here.
54
(a) CFFCS
(b) NFFCS
Fig. 4.10 The section view of the stress of the couplings
55
(a) CFFCS
(b) NFFCS
Fig. 4.11 The internal structure of the stress of the couplings
56
Fig. 4.12 and Fig. 4.13 show the distribution of equivalent elastic strains of coupling in
the structural analysis. The maximum equivalent strains of CFFCS and NFFCS are 0.00057
and 0.00087 mm/mm respectively, which are less than the maximum allowable equivalent
elastic strain of the material. In CFFCS, the equivalent strain has obvious stratified distribution
characteristics, and the maximum strain appears at the maximum stress. In addition, the strain
of the inner wall is obviously greater than the strain of the outer wall. The equivalent strain
distribution in NFFCS is relatively uniform. The position where the maximum equivalent
elastic strain appears is shown in the enlarged view in Fig. 4.13. It can be seen that the
maximum equivalent strain occurs at the groove inside the plug, which does not occur at the
maximum equivalent stress. This is because in practical applications, the chamfering design is
required here, and the maximum equivalent strain is generated at the thin-walled surface due
to the existence of the concave groove.
57
(a) CFFCS
(b) NFFCS
Fig. 4.12 The equivalent elastic strain of the couplings
58
(a) CFFCS
(b) NFFCS
Fig. 4.13 The position of the maximum equivalent elastic strain in the couplings
59
Fig. 4.14 and Fig. 4.15 show the distribution of the safety factors of coupling. The
minimum safety factors of the CFFCS and NFFCS are around 2.184, which meets the safety
characteristics of their materials. It can be seen that the safety factor on the outside and the
internal valve of CFFCS and NFFCS is very high, which is around 15. This is because the
impact and stress on the external components are very small, resulting in a high safety factor.
The internal valve components are subject to less stress due to impacts in all directions, which
results in a higher safety factor. It can be seen from Fig. 4.15 that the safety factor of the inner
wall of the pipeline is low and the lowest safety factor appears at the maximum stress. However,
through the distribution characteristics of the safety factor on the inner wall of the pipeline,
the safety factor distribution inside the NFFCS is relatively uniform, while the safety factor
distribution on the inner wall of the CFFCS pipeline has obvious stratification and the safety
factor is low. Therefore, in terms of safety factor, NFCCS has higher performance.
60
(a) CFFCS
(b) NFFCS
Fig. 4.14 The internal structure of the safety factor of the couplings
61
(a) CFFCS
(b) NFFCS
Fig. 4.15 The position of the minimum safety factor of the couplings
62
4.3 High pressure burst experiment
Numerical investigation methods have saved a lot of time and cost for the design of
mechanical products, and have provided a mechanical theoretical basis for product
optimization. In recent years, it has become an indispensable program in the development
stage of mechanical products. According to the above-mentioned series of analyses of NFFCS,
it can be known that the improved NFFCS has good performance, safety, reliability, and
feasibility in terms of fluid mechanics characteristics and structural stability. Therefore, we
made samples based on this design as shown in the Fig. 4.16, the Fig. 4.16(a) shows the
uncoupled state and the Fig. 4.16(b) shows the coupled state. However, there are still some
uncontrollable or unpredictable factors in the simulation calculation, which affect the
correctness of the results. This paper uses experimental methods to verify the correctness of
the previous simulation results.
When verifying the structural stability of hydraulic pipelines or components in pipelines
under high pressure, experiments are usually carried out by using the high pressure burst
method. The high pressure burst experiment is to test the maximum pressure when the NFFCS
burst suddenly by continuously pressurizing the NFFCS. Due to the danger of the explosion
experiment, this experiment was carried out in an experimental well, as shown in the Fig.
4.17(a). In the experiment as shown in the Fig. 4.17(b), one end of the coupled NFFCS is in a
closed state, and the other end is pressured by a high-pressure pump (Fig. 4.17(c)). As shown
in the Fig. 4.17(d), the input pressure is gradually increased manually. So that the internal
pressure continues to rise until it bursts. The change of the input pressure during the whole
process will be transmitted and recorded to the computer (Fig. 4.17(f)) through the pressure
sensor and signal converter (Fig. 4.17(e)).
63
(a) Uncoupled state
(b) Coupled state
Fig.4.16 The prototype of the modified NFFCS
64
(a) Experimental well
(b) Experimental method
65
(c) High pressure pump
(d) Pressure setting
66
(e) Signal converter
(f) Accord data
Fig. 4.17 The experimental device: experimental
67
In order to ensure the accuracy of the experimental data, a total of 5 samples were subjected
to bursting experiments. The experimental results are shown in the Fig. 4.18. The maximum
burst pressure is 192.8MPa, the minimum burst pressure is 190.4MPa, the average burst
pressure is 191.7MPa and the maximum deviation is 1.3MPa. It can be seen from the
experimental data that the safety critical pressure of NFFCS is 190MPa, which is about 7.6
times of the actual working pressure of 25MPa. It can be speculated that this is due to the heat
treatment of the parts used in the production of the NFFCS prototype, which has improved its
mechanical properties such as hardness, wear resistance and strength. The reliability of NFFCS
in compressive strength performance is verified through burst experiments, and the maximum
burst pressure fully meets the design goals and safety requirements in practical applications.
Fig. 4.18 The burst pressure of 5 NFFCS samples in experiment
68
4.4 Chapter summary
In this chapter, we analyzed the energy loss during the flow of the coupled CFFCS and
NFFCS. During the flow process, the pressure drop of NFFCS is about 49.49kPa, and the
pressure drop of CFFCS is about 50.76kPa, both in line with international standards ISO 16028.
Although the internal design of the NFFCS is more complicated than that of the CFFCS, the
pressure drop value is reduced by 2.5%. This is due to the sudden decrease in the flow
cross-sectional area at the guide pin position near the inlet of the CFFCS. Then a sharp
increase in the velocity of the hydraulic oil leads to greater energy consumption. In
addition, by observing the distribution of turbulent kinetic energy inside the NFFCS, a series
of optimizations have been carried out on the parts with large energy loss. The modified
NFFCS not only reduces the pressure drop to 42.79kPa, but also has a uniform pressure drop
gradient distribution, and the vortex area at the outlet is also significantly improved.
In addition, the fluid-structure interaction was performed on the CFFCS and the modified
NFFCS, the pressure load calculated by Fluent was applied to the entire coupling. It can be
found that the maximum deformation of CFFCS is 0.0045mm, and that of NFFCS is
0.0049mm, which meets the safety standards specified by the components. The maximum
deformation of CFFCS appears at the junction of plug and socket, which is caused by the
thinner wall. The maximum deformation of NFFCS appears at the outlet end, which is caused
by the large impact on the wall due to the large flow velocity of the outlet.
In the distribution of equivalent stress, the maximum stresses of CFFCS and NFFCS are
114.46 MPa and 114.45 MPa respectively, and the maximum strains are 0.00057mm/mm and
0.00087 mm/mm respectively, which meet the material characteristics. The maximum stress
and strain of CFFCS appear at the maximum of deformation, which is caused by the thin wall.
The maximum stress of NFFCS appears in the socket, which is caused by the large velocity
and pressure here. However, the maximum strain appears in the groove of the plug, which is
69
caused by the thin wall caused by the chamfering design in actual operation. The minimum
safety factor of CFFCS and NFFCS is almost the same around 2.184, which satisfies the safety
characteristics of materials.
In order to verify the correctness of the simulation results, we conducted a high-pressure
burst experiment on the NFFCS sample in the experimental well. Five sets of experimental
data show that the safety critical pressure of NFFCS is 190MPa, which is about 7.6 times the
actual working pressure. It is speculated that the heat treatment process is used for the parts,
which improves the mechanical properties such as wear resistance and strength.
70
CHAPTER 5
DESIGN AND STRUCTURE ANALYSIS ON MULTICOUPLING DRIVE PART
5.1 Design and structure analysis of 4-circuit hydraulic quick
action multi-coupling
With the rapid development of science and technology, hydraulic technology has been
widely used in agriculture, construction, manufacturing industry, aerospace and other fields.
Multifunctional hydraulic machinery requires the joint action of multiple hydraulic circuits to
drive hydraulic equipment to move in different degrees of freedom. Although the NFFCS
mentioned in the previous article is faster and more labor-saving than the connection process
of CFFCS, it still needs to be connected one by one in practical applications that require
multiple hydraulic circuit connections. In order to solve the time-consuming and laborious
problem of multi-hydraulic circuit connection, this paper proposes 4-circuit and 6-circuit
hydraulic quick action multi-coupling drive part according to the extensive needs of industrial
applications. The next chapter will explain the design mechanism of the 4-circuit hydraulic
quick action multi-coupling, and verify its structural stability through numerical investigation.
71
5.1.1 Mechanism of 4-circuit hydraulic quick action multi-coupling
The design mechanism diagram of the 4-circuit is shown in Fig. 5.1. It consists of an
integrated female with 4 sockets (upper side of the Fig. 5.1), an integrated male with 4 plugs
(lower side of the Fig. 5.1), and a handle for quick connection. In order to quickly align the
four integrated couplings at the same time, two guide pins are designed in the integrated female,
and two guide pin holes are designed in the corresponding integrated male. During installation,
only need to insert the guide pin into the guide pin hole, the 4 couplings can be aligned at the
same time. It can also be found that both the guide pin and the guide pin hole adopt
asymmetrical design. This is to ensure that the handle and the guide pin 2 are on the same side
in the installation process, so that the next installation can be carried out smoothly.
Fig. 5.1 The modeling of 4-circuit hydraulic multi-coupling
72
In order to make the installation process more labor-saving, the handle module based on
the principle of leverage has been designed as shown in Fig. 5.1. When the coupling first
touches, the guide pin 2 is located at the starting point A of the guide curve. Due to the
constraints of the guide pin, the guide pin 2 can only move with a single degree of freedom
along the axis of the coupling. With the rotation of the handle, the guide pin 2 moves to the
point B nearest to the rotation center of the handle under the guidance of the guide curve. At
this time, the entire coupling is connected, and the hydraulic circuit is completely turned on.
As mentioned earlier, the guide cure needs to have the function of guiding the smooth
movement of the guide pin 2 closer and closer to the center of rotation, so the design of the
guide curve is extremely critical. Fig. 5.2 is the design principle diagram of the guide curve,
where R is the radius of the center circle, a is the distance from point A to the lever rotating
center, and b is the distance from the lever rotating center to the guide pin 2 located in B.
According to Pythagorean Theorem, the relationship between a, b and R can be determined as
shown in formula (5-1) and (5-2).
(𝑅 − 𝑎)2 + 𝑏 2 = 𝑅 2
(5-1)
𝑎2 + 𝑏 2
𝑅=
2𝑎
(5-2)
73
Fig. 5.2 The Schematic diagram of Pythagorean Theorem
In addition, since the flowing medium is high-pressure hydraulic oil, it is difficult to avoid
the disconnection of the integrated male and the integrated female of the coupling caused by
the greater impact created by the sudden change of the internal hydraulic pressure. In order to
prevent this problem, a locking module to the handle has been subjoined. When the coupling
is fully coupled, the locking bolt is applied for final fixation. Ultimately, the entire 4-circuit
hydraulic quick coupling connection process is completed.
Therefore, according to this design, we have produced a prototype as shown in Fig. 5.3(a),
the integrated female is in the upper while the integrated male is in the below which are
uncoupled. When the coupling first touches, the guide pin 2 is located at the starting point of
the guide curve, which shows in Fig. 5.3(b). Fig. 5.3(c) shows the coupled state. The successful
production and connection of the prototype confirms the correctness and rationality of the
design.
74
(a) The state of uncoupled
(b) The state of alignment
75
(c) The state of coupled
Fig. 5.3 The connection process of prototype
76
5.1.2 Analysis conditions of 4-circuit hydraulic quick action multi-coupling
Due to the instability of high-pressure hydraulic oil, higher requirements are placed on the
safety of components, especially the connecting parts. If the connection is not firm, safety
accidents such as wear and leakage will occur. Therefore, it is necessary to analyze the
structure of the connecting parts. This section uses the finite element method to evaluate and
discuss the safety and stability of 4-circuit hydraulic quick action multi-coupling. The material
of each part still adopts S45C with higher strength. The 3D model and grid are shown in Fig.
5.4 and Fig. 5.5. In the simulation, due to the complexity of the structure, unstructured grids
are still used and the total number of grids is about 1 million. In the structural simulation, the
boundary conditions are set as shown in Fig. 5.6, the fixed constraints are imposed on the
bottom surface of the integrated male as shown by the blue surface in the Fig. 5.6. A separation
load of 1000N is applied to the bottom of the integrated female terminal (red area). The
connection methods of all components are constrained in accordance with the actual design.
Fig. 5.4 The 3D modeling of the 4-circuit hydraulic multi-coupling
77
Fig. 5.5 The mesh of the 4-circuit hydraulic multi-coupling
Fig. 5.6 The boundary conditions of the 4-circuit hydraulic multi-coupling
78
5.1.3 Structural analysis of the 4-circuit hydraulic quick action multi-coupling
Fig. 5.7 is the simulation results of the structural stability of the coupled 4-circuit hydraulic
quick action multi-coupling. Fig. 5.7(a) shows the stress distribution, the maximum stress is
1.03MPa where appears in the contact position of the handle and the guide pin2. This is due
to the guide pin 2 is on the integrated male, which is fixed relative to the integrated female and
handle. Handle is located on the integrated female, and the separation force applied on the
integrated female is transferred to the handle through the rigid connection, so that the contact
position of the handle and the guide pin 2 produces the maximum stress. Fig. 5.7(b) is the
strain distribution, it can be seen that the overall distribution is similar to stress, and the
maximum strain appears at the maximum stress. The Fig. 5.7(c) shows the distribution of
deformation, due to the fixed constraints are imposed on the integrated male, there is almost
no deformation. The deformation area is mainly concentrated on the integrated female and the
handle. Because of the mutual constraint between the guide pin 2 and the guide curve, the
amount of deformation in the contact place between the handle and the guide pin 2 is small,
and the farther away from the contact point on the handle, the amount of deformation is bigger.
The maximum deformation of 0.038mm occurs at the farthest distance from the contact point.
Since the opening direction of the guide curve is on the right side, the left side of the integrated
female has a greater amount of deformation than the right side. Combining the yield strength
and stress distribution of the material, the safety factor distribution shown in Fig. 5.7(d) can
be obtained. It can be seen that the smallest safety factor 4.75 appears at the maximum stress,
which is much higher than the allowable stress of the material. In addition, the safety factor is
evenly distributed throughout the component with a higher value. By means of the above
analysis, the structural safety and stability of 4-circuit hydraulic quick action multi-coupling
is fully in line with industrial production requirements. From the above structural stability
analysis, it can be seen that 4-circuit hydraulic quick action multi-coupling satisfies various
safety characteristics.
79
(a) The distribution of stress
(b) The distribution of strain
80
(c) The distribution of displacement
(d) The distribution of safety factor
Fig. 5.7 The structural analysis results of the 4-circuit hydraulic multi-coupling
81
5.2 Design and structure analysis of 6-circuit hydraulic quick
action multi-coupling
In the industry, the demand for the number of pipes varies according to specific
requirements. Four pipes and six pipes are generally used. In the previous chapter, the
structural safety analysis of the 4-circuit hydraulic quick action multi-coupling has been
carried out, the result shows that the 4-circuit hydraulic quick action multi-coupling meets the
requirements of engineering applications. In this chapter, the structural safety analysis of the
6-circuit hydraulic quick action multi-coupling will be carried out.
5.2.1 Mechanism of 6-circuit hydraulic quick action multi-coupling
Compared with the 4-circuit hydraulic quick action multi-coupling, the 6-circuit hydraulic
quick action multi-coupling has greater resistance to overcome during the installation process
due to the increase in pipelines. Therefore, the hydraulic quick coupling is designed by the
principle of leverage, as shown in Fig. 5.8. The 6-circuit hydraulic quick action multi-coupling
consists of an integrated female with 6 sockets, an integrated male with 6 plugs, a handle
module, a locking module and others.
Fig. 5.8 The modeling of 6-circuit hydraulic multi-coupling
82
Different from the 4-circuit hydraulic quick action multi-coupling, the guide pin (Fig. 5.9)
in the 6-circuit hydraulic quick action multi-coupling not only has a guiding function, but also
integrates the traction function. As shown in Fig. 5.9, a trapezoidal groove is designed on the
front end of the guide pin, and a groove is also designed on the corresponding rotation lock.
When the guide pin of the integrated female is put into the guide pin hole of the integrated
male, due to the resistance of the internal elasticity and back pressure of coupling, only a part
of the groove of the guide pin is in contact with the groove of the rotation lock, as shown in
Fig. 5.10(a). As the rotation lock rotates, the guide pin is dragged to move axially, which is
shown in Fig. 5.10(b). During this process, multiple hydraulic circuits begin to couple. When
the rotation lock rotates to the state shown in Fig. 5.10(c), the entire coupling process ends.
When the two grooves correspond to each other, the integrated female connected to the guide
pin can move in translation. In other states, it is in the locked state, thus ensuring the locking
function between the male and the female.
Fig. 5.9 The design of the locked function
83
(a) The state of contact
(b) The state of rotating
(c) The state of coupled
Fig. 5.10 The coupling process of guide pin
84
Fig. 5.11 shows the working principle of the locking module. Fig. 5.11(a) shows that in the
uncoupled state, under the action of the spring, the position locking plate is inserted into the
locking slot on the right side and is in a locked state. When the lever button on the handle is
pressed, the position locking plate is separated from the locking slot, and the locked state is
released, as shown in Fig. 5.11(b). Subsequently, during the rotation of the handle, the position
locking plate will also rotate due to the restriction of the hole position, as shown in Fig. 5.11(c).
After the male is coupled with the female, releasing the lever button, the position locking plate
will be inserted into the upper locking slot under the action of the spring, as shown in Fig.
5.11(d). At this time the coupling is firmly locked, the problems such as slide and slip will not
occur which ensures the safety of the connection during the delivery of high-pressure hydraulic
oil. Rotation lock and connecting lever are rigidly connected, so that the rotation handle can
use the principle of leverage to complete the entire coupling process of the coupler with a small
torque.
Similarly, based on the above mechanism analysis, a prototype of 6-circuit hydraulic quick
action multi-coupling was produced as shown in Fig. 5.12, Fig. 5.12(a) represents the
integrated female while Fig. 5.12(b) means the integrated male. In the process of connection,
under the guidance of the guide pin, the integrated female and the integrated male are quickly
aligned which shows in Fig. 5.13(a). When the red button in the handle module is pressed, the
locked state is released. Subsequently, by turning the handle, it is easier to rotate the handle to
the other side under the action of the principle of leverage. When the coupled process is
finished, then releasing the red button, the coupling is firmly locked under the action of spring
as shown in Fig. 5.13(b). The successful production and connection of the prototype confirms
the correctness and rationality of the design.
85
(a) The state of uncoupled
(b) The state of unlocked
86
(c) The process of turning
(d) The state of coupled
Fig. 5.11 The principle of the locking module
87
(a) The integrated male
(b) The integrated female
Fig. 5.12 The prototype of 6-circuit hydraulic multi-coupling
88
(a) The state of alignment
(b) The state of coupled
Fig. 5.13 The connecting process of prototype
89
5.2.2 Analysis conditions of 6-circuit hydraulic quick action multi-coupling
In order to ensure the safety during use, this research conducted a structural analysis on the
coupled 6-circuit hydraulic quick action multi-coupling, and the analysis method also adopted
numerical investigation based on the finite element method. All parts are also made of S45C
material, and the grid is shown in Fig. 5.14. In the simulation, due to the complexity of the
structure, unstructured grids are still used, and the total number of grids is about 0.26 million.
In the structural simulation, the setting of the boundary conditions is shown in Fig. 5.15.
Because it is difficult to directly apply the contact pair to the guide pin and the rotation lock,
separate loads are applied separately in two areas that are opposite to each other. In order to
observe the structural stability of the integrated female, as shown in Fig. 5.15(a), the A surface
where the contact area between guide pin groove and the rotation lock is the fixed area, and a
separate load of 5000N is applied to the integrated female. In order to observe the structural
stability of the integrated male, the blue area of the integrated male in Fig. 5.15(b) is the fixed
area, and a separate load of 1000N is applied to each of the two rotation locks.
90
Fig. 5.14 The mesh of the 6-circuit hydraulic multi-coupling
(a) The fixed support
91
(b) The fixed support
Fig. 5.15 The boundary conditions of the 6-circuit hydraulic multi-coupling
92
5.2.3 Structural analysis of the 6-circuit hydraulic quick action multi-coupling
Fig. 5.16 is the simulation results of the structural stability of the coupled 6-circuit hydraulic
quick action multi-coupling. Fig. 5.16(a) shows the distribution of stress, the maximum stress
is 80.8MPa, appearing on the guide pin which is caused by the shape of the groove. The stress
distribution on the integrated plug in Fig. 5.16(b) is relatively uniform, and its value is almost
zero. The maximum stress is 97.8MPa caused by the force application point, which appears
on the contact surface between the guide pin and the rotation lock. Fig. 5.16(c) and Fig. 5.16(d)
are the strain distributions of the 6-circuit hydraulic quick action multi-coupling respectively.
It can be found that the maximum strain appears at the maximum stress no matter whether it
is at the integrated male or the integrated female, and the maximum strain value is about
0.0005mm/mm which is less than the maximum allowable strain of the material. Fig. 5.16(e)
is the deformation distribution of the integrated female. The maximum deformation 0.0135mm
appears on the side farthest from the A surface where the guide pin groove and the rotation
lock contact, and it is shown as the red surface in the Fig. 5.16(f) is the deformation distribution
of the integrated male where the deformation at the bottom is almost 0. The deformation on
the handle has obvious layered distribution characteristics. It gradually increases from the
point of force along the handle. At the same time, the deformation distribution on the levers
on both sides of the handle is not symmetrical, it can be speculated that this is caused by the
connecting lever on the handle side. The maximum deformation of 0.0025mm appears at the
top end on the side with the connecting lever. Fig. 5.16(g) and Fig. 5.16(h) show the safety
factor distribution of 6-circuit hydraulic quick action multi-coupling. It can be seen that the
minimum safety factor of the integrated female is 6.06, and the minimum safety factor of the
integrated male is 5.01, which are much higher than the safety characteristics of the material.
Both the minimum safety factor in the integrated male and female appear at the maximum
stress. Not only on the integrated male, but also on the integrated female, the safety factors are
evenly distributed throughout the component with a higher value. From the above-mentioned
93
parameter data, the 6-circuit hydraulic quick action multi-coupling in this study is very safe
and durable.
(a) The stress of integrated socket
(b) The stress of integrated plug
94
(c) The strain of integrated socket
(d) The strain of integrated plug
95
(e) The displacement of integrated socket
(f) The displacement of integrated plug
96
(g) The safety factor of integrated socket
(h) The safety factor of integrated plug
Fig. 5.16 Structural analysis results of the drive part of the 6-circuit hydraulic multi-coupling
97
5.3 Chapter summary
In this chapter, the 4-circuit and 6-circuit hydraulic quick action multi-coupling are
analyzed from the perspective of design mechanism, and the structure stability is analyzed.
The 4-circuit hydraulic quick action multi-coupling uses the leverage principle to design the
handle module, which makes the connection process more labor-saving. The asymmetric guide
pin design not only ensures the correct installation direction, but also facilitates the
simultaneous alignment of 4 pairs of coupling.
In the structural analysis, the contact position of the handle and the guide pin 2 produced a
maximum stress of 1.03MPa and a maximum strain of 0.00057mm/mm. The deformation area
is mainly concentrated on the integrated female and the handle, and the farther away from the
contact point on the handle, the amount of deformation is larger. The maximum deformation
of 0.038mm occurs at the farthest distance from the contact point. The smallest safety factor
is 4.75, which appears at the maximum stress and is much higher than the allowable safety
factor of the material.
In addition, the guide pin in the 6-circuit hydraulic quick action multi-coupling not only has
a guiding function but also a traction function. By adding a locking module to prevent
problems such as slippage of the 6-circuit hydraulic quick coupling. The same leverage
principle is used to complete the coupling connection with a smaller torque.
In its structural analysis, the maximum stress of the integrated female is 80.8MPa located
on the guide pin, which is caused by the groove on the guide pin. The maximum stress of the
integrated male is 97.8MPa, which appears on the contact surface between the guide pin and
the rotation lock. This is due to the point of force application here. In the maximum strain
distribution, regardless of the integrated female or the integrated male, the maximum strain
appears at the maximum stress, and its value is about 0.0005mm/mm, which is less than the
maximum required strain of the material. The maximum deformation of the integrated female
is 0.0135mm, and the maximum deformation of the integrated male is 0.0025mm. The
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minimum safety factor of the integrated female is 6.06, and the integrated male is 5.01, both
appearing at the maximum stress. In this study, whether it is 4-circuit or 6-circuit, both meet
the safety characteristics of the material.
In summary, the 4-circuit and 6-circuit hydraulic quick action multi-coupling drive parts
designed on the basis of the improved NFFCS are in line with the safety characteristics of the
material. This design will save a lot of time and cost for the connection and disconnection of
multi-circuit hydraulic coupling, and is expected to be mass-produced.
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CHAPTER 6
CONCLUSIONS
A novel flat-face coupling system (NFFCS) design incorporating an instantaneous
pressure relief device based on Pascal's law was presented in this research. The mechanism
and kinematics of the NFFCS were explained and compared with a similar sized CFFCS. The
flow characteristics and axial force during the opening process of the internal valve of the
coupling systems were simulated using the CFD dynamic mesh algorithm. Combining with
the ANSYS Mosaic meshing technology, a series of numerical investigations were carried out
on the flow characteristics of the entire system. Based on the distribution of turbulent kinetic
energy, the energy-consuming locations of the initial design of NFFCS were determined and
optimized. Thus, a design with optimal flow characteristics and hydraulic power transmission
efficiency was obtained. In addition, the mechanical properties of CFFCS and NFFCS are
compared and analyzed by CFD fluid-structure interaction method, and the correctness of the
numerical investigation is verified by burst experiment. Based on the structural analysis of
optimal NFFCS, the 4-circuit and 6-circuit hydraulic quick action multi-coupling were
designed and investigated. The conclusions drawn from these investigations are summarized
as follows:
1.
The kinematics of the CFFCS and NFFCS are explained through the establishment of
theoretical models of their coupling processes. In the NFFCS design, there is no major
design change in the socket connector, while a high-pressure relief pin valve based on
Pascal's law is incorporated in the plug connector. During coupling, the pin valve connects
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the two chambers by overcoming a relatively small resistance force, which equalizes the
pressures on both sides of the main valve so that it can be easily opened.
2.
In order to verify the correctness of the NFFCS design, comparative simulations of the
NFFCS and CFFCS coupling processes based on the novel CFD dynamic mesh
technology were carried out. Analysis of the transient simulation results showed that the
opening of the pin valve in the NFFCS has a balancing effect on the pressure difference
between the neighboring chambers. Monitoring of the axial resistance during the opening
of the valves of the two systems showed that the resistance generated by the NFFCS pin
valve is 34% lower than that of the CFFCS. This observation verifies the correctness of
the NFFCS design.
3.
Comparison of the steady-state flow characteristics of the CFFCS and the initial NFFCS
design revealed that although the internal geometry of the NFCCS is more complicated
due to the incorporation of the instantaneous pressure relief device, its pressure drop is
about 2.5% less than that of the CFFCS. This confirms the feasibility of replacing CFFCS
with NFFCS in future hydraulic power transmission applications.
4.
To reduce further the energy consumed during the process of hydraulic power
transmission through the NFFCS, the distribution of turbulent kinetic energy in the initial
NFFCS design was observed. A series of geometric modifications and CFD simulations
were carried out to devise the modified NFFCS design with excellent hydraulic power
transmission efficiency. The pressure drop of the modified design is 42.79kPa, which is
13.5% lower than that of the initial design.
5.
The maximum deformation of CFFCS is 0.0045mm, and the maximum deformation of
NFFCS is 0.0049mm. The maximum deformation of both is far in line with the safety
standards specified by the components. In CFFCS, the maximum deformation occurs at
the connection between the plug and the socket, which is caused by the thin wall. The
largest deformation of NFFCS appears at the outlet, which is caused by the large flow
velocity at the outlet and then leads to the greater impact on the wall.
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6.
The maximum stresses of CFFCS and NFFCS are 114.46 and 114.45 MPa respectively,
which are almost the same, and both are less than the maximum allowable stress of the
material. The minimum stress value of NFFCS is much smaller than CFFCS, and the
distribution range is wide, indicating that the performance of NFFCS is stronger than
CFFCS in stress analysis.
7.
The maximum equivalent strains of CFFCS and NFFCS are 0.00057 and 0.00087
mm/mm, respectively, which are less than the maximum equivalent elastic strain allowed
by the material. In CFFCS, the maximum strain occurs at the maximum stress. In NFFCS,
the maximum equivalent strain appears at the groove inside the plug, which is caused by
the design of chamfering in practical applications.
8.
The minimum safety factor of CFFCS and NFFCS is almost the same, which is 2.184 and
meet the safety characteristics of their materials. Moreover, the minimum safety factor
appears at the maximum stress on the inner wall of the pipeline
9.
In the structural analysis of the 4-circuit hydraulic quick action multi-coupling, it is found
that the maximum deformation is 0.00057mm/mm, the maximum stress is 103MPa, the
maximum strain is 0.00057mm/mm, and the minimum safety factor is 4.75. The 4-circuit
hydraulic quick action multi-coupling drive part has strong structural strength, which can
avoid problems such as disconnection or failure of the connector due to sudden changes
in hydraulic pressure.
10. In the structural analysis of the 6-circuit hydraulic quick action multi-coupling, it is found
that the maximum displacement occurs at the lever handle (0.013477mm), the maximum
strain (deformation) is 0.0005mm/mm, and there is no excessive deformation or fracture.
The maximum stress occurs on the lock core of the guide pin (97.804 MPa), and the
minimum safety factor is 5.01, which meets the level 1 required by the industry.
102
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ACKNOWLEDGEMENT
At the completion of this research, I would like to thank all of those who have given me a
favor to solve some problems and complete my work smoothly.
I would especially like to express my deepest gratitude to my advisor, professor Sung-Ki
Lyu, for his tireless support and guidance throughout my graduate study years, which made
this thesis possible. He was always ready to answer my numerous questions and provide his
technical expertise. I learn a great deal from his expertise in the area of machinery design. I
am grateful for his excellent guidance and invaluable support during the course of my research.
His perseverance and dedication have shaped me to be a better researcher. I also thank the
members of my dissertation committee Prof. Jungwon Yoon, Prof. Su-Jin Kim, Prof. DoYeong Lee and Prof. Long Lu who have generously given their time and expertise to improve
my work. I deeply thank them for their contributions and their supports.
In the division of mechanical and aerospace engineering, I had the good fortune to learn the
knowledge of mechanical design deeply from a great number of professors and seniors. I
would like to thank my dear senior Amre Eizad. His favors have been critical in giving me the
opportunity to pursue further studies and I am very grateful. I also extend my thanks to YongMin Cho, the CEO of Chunma Machinery Co., Ltd., for his help of the production of samples
and test bench in this research.
Besides, this work was supported by the Regional Leading Research Center of NRF and
MOCIE (NRF-2019R1A5A8083201) and Brain Korea 21 Four project of Gyeongsang
National University.
Finally, I sincerely want to thank my husband, my lovely son and my parents for their
everlasting love and support. They have served as a crucial role model and inspiration of my
life.
Thank all of those who helped me during my life in Korea.
2021.08.25
Yuting Wu
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