Research on service behavior of brake pad fastener snap ring during high-speed train braking

process

Abstract
The service behavior of the snap ring was investigated with the goal of resolving the issue of the brake
pad fastener snap ring failing in the high-speed train braking system. The acquisition and imparting of
the snap ring pretension force were taken into consideration when creating a finite element model of
the brake pad braking process. The train's 50 km/h-23 kN-13.5 s braking circumstances were
simulated using the finite element thermal-mechanical coupling explicit analysis approach. From this,
it was enabled to assess how the movement of the relevant brake pad components affected the snap
ring's displacement and stress. The findings demonstrate that the brake disc temperature rises and then
falls during the braking operation, peaking at 73.34°C. The brake disc temperature band is distributed
in an annular band. The brake pad components are affected by high heat and friction torque during the
braking process, and will adjust their position through the floating structure, causing displacement and
rotation. The displacement and stress variations of the snap ring are influenced by the surrounding
components. The highest displacement and stress fluctuations are seen in the Group VI snap ring,
which is located near the inner side of the brake pad and the speed inlet area. The average
displacement changes in the x and z-directions are 0.1967mm and 0.1233mm, and the average contact
stress and internal stress changes are 9.8MPa and 15.5MPa. The findings of the snap ring's service
behavior serve as the foundation for the investigation of snap ring failure.
Keywords: High-speed train brake pad, Fastener snap ring, Finite element, Preload force calculation,
Service behavior
1 Introduction
High-speed trains' safety concerns have gained considerable attention as their speed rises. One crucial
tool for guaranteeing train operating safety is the stability of high-speed train braking systems.
Disc brakes are used in most train braking systems. Disc brakes are known for their substantial
braking capability, robust heat dissipation capabilities, ease of installation and maintenance, and
straightforward design. Their method of operation is to provide braking power by using the contact
friction between the brake disc and the brake pads in order to slow down. As a result, the brake pad's
steadiness during braking is essential. A high-speed train's disc brake is depicted in Fig. 1.
Fig. 1 High-speed train disc brake
A high-speed train braking pad is depicted in Fig. 2. The service performance of this kind of brake
pad, which is fastened in part by snap rings, has an impact on the brake pad's stability. The snap ring is
an elastic component that controls the movement and positioning of pieces by converting elastic
potential energy into mechanical energy using its structural properties. When the brake disc makes
contact with the fast-rotating brake disc during braking, heat is produced, creating a high-temperature
condition that might cause the components to rotate and make unstable contact with one another.
Because it is a fastening element in close proximity to the component, the snap ring spring's elastic
balance is harmed by the component's rotational displacement during the braking process. The
displacement and stress condition of the snap ring will alter when the vehicle brakes. In extreme
circumstances, the snap ring will fail and stop functioning normally, which will impair the brake pads'
ability to brake and threaten the security of high-speed train operations. As a result, it's essential to
examine the snap ring's service behavior and determine how its movement varies when braking.
Fig. 2 High-speed train brake pad: (a) front side (b) back side
During the braking process, frictional heat will generate high temperatures, and the thermal load will
reduce the performance of the disc brake, affect the contact performance between components, and
have an impact on the normal service of the braking system components. Wang Z [1] et al. studied the
temperature evolution of train brake discs during high-speed braking through field tests, theoretical
analysis, and finite element modeling. Temperature changes caused corresponding fluctuations in the
instantaneous friction coefficient and thermal stress distribution in the disc surface, thus producing
thermal damage. Suxia Zhou [2] et al. studied the effect of friction ring thickness on the heat capacity
and thermal stress of cast steel brake discs of new intercity EMUs. They found that the effect of
thickness on the stress field is higher than the effect on the temperature field, and the thickness has a
linear relationship with the two. Hong H [3] conducted thermo-mechanical friction analysis on brake
discs and brake discs to find out the causes of thermal cracks and verified the analytical model through
a generator. Ziyun W [4] conducted a numerical simulation on the thermal failure of a wet multi-disc
brake during emergency braking caused by thermoelastic instability caused by thermomechanical
coupling, and obtained the contact pressure distribution, temperature field, and stress during the
emergency braking field. The results show that during emergency braking, the temperature at the outer
diameter of the friction plate is high, and a large temperature gradient and stress gradient are generated
at the edge of the radial groove, causing the friction plate to warp. Vishvajeet[5] et al. found the most
suitable material based on thermo-mechanical analysis, which can not only maintain heat generation
but also withstand other mechanical loads. Juraj G [6] et al. elaborated on the temperature rise problem
of friction elements during the braking process, and proposed a temperature stabilization method at the
contact point of the tribological elements during the braking process. In order to reveal the contact
characteristics of the hydraulic brake friction pair in the initial stage of the braking process. Qingrui M
[7] et al. conducted a numerical analysis of the contact characteristics of the friction pair, including the
change rules of contact surface temperature, contact stress, and heat flow density. The theoretical
analysis has been verified by experiments. Topczewska K [8-9] based on the known unsteady
temperature field, determined the spatial and temporal distribution of the quasi-static temperature
stress of the friction element, and analyzed the impact of the friction power time history on the disk
stress state. The results show that at the end of the braking process, tensile normal stress is formed on
the working surface of the brake disc, which may lead to radial cracks on the surface; the nominal
value of the contact pressure in the contact area between the cermet pad and the cast iron disc was
studied. The effect of time on temperature, the maximum temperature decreases linearly with the time
to reach the nominal value of the contact pressure, and the time to reach this temperature increases.
Unstable contract performance during the braking process will produce undesirable torque between
components, which will have a negative impact on the service environment of the fastener and affect
its service performance. Junsheng Q [10-11] et al. established a three-dimensional thermal-mechanical
coupling model of high-speed wheel-mounted brake discs including bolted connections and contact
relationships, which provided support for the design of brake discs and connecting bolts; tested at
different initial speeds Axial load on the bolts connected to the brake disc under emergency braking
conditions. A finite element model was established to analyze in detail the evolution of the bolt axial
load during the braking process. The results show that as the initial speed of emergency braking
increases, the variation range of the axial load of the bench test bolt first increases and then decreases.
The simulated bolt axial load variation range is 55.5kN, and the error is 7% compared with the test.
Zeng Z [12] et al. studied the longitudinal resistance characteristics of WJ-8 fasteners under different
torques and vertical loads. P. D H [13] analyzed the loosening moment under the action of axial
external load, gave a calculation example, and defined the fastener locking requirements. Xiaolu C
[14] et al. studied the fracture mechanism and control method of fastener spring bars in the high-risk
section of rail corrugation, established a numerical simulation model of the wheel-rail-fastener system,
considered the resonance response and fatigue failure of fastener spring bars, and explored The
fracture mechanism of fasteners and corresponding control methods are proposed. The results show
that the wheel-rail friction coupling vibration in the corrugation zone causes the resonant response of
the fastener spring bar, which intensifies the vibration response of the fastener spring bar. The
intensified vibration response of the fastener spring bar leads to fatigue of the elastic bar. The outer
spring bar on the small radius curve low rail is smaller than the innerspring bar.
Spring strips are more likely to break. Wei L [15] et al. studied the causes of rail corrugation. Research
results show that short-pitch rail corrugation is caused by the wavelength-fixing mechanism of vertical
rail bounce on elastic fasteners. The corrugation wavelength is mainly related to the vertical stiffness
of the rail fasteners. Loiseau Marthe[16] et al. studied the creep behavior of bonded fasteners under
different load levels. Creep modeling using the nonlinear and analytical methods of the Burger model
to identify material creep properties through real-scale experimental studies of bonded assemblies, a
numerical model developed to evaluate the consistency of analytical methods. Gao Xiaogang[17] et al.
took the W300-1 fastener as the research object, tested and summarized its service performance and
online track mode identification, and obtained the dynamic characteristics of fasteners with pre-
tightening force under temperature-related and frequency coupling conditions. Change patterns.
Research has found that the stiffness of the elastic backing plate in the fastener system is sensitive to
low temperatures and high frequency. Feifei Huang[18] found that the elastic force of the U-shaped
snap ring of the CRH2A EMU brake plate support became weak and could not closely adhere to the
gear iron. The snap ring could even be opened with bare hands without using tools during
maintenance.
Jixia Zhen[19] et al. analyzed the failure of the input shaft snap ring of the wind power yaw reducer
and found that the friction between the various parts was too frequent, resulting in the snap ring and
the groove not being better connected and running in during operation changes, resulting in The snap
ring fell off. Liang Ji[20] et al. also studied the wind power yaw reducer and found that passive yaw is
the main reason for the failure of the snap ring.
Scholars have studied and analyzed the problems caused by frictional heat generation during the
braking process, and found that thermal cracks and other undesirable phenomena will appear on the
surface of braking parts, affecting their service performance; at the same time, the contact performance
is poor, resulting in poor torque. The problems that lead to the harsh service environment of fasteners
were studied and analyzed, and the load change rules of fasteners and the adverse consequences aused
by the failure of fasteners to serve normally were summarized. Because the braking process involves a
complex coupling of thermal and mechanical interactions, there aren't many studies on the service
behavior of snap rings under thermal-mechanical coupling. The heat in the environment will not only
cause adverse effects on components that are in direct contact with each other, but will also be
transmitted to other components in contact, resulting in poor contact and vicious torque, which is
detrimental to the service environment of the snap ring.
This article does a braking finite element simulation analysis and models the brake pad using the finite
element method. First, the load that the snap ring experienced during service was studied, and its
detrimental consequences on the snap ring's service behavior were analyzed. Second, the pre-
tightening force of the snap ring was determined by running a tooling finite element simulation on it.
Next, the preloaded and distorted snap ring is added to the brake pad's finite element model for
simulation. It has been determined that temperature variations during braking lead to poor component
contact and displacement changes, both of which have an impact on the snap ring. The torque effect is
produced, which causes the snap ring to shift in stress and displacement to different degrees. Lastly, a
summary of the snap ring's displacement and stress variations during braking is provided. The snap
ring groups and service locations experiencing significant motion changes are identified, and their
causes are analyzed.
2 Load during service of snap ring
The brake pad fastener snap ring plays a role in connecting the steel back, support ing the positioning
seat and friction body, and fixing the position of the components during service. Preload force is
supplied by the snap ring in both the axial and radial axes. The high-speed spinning brake disc and the
brake pads make contact throughout the braking process. As a result, the snap ring bears a variety of
adverse loads, namely impact and braking torque, which have an impact on the snap ring's axial and
radial service.
2.1 Braking torque
The primary source of braking force for disc brakes is the contact friction between the brake discs and
brake pads. The braking torque is produced by the frictional contact force between the components:
（1-1）
The braking torque will produce a radial load on the fastener snap ring:
（1-2）
Among them, is 𝐹 the braking pressure of the brake pad; 𝐷𝑡 is the friction radius; 𝜇 is the friction
coefficient between the brake pad and the brake disc; 𝑖 is the number of snap rings.
2.2 Impact load
The IEC 61373 standard is used to determine the impact load. The maximum impact acceleration in all
directions for components mounted on shafts is 1000 m/s2. The purpose of the fastener is to prevent
the steel back, support positioning seat, and friction body of the brake pad from separating from one
another when subjected to an axial acceleration of 1000 m/s2. This separation is caused by inertia. The
axial load sustained by each snap ring is determined by formula (1-3).

（1-3）
Make that the friction body, steel back, and support positioning seat in the brake pad don't slide in
relation to one another while exposed to a radial acceleration of 1000 m/s2. One can compute the
radial load supported by the snap ring fastening using the formula (1-4).
（1-4）
Among them, is the 𝑀𝑓𝑟 mass of the parts to which the brake pad is fastened; 𝑎 is the impact
acceleration; 𝑖 is the number of snap rings.
3 Calculation of brake pad fastener snap ring pre-tightening force
A unique fastener that is essentially an elastic component is the snap ring. In order to guarantee that it
has a pulling impact on the friction body during the braking process and fulfills the purpose of
tightening, it must be tooled before being formally placed into service in order to obtain pre-tightening
force. For the purpose of ensuring that the snap ring has an initial preload force in the brake finite
element model, the snap ring must be imported into the model in a deformed form and assigned an
initial state.
3.1 Brake pad structure
The friction body, steel backing, snap ring, and support positioning seat make up the brake pad.
There is 1 steel back, 6 support positioning seats, 18 friction bodies, and 18 snap rings altogether.
Each support positioning seat, three friction bodies, and three snap rings make up a group. The braking
pressure is applied to the steel back through the brake piston, transmitted to the friction body through
the support positioning seat, and comes into contact with the high-speed rotating brake disc to generate
braking force.
A unique structure of the brake pad is formed by the friction body, the support positioning seat, and
the steel back adopting a ball pair type contact, as seen in Fig. 3. These components of this contact
type have gaps between them. When the brake pad comes into contact with the high-speed rotating
brake disc during the braking process of high-speed trains, the static friction force is changed into
sliding friction force, damaging the balance of each component. Friction heat is also produced on the
contact surface between the brake disc and the friction body. The material swells when heated to high
temperatures, creating an unstable contact between the friction body and the brake disc. The ball pair
contact between the floating structure can cause the friction body and the support positioning seat to
distort and deflect. Stable braking performance is ensured by the floating structure, which keeps the
friction body and brake disc in close touch with one another throughout the braking process.
Fig. 3 Cross-sectional change diagram of floating structure
There are three friction bodies and three snap rings in the support positioning seat. The pin portion is
attached to the steel back's guide hole in order to perform a positioning function that restricts each
group of friction bodies' range of motion and guarantees the stable contact of each group of friction
bodies. A crucial element linking the friction body and the snap ring is the support positioning seat,
whose displacement changes during braking will impact the friction body via the floating structure and
alter the elastic balance of the snap ring.
3.2 Snap ring modeling and meshing
With an upper and lower structure joined by a space curve, the snap ring has a unique structure.
To draw a curve in modeling, the top and bottom portions are separated into two planes. Bridge curves
are used to join the top and lower connecting sections, and the entire transition is seamlessly
integrated.
Finally, the snap ring modeling is finished by drawing a circular surface on one end of the curve and
using the sweep command. The snap ring's mesh division diagram is displayed in Fig. 4. The snap ring
mesh type of choice is the hexahedral mesh. In contrast to other mesh types, the hexahedral mesh
exhibits superior quality, reduced quantity, and increased computation accuracy. There are 5,508
meshes in all.
Fig. 4 Mesh division of the snap ring
3.3 Basic assumptions and boundary conditions
The following assumptions are taken in the process of establishing the finite element of the snap ring
tooling, taking into account that the tooling is simply utilized to acquire the preload force of the snap
ring for future braking analysis:
1) Without taking into account the effects of force and component deformation, the snap ring is a
flexible body component and the other components are set as rigid bodies;
2) The snap ring's wear is not taken into account, and the friction coefficient between the components
is 0.2;
3) The impact of heat is not taken into consideration when tooling the snap ring.
The process of tooling the snap ring is divided into two steps: 1) The guide rod, steel back, support
positioning seat, and friction body are fully restrained, and the pressure block is displaced downward
to press the snap ring into the groove; 2) Release all Constraints use the structural characteristics and
relationships between components to achieve adaptive adjustment of the snap ring state.
The brake pad structure is shown in an exploded view in Fig. 5, and the boundary condition setup for
the finite element model of the snap ring tooling is shown in Fig. 6. The guide rod, steel back, support
positioning seat, and friction body are all fully fixed, and the snap ring is installed by applying
downward displacement to the pressure block.
Fig. 5 Exploded view of brake pad structure
Fig. 6 Finite element model of snap ring tooling
3.4 Analysis of snap ring tooling results
The stress cloud chart of the snap ring tooling is shown in Fig. 7, and the stress cloud chart of a single
snap ring is shown in Fig. 8. The snap ring's overall stress condition is good, and its maximum stress
result, 611.7MPa, is a local flash point value. The location is where the snap ring and pressure block
come into contact. Here, the pressure of the pressure block creates the local stress flash point.
The maximum stress value of the snap ring mostly occurs in the radius expansion area, which is
subject to the expansion pulling force; secondly, it is at the contact area between the upper plane of the
snap ring and the pin at the upper end of the friction body, where the snap ring provides upward
pulling force to tighten the friction body. With the snap ring's local stress flash point value excluded,
the overall stress is around 360MPa, and the snap ring material's stress yield value is 605MPa, which
is less than the yield strength. It is thought that brake simulation can be performed using the snap ring
tooling results.
Fig. 7 Stress cloud diagram of snap ring tooling results
Fig. 8 Preloading form of a single snap ring
4 Establishment of finite element model of brake pad braking process
4.1 Basic assumptions
When creating the brake finite element model, the following presumptions are made because the finite
element simulation cannot be entirely compatible with the real working conditions:
1) The contact interface between the brake disc and the brake pad is ideal;
2) The motion during the braking process is uniform deceleration motion;
3) The friction coefficient between the brake disc and the brake disc remains constant, disregarding the
impact of brake disc wear on brake disc temperature;
4) During the braking process, 90% of the kinetic energy of the system is converted into friction heat;
5) The materials of the brake disc and brake pad are all isotropic.
The relevant material parameters of each component of the brake pad and the brake disc are presented
in Tables 1 and 2.
Table 1 Material properties of each component
Table 2 Material parameters of brake discs changing with temperature
4.2 Boundary conditions
The boundary condition settings for the braking simulation are displayed in Fig. 9. 50 km/h, 13.5 s of
braking time, 23kN of braking pressure applied through the brake piston, and 7.26MPa of converted
pressure are the parameters for the brakes. The thermal-mechanical coupled explicit analysis approach
is the analysis technique used. An initial room temperature of 20°C and an initial angular velocity of
36.2rad/s is applied to the brake disc before the analysis starts. Following the analysis, the steel back
only releases the x-direction degree of freedom, limiting the other degrees of freedom, and the brake
pressure is applied to the brake piston. The brake disc defines the angular velocity amplitude to cause
uniform deceleration. To ensure that both brake pads have the same degree of freedom in a braking
situation, x-direction symmetry and anti-symmetric restrictions must be added to the brake disc's back
during the simulation of a single brake pad.
Fig. 9 The braking finite element model
4.3 Number and group the snap rings
The supporting positioning seat serves as the pivot for the grouping of 3 friction bodies and 3 snap
rings that make up the brake pad's structural features. Eighteen snap rings are numbered and arranged
together as seen in Fig. 10. The snap ring is divided into the speed inlet area (groups IV and VI), the
transition area (groups II and III), and the speed outlet area (groups I and V) based on the brake disc's
rotation direction; the snap ring is further divided into inner, middle, and outer rings based on the
brake disc's diameter direction.
Fig. 10 Snap ring position numbering and grouping
5 Analysis and discussion of simulation results
5.1 Analysis of brake disc temperature results
The temperature cloud diagram of the brake disc at different times throughout the braking operation is
displayed in Fig. 11. Fig. 11(a) shows that in the early stage of braking, the highest temperature band
of the brake disc appears on the outside. Fig. 11(b) shows that in the middle stage of braking, the brake
disc forms three temperature bands. This is because the brake disc's speed reduces with increasing
braking duration, its exterior heat dissipates more effectively than its interior, and its interior
temperature rises more steadily than its exterior temperature. The brake disc's maximum temperature
now shows up in the interior temperature zone. Fig. 11(c) is at the end of braking, the brake disc
temperature shows a flake distribution, which is a manifestation of the heat transfer effect of the brake
disc.
(a) (b) (c)
Fig. 11 Brake disc temperature cloud diagram at different times: (a) 4.05s, (b) 7.425s, (c) 13.5s
The brake disc's maximum temperature curve is shown in Fig. 12. The brake disc's maximum
temperature, at 4.05s of braking, is 73.34°C, which is in the zone outside the disc. Because the heat
dissipation effect is better on the outside of the brake disc than it is on the inside, the maximum
temperature value of the brake disc shows an upward trend at 7.425s. The highest temperature point of
the brake disc appears in the temperature zone on the inside of the brake disc, and the value changes
slightly. The maximum temperature of the brake disc demonstrated an overall tendency of initially
rising and then falling during the entire braking procedure, which is in line with the real
circumstances.
Fig. 12 Maximum temperature curve of brake disc
5.2 X-direction displacement change of snap ring
5.2.1 X-direction displacement phenomenon of snap ring
The snap ring produces mechanical force to tighten the friction body by structural deformation at the
start of braking, and the warp height of the deformed snap ring is h1. Because of its rounded bottom,
the snap ring is susceptible to the force and movement of the surrounding components when braking,
which will cause a change in the snap ring's x-direction displacement. The warp height of the
deformed snap ring becomes h2 at the end of braking. Take the snap ring's x-direction displacement
cloud diagram at the beginning and end times. Because of the small change, the snap ring's degree of
warp is magnified ten times. Fig. 13(a) shows a schematic diagram of the snap ring's warp height
decreasing in the x-direction, and Fig. 13(b) shows a schematic diagram of the snap ring's warp height
increasing in the x-direction. Δh=h1-h2, Δh>0, indicates that the component's force applied to the snap
ring decreases as the snap ring's height decreases during braking; Δh <0, on the other hand, indicates
that the component's force increases as the snap ring's height increases during braking.
(a) (b)
Fig. 13 Displacement changes in the x-direction of the snap ring: (a) height decreases, (b) height
increases
5.2.2 Causes of x-direction displacement of snap ring
Due to the existence of a floating construction, there is a space between the steel back and the brake
pad support positioning seat. The high temperature of the friction interface will have an impact on the
performance of the contact during braking. There is instability in the friction body-brake disc
interaction. This is sent to the support positioning seat, where it acts on the snap ring and modifies its
displacement in the x-direction. The support positioning seat is a hub component and has the most
obvious changes.
A cloud chart representing the displacement change of the support positioning base in the x-direction
is shown in Fig. 14. Fig. shows that the warp of the seats in groups V and VI for support positioning is
greater in the x-direction. The snap ring's displacement in the x-direction will be impacted by the
warping of the support positioning seat, and the degree of variation in the snap ring will vary.
Fig. 14 Displacement changes in the x-direction of the support positioning base
5.2.3 X-direction displacement results of snap ring
The x-direction displacement change of 18 snap rings throughout the braking process is given in mm,
as Table 3 illustrates. The height at the beginning moment less the height at the end time is the change
in the snap ring spring height during the braking process. In other words, Δh=h1-h2, where Δh is the
change in the snap ring's x-direction displacement during braking.
Table 3 Displacement changes in the x-direction of the snap ring during braking
A cloud chart showing the change in height at various snap ring locations is shown in Fig. 15.
Three snap rings form a group. together. When there is a positive height difference, the snap ring's
final height is lower than its starting height. There is less force applied to the snap ring by the
component.
When the height difference is negative, it means that the component is applying more force to the snap
ring. The results show that the majority of the changes in the snap ring's height difference are positive,
which suggests that the majority of the forces applied to the snap ring by the components during the
braking process tend to decrease.
Fig. 15 Cloud diagram of the displacement results of the snap ring in the x-direction
The positive and negative changes in the height of the snap ring represent the increase or decrease in
the force exerted by the component on the snap ring. Since the displacement change of the snap ring
during the braking process is the main focus of this research and analysis, the absolute value change is
taken into consideration when analyzing the snap ring displacement change results.
Analysis of snap ring x-direction displacement results:
(1) The snap rings are divided into six groups. The average displacement change in the x-direction of
each group of snap rings is calculated. Group VI snap rings have an average displacement shift in the
x-direction of 0.1967mm, with the highest change.
(2) Snap rings in the speed entrance area have an average x-direction displacement change of
0.1567mm; in the transition area, the average x-direction displacement change is 0.1417mm; in the
speed exit area, the average x-direction displacement change is 0.1167mm. Groups IV and VI
comprise the speed entrance area, where the snap ring moves most forcefully in the x-direction.
(3) The inner ring snap ring's average x-direction displacement change is 0.1575mm, and the middle
ring snap ring's average x-direction displacement change is 0.1733mm, the outer ring snap ring's
average x-direction displacement change is 0.1025mm. The middle ring snap ring has the biggest
average displacement change in the x-direction, followed by the inner ring snap ring's average
displacement change in the x-direction, and the outer ring snap ring's average displacement change in
the x-direction, which is the smallest. This is so because the piston, which is housed in the inner and
middle rings, is the one that receives the braking pressure. The contact between the components is
better, which has a greater impact on the movement of the snap ring.
5.3 Z-direction displacement change of snap ring
5.3.1 Z-direction displacement phenomenon of snap ring
Through the clamping component, the snap ring accomplishes its goal of regulating the positioning of
its movement range. The z-direction displacement of the snap ring, meaning the snap ring's opening
distance, will vary as a result of displacement changes and component impacts during the braking
process. The degree of expansion of the snap ring is amplified ten times due to the modest quantity of
change. The displacement cloud diagrams of the snap ring at the beginning and end of the process are
displayed in Fig. 16. By calculating the difference between the displacement values of two
symmetrical places at the snap ring opening end, the distance of the snap ring opening is found. The
snap ring's opening distance at this point is d1, and it will be d2 at the end of the time. It is specified
that when Δd=d2-d1, Δd>0, the snap ring expands while braking, and when Δd<0, the retraction will
occur.
Fig. 16 Displacement changes in the z-direction of the snap ring
5.3.2 Causes of z-direction displacement of snap ring
The disc brake's operation relies on the frictional contact between the brake disc and friction body to
produce braking force. Each component will exhibit a fairly noticeable displacement along the
tangential direction of the brake disc rotation, that is, in the z-direction, following the conversion of
static friction into sliding friction. The fastener snap ring will be negatively impacted by this and will
be shifted in the z-direction.
The support positioning seat's z-direction displacement cloud diagram may be shown in Fig. 17.
The support placement base exhibits an anticlockwise rotation tendency, as can be observed in the
image. The support positioning seat serves as a pivot component. The friction body will deflect as a
result of the support positioning seat's displacement in the z-direction. The snap ring's equilibrium
state will be harmed by component displacement, as seen in Fig. 18, and the snap ring's displacement
in the z-direction will alter.
Fig. 17 Z-direction displacement cloud diagram of the support positioning base
Fig. 18 Effect of friction body displacement on snap ring service
5.2.3 Z-direction displacement results of snap ring
The opening variations of the 18 snap rings during the braking process are displayed in mm, as
indicated in Table 4. The change in the snap ring's opening end during the braking process is equal to
Δd=d2-d1, which is the snap ring's opening distance at the end time minus the opening distance at the
beginning time. The snap ring's displacement shift in the z-direction during braking is represented by
Δd.
Table 4 Displacement changes in the z-direction of the snap ring during braking
The z-direction displacement change cloud diagram for each snap ring at various places is displayed in
Fig. 19. Three snap rings form a group. The snap ring opening distance at the end time is subtracted
from the snap ring opening distance at the beginning time to determine the opening change.
The results demonstrate that all of the opening changes are positive, indicating that every snap ring
exhibits an expansion tendency when the brakes are applied.
Fig. 19 Cloud diagram of the displacement results of the snap ring in the z-direction
Analysis of the displacement results of the snap ring in the z-direction:
(1) The snap rings are divided into six groups. The average displacement change in the x-direction of
each group of snap rings is calculated. Group VI snap rings have an average displacement shift in the
z-direction of 0.1233mm, with the highest change.
(2) Snap rings in the speed entrance area have an average z-direction displacement change of
0.1083mm; in the transition area, the average z-direction displacement change is 0.0733mm; in the
speed exit area, the average z-direction displacement change is 0.0983mm. Groups IV and VI
comprise the speed entrance area, where the snap ring moves most forcefully in the z-direction.
(3) The inner ring snap ring's average z-direction displacement change is 0.1575mm, and the middle
ring snap ring's average z-direction displacement change is 0.0833mm, the outer ring snap ring's
average z-direction displacement change is 0.0678mm. The inner ring snap ring has the biggest
average displacement change in the z-direction, followed by the middle ring snap ring's average
displacement change in the z-direction, and the outer ring snap ring's average displacement change in
the z-direction, which is the smallest. This is so because the piston, which is housed in the inner and
middle rings, is the one that receives the braking pressure. The contact between the components is
better, which has a greater impact on the movement of the snap ring.
5.4 Changes in the stress of snap ring
5.4.1 Results of change in contact stress of snap ring
As a fastening element, the snap ring plays a clamping and positioning role in the brake pad and is in
close contact with the component. There is reciprocal displacement between components during the
braking process, as well as variations in motion. Because of component interaction, there will be a
concentration of stress. Fatigue failure of fasteners will result from prolonged concentration of stress.
As a result, analysis of the snap ring's contact stress results is required.
Fig. 20 shows the contact stress distribution at the end of the snap ring braking. It can be seen from the
stress cloud diagram of the snap ring that the stress concentration flash points are mainly concentrated
at the front end and middle arc position of the snap ring. The stress concentration flash point at the
front end of the snap ring is caused by the contact between the pressure block and the snap ring during
the tooling in the previous analysis step, which corresponds to the results of the snap ring tooling; The
stress concentration flash point appears in the middle arc of the snap ring because it is in contact with
the pin of the friction body. During the braking process, the snap ring and the friction body will have
an interaction force. The contact surfaces rub against each other, and stress concentration occurs in the
snap ring.
The distribution of contact stress at the end of snap ring braking is depicted in Fig. 20. It can be seen
from the stress cloud diagram of the snap ring that the stress concentration flash points are mainly
concentrated at the front end and middle arc position of the snap ring. The contact between the
pressure block and the snap ring during tooling in the preceding analytical phase is what causes the
stress concentration flash point at the front end of the snap ring, which is consistent with the snap ring
tooling results. The stress concentration flash point appears in the middle arc of the snap ring because
it is in contact with the pin of the friction body. During the braking process, the snap ring and the
friction body will have an interaction force. The contact surfaces rub against each other, and stress
concentration occurs in the snap ring.
Fig. 20 Contact stress cloud diagram of snap ring
The maximum contact stress values at the beginning and end moments of the 18 snap rings and
changes are displayed in Table 5.
Table 5 Maximum contact stress value amount at the initial and end moments of the snap ring and
Change
The contact stress cloud diagram and the maximum contact stress change at the end of the 18 snap
rings are displayed in Fig. 21.
Fig. 21 Contact stress distribution cloud diagram and variation of snap ring
The positive and negative changes in the contact stress of the snap ring represent the increase or
decrease in the maximum contact stress of the snap ring. Since the focus of this research and analysis
is on the stress change during the snap ring's braking process, the absolute value is taken into
consideration when analyzing the results of the change in contact stress of the snap ring.
Analysis of snap ring contact stress results:
(1) The snap rings are divided into six groups. An average is computed for the maximum contact stress
change for every set of snap rings. Group VI snap rings had the biggest change in maximum contact
stress, with an average of 9.8MPa.
(2) The snap ring's average maximum contact stress change in the speed inlet zone is 5.1MPa, while
the average maximum contact stress change in the transition zone is 0.5MPa and the average
maximum contact stress change in the speed exit zone is 3.1MPa. In the speed entrance area (group IV
and group VI), there is better contact between the snap ring and the components, and the stress change
is more noticeable.
(3) The inner ring snap ring's average maximum contact stress change is 5.4MPa, the middle ring snap
ring's average maximum contact stress change is 4.4MPa, and the outer ring snap ring's average
maximum contact stress change is 0.5MPa. The inner ring snap ring has the biggest maximum contact
stress change. This is because the braking pressure is applied at this position, the contact between the
snap ring and the components is better, and the impact is greater. The snap ring moves violently, and
the average contact stress change is larger.
5.4.2 Result of internal stress change of snap ring
The snap ring is an elastic element that relies on its own structural characteristics to convert elastic
potential energy into mechanical energy. It is in a state of deformation during the entire service
process. There are areas of stress concentration inside it. Long-term stress concentration will lead to
fatigue damage to the internal structure. Therefore, it is necessary to analyze the internal stress
distribution of the snap ring and obtain the changing trend of the internal stress of the snap ring at
different positions.
As can be seen in Fig. 22, the snap ring's service height is h. To produce the snap ring's stress profile
cloud diagram, a section with a height of h/2 is constructed on the y-z plane. The snap ring's internal
stress cloud diagram is displayed in Fig. 23. The snap ring's internal stress cloud diagram illustrates
that the snap ring's maximum internal stress is primarily concentrated in the snap ring structure's
bending area. This is due to the snap ring's constant expansion and tightness, which serves to tighten
the friction body. Furthermore, it is also very evident where the stress is at the point of contact
between the friction body pin and the snap ring. This is a result of the snap ring's constant contact with
the pin portion, which is similar to the snap ring's contact stress result.
Fig. 22 Schematic diagram of the position of the snap ring cross-section
Fig. 23 Internal stress cloud diagram of the snap ring
The maximum internal stress values at the beginning and end moments of the 18 snap rings and
changes are displayed in Table 6.
Table 6 Maximum stress value amount inside the snap ring at the initial and final moments and
Change of 18 snap rings are displayed in Fig. 24.
Fig. 24 Cloud diagram and variation of stress distribution inside snap ring
The positive and negative changes in the internal stress of the snap ring represent the increase or
decrease in the maximum internal stress of the snap ring. Since the focus of this research and analysis
is on the stress change during the snap ring's braking process, the absolute value is taken into
consideration when analyzing the results of the change in internal stress of the snap ring.
Analysis of internal stress results of snap ring:
(1) The snap rings have been divided into six groups. The average maximum stress change inside each
group of snap rings is calculated. Group VI's snap rings have the largest average maximum stress
change, at 15.5MPa.
(2) The average maximum stress change inside the snap ring in the speed inlet zone is 15.1MPa, the
average maximum stress change inside the snap ring in the transition zone is 12.7MPa, and the
average maximum stress change inside the snap ring in the speed exit zone is 10.8MPa The snap ring
at the velocity entrance area (groups IV and VI) has the most noticeable changes in internal stress, and
it also undergoes the greatest geometrical changes.
(3) The average maximum stress change inside the inner ring snap ring is 19.0MPa, the average
maximum stress change inside the middle ring snap ring is 7.4MPa, and the average maximum stress
change inside the outer ring snap ring is 13.9MPa. The maximum stress change inside the inner ring
snap ring has the largest average value. This is because the braking pressure is applied at this position,
the contact between the snap ring and the components is better, and the influence is greater. The
change in the shape of the snap ring is the most obvious, and the average internal stress variation is
larger.
6 Conclusions
This paper creates a finite element model of the brake pad, simulates the braking of high-speed trains,
studies the fastener snap ring's service behavior during the braking process, and analyses and
characterizes the changes in the snap ring's service behavior from the standpoints of displacement and
stress. From these analyses, the following conclusions are drawn:
(1) The brake disc's maximum temperature, 73.34°C, is found on its outside when braking to 4.05s; as
braking time grows, the maximum temperature band slides inside, and at 7.425s, it is found on the
inside of the brake disc; The brake disc's heat dissipation is manifested at the end of braking when the
disc surface temperature is dispersed in a sheet form. The highest temperature of the brake disc surface
exhibited a trend of initially increasing and then dropping during the entire braking procedure;
(2) During the braking process, affected by high heat and friction torque, the support positioning seat
will be displaced in the x- and z-directions with the help of the floating structure to ensure that the
friction body and the brake disc are closely adhering to maximize braking performance. The
movement of the brake pad components will affect the service stability of the fastener snap ring, and
the snap ring will have changes in displacement and stress;
(2) High heat and friction torque during the braking process will cause the support positioning seat to
move in the x- and z-directions with the assistance of the floating structure. This movement will
improve the braking performance by ensuring that the friction body and the brake disc are closely
adhered to. The service stability of the fastener snap ring will be affected by the displacement and
stress changes of the snap ring due to the movement of the brake pad components.
(3) During the braking operation, the fastener snap ring will experience displacement and stress
changes that are influenced by the displacement of the brake pad components. Among them, the
displacement and stress changes of the snap ring in the velocity entrance area are always greater than
those of the snap ring in the velocity exit area. The displacement change of the inner ring and the
middle ring snap ring of the brake pad is greater than that of the outer ring snap ring, and the stress
change of the inner ring snap ring is greater than that of the middle ring and outer ring snap ring. On
the inside of the brake pad and at the speed inlet area, the Group VI snap rings' displacement and stress
changes are most obvious.