Abstract
To investigate the motion characteristics and vibration characteristics of elastic composite cylindrical roller bearings (ECCRBs), a simulation model of the ECCRB is established in ABAQUS software. The influences of filling degrees and working conditions on motion characteristics of the ECCRB are compared, and the modal characteristics and harmonic response of the ECCRB are analyzed. Calculation results show that when the filling degree increases from 0% to 60%, there is a nonlinear relationship between the filling degree and the slip rate of cage, the natural frequency of each order decreases, and the vibration displacement of ECCRB increases under the excitation load; increasing the rotation speed and decreasing the radial load reasonably is beneficial to the motion stability of the ECCRB. At the same time, with the increase of the excitation load, the vibration displacement of the ECCRB increases. The analysis results can present some theoretical basis for the structural optimization of the ECCRB.
Keywords
Introduction
Cylindrical roller bearings (CRBs) are essential supporting components, which mainly use the rolling contact between the inner ring, outer ring, cage, and rolling elements to transfer load and motion. CRBs are widely used in rotating machinery such as aero-engine spindles and speed change devices because of their strong radial bearing capacity, large radial stiffness, and small friction factor. The motion and vibration characteristics of CRBs have an essential influence on the working performance and reliability of bearings.
Patra et al. 1 developed a dynamic model of the CRB and analyzed theoretically the influence of the rotation speed on the dynamic characteristics of the CRB under balanced and unbalanced conditions. Cui et al. 2 built a dynamic analysis model considering the cage dynamic unbalance to investigate the influence of the cage dynamic unbalance on the dynamic characteristics. Han et al. 3 presented a nonlinear dynamic model of the CRB and the influences of different working conditions on the motion characteristics of the CRB considering actual working conditions were studied. Rubio et al. 4 used Algor to analyze the dynamic characteristics of the CRB, ignoring the contact impact of the cage. Chen et al. 5 developed a dynamic model of a high-speed CRB to investigate the influences of four aviation lubricants on the centroid trajectory of the cage, and used the Poincaré diagram to analyze the running stability of the cage. Hao et al. 6 built a finite element model of the shaft-CRB-bearing pedestal system considering temperature effects and clearance changes. The dynamic characteristics of the bearing were systematically studied. Considering the roller angular misalignment, the geometric interference model of the cylindrical contact pair was established by Wu et al. 7 to study the contact characteristics of the cylindrical roller with these two logarithmic profiles. Ma et al. 8 developed a theoretical method for calculating roller-pocket oil film pressure and film thickness, and the influences of working conditions and structural parameters on roller-pocket oil film performance were analyzed. Tong 9 presented a quasi-static model with four degrees of freedom of Double-row Cylindrical roller bearings (D-CRBs), and investigated the influences of structural parameters on dynamic characteristics and fatigue life of the D-CRB.
The above studies are all based on traditional solid CRBs. However, when the solid CRBs are under load, the phenomenon of stress concentration on the edges of the rolling elements often occurs. The centrifugal force of the rolling elements is significant when the bearing is running at high speed, which leads to contact fatigue and surface wear of the inner ring, the rolling elements and outer ring. To overcome the above shortcomings, some scholars have done a lot of studies on the structure of rolling elements. A new cylindrical roller model with the logarithmic profile was proposed by Cui. 10 The contact stress of the bearing and the modified reference rating life of the bearing were analyzed. Liu 11 established a dynamic model of the CRB with hollow rollers considering lubrication and analyzed the impacts of different working conditions and hollow roller percentage on bearing vibrations. Solanki 12 proposed a layered cylindrical hollow rolling element, and studied the effects of different hollowness percentages on contact pressure. Yao et al.13,14 investigated the effect of the rolling element of filling degree on dynamic equivalent stress and inner wall bending stress of the ECCRB.
Based on the above research, to study the motion and vibration characteristics of the ECCRB, according to the complex nonlinear dynamic contact relationship between the bearing parts, a finite element model of the ECCRB is built in ABAQUS software. The dynamic and vibration characteristics of the ECCRB are studied. The results can give some theoretical guidance for the engineering application of the bearing.
Theoretical foundation
Cage slip rate
As an evaluation criteria for the motion stability of the bearing, the cage slip rate is used to represent the relative error between the theoretical rotation speed and the actual rotation speed of the cage.
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Assuming that the rollers do not slip, the theoretical rotation speed can be calculated by the kinematic theory of the rolling bearing. Under a given working condition, the actual rotation speed of the cage can be calculated by using the central difference algorithm in ABAQUS software. In the calculation, it is assumed that the inner ring and outer ring of the bearing rotate at the same time. The contact angle α of the rolling elements to the inner and outer rings is the same. Equation (1) is used to represent the linear speed
The cage linear velocity can be expressed as equation (4):
Combining equations (1)–(4), the theoretical angular velocity
The calculation formula of slip rate S of the cage is written as equation (6)
In equation (6),
Mechanical vibration theory
Mode is the evaluation index for the vibration characteristic of a structure as well as a natural property of a structure. When the natural frequency of a structure approaches the excitation frequency, the structure will be damaged by resonance. Through the modal vibration analysis of the ECCRB, the natural frequency of the bearing can be obtained, which is helpful to avoid resonance effectively. 16
Equation (7) is used to calculate the vibration equation of a linear system with n degrees of freedom as follows
Neglecting damping and external loads, the vibration equation of the system can be expressed as equation (8):
Since
Equation (11) can be expanded to obtain the nth degree equation about the frequency
Harmonic response analysis
Harmonic response analysis is the theoretical method to study the vibration displacement of a linear system under excitation load with different frequencies, and obtain the frequencies with the significant influence on the vibration displacement. The equation of motion is determined as equation (9).
In equation (9), the force vector
Analysis of bearing motion characteristics
The establishment of finite element model
Structure dimensions of N2208 roller bearings.
According to the parameters, a three-dimensional model is established in SolidWorks software and imported into ABAQUS software, and the material parameters of each component are set in ABAQUS software. To fit the deformation of all parts during the actual bearing operation, all aspects of the bearing are set as elastomers. The materials of hollow cylindrical rollers and inner and outer rings are bearing steel, which the elastic modulus is 2.0 × 105 MPa as well as the density is 7800 kg/m3, and the Poisson’s ratio is 0.3; The cage is made of nylon, with an elastic modulus of 2600 MPa, a density of 1200 kg/m3 and a Poisson’s ratio of 0.35; The filling body is made of PTFE, with an elastic modulus of 280 MPa, a density of 2200 kg/m3 and a Poisson’s ratio of 0.4. Considering the accuracy and efficiency of calculation, the element type of each bearing component adopts hexahedral element, and the type of hexahedral element is C3D8R.
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The finite element model of the ECCRB with a filling degree of 50% is shown in Figure 1. Mesh generation of bearing with a filling degree of 50%.
Setting of contact properties of bearing parts.
The application of bearing load and speed is completed in the load unit. Due to the way in which the studied bearing is guided by the inner ring, the outer ring in the boundary conditions is steady. In addition to the X, Y direction of the translational degree of freedom and the rotational degree of freedom in the Z direction, the other directions of the cage and the inner ring are limited. The inner ring is applied with rotation speed in the Z direction and radial load in the Y direction, and the gravitational acceleration of y-direction with a magnitude of 9.8 m/s2 is applied to the whole model. The smooth step is chosen as load amplitude curve and speed amplitude curve.
Validation of the model
Structure dimensions of N310E roller bearings.

Comparison of vibration displacement of experimental value and simulation value.
According to the analysis in Figure 2, the average relative error between the finite element simulation value and the experimental value is 5.71%, and the maximum relative error is 8.69%. The change rule of the two is similar, which verifies the correctness of the established finite element model to a certain extent.
Comparison between simulation value and theoretical value of part velocity.
Computational results and analysis
With a filling degree K of 50% and guide clearance c of 0.55 mm, under the working conditions in which the radial load Fr is 2500 N, and the bearing rotation speed n is 2000 r/min, the dynamic response of the ECCRB is calculated and gained.
On the finite element model, node No. 558 on the outer diameter surface of the cage, node No. 235 on the inner ring raceway, and node No. 445 on the end face of the rolling element are selected respectively. The displacements of the cage, the inner ring raceway and the rolling element are shown in Figures 3 and 4 displays the node velocity of the cage. It can be obtained from the figures that at 0.015 s, the displacements of the node No. 558, No. 235 and No. 445 gradually increase, and the velocity of the node No. 445 gradually increases, which means that at this time, the inner ring starts to rotate and the rolling element starts to move driven by the inner ring. The displacements of node No. 558, node No. 235 and node No. 445 change sinusoidally with time. The node displacement period of the inner ring is about 0.03 s, and that of the rolling element and cage node is about 0.0736 s. The node velocity of the rolling element changes sinusoidally with time. The peak represents the velocity when the node touches the inner raceway, and the trough represents the velocity when the node touches the outer raceway. The velocity change period of the rolling element is about 0.01 s, and the average amplitude is about 5.5 m/s. Node displacements of inner ring, rolling element and cage. Node velocity of the rolling element.

Effects of the filling degree on cage stability
When the radial load Fr is 2500 N, the rotation speed of inner ring n is 2000 r/min, and the guide clearance c is 0.55 mm, the finite element models of the ECCRBs with filling degrees of 0%, 40%, 50% and 60% are established respectively. The actual angular velocity and the average value of the slip ratio of the cage are calculated and displayed in Figures 5 and 6. Angular velocity of the cage with different filling degrees. (a) K = 0%; (b) K = 40%; (c) K = 50%; (d) K = 60%. Average slip rate of the cage with different filling degrees.

It can be discovered from Figures 5 and 6 that there is a nonlinear relationship between the filling degree and the average value of cage slip rate. When the filling degree is less than 50%, with the increase of filling degree, the fluctuation range of angular velocity decreases and the average value of the cage slip rate decreases; as the filling degree increases from 50% to 60%, the fluctuation range of angular velocity and the average value of the cage slip rate increases. That is because as the filling degree increases, the mass and stiffness of rolling element decreases, the centrifugal force of rolling element is reduced and the inner ring’s drag effect on the roller is enhanced, which enhances the motion stability of the rolling element. When the filling degree is more than 50%, the stiffness and bearing capacity of the rolling element decrease sharply, which reduces the motion stability of the rolling element.
Influence of rotation speed on cage stability
Under the working conditions in which the radial load Fr is 2500 N, with the filling degree K of 50% and the guide clearance value c of 0.55 mm, the finite element models with the inner ring rotation speeds which are set as 1000 r/min, 2000 r/min, 3000 r/min, and 4000 r/min are established respectively. The actual angular velocity and the average value of the slip ratio of the cage are calculated, as shown in Figures 7 and 8. Angular velocity of the cage with different rotation speeds. (a) n = 1000 r/min; (b) n = 2000 r/min; (c) n = 3000 r/min; (d) n = 4000 r/min. Average slip rate of the cage with different rotation speeds.

Combined with Figures 7 and 8, it can be found that reducing the rotation speed is helpful to suppress the sliding of the cage. As the inner ring rotation speed decreases from 4000 r/min to 1000 r/min, the fluctuation range of angular velocity of the cage is reduced by 27.97 rad/s, and the average value of the slip rate decreases is reduced by 1.09%. The reason for this phenomenon is that as the inner ring rotation speed increases, the rolling element centrifugal force gradually increases. Therefore, the traction of the inner ring acting on the rolling elements is gradually weakened, and the impact frequency between roller and cage increases, leading to the decrease in the overall motion stability of the bearing.
Influence of radial load on cage stability
With the conditions that the inner ring rotation speed n is 2000 r/min, the filling degree K is 50% and the guide clearance c is 0.55 mm, the finite element models with different radial loads are established respectively. The actual angular velocity and the average value of the slip ratio of the cage are calculated, as displayed in Figures 9 and 10. Angular velocity of the cage with different radial loads. (a) Fr = 1500 N; (b) Fr = 2500 N; (c) Fr = 3500 N (d) Fr = 4500 N. Average slip rate of the cage with different radial loads.

Figures 9 and 10 has revealed that the relationship between the bearing motion stability and the radial load acting on the ECCRB is monotonically decreasing. With the increase of the radial load, the cage fluctuation range of angular velocity cage is reduced by 3.52 rad/s, and the average value of the slip rate decreases is reduced by 1.01%. This is because following the increase of radial load, the traction on the rolling element increases, and the movement of the roller tends to be stable, which inhibits the bad impact between roller and cage. As a consequence, the overall operation of the bearing becomes stable.
Analysis of bearing vibration characteristics
The establishment of finite element model
Modals of ECCRB with filling degrees of 0%, 40%, 50% and 60% are established and material properties are set in ABAQUS software, and the specific material parameters are shown in Table 1. Then a modal analysis step is created. The Block Lanczos method is selected to calculate, and the numerical method is used to select the number of eigenvalues requested. 19 Considering that the natural frequency corresponding to the higher-order mode is large, which has little influence on the vibration of the bearing, the first 12 orders of free mode of the bearing are extracted as reference results. The contact between the parts of the bearing is established in the interaction module, and binding constraints are set between the filling body and the hollow cylindrical roller.
Based on the above modal analysis, a harmonic response analysis step is established, and the excitation frequency is set. According to the actual load condition of the bearing, with coupling the inner surface of inner ring to center point of the inner surface, the excitation load is exerted at the bearing center point, and outer surface of the outer ring is fixed. Because the shapes of the bearing components are relatively regular, the mesh of each part of the bearing is divided into a hexahedral mesh with the element type C3D8R.
Modal calculation results and analysis
Natural frequencies of the first 12 orders.
From this table, it can be detected that the natural frequencies at the first six orders of the bearing in the free mode are 0, which is called the rigid mode. A higher-order mode of each bearing tends to correspond to a larger frequency. In addition, the data in the table shows that as the filling degree increases, the natural frequencies of each order of the ECCRB decrease.
Figure 11 shows the seventh-order modal vibration shapes of the ECCRB with different filling degrees, and vibration modal shapes of different orders of the ECCRB with a filling degree of 50% are shown in Figure 12. It can be discovered that for the vibration modal shape of the same order, as the filling degree increases, the vibration displacement of the bearing increases slightly. Each vibration modal shape of the bearing is different. Vibration modal shape of the seventh order with different filling degrees. Vibration modal shape of bearings with a filling degree of 50%.

Harmonic response analysis results
Analysis of bearing harmonic response under variable filling degree
To explore the effect of filling degree on vibration of bearing under excitation load, the same nodes of the cage and the rolling element of each finite element model are selected for comparative analysis, as shown in Figure 13. When the excitation load F is 2500 N, the vibration responses of ECCRBs with the filling degrees of 0%, 40%, 50% and 60% are calculated and compared. Node selection of the bearing finite element model.
Figures 14 and 15 show the vibration displacement of the cage node with excitation frequency and the vibration displacement of the rolling element node with excitation frequency respectively. It can be found from the figures that there are two peaks in the vibration displacements of the cage node and the rolling element node with different filling degrees. The change curve of the node displacement is as follows: increases from 0 to the maximum value at the beginning, then decreases rapidly, soon afterwards increases rapidly to the second peak value, and finally decreases slowly. The maximum peak values and the corresponding frequencies are shown in Table 6. Vibration displacement of cage node with different filling degrees. Vibration displacement of rolling element node with different filling degrees. Maximum peak values and the corresponding frequencies.

It is obvious to see from the table that the relationship between filling degree and vibration displacement is monotonically increasing. This is because the increase of filling degree would lead to the decrease in rolling element stiffness. The frequency corresponding to the peak displacement of the cage node is the same as that of the rolling element node, and the vibration displacement of the cage node is greater than that of the rolling element node. The reason is that the cage material has a smaller elastic modulus and a lower density compared with the rolling element, resulting in the larger elastic deformation of the cage under the excitation load.
Analysis of bearing harmonic response under variable excitation load
With a filling degree of 50%, the vibration response of the ECCRB under different excitation loads F of 1500 N, 2500 N, 3500 N, and 4500 N are carried out respectively. The vibration displacements at the same node of the cage and the rolling element with excitation load are obtained, as shown in Figures 16 and 17. Vibration displacement of cage node with different excitation loads. Vibration displacement of rolling element node with different excitation loads.

From the Figures 16 and 17, it is clear to know that the vibration displacements of the cage node and the rolling element node are consistent with the excitation frequency. At frequencies of 6578.4 Hz and 7214.7 Hz, the vibration displacement of the cage node and the rolling element node reaches the maximum value and the second peak. As the excitation load increases, the node vibration displacement increases. By comparison with the cage node, the vibration displacement of the rolling element node is smaller than that of the cage node, which shows that the excitation load plays a crucial effect on the vibration of the cage.
Conclusions
On the basis of kinematic theory and modal vibration theory, the finite element model of the ECCRB is established, and the influences of different filling degrees and different working conditions on the motion characteristics of the bearing are studied. The modal analysis and harmonic response analysis of the bearing are carried out. The conclusions are as follows: 1. There is a nonlinear relationship between the filling degree and the cage rate, as the filling degree increases, the bearing motion stability increases at the beginning and then decreases. Bearing inner ring speed and radial load affect significantly on the cage rate. Reasonable decrease of rotational speed and increase of radial force could enhance stability of the ECCRB. 2. The relationship between the filling degree and the natural frequency of the ECCRB is monotonically decreasing, and the decrease of the filling degree could decrease the vibration displacement of the ECCRB. With the increase in the excitation load, the vibration displacement of the ECCRB increases.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to thank the Major scientific and technological projects in Shanxi Province (20201102003), Shanxi coal based low carbon joint fund (No. U1610118), National Natural Science Foundation of China (No. 51375325), National Key R&D Program of China (2018YFB1308701), Henan Province Science and Technology Research Project (No. 222102240017) for their support to this research.
