Abstract
Nonlinear dynamic analysis was performed on a planetary gear transmission system with meshing beyond the pitch point. The parameters of the planetary gear system were optimized, and a two-dimensional nonlinear dynamic model was established using the lumped-mass method. Time-varying meshing stiffness was calculated by the energy method. The model consumes the backlash, bearing clearance, time-varying meshing stiffness, time-varying bearing stiffness, and time-varying friction coefficient. The time-varying bearing stiffness was calculated according to the Hertz contact theory. The load distribution among the gears was computed, and the time-varying friction coefficient was calculated according to elastohydrodynamic lubrication (EHL) theory. The dynamical equations were solved via numerical integration. The global bifurcation characteristics caused by the input speed, backlash, bearing clearance, and damping were analyzed. The system was in a chaotic state at natural frequencies or frequency multiplication. The system transitioned from a single-period state to a chaotic state with the increase of the backlash. The bearing clearance of the sun gear had little influence on the bifurcation characteristics. The amplitude was restrained in the chaotic state as the damping ratio increased.
As a standard meshing gear transmission system, the gear transmission meshes in the actual mesh zone AC and EC, as shown in

Fig.1 Tooth surface friction of a standard meshing gear transmission system
As a non-standard meshing gear transmission system, the gear transmission system only meshes in one side of the C point, as shown in Figs.

Fig.2 Tooth surface friction of a gear transmission system meshed upside of the pitch point

Fig.3 Tooth surface friction of a gear transmission system meshed underside of the pitch point
The non-standard meshing gear transmission system with meshing beyond the pitch point can avoid the change of the tooth surface friction force direction.
As a non-standard meshing gear transmission system, the gear transmission system with meshing beyond the pitch point can avoid the change of the tooth surface friction force and has been investigated by scholars in recent years. In 1997, Gao and Zho
In the process of planetary gear transmission, which is inevitably affected by nonlinear factors such as the backlash, bearing clearance, and single and double teeth alternately meshing, the system may be in a multi-period or even a chaotic state, aggravating the vibration and noise and affecting the stability of the system. Domestic and foreign scholars have performed substantial research on the nonlinear dynamics of gears. The scholar
Taking the spur gear as the research object and considering the influence of the tooth surface friction, backlash, and time-varying meshing stiffness on the dynamic characteristics of the system, Wang et al
The effects of the friction coefficient, damping ratio, and clearance on the bifurcation characteristics were examined by using the Poincaré section. In 2015, taking a higher-contract ratio planetary gear system as the research object, L
Thus, the dynamic response, inherent characteristics and load-sharing characteristics of the gear transmission system with meshing beyond the pitch point have been studied. However, domestic and foreign scholars mainly studied the nonlinear dynamics of the standard gear transmission system; nonlinear research on PGTSMPP has rarely been reported and is therefore the focus in this paper.
According to Refs.[
λspi is the meshing phase coefficient between the ith sun‑planet meshing pair and the first sun‑planet meshing pair, λrpi the meshing phase coefficient between the ith ring‑planet meshing pair and the first ring‑planet meshing pair, and λsr the phase difference between the sun and the ring. The corresponding formulas can be written as
(1) |
where zs and zr are the numbers of teeth for the sun and ring gears, respectively; φi is the position angle of the planet, ppb the base pitch of the planet, the point of the opposite tooth surface of the planet base circle relative to the meshing starting point of the sun, the meshing starting point of the ring gear, and dec the decimal part.
A schematic of the friction arm of the planet is shown in

Fig.4 Friction arm of PGTSMPP
In the model established in this paper, the external meshing involves the standard gear meshing pairs, and the inner meshing involves the meshing pairs after the pitch point. The actual meshing line Bpri_1Bpri_2 is located on the side of the pitch point, and the friction arms at any moment according to the geometric relationship are given as follows
(2) |
where hspi_1 and hspi_2 are the friction arms of the sun in the sun‑planet meshing pairs, and hpsi_1 and hpsi_2 the friction arms of the planet in the sun-planet meshing pairs; εsp is the contact ratio of the sun-planet meshing pairs, ωsc the relative angular velocity of the sun relative to the carrier, pb the base pitch; and Nspi_1Nspi_2 the theoretical meshing line of the sun-planet meshing pairs.
(3) |
where hpri_1 and hpri_2 are the friction arms of the planet in the ring-planet meshing pairs, and hrpi_1 and hrpi_2 the friction arms of the ring gear in the ring-planet meshing pairs; εpr is the contact ratio of the ring-planet meshing pairs, ωpc the relative angular velocity of the planet relative to the carrier, and Npri_1Npri_2 the theoretical meshing line of the sun-planet meshing pairs.
The time-varying meshing stiffness of the internal and external meshing pairs was determined using the energy metho

Fig.5 Time-varying stiffness of the external meshing gears of planetary gear transmission system

Fig.6 Time-varying stiffness of the internal meshing gears of planetary gear transmission system
Because of the periodicity of the time-varying meshing stiffness, the Fourier series is used. To simplify the calculation of the dynamical equations, the higher-order terms are usually ignored, and the second-order Fourier series is taken. The formula is shown in Eq.(4), and the results are shown in Figs.
(4) |
where k0 is the average stiffness of the gear pairs, ω the meshing frequency, and φ the initial phase of the meshing stiffness.
In the process of gear transmission, there will be single and double teeth alternating meshing. In the single-tooth meshing area, the load is borne by a pair of teeth. In the double-teeth meshing area, the load is shared by two pairs of teeth. Because of the different meshing positions in the double-teeth meshing area, the distribution of the load between the two pairs of teeth differs. In the double-teeth meshing zone, the total deformation of each pair of meshing teeth is considered to be equal. The load-distribution ratio is the ratio of the maximum load to the total load between the simultaneous meshing teeth. The results of the load distribution calculated by MATLAB are shown in

Fig.7 Load distribution among the teeth of every meshing gear of the planetary gear transmission system
The correct calculation of the load distribution among the teeth lays the foundation for the calculation of the time-varying friction coefficient, which is described in the next section.
The elastohydrodynamic lubrication (EHL) model comprehensively considers the effects of the load distribution, the relative sliding velocity, the rolling speed, the surface morphology, and the lubrication condition of the gear teeth during the meshing process. Comparing the results obtained by calculation models with different friction coefficients with the experimental value
This calculation model is expressed as
(5) |
where Ph is the maximum Hertz contact stress (GPa), R the comprehensive radius of curvature at the contact point (m), SR the slip ratio at the contact point, and Ve the convolution rate (m/s). f(SR,Ph,ηm,Savg ) can be written as
(6) |
The maximum Hertz contact stress is defined as
(7) |
where is the unit normal load (GN/m). is the comprehensive elastic modulus (GPa), which can be calculated as
(8) |
where μ1 and μ2 are Poisson ratios of the driving and driven wheels, respectively, and E1 and E2 the elastic moduli (GPa) of the driving and driven wheels, respectively.
The instantaneous velocity of the two gears at any meshing point can be written as
(9) |
where ρp and ρg are the radii of curvature (m) of the driving and driven wheels, respectively, and s is the distance from the instantaneous meshing point to the actual starting point.
The formulas for the slip ratio, relative sliding velocity, convolution rate, and rolling speed are as follows
(10) |
The root-mean-square value of the roughness is Savg = 0.6 μm. The values of b1, b2,…, b9 are shown in
Bearings are important supporting elements in the gear transmission system and have the function of transferring motion and force. According to analysis and application of rolling bearin
(11) |
where is the outer diameter of rolling body, Z the number of rolling body, the contact angle, and the elastic displacement of bearing rings.
According to the actual conditions, the sun selects a 6213 radial ball bearing. The carrier selects a 6020 radial ball bearing. The unknown quantities in Eq.(11) are shown in
The dynamic model was established via the lumped-mass method, as shown in

Fig.8 Dynamic model of the planetary gear transmission system
As shown in
(12) |
The gear meshing force is caused by the relative displacement along the direction of the meshing line. Therefore, it is necessary to analyze the relative displacement and the force and then derive the dynamic equations of the system. The relative position relationship are shown in Figs.

Fig.9 Relative position relationship of the external meshing gears

Fig.10 Relative position relationship of the internal meshing gears
In the figures, , where is the actual meshing angle of the sun-planet meshing pairs. , where is the actual meshing angle of the ring-planet meshing pairs.
The projection of the relative displacement of the ith sun-planetary gear along the direction of the meshing line can be written as
(13) |
The projection of the relative displacement of the ith ring-planetary gear along the direction of the meshing line can be written as
(14) |
The relative position relationship of the planetary‑planet carrier is shown in

Fig.11 Relative position relationship of the planetary‑planet carrier
Therefore, the projection of the relative displacement of the ith planetary-planet carrier along the X, Y, and tangential directions can be expressed as
(15) |
To facilitate the analysis, the vibration angular displacements (θs, θpi, θr, θc) of the sun, planet, inner ring gear, and carrier are transformed into line displacements (us, upi, ur, uc).
(16) |
where rbs, rbpi, and rbr are the radii of the base circle of the sun, planet, and ring gears, respectively; and rbc is the center distance between the sun and the planet.
According to the force relationship of Figs.
(17) |
where Is is the moment of inertia of the sun; Ms the quality of the sun; and TD the input torque. Fspi and Ffspi are the dynamic meshing force and dynamic friction force of the sun in the ith sun-planet meshing pair, respectively.
(18) |
where Ipi is the moment of inertia of the ith planet and Mpi the quality of the ith planet. Fpsi and Ffpsi are the dynamic meshing force and dynamic friction force of the planet in the ith sun-planet meshing pair, respectively; and Fpri and Ffpri the dynamic meshing force and dynamic friction force of the planet in the ith planet‑ring meshing pair, respectively.
(19) |
where Ir is the moment of inertia of the ring and Mr the quality of the ring. Frpi and Ffrpi are the dynamic meshing force and dynamic friction force of ring in the ith planet‑ring meshing pair, respectively.
(20) |
where Ic is the moment of inertia of the carrier, Mc the quality of the carrier, Tc the output torque, kcu the torsional stiffness, and ccu the torsional damping.
c represents bearing damping; sx, sy, piy, pix, ry, rx, cx, cy are the subscript of the sun gear, the planet, the ring gear and the carrier, respectively. c is determined as Ref.[
Because the dynamic Eqs.(17)—(20) are positive semidefinite, there is rigid-body displacement, and the solution is uncertain. Therefore, the relative coordinates δspi and δrpi are introduced, and the concrete expressions are Eqs.(13) and (14). Thus, we can conclude the following
(21) |
After the elimination of the rigid-body displacement, the system has 3N + 8 DOFs, which can be written as
(22) |
Because the numerical gap of the stiffness value and the vibration micro-displacement value is too large, the calculated results cannot converge under the numerical integration method. To obtain the ideal results, the dimensionless displacement (bc= 1
(23) |
where ksp is the average meshing stiffness of the sun-planet meshing pairs. Then, the dimensionless time and dimensionless displacement are
(24) |
The dimensionless acceleration and velocity can be written as
(25) |
After the elimination of the rigid-body displacement and dimensionlessness, the dynamic differential equations can be expressed as
(26) |
The specific expressions for are
(27) |
According to the above analysis, the effects of the input speed, backlash, bearing clearance, and damping ratio on the global bifurcation characteristics were investigated using the 4—5-order Runge‑Kutta method.
As shown in the previous section, the vibration and instability of the planetary gear transmission system are mainly related to the change of the meshing force. Therefore, it is necessary to study the bifurcation characteristics of the relative displacement along the meshing line. To determine the effect of the velocity on the global bifurcation characteristics, the gear parameters and operation conditions shown in

Fig.12 Bifurcation along the external meshing line direction

Fig.13 Bifurcation along the internal meshing line direction
As shown in Figs.
Because of the similarity of the bifurcation characteristics along the external and internal meshing lines, it is only necessary to analyze the system entering the chaotic channel with the change of the speed, as shown in
Therefore, the appropriate speed can effectively prevent the system from entering the chaotic state, improve the stability, load-sharing properties and lifetime of the system, and reduce the vibration and noise.
To analyze the influence of damping ratio on the global bifurcation characteristics, without losing generality, we consider Ωsp = 1.21 and Ωsp = 2.83, with typical nonlinear dynamic characteristics. The operating conditions are shown in

Fig.14 Bifurcation with different damping ratios
As shown in
To confirm that the increase of the damping ratio can suppress the vibration amplitude of the chaos, the damping ratio coefficients ζ of 0.05, 0.07, and 0.09 are selected; the corresponding bifurcation results are shown in Figs.

Fig.15 Bifurcation with different damping ratios
Therefore, the damping can be increased by changing the material of the meshing pairs, which can effectively ameliorate the vibration characteristics of the system, reduce the noise, and improve the system stability.
To analyze the influence of the sun-planet meshing backlash on the global bifurcation characteristics, without losing generality, we consider Ωsp=0.78 and Ωsp=1.17, with typical nonlinear dynamic characteristics. The operating conditions are shown in

Fig.16 Bifurcation with respect to the backlash
As shown in
Thus, in the design, manufacture, and installation of the gear system, reasonable backlash can effectively prevent the system from entering the chaotic state, which suppresses the system vibration and noise, improves the system stability, and extends the service life of the product.
To analyze the influence of the bearing clearance on the global bifurcation characteristics, without losing generality, we consider Ωsp = 0.85 and Ωsp = 2.05. The operating conditions are shown in

Fig.17 Bifurcation in different bearing clearance
As shown in
(1) With the increase of the damping ratio, the amplitude of the system can be significantly restrained in the chaotic state. The system near the resonant frequency region is often accompanied by an unstable motion state, such as violent change and chaos, and the proper selection of the system speed parameters can effectively prevent the chaotic motion and improve the stability of the system.
(2) The backlash has a great impact on the bifurcation characteristics of the system. With the increase of the backlash, the system transitions from the single-period state to the multi-period and chaotic states.
(3) Compared with the backlash, the bearing clearance of the sun has little effect on the bifurcation characteristics. The influence of the nonlinear characteristics of the system is obvious only in the segment with small parameters of the sun bearing clearance, which indicates the superiority of the planetary gear transmission system.
The results can provide theoretical support for the parameter selection and operating conditions of PGTSMPP.
Contributions Statement
Mr. TANG Xin designed the study. Dr. BAO Heyun wrote the manuscript. Dr. LU Fengxia established the dynamic model. Dr. JIN Guanghu solved the dynamic equations. All authors commented on the manuscript draft and approved the submission.
Acknowledgements
This work was supported by the National Natural Science Foundation of China(No.51975274) and National Key Laboratory of Science and Technology on Helicopter Transmission(Nanjing University of Aeronautics and Astronautics)(No. HTL-A-19K03).
Conflict of Interest
The authors declare no competing interests.
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