Elsevier

Tribology International

Volume 138, October 2019, Pages 215-238
Tribology International

Numerical study of the effects of textured shaft on the wear of rotary lip seals

https://doi.org/10.1016/j.triboint.2019.05.037Get rights and content

Highlights

  • A numerical study on the effect of shaft texture on rotary lip seal wear is proposed.

  • The effect of texture depth and parameters on the seal wear is analyzed numerically.

  • The seal wear rate decreases with the increase of the textured depth.

  • The seal wear rate decreases with the increase of the textured area ratio.

  • Generally, the seal wear is susceptible to the length of textures in axial direction.

Abstract

The effects of the textured shaft on the wear and sealing performance of rotary lip seals have recently attracted considerable attention. This paper presents a numerical method for analyzing the effects of the textured shaft on the seal performance, especially the seal wear. The proposed numerical analysis method has been validated by comparing the simulation results to experimental observations. Based on the simulation results, this study finds that: increasing the textured feature depth and increasing the textured area ratio can increase the seal life and improve the torque of the seal, especially in the axial direction. The reverse pumping rate decreases with the increase of textured area ratio, especially in the axial direction.

Introduction

Rotary lip seals are extensively used in aircrafts, such as fuel system and hydraulic system, due to their advantages, which include low friction and zero leakage [1]. Typical structure of the rotary lip seals consists of dust lip, metal frame, garter spring, and rubber sealing lip.

In order to prevent the leakage, the rotary lip seal is always fitted on a shaft with interference. However, during operation, a lubricating film clearance forms between the shaft and the seal. Its existence has been experimentally verified [2]. A series of experimental studies have been conducted. The asymmetric deformation of the seal in axial direction has been found and the reverse pumping effect has been discovered [3,4]. Based on these experimental findings, to explain the sealing mechanism, Müller [5] has proposed the reverse pumping principle. The reverse pumping principle has been served as a basis for numerous lubrication models of rotary lip seal, such as hydrodynamic lubrication models [6,7], elastohydrodynamic (EHD) lubrication models [8,9], mixed EHD lubrication models [10,11], thermoelastohydrodynamic (TEHD) models [12,13], and etc. Generally, the sealing performance is correlated with the reverse pumping rate, where the leakage tends to occur at low pumping rates. Furthermore, it has been observed that the asymmetric profile of the sealing lip affects the reverse pumping rate [5].

The rotary lip seal performance is continuously degraded by wear as a result of the changes in the sealing lip profile and shaft-seal interference [14]. Specifically, wear affects the asymmetry of sealing lip and reduces the reverse pumping effect. The change in the shaft-seal interference affects the contact width and the hydrodynamic pressure distribution, which are important for the reverse pumping effect and seal performance [15]. It has been noted that accurate prediction and modeling of the seal wear process are essential for improving the service life and reliability of the seals [14].

Researchers have analyzed the effects of sliding speed, temperature, lubrication conditions, and load on the seal wear experimentally. The experimental results indicate significant influence of these conditions on the seal wear [[16], [17], [18]].

In addition to extensive experimental studies on the seal wear, many analytical models and numerical simulation methods have been proposed to analyze the seal wear phenomena. The common used approaches are always based on finite element(FE) method, macroscale modeling strategies and Archard wear equation. The wear process can be modeled by uploading-geometry strategy, moving-node strategy, and element-death strategy, respectively. The updating-geometry based strategy describes the material removal process by updating the geometry of the seal and re-meshing the FE model [[19], [20], [21]]. The moving-node based strategy describes the seal wear process by adjusting the node-displacements of the FE model [22,23]. The element-death strategy describes the seal wear process by deactivating the elements of the FE model. Consequently, this approach cannot model a continuous wear process. Another downside is the fact that the simulation accuracy is highly affected by the element size [24,25]. In addition, the effects of temperature are taken into account in Refs. [21,23] and the effects of the shaft roughness on the seal wear are considered in Ref. [26].

Gao et al. [27] experimentally analyzed the effects of different types of seal-friction couples on the seal wear. The experimental results indicate that thermo-physical phenomena need to be considered when analyzing seal wear. Sui et al. [28] presented a FE based simulation method to optimize the seal profile to improve seal wear and increase service life. The simulation results indicate that the service life can be improved by optimizing the seal profile. Belforte et al. [29] applied the FE method to analyze the seals used in pneumatic actuators and optimized the seal shape to improve seal wear and service life. The accelerated life tests were performed and the results show that the multi-lobed seal is the best configuration for this application. Weber and Haas [30] applied FE based wear simulation method and endurance tests to optimize the profile of the PTFE seal and improve the seal wear.

Furthermore, the effects of textured shafts on the sealing performance, including friction torque and reverse pumping rate of the rotary lip seals, have attracted considerable attention recently. Jia et al. [31] presented a stationary EHD simulation method and analyzed the effects of the textured shaft on the pumping action. The presented model has been verified by experiments. It can be seen that the shaft with oblique grooves can significantly enhance pumping rate. Fatu et al. [32] proposed a transient EHD model to study the transient behavior of the seal. The effectiveness of the proposed model has been analyzed by comparing experimental results and numerical results between the stationary and transient conditions. The effects of the grooved shaft on the sealing performance have been analyzed, and the results indicate that the oblique grooved shaft can improve the reverse pumping rate obviously.

Li et al. [33] performed a numerical study on the rotary lip seals based on a stationary EHD model. The authors have considered the cavitation conditions by modifying the Reynolds equation to include the Jakobsson-Floberg-Olsson cavitation model. The numerical reverse pumping rates indicate good correlation with experimental results. The study is focused on the effects of rotation angle on the reverse pumping rate, and the analysis was performed on two types of triangular shaft surfaces, surface cavities and surface asperities. It has been found that the triangular asperity tends to pump fluid toward its apex while triangular cavity tends to pump fluid toward its base. Guo et al. [34,35] presented a stationary model which includes the relationship between viscosity and temperature of the lubricant. The model has been applied to analyze the effects of three different types of surface cavity textures (circle, square, and triangle) on the sealing performance, especially the friction torque and reverse pumping rate. It was concluded that both texture geometry parameters and contact zone position are important factors for analyzing the effects of the textured shaft on the sealing performance.

Because of the above researches on the textured and grooved shafts, focus on reverse pumping and friction torque, the effects of the textured (and grooved) on the seal wear began to be concerned about.

Thielen et al. [36] has studied the effects of the textured shaft on the seal wear and sealing performance by testing and analyzed six different kinds of the textured shaft. The seal wear was obtained by comparing the sealing lip profiles before and after the long-term tests. The experimental results indicate that texturing shaft can improve the seal wear.

The experimental studies need lots of manpower, material and financial resources and it is difficult to get enough experimental data to evaluate the seal wear for different applications, including different operating conditions, different designs of the rotary lip seal, and etc. Although there are a number of experimental studies focusing on the effect of the textured shaft on the seal wear, it is hardly to find a numerical study focusing on the seal wear. Hence, a numerical analysis method is in need to analyze the effects of the textured shafts on the rotary lip seals for different applications, especially focus on the seal wear. In the presented work, a TEHD seal wear simulation method is presented to evaluate the seal wear with textured shafts. The proposed method consists of a modified Archard wear model and a mixed TEHD lubrication model. The asperity contact is numerically determined in the mixed lubrication analysis part and used in the seal wear simulation part, where the seal wear is calculated based on the modified Archard wear equation. The proposed method has been verified by comparing the simulation results to experimental observations. The simulation studies are performed to study the effects of the textured shaft on the seal wear and to lay a foundation to determine the type of the texture, which can improve the seal wear, for different applications.

Section snippets

Textured shafts

Since the clearance thickness is significantly smaller than the shaft radius, the rotational motion is approximated as translational motion. As a result, a Cartesian coordinate system is applied to study the system behavior, Fig. 1, where z is the radial coordinate, y is the axial coordinate, and x is the circumferential coordinate. Also, the shaft roughness is much smaller than the seal roughness, hence the shaft surface is assumed to be perfectly smooth, excluding manufactured surface

Lubrication analysis

The mixed TEHD lubrication model integrates the effects of multi-domain phenomena, including fluid phenomena, asperity contact phenomena, and deformation phenomena, all of which are strongly coupled, as shown in Fig. 3. The thermo-mechanics affects the fluid mechanics through fluid viscosity, μ, and itself is affected by the friction dissipation, Фa, and the viscous dissipation, Фh, which, on the other hand, are affected by the fluid phenomena and asperity contact phenomena, respectively. In

The modified Archard wear equation

The wear of rotary lip seal is predicted based on a commonly used simple phenomenological model, Archard wear equation. The volume of removed material is given by [44].Vw=KHWnSwhere Wn is the total normal load, S is the sliding distance, H is the hardness of the seal, and K is the dimensionless Achard wear coefficient. The Archard wear equation can be rewritten asVw=kLWnSwhere kL = K/H is defined as the specific wear modulus under specific lubricating conditions, which is usually experimentally

Tested seal, shaft and oil

The tests have been performed on a typical kind of rotary lip seals (GB/T 9877–2008), the main information of the tested seal is as follows: rubber ring material is nitrile butadiene rubber (NBR), elastic modulus of the rubber material is 10.2 MPa, internal diameter is 16 mm (actual one is 15.2 mm), external diameter is 30 mm, and interference is 0.4 mm. Fig. 11 shows the actual dimensions of the seal and the main geometric parameters of the seal.

The shaft surface roughness and sealing lip

Numerical study

The presented seal wear simulation method is used to evaluate the seal wear and sealing performance of rotary lip seal under mixed lubrication conditions and to analyze the effect of the textured shaft on the seal wear and sealing performance. The main simulation parameter values are displayed in Table 2. It should be noted that the wear of the shaft is not measurable and much smaller than the seal wear. Neglecting the shaft wear is quite common in recent published related papers, in which the

Conclusions

A numerical study on the effects of the textured shaft on the performance of rotary lip seals is presented. The proposed method not only can be used to analyze the effects of the textured shaft on the reverse pumping rate and torque numerically, but also the seal wear can be analyzed numerically by the presented method. Hence the proposed numerical analysis method is more effective and practical, especially for analyzing the seal wear.

The proposed numerical analysis method has been validated by

Acknowledgments

This study was co-supported by the National Natural Science Foundation of China (51620105010, 51505015, 51575019), Natural Science Foundation of Beijing Municipality (17L10011), and Program 111.

Nomenclature

A
simulation space, μm x μm
a
structure geometry parameter(length of structure in x direction), μm
b
structure geometry parameter(length of structure in y direction), μm
Cp
specific heat of the fluid, J/kg K
E
elastic modulus of the seal, MPa
Fai
asperity contact force, N
Fs
spring force, N
H
seal hardness, MPa
h
clearance thickness, μm
havg
initial average clearance thickness (guessed based on experience), μm
hc
coefficient of heat transfer, W/m2 K
hshaft
shaft surface height distribution, μm
hw
wear depth, m
Ix
normal

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