Influence of the real dimple shape on the performance of a textured mechanical seal
Introduction
A mechanical seal is a sealing device that is widely used in industry such as those utilizing pumps and compressors. It mainly consists of two rings (the stator and the rotor), where one of them is linked to the housing and the other to the shaft. The sealing function is ensured by the mating faces of these two rings, which constitute a mechanical seal. A thin fluid film that is a few microns thick is created in the gap between the rotating faces, which reduces the risk of contact, and therefore, the risk of wear. To preserve the main function of mechanical seals (that is, sealing), the induced leakage should be eliminated or minimized to an acceptable level. Thus, the optimal configuration of a mechanical seal is one that minimizes the friction as well as the leakage.
One of the main methods studied and discussed in tribology literature is the performance enhancement of mechanical seals through surface texturing. In 1966, Hamilton et al. [1] discovered the effect of surface texturing when studying roughness effects by means of artificial asperities possessing a cylindrical shape. The authors explained that each pillar is like a micro-bearing that assists in fluid film generation and friction reduction. A few years later, Anno et al. [2] showed that cavities can be more beneficial for mechanical seals because they preserved a low leakage rate and allowed hydrodynamic fluid film generation between the seal faces. In 1997, Etsion et al. [3], showed that it was possible to increase a seal life when dimples were created on the seal surfaces with a laser texturing method. The possibility of enhanced performance in terms of lower friction and a higher critical load before seizure has been confirmed by several other studies [4], [5], [6]. However, the experimental results yielded a great dependence on texture geometrical parameters [6], such as the area ratio covered by the cavities, and the diameter and depth of the dimples. For certain parameters, it is possible to obtain worse performance with textured surfaces than with flat surfaces [7]. This is because the shapes of the dimples are unlimited [8], which highlights the need for an optimization process.
Numerical simulation offers a good solution for optimizing dimple shapes [8], [9]. A recent review of Gropper et al. [10] revealed that several papers were dedicated to simulating the hydrodynamic lubrication of textured surfaces. Additionally, Etsion [11] provided some advice for solving the Reynolds equation, which governs fluid flows in the thin film when the surfaces are being textured. During this process, some pressure peaks are created at the trailing edge of cavities, whereas pressure decreases occur in the dimples. If the pressure decreases below a critical value, cavitation or film rupture could occur. It is important to model this phenomenon using a mass conserving algorithm to obtain relevant results, more particularly with seals, as shown by Qiu and Khonsari [12].
Generally, perfect dimple shapes are considered in simulations by using very thin meshes [13]. However, the geometrical shape of real cavities on real textured surfaces is not perfect. Even if laser surface texturing was used, which is one of the most commonly used methods [14], there are many alternative techniques for performing surface structuration [15], [16]. Each of these methods produces imperfect dimples, where Ref. [17] demonstrated that the performance of textured surfaces greatly depended on their geometrical parameters. It is, therefore, necessary to consider shape imperfections to obtain accurate results. Although some researchers have simultaneously considered surface roughness and surface texture [10], [17], [18], very few have focused on the effects of dimples.
In the present work, the real dimple shapes are analyzed and introduced into a hydrodynamic model. The dimples are created through the ion etching of stainless steel sealing rings (Fig. 1). Once the surface texturing has been performed, a plasma-assisted, thermo-chemical surface treatment is applied to enhance the surface hardness and corrosion resistance [16]. An optimization of the size, shape, and distribution of the texture patterns were performed in previous studies [9], [13] that used a triangular dimple shape. In contrast, this work investigates the influence of several types of defects: the roughness at the bottom of the dimples, the absence of sharp angles, and deformed boundaries.
Section snippets
Theoretical model
The previously mentioned model using two rings in relative sliding motion, which has been used in previous studies [9], [13], [17], is presented in Fig. 2. The upper ring is smooth and possesses an angular velocity ω. The lower ring is fixed and textured. The periodicity permits that only one small radial domain be studied. Fig. 2 illustrates that this domain is located between the inner radius Ri and the outer radius Ro. A periodic condition on the lateral boundaries is necessary to ensure
Optimization studies
Before treating the influence of the real dimple shapes, a previous work by Adjemout et al. [9], which explores texture pattern optimization, is summarized in this section. Both local and global effects in this study [9] were investigated in regards to different textured shapes and orientations on the hydrodynamic performance of a mechanical seal. The results showed that an area density of 0.3 yielded the highest load carrying capacity for each textured shape, while the effective value of the
Influence of the real dimple shapes on the seal behavior
To assist the reader in understanding the remainder of this study, the surface texturing process used herein is briefly described. Fig. 4 provides an overview of the main steps involved in texturing process. The texture was obtained by means of the ion etching method. In the reactor, a mask having holes with the desired shape pattern was used to limit etching to areas not protected by the mask. After etching, a thermo-chemical post-treatment was used to increase the hardness of the surfaces.
The
Conclusions
The influence of the real shape of triangular dimples on a mechanical seal's performance was investigated using a hydrodynamic model coupled to a mass conserving cavitation model. The defects were selected among real problems encountered after surface texturing. In this case, surface texturing was performed with ion etching through a protecting mask. The impacts of the three main defects on the hydrodynamic performance of the mechanical seal were numerically analyzed, which led to the following
Acknowledgments
This work was supported by the ANR under the reference ANR-11-RMNP-0008. This work pertains to the French Government program “Investissements d’Avenir” (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01).
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