MINIATURE FLUID DYNAMIC BEARING WITH IMPROVED LOAD CAPACITY Chien-Sheng Liu 1, Min-Kai Lee 1,2 and Ying-Chi Chuo 3 1 Department of Mechanical Engineering and Advanced Institute of Manufacturing with High-tech Innovations, National Chung Cheng University, Chia-yi County, Taiwan 2 Additive Manufacturing and Laser Application Center, Industrial Technology Research Institute, Tainan, Taiwan 3 Opto-Electronic Actuator Module Business Division, Wah Hong Industrial Corp., Tainan, Taiwan E-mail: imecsl@ccu.edu.tw; minkeilee@itri.org.tw; YingChiChuo@wahhong.com ICETI-2014, J1049 No. 15-CSME-30, E.I.C. Accession 3806 ABSTRACT In this paper a novel design is proposed to improve the load capacity of fluid dynamic bearing (FDB) for miniature spindle motors and small-form-factor data storage applications. In contrast to conventional miniature FDB with two sets of herringbone grooves on its inner surface, the proposed miniature FDB comprises another one set of herringbone grooves on its outer surface. The proposed miniature FDB is verified numerically utilizing commercial software Advanced Rotating Machinery Dynamics (ARMD). The simulation results show that compared to the conventional miniature FDB, the proposed miniature FDB can obviously improve the load capacity of the bearing system. Overall, the results presented in this study show that the proposed miniature FDB provides another solution for miniature spindle motor applications. Keywords: stiffness; load capacity; herringbone-grooved journal bearing; fluid dynamic bearing; FDB; miniature spindle motor. ROULEMENT À BILLES MINIATURES À PALIER FLUIDE AVEC CAPACITÉ DE CHARGE AMÉLIORÉE RÉSUMÉ Cette étude propose une conception innovatrice pour améliorer la capacité de charge du roulement à billes à palier fluide pour arbre à moteur miniature, et applications de petit format de stockage de données. À l opposé du roulement à billes miniatures à palier fluide conventionnel avec deux paires de rainures en chevron sur sa surface interne, notre roulement à billes à palier fluide inclus une autre paire de rainures en chevron sur sa surface externe. Cette proposition a fait l objet de vérification avec un logiciel commercial Advanced Rotating Machinery Dynamics (ARMD). Les résultats de simulation démontrent qu en comparaison avec le roulement à billes miniatures à palier fluide conventionnel, les résultats de notre proposition peuvent de toute évidence améliorer la capacité de charge du système d engrenage. Globalement, les résultats de cette étude, démontrent que cette proposition procure une autre solution pour des applications d arbre à moteur miniature. Mots-clés : rigidité; capacité de charge; engrenage à rainures en chevron de palier lisse; engrenage à billes à palier fluide; FDB; arbre à moteur miniature. Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015 527
1. INTRODUCTION Bearings are very important parts of spindle motors and their condition is often critical to success of an operation or rational motion for spindle motors [1 4]. There are many various forms of bearings in current use, such as oil-lubricated bearings, ball bearings, magnetic bearings, fluid dynamic bearings (FDBs), and so on. For existing spindle motors, the FDBs, also called hydrodynamic bearings (HDBs), provide the most promising option for further improving the overall performance. Therefore, instead of oil-lubricated bearings or ball bearings, FDBs are often used in the precise spindle motors [5]. The FDB is machined with a herringbone-grooved pattern which spreads shock loads over the bearing area. During operation, the herringbone-grooved pattern pumps the lubricant in the inward direction and then increases the pressure with the bearing. Therefore, the FDB has excellent performance of high stability, shock resistance, low friction, low noise, and low side leakage [6 8]. Therefore, the use of FDB systems has attracted increasing interest in the spindle motor applications, such as polygon motors, cooling fans, HD and DVD drives, and so on [9]. As a result, many studies in the field have been proposed to investigate the performance and design of FDBs [10 19]. The literature contains many proposals for improving the load capacity of the FDBs. It is noted that the load capacity of the FDB decreases with a decreasing dimension (i.e. size effect), thus the spindle stability and stiffness could be insufficient for miniature FDB spindle motors [7, 20]. However, to the best of the current authors knowledge, very few methods have been proposed for the case of the FDB designed for miniature spindle motor applications [5, 21, 22]. Accordingly, this study is aimed to overcome this problem and propose a novel design to improve the load capacity of FDB for miniature spindle motors and small-form-factor data storage applications. This study utilizes commercial software Advanced Rotating Machinery Dynamics (ARMD) to numerically verify the proposed miniature FDB. The remainder of this study is organized as follows. Section 2 describes the groove pattern of a conventional miniature FDB and introduces the structure of the miniature spindle motor proposed in a previous study [22]. Section 3 introduces the detailed design of the novel miniature FDB proposed in the present study. Section 4 describes the numerical evaluation of the proposed miniature FDB and compares its performance with that of the miniature FDB proposed in [22]. Finally, Section 5 provides some brief concluding remarks. 2. CONVENTIONAL MINIATURE FDB AND SPINDLE MOTOR This section reviews the basic structure and background of the miniature FDB spindle motor proposed by the present group in [22]. Figure 1 shows the basic structure of the miniature FDB spindle motor. Instead of oil-lubricated bearings or ball bearings, a miniature FDB is used in the motor in order to obtain better dynamic properties. In FDB motors, the design of the herringbone groove pattern plays a crucial role in smoothing the pressure distribution within the bearing system and therefore has a direct effect upon its load capacity. Among the aspects contributing to bearing performance, the load capacity is the most critical issue in FDB design. Figure 2 illustrates the structure of the conventional miniature FDB. As shown, the conventional miniature FDB contains two sets of herringbone-grooved patterns on its inner surface, namely front grooves and rear grooves, respectively. Many studies have been proposed to investigate the correlation between the load capacity of the FDB and the groove pattern parameters (e.g., the groove angle, the groove width, the groove depth, the groove width ratio, the groove length ratio, clearance, etc.) or operating conditions (e.g., the rotational speed, the eccentricity ratio, unbalanced force, etc.). For example, as shown in Fig. 3, a previous study indicated that the FDB has the maximal load capacity if the relations between groove widths A, B, C, and D in the front grooves and rear grooves are expressed by 0 < A B < 0.2(A + B) (1) 528 Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015
Fig. 1. Structure of miniature FDB spindle motor in [22]. Fig. 2. Basic layout of conventional miniature FDB. and 0 < D C < 0.2(C + D). (2) Figure 4 shows the photograph of conventional miniature FDBs for different dimensions. Note that for a more comprehensive description of the miniature FDB motor, the reader is referred to [22]. 3. STRUCTURE OF PROPOSED MINIATURE FDB In order to overcome this size effect and improve the rotating stiffness of the conventional miniature FDB, this paper proposes a novel design of miniature FDB to increase its load capacity. Figures 5 and 6 illustrate the structure and the groove pattern of the proposed miniature FDB, respectively. From Figs. 5 and 6, it is seen that in the proposed miniature FDB, there are not only two sets of herringbone grooves on its inner surface, but also one set of herringbone grooves on its outer surface. By contrast, the conventional miniature FDB only contains two sets of herringbone grooves which locate on its inner surface (as shown in Fig. 2). Comparing Figs. 2 and 5, it is implied that in the proposed miniature FDB, the herringbone-grooved pattern pumps the lubricant in the inward direction and induces two fluid dynamic pressure areas during operation. One locates between the shaft and the inner surface of the bearing (it induces a resultant force F 1 ), and another locates between the turntable and the outer surface of the bearing (it induces another resultant force F 2 ) (as shown in Fig. 7). By contrast, the conventional miniature FDB only induces one fluid dynamic Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015 529
Fig. 3. Groove widths A, B, C, and D in the front grooves and rear grooves [23]. Fig. 4. Basic layout of conventional miniature FDB. Fig. 5. Basic layout of proposed miniature FDB. 530 Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015
Fig. 6. Groove pattern of proposed miniature FDB. Fig. 7. Simplified force model of proposed miniature FDB. Fig. 8. Designed groove pattern and associated pressure distribution of conventional miniature FDB. Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015 531
Table 1. Optimal groove parameters of conventional miniature FDB. Variable Corresponding Value Fluid density 915 kg/m 3 Fluid viscosity 0.0297 Ns/m 2 Groove angle 21 Groove width 0.08 mm Groove depth 0.005 mm Groove width ratio 0.32 Groove length ratio 1.2 Bearing clearance 0.002 mm Groove number 8 Front groove length 1.55 mm Rear groove length 0.85 mm Interval between the two grooves 0.4 mm Height of FDB 3 mm Outer diameter of FDB 3.5 mm Inner diameter of FDB 1.5 mm Table 2. Designed groove parameters of proposed miniature FDB. Variable Corresponding Value Fluid density 915 kg/m 3 Fluid viscosity 0.0297 Ns/m 2 Groove angle 21 Groove width 0.08 mm Groove depth 0.005 mm Groove width ratio 0.32 Groove length ratio 1.2 Bearing clearance 0.002 mm Groove number 8 Outer groove length 2 mm Front groove length 1.55 mm Rear groove length 0.85 mm Interval between the two grooves 0.4 mm Height of FDB 3 mm Outer diameter of FDB 3.5 mm Inner diameter of FDB 1.5 mm pressure area between the shaft and the inner surface of the bearing (as shown in [22, fig. 3]). Therefore, the proposed miniature FDB can essentially provide greater load capacity according to such a novel design. 4. NUMERICAL EVALUATION In this section, the performance of the proposed miniature FDB is verified numerically utilizing commercial software ARMD, and the load capacities of the proposed miniature FDB and the conventional miniature FDB (presented in [22]) are compared and discussed. For comparison purposes, the proposed miniature and the conventional miniature FDBs have the same height, outer and inner diameters. Tables 1 and 2 summarize 532 Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015
Fig. 9. Simplified force model for conventional miniature FDB. Fig. 10. Designed groove pattern and associated pressure distribution of proposed miniature FDB. the optimal groove parameters of the conventional miniature FDB [22] and the proposed miniature FDB, respectively. Figure 8 shows the optimal groove pattern and associated pressure distribution on the conventional miniature FDB under the specified design conditions of a rotational speed of 4000 rpm and an eccentricity ratio of 0.4. From the simulation results, it is observed that the peak pressure occurs at the tip of the herringbone groove and the total load capacity contributed by the front and rear grooves on the inner surface of the bearing is 1.3 N. The simplified force model for the conventional miniature FDB under the specified design conditions is illustrated in Fig. 9. Figure 10 shows the designed groove pattern and associated pressure distribution on the proposed miniature FDB also under the specified design conditions of a rotational speed of 4000 rpm and an eccentricity ratio of 0.4. From the simulation results, the total load capacity of the proposed miniature FDB is 1.7 N. The simplified force model for the proposed miniature FDB under the specified design conditions is illustrated in Fig. 11. The numerical simulation results show that the proposed miniature FDB improves the load capacity of the bearing and therefore yields an effective improvement in the stiffness of the bearing. Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015 533
Fig. 11. Simplified force model for conventional miniature FDB. This represents an improvement of around 30.7% compared to the conventional miniature FDB and can be directly contributed to the effectiveness of such a novel FDB design. It is noted that this simulation results are in good agreement with theoretical analysis in Section 3. 5. CONCLUSIONS In this paper we investigated the design and numerical characteristics of a novel miniature FDB. The proposed miniature FDB is designed to improve its load capacity for miniature spindle motors and small-formfactor data storage applications. The performance of the proposed miniature FDB is verified numerically utilizing commercial software ARMD. The numerical simulation results show that the proposed miniature FDB has an improved load capacity of around 30.7% compared to the conventional miniature FDB. All of the above results prove that the proposed miniature FDB has the potential for miniature spindle motor applications to improve its dynamic performance. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support provided to this study by the Ministry of Economic Affairs of Taiwan under Grant D327HJ1200, the National Science Council of Taiwan under Grant NSC 101-2221-E-194-065, and the Ministry of Science and Technology of Taiwan under Grant MOST 103-2221-E- 194-006-MY3. REFERENCES 1. Wang, C.C., Yao, Y.D., Liang, K.Y., Huang, C.C. and Chang, Y.C., Development of a miniature fan motor, Journal of Applied Physics, Vol. 111, No. 7, pp. 07E718-1 07E718-3, 2012. 2. Liu, C.S. and Lin, P.D., Analysis and validations of fluid dynamic bearing for spindle motors of high-density optical disc players, Japanese Journal of Applied Physics, Vol. 47, No. 10, pp. 8101 8105, 2008. 3. Niknam, S.A., Songmene, V. and Au, Y.H.J., Proposing a new acoustic emission parameter for bearing condition monitoring in rotating machines, Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 4, pp. 1105 1114, 2013. 4. Nes, A.S., Braat, J.J.M. and Pereira, S.F., High-density optical data storage, Reports on Progress in Physics, Vol. 69, No. 8, pp. 2323 2363, 2006. 534 Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015
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