Instability of Aerostatic Journal Bearings with Porous Floating Bush at High Speeds Masaaki MIYATAKE *4, Shigeka YOSHIMOTO, Tomoaki CHIBA and Akira CH

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Transcription:

Instability of Aerostatic Journal Bearings with Porous Floating Bush at High Speeds Masaaki MIYATAKE *4, Shigeka YOSHIMOTO, Tomoaki CHIBA and Akira CHIBA Department of Mechanical Engineering, Tokyo University of Science, 1-14-6 Kudan-kita, Chiyoda-ku, Tokyo, 102-0073 Japan Recently, demands of higher rotational speeds and rotational accuracy to aerostatic spindles used in machine tools such as a drilling machine for printed circuit board and a milling machine for manufacturing very small precise parts have been remarkably increasing. In this paper, to achieve higher rotational speed and accuracy, a new design of an aerostatic journal bearing with a porous floating bush was proposed and the instability of the proposed bearing at high speeds was investigated experimentally and theoretically. The optimum design parameters of the proposed aerostatic journal bearing with a porous floating bush were obtained by theoretical calculation and a test spindle of a 12 mm diameter rotor supported by proposed porous floating bush was constructed for experimental investigation of the instability. It was consequently found that the proposed porous floating bush journal bearings could greatly improve the whirling stability of a rotor at high speeds than the conventional externally pressurized porous journal bearings. Key Words : Aerostatic Bearing, Journal Bearing, Porous Bearing, Floating Bush, Whirling Instability E-mail:miyatake@rs.kagu.tus.ac.jp

(a) Porous bush (b) Cross sectional view Fig. 1 Aerostatic journal bearings with porous floating bush Fig.2 Coordinates system of the proposed bearings

Fig. 3 Porous floating bush Fig. 4 Experimental apparatus (Test spindle)

Table 1 Principal dimensions of the aerostatic journal bearings with porous floating bush Fig. 5 Relationship between rotor speed and the whirling vibration of a rotor Fig. 6 Conventional aerostatic porous spindle

(a) Moderate surface restriction a1 a2=0.4 (b) Strong surface restriction a1=a2=0.04 Fig.7 Variation of hl vs. Nmax and 1/v (Effects of the permeability of the bulk of the porous bush and surface restriction rate)

Fig. 9 Variation of h1ƒës. and 1/ƒË Fig. 8 Variation of h1 vs. Nmax and 1/v (Effects of the difference between ƒ 1 and ƒ 2) (Comparison between experimental results and calculation results)

(1) Yoshimoto, S. and Nakano,Y., Stability of a Rigid Rotor Supported by Aerostatic Journal Bearings with Circular Slot Restrictors, Transactions of the Japan Society of Mechanical Engineers, Series C, Vol. 49, No.443 (1983-7), pp. 1200-1209. (2) Miyatake, M.,Yoshimoto, S. and Sato, J., Whirling Instability of a Rotor Supported by Aerostatic Porous Journal Bearings with a Surface Restricted Layer, Proceedings of the Institution of Mechanical Engineers, Part J, Vol. 220, No.2 (2006-2), pp. 95-103 (3) Mori, A., Aoyama, K. and Mori, H., Influence of the Gas-film Inertia Forces on the Dynamic Characteristics of Externally Pressurized, Gas Lubricated Journal Bearings, Part I : Proposal of Governing Equations, Bulletin of the Japan Society of Mechanical Engineers, Vol. 23, No.178 (1980-4), pp. 582-586. (4) Mori, A., Aoyama, K. and Mori, H., Influence of the Gas-film Inertia Forces on the Dynamic Characteristics of Externally Pressurized, Gas Lubricated Journal Bearings, Part II: Analyses of Whirl Instability and Plane Vibration, Bulletin of the Japan Society of Mechanical Engineers, Vol. 23, No.180 (1980-6), pp. 953-960. (5) Mori, A. and Mori, H., Analysis of Stability of Externally Pressurised, Gas Lubricated Floating bush Journal Bearings, Bulletin of the Japan Society of Mechanical Engineers, Vol. 28, No.239 (1985-5), pp. 1000-1003. (6) Yoshimoto,S., Anno,Y. and Iiya,T., Aerostatic Floating-Bush journal Bearings (1St Report, Bearing Employing Slot Restrictors), Transactions of the Japan Society of Mechanical Engineers, Series C, Vol. 56, No.521 (1990-1), pp. 116-419. (7) Malik, M., Externally pressurized gas- lubricated floating-ring porous journal bearings. Part 1: steady state analysis, Proceedings of the Institution of Mechanical Engineers, No.198 (1984), pp. 213-224. (8) Malik, M. and Hori, Y., Externally pressurized gaslubricated floating-ring porous journal bearings. Part 2: dynamic state analysis, Proceedings of the Institution of Mechanical Engineers, No.200 (1986), pp. 101-109. (9) Miyatake, M., Yoshimoto, S. and Yagi, K., Static and Dynamic Characteristic of Aerostatic Porous Thrust Bearings with Deep Feed Groove, Transactions of the Japan Society of Mechanical Engineers, Series C, Vol. 71, No.702 (2005-2), pp. 739-744.

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