Comparison between Different Supply Port Configurations in Gas Journal Bearings

Abstract

Because of their precision, gas bearings are widely used for very high speed spindle applications. Compared to conventional oil bearings, gas bearings generate less heat and do not pollute the environment. Air viscosity is three orders of magnitude lower than oil, so the power dissipated in gas bearings is very low. The major disadvantage of these bearings is rotor whirl instability, which restricts the possible range of applications. Researchers have studied this problem using different methods since the '60s. Gross first applied a perturbation method to evaluate the stability of an infinitely long journal bearing (Gross & Zachmanaglou, 1961). Galerkin’s method was used by others to calculate rotor speed and mass at the stability threshold (Cheng & Pan, 1965). Lund investigated the stiffness and damping coefficients of hydrostatic gas bearing, and used these coefficients to investigate whirl instability (Lund, 1968). Wadhwa et al. adapted the perturbation method to calculate the dynamic coefficients and to study the stability of a rotor supported by orifice compensated gas bearings (Wadhwa et al., 1983). Results show that aerostatic bearings have a larger load capacity and higher stability than plain journal bearings. Han et al. proved that more circumferential supply ports result in increased stiffness coefficient but reduced damping (Han et al., 1994). Others found that orifice-compensated and shallow-pocket type hybrid gas journal bearings offer better stability than eight-orifice type bearings (Zhang & Chang, 1995). Also porous journal bearings were studied (Sun, 1975) and compared against hybrid gas bearings with multi-array entries (Su & Lie, 2006), (Heller et al., 1971). Despite the fact that damping is generally higher in porous bearings than in aerostatic bearings, the results of (Su & Lie, 2006) suggest that at high operating speeds, multi-array entry bearings are more stable than porous bearings. Other studies (Andres, 1990), (Sawcki et al., 1997), (Yoshikawa et al., 1999) considered various pressurized air compensated configurations, but very few papers analysed the influence of the number and location of entry ports. In (Su & Lie, 2003) hybrid air journal bearings with multi-array supply orifices were compared to porous bearings. One to five rows of orifices were considered. It was found that five rows of supply orifices perform as well as porous bearings, whilst supply orifice feeding has the advantage of consuming less power than porous feeding. Paper (Yang et al., 2009) compared bearing systems with double-array orifice restrictions to three and six entry