Study on the Pad Posture and Static Characteristics of Fluid Pivot Journal Bearing

Mingyang Lou^ab*, Xing Xu^a, Wei Chen^a, Olivier Bareille^b, Mohamed Ichchou^b

a Key Laboratory of Ministry of Education for Modern Design & Rotor-Bearing System, Xi’an Jiaotong University, Xi’an Shannxi 710049, China

b Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, 69310 Ecully Cedex France

INTRODUCTION: The Fluid Pivot Journal Bearing (FPJB) is a new type tilting pad journal bearing with both hydrodynamics and hydrostatic characteristics. In 1970, L. W. Hollingsworth [1] published his first patent on FPJB which described the basic configuration and the principle operation of this type journal bearing. In 1977, L. W. Hollingsworth and D. V. Nelson [2] summarized large numbers test results on static and dynamic characteristics of FPJB, they also introduced the design and development history of FPJB. From then, FPJB experienced twice improvement. In 1977, L. W. Hollingsworth [3] improved the pad stop of FPJB in his claimed patent, by designing the head of the pad stop to be a convergence shape at the top of it, respectively an arcuate notch on the opposing end of the tilting pad, the tilting pads are better to float freely and better track the shaft that allowing the pad to pivot about their centers. In 2000, A. M. George [4] improved the FPJB by machining a clearance between the outer surface of the pads and the inner surface of the shell, these clearances allow the tilting pads to align themselves with the shaft which changes position due to a variety of dynamic influences, thermal and mechanical distortions. Furthermore, A. V. Harangozo and T. A. Stolarski [5] used an analytical model to predict the unbalance response of a single mass rotor and modeled the FPJB as a two-mass-spring and damper system to study the stability. The results show that with a specific range of frequencies, the FPJB exhibits fine rotor dynamics performance. Many of the configuration design and the test data about FPJB are published, while the analysis on the operation mechanism and the performance of FPJB are inadequate. The present paper will address on this subject.

METHOD: Figure 1 shows the configuration of FPJB, each pad is supported on a self-generated hydrostatic oil film which is created by tapping off a small portion of hydrodynamic oil to pressurize a central cavity on the back of each pad. The pads are geometrically preload     to promote a better hydrodynamic wedge action. By                    assuming that there’s not tilt axially in response to           misalignment, tilting pad in FPJB contains two degrees of freedom, lift and pitch. Under the given condition, shaft and all pads reach to a state of equilibrium which comes from the force and the moment balance of tilting pads. We study this steady state by staring with force balance and moment balance. Furthermore for the lift movement, we should also take flow balance into consideration. Because all pads surrounded by lubricant oil that lift and pitch based on hydrostatic oil film determined by the hydrodynamic pressure distribution. By conducting an   analysis of the range of lifting and pitch of the pads, a computer program based on finite difference method was developed to predict the static characteristics of FPJB including capacity, power loss and oil flow. Then the results of static characteristics are compared with reference [2], also with the results of lobed journal bearing and tilting pad journal bearing by reducing the model of FPJB. Agreement was obtained to prove the valid of the program.

RESULTS & DISCUSSION: Figure 2 shows the hydrodynamic pressure distribution of tilting pad 1 under the condition of e (eccentricity) = 2um, Ω (shaft speed) = 3000 rpm. In Figure 2(a) a valley value showing up at the position of the central lubricant passage, which make the shape of the pressure distribution transform into a two-peak distribution and Figure 2(b) gives the contour map of pressure distribution in dimensional formation. The peak value of pressure is about 35 MPa and the pressure at the position of central lubricant passage is 10~15MPa. Figure 3(a) shows the non-dimensional hydrodynamic pressure distribution under the same operating condition, because the pad lead edge contact with the bearing shell, which result in a parabolic-shape section pressure in this position, this is agree with the previous analysis of the pad “posture”. Figure 3(b) shows the contour map of pressure distribution in dimensional formation. The pressure in the cavity is 14 MPa. Considering the varying eccentricity affect static characteristic, as the increase of the eccentricity of the bearing, the eccentricity of pad 1 increases and that of pad 2 and 3 decreases, the capacity of the bearing increases. For the main loaded pad 1, as the eccentricity increases, the lift height is decreases with a larger counterclockwise pitch angle and constant flow in the central lubricant passage.

CONCLUSION: A computer program was developed to predict the static characteristics of FPJB. We compared the results with reference [2], also with the results of lobed journal bearing and tilting pad journal bearing by reducing the model of FPJB. The agreement proved the program is valid to calculate the bearing static performance.

REFERENCE: [1] Hollingsworth, us patent 1970, [2] Nelson, Trans ASME 1977, [3] Hollingsworth, us patent 1977, [4] George, us patent 2000, [5] Harangozo, Tribo internationa


Figure 1 Configuration of FPJB (schematic diagram)


Figure 2 Hydrodynamic pressure distribution of tilting pad 1