The Effect of the Pivot Stiffness on the Performances of Five-Pad Tilting Pad Bearings

Abstract

The role of the pivot flexibility in tilting-pad journal bearings (TPJBs) has become essential, particularly for bearings working at high applied load and relatively high rotor speeds. Predictions from a simple bearing model with rigid pivots show incorrect estimation of the dynamic coefficients in comparison with the experimental results. Normally, the more flexible the pad pivot, the lower the dynamic coefficients because the stiffness of the pivot takes in series with the stiffness and damping of the oil film. This paper investigates the influence of pivot stiffness on the dynamic force coefficients of two different five-pad TPJBs as a function of the applied static load and excitation frequency: rocker-backed pivot and spherical pivot bearings. In order to highlight the effect of the pivot stiffness in the spherical pivot bearing, displacement restriction components and elastic copper made shims have been used. Firstly, a thermo-elasto-hydrodynamic model for the static and dynamic characteristics of the two bearings is described. This model takes into account the flexibility of both pad and pivot. The pivot stiffnesses calculated by means of the Hertz theory and those obtained by experiments have been introduced and compared in the model. The clearance profiles of two tested bearing and the shaft center loci obtained by measurement and prediction are also shown. The dynamic coefficients of the two bearings obtained from the numerical simulation were compared with the experimental results. By the analysis it can be concluded that the effect of the pivot flexibility on the clearance profile, the shaft locus and on the dynamic coefficients is very significant. More important, it is important to estimate the pivot stiffness of each single pad using experimental measurements.

Static Characteristics of a Tilting Five-Pad Journal Bearing with an Asymmetric Geometry

“…The TEHD model of TPJBs was developed and validated by experimental data in [28] and was improved for the elastic pivot stiffness and thermal feature, as partially done in [29] for two-axial groove journal bearings. Figure 1 shows the scheme of a pad-shaft system in a rocker-backed TPJB, where O b and O j are the center of the bearing and the shaft, respectively.…”

“…It was found that the effect of the flexibility of the pivot along the η direction in the prediction of the performances of TPJBs is very important. The pivot stiffness estimated from the Hertz contact theory is always higher than that obtained by experiments, especially for rocker-backed bearings [29]. In order to improve the numerical simulation, the pivot stiffness estimated by experiment is introduced to the code.…”

“…In order to improve the numerical simulation, the pivot stiffness estimated by experiment is introduced to the code. The experimental procedure for the stiffness pivot calculation of each pad has been described in detail in [29].…”

“…− ( ) = 0 Readers are strongly recommended to refer to [11,29] for a detailed description of the model. Each pad consists of two different parts: a base part (steel) and an anti-friction layer (Babbit metal with a thickness of 3 mm).…”

“…In this study, a convection boundary condition with a constant coefficient of q = 50 W/(m 2 •K) is applied on the pad surfaces contacting lubricating oil at supply temperature (T supply = 40 • C and q air = 20 W/(m 2 •K)) in order to evaluate the distribution of the pad temperature. The constant convection coefficient q = 50 W/(m 2 •K) was also used in some previous publications [11,29,31,32]. Actually, this value is not uniform at the pad peripheries and it changes with the conditions of the oil flow around the pad [19].…”