Noriaki Ishii

Theoretical Model of Lubrication Mechanism in the Thrust Slide-Bearing of Scroll Compressors

This study presents a theoretical model of the remarkably good lubrication characteristics of the thrust slide-bearings found in scroll compressors experimentally by Ishii et al. (2007). It has been suggested that the thrust plate undergoes elastic deformation due to axial loading. As a result of this deformation, a fluid wedge is believed to form at the periphery of the thrust plate. The fluid wedge angle between the sliding surfaces was calculated with a finite element method analysis and then used in the average Reynolds equation from Patir and Cheng (1978, 1979) to analyze the fluid lubrication on rough sliding surfaces. The plastic and elastic contacts between the orbiting and fixed thrust plates were analyzed using the solid contact theory from Greenwood and Williamson (1966). The derived theoretical model permitted the calculation of the oil film pressure, the solid contact force, the fluid frictional force, and the solid shearing drag force. These results were used, in turn, to determine the resultant friction coefficient at the thrust slide-bearing. Finally, it is shown that the predicted theoretical results agree well with the experimental lubrication test results of Ishii et al. (2007). The proposed theoretical development appears to model accurately the essential mechanism for the remarkably good mixed fluid-and-solid lubrication in a thrust slide-bearing.

Experimental Study of the Lubrication Mechanism for Thrust Slide Bearings in Scroll Compressors

This study focuses on the effect that a pressure difference across the orbiting thrust plate of a thrust-slide bearing has on the improved lubrication of the bearing in scroll compressor applications. A thrust slide bearing model submerged in a refrigerant oil, VG-56, was operated under pressurized conditions using R-22 as the pressurizing gas, where the pressure difference across the friction surface of the thrust bearing was adjusted from 0 to 1.0 MPa and the friction force and friction coefficient at the thrust slide bearing were measured over a range of orbital speeds. As a result, significant improvement in lubrication at the thrust slide bearing due to the pressure difference was identified. Furthermore, careful observation of the wear state of the thrust slide bearing revealed the formation of a fluid wedge between the sliding surfaces due to axial loadings. This fluid wedge appears responsible for the observed improvement in lubrication. In addition, the wedge formation was quantitatively investigated using finite element method (FEM) analysis of the elastic deformation of the thrust plate. The FEM analysis was subsequently validated through strain measurement on the thrust plate.

Field Measurement of Dynamic Instability of a 50-Ton Tainter-Gate

The 2-degrees-of-freedom coupled-mode self-excited vibration of Tainter gate has been demonstrated in hydraulic model studies of 2D and 3D gate models. The concurrent development of a theoretical model has permitted the prediction that some full-scale Tainter-gates could be potentially susceptible to this coupled-mode self-excited vibration. However, there are many people who regard with suspicion the prediction that a massive Tainter-gate might actually undergo this type of vibrations. In response to these skeptics, this paper presents the test results from an actual operational 50-ton Tainter-gate to determine its propensity for large-amplitude self-excited vibration. Further, the previously developed theoretical analysis was applied to the tested gate to uncover an essential dynamic instability for this installation, as well as to provide a framework for the design of a retrofit and an optimized maintenance plan.Copyright

Vibration Tests With a 1/13-Scaled 3-D Model of the Folsom Dam Tainter-Gate and Its Prediction by Theory

This study presents 3-D model gate vibration test results demonstrating violent spontaneous vibrations and validating the basic assumptions made in previously published theoretical analyses. First, the design of a 1/13-scaled 3-D model of Folsom dam Tainter-gate is presented, in which the streamwise natural bending vibration mode of the skinplate, measured in the field vibration tests on the remaining Folsom gate, is shown to be correctly replicated with the aid of FEM simulations. Secondly, in-air and in-water vibration test results with the 1/13-scaled 3-D model are presented, reproducing the intense coupled-mode self-excited vibrations. Thirdly, test results are plotted on a theoretically calculated stability criterion diagram to confirm the validity of the theoretical analysis. Finally, the intense dynamic instability of the Folsom gate, which could have caused its failure, is presented.Copyright

Friction-Maintained Dynamic Stability

Mechanical systems relying on Coulomb friction to maintain dynamic stability may suffer a dynamic instability if exposed to an initial displacement exceeding a system-specific threshold. In fluid systems, even small values of negative damping are sufficient to drive the dynamic instability with sufficiently large initial displacement. The Tainter gate failures at the Folsom dam in 1995 and at the Wachi dam in 1967 are two well-known failures. To aid in preventing a recurrence, the authors engaged in a decade long research program that provided evidence that both gates failed due to an essential dynamic instability mechanism that all Tainter-gates may possess. This paper presents measurements suggesting “friction-maintained dynamic stability” of a full-scale 50-ton Tainter gate. Accompanying gate model studies showed that the gate can fail when exposed to an initial displacement exceeding a threshold value. The present study should serve to alert gate designers, owners and operators that many Tainter gates which have not yet failed may, nonetheless, have a high susceptibility to failure if and when they are exposed to a sudden input of energy resulting in an initial displacement exceeding the gate-specific threshold displacement.

Measurement of Instantaneous Flow-Rate Coefficients and FIV Characteristics of Tainter Gates at Large Openings

Tainter gates become dynamically stable at large gate openings, even though they usually exhibit intense dynamic instability at small gate openings. To assess which factors govern this stability at large openings, model investigations were conducted. The present study focuses on discharge characteristics and hydrodynamic pressures at large gate openings. The model tests were conducted using a 1/27-scaled 2-dimensional model of the Folsom Dam Tainter gate. The instantaneous flow-rate coefficient was obtained from LDV measurements of flow velocity accompanying periodic changes in the gate opening. Measurements of the hydrodynamic pressure associated with the flow-rate variation were also used to determine instantaneous flow-rate coefficients. Comparison of the directly measured instantaneous flow-rate coefficient form LDV measurements with that obtained from pressure measurements suggests that the flow field is less receptive to disturbances due to gate motion at larger gate openings. With increased gate openings, the fluctuating fluid force acting on the gate decreases, making the gate dynamically stable.Copyright

Coupled-Mode Dynamic Instability of Tainter Gates With Parallel Bending Vibration of the Skinplate

As part of the investigation of the dynamic instability of the gate closely related to the Folsom Dam Tainter-gate failure, and in order to assure the dynamic stability of the gate, the field vibration tests on three full-scale operational Tainter-gates were conducted. From these tests, the possible existence of another coupled-mode self excited vibration mechanism, which involves the dangerous dynamic coupling of the whole gate rigid-body rotational vibration with a “parallel” bending vibration of the skinplate was suggested. This paper presents the mechanism of the suggested coupled-mode self-excited vibration, theoretical analysis for the suggested dynamic instability, and 2-dimensional laboratory model tests results. Further, the need for retrofit countermeasures for Tainter gates which are currently installed in both Japan and the USA and susceptible to this dangerous coupled-mode dynamic instability is emphasized.Copyright

Effects of Weir Plate Inclination Angle on Flow-Induced Vibration of Long-Span, Shell-Type Gates.

Here we present detailed experimental results concerning flow-induced vibrations of long-span, shell-type gates in which the upstream gate face consists of vertical and inclined skin plates (also referred to as weir plates). Such shell-type gates possess two degrees-of-freedom, one each in the streamwise (horizontal) and vertical directions, due to bending flexibility in those two directions.The streamwise and vertical vibrations can become closely coupled with each other through the hydrodynamic forces acting on the weir plates, resulting in severe self excited vibrations. A two-dimensional laboratory model of a long-span, shell-type gate was operated with underflow only (i.e., no overflow) at small gate openings with several different inclined weir plate geometries, ranging from a 17.5°to a 65°inclination angle (relative to the horizontal) for the inclined weir plate. By measuring the gates vertical and horizontal displacements, it was possible to determine the vibration frequency, the excitation ratio (negative damping ratio) and the trajectories of gate motion. These results show that inclination angle of the inclined weir plate angle plays a significant role in determining the gates susceptibility to this type of dynamic instability. Long-span, shell-type gates with an inclined weir plate angle of about 60°relative to the horizontal were found to be the most unstable.