Anh Kiet Tieu

Identification of sixteen force coefficients of two journal bearings from impulse responses

Abstract A method to determine all 16 dynamic coefficients of two journal bearings from impulse responses is described. From the transfer functions of the displacements to the excitation forces, all 16 coefficients of two journal bearings can be obtained based on least-squares estimation. Computer simulation verifies that the experimentally identified coefficients approximate well the measured rotor-bearing dynamic characteristics in both time and frequency domains. Oil whirl phenomena were found in the experiments. The whirling frequencies correlate well with the values predicted from the estimated bearing coefficients and agree very well with the linear stability theory. Detailed formulae, experimental procedures, measured signals and estimated bearing coefficients are presented.

Identification of sixteen dynamic coefficients of two journal bearings from experimental unbalance responses

Abstract A method to determine all sixteen linear dynamic coefficients of two journal bearings from two or more sets of unbalance responses is proposed. The displacement responses under trial unbalance are transferred to the frequency domain, from which a linear estimator for all coefficients is developed based on the least-square estimation. The unbalance method can be conveniently applied to estimate the coefficients of large journal bearings, without relying on expensive excitation equipment to apply a large dynamic force to the rotor as required by other methods. The method is thoroughly tested by the experiments on a bearing test rig. The estimated coefficients of two journal bearings from the experimental unbalance responses correlate well with the theory.

Stability of Finite Journal Bearings–-from Linear and Nonlinear Bearing Forces

This paper investigates the relationship between the stability contour determined from the nonlinear simulation and that from the linear theory. The nonlinear bearing forces are directly obtained from the bearing pressure distribution which is solved from the Reynolds equation at each journal position. It is found that the critical speeds in the case of nonlinear bearing forces are the same as those predicted by the linear theory, although the whirl loci under large dynamic excitations are significantly different from those arising from the linear bearing forces. Typical whirling trajectories under impact excitation, position perturbation and synchronous unbalance excitations are simulated and presented to explain the stable, critical and unstable phenomena. The whirl displacement signals are also transformed to the frequency domain, and their whirling frequencies are analyzed according to their frequency characteristics. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/S...

The effect of perturbation amplitudes on eight force coefficients of journal bearings

This paper investigates the relationship between eight linear oil-film force coefficients of circular journal bearings and the perturbation amplitudes. The force coefficients are calculated by the finite perturbation method and compared with those calculated by the infinitesimal perturbation method. Numerical experiments show that the calculated results from both finite perturbation and infinitesimal perturbation methods are very close (with a variation less than 0.1 %) when the perturbation amplitudes are less than 0.02c (displacement) or 0.02ωc (velocity) for normal bearing eccentricities. For eccentricities between 0.2 and 0.8, the calculated coefficients by the finite perturbation method will differ less than 2.5 % from the results by the infinitesimal perturbation method if the perturbation amplitude is less than 0.05c or 0.04ωc. The bearing coefficients calculated by the finite perturbation method under different perturbation amplitudes are presented graphically.

FE analysis of cutting tool temperature field with adhering layer formation

Abstract Research work is carried out on the cutting tool temperature field with adhering layer formation when machining CaS free-cutting stainless steel. By means of combining thermocouple measuring method and finite element (FE) analysis, the cutting temperature distributions and their relation with the formation of adhering layer are investigated. The adhering layer, which has a protective effect on cutting tools, is basically composed of a ternary compound oxide similar to the inclusions in the steel. When there is an adhering layer formed during machining, the temperature gradient in the direction vertical to the tool rake face is smaller than that without the layer. The small temperature gradient is favorable for the stable formation of the adhering layer on cutting tools. The stable cutting temperature field keeps the relatively stable adhering layer and influences the layer composition distributions on the tool face.

A direct method to evaluate hole-alignment error in residual-stress measurement

In this paper, a direct method based on the Sandifer and Bowie theory was derived to take hole-alignment error into account in residual-stress measurement. The principal stresses and their orientation are calculated directly from this method when the relaxed strains, hole diameter, eccentricity and orientation are known. It is more convenient to use than other iterative methods.

Excellent melt lubrication of alkali metal polyphosphate glass for high temperature applications

A new class of high temperature lubricant of inorganic alkali metal polyphosphate was developed for hot metal forming, which involved a fundamental knowledge of melt lubrication. At elevated temperature a 55% reduction in friction and a 50% decrease in wear were achieved by the presence of the molten polyphosphate lubrication, as well as the excellent anti-oxidation capability at the rubbing steel/steel interface, which closely correlated with the tribo-induced hierarchical structure at the rubbing interface. This inorganic polyphosphate lubricant can be called a smart polymeric material, which will be an eco-friendly alternative for high temperature applications.

Volume-grating phase-shifting digital speckle pattern interferometry used for measurement of out-of-plane displacement field

A new digital speckle pattern interferometry, called volume-grating phase-shifting digital speckle pattern interferometry, is discussed in this paper. The out-of-plane displacement field of a bent plate can be quantitatively measured using volume-grating phase-shifting digital speckle pattern interferometry proposed in this paper. Theoretical and experimental results, as well as absolute errors, are given.

Influence of in-plane displacement and strain components on slope fringe distributions in double-aperture speckle wedge-shearing interferometry

Abstract The influence of in-plane displacement and strain components on slope (first-order derivative of out-of-plane displacement component) fringe distributions in double-aperture speckle wedge-shearing interferometry is discussed in detail. The research results show that only the in-plane displacement component parallel to the centre line of double apertures has an influence on the slope fringe distributions. It is also shown that the in-plane strain components have no influence on the slope fringe distributions when utilising normal illumination and an axisymmetric system. A theoretical analysis and an experimental demonstration are given. The experimental results are in good agreement with the quantitative analysis.

Molecular dynamics simulation of crack propagation on different slip planes of BCC iron

In this paper, molecular dynamic simulations of crack propagation in body centre cubic (bcc) single crystal have been performed. The crack propagation behaviors on two different slip planes ({1 1 0} and {2 1 1}) have been investigated. A self-adaptive time step algorithm has been proposed to increase the stability of the simulations. It has been found that the slip plane significantly affects the propagation speed of the crack.