Laurentiu Moraru

A Deterministic Elastohydrodynamic Lubrication Model of High-Speed Rotorcraft Transmission Components

This paper describes a deterministic approach to analyze elastohydrodynamic lubrication (EHL) of helicopter transmission gear surfaces with asperities. A thermal, non-Newtonian, model, accompanied by three specialized models is used to study the mechanisms of film generation, pressure distribution and temperature distributions within the fluid and on the surface, in an EHL line contact. A two-slope viscosity-pressure model is used to avoid overestimation of the viscosity of the lubricant under heavy loads. The three-dimensional roughness description of an actual gear tooth surface is measured using a non-contact surface profiler. The pressure distribution, film thickness distribution and the temperature rise at the contact surface are predicted as a function of the gear operating conditions including normal load, lubricant temperature and rolling/sliding speed. Presented at the 57th Annual Meeting in Houston, Texas May 19–23, 2002

Dynamic Modeling of a Dual Clearance Squeeze Film Damper—Part I: Test Rig and Dynamic Model with One Damper

This paper presents a description of a dual clearance squeeze film damper (SFD) test rig and a dynamic model of a single SFD. The purpose of a dual SFD is to provide protection at high levels of vibration, when conventional devices are no longer effective. An experimental facility was designed and built for tire study of SFD behavior within mechanically controlled orbits. It can be used for both single and dual squeeze film dampers. In Part 1 of this paper, measured and computed responses are presented while operating as a single squeeze film device. Numerical and experimental results for undamped cases are presented, i.e., when no oil film is present in the damper. Subsequently, the effects of the oil film are considered. Good agreement was obtained between experimental data and predictions. Presented at the 57th Annual Meeting in Houston, Texas May 19–23, 2002

Aspects Regarding the Use of Probabilistic Models for Isothermal Full Film Rough Line Contacts

This article presents a method to improve the classical probabilistic EHL models by incorporating deterministic EHL results. We first provide a review of the latest developments in the probabilistic approach for the solution of a mixed EHL problem, as well as a review of the latest deterministic EHL results that can impact the probabilistic formulations. Next, we present a probabilistic algorithm for the computations of the load supported by the fluid, the elastically deformed asperities, and the plastically deformed asperities in a mixed EHL contact. The Chang-Etsion-Bogy elastoplastic micro contact model is used together with a corresponding probabilistic fluid flow model. We also present an approach in which the elastic roughness deformation induced by the oil film can be incorporated into the probabilistic model via the use of the amplitude reduction technique.

Dynamic Modeling of a Dual Clearance Squeeze Film Damper. Part II

Squeeze film dampers (SFD) have been used for many years to control the vibrations of the shafts in high-speed rotating machinery. Dual squeeze film dampers are essentially a combination of two SFDs separated by a sleeve. Normal operation utilizes only one oil film, as in a conventional damper, and the sleeve is fixed in place. However, under high load conditions, the sleeve is released and both oil films become operational. An experimental facility was designed and built to study both single and dual SFD behavior within mechanically controlled orbits. The first part of this paper presents a description of this test rig and provides a dynamic model of a single SFD. In this portion of the paper, we present results obtained while the inner sleeve is released and both oil films are operational. Here the inner sleeve is not supported by any mechanical device; the anti-rotational effect is provided by the opposed actions of the inner oil film and of the outer oil film. The modeling covers the inner and the outer oil films, the elastic fixtures, the unbalanced shaft, and the sleeve. Numerical results are compared with the experimental data. Presented at the ASME/STLE Tribology Conference, in Long Beach, California October 24-27, 2004 Review led by Luis San Andres

Application of the Amplitude Reduction Technique Within Probabilistic Rough EHL Models

Over the years, the deterministic EHL approach has been widely used. This technique is very powerful in capturing details of asperity deformation and interaction. The probabilistic EHL methodology is still used when the main interest of the engineer is directed toward computations of bulk properties. During recent years, the results of many deterministic analyses have been published. The reduction of the waviness amplitude in EHL contacts under rolling-sliding was systematically studied and it was showed that the amplitude reduction is completely described by a single parameter that includes the relative wave length and the operating conditions. This approach, usually referred to as the amplitude reduction technique has opened a way for developing improved probabilistic EHL models by incorporating the effects of fluid-induced roughness deformation, which is calculated using the Fast Fourier Transform. Such a probabilistic model has already been presented for the case of isotropic roughness, (30), (31). Here we provide a review of the lastest developments in the amplitude reduction technique and we present a probabilistic EHL algorithm for the computations of the load supported by the fluid, the elastically deformed asperities and the plastically deformed asperities in a mixed EHL contact with either isotropic of non-iotropic roughness. The fluid-induced roughness deformation is incorporated into the probabilistic model via the use of the amplitude reduction technique.Copyright

An Approximate Analytical Solution for the Elasto-Plastic Contact of the Non-Isotropic Rough Surfaces

The contact mechanics models incorporate various integral expressions that involve the probability density functions of the asperity heights. These integrals can be calculated numerically with the desired accuracy, however, in the practical applications of the contact mechanics, the computations are already quite time consuming. Consequently, various authors have proposed approximate solutions, mostly for the isotropic contact mechanics models. The current paper presents an approximate solution for the non-isotropic elasto-plastic model proposed by Horng. The probability density of the asperity heights is approximated, so the integrals that describe the elastic contact, as well as the integrals that describe the plastic contact, can be obtained as closed form expressions. Finally, the plastic supported load and the elastic supported load for the non-isotropic roughness are expressed using the gap, the spectral moments of the roughness and the plasticity index.Copyright

Dynamic Modeling of a Dual Clearance Squeeze Film Damper: Part III

Squeeze film dampers (SFD) are devices utilized to control vibrations of the shafts of high-speed rotating machinery. A dual squeeze film damper (DSFD) consists of two squeeze film bearings that are separated by a sleeve, which is released when the rotor experiences abnormal operating conditions. In this part of our study of DSFD we analyze the case when both the inner and the outer oil films are active and the separating sleeve is supported by a squirrel cage. Numerical results are compared with the experimental data.© 2005 ASME