Hydrodynamic Load Support Research Articles

Development of a Mechanical Seal Closed Design Model

The operating experience of mechanical seals shows that as a result of angular deformations of the rings, the wear of the contact surfaces along the radius is uneven. When designing mechanical seals, it is necessary to calculate the amount of expected leakage through the seal, friction power consumption, as well as the possible durability of the unit. These calculated dependencies and estimates are obtained based on the constructed mechanical seal model. In the area of hydrodynamic load support in the case of a confusor joint or a converging film, the slope of the pressure diagram is such that a decrease in the film thickness increases the hydrodynamic support. This is referred to as enabling stable non-contact sealing operation. The execution of the rubbing surfaces of the interface of the end pair should ensure the formation of the calculated confusor shape of the sealing gap in all modes of operation and axisymmetric pressure fields during the rotation of the rotor.

Multiphase Flow Analysis of Three-Dimensional Journal Bearing with Roughness Using Computational Fluid Dynamics (CFD)

Investigating the impact of surface roughness characteristics on three-dimensional journal bearings while adjusting the eccentricity ratio is the primary goal of this work. Based on computational fluid dynamics, the rotor-bearing system with sand-grain roughness distribution surface is investigated (CFD). To study the tribological performance, the Reynolds-Average Navier-Stokes equations are combined with a multi-phase cavitation model (hydrodynamic pressure, load support and friction). The calculation demonstrates that the surface roughness level in relation to friction and load support is a determining factor for journal bearing performance

Interaction of inertia and cavitation in textured hydrodynamically lubricated bearing in very low sliding velocity considering slip

Surface texturing is becoming an effective way for increasing the performance of hydrodynamic bearings. However, introducing the texture on the lubricated sliding contact may lead to the presence of the inertia effect as well as the cavitation effect. In this study, the correlation between the inertia and the cavitation in textured bearing is investigated in terms of hydrodynamic pressure and load support. The Navier-Stokes equation based on CFD and modified Reynolds equation are applied. In addition, the use of slip condition and its effect on the performance of bearing is of particular interest. The simulations are conducted in very low sliding velocity to address the low inertia effect. The result shows that increasing inertia effect can increase the load support, but the cavitation effects is not found in the bearing pattern in which the inertia is not significant. The other interesting result concludes that slip reduces the load support, but on the other hand slip also increases the load support depending on the texture length.

Hydrodynamic Lubrication of Textured Journal Bearing Considering Slippage: Two-dimensional CFD Analysis Using Multiphase Cavitation Model

Partial texturing of the surface of journal bearings have been proven very beneficial in terms of friction coefficient. In the present work, the load support of the hydrodynamic textured journal bearing combined with artificial slippage is fully characterized by means of computational fluid dynamics (CFD) simulations based on the numerical solution of the Navier–Stokes equations for incompressible flow. In order to model slippage, the enhanced user-defined-function (UDF) code is developed. Realistic boundary condition is employed by implementing the mixture multiphase model to model a cavitation in the bearing. The numerical analysis is performed under the condition of different groove depths, eccentricity ratios and slippage placements along the textured area of bearing. The simulation results including hydrodynamic pressure and load support are gained and compared for conventional smooth parameters. A reference to determine optimal groove depths as well as best artificial slippage placement of textured bearing under different conditions of loading are proposed. Based on the present results, favorable slippagetextured journal bearing design can be assessed.

Investigation of Effective Groove Types for a Film Riding Seal

Over the past two decades, significant efforts have been made to introduce film riding sealing technology on large industrial or aerospace gas turbines. The main challenge comes from the high surface speeds and high temperatures, which lead to large thermal distortions. One approach to tackle the effect of thermally induced distortion is to design a seal to operate at a large film to limit the viscous heat generation. To design a seal pad that maximizes force at relatively high film heights, it is important to select the seal groove type that looks the most promising to deliver this characteristic. Several groove types have been assessed as part of this study. The most promising groove type is the Rayleigh step, which gives the strongest level of combined hydrostatic and hydrodynamic load support while also being easier to tessellate on individual seal segments. The results generated using a uniform grid Reynolds equation method show reasonable agreement with computational fluid dynamics (CFD) calculations. This provides confidence in the validity of the method, approach, and results.

Influence of wall slip on the hydrodynamic behavior of a two-dimensional slider bearing

In the present paper, a multi-linearity method is used to address the nonlinear slip control equation for the hydrodynamic analysis of a two-dimensional (2-D) slip gap flow. Numerical analysis of a finite length slider bearing with wall slip shows that the surface limiting shear stress exerts complicated influences on the hydrodynamic behavior of the gap flow. If the slip occurs at either the stationary surface or the moving surface (especially at the stationary surface), there is a transition point in the initial limiting shear stress for the proportional coefficient to affect the hydrodynamic load support in two opposite ways: it increases the hydrodynamic load support at higher initial limiting shear stresses, but decreases the hydrodynamic load support at lower initial limiting shear stresses. If the slip occurs at the moving surface only, no fluid pressure is generated in the case of null initial limiting shear stress. If the slip occurs at both the surfaces with the same slip property, the hydrodynamic load support goes off after a critical sliding speed is reached. A small initial limiting shear stress and a small proportionality coefficient always give rise to a low friction drag.

Hydrodynamics of slip wedge and optimization of surface slip property

The hydrodynamic load support generated by a slip wedge of a slider bearing was studied. The surface slip property was optimized so that a maximum hydrodynamic load support could be obtained. A multi-linearity method was given for the slip control equation of two-dimensional (2-D) wall slip. We investigated 2-D wall slip and the hydrodynamics of a finite length bearing with any values of the surface limiting shear stress. It was found that the hydrodynamic effect of the slip wedge is greater than the traditional geometrical convergent-wedge. Even though the geometrical gap is a parallel or divergent sliding gap, the slip wedge still gives rise to a very big hydrodynamic pressure. The optimized slip wedge can give rise to a hydrodynamic load support as high as 2.5 times of what the geometrical convergent-wedge can produce. Wall slip usually gives a small surface friction.

Entrainment and Inlet Suction: Two Mechanisms of Hydrodynamic Lubrication in Textured Bearings

A new mechanism of hydrodynamic lubrication termed “inlet suction,” applicable to low convergence, micropocketed bearings, has been identified. In this, sliding of one of the bearing surfaces generates a subambient pressure in pockets close to the bearing inlet. Because this pressure is less than the external atmospheric pressure, lubricant is “sucked” into the bearing through the inlet land. This is quite a different mechanism from classical entrainment due to shear. In the current paper flow, hydrodynamic load support and friction are calculated using analytical solutions for simple pocketed bearings having a wide range of convergence ratios, including parallel surfaces. It is found that for the parallel case, inlet suction provides the only mechanism of hydrodynamic load support, and that inlet suction continues to play a major role in load support and friction reduction up to quite high convergence ratios. This mechanism of lubrication is believed to be responsible for the enhanced lubricant film formation and reduced friction of textured bearings, previously reported by a number of authors.

Low Friction and High Load Support Capacity of Slider Bearing With a Mixed Slip Surface

The classical Reynolds theory reveals that a converging gap is the first necessary condition to generate a hydrodynamic pressure in a viscous fluid film confined between two solid surfaces with a relative sliding/rolling motion. For hundreds of years, the classical lubrication mechanics has been based on the frame of the Reynolds theory with no slip assumption. Recent studies show that a large boundary slip occurs on an ultrahydrophobic surface, which results in a very small friction drag. Unfortunately, such a slip surface also produces a small hydrodynamic pressure in a fluid film between two solid surfaces. This paper studies the lubrication behavior of infinite width slider bearings involving a mixed slip surface (MSS). The results of the study indicate that any geometrical wedges (gaps), i.e., a convergent wedge, a parallel gap, and even a divergent wedge, can generate hydrodynamic pressure in an infinite slider bearing with a mixed slip surface. It is found that with an MSS, the maximum fluid load support capacity occurs at a slightly divergent wedge (roughly parallel sliding gap) for an infinite width slider bearing, but not at a converging gap as what the classical Reynolds theory predicts. Surface optimization of a parallel sliding gap with a slip surface can double the hydrodynamic load support and reduce the friction drag by half of what the Reynolds theory predicts for an optimal wedge of a traditional slider bearing.

Pore Ensemble Statistics in Application to Lubrication Under Reciprocating Motion

The concept of pore ensemble is applied to piston ring lubrication. The solution of the two-dimensional Reynolds equation is given for reciprocating motion of a compression piston ring with circular micropores of rectangular profile on its sliding surface. Minimal film thickness between ring and cylinder is considered for identical and diverse pore ensembles. It was shown that the pores provide hydrodynamic load support, sufficient to keep the ring and cylinder apart even in cases of a linear ring profile and ring surface parallelism to the cylinder wall, through the whole piston stroke including the dead points. Calculations for a id pore cluster with diverse pore radius and depth (each with not less than 20 percent variance) show that fluctuation of the resulting film thickness is reduced up to 0.5 percent variance, while the thickness values of ensembles of diverse pores differ by more than 1. 5 times from that found for an identical pore population. There is apparently no need for perfect identity of the pores during production, but pore ensemble statistics must be taken into consideration in proper lubrication calculations. In general, the pore ensemble is an essential aspect in exact determination of the load support and better insight is provided into the tribological behavior of pore-covered surfaces. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

Mixed lubrication in mechanical face seals with plain faces

For the purposes of design and analysis of plain-face contacting mechanical face seals, it is necessary to predict the seal leakage and the friction power as functions of the face profile and the operating conditions. Thus a model of mixed friction behaviour for face seals is required. A complete model is presented. Load support is based on the sum of three components of pressure: hydrostatic, hydrodynamic and contact. Prediction of the hydrostatic pressure is straightforward. Empirical data are developed for the prediction of hydrodynamic load support. Both a plastic and an elastic model for contact pressure are presented. Complete equations are given for load support, load support equilibrium and friction. It is shown how the model is used to predict the film thickness and leakage for any combination of fluids and operating conditions. Predictions from the model are compared with some test data and agreement is encouraging. While the model is currently used as a practical tool to predict the seal performance, the weaknesses of the model are discussed, and it is shown where further research is needed.

Hydrodynamic Lubrication of Face Seal in a Turbulent Flow Regime

Abstract Models for thermohydrodynamic lubrication in the turbulent regime are developed for a mechanical end face seal with various combinations of asperity height and roughness pattern. A surface wear model, based on deformation, is established for mixed lubrication such that the displacement is at most equal to the mean asperity height. Only normal load is involved in the solution of asperity deformation, and the mean film thickness is determined based on a total volume conservation hypothesis, in conjunction with an elastic-exponential hardening model. The singularity problem, present in the expected form of the Reynolds equation for a seal surface with circumferentially-oriented roughness grain sphere grooves, is avoided by viewing the seal roughness as porous material, thereby introducing roughness permeability. Flow permeability is thus obtained by combining Darcy’s law for porous material with the average flow model developed by Patir and Cheng for mixed lubrication. The hydrodynamic pressure and thereby the hydrodynamic load support are relatively higher from a seal with radially-oriented roughness. Both the mean film thickness and the hydrodynamic load support are substantially elevated by increasing the composite rms roughness, raising the inlet-flow pressure, and decreasing the rotational speed. Good agreement has been obtained from the comparison between the results herein and Lebeck’s experimental results.

Moving-Wave Gas Seal

This paper presents a new concept for a self-acting, rotating-shaft gas seal. One of the seal faces is elastically distorted to provide a converging tilt and wavy shape on the face. The tilt enhances hydrostatic load support while the waviness enhances hydrodynamic load support. By moving the wave slowly elastically, this unique shape is preserved in spite of any wear. Leakage is essentially blocked by a continuous sealing dam around the seal. Advantages claimed are low leakage and tolerance to liquid contamination. Theory is presented for the operation of the seal. A model is developed to predict performance. Friction, leakage, film thickness, and relative wear rate are predicted. The predictions show that load support is adequate under pressurized operation. Friction is very low. When operating at zero pressure differential, a very smooth surface (low roughness) is necessary so that waviness can produce enough hydrodynamic load support to lift off. Seal leakage is very low and is insensitive to speed. E...

Hydrodynamic Lubrication and Wear in Wavy Contacting Face Seals

A model of face seal lubrication is proposed and developed. Hydrodynamic lubrication for rough surfaces, surface waviness, asperity load support, elastic deflection, and wear are considered in the model. Predictions of the ratio of hydrodynamic load support to asperity load support are made for a face seal sealing a low viscosity liquid where some contact does occur and surface roughness is important. The hydrodynamic lubrication is caused by circumferential surface waviness on the seal faces. Waviness is caused by initial out of flatness or any of the various distortions that occur on seal ring faces in operation. The equilibrium solution to the problem yields one dimensional hydrodynamic and asperity pressure distributions, mean film thickness, elastic deflection, and friction for a given load on the seal faces. The solution is found numerically. It is shown that the fraction of hydrodynamic load support depends on many parameters including the waviness amplitude, number of waves around the seal, face width, ring stiffness, and most importantly, surface roughness. For the particular seal examined the fraction of load support would be small for the amount of waviness expected in this seal. However, if the surface roughness were lower, almost complete lift-off is possible. The results of the analysis show why the initial friction and wear rates in mechanical face seals may vary widely; the fraction of hydrodynamic load support depends on the roughness and waviness which are not necessarily controlled. Finally, it is shown how such initial waviness effects disappear as the surface profile is altered by wear. This may take a long or short time, depending on the initial amount of hydrodynamic load support, but unless complete liftoff is achieved under all operating conditions, the effects of initial waviness will vanish in time for steady state conditions. Practical implications are drawn for selecting some seal parameters to enhance initial hydrodynamic load support without causing significant leakage.