Engineering topics

Dowson, D.

Publications and source records attributed to Dowson, D..

32 records · Page 2

NASA NTRS1977

Elastohydrodynamic lubrication of elliptical contacts for materials of low elastic modulus. I - Fully flooded conjunction

A complete numerical solution is presented to the problem of isothermal elastohydrodynamic lubrication of elliptical contacts for low-elastic-modulus materials operating under fully loaded conditions. No assumption is made for the pressure or film thickness within the contact, and compressibility and viscous effects are taken into account. Because of the dimensionless representation of the coordinates, the actual Hertzian contact ellipse becomes a circle regardless of the value of the ellipticity parameter. A minimum-film-thickness relation and a central-film-thickness relation are derived from examining 17 different cases. Contour plots for detailed illustration of the pressure distribution and film thickness in the conjunction are provided.

Hamrock, B. J.↗

NASA NTRS1977

Elastohydrodynamic lubrication of elliptical contacts for materials of low elastic modulus. 1: Fully flooded conjunction

Elastohydrodynamic lubrication concepts of conjunctions of elliptical form are applied to materials of low elastic modulus. The influence of the ellipticity parameter and the dimensionless speed, load, and material parameters on minimum film thickness for these materials has been investigated. The ellipticity parameter was varied from a ball on plate configuration to a configuration approaching a line contact. The dimensionless speed and load parameters were varied by 1 order of magnitude. Seventeen different cases were used to generate minimum and central film thickness relations. Contour plots are presented that illustrate the pressure distribution and film thickness in the conjunction.

Hamrock, B. J.↗

NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. 4: Starvation results

The influence of lubricant starvation on minimum film thickness was investigated by moving the inlet boundary closer to the contact center. The following expression was derived for the dimensionless inlet distance at the boundary between the fully flooded and starved conditions: m* = 1 + 3.06 ((R/b)(R/b)H) to the power 0.58, where R is the effective radius of curvature, b is the semiminor axis of the contact ellipse, and H is the central film thickness for fully flooded conditions. A corresponding expression was also given based on the minimum film thickness for fully flooded conditions. Therefore, for m m*, starvation occurs and, for m m*, a fully flooded condition exists. Two other expressions were also derived for the central and minimum film thicknesses for a starved condition. Contour plots of the pressure and the film thickness in and around the contact are shown for the fully flooded and starved lubricating conditions, from which the film thickness was observed to decrease substantially as starvation increases.

Hamrock, B. J.↗

NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. III: Fully flooded results

The influence of the ellipticity parameter and the dimensionless speed U, load W, and material G parameters on minimum film thickness was investigated. The ellipticity parameter k was varied from 1 (a ball-on-plate configuration) to 8 (a configuration approaching a line contact). The dimensionless speed parameter was varied over a range of nearly two orders of magnitude. And the dimensionless load parameter was varied over a range of one order of magnitude. Conditions corresponding to the use of solid materials of bronze, steel, and silicon nitride and lubricants of paraffinic and naphthenic mineral oils were considered in obtaining the exponent on the dimensionless material parameter.

Hamrock, B. J.↗

NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. III - Fully flooded results

Procedures outlined by the authors in an earlier publication (1976) on theoretical formulation for isothermal elastohydrodynamic lubrication of point contacts are used to assess the influence of the ellipticity parameter and the dimensionless speed, load, and material parameters on minimum film thickness. Conditions equivalent to using solid materials of bronze, steel, and silicon nitride, and lubricants of paraffinic and naphthenic mineral oils are considered in obtaining the exponent on the dimensionless material parameter. Thirty-four different cases are used to generate the fully flooded film thickness formula. A fully flooded condition is said to exist when the inlet distance of the conjunction ceases to influence in any significant way the minimum film thickness. Contour plots are also shown that indicate in detail the pressure spike and two side lobes in which the minimum film thickness occurs.

Hamrock, B. J.↗

NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. IV - Starvation results

Previously developed basic theory for isothermal elastohydrodynamic lubrication of point contacts is used to study the effect of lubricant starvation on pressure and film thickness within the conjunction. A simple expression for the dimensionless inlet boundary distance is derived. This inlet boundary distance defines whether a fully flooded or a starved condition exists in the contact. Simple expressions for the minimum and central film thickness under starvation condition are presented. Contour plots of pressure and film thickness in and around the contact are shown in order to gain a better insight into what occurs in going from fully flooded to lubricant starvation condition. Theoretical predictions are compared with earlier results.

Hamrock, B. J.↗

NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. 2: Ellipticity parameter results

A numerical solution of the isothermal elastohydrodynamic problem for point contacts is presented which reproduces all the essential features of experimental observations based upon optical interferometry. In particular, the two side lobes, in which minimum film thickness regions occur, emerge in the theoretical solutions. The influence of the ellipticity parameter on solutions to the point contact problem is explored. The ellipticity parameter k was varied from 1 (a ball on a plate) to 8 (a configuration approaching line contact). It is shown that the minimum film thickness can be related to the well known line contact solutions by a remarkably simple expression involving either k or the effective radius of curvature ratio R sub y/R sub x.

Hamrock, B. J.↗

NASA NTRS1975

Isothermal elastohydrodynamic lubrication of point contacts. II - Ellipticity parameter results

A numerical solution of the isothermal elastohydrodynamic problem for point contacts has been presented which reproduces all the essential features of the previously reported experimental observations based upon optical interferometry. In particular, the two 'side lobes' in which minimum film thickness regions occur are shown to emerge in the theoretical solutions. The influence of the ellipticity parameter upon solutions to the point contact problem has been explored in the present paper. The ellipticity parameter (k) was varied from one (a ball on a plate) to eight (a configuration approaching line contact), and it has been shown that both the central and minimum film thicknesses can be related to the well known line contact solutions by remarkably simple expressions involving either (k) or the effective radius of curvature ratio (Ry/Rx).

Hamrock, B. J.↗

NASA NTRS1975

Isothermal elastohydrodynamic lubrication of point contacts. I - Theoretical formulation

The analysis of an isothermal elastohydrodynamic lubrication (EHL) point contact was evaluated numerically. This required the simultaneous solution of the elasticity and Reynolds equations. In the elasticity analysis the contact zone is divided into equal rectangular areas and it is assumed that a uniform pressure is applied over each element. In the numerical analysis of the Reynolds' equation a phi analysis where phi is equal to the pressure times the film thickness to the 3/2 power is used to help the relaxation process. The EHL point contact analysis is applicable for the entire range of elliptical parameters and is valid for any combination of rolling and sliding within the contact.

Hamrock, B. J.↗

NASA NTRS1975

Isothermal elastohydrodynamic lubrication of point contacts. 1: Theoretical formulation

The isothermal elastohydrodynamic lubrication (EHL) of a point contact was analyzed numerically by simultaneously solving the elasticity and Reynolds equations. In the elasticity analysis the contact zone was divided into equal rectangular areas, and it was assumed that a uniform pressure was applied over each area. In the numerical analysis of the Reynolds equation, a phi analysis (where phi is equal to the pressure times the film thickness of the 3/2 power) was used to help the relaxation process. The EHL point contact analysis is applicable for the entire range of elliptical parameters and is valid for any combination of rolling and sliding within the contact.

Hamrock, B. J.↗

NASA NTRS1975

Numerical evaluation of the surface deformation of elastic solids subjected to a Hertzian contact stress

The elastic deformation of two ellipsoidal solids in contact and subjected to a Hertzian stress distribution has been evaluated numerically as part of a general study of the elastic deformation of such solids in elastohydrodynamic contacts. In the analysis the contact zone is divided into equal rectangular areas and it is assumed that a uniform pressure is applied over each rectangular area. A study has been made of the influence on the size of the rectangular area upon accuracy. The results also indicate how far from the center of the contact one needs to go before elastic deformation becomes insignificant.

Dowson, D.↗

NASA NTRS1975

Numerical evaluation of the surface deformation of elastic solids subjected to a Hertzian contact stress

The elastic deformation of two ellipsoidal solids in contact and subjected to a Hertzian stress distribution was evaluated numerically as part of a general study of the elastic deformation of such solids in elastohydrodynamic contacts. In the analysis the contact zone is divided into equal rectangular areas and it is assumed that a uniform pressure is applied over each rectangular area. A study was made of the influence on the size of the rectangular area upon accuracy. The results also indicate how far from the center of the contact one needs to go before elastic deformation becomes insignificant.

Dowson, D.↗

NASA NTRS1974

Numerical evaluation of the surface deformation of elastic solids subjected to a hertzian contact stress

The elastic deformation of two ellipsoidal solids in contact and subjected to Hertzian stress distribution was evaluated numerically as part of a general study of the elastic deformation of such solids in elastohydrodynamic contacts. In the analysis the contact zone was divided into equal rectangular areas, and it was assumed that a uniform pressure is applied over each rectangular area. The influence of the size of the rectangular area upon accuracy was also studied. The results indicate the distance from the center of the contact at which elastic deformation becomes insignificant.

Hamrock, B. J.↗

NASA NTRS1970

Elastohydrodynamic lubrication

The history of this relatively new topic in tribology is presented, and attention is drawn to the significant theoretical and experimental developments. The position of elastohydrodynamics in the spectrum of lubrication regimes is discussed, and the essential features of elastohydrodynamic contacts are described. Attention is focused upon film thickness calculations and measurements since this represents one of the most important and best charted fields within the subject, but friction and the distribution of pressure are also considered. Transition from hydrodynamic to elastohydrodynamic and boundary lubrication conditions is considered, and attention is drawn to the importance of the ratio of film thickness/surface roughness. A number of successful engineering applications of elastohydrodynamics are noted, and the present trends in research are outlined.

Dowson, D.↗