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  • 1
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    In:  Other Sources
    Publication Date: 2011-08-16
    Description: The life of many highly loaded machine elements relies on the formation of a very thin elastohydrodynamic (EHD) oil film between surfaces in motion. Its formation results from the combination of a remarkable set of circumstances rarely appreciated. This article endeavors to explain the mechanism of elastohydrodynamic lubrication with simple physical concepts that are easy to understand.
    Keywords: MECHANICAL ENGINEERING
    Type: Lubrication Engineering; 31; June 197
    Format: text
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  • 2
    Publication Date: 2011-08-18
    Description: Changes in traction, caused by dents and grooves on a highly polished ball, are investigated as these defects approach and go through sliding elastohydrodynamic point contacts. The contacts are formed with the ball loading against a transparent disk. The ball and thus the topographical features are held stationary at various locations in the vicinity and within the contact while the disk is rotating. These topographical features can cause substantial changes in the traction when compared to traction obtained with smooth surfaces. Previously announced in STAR as N82-32734
    Keywords: MECHANICAL ENGINEERING
    Format: text
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  • 3
    Publication Date: 2019-06-28
    Description: The high temperature performance of solid lubricated rolling elements was conducted with a specially designed traction (friction) test apparatus. Graphite lubricants containing three additives (silver, phosphate glass, and zinc orthophosphate) were evaluated from room temperature to 540 C. Two hard coats were also evaluated. The evaluation of these lubricants, using a burnishing method of application, shows a reasonable transfer of lubricant and wear protection for short duration testing except in the 200 C temperature range. The graphite lubricants containing silver and zinc orthophosphate additives were more effective than the phosphate glass material over the test conditions examined. Traction coefficients ranged from a low of 0.07 to a high of 0.6. By curve fitting the traction data, empirical equations for slope and maximum traction coefficient as a function of contact pressure (P), rolling speed (U), and temperature (T) can be developed for each lubricant. A solid lubricant traction model was incorporated into an advanced bearing analysis code (SHABERTH). For comparison purposes, preliminary heat generation calculations were made for both oil and solid lubricated bearing operation. A preliminary analysis indicated a significantly higher heat generation for a solid lubricated ball bearing in a deep groove configuration. An analysis of a cylindrical roller bearing configuration showed a potential for a low friction solid lubricated bearing.
    Keywords: URBAN TECHNOLOGY AND TRANSPORTATION
    Type: NASA-CR-175114 , DOE/NASA/0323-2 , NAS 1.26:175114 , AT86D002
    Format: application/pdf
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  • 4
    Publication Date: 2019-01-25
    Description: The characteristics of the initial surface topography is intimately connected to the machining process by which it is produced. Both processes create a near surface region of residual stresses, microstructure and hardness that is different from the bulk. The material properties in this rather undefined region can significantly influence the mode of failure, such as wear, scuffing or fatigue, as well as the degree of failure resistance for a given material. Under full film elastohydrodynamic (EHD) conditions, where shear is accomodated within a relatively thick lubricant film, the normal and shear stresses are distributed uniformly over the near surface region, and the surface topography has little influence on the lubrication or failure process. Under more typical conditions where surface roughness and lubricant film thickness are of the same order of magnitude, the surface topography not only emerges as an important parameter in failure initiation, but it also becomes intimately involved in the lubrication process itself.
    Keywords: MECHANICAL ENGINEERING
    Type: Delaware Univ. Abstr. of the 20th Ann. Meeting, Society of Engineering Science, Inc.; p 146-147
    Format: text
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  • 5
    Publication Date: 2019-06-28
    Description: Scoring and scuffing are defined as two distinct but related forms of failure of hardened ferrous components lubricated by liquids. Experimental observations of these phenomena are described, and criteria for scoring and scuffing are discussed. The mechanisms proposed by various authors to explain these observations are enumerated. The view presented here is that, under conditions yet to be defined, scoring is a gradual breakdown of the lubrication of interacting asperities, such lubrication being boundary or microelastohydrodynamic in nature, or a mixture of the two. The final scuffing stage represents a breakdown of the main elastohydrodynamic system, but this may be triggered by the deterioration in surface topography as a result of scoring. An extension of published theoretical treatment of elastohydrodynamic breakdown is proposed, and a critical experiment to assess the importance of edge effects in band contacts is suggested. The practical implications of the proposed mechanism are reviewed, and recommendations for further work are made. A possible thermal instability mechanism for the breakdown of boundary lubrication is outlined.
    Keywords: MECHANICAL ENGINEERING
    Type: NASA-TM-83074 , E-1478 , NAS 1.15:83074
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  • 6
    Publication Date: 2019-06-28
    Description: Traction and film thickness were simultaneously measured under sliding elastohydrodynamic (EHD) conditions. The influence of surface topography was investigated by using simulated surfaces produced by depressing dents and grooves in highly polished steel balls. Significant changes in traction occurred depending on the orientation of the surface defects and their location with respect to the contact region. The results can be explained in terms of changes in overall film thickness and redistribution of pressure within the contact region due to micro-EHD effects. It can be concluded that the traction capability of mechanical components operating in thin film lubrication can be enhanced particularly by surface topographical orientation perpendicular to the surface motion. Associated with the higher traction are increases in local shear stress and normal stress as well as an increase in temperature at asperity sites. It is postulated that the local surface topography can become involved in run in or failure initiation even without actual asperity contact.
    Keywords: MECHANICAL ENGINEERING
    Type: NASA-TP-2175 , E-1333 , NAS 1.60:2175
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  • 7
    Publication Date: 2019-06-27
    Description: A heat transfer device consisting of a heated rotating cylinder in a bath was analyzed for its effectiveness to determine heat transfer coefficient of fluids. A time dependent analysis shows that the performance is insensitive to the value of heat transfer coefficient with the given rig configuration.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-TM-X-71809 , E-8156
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  • 8
    Publication Date: 2019-06-27
    Description: Traction measurements under starved elastohydrodynamic conditions were obtained for a point-contact geometry. Simultaneous measurements of the film thickness and the location of the inlet lubricant boundary were made. Optical interferometry was used to measure film thickness. The thickness of a starved film for combined rolling and sliding conditions varies with the location of the inlet boundary in the same way as previously found for pure rolling conditions. When the fluid velocity distribution is calculated in the inlet region by a Reynolds lubrication analysis, backflow is seen to occur over a portion of the inlet region. Backflow is essential for the establishment of a flooded condition. The location of certain fluid velocity conditions within the inlet region, as suggested in the literature, does not adequately describe the onset of starvation. For the same slide-roll ratio a starved film was observed to possess greater traction than a flooded film. Traction measurements under starved conditions were also compared with those under flooded conditions for equivalent shear rates in the Hertzian region. When the shear rates within the Hertzian region were low and the film was severely starved, the measured tractions were lower than expected. This may be due to large shear stresses developed by the large pressure gradients that are generated in the inlet region when it is severely starved.
    Keywords: MECHANICAL ENGINEERING
    Type: NASA-TN-D-8087 , E-8412
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  • 9
    Publication Date: 2019-06-28
    Description: Changes in traction, caused by dents and grooves on a highly polished ball,are investigated as these defects approach and go through sliding elastohydrodynamic point contacts. The contacts are formed with the ball loading against a transparent disk. The ball and thus the topographical features are held stationary at various locations in the vicinity and within the contact while the disk is rotating. These topographical features can cause substantial changes in the traction when compared to traction obtained with smooth surfaces.
    Keywords: MECHANICAL ENGINEERING
    Type: NASA-TM-82943 , E-1345 , NAS 1.15:82943
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  • 10
    Publication Date: 2019-06-27
    Description: Differential-maximum-bubble-pressure apparatus used for surface tension measurements in air of liquid lubricants to 200 C
    Keywords: MACHINE ELEMENTS AND PROCESSES
    Type: NASA-TN-D-6450 , E-6205
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