The effect of working parameters upon elastohydrodynamic film thickness under periodic load variation

There are a number of widely used machine components, such as rolling element bearings, gears and cams, which operate in the lubrication regime known as Elastohydrodynamics (EHD), where lubricant film thickness is governed by hydrodynamic action of convergent geometry, elastic deformation between non-conformal contacting surfaces, and the increase of lubricant viscosity with pressure. Variable loading conditions occur not only in all the machine components mentioned above, but also in natural joints such as hip or knee joints of humans or many vertebrates. Experimental studies of the behaviour of EHD films under variable loading are scarce and to authors’ knowledge systematic studies of the evolution of lubricant film thickness in EHD contacts subjected to forced harmonic variation of load are even less common. The aim of the present study is to explore the effect of load amplitude on the EHD film behaviour. This is done in alternating cycles with the load varying about a fixed, preset value at various amplitudes. Experimental results are compared with a simple theoretical analysis based on the speed of change of contact’s dimensions, a semi-analytical solution which includes both speed variation and squeeze effect, and finally with a full numerical solution

Lubrication Quality Of Compressor Rolling Bearings With Oil-Refrigerant Mixtures

Rolling bearings in refrigerant compressors are lubricated with a mixture of oil and refrigerant. This has always represented a challenge for the bearing lubrication quality estimation (e.g.: kappa or lambda parameters). Even if the mixture rate is known, the exact effect on the lubricant film thickness remains doubtful due to the unknown piezoviscosity and compressibility of the refrigerant and their variation with pressure and temperature. In this work, existing mixing laws for viscosity and piezo-viscosity are examined and adapted to better represent existing measurements. The results are compared with published Daniel plots showing reasonable agreement. Once this is done a modification to the bearing lubrication quality parameter kappa is proposed to better reflect the effect of the refrigerant on the lubrication quality of compressor rolling bearings. This is a first step in the direction of predicting the bearing life for this challenging application. Considerations of machine efficiency and environmental impact lead to ever new development of refrigerants and refrigeration oils. The refrigeration industry must adopt quickly to these changes. The capability to predict accurately the film thickness in lubricated contacts and having tools available to perform a reliable bearing selection are prerequisites for a robust machine design. The modelling capability of the lubrication quality does not only support the bearing life prediction but will facilitate the assessment of other fundamental bearing performance ratings such as friction and heat generation. The potential of the lubrication quality prediction is outlined in an example of a screw compressor in an air conditioning application

Elastohydrodynamic lubrication of smooth and rough surfaces

Different aspects of Elastohydrodynamic Lubrication (EHL) are studied. For smooth surfaces, a novel approach which solves separately the inlet and outlet regions by using the fracture mechanics equations, is proposed to solve EHL line contacts for shape and pressure. For rough surfaces, the full EHL geometry is reduced to an infinitely long contact with known mean film thickness and pressure; so real-roughness steady state analyses are carried out by considering the separate Fourier components of roughness and pressures, transient analysis by applying general finite difference methods. The subsurface stresses under micro-EHL are also calculated and given in form of a probability rather than a specific value and location. Initially, full-geometry EHL line contacts of smooth surfaces are studied. The spike of pressures is assumed to be singular and the idea is to start with an original Hertzian pressure distribution, then the edges of this pressure are truncated and the effects calculated via linear fracture mechanics; after this, the removed pressures are replaced by the converged inlet and outlet pressures, previously obtained by iterating the Reynolds and fracture mechanics equations. It is found that the outlet pressures follow a modified logarithmic function and therefore the exit bump in the shape joins the parallel film zone with a finite value of slope, unlike the Greenwood extension of Grubin's theory. From a set of solutions, the behaviour of the pressure spike as a function of two dimensionless numbers is followed. Comparisons with results from full numerical solutions are shown, giving good agreement. The scheme is later extended to consider compressibility and the Roelands viscosity law. After reducing full EHL geometry, the effects of real and wavy roughness in microEHL of Newtonian and Eyring fluids with or without compressibility are studied. Steady state analyses of real roughness show that only the high frequency components remain after deformation. By linearizing the Reynolds- Eyring equation an analytical solution is obtained and a criterion for the deformation of the roughness in EHL is given; from this, it is shown that the deformation is very much dependent on the ratio λ/ħ, obtaining little deformation for low values of it. Transient analyses of roughness in lubrication are also carried out considering the infinitely long contact. It is found that the transient pressure and film distributions are made of two parts: a) the well known steady state solution, plus b) a complementary function depending only on the modulation of film and pressures from the inlet. It is shown that the conclusions outlined for some authors (e.g. Venner and Lubrecht) about pressures travelling with the velocity of the roughness but shape with the average velocity of the lubricant, are only a particular case of a more general understanding. It is now believed that there is no a real physical damping in the transient shape

Rolling Bearings Lubricated With Refrigerant

Oil-free systems gain increased popularity in the chiller industry. This technology is often related to reduced maintenance, higher system efficiency and smaller environmental footprint. In refrigeration compressors which use oil for the lubrication it is impossible to avoid mixing of refrigerant and oil in the system. The system efficiency is affected by the contamination of the refrigerant by oil. One possibility to operate a compressor of a chiller without oil is the pure refrigerant lubrication (PRL) bearing technology. With this technology the traditional oil lubrication system can be removed completely and the bearings are lubricated only with refrigerant. This removes the challenges related to mixtures of fluids - on the chiller performance side (efficiency reduction due to oil contamination of the working fluid) as well as on the lubrication side (lubricant diluted by refrigerant). The challenge which pure refrigerant lubrication poses to rolling bearings was overcome by a special bearing development. Energy efficiency and total cost of ownership are major drivers of the industry. Therefore, accurate predictions of bearing power losses became a fundamental component of the bearing performance rating. For bearings operating in refrigerant environment, friction is even more important, since the heat generation affects the thermodynamics of the refrigerant, and can lead to refrigerant evaporation and starvation. A special bearing test rig has been developed, which allows the investigation of the bearing performance under pure refrigerant lubrication conditions. Its main components are the hermetic bearing test head and the refrigerant supply system. The test rig was designed to withstand the conditions present in refrigerant environments and enables the investigation of refrigerant condition and injection properties of typical bearing operating conditions. The refrigerant is pumped, filtered and conditioned in the lubrication circuit before it is injected into the bearing to provide cooling and lubrication. The bearings inside the stainless steel housing can be subjected to various loads and rotational speeds. The refrigerant properties such as film building ability, density and the boundary friction coefficient as well as possible evaporation influence the bearing friction characteristics. With the test rig the influence of the thermodynamic behaviour can be analysed. The mapping of the friction behaviour under various operating conditions allows the optimisation of the injection properties. PRL bearings enable machine designs which offer excellent overall system efficiency

Vulnerabilidad de las tierras secas a la desertificación, degradación de las tierras e impactos económicos y sociales

El documento presenta varios estudios de caso que analizan la utilidad de diversos indicadores utilizados para evaluar la vulnerabilidad a la desertificación, la degradación de las tierras y la sequía con un enfoque sobre los impactos socio-económicos. Así mismo se aplican, analizan y evalúan diferentes metodologías aplicadas a diferentes escalas que van desde lo nacional hasta lo local y que incluso pueden ser replicadas fuera de la región latinoamericana y caribeña con los ajustes y adaptaciones pertinentes.JRC.D.6-Knowledge for Sustainable Development and Food Securit

La cultura del arte como multiconexión digital de audiencias

En tiempos de pandemia, el sector cultural en México fue gravemente afectado, las personas dejaron de asistir al cine, teatro, museos, y de más foros artísticos para resguardarse en sus casas y hacer frente a la enfermedad Covid-19. Por otro lado, como espectadores revaloramos la importancia del arte, nos dimos cuenta que es un medio para imaginar, crear, disfrutar y recomponer el tejido social que el confinamiento dejó a su paso. A pesar de estas adversidades, la cultura resurgió, pero ahora en un formato diferente, la digitalización del arte fue un pilar para amenizar el contexto histórico por el que atravesamos este 2020. La manera de lidiar con ello fue la estrategia digital que algunas instituciones dedicadas a la cultura en la República Mexicana implementaron para respetar los derechos culturales de los ciudadanos. Algunos de ellos fueron los recorridos virtuales a museos, exposiciones digitales, obras de teatro online, conciertos vía streaming, e-books, conferencias y plataformas gratuitas para ver películas, formaron parte de la nueva realidad cultural en México. Durante este periodo tanto artistas como espectadores se tuvieron que adaptar a esta digitalización forzada provocada por la pandemia. Muchos creadores artísticos y culturales tomaron de referente este contexto histórico para ofrecerle al público nuevas maneras de crear arte a través de la virtualidad. Una de las ventajas que se obtuvo en estos experimentos fue la multiconexión masiva de las audiencias, pues con las nuevas tecnologías fue más fácil poder enlazar al público directamente con los artistas en tiempo real desde cualquier parte del mundo. Ante la situación que muchas organizaciones enfrentaron, se dieron a conocer una variedad de plataformas para videoconferencias, que casi nunca antes fueron mencionadas. De este modo los usuarios disfrutaban desde la comodidad de sus hogares puestas en escena que tal vez nunca se llegaron a imaginar. Una estrategia más fueron las convocatorias de creación artística para todo tipo de edades, inspiradas en el confinamiento; en la pandemia; en la pérdida de seres queridos; en los médicos; entre otros temas relacionados. La unión de contenidos de diferentes organizaciones dedicadas a la cultura también destacó en este periodo, en donde sin importar la institución, o medio de comunicación, se abrió paso a nuevas alianzas en favor del sector cultural para beneficio de las audiencias. Sin duda este hecho histórico dejó una huella importante en la industria cultural, ahora solo queda esperar para que estos recintos dedicados al arte abran de una manera paulatina, cumpliendo todos los estándares de salubridad y se reincorporen a esta nueva normalidad que ha dejado la pandemia por Covid-19.Facultad de Periodismo y Comunicación Socia

Response of elastic/viscoelastic layers on an elastic half-space in rolling contacts:Towards a new modeling approach for elastohydrodynamic lubrication

The 3D quasi-static contact problem of a rigid sphere rolling on an elastic half-space covered by a thin viscoelastic/elastic layer is studied as representative of soft layered contacts in engineering, physics, and biomedical applications, as well as for its potential merit to model Elasto-Hydrodynamically Lubricated (EHL) contacts in specific EHL film behavior suggested in earlier research. The viscoelastic layer behavior is modeled with a standard linear solid (SLS) model with a single relaxation time. Two approaches are used, the foundation (Winkler) reduced model assuming unidirectional stress–strain behavior normal to the surface only and the Papkovich–Neuber potential model which, based on the complete Navier–Cauchy equations, accounts for non-local support and bending effects. The models are validated against literature and compared. Whereas coated/layered contact problem studies mostly consider relatively thick coatings, and results focus on the pressure distribution and the contact area size, in this paper we consider layers with a thickness much smaller than the contact radius, i.e. of the order of a thickness of a conventional EHL film. The details of the layer deformation, pressure profiles and subsurface stresses are presented and interpreted in terms of the underlying physics, in particular the thin layer limit common also in thin fluid layers. It is shown that two dimensionless parameters, the ratio of the elastic modulus between the layer and the substrate and the ratio of the layer thickness to the corresponding Hertz contact width, dominate the systematic response for elastic layered contact problems. And four dimensionless parameters with two extras, the Deborah number based on the Hertz contact width and the ratio of the two elastic limits of the SLS viscoelastic material, are needed and analyzed for viscoelastic layered contacts. The results presented provide a good framework for the understanding and interpretation of the effects of viscoelastic layers on the deformation and pressure distribution of contact problems. Finally, the capability of actually modeling EHL film behavior via a viscoelastic layer suggested by van Emden et al. (2017) is discussed.</p

The effect of non-traditional cooling on dog sperm cryosurvival and ability to perform the acrosome reaction

The objective of this study was to assess cryosurvival, plasma membrane fluidity, and capability of cryopreserved dog (Canis lupus familiaris) spermatozoa, cooled to –5 °C before freezing, to perform the acrosome reaction under the effect of progesterone and calcium ionophore. In the first experiment, fresh spermatozoa diluted in Tyrode’s medium plus albumin, lactate, and pyruvate (TALP) were incubated at 38 °C in 5% CO2 in air, with progesterone or calcium ionophore added at 2, 4, and 6 h after incubation and sampled 30 min later to assess the acrosome reaction. In the second experiment, diluted sperm were packaged in plastic straws, cooled to either +5 °C or –5 °C and cryopreserved. Progressive motility, plasma membrane integrity and fluidity, capacitation status and acrosome integrity were assessed before and after freeze-thawing. After thawing, sperm were assessed, resuspended in TALP and incubated to assess the acrosome reaction. Parameters for sperm cryosurvival were similar in sperm cooled to either +5 °C or –5 °C, except in the percentage of hyper-fluid membranes which was lower (P&lt;0.05) in sperm cooled to –5 °C. There were no differences in the percentages of frozen-thawed spermatozoa with acrosome reaction, induced by progesterone or calcium ionophore, between cooling treatments. In conclusion, cooling of dog spermatozoa to –5 °C did not improve sperm cryosurvival but had a positive effect on plasma membrane fluidity

Prediction of micropitting damage in gear teeth contacts considering the concurrent effects of surface fatigue and mild wear

The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts for mixed lubrication conditions, stress history, and fatigue damage accumulation, is adapted here to deal with transient contact conditions that exist during meshing of gear teeth. The model considers the concurrent effects of surface fatigue and mild wear on the evolution of tooth surface roughness and therefore captures the complexities of damage accumulation on tooth flanks in a more realistic manner than hitherto possible. Applicability of the model to gear contact conditions is first confirmed by comparing its predictions to relevant experiments carried out on a triple-disc contact fatigue rig. Application of the model to a pair of meshing spur gears shows that under low specific oil film thickness conditions, the continuous competition between surface fatigue and mild wear determines the overall level as well as the distribution of micropitting damage along the tooth flanks. The outcome of this competition in terms of the final damage level is dependent on contact sliding speed, pressure and specific film thickness. In general, with no surface wear, micropitting damage increases with decreasing film thickness as may be expected, but when some wear is present micropitting damage may reduce as film thickness is lowered to the point where wear takes over and removes the asperity peaks and hence reduces asperity interactions. Similarly, when wear is negligible, increased sliding can increase the level of micropitting by increasing the number of asperity stress cycles, but when wear is present, an increase in sliding may lead to a reduction in micropitting due to faster removal of asperity peaks. The results suggest that an ideal situation in terms of surface damage prevention is that in which some mild wear at the start of gear pair operation adequately wears-in the tooth surfaces, thus reducing subsequent micropitting, followed by zero or negligible wear for the rest of the gear pair life. The complexities of the interaction between the contact conditions, wear and surface fatigue, as evident in the present results, mean that a full treatment of gear micropitting requires a numerical model along the lines of that applied here, and that use of overly simplified criteria may lead to misleading predictions

Responsabilidad Social y Ciudadanía: Una perspectiva desde la universidad y la administración pública

La importancia de los estudios sobre ciudadanía se vincula con la imperiosa necesidad de fortalecer la democracia en todos aquellos espacios donde se manifiesta, principalmente en países como México, donde la falta de credibilidad en las instituciones democráticas ha permeado a la sociedad.Los desafíos contemporáneos requieren nuevos ajustes en diversos sentidos; al interior de las organizaciones es necesario adecuar los mecanismos con que interactúan frente a la sociedad en la que se desarrollan. Es requisito indispensable que el grueso de las organizaciones sociales se vinculen de manera directa con los problemas globales y nacionales: cambio climático, guerras, exigencias democráticas, movimientos sociales, pobreza, desempleo, inestabilidades políticas etc., ello les exige que se asuman como parte del complejo social, donde sus acciones repercuten de forma directa o indirecta