Inverse approach for estimating rheological characteristics of adsorption layer in thin film EHL contacts

A modified Reynolds equation is derived for thin film elastohydrodynamic lubrication (TFEHL) by means of the viscous adsorption theory. This TFEHL theory can be used to explain the deviation between the measured film thickness and that predicted from the convenient elastohydrodynamic lubrication (EHL) theory under very thin film conditions. Results show that the thinner the film, the greater the ratio of the adsorption layer to the total film thickness becomes, and the greater the value of the pressure–viscosity index (z′). An inverse approach is proposed to estimate the pressure distribution based upon the film thickness measurement and to determine the pressure–viscosity index of oil film, and the thickness (δ) and the viscosity ratio (η^*) of the adsorption layer in TFEHL circular contacts. Based on TFEHL theory, the inverse approach can reduce z′ error, and provides a reasonably smooth curve of pressure profile by implementing the measurement error in the film thickness. This algorithm not only estimates the pressure, but also calibrates the film shape. Consequently, it predicts z′, η^*, and δ with very good accuracy. It can also be used to evaluate the lubrication performance from a film thickness map obtained from an optical EHL tester. Results show that the estimated value of z′ is in very good agreement with the experimental data.     

A simple formula for the dielectric relaxation time—temperature—pressure relationship of lubricating oils and its application to viscosity prediction

The dielectric constant and the loss factor for several lubricating oils were both measured within the frequency range of 100 Hz–1.5 MHz. Measurements were made at atmospheric pressure with varying temperature, and at fixed temperature with varying pressure. The temperature dependence of the dielectric relaxation time could be expressed by the Vogel—Fulcher—Tammann (VFT) equation. In order to express the dielectric relaxation time as a function of temperature and pressure, a pressure‐dependent term was introduced into the characteristic temperature of the VFT equation. The experimental relaxation time could then be represented by a simple formula. Moreover, high‐pressure viscosity was calculated from the dielectric relaxation data by introducing the pressure dependence of the bulk modulus. The predicted results showed fairly good agreement with the viscosity data.     

Composition—property correlations in some lubricating oil base stocks

The dependence of properties on the composition of lubricating oil base stocks (LOBS) available from Indian refineries has been studied. It has been observed that pour point is greatly influenced by the saturated component of the base stocks, whereas viscosity and viscosity index are greatly influenced by the aromatic content. The effect of various components on the viscosity/shear stress versus temperature behaviour of LOBS samples has also been investigated.     

Molecular dynamics characterization of thin film viscosity for EHL simulation

Molecular simulations were used to characterize changes in lubricant viscosity that may occur during thin film elastohydrodynamic lubrication (EHL). Molecular dynamics simulations were performed at variable wall speed and film thickness such that the effects of both parameters could be evaluated. Using this approach it was found that the viscosity of thin films under large shear is subject to both shear thinning and oscillation with film thickness. A composite model was developed that incorporated both effects. The expected impact that this model might have on an EHL interface was evaluated using a continuum simulation. An overall decrease in viscosity with some oscillation near the interface edges was predicted due to the molecularly modeled thin film effects.     

Performance characteristics of a centrally loaded 120° partial journal bearing using pseudoplastic lubricants

The additive mixed oils, which are classified as non-Newtonian fluids, are widely used in many lubrication systems. During the past decade, several analyses of plane journal bearings using non-Newtonian lubricants have been reported, although no work has been reported on partial bearings. This paper attempts to fill this void. The non-Newtonian lubricant is taken to be pseudoplastic and two types of rheological model, based on a cubic shear stress law and a shear strain rate power law, have been considered. The computed results reported here are dimensionless and include the static as well as the dynamic characteristics of the bearing     

Experimental observation of a dimple-wedge elastohydrodynamic lubricating film

One of the main features of typical elastohydrodynamic lubricating (EHL) contacts is the unique horseshoe film shape, which can be readily observed by using interferometry and quite accurately modelled by the well-established EHL theory. However, an anomalous EHL film, characterized by a wedge shape together with a tiny dimple at the inlet region, is observed under pure sliding conditions with ultra slow speeds of 3–800 μm/s in an optical EHL test rig. The variations of the wedge and the inlet dimple with different sliding speeds and loads are investigated using a series of polybutene oils of high viscosities. It is found that the inclination of the wedge is dependent on sliding speeds, loads and oil viscosities. The dimple always occurs at the inlet. The appearance of an inlet dimple together with a wedge film shape is reported for the first time. The phenomenon can be attributed to a non-Newtonian characteristic of the lubricant: the limiting shear strength. Additionally, the influence of starvation on the film shape is also examined.     

Power law fluid model incorporated into elastohydrodynamic lubrication theory of line contact

An algorithm is developed for the study of the infinitely long slider bearing in general form, considering the lubricant to be an incompressible power law fluid in isothermal conditions. The earlier works on this topic were considered by taking cavitation boundary conditions when a cylinder moves over a plane lubricated with a power law fluid and in EHL solution in a particular case, viz. pure rolling of a cylinder over an identical cylinder. We have considered a general solution including elastohydrodynamic lubrication (EHL) for different values of power law exponent. Deviation of values of central film thickness for different values of power law exponent from those for Newtonian lubricants are presented. The effects of the power law exponent on the central film thickness, minimum film thickness and load capacity are analysed. The effects of rolling and sliding velocities of contact surfaces are also analysed in terms of an equivalent radius of a cylinder moving over a moving plane. Film shapes and pressure distributions are also calculated numerically and presented graphically for various values of central film thickness considered in this paper. A number of observations obtained here with pseudoplastic nature of lubricants are in good agreement with the experimental results. The theoretical observations suggest the behaviour of common lubricants as pseudoplastic fluids in the cases of slowly moving surfaces and motion under heavy load.     

Biobased dry‐film metalworking lubricants

Metalworking lubricants must allow the manufacture of acceptable products at competitive cost without causing harm to operators or the environment. One way of attaining such a goal is through the use of biobased raw materials in lubricant formaulations. Biobased materials are generally non‐toxic, easily biodegradable, and abundantly available from renewable agricultural sources. However, successful application of biobased marterials in lubrication requires a thorough understanding of the tribochemical properties of these agricultural products. Recent studies have shown that biobased lubricants comprising starch and vegetable oils have promising lubrication properties. This paper discusses investigations into the effect of film thickness on the friction properties of dry‐film lubricants formulated from starch‐soybean oil composites.     

Bore polishing — identification and simulation

Bore polishing resulting from the abrasive wear of the bore of diesel engines has become more evident with the introduction of turbo-charging and consequent high mileage. This paper identifies the principal characteristics of bore polish, suggests causes and records the examination of a particular used liner. A laboratory wear machine was then used to simulated bore polish conditions. The results are analysed and discussed.     

Non-Newtonian effects on the dynamic characteristics of one-dimensional slider bearings: Rabinowitsch fluid model

The non-Newtonian effects of an isothermal incompressibe laminar-flow lubricant on the dynamic stiffness and damping characteristics of one-dimensional slider bearings are theoretically examined. On the basis of Rabinowitsch fluid (cubic equation) model, the modified Reynolds equation considering bearing-squeeze action is derived to take into account the transient motion of the slider, and the non-Newtonian properties of lubricants. Applying a small perturbation technique, both the steady-state performance and the dynamic characteristics are evaluated. According to the results, the steady film pressure, load-carrying capacity, and the dynamic stiffness and damping behaviors are significantly affected by the values of the dimensionless nonlinear factor accounting for non-Newtonian effects, the wedge parameter of a slider profile and the squeeze number of bearing-squeeze action.     

Parametric elastohydrodynamic study of a linear contact applied to SiC lubricated with a low‐viscosity fluid

Ceramic materials are often used in tribological applications because of their good mechanical properties. Silicon carbide (SiC) has a low density, high Young's modulus, high corrosion resistance, and very low friction in water. It is a very good candidate for replacing waterproof oil‐lubricated bearings in water pumps and for applications when water is used as a lubricant. In such applications the SiC dissolves very slowly in water to form silicilic acid. After running in, journal and bearing surfaces look like mirrors with very low roughness and very small friction coefficient. This paper reports on a parametric study to model the cylinder—plane configuration, which is then utilised in the specific case of SiC in water. The study was carried out in order to reduce the number of parameters and to ensure good precision of the results. The numerical model is applied to an SiC contact with a very low‐viscosity lubricant (water) using elastohydrodynamic (EHD) conditions. Multigrid techniques for the Reynolds equation and multilevel, multi‐integration methods for the elasticity equations are used to solve the EHD problem. These results are given in non‐dimensional form using the Moes parameters M and L for several velocities and loads. Comparison is made between the two‐ and three‐dimensional cases in order to assess the effects for the cylinder ends.     

Rerefining of used oils — a review of commercial processes

The total demand for lubricants in India is in the region of 855,000 tonnes a year. This constitutes 1.5% of total petroleum products consumption. Presently, there are three lubricant refineries in the country with a total installed capacity of 535,000 tonnes. The shortfall, of about 370,000 tonnes (approximately 45% of demand), is met through imports. In addition, there are generated 60,000–65,000 tonnes a year of base oil through rerefining of used oils. The three refineries are being expanded in order to raise base oil production to 810,000 tonnes per annum. Some other refinery projects are also under consideration and approval by the government. The current expansion programmes, together with the projected new refineries, are expected to make India self-sufficient in terms of base stock production lubricant. Overall, lubricant production in India is entirely dependent on imported lubricant-bearing crudes, and not on the indigenous crudes, which are non-lube bearing. With recent advances in engine design, together with improved lubricant quality through high-performance lubricants, the consumption, or demand rate, has been in decline or unchanging in most countries. In India, a gap still exists between production and demand of lubricants, necessitating import. Since they are a high-value, non-energy product, lubricant conservation measures are therefore essential. Interest in rerefining of used oil is increasing, with more and more emphasis on making the processes environmentally friendly. The conventional acid-clay rerefining process is environmentally unfriendly, and hence needs modification. An attempt is made in this paper to highlight various rerefining processes available. The relative merits and demerits of each process are discussed, and among the various processes, that based on molecular/high vacuum distillation is covered in detail.     

Squeeze film lubrication between circular stepped plates: Rabinowitsch fluid model

In this paper, a theoretical analysis on the squeeze film characteristics between circular stepped plates lubricated with Rabinowitsch fluid is presented. By using Rabinowitsch fluid model, the modified Reynolds type equation is derived to study the dilatant and pseudoplastic nature of the fluid in comparison with Newtonian fluid. The closed form solution is obtained by using perturbation method. According to the results obtained, the load-carrying capacity and squeeze film time increases for dilatant fluids as compared to the corresponding Newtonian fluids whereas the reverse trend is observed for pseudoplastic fluids. Further, it is observed that the response time decreases as the step height increases.     

Tribochemistry of aluminium and aluminium alloy systems lubricated with liquids containing alcohol or amine additive types and some other lubricants — a review

The ability of a lubricating oil to reduce wear and prevent damage of interacting solids is a crucial factor controlling lubricant formulation. It is well known that friction produces local high temperatures. Many chemical reactions that are initiated by the friction process itself occur at much lower temperatures than those needed to provide the activation energy. Under boundary lubrication conditions, a clean surface exposed as a result of mechanical activity of the solid surface is extremely reactive, especially in the case of metals. This review mostly relates to the tribochemistry of aluminium, and discusses the tribological characteristics of alcohol‐ and amine‐type liquids used as either additives or lubricants to lubricate aluminium and its alloys under boundary friction conditions. Although tribochemical reactions during sliding are perceived in various ways, here the emphasis is on the negative‐ion‐radical action mechanism (NIRAM) approach. This review addresses the question as to how present knowledge of tribochemistry can be applied to the elucidation of the mechanisms of action by which the boundary lubricant compounds considered reduce aluminium‐on‐aluminium, steel‐on‐aluminium, and aluminium‐on‐steel wear. Also, information and a discussion on the tribological behaviour of other additives and/or lubricants in relation to the friction and wear of aluminium and its alloys are presented. A concise review of the most recent work on the tribochemistry of selected fluorinated alcohols is also included.     

Triboreactions of graphite with moisture—a new model of triboreactor for integrating friction and wear

In order to account for the influence of environment on the frictional properties of graphite, a new model of tribocontact considering the confined volume under the pin as a triboreactor is proposed. The heart of the reactor is the mechanical production of debris displaying fresh prismatic surfaces. Both the rate of formation and the rate of deactivation of these latter are influenced by the environment and are mutually influencing each the other. The model combines three distinct mechanisms, seldom considered before: (a) the one step embrittlement of graphite governed by the amount of moisture within the contact; (b) the kinetics of water adsorption outside the contact, considered to obey the Elovich equation; (c) the triboreactions between the water and the active surfaces associated with the prismatic surfaces of the graphite debris. The validity of the model is supported by various experimental results.     

Study of surface roughness effects in elastohydrodynamic lubrication of rolling line contacts using a deterministic model

A sinusoidal surface roughness model is adopted for the analysis of the effects of roughness amplitude and wavelength on pressure profile, film shape, minimum film thickness and coefficient of friction in a steady state EHL line contact. The influence coefficients used for the evaluation of surface displacements are calculated by utilizing a numerical method based on Fast Fourier Transform. Significant reduction is observed in the minimum film thickness due to surface roughness. Such reduction is quantified by roughness correction factor, C_R, and a relationship between C_R and non-dimensional surface roughness amplitude A is derived as: C_R=1−0.7823A^0.8213. This equation may prove to be of interest from designer's viewpoint. The friction coefficient is found to increase appreciably with increasing amplitude and decreasing wavelength of surface roughness.     

Calculation of oil‐film thickness in journal bearings running at varying rotational speeds

There are many methods of calculating the properties of lubricated hydrodynamic sliding bearings running at variable loads, but, in some cases, sliding bearings of heavy‐duty machines run under constant load and at variable speeds; this can lead to reduced film thickness in the bearing and boundary lubrication. The minimum oilfilm thickness in such a bearing needs to be calculated so that the proper lubricant viscosity can be selected to reduce solid‐solid contact during machine operation. In this study, a method is presented for calculating the film thickness of sliding bearings operating with alternating rotational directions. The results of the calculations show that the squeeze effect can develop enough load‐carrying capacity to prevent solid‐solid contact between the rubbing surfaces during operation, provided the change of direction of rotation is quick enough.     

The lubrication mechanism in low speed plastic deformation with a synthetic lubricant — infiltration of lubricant into the boundary contact region

To investigate and evaluate the micro-pool mechanism in a mixed lubrication regime using polybutene lubricants, experiments were performed using 1 mm brass sheet, for various surface rough nesses of a work piece and die, lubricant viscosities and extrusion speeds. Systematic study showed that (1) the lower viscosity lubricant is easily squeezed out from the micro-pools into the flat are a through the troughs between the tool and work piece caused by plastic deformation of work piece, and (2) with decrease in lubricant viscosity and speed, or the larger the tool surface roughness becomes, the more lubricant infiltrates into the tool-work piece interface, which reduces the actual friction coefficient.     

Monolayer freeze-fracture—a modified procedure

Monolayer freeze-fracture of biological membranes is a valuable tool for integrating membrane morphology with biochemical analysis of membrane components. This correlation has been restricted by the purity of the biochemical sample. In this article, the method is reviewed, and an improved method is described. The essential modification was the use of a polysaccharide-coated microscope slide, instead of a copper plate, to cover cells attached to a polylysine-coated coverslip. It was found that proper freeze-fracture will not occur unless there is a distinct temperature gradient, with its accompanying stresses, across the cell monolayer during the freezing process. This gradient is achieved by using glass slides of different thickness to cover each side of the monolayer. Comparison of the results with those obtained when using a copper-glass system demonstrated a consistently purer sample for the glass-glass system, with whole-cell contamination of the external membrane leaflet being reduced to 0.4%. Problems associated with obtaining pure samples for biochemical analysis are discussed, and the results of freeze-fracture with the glass-glass and glass-copper systems are compared. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of polypeptides associated with the separate halves of the erythrocyte membrane demonstrated that band 3, the anion transport protein, separates with the cytoplasmic face, whereas only sialoglycoproteins and their fragments are retained in the external face. This finding, obtained with the glass-glass system, is consistent with results of our earlier freeze-fracture study that used a copper-glass system which showed that covalent bonds may be broken during this procedure.     

Stochastic model updating: Part 2—application to a set of physical structures

The application of a stochastic model updating technique using Monte-Carlo inverse propagation and multivariate multiple regression to converge a set of analytical models with randomised updating parameters upon a set of nominally identical physical structures is considered. The structure in question is a short beam manufactured from two components, one of folded steel and the other flat. The two are connected by two rows of spot-welds. The main uncertainty in the model is concerned with the spot-weld but there is also considerable manufacturing variability, principally in the radii of the folds.