Relation between low temperature fluidity and sound velocity of lubricating oil

Low temperature fluidity is important for lubricating oil. Viscoelastic solid transition temperature at atmospheric pressure T_VE0 represents the low temperature fluidity of lubricating oil, which is estimated from the occurrence of photo elastic effect by lowering the temperature using liquid nitrogen. Sound velocity in lubricating oil is measured using Sing around technique. Then the adiabatic Bulk modulus K is calculated from the measured sound velocity. A relation is found between the adiabatic bulk modulus and the viscoelastic solid transition temperature of lubricating oil. The relation depends on the molecular structure of lubricating oil. The oils of a group belong to almost same molecular structure, follows the same relation. For same molecular structure T_VE0 decreases as decreasing the adiabatic bulk modulus of lubricating oil. It is also found that, the viscoelastic solid transition temperature of blend oils can be predicted from the base oils’ T_VE0 and K.     

Effect of high-pressure rheology of lubricating oils on boundary lubrication

High-pressure rheology of lubricating oil was determined using different experiments, and the phase diagram was drawn. The four-ball wear tests were used to evaluate anti-wear characteristics of oils in boundary lubrication condition. The bridged ring compound oils showed the minimum wear scar in the four-ball wear tests. The diameter of wear scar decreases with increasing the elastohydrodynamic film-forming capability. Next, we considered the molecular packing parameter T_VE−T at the four-ball wear test. The T_VE−T values of bridged ring compound oils were in the range 250-360 and oils were elastic-plastic solid. It is concluded that the solidified oil film under boundary lubrication conditions has the anti-wear action.     

Prediction of pressure–viscosity coefficient of lubricating oils based on sound velocity

The pressure–viscosity coefficient is an important parameter in tribology. Experimentally, it is calculated using the high‐pressure viscosity measurement. Also, the adiabatic bulk modulus is calculated using the sound velocity in the lubricating oil. Several lubricating oils are considered on the group basis such as traction oil, mineral oil, polyalphaolefin oil, perfluoropolyether oil and glycerol, depending on their molecular structure. Experimental pressure–viscosity coefficient is compared with the adiabatic bulk modulus. It is found that the pressure–viscosity coefficient increases exponentially with the adiabatic bulk modulus, and the relationship depends on the molecular structure of the lubricating oils. This study proposes two equations to predict the pressure–viscosity coefficient from the adiabatic bulk modulus based on sound velocity, one for the traction oil, and another for the paraffinic mineral oil and the polyalphaolefin oil. Copyright © 2009 John Wiley & Sons, Ltd.     

航空润滑油拖动特性动态数据库的建立

建立的润滑油拖动特性动态数据库具有查询、计算、绘图、比较、维护和设置步长等多项功能.通过该数据库,可以查到多种航空润滑油在多种工况条件下的拖动系数及拖动系数计算公式中的系数值,可以对不同润滑油的拖动系数进行比较及同一种油的计算值和试验值进行比较.     

Effect of EHL Contact Conditions on the Behavior of Traction Fluids

New infinitely variable transmission (IVT) systems are under development for the automotive industry as a means to achieving significant fuel economy benefits. These systems rely on the lubricating fluid to transmit the drive train loads across the interface of the transmission components. This requires the development of new fluids that exhibit high traction properties under elastohydrodynamic lubrication (EHL) conditions. However, it has been reported recently that the traction performance of some fluids can reduce dramatically as temperature is reduced. This may place severe operational limits on IVT systems and suggests that the low-temperature traction properties of fluids for these systems should be studied in order to understand the mechanism for the observed reduction in traction.

The work reported here is an experimental study aimed at identifying whether low temperature traction reduction is related to a fundamental change in rheological behavior specific to the fluids tested or to more generic changes in the EHL contact conditions. A series of model experiments were performed using a mini traction machine (MTM) on three high-viscosity polybutene samples. The results have been mapped against previously reported non-dimensional parameters used to identify different EHL regimes. The results show that dramatic reductions in traction occur when the contact transitions from the rigid piezo-viscous (RP) toward the rigid iso-viscous (RI) region. Similar results were also found for two other high-viscosity fluids of different molecular structure and lower traction properties. The results support the hypothesis that the reduction in traction observed at low temperature is due to a change in EHL contact conditions rather than being solely due to a change in the rheological performance of the test fluids.     

Wear and friction behaviour of CaCO3 nanoparticles used as additives in lubricating oils

For some years, reports have been published on adding solid lubricant powder to oil to improve the tribological properties of the latter, but the results have not been satisfactory. In this paper, we describe the preparation of CaCO₃ nanoparticles in a microemulsion consisting of sodium dodecyl‐sulphate (SDS)/isopentanol/cyclohexane/water, and assessment of the tribological behaviour of CaCO₃ nanoparticles as additives for lubricating oils. The CaCO₃ nanoparticles were characterised by transmission electron microscopy (TEM), and their tribological performance was tested in a four‐ball machine; the rubbing surface was analysed with X‐ray photoelectron microscopy (XPS). The results indicate that the size of CaCO₃ nanoparticles increased with the concentration of aqueous reactant, and that CaCO₃ nanoparticles exhibited good load‐carrying capacity, antiwear and friction‐reducing properties. The tribological properties of lubricating oils could be improved significantly by dispersing CaCO₃ nanoparticles in 500SN base oil containing dispersants such as polyisobutene‐butanediimide (T154), calcium alkylsulphonate (T101) and methyl‐tricaprylamine chloride (aliquat 336). The improvements in friction and wear were concluded to be due to the formation of a film containing CaCO₃ and CaO in the rubbing region, and the presence of nanoparticles, which may act in the same way as ball bearings, to facilitate sliding.     

EHL traction and related rheological parameters under high temperature conditions

An approximate formula is presented for the maximum traction coefficient in EHD conditions, by making some reasonable simplifications of an Eyring fluid model. Experiments are conducted for some mineral and synthetic hydrocarbon oils over a wide range of temperatures. Among the rheological parameters related to maximum traction, the representative stress τ₀, is determined from the experimental traction curve while the effective viscosity-pressure coefficient α is obtained in a high pressure viscometer. When the temperature is raised, α falls and τ₀ rises. The behaviour of τ₀ for some paraffinic oils follows the Eyring theory while the effect of the dissociation of molecular clusters appears with the naphthenic oils. The increase in temperature causes a reduction in the maximum traction coefficient, which can be predicted with sufficient accuracy by the formula using the rheological parameters expressed as a function of temperature.     

Energy conservation in stainless steel cold rolling applications

In order to predict the performance of rolling oils in actual production mills, from tests in laboratory mills, the influence of various factors, such as shape factors work roll diameter (D), strip thickness (h₁), operating conditions (reduction rate (r), peripheral velocity of roll (U₀), strip velocity (U₁)) and kinematic viscosity of the rolling oil (υ₀) was investigated. A parameter RLI (Rolling Lubrication Index = u₀(U₀+U₁)(1−r)(D/2h₁.r)^½) was found to be useful in predicting lubricating conditions in actual production mill applications. The coefficients of friction of mineral oils, some synthetic hydrocarbons including polybutene, and several kinds of additives were obtained from laboratory mills under the same condition of RLI value as that for finish rolling in actual production mills. With hydrocarbons, paraffins showed the lowest coefficient offriction, while aromatics exhibited a higher coefficient of friction, with naphthenes showing the highest. A high-quality rolling oil was formulated using a combination of ester and paraffinic mineral oil. It is observed that this new oil can save 14% of energy consumed by a laboratory mill compared with conventional rolling oils. In production mills, nearly the same energy conservation level can be achieved.     

An Investigation into Grease Behavior in Thermal EHL Circular Contacts

The behaviors of two lithium lubricating greases were investigated under EHL circular contact through measurements of traction coefficients on a self-made rig in which the contact was continuously fed with fresh grease. The average values of Erying shear stress and shear modulus of the two lithium greases were obtained from traction experimental data using this rig. Based on the Evans-Johnson model and thermal analysis, we calculated the values of shear stress and traction coefficients of the two greases. The results show that the calculated traction coefficients agree fairly well with the measured data.     

Film-forming tendencies of some base fluids and viscosity modifiers

The film-forming tendencies of selected mineral base oils and synthetic base fluids were investigated with and without conventional GL-5 additive packages. A pressurised falling-body viscometer and a concentrated contact simulator were used to measure low-shear viscosities, central film thicknesses, and traction coefficients. Analysis of the mineral—based oils showed that a paraffinic base oil and a naphthenic base oil had similar film thicknesses, even though the naphthenic base oil has higher pressure-viscosity coefficients. A very high viscosity index oil gave thinner film thicknesses and lower pressure—viscosity coefficients than the paraffinic or naphthenic base oils. Analysis of the synthetic base fluids showed that a PAO-4 base fluid gave thicker film thicknesses than an ester base fluid. The analysis of fully-formulated oils showed that the PAO-4 oil containing a proprietary polyolefin provided a similar filnz thickness to the PAO-4 oil containing a more expensive PAO-100.     

A naphthenic oil of hydrogenated coal tar distil- late as a lubricant with low solidification pressure

A new type of lubricant, TN, composed mainly of condensed polycyclic naphthenic hydrocarbon rings, was developed from coal tar through a complete hydrogenation process. This oil is characterised by a high pressure-viscosity coefficient and low solidification pressure. The tribological characteristics of this new oil were investigated in an elastohydrodynamic lubrication regime under sliding, rolling/sliding, and rolling contact conditions. The results for the TN oil were compared with those of several oils of different molecular structures: polyalphaolefin of paraffinic hydrocarbon, and naphthenic and paraffinic petroleum oils. It was found that the TN oil was superior to the other oils tested in its ability to form thick oil films, to reduce wear, to increase the coefficient of traction, and to prolong fatigue life.     

Correlation of structure and properties of groups I to III base oils

The understanding of the relationship between molecular structure and viscosity–temperature behaviour of a lubricant system is a subject of considerable importance. The quantitative distribution and types of different classes of hydrocarbons such as aromatics, paraffins (normal and iso) and naphthenes determine the physico‐chemical behaviour of a lubricant system. The study of molecular structure and molecular alignment of hydrocarbons constituting a lubricant helps in the development of lubricating oil with desired physico‐chemical properties. The present study highlights the application of nuclear magnetic resonance spectroscopic technique for deriving detailed hydrocarbon structural features present in API groups II and III base oils produced through catalytic hydrocracking/isodewaxing processes. The viscosity–temperature and viscosity–pressure properties, such as viscosity index, pour point, elastohydrodynamic film thickness and cold cranking simulator viscosity, were determined. The structural features of these base oils such as various methyl branched structures of isoparaffins and branching index, which are characteristics of high performance molecules, were correlated with the above‐mentioned properties to explain their physico‐chemical properties, particularly low temperature properties. The molecular dynamics parameters such as diffusion coefficient and T₁ relaxation times estimated from the nuclear magnetic resonance spectral studies have provided sufficient evidence for the dependence of these properties on these high performance molecules present in various types of methyl structures of isoparaffins of groups II and III base oils compared with conventional group I base oils. Results are explained on the basis of molecular structural differences of hydrocarbons present in these base oils and diffusion measurement studies. On the basis of the studies, molecular engineering concept for the designing of a high performance base oil molecule is proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

Base fluid parameters for elastohydrodynamic lubrication and friction calculations and their influence on lubrication capability

In this study, base fluid parameters for elastohydrodynamic lubrication (EHL) and friction analyses have been determined experimentally. The viscosity at atmospheric pressure, η₀, the pressure‐viscosity index, α, and the EHL friction coefficient, γ, are important parameters in EHL theory and they are crucial in the selection of efficient lubricants for different applications. This investigation focuses on three important lubrication mechanisms: the capability of forming a separating lubricant film, the friction generated in a lubricated contact, and the height of pressure peaks, such as the outlet pressure spike and pressure ripple caused by surface roughness. The influence of different lubricant parameters on these three mechanisms is discussed. The value of α is measured in a Couette high‐pressure viscometer, and the value of γ is obtained from a jumping‐ball device. Other parameters discussed are temperature‐viscosity coefficient, β, bulk modulus, B₀, thermal conductivity at atmospheric pressure, λ₀, and heat capacity unit volume, ρc_p0. A comparison between traditional mineral base oils and environmentally adapted oil based on rapeseed oil and synthetic esters contributes to the further understanding of the performance of these new materials in lubrication applications. It is shown that rapeseed oil and synthetic esters have good lubricating properties and are, in most cases, better than mineral oils.     

New Lubricating Oils by Graphite Treatments of Petroleum Distillates

High-surface-area graphites, that can be prepared by grinding synthetic graphites in vibratory ball mills, can be used in a novel refining technique which can markedly reduce pour points of various petroleum distillates and can be used to prepare oil fractions with high viscosity indices. The treatment also lowers the content of aromatic and sulfur compounds and completely eliminates polycyclic aromatics.

Some of the oils obtained by selective adsorption on graphite may well have some of the desirable properties of naphthenic oils and also constitute improved base stocks for multigrade lubricating oils, oils for gas-turbine engines, hydraulic oil and transmission lubricants. Results obtained to date indicate that graphite refining can produce oils similar to those that can be obtained by super-refining treatment i.e., high pressure hydrogenation and deep dewaxing. Otherwise, the process can be used to prepare good quality base oils in one step, thus eliminating furfural extraction, dewaxing and ferrofining or clay contacting.

A notable feature of base oils prepared by this new process is then relatively low viscosity at low temperatures (0°F) which makes them good candidates for the preparation of lubricating oils with fuel saving properties.     

Prevention of oxidative degradation of ZnDTP by microcapsulation and verification of its antiwear performance

Prevention of oxidative degradation of zinc dialkyldithiophosphate (ZnDTP) is very important to prolong the antiwear performance of the engine oil. The microcapsulation of ZnDTP was attempted so as to isolate ZnDTP from peroxides in bulk lubricating oil during use. Microcapsulation allows for the isolation and protection of ZnDTP in bulk lubricating oil, and the ability to release control of ZnDTP at the contact surfaces. The protection of ZnDTP from oxidative degradation was evaluated by adding cumenhydroperoxide to the microcapsule-containing sample oil or by blowing NO_x gas into the sample, followed by analysis with ³¹P NMR. The antiwear performance of the microcapsule-containing sample oil was studied using a 4-ball tribometer. It was confirmed that the microcapsulation of ZnDTP significantly reduced the oxidative degradation of ZnDTP additive, and antiwear performance was effectively maintained.     

Effects of transverse atomic steps on bilayer lubricating films of simple hydrocarbons

Molecular dynamics (MD) simulations were conducted in order to study the dynamic behavior and traction of bilayer lubricating films of n-hexane, cyclohexane, and n-hexadecane. Lubricants were confined between bcc iron surfaces with and without transverse grooves of mono-atomic depth. Once the system equilibrated statically, one of the solid surfaces was moved to shear the film. The results demonstrated that the traction coefficient was governed by structures of the films, which depended on the molecular structures of the lubricants and on the atomic scale geometry of the solid surfaces. Traction was high when interfacial slip between lubricant layers and solid walls occurred. Evolution of the layered structure by gradual rearrangement of the molecules and resulting slip between the lubricant layers, caused significant reduction in the traction coefficient. The atomic steps enhanced the molecular rearrangement of n-hexadecane, while they retarded or inhibited those of n-hexane and cyclohexane resulting in a relatively higher traction coefficient for stepped surfaces. Molecular orientation of the normal alkanes under shear is described by the orientational order parameter, which has a strong correlation with the traction coefficient. The steady state traction coefficient of all the three simple hydrocarbons was highest when both of the surfaces had steps, and lowest when both of the surfaces were flat.     

The Lubricating Properties of Human Whole Saliva

We demonstrate the efficient boundary lubricating properties of human whole saliva (HWS) in a soft hydrophobic rubbing contact, consisting of a poly(dimethylsiloxane) (PDMS) ball and a PDMS disk. The influence of applied load, entrainment speed and surface roughness was investigated for mechanically stimulated HWS. Lubrication by HWS results in a boundary friction coefficient of μ ≈ 0.02, two orders of magnitude lower than that obtained for water. Dried saliva on the other hand results in μ ≈ 2–3, illustrating the importance of hydration for efficient salivary lubrication. Increasing the surface roughness increases the friction coefficient for HWS, while it decreases that for water. The boundary lubricating properties of HWS are less sensitive to saliva treatment than are its bulk viscoelastic properties. Centrifugation and ageing of HWS almost completely removes the shear thinning and elastic nature observed for fresh HWS. In contrast, the boundary friction coefficients are hardly affected, which indicates that the high-M _w (supra-)molecular structures in saliva, which are expected to be responsible for its rheology, are not responsible for its boundary lubricating properties. The saliva-coated PDMS surfaces form an ideal model system for ex-vivo investigations into oral lubrication and how the lubricating properties of saliva are influenced by other components like food, beverages, oral care products and pharmaceuticals.     

Relation Between Shear Relaxation and Molecular Structure of Lubricating Oils

The viscoelastic property of various lubricating oils was measured with the oscillating crystal technique. The relation between the shear relaxation behavior and the molecular structure of the lubricating oils is discussed. Eyring's theory for viscous flow is used to explain the relaxation behavior from a molecular point of view. Some insight into a procedure for estimating the relaxation time from the molecular structure is presented.     

Biodeterioration potentials of microorganisms isolated from car engine lubricating oil

Biodeterioration potentials of bacteria isolated from used car engine lubricating oil were examined using both used and unused oils as substrate. Used oil served as a better substrate for growth of the bacteria than unused oil. The bacterial isolates were identified as Bacillus, Corynebacterium, Actinomyces, Micrococcus, Serratia, Citrobacter, Edwardsiella, Pseudomonas, Nocardia and Acinetobacter species. Fungal genera isolated from the oil were Cladosporium, Aspergillus, Cephalosporium, Mucor, Monosporium, Penicillium and the yeast Saccharomyces. Incubation of the bacteria at different temperatures for 48 hours showed that 100% grew at 30°C, 56% grew at 45°C, 44% grew at 55°C, 31% grew at 60°C while 25% grew at 70°C and survived at 80°C. These results indicate that lubricating oil in service is more prone to biodeterioration than unused oil.     

Molecular interaction originating from polar functional group in lubricants and its relationship with their traction property under elastohydrodynamic lubrication

The correlation between molecular interaction and traction properties was investigated using a traction tester and in situ observation of elastohydrodynamic lubrication film with a micro‐Fourier transform infrared spectrometer. The sample oils used were polypropylene glycols (PPGs) with the end‐group of alcohol or ether and a synthetic hydrocarbon oil, poly‐α‐olefin. From the traction tests, it was found that the traction coefficient of PPG was sensitive to the end‐group. PPG with alcohol as the end‐group showed a higher traction coefficient than that with the ether group. In situ observation with a micro‐Fourier transform infrared was performed in order to investigate the molecular interaction of the lubricant oil. It was found that the hydrogen bonding of hydroxyl groups in PPG was strengthened by high pressure in the Hertzian contact region. These results suggest that the rheological properties in the elastohydrodynamic lubrication contact region were affected by the strengthened hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd.