Influence of lubricant on gear failures — test methods and application to gearboxes in practice

Base oil type, oil viscosity, and additive type and content have a strong influence on typical gear failures. As it is not possible to quantify the influence of a lubricant on load‐carrying capacity simply from a knowledge of the physical or chemical oil data, many test methods have been developed for the evaluation of mechanical—technological lubricant properties. Simple low‐cost bench test methods often show poor correlation with practice. From both experience and systematic investigation, it can be seen that testing of gear lubricants can be performed adequately only in gear test rigs using specified test gear geometry. The standard FZG back‐to‐back gear test rig has been developed over many years and improved for different types of gear failure simulation. The standard FZG oil test A/8.3/90 is widely used for the evaluation of the scuffing properties of industrial gear oils. Automotive gear oils of GL4 level can be tested in the step test A10/16.6R/90, and axle oils of GL5 level in the shock test S‐A10/16.6R/90. For slow‐speed regimes, the C/0.05/90:120/12 wear test can be used. The influence of lubricants on the micropitting performance of gears can be evaluated in the GF‐C/8.3/90 micropitting test. Different pitting tests are available, as single‐stage (PT‐C/9:10/90) or load spectrum (PT‐C/LLS:HLS/90) tests. The aim of this paper is to describe the influence of the lubricant on the different failure modes in gears, how to quantify this effect in adequate test methods, and how to introduce the results of such tests as determining values of the lubricant into load‐carrying capacity rating methods.     

Molecular dynamics studies of lubricant depletion under moving laser heating: Effects of laser power and film thickness

Molecular dynamics simulation is employed to study the depletion behaviors of perfluoro-lubricants under scanning laser heating for heat-assisted magnetic recording hard disk drives. A partial lubricant near the substrate is irradiated by the laser beam to mimic nano-scale heat transfer from disk to lubricant. The lubricant surface morphology and thickness profiles are examined to reveal the dynamic depletion behaviors. The localized temperature evolution is also evaluated to illustrate the direction-dependent ridge formation around the depletion zone. In addition, the effects of laser power and film thickness on lubricant depletion are explored. Although evaporation is enhanced significantly at high laser powers or for lubricant with thickness around one monolayer, thermodiffusion is the primary mode of lubricant depletion under scanning laser heating.     

Gene expression profile analysis reveals the effect of metformin treatment on HepG2 cells

Metformin is a first-line drug in the fight against type 2 diabetes. In recent years, studies have shown that metformin has some preventive and therapeutic effects on liver cancer, but the effects of metformin on the gene expression of liver cancer cells are not fully known. This study focused on the differences in the gene expression profiles in liver cancer cells treated with or without metformin. A total of 153 differentially expressed genes (DEGs) (FC > 2 and q-values < 0.001) were found, including 77 upregulated genes and 76 downregulated genes. These DEGs are involved in mitogen-activated protein kinase (MAPK), nuclear factor-kappa B (NF-κB), cell adhesion molecules (CAMs), and leukocyte transendothelial migration signaling pathways. These findings reveal the effects of metformin treatment on gene expression profiles in liver cancer cells and provide new clues for unveiling the mechanism of the antitumor effects of metformin.     

Gene expression profiling of HepG2 cells after treatment with black tea polyphenols

This study aimed to determine the effects of black tea polyphenols on gene expression in hepatocellular cancer cells. The total RNA from HepG2 hepatocellular cancer cells treated with black tea polyphenols was subjected to Human 14K cDNA microarray analysis. Real-time PCR and Western blot analysis were conducted to verify microarray data. Black tea polyphenols treatment at the dose of 20 mg/L, 40 mg/L or 80 mg/L for one to three days inhibited the growth of HepG2 cells in a dose and time dependent manner. A total of 48 genes showed more than two-fold change after black tea polyphenols treatment, including 17 upregulated genes and 31 downregulated genes, and they were involved in the regulation of cell growth, cell cycle, apoptosis, signaling, angiogenesis, tumor invasion and metastasis. Real-time PCR analysis of the selected genes showed that their mRNA expression changes were consistent with the microarray data. In addition, Western blot analysis of the selected genes showed that their protein expression changes were consistent with mRNA expression. In conclusion, gene expression profiles provide comprehensive molecular mechanisms by which black tea polyphenols exerts growth inhibition effects on cancer cells. The novel molecular targets identified in this study may be further exploited as therapeutic strategies for hepatocellular cancer.     

Behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives

In this paper, we describe the behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives. In the experiments, the ultrathin liquid lubricant film behavior is investigated using Zdol and cyclotriphosphazene-terminated PFPE lubricant which have different end groups as a function of lubricant film thickness. The disks are examined with a scanning microellipsometer before and after contact slider experiments. It is found that the lubricant film thickness profiles almost do not change, when the lubricant film thickness is less than one monolayer. It can also be observed that lubricant film thickness instability due to dewetting occurs as a result of slider-disk contacts for the tested lubricants and the films undergo spontaneous redistributions, resulting in significantly nonuniform film thickness profiles, when the lubricant film thickness is thicker than one monolayer. In addition, it is found that the observed behavior of ultrathin liquid lubricant films for cyclotriphosphazine-terminated PFPE lubricant contrasts markedly with that for Zdol. The difference between cyclotriphosphazene-terminated PFPE lubricant and Zdol is only the functional end group. Therefore, it may be concluded that their unstable lubricant behavior depends on the chemical structure of functional end groups.     

Geometric effects in high‐temperature vapour‐phase lubrication using hydrocarbon feed gases

The effectiveness of ‘far‐field’ vapour‐phase lubrication, in which areas of a bearing surface that are cycled through the contact are exposed to vapour while outside the contact, has been demonstrated in both sliding and combined roll slide tests using acetylene vapours to deposit pyrolytic graphite. Friction coefficients as low as μ = 0.008 have been measured for steel at 540°C with far‐field acetylene concentrations as low as 5%. Effective vapour‐phase lubrication depends on solid lubricant deposition that exceeds the contact's capacity to remove solid lubricant through wear. While the rate of removal is increased by increasing the sliding velocity, in far‐field vapour‐phase lubrication the rate of lubricant deposition, and therefore the lubrication effectiveness, is augmented by increased areas available for far‐field deposition, such as those provided by performing wear tests with increased wear‐track diameters. These geometric concepts may be considered in rolling‐element bearing and gear set applications where vapour‐phase lubrication is to be employed.     

Lubrication of Microelectromechanical Systems (MEMS) Using Bound and Mobile Phases of Fomblin Zdol®

A lubrication scheme for MEMS electrostatic lateral output motors based on a mixture of bound and mobile lubricant was studied. Lubrication by bound monolayer alone provided some increase in operational life, but after a short time, the film wore away and the device failed in the unlubricated mode. A mobile phase was used to provide lubricant replenishment. Tribological studies were conducted on Si(100) wafers, as well as on MEMS electrostatic lateral output motors, dip-coated with a mixture of bound and mobile phases of Fomblin Zdol. Accelerated screening tests on Si(100) wafers were undertaken using a pin on disk tribometer. However, the optimum balance of bound and mobile phases was determined by studies on the device itself. The fractional surface coverage of lubricant and the ratio of bound to mobile phase was varied through selection of reaction temperature and rinse chemistry. The mobile phase on model surfaces and devices acted as a source of lubricant replenishment, and together with the bound phase provided dramatic improvement in performance. The wide variation seen in the performance of individual devices suggests that dip coating does not provide a uniform coating on the contacting surfaces of these devices.     

The Influence of Lubricant Upon EHD Film Behavior During Sudden Halting of Motion

Although steady state elastohydrodynamic (EHD) lubrication is quite well understood both from the theoretical and from the experimental point of view, studies of transient effects in EHD are currently far less developed. This paper describes an experimental investigation into EHD film behavior during sudden halting of motion. A technique has been devised which enables both central lubricant film thickness and film thickness profiles to be measured every millisecond during halting of a ball on flat, sliding contact. This has enabled detailed information of influence of lubricant on film collapse during halting to be obtained. It is shown that film collapse occurs in two stages. The first is a very rapid reduction in film thickness with only very small changes in film geometry and thus pressure distribution. This is followed, as soon as entrainment ceases, by the formation of a lubricant entrapment, and subsequent slow leakage of fluid from the central film region. This paper focussed on the formation of this entrapment and the influence of the rheological properties of the lubricant, i.e. viscosity and pressure-viscosity coefficient, on its development and behavior.     

The Size Effect of Boron Nitride Particles on the Tribological Performance of Biolubricants for Energy Conservation and Sustainability

The size of particulate additives in a bio-based lubricant influences their friction and wear performance during sliding contact. The present investigation evaluates the effect of boron nitride particle size on the tribological performance of canola oil-based lubricant mixtures. During sliding experiments, micron-, submicron-, and nanometer-sized boron nitride particle additives were considered. Friction and wear measurements were carried out on the prepared lubricant mixtures using a pin-on-disk tribometer at ambient conditions. A scanning electron microscope and optical profilometer were used for topographical studies to evaluate the influence of particle size on wear damage and surface roughness. The results revealed that the nanometer-sized particulate mixture outperformed micron- and submicron-sized particulate combinations in terms of friction and wear performance and provided a 90 % smoother surface finish. Furthermore, the tribological response of canola oil containing micron- or submicron-sized particles was found to be significantly enhanced by the addition of nano-sized particles, where the friction and wear were reduced by 40 and 70 %, respectively. It was inferred that the nano-sized particles were able to better coalesce in the asperity valleys due to their small size and spherical shape, which provided them with enhanced tribological properties in comparison with the micron- and submicron-sized particles that were larger and exhibited a plate-shaped in geometry. Newly developed non-dimensional surface roughness parameters were introduced to quantify the influence of particle size and the mechanisms involved in the tribological phenomena. The state of lubricants derived from bio-based feedstock were subsequently explored for their influence on energy conservation and sustainability, as well as their potential impact on the lubricant market place.     

Non-Newtonian Effects of Powder-Lubricant Slurries in Hydrostatic and Squeeze-Film Bearings

This paper presents the result of pressure measurements made in hydrostatic and squeeze-film bearings lubricated with a powder-lubricant slurry. The powder lubricant was powdered graphite and the carrier fluid was ethylene glycol. Pressure measurements compared favorably with analytical predictions based on a power law rheological model.

The behavior of ethylene glycol acting alone as a lubricant was also analyzed to provide a baseline for comparing the effects of the graphite powder additive on pressure and flow rate.

The hydrostatic bearing test showed that the addition of graphite powder into ethylene glycol can raise the load-carrying capacity of a lubrication system however pumping requirements are also raised. Squeeze-film bearing tests showed that the damping factor is increased with the addition of powdered graphite into the carrier fluid.     

Sine Sweep Load vs. Impact Excitations and Their Influence on the Damping Coefficients of a Bubbly Oil Squeeze Film Damper

Squeeze Film Dampers (SFD) suppress excessive vibrations and rotordynamic instabilities in turbomachinery. However, air ingestion into the oil film is a pervasive phenomenon that affects their performance, complicating their analysis and design, and demanding careful experimentation. The type of force excitation affects the damping coefficients since the ensuing dynamic journal motions may lead to a rapid expulsion or to coalescence and entrapment of the air within the lubricant matrix. Experimental force coefficients from a small rotor-SFD apparatus operating with controlled mixtures of air and oil, i.e. emulating degrees of air entrainment, are obtatined from the dynamic response to sine sweep forces and impact loads. The parameter identification procedure renders damping coefficients that are sensitive to the type of force excitation. For impact tests, damping coefficients steadily increase for lubricant mixtures up to 50 % in air volume content. For unidirectional sine sweep load excitations, the damping coefficients are nearly constant even for mixtures with larger air volume fractions. The larger and sustained amplitudes of periodic journal motion induced in the sweep sine tests expel rapidly the air from the mixture, thus leaving a lubricant film that generates invariant dumping coefficients. Conversely, in the impact tests, the journal motions are of small amplitude and short duration thus providing larger damping values; the mixture behaves as a nearly incompressible fluid of larger viscosity.     

Non-Newtonian Temperature and Pressure Effects of a Lubricant Slurry in a Rotating Hydrostatic Step Bearing

The purpose of this research was to investigate the pressure and temperature effects of graphite powder lubricant when added to a Newtonian carrier fluid and applied in a rotating hydrostatic step bearing. Temperature and pressure profiles were determined both analytically and experimentally. The rheological behavior of the non-Newtonian lubricant was modeled using a power law model previously shown to approximate experimental data for this fluid. Ethylene glycol was used as the Newtonian lubricant, providing a check on the test apparatus and a base line for comparison with the non-Newtonian graphite slurry.

Data revealed a temperature increase with bearing rotational speed for both fluids and compared favorably with the mathematical predictions. A significantly higher temperature rise was seen in the non-Newtonian lubricant due to the higher shear rates. The pressure profile was not directly dependent on bearing rotational speed in the mathematical model, but experimental data demonstrated a reduction in pressure at higher rotation speeds. This loss was greater for the non-Newtonian lubricant and attributed to temperature dependence of power law constants. It was concluded that the effects of operating speed and temperature on a non-Newtonian lubricant should be considered as well as their greater load-carrying capacity.     

Molecular Dynamics Simulation of Thermal-Induced Local Heating and Depletion of Ultrathin Perfluoropolyether Lubricant Under Moving Laser Heating

Molecular dynamics simulations of nanostructured perfluoropolyether lubricant are performed to investigate localized heating and lubricant depletion instability under moving laser heating. The evolution of the lubricant surface morphology indicates that a valley-like depletion track is formed along the path of the laser beam, resulting in aggravated depletion with heating time. The depletion profiles at various laser durations are characterized by the film thickness, in which a raised ridge is formed around the depletion zone signifying that the thermocapillary stress has a non-negligible effect on lubricant depletion. The recovery process is studied as well by further equilibrating the entire system with the laser beam turned off. During the cooling stage, the lubricant undergoes a slow recovery when compared with the laser-induced depletion. In addition to the attractive lubricant-to-disk interaction, the strong polar coupling between end beads prevents the recovery of lubricant beads back to the depleted surface. Moreover, the nonuniform surface tension and the nonequilibrium thermocapillary stress are expected to account for the mechanisms of lubricant depletion under moving laser heating.     

Non‐Newtonian and thermal effects in constant flow valve compensated symmetric hole‐entry hybrid journal bearing

Every high speed machine, demanding high level of perfection, can operate successfully through a precise design of bearings. Such a design can be formulated after carefully studying both static and dynamic characteristics of the journal bearing. The present paper described the study of static and dynamic performance of a hole‐entry hybrid journal bearing system compensated with constant flow valve restrictor by considering the combined influence of thermal effects and non‐Newtonian behaviour of the lubricant. The variation of the viscosity due to the non‐Newtonian behaviour of the lubricant and temperature rise has been considered in the study. The numerical solution of the generalized Reynolds equation governing the flow of the lubricant, having variable viscosity along with the energy and heat conduction equations, was obtained using finite element method. The non‐Newtonian lubricant has been assumed to follow the cubic shear stress law. The study included the performance of a double row symmetric hole‐entry hybrid journal bearing configuration containing 12 holes per row. The results presented in this paper indicate that change in viscosity of lubricant affects the performance of the hole‐entry hybrid journal bearing system quite significantly. Copyright © 2007 John Wiley & Sons, Ltd.     

A Study of Wear Chemistry and Contact Temperature Using a Microsample Four-Ball Wear Test

This study demonstrates that insight into the tribological reactions taking place in mixed and boundary lubrication can be provided by combining microsample four-ball wear tests with chemical analysis using gel permeation chromatography (GPC) as the principal analytical tool. At the end of the microsample four-ball wear test the lubricant turns into a grease-like mixture preventing the liquid lubricant from recirculating into the wear track and thus causing failure. Analyses of the various lubricant samples after their failure in the microsample test all show a relatively small amount of insoluble deposits and a large quantity of unreacted fresh lubricant. Virtually no intermediate reaction products were found. Combining this information with lubricant stability and the fact that a large quantity of lubricant flowed through the sliding junction while only a small portion was oxidized suggests that two very different thermal environments exist in the concentrated contact. The insoluble deposits are typical of thermal oxidative reactions that require temperatures of400°C or above. The unreacted lubricant found at failure indicates that this portion of the lubricant sample was maintained at temperatures of 150°C or below. The formation of grease-like mixture with as little as four percent reacted material indicates the remaining liquid lubricant and its insoluble reaction deposits were well mixed throughout the test. These findings suggest that the hot zones causing severe lubricant degradation are in the immediate vicinity of the asperity-asperity contacts while the low temperature zone - the valleys between asperities which are in the majority - are much cooler.     

Investigation of subsequent deposit growth on pre‐existing lubricant deposits: a substitutional growth model

The Penn State Micro‐Oxidation (PSMO) test was used in an inverse manner to pre‐cover metallic pan surfaces with polymeric, transitioning and carbonaceous films. These pre‐coated pans were then used as the initial test specimen/surface upon which fresh lubricant samples were aged. The effects of pre‐deposits of varied ages were gauged against the baseline of a virgin metallic surface to decouple the lubricant‐deposit system towards resolving their effect upon further deposit growth. From such data, a uniform deposition model describing deposit formation and aging was developed. Chemical characterisation of PSMO deposits by Fourier transform infrared‐attenuated total reflectance and energy‐dispersive X‐ray spectroscope analyses provide additional supporting evidence of changes in chemical bonding (alkyl C–H and carbonyl C=O bond stretching vibrations) and composition (C‐ and O‐atom content) as the deposits undergo deoxygenation and dehydrogenation reactions. Across different aged oils and films, the substitution tests show a declining activity towards mass deposition with film age, interpreted as decline in reactivity. Copyright © 2016 John Wiley & Sons, Ltd.     

A method for testing lubricants under conditions of scuffing. Part I. Presentation of the method

A method for the tribological assessment of lubricants under conditions of scuffing is presented. The method uses a four‐ball tester, and allows one to assess the effect of lubricant on scuffing intensity through an analysis of changes in the friction torque and wear of the stationary balls, at continuously increasing load. The behaviour of a lubricant under scuffing conditions can be characterised using the so‐called limiting pressure of seizure p_oz, which depends on the load at which the balls seize and the average value of the wear area calculated from the wear‐scar diameters measured on the stationary balls. A comparison is made ‐ from the point of view of the resolution, time consumption, and cost ‐ of the new method with the existing, standard tests, using a four‐ball tester and a gear test rig (FZG). It is concluded that the proposed method, unlike standard FZG and certain four‐ball tests, enables one to differentiate between gear oils, in agreement with their API GL performance level. The very short run‐time of the new method enables one to perform more tests and obtain a low standard deviation. The new method is much cheaper than the standard four‐ball and FZG methods.     

Determination of the friction factor of Ti-6Al-4V titanium alloy in hot forging by means of ring-compression test using FEM

The friction factor of Ti-6Al-4V titanium alloy under hot forging situation was determined by the combined approach of ring-compression tests and finite element (FE) simulations. It is noticed in particular that the heat-transfer (HT) coefficient has significant effects on the metal flow and calibration curves, thereby affects the measurement of interfacial friction factor. Moreover, the HT coefficients are different for glass lubricant and dry friction conditions. Therefore, different HT coefficients should be employed to generate the calibration curves when both of the lubricant conditions were applied for determining the interfacial friction coefficients in hot ring-compression of Ti-6Al-4V titanium alloy.     

Scuffing of Area Contacts Under Starved Lubrication Conditions

The scuffing behavior of 390-T6 and DHT3 aluminum alloys, Si-Pb brass and gray cast iron, sliding against 1018 carburized steel, is experimentally studied under starved lubrication conditions. The major emphasis of the study is on the 390-T6 aluminum alloy. All tests are conducted in a high pressure tribometer (HPT) under RI34a (tetrafluoroethane) environment with polyalkylene glycol (PAG) and polyolester (POE) lubricants to simulate failures of critical tribocontacts in refrigerant compressors. An area contact, pin-on-disc geometry, is used to examine the effects of degree of lubricant starvation, sliding velocity, materials, and lubricant/refrigerant mixtures on scuffing. The scuffing transitions characteristics of 390-T6 aluminum as a function of lubricant supply rate are also examined. The processes leading to scuffing and failure mechanisms are studied by examination of scuffed surfaces and subsurfaces. Based on the experimental observations, it is hypothesized that bulk material failure during scuffing is due to plastic shearing or smearing.     

Determination of the friction factor of Ti-6Al-4V titanium alloy in hot forging by means of ring-compression test using FEM

The friction factor of Ti-6Al-4V titanium alloy under hot forging situation was determined by the combined approach of ring-compression tests and finite element (FE) simulations. It is noticed in particular that the heat-transfer (HT) coefficient has significant effects on the metal flow and calibration curves, thereby affects the measurement of interfacial friction factor. Moreover, the HT coefficients are different for glass lubricant and dry friction conditions. Therefore, different HT coefficients should be employed to generate the calibration curves when both of the lubricant conditions were applied for determining the interfacial friction coefficients in hot ring-compression of Ti-6Al-4V titanium alloy.