Mixed Lubrication Conditions Research Articles

Contact modeling and stiffness of a rough surface under mixed lubrication condition

Contact modeling and stiffness of a rough surface under mixed lubrication condition

A Wear Calculation Method for Helical Gear Based on Irreversible Thermodynamics

Abstract The accurate prediction of wear depth is of great significance to the failure evaluation of gear transmission. At present, the traditional wear calculation method is still unable to accurately predict the wear depth of helical gears under variable working conditions. The present work proposes a novel wear depth prediction method of helical gear, which is based on the theory of thermodynamics and the degradation entropy generation (DEG) theorem. The method can be used in not only dry contact friction but also mixed elastohydrodynamic lubrication (EHL) condition. The degradation coefficient of helical gear material was determined by wear experiment. The advantages of using DEG theorem to calculate wear loss under variable operation conditions are demonstrated by wear experiment. Moreover, the influence of ignoring the update of tooth surface load on the calculation results in wear process is further studied. The results reveal that the wear depth prediction method of helical gear proposed in this work can obtain the consistent wear depth distribution with the traditional method. The results calculated by the traditional method are generally higher than the method in this paper, and ignoring the load update in the wear process will make the results larger. The prediction method of helical gear wear depth presented in this paper will be useful for accurately predicting the wear of helical gear.

Condition monitoring of pitting evolution using multiple sensing

Pitting on surfaces is a type of rolling contact fatigue (RCF) occurs in rolling-sliding contacts operating under mixed or boundary lubrication conditions. The early detection of pitting is of great importance due to its potential detrimental effects on the performance and reliability of machinery components, such as rolling element bearings and gears. This research aims to investigate the responses of multiple sensors to the progression of pitting and achieve early detection of pitting initiation. Experiments were conducted on a TE74 twin-disc tribometer to investigate the behaviour of bearing steel discs. Mild wear and pitting fatigue were obtained with specimens of different roughness combinations. During testing, vibration, acoustic emission (AE) and electrostatic (ES) data were recorded, and post-test signal analysis was conducted in both the time domain and frequency domain. After testing, the worn surfaces were examined to determine the mechanisms responsible for specific features seen in the sensor data. The presence of pitting in the near-surface region was observed, and its development was effectively monitored using the employed sensing techniques. The stages of running-in, pitting initiation, and pitting formation were identified through the analysis of time-domain parameters and frequency spectrums. Vibration signal analysis exhibited a more prominent indication of pitting formation, whereas AE and ES methods demonstrated an ability to detect the onset of pitting at an earlier stage.

Evaluating the Time-Varying Mesh Stiffness of Planetary Gear Set With Gear Crack Considering the Friction Force Under Mixed Elastohydrodynamic Lubrication Condition

Abstract The mesh stiffness calculation of the planetary gear system with gear crack is investigated, and gear crack is modeled into end-face and addendum extended crack. Furthermore, the mesh stiffnesses are calculated under the effect of friction force obtained under the mixed elastohydrodynamic lubrication (EHL) condition. The nonuniformly distributed gear cracks on external and internal gear pairs are comprehensively modeled and analyzed, which is rarely investigated in previous studies. Moreover, the influence of crack propagation conditions on the meshing stiffness of the planetary gear system is analyzed. The obtained results show that the mesh stiffnesses experience abrupt variation in the pitch area during meshing, the extent of change is determined by the amplitude of the friction coefficient; in addition, gear crack position and extent could cause mesh stiffness reduction at varying levels at different stages of gear meshing.

Friction of Coated and Uncoated Rough Surfaces in Additive-Free and Fully Formulated Engine Oils Under Boundary and Mixed Lubrication Conditions

Abstract Downsizing in spark-ignited engines becomes one of the most effective concepts to achieve fuel economy and emissions requirements and drives operating conditions of major moving engine components into more mixed and boundary lubrication regimes. Piston ring design faces considerable challenge to reduce friction losses, oil consumption, blow-by, and wear. Surface coatings have received great attention in applications of automotive engine components due to their unique friction properties. In this study, an elastic–plastic contact model of rough surfaces with hard coatings is used to provide proper design of the ring coating thickness concerning the plasticity-yielding resistance of stiff coating. A diamond-like carbon (DLC) coating with surface hardness higher than current production ring coatings has been evaluated in additive-free base oil and fully formulated engine oil under boundary and mixed lubrication conditions. The friction behavior of the coated material relative to uncoated materials is compared in boundary and mixed lubrication regimes. A traction-merging rolling speed in the mixed lubrication regime is observed, at which friction coefficient is independent of applied loads. By using a mixed lubrication model previously published, calculations indicate that the lower the traction-merging rolling speed, the earlier the contact approaches elastohydrodynamic lubrication (EHL), therefore reducing the risk of asperity-interacting fatigue in boundary and mixed lubrication regimes. Effects of tribofilm on friction behavior for DLC coating and uncoated steel in fully formulated oil are investigated by using measured Stribeck curves in conjunction with the model. The results from this study analytically support one of the possible mechanisms of tribofilm-dependent friction enhancement in mixed lubrication conditions proposed in the earlier publication.

Calculation and influence analysis of tooth surface friction coefficient of planetary gear system in mixed lubrication condition

Based on the gearing theory, the calculation formulas of comprehensive curvature radius, relative sliding speed and tooth surface contact stress of internal and external meshing gear pair of the planetary gear system are deduced in this paper. Combined with load sharing theory and thermal elastohydrodynamic lubrication analysis method, the oil film thickness, film bearing ratio and friction coefficient are solved and the effects of roughness and input power on the friction coefficient are studied. The results show that with the increase of tooth surface roughness, the film bearing ratio decreases and friction coefficient increases; As the input power increases, the film temperature increases, the oil viscosity and film thickness decreases, which causes the tooth surface friction coefficient increases.

A performance degradation analysis method for a reciprocating rod seal in the wear process under mixed lubrication conditions

The sealing performance of a reciprocating seal degrades during service. The analyses developed based on the initial state of a seal cannot express the performance degradation due to seal wear in detail. In this research, the Archard model in dry friction is extended to mixed lubrication by introducing the lubrication coefficient verified by an experiment. The nonlinear adaptive meshing technique is used in finite element analysis (FEA) to simulate the material removal to obtain the static contact pressure during the wear process. Hence, a sealing performance degradation analysis method is proposed based on the modified Archard model for reciprocating seals under mixed lubricating conditions. This method can be used to analyse the sealing, lubrication, and wear characteristics of seals at any service time. The simulation results indicate that the static pressure becomes more uniform with the occurrence of seal wear, and the lubricating property of the oil film reduces the seal wear. The variation trends of the sealing performance parameters such as contact pressure, friction, and net leakage at different service times under specific operating conditions are obtained.

DLC-Coated Thermoplastics: Tribological Analyses under Lubricated Rolling-Sliding Conditions

The goal of this work is to evaluate the potential of diamond-like carbon (DLC) coatings on thermoplastic polymers for friction and wear reduction in highly stressed rolling–sliding contacts. Therefore, hydrogen-containing DLC coatings were deposited on the polymer surface by a low-temperature high power pulsed magnetron sputtering (HPPMS) physical vapor deposition (PVD) process. The rolling-sliding contact between coated polyamide 66 (PA66) or coated polyether ether ketone (PEEK) against case-hardened steel 16MnCr5 is investigated in a twin-disk tribometer at normal loads up to FN = 1,000 N, sum velocities between 1 m/s ≤ vΣ ≤ 16 m/s and slip ratios up to s = 50%. Results show a friction reduction with the application of DLC on the considered polymers compared to uncoated polymers under specific lubrication conditions. High solid losses caused by the polymer’s internal damping properties dominate the temperature behavior of the polymer, even when coated with DLC. Regarding the wear behavior, DLC coatings show potential especially under severe mixed lubrication conditions with high-solid load portion and sliding. The knowledge gained about coated polymers can be used to improve the overall tribological performance in terms of friction and wear of thermoplastic machine elements like gears.

Tribochemistry of phenol and benzyl alcohol between YG8 self‐mated interfaces

In this work, the tribological performance of mixed lubricant consisting of phenol and benzyl alcohol between YG8 self‐mated interfaces was systematically investigated under 98 N and 1450 rpm. The results showed that with increasing concentration of phenol in the mixed lubricant, the friction coefficient initially decreases and then increases; however, anti‐wear performance initially was enhanced and then weaken. As the concentration of phenol was 1.0 wt.%, the tribo‐system exhibited the lowest friction coefficient and wear scar diameter. TEM, SEM and XPS techniques were employed to probe the possible tribological mechanism sliding in the mixed lubricant containing 1.0 wt.% of phenol. The phenol effectively slowed down the oxidation of the mixed lubricant, combination of friction‐induced tungsten oxides and carbon quantum dots, assuring the tribo‐system still in mixed lubrication condition, which made the tribo‐system maintain low friction coefficient and wear.

Investigation of the possible applications for microtextured rolling bearings

The application of microdimple-textured surfaces for rolling bearings is not very common but can be beneficial for various usage scenarios. In contrast to the applications for plain bearings or cylinder running surfaces, however, surface patterns for rolling bearings only offer advantages under certain conditions. For example, for use with start-stop cycles, in mixed lubrication conditions, or under sliding conditions in a bearing as well as on a roller—bearing flange contact, friction can be reduced with the targeted use of microtextures. The geometry of the microtextures must be chosen so that individual dimples fit in the contact area between the ball or roll and the bearing surface in order to act as a reservoir for the lubricant. First applications for microtextured angular contact ball bearings under oscillating movement conditions proved friction reducing effects under reciprocating motion. In this case the microtextures served as lubricant reservoirs. The idea is transferred to the sliding contact for tapered roller bearings; it is investigated whether the measured reductions in friction are due to the microtextures serving as lubricant reservoirs or whether there is even a positive hydrodynamic effect caused by the microtextures. By means of a calculation approach as well as some exemplary test rig measurements, the behavior for microtextured tapered roller bearings is investigated.

Effects of wall slip on the dynamic characteristics of water-lubricated bearing considering rough contact

PurposeThis paper aims to identify the role of the wall slip on the dynamic characteristics of the multi-groove water-lubricated bearing considering rough contact, including stiffness and damping coefficients of the water film and contact stiffness coefficient of the asperity contact.Design/methodology/approachThe modified perturbed average Reynolds equations with the wall slip are derived, and the calculated perturbed hydrodynamic pressures are integrated to obtain the stiffness and damping coefficients of the water film. The elastic-plastic contact model of Kogut and Etsion is used to determine the contact stiffness coefficient.FindingsNumerical results reveal that the wall slip has the more significant impact on the water film stiffness coefficients compared with the damping and contact stiffness coefficients. When the slip angle lies in a reasonable range, the lubrication performance can be effectively improved, especially in the mixed lubrication condition. In addition, it is worth emphasizing that the abrupt change of the water film stiffness coefficients occurs at the region II (pressure zone) in this study.Originality/valueThe influence mechanism of the wall slip on the dynamic characteristics of the water-lubricated bearing considering rough contact is first revealed.

Wind turbine main-bearing lubrication – Part 2: Simulation-based results for a double-row spherical roller main bearing in a 1.5 MW wind turbine

Abstract. This paper is the second in a two-part study on lubrication in wind turbine main bearings. Where Part 1 provided an introductory review of elastohydrodynamic lubrication theory, this paper will apply those ideas to investigate lubrication in the double-row spherical roller main bearing of a 1.5 MW wind turbine. Lubrication is investigated across a “contact conditions dataset” generated by inputting main-bearing applied loads, estimated from hub loads generated using aeroelastic simulation software, into a Hertzian contact model of the main bearing. From the Hertzian model is extracted values of roller load and contact patch dimensions, along with the time rate of change of contact patch dimensions. Also included in the dataset are additional environmental and operational variable values (e.g. wind speeds and shaft rotational speeds). A suitable formula for estimating film thickness within this particular bearing is then identified. Using lubricant properties of a commercially available wind turbine grease, specifically marketed for use in main bearings, an analysis of film thickness across the generated dataset is undertaken. The analysis includes consideration of effects relating to temperature, starvation, grease thickener interactions and possible non-steady effects. Results show that the studied main bearing is at risk of operating under mixed lubrication conditions for a non-negligible proportion of its operational life, indicating that further work is required to better understand lubrication in this context and implications for main-bearing damage and operational lifetimes. Key sensitivities and uncertainties within the analysis are discussed, along with recommendations for future work.

Online Friction–Thermal–Load Coupling Model of an Arbitrary Curve Contact Under Mixed Lubrication Considering Actual Operating Conditions

Abstract The existing fractal contact model fails to solve the problem of online real-time irregular curvilinear contact under mixed lubrication conditions. In this study, a novel arbitrary curve contact model is established, considering the actual operating condition that occurs under mixed lubrication. Furthermore, a new online friction–thermal–load coupling model for an arbitrary curve contact under mixed lubrication conditions is presented, considering the actual operating condition. To investigate the effects of the proposed online friction–thermal–load coupling model, a 30,205 tapered roller bearing with curved contact is examined by reading the real-time key point temperatures using thermocouple. Finally, the effectiveness of the presented model is verified through experiments and comparison.

Research on tribological performance of piston ring/liner conjunction considering non-Gaussian roughness and cavitation

The tribological performance of piston ring-cylinder liner conjunction (PRCLC) notably depends on the surface finish of cylinder liner. This study investigates the mixed lubrication condition near dead centres considering non-Gaussian roughness and cavitation. The non-Gaussian flow factors are incorporated in the average Reynold's equation to include the effect of non-Gaussian roughness. Whereas, the Weibull probability distribution function is employed to model the asymmetry in asperity heights. It is found that surfaces with more negative skewness exhibit lower engine total frictional force in the vicinity of the dead centres. No substantial effect of the skewness and surface roughness is observed in the mid-stroke of the piston. It is shown that PRCLC operates in mixed-EHL regime near the dead centres. The lambda ratio is found to increase with an increase in skewness of the rough surface.

On the effectiveness of Ni alloy-bronze composite lattice structures used in slide bearings operated under heavy loads

Lattice structures with graded composition and microstructure in function of position allow for a smooth transition and due to their combination of multiple desirable features show great prospects in various applications. This study describes the characteristics of Ni alloy-bronze composite lattice structures (CLS) which can be used for manufacturing slide bearings intended for operation under heavy loads. The CLS were fabricated via directed energy deposition method on a substrate of AISI 1045 structural steel. The basic microstructural characteristics as well as microhardness were analyzed to study the features of composite structures depending on the speed of the laser beam as well as the laser spot step. Operational tests of the studied CLS were carried out under different loads and lubrication conditions. It was confirmed that the CLS wear resistance is 5–12 times higher under dry friction conditions and ∼65 times higher under mixed lubrication conditions as compared to the initial Ni alloy and bronze. It was claimed that CLS tested and formed using modern directed energy deposition technology exhibit improved material and tribological characteristics. These results are important both from the standpoint of practical applications and future research.

Wear and the Transition from Static to Mixed Lubricated Friction of Sorption or Spreading Dominated Metal-Thermoplastic Contacts

Stiction, run-in wear and friction of lubricated polyoxymethylene homopolymer (POM)- and aliphatic polyamide (PA46)-steel tribosystems were investigated for mild-loaded mixed lubrication conditions with and without thermal conditioning of the polymers in the lubricant prior to testing. Macroscopic oscillatory tribometry and standard gliding experiments were carried out. The hypothesis that sorption of a lubricant into a thermoplastic polymer and partial solving of the surface by the lubricant can change wear rate and friction was tested. It was found that for POM-lubricant-pairings, the tribological behavior is dominated by the sorption of the lubricant into the polymer; it is not influenced by the spreading energy. For the PA46-lubricant pairings, no mass uptake by sorption was measured, and the tribological behavior is influenced by spreading and changes in hardness due to thermal aging. For mild loading in mixed lubricated conditions, friction and wear properties seem to be primarily determined by the hardness-dependence of abrasive contact and less by adhesion or hysteretic mechanisms.

Effects of surface roughness on the mixed thermal elstohydrodynamic lubrication characteristics of surface textured slipper bearing in axial piston pump

In this paper, a mixed thermal elstohydrodynamic lubrication model is presented for the rough textured slipper bearing in axial piston pump. The autocorrelation function is employed to reveal more detailed information about the tribological performance of the rough textured surfaces in terms of the Skewness and Kurtosis effects and the viscosity change. The effects of surface texturing is represented by the combined root mean-square roughness and asperity height on the optimum texture parameters under the mixed lubrication condition. It is shown that Kurtosis and Skewness have significant effects on the contact performance under different rotational speeds. The optimum dimple depth-over-diameter ratio under mixed lubrication condition are determined at maximized load-carrying capacity and minimized friction coefficient.

Influence of Cross-Grooved Texture Shape on Tribological Performance under Mixed Lubrication

Surface texture plays an important role in improving the tribological properties of materials. In this paper, the effect of different shapes (i.e., triangle, square, hexagon, round) on the tribological performance of cross-grooved texture was investigated. First, the mixed lubrication condition was used for the pin-on-disc rotating sliding tests. Then, the stress distribution of the four textures was analyzed to better explain the experimental results. Overall, the hexagon-textured specimens exhibited lower friction coefficients than the other shape-textured specimens under the examined conditions. Simulation results indicate that the contact stress can be reduced on the surface of hexagon-textured specimens, and this leads to a better oil film for lubrication. Furthermore, the hydrodynamic lubrication stood out with the increase of speeds to 250 rpm. However, as the test loads further increased, the film thickness decreased, resulting in the increase in the asperity contact areas, which dropped the above advantage of hexagon-textured specimens. This study would be beneficial for the texturing tribological and lubrication design.

Micro-pitting and wear damage characterization of through hardened 100Cr6 and surface induction hardened C56E2 bearing steels

Rolling-sliding contact fatigue experiments were performed on through hardened (TH) 100Cr6 and surface induction hardened (SIH) C56E2 bearing steels to study the effect of heat treatment procedure and hardness difference on their wear and/or surface-initiated damage. The heat-treated microstructures included tempered martensite without the presence of retained austenite. It was found that initial stress distribution below the surface of TH specimens remains close to zero with respect to the depth, while, employing SIH resulted in relatively high compressive residual stresses. The wear damage of the specimens was characterized for negative ΔH (i.e., specimen's hardness minus counterpart's hardness) subjected to rolling-sliding contact in a twin-disc configuration operated under a mixed lubrication condition. According to the wear mechanisms and damages assessed, three regions were distinguished: i) mild wear and onset of micropitting, ii) transition region from mild to severe micro-pitting and iii) severe micro-pitting wear. Decreasing the ΔH resulted in a gradual increase in the wear rate of TH specimens, while, the increase in the wear rate of SIH specimens was delayed; with a same absolute hardness, TH specimen with a ΔH of −84 HV already reached the third wear region, while a SIH specimen with a ΔH of −150 HV still operates in the second region. Inspection of the affected subsurface unveiled the response of worn microstructures to the etchant and how the micro-cracks could possibly form within the wear affected zone. The results obtained were mainly explained based on the state of the residual stresses, possible contributions of the microstructural features, and wear behavior (material removal) of the specimen.

Influence of temperature on wear performance of greases in rolling bearings

Purpose Rolling bearing operation under mixed and boundary lubrication conditions may lead to heavy adhesive or abrasive wear, which may lead to wear-induced rolling bearing failure. The purpose of this paper is to investigate the wear protection capabilities of different grease compositions at varying temperatures. It is considered that the temperature influences the lubrication conditions, the behaviour of grease components, namely, bleed oil and thickener, as well as the tribofilm formation due to tribo-chemical interactions between additives and surfaces. Design/methodology/approach In this study, four different greases were produced on the basis of a mineral base oil by varying the thickener and the addition of ZDDP. Various grease-lubricated rolling bearing experiments were conducted in a wide temperature range from 0°C to 120°C. Subsequently, the wear pattern, tribofilm formation and grease structures were analysed. Thereby, the influence of the different grease thickeners and the performance of ZDDP as a common antiwear and extreme pressure additive was evaluated. Findings The results show a strong temperature-dependency and allow a classification of temperature ranges concerning wear protection. At low temperatures, all greases provide a very good wear protection without the evidence of additive-based tribofilm formation. In the experiments at elevated temperatures, ZDDP tribofilms were formed. The formation depends on the thickener type: in comparison to lithium thickener, polyurea thickener favours more protective tribofilms at the same temperature. The experimental results show that medium temperatures in the range of 40°C–60°C are critical concerning wear due to the insufficient tribolayer formation and limited load carrying capacity of the grease. Originality/value Temperature is a key operating parameter for grease lubrication in roller bearings. The experimental work enables consideration of different impact pathways of temperature by combining roller bearing tests and microanalysis.