Lubrication Research Articles

A methodology for predicting and optimizing the lubrication of an off-road machine transmission

This article presents an analysis method based on lumped-parameters (LP) and computational fluid dynamics (CFD) to evaluate the behavior and calibrate the lubrication (LUB) circuit in an agricultural dual-clutch transmission. Gearbox LUB is crucial because it affects reliability, life, and transmission efficiency. The transmission’s hydraulic circuit consists of the higher pressure power transmission control and actuation circuit and the low pressure LUB circuit. The former was analyzed with LP approach, instead the latter was modeled with CFD approach due to its geometric complexity. The results will be contextualized by commenting on the advantages and limitations of LP and CFD analysis, finally outlining a mixed analysis methodology. The simulations are solved for different engine speed. The results are used to calibrate a comprehensive LP model (model) and to optimize the LUB circuit by adjusting some critical design parameters. As an experimental approach, simplified tests are usually performed ignoring the action of the centrifugal force, which has a great impact on the flow regime inside the LUB ducts. This work aims to develop a methodology for analyzing LUB in all parts of the gearbox, which can help designers test various combinations within industrial timeframes and obtain reliable and the most efficient configuration.

Effect of Indentation at Rolling Body Surfaces on the Film Formation in Elastohydrodynamic Lubrication Contact under Dynamic Loads

Vibration is one of the phenomena that can occur during the operation of roller bearing which can lead to dynamic loads being subjected to the contact and thus, affect the film formation in elastohydrodynamic (EHD) contacts. On the other hand, surface textures such as artificial indentations and grooves greatly affect the film formation as well as the corresponding pressure distribution in the contact area, since the depths of the textures are usually much greater than the film thickness in the contact. This paper investigates the influence of an artificial indentation located on the rolling element, which passes through a point contact under dynamic loads by means of numerical simulations. Solutions to the Reynolds equation are performed and calculations of the elastic deformation equation, force balance equation and lubricant properties equations to show the effects of both indentation passing through the contact and a sinusoidal dynamic load on the film thickness as well as pressure profile of EHD point contact. Moreover, the effect of frequency and amplitude excitation of the dynamic load on the film formation was investigated. The results revealed that the artificial indentation under sinusoidal dynamic load of EHD point contact induced a significant effect on the thickness of the film formation and pressure distribution.

Wear protection assessment of ultralow viscosity lubricants in high-power-density engines: A novel wear prediction algorithm

Durability and reliability have been studied for decades through intensive trial-error experimentation. However, there are numerous fields of application where the costs associated with this approach are not acceptable. In lubricated machines with severe dynamic loads, such as high-power-density engines, simulation tools offer clear advantages over intensive testing. Prototypes and multiple scenarios can be cost-effectively simulated to assess different lubricants and engine configurations. The work presented here details the study of wear based on a validated elastohydrodynamic (EHD) simulation model of the connecting rod journal bearing. This model accounts for elastic deformation through a connecting rod finite element model (FEM). In addition, multiple lubricant rheological and tribological dependences, determined by specific experimental tests, are applied in the model through their interaction with the simulation software. Correspondingly, a novel wear algorithm is proposed to predict wear depth over time evolution along a proposed wear cycle based on the typical working ranges of high-performance engines. A final assessment is presented to compare 4 different ultralow-viscosity lubricants in their protective performance under severe conditions. The results show the evolution of the wear load and wear depth over the wear cycle. This evaluation is key to describing a lubricant selection procedure for high-power-density engines.

Implementation of a New Smart-Lubricant Model Using Generalized Newtonian Approach in Soft Elastohydrodynamic Lubrication

Abstract This is an inceptive attempt to replace the classical Bingham fluid model with a continuous double Newtonian power-law-based constitutive equation for smart lubricants like magneto-rheological, electro-rheological, and ferro-fluids. The implementation of Bingham model in hydrodynamic (HD) and elastohydrodynamic (EHD) lubrication analyses is highly challenging and inconvenient due to its inherent discontinuity. Therefore, the present work demonstrates the use of an already existing rheological model with an appropriate set of parameters to describe the flow behavior of smart lubricants in a soft-EHD lubrication algorithm based on the generalized Newtonian approach. The formation of both floating and adherent cores validates the proposed model. An extensive parametric study is also performed to explore the effects of operating speed, load, and slide-to-roll ratio on the soft-EHL characteristics. The results are very promising, showing that it's possible to customize smart lubricants to match specific operating conditions. This is achieved by adjusting the yield stress value accordingly, allowing for the desired variation in lubrication characteristics.

Dynamic modeling and force-vibration mapping mechanism construction of rolling bearings considering cage flexibility and local faults

Rolling bearings are one of important components of rotating machinery transmissions and thus its health condition is important for the safety of machinery. Thus, rolling bearing vibration analysis is critical for bearing health condition monitoring. Current bearing vibration analysis ignores multiple compound influencing factors for dynamic modeling. Hence, a more accurate bearing dynamic model, considering multiple factors including cage flexibility, time-varying displacement excitations caused by local defects and elastohydrodynamic (EHD) lubrication, is proposed to accurately reflect force states among bearing elements and vibration responses of bearings. Furthermore, the force states and vibration responses of bearings with localized defects in the proposed model are analyzed in details: (1) the main excitation source of the dynamic model under faulty conditions is identified as the contact force between the rolling element and the raceway by analyzing the vibration signals and main physical variables (the contact forces and the friction forces) in numerical values and change trends; (2) the mapping mechanism between the main excitation source and the vibration response is constructed. The force states among bearing elements are detailedly analyzed in the proposed model and the relationship between the forces and bearing vibration responses is revealed, facilitating the bearing health condition monitoring and vibration control of machinery.

Thermoelastohydrodynamic Mixed Lubrication of Combined Rod Seals Operating at High Pressures and Speeds: Mathematical Modeling and Numerical Analysis

Abstract Thermoelastohydrodynamic (TEHD) mixed lubrication characteristics of a step-combined rod seal under high-pressure and high-speed conditions are analyzed in this article. A novel TEHD mixed lubrication model for combined rod seals is innovatively established from the perspective of “seal-film-rod” system for the first time. Parameterized studies are conducted to evaluate the thermal effect on seal behavior with the comparison of isothermal elastohydrodynamic (EHD) lubrication analysis. Numerical results show that the interface friction heat is quite remarkable and mainly concentrated on the sealing lip, especially in high pressure and speed cases. With the increasing sealed pressure or rod speed, the temperature rise becomes more obvious and has a more significant impact on the sealing performance. The excessively rising temperature will even exceed the melting point of the sealing material, causing thermal damage.

Elastohydrodynamic Rotational Lubrication Analysis on the Multi-Body Dynamic Properties of Journal-Bearing Systems

This research aims to investigate the properties of elastohydrodynamic rotational lubrication analysis on journal-bearing systems. To simulate elastohydrodynamic lubrication on journal-bearing systems, the Elasto-Hydro-Dynamic (EHD) solver is combined with the Multi-Body Dynamic (MBD) solver to create MBD virtual environment with lubricant. The hydrodynamic lubricant is governed by using the Reynolds equation, whereas the elastic contact is governed using Greenwood and Tripp theories. The simulation is performed by changing the operating conditions such as the speed, load, and clearance between two surfaces. One can find these parameters’ effects such as film thickness, hydrodynamic pressure, and friction. The result shows that the friction induced by shaft speed is similar to the Stribeck curve on mixed lubrication regime. Consequently, the clearance, speed, and load will not only affect the friction but also affect the hydrodynamic pressure and film thickness.

A comparison of wear between unidirectional and reciprocating sliding motions under different applied loads and lubricants

Exploring the wear mechanisms of tribo-pairs is essential to reducing friction and wear. Although it has been known that the wear mechanisms between reciprocating sliding motion (RSM) and unidirectional sliding motion (USM) are different, the differences are seldom reported, which causes trouble for many researchers in selecting proper tribo-pairs or wear test methods. Herein, we systematically explore the effect of RSM and USM on the wear mechanism for several typical friction and lubricant materials, such as alloys, liquids, and solid lubricants. It is found that tribo-pairs suffer unstable sliding under RSM, and sliding speed as well as sliding direction change frequently, which makes it difficult to form a transfer layer or tribo-chemical reaction film, leading to high friction and abrasive wear for hard alloys and DLC coating. Additionally, the quasi-static stage under RSM, when lubricated with polyalphaolefin (PAO) 40, does not facilitate elastohydrodynamic (EHD) lubrication, which typically prevents wear. However, stable sliding under USM could lead to adhesive wear, increasing the friction coefficient for soft TC4 alloys. Although wear under RSM is generally higher than that under USM, increasing the applied load could diminish the wear difference for hard metals.

Physics-Informed Deep Learning-Based Proof-of-Concept Study of a Novel Elastohydrodynamic Seal for Supercritical CO2 Turbomachinery

Abstract Supercritical carbon dioxide (sCO2) power cycles show promising potential of higher plant efficiencies and power densities for a wide range of power generation applications such as fossil fuel power plants, nuclear power production, solar power, and geothermal power generation. sCO2 leakage through the turbomachinery has been one of the main concerns in such applications. To offer a potential solution, we propose an elastohydrodynamic (EHD) seal that can work at elevated pressures and temperatures with low leakage and minimal wear. The EHD seal has a very simple, sleeve-like structure, wrapping on the rotor with minimal initial clearance at micron levels. In this work, a proof-of-concept study for the proposed EHD seal was presented by using the simplified Reynolds equation and Lame’s formula for the fluid flow in the clearance and for seal deformation, respectively. The set of nonlinear equations was solved by using both the conventional Prediction–Correction (PC) method and modern Physics-Informed Neural Network (PINN). It was shown that the physics-informed deep learning method provided good computational efficiency in resolving the steep pressure gradient in the clearance with good accuracy. The results showed that the leakage rates increased quadratically with working pressures and reached a steady-state at high-pressure values of 15∼20 MPa, where Q = 300 g/s at 20 MPa for an initial seal clearance of 255 μm. This indicates that the EHD seal could be tailored to become a potential solution to minimize the sCO2 discharge in power plants.

Influence of lubricant material in the point contact zone of rolling friction on fatigue life for friction bearing units

Although lubrication is necessary for the satisfactory operation of rolling bearings, the effect of lubricant on the fatigue life of the bearing has not been sufficiently studied. In recent times, the theory of elastohydrodynamic (EHD) lubrication [1] has been used to explain the different effects of lubricants. According to this theory, the thickness of the lubricating layer separating the moving elements of the bearing is determined by the viscosity-pressure dependence of the lubricant. The contact of surface micron irregularities does not occur if it is possible to maintain a sufficient thickness of the lubricating layer - in this case, the long-term durability of the bearing is ensured. If the film thickness is reduced to a level where surface irregularities are encountered, the fatigue life rapidly decreases with increasing contact frequency. In any case, a comprehensive calculation methodology is needed that would allow to take into account the influence of lubricant on the fatigue life of bearing units.

Lubrication Condition Monitoring in EHD Line Contacts of Thrust Needle Roller Bearing Using the Electrical Impedance Method

In this study, we developed the electrical impedance method which simultaneously measures the thickness and breakdown ratio of oil films in elastohydrodynamic (EHD) line contacts within thrust needle roller bearings. Initially, we theoretically demonstrated that the oil film thickness and breakdown ratio can be simultaneously measured using the complex impedance that is produced when an AC voltage is applied to EHD line contacts. To verify the measurement accuracy of the electrical method, we monitored the oil film thickness of a thrust needle roller bearing and compared it with the theoretical value. The results revealed that the oil film thickness was thinner than the theoretical value immediately after starting the test, with the breakdown ratio being greater than 0 (indicating mixed lubrication); however, the breakdown ratio decreased over time, and the oil film thickness nearly matched the theoretical value one hour after starting the test, when it is believed that running-in wear is complete (i.e., breakdown ratio ≈ 0). Furthermore, following the test, after examining the race surface, we confirmed that running-in wear had indeed occurred. These results suggest that the developed method can monitor the lubrication conditions in EHD line contacts, such as those in thrust needle roller bearings, in detail.

Friction assessment of ultralow viscosity lubricant formulations based on a validated elastohydrodynamic simulation

High-performance engines are highly optimized machines; therefore, any identification of potential improvement requires a detailed study to assess different performance scenarios, working fluids and engine configurations. Simulation tools are among the best ways to study all possible options. In this framework, a validated elastohydrodynamic (EHD) simulation model of the connecting rod journal bearing is used to assess the performance of different lubricant formulations in mixed lubrication scenarios. To simulate a realistic scenario, a finite element analysis (FEA) is performed, where the assembly of connecting rod and shaft supports are modelled with finite element models (FEM) to account for elastic deformations on journal bearings. Furthermore, a set of oil properties is measured and collected to import into GT-Suite commercial software. This set enables us to consider the dependence of variables such as viscosity, density, specific heat transfer, and coefficient of friction on the shear rate, pressure, temperature, and air solution. Moreover, a friction assessment methodology is presented to choose the best lubricating oil for severe working conditions. This test method accounts for several factors: tribology, rheology, wear, and lubrication regime. The results are presented for three ultralow viscosity lubricants and the reference oil from validation. This assessment gives a complete comparison of lubricants performance, where lubrication regime and tribological losses are key factors to select the best candidate.

Simulation methodology for the identification of critical operating conditions of planetary journal bearings in wind turbines

The usage of journal bearings as planetary bearings in wind turbines instead of roller bearings has become more common in recent years. Their usage is advantageous, due to smaller installation space needed compared to roller bearings allowing for higher power densities of wind turbine drive trains. However, this technology presents a challenge since there is currently no standardized approach for the design of planetary journal bearings regarding wear. Due to varying wind speeds and dynamic operating events a large variation of loads has to be considered in the design process of a planetary journal bearing for wind turbines. Some of these loads are considered potentially critical to the journal bearing in terms of wear. Identifying these critical load areas early in the design phase supports a reliable bearing design and wind turbine operation.This paper introduces a method to identify critical operating conditions for planetary journal bearings using a simulation tool chain, which couples a multi body simulation (MBS) model of a wind turbine with an elasto-hydrodynamic (EHD) model of the planetary journal bearing. Based on the EHD results critical operating conditions are determined for the planetary bearing. Furthermore, methods are implemented to reduce the number of required EHD simulations for analysing the bearing design. The combination of the identification of critical operating conditions, while reducing the computational effort leads to a simulation methodology, which enables a faster bearing design assessment considering the wide variation of wind turbine operating conditions. The applicability of this method is demonstrated by a simplified use case.Firstly, this paper introduces the MBS model and the parameter space that describes possible combinations of bearing loads such as forces, moments and rotational speed. Due to the number of combinations and the EHD computing effort, the identified parameter space is secondly sampled statistically to reduce the simulation effort. A risk map is derived from the EHD results, to easily indicate potentially critical operating conditions for the planetary journal bearing.

Influence of Real Lubricant Density–Pressure Behavior on the Dynamic Response of Elastohydrodynamic Lubricated Conjunctions

Abstract The current work investigates the influence of real lubricant density–pressure behavior on the dynamic response of elastohydrodynamic lubricated conjunctions. Such a response is often based on a nonrealistic universal equation of state, despite longstanding evidence of its lack of support by measurements. A finite element framework is employed to investigate the damping and stiffness characteristics of line contact elastohydrodynamic (EHD) lubricating films, subject to a harmonic loading. Both the equivalent stiffness and damping coefficients of lubricating films are found to increase with the base applied external load and its amplitude of oscillation. They decrease however with increasing mean entrainment speed and load oscillation frequency. That is, they both increase as lubricant films get thinner. By comparison with the real density–pressure response of a highly compressible silicon oil, the universal equation of state is shown to underestimate the lubricant film’s stiffness and damping characteristics. The relative deviations in equivalent damping and stiffness coefficients can reach up to about 12% and 25%, respectively. Therefore, realistic lubricant characteristics should always be considered. In particular, the use of the universal equation of state should not be taken for granted, as is customary in the elastohydrodynamic lubrication (EHL) literature. Lubricant density–pressure response is not of a secondary nature when it comes to predicting the dynamic performance characteristics of EHL conjunctions.

Update on ISO standards in condition monitoring and vibration

This paper presents an update on further progress in International Standards on Condition Monitoring (CM), and International Standards related to CM techniques including: vibration monitoring (VM), infra-red thermography (IRT), acoustic emission (AE), ultrasonics (UT), tribology and tribology and lubrication (LUB). Vibration and Condition Monitoring Standards are within the scope of ISO Technical Committee 108 ‐ Mechanical vibration, shock and condition monitoring (ISO/TC 108). ISO/TC 108 has a portfolio of 196 ISO Standards including 28 relating to condition monitoring and 56 relating to vibration of machines, vehicles and structures. (at March 2023) Since 2020, 29 documents have been issued revised or renumbered in ISO/TC 108's portfolio. This paper gives the latest status of a selection of these documents.

Analysis of an Elasto-Hydrodynamic Seal by Using the Reynolds Equation

This paper reports numerical studies of an Elasto-Hydrodynamic (EHD) seal, which is being developed for supercritical CO2 (sCO2) turbomachinery applications. Current sCO2 turbomachinery suffers from high leakage rates, which is creating a major roadblock to the full realization of sCO2 power technology. The high leakage rates not only penalize the efficiencies but also create environmental concerns due to greenhouse effects caused by the increased CO2 discharge to the atmosphere. The proposed EHD seal needs to work at elevated pressures (10–35 MPa) and temperatures (350–700 °C) with low leakage and minimal wear. The unique mechanism of the EHD seal provides a self-regulated constriction effect to restrict the flow without substantial material contact, thereby minimizing leakage and wear. This work utilizes a physics-based modeling approach. The flow through the gradually narrowing seal clearance is modeled by the well-known Reynolds equation in EHD lubrication theory, while the deformation of the seal is modeled by using the governing equations of three-dimensional solid mechanics. As for the solution methodology, COMSOL’s Thin-Film Flow and Solid Mechanics modules were employed with their powerful capabilities. The numerical results were presented and discussed. It was observed that the Reynolds equation fully coupled with the surface deformation was able to successfully capture the constriction effect. The maximum and minimum leakages were calculated to be 2.25 g/s and 0.1 g/s at P = 5.5 MPa and P = 11 MPa for the design seal, respectively. It was interesting to observe that the seal leakage followed a quadratic trend with increasing pressure differential, which can become advantageous for high-pressure applications such as sCO2 power generation technology.

Dynamic modelling of gearbox with multiple localized defects and its coupled vibration analysis

Gearboxes are one of critical components of railroad vehicles, aero-engines and other rotating machinery, and their health condition is crucial for the safe operation of these rotating machines. Fault signature extraction is a vital step for gearbox health condition monitoring. To this end, knowing the characteristics of fault signature is a prerequisite. Dynamic model is an effective tool for analysing the fault features of gearbox. However, in reality, the vibrations measured from the gearbox housing are usually coupled with the vibrations of the rolling bearings, the gear mesh and the gearbox housing, which are influenced by the friction, lubrication, surface roughness, etc., of each contact pair. Consequently, any dynamic model of individual components cannot accurately reflect the actual vibration behavior of the gearbox and also the resulted vibration response cannot match the vibrations measured from the gearbox housing. To resolve this problem, this paper develops a comprehensive dynamic model to analyze the vibration response of a gearbox with multiple localized defects by coupling the effects of spur gear mesh, deep groove ball bearings, time-varying displacement excitation of the bearing caused by localized defects, time-varying mesh stiffness (TVMS) of the gear, and mixed elastohydrodynamic (EHD) lubrication. The coupled vibrations of the gearbox housing can then be obtained to analyze the vibration behavior of a gearbox with multiple localized defects. In addition, the influences of the relative motion of the bearing rollers and flexible cage on the gearbox vibration are analyzed. At last, the proposed dynamic model is validated by experiments.

Prediction of Wear Failure in Journalpin Bearings

Wear damage is a common failure in engine bearing shells. Wear failure induces changes in the bearing geometry and affects the oil film pressure and the durability of bearing shells. In this paper, wear failure in journalpin (main) bearings of a reciprocating engine is studied. Using a dynamic model, forces and torques in the journalpin bearings have been calculated and used in the analysis. To calculate the lubricating characteristics of the bearing, such as minimum oil film thickness and maximum oil film pressure, the Elasto-HydroDynamic (EHD) model that incorporate mass conservation algorithms is utilized. Wear failure in journalpin bearings of a reciprocating engine is assessed and the results are presented. Archard’s model is used as a wear model of journalpin-bearing material. The asperity interaction of two rough surfaces is considered by the boundary lubrication model. The results show that the wear rate at the initial stage of engine running is high. The results indicate that the main reason for wear in the journalpin bearings is the applied torque on the bearing, leading to edge wear. In a V-12 or 6Inline engine, journalpin bearing number 4 has the highest amount of torque applied and therefore has the highest amount of wear.

Investigation of manufacturing-related deviations of the bearing clearance on the performance of a conical plain bearing for the application as main bearing in a wind turbine

Abstract The use of a segmented conical plain bearing for the application as main bearing in a wind turbine offers multiple advantages. The main bearing can be designed very compact. In addition, in case of a main bearing damage, individual bearing segments can be exchanged on the tower without the use of an external crane, in contrast to the repair procedure of rolling bearings. It is well known from conventional applications of plain bearings that the bearing clearance has a major influence on the load carrying capacity. Therefor the realization of narrow tolerances in the manufacturing of the bearing components is necessary. In this paper, the influence of the manufacturing tolerances on the performance of conical plain bearings is investigated. The influence of the tolerances on the load carrying capacity can be determined by conducting elastohydrodynamic (EHD) simulations. Finally, an evaluation of the robustness of the conical bearing concept against manufacturing-related deviations is carried out.

Development of a Machine Learning Model for Elastohydrodynamic Pressure Prediction in Journal Bearings

Abstract This paper presents a machine learning neural network capable of approximating pressure as the distributive result of elastohydrodynamic (EHD) effects for a journal bearing at steady state. Design of efficient, reliable fluid power pumps and motors requires accurate models of lubricating interfaces; however, many state-of-the-art simulation models are structured around numerical solutions to the Reynolds equation which involve nested iterative loops, leading to long simulation durations and limiting the ability to use such models in optimization studies. This study presents a machine learning model capable of approximating the pressure solution of the Reynolds equation for a journal bearing with given distributive geometric boundary conditions and considering cavitation and elastic deformation at steady-state operating conditions. A 1024-sample training set was generated using an in-house multiphysics simulator. A hyperparameter optimization study was conducted, leading to the six-layer U-Net convolutional neural network architecture proposed. After training, the neural network accurately predicted pressure distributions for test samples with different geometric inputs from the training data, and accurately estimated resultant journal bearing loads, showing the feasibility of post-processing the machine learning output for integration into other fluid power models. Additionally, the neural network showed promise in analyzing geometric inputs outside the space of the training data, approximating the pressure in a grooved journal bearing with reasonable accuracy. These results demonstrate the potential of a machine learning model to be integrated into fluid power pump and motor simulations for faster performance evaluation and optimization.