5.3 Elastohydrodynamic lubrication
8 min read•august 21, 2024
() is a critical concept in friction and wear engineering. It describes how heavily loaded, non-conforming contacts are lubricated, combining fluid mechanics and elastic deformation principles to explain thin-film lubrication under high pressure.
EHL is essential for reducing friction and extending component life in applications like gears and bearings. Understanding EHL helps engineers optimize lubricant selection, component design, and operating conditions to improve machine performance and durability.
Fundamentals of elastohydrodynamic lubrication
- Elastohydrodynamic lubrication plays a crucial role in reducing friction and wear in heavily loaded, non-conforming contacts
- Combines principles of fluid mechanics and elastic deformation to describe lubrication in high-pressure, thin-film conditions
Definition and basic principles
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- Lubrication regime characterized by elastic deformation of contacting surfaces and pressure-induced viscosity changes in the lubricant
- Occurs in non-conforming contacts with high loads and relative motion (, gears)
- Relies on the formation of a thin lubricant film (typically 0.1-1 μm thick) to separate surfaces and reduce friction
Historical development
- Concept introduced in the 1940s by Ertel and Grubin to explain unexpected film thickness in rolling contacts
- Dowson and Higginson developed the first comprehensive EHL theory in the 1960s
- Advancements in computational methods and experimental techniques led to refined models in subsequent decades
Importance in engineering
- Enables efficient operation of heavily loaded machine elements with minimal wear
- Critical for extending the lifespan of components in automotive, aerospace, and industrial applications
- Allows for higher -carrying capacity and reduced energy losses in mechanical systems
Fluid film formation
- Formation of a lubricant film in EHL contacts depends on the balance between and elastic deformation
- Understanding fluid film formation helps engineers optimize lubricant selection and component design for improved performance
Pressure-viscosity relationship
- Describes how lubricant viscosity increases exponentially with pressure in EHL contacts
- Barus equation: where η is viscosity, η_0 is ambient viscosity, α is , and p is pressure
- More accurate models (Roelands equation) account for limitations of Barus equation at very high pressures
Film thickness equations
- (hc) equation for line contacts:
- (hmin) equation for point contacts:
- R is equivalent radius, U is entrainment velocity, W is load, E' is reduced , k is ellipticity parameter
Minimum film thickness
- Occurs near the outlet of the contact zone where pressure gradient is highest
- Critical parameter for determining the onset of asperity interactions and potential surface damage
- (λ) compares minimum film thickness to composite surface roughness to assess lubrication regime
Contact mechanics
- Understanding helps predict stress distributions and deformations in EHL contacts
- Crucial for analyzing fatigue life and wear resistance of machine elements
Hertzian contact theory
- Describes elastic deformation and stress distribution in idealized, smooth contacting bodies
- Assumes perfectly elastic, frictionless contact with small strains and continuous surfaces
- Provides analytical solutions for contact area, pressure distribution, and maximum contact pressure
Non-Hertzian contacts
- Account for real-world deviations from idealized Hertzian conditions
- Include effects of surface roughness, non-elliptical contact geometries, and plastic deformation
- Require numerical methods or semi-analytical approaches for accurate analysis
Surface roughness effects
- Influences local pressure distribution and film thickness in EHL contacts
- Can lead to asperity interactions and mixed lubrication conditions
- Affects friction, wear, and fatigue life of contacting surfaces
- Characterized by parameters such as Ra (average roughness) and Rq (root mean square roughness)
Lubricant properties
- Lubricant properties significantly influence EHL performance and film formation
- Selection of appropriate lubricants requires consideration of operating conditions and desired tribological outcomes
Viscosity vs temperature
- Viscosity generally decreases with increasing following an exponential relationship
- (VI) quantifies the rate of viscosity change with temperature (higher VI indicates less temperature sensitivity)
- ASTM D341 equation models viscosity-temperature relationship:
Pressure-viscosity coefficient
- Measures the sensitivity of lubricant viscosity to pressure changes
- Typically ranges from 10-20 GPa^-1 for mineral oils to 5-10 GPa^-1 for synthetic oils
- Determined experimentally using high-pressure viscometers or inferred from EHL film thickness measurements
Thermal conductivity
- Affects heat dissipation and temperature distribution in EHL contacts
- Generally increases with pressure and decreases with temperature
- Typical values range from 0.1-0.2 W/mK for mineral oils to 0.2-0.4 W/mK for some synthetic lubricants
Operating conditions
- Operating conditions significantly impact EHL performance and film formation
- Understanding these effects helps engineers optimize component design and lubrication strategies
Speed and load effects
- Increasing generally increases film thickness due to enhanced hydrodynamic action
- Higher loads lead to larger contact areas and higher pressures, potentially reducing film thickness
- Speed and load effects combined in dimensionless speed (U) and load (W) parameters in EHL equations
Temperature influence
- Higher temperatures reduce lubricant viscosity, potentially leading to thinner films
- Thermal effects can cause viscosity variations across the film thickness (thermal EHL)
- Temperature changes affect material properties of contacting surfaces (thermal expansion, elastic modulus)
Starvation vs fully flooded
- ensure adequate lubricant supply to the contact inlet
- occurs when lubricant supply is insufficient, leading to reduced film thickness
- Starvation effects more pronounced at high speeds and in grease-lubricated contacts
- Degree of starvation quantified by film thickness reduction factor or inlet distance parameter
Numerical modeling
- Numerical modeling enables detailed analysis of complex EHL problems
- Helps predict performance and optimize designs for various operating conditions
Reynolds equation
- Governs pressure distribution in thin lubricant films
- Modified for EHL to include elastic deformation and pressure-viscosity effects
- General form for incompressible, isoviscous flow:
Energy equation
- Describes temperature distribution in EHL contacts
- Accounts for heat generation due to shearing and compression of the lubricant
- Coupled with and elastic deformation equations for thermal EHL analysis
Finite element analysis
- Enables solution of complex EHL problems with irregular geometries and non-linear material behavior
- Can incorporate multiphysics effects (thermal, structural, fluid dynamics)
- Allows for detailed stress analysis and optimization of component designs
- Commercial FEA software packages (ANSYS, COMSOL) offer specialized EHL modules
Measurement techniques
- Experimental measurements provide crucial validation for EHL theories and numerical models
- Help characterize lubricant properties and evaluate component performance
Film thickness measurement
- Optical interferometry measures film thickness in transparent EHL contacts (glass or sapphire discs)
- Electrical capacitance technique for opaque contacts
- Ultrasonic methods enable in-situ measurements in real machine elements
Friction measurement
- Measures overall friction in EHL contacts using load cells or torque sensors
- Enables calculation of friction coefficient and evaluation of lubricant performance
- Mini traction machine (MTM) commonly used for laboratory-scale friction measurements
Traction coefficient determination
- Traction coefficient relates tangential force to normal load in EHL contacts
- Measured using specialized test rigs (disc machines, ball-on-disc tribometers)
- Provides insights into lubricant rheology and shear behavior under EHL conditions
Applications in engineering
- EHL principles apply to numerous engineering applications involving heavily loaded, non-conforming contacts
- Understanding EHL helps optimize component design and lubrication strategies for improved performance and longevity
Rolling element bearings
- EHL crucial for efficient operation of ball and roller bearings
- Film thickness predictions used to determine appropriate lubricant selection and bearing design
- EHL analysis helps estimate fatigue life and predict potential failure modes
Gears and cam-follower systems
- EHL governs lubrication in gear tooth contacts and cam-follower interfaces
- Film thickness calculations used to optimize gear geometry and surface finish
- EHL models help predict resistance and in gears
Metal forming processes
- EHL principles apply to lubrication in cold rolling and wire drawing processes
- Film thickness predictions used to optimize lubricant selection and process parameters
- EHL analysis helps reduce friction and improve surface quality in formed products
Failure modes
- Understanding EHL-related failure modes helps engineers design more reliable and durable machine elements
- Proper lubrication and operating conditions can mitigate these failure mechanisms
Micropitting and wear
- Occurs when asperity interactions lead to localized surface fatigue
- More prevalent in mixed lubrication regimes with insufficient film thickness
- Characterized by shallow pits (typically <10 μm deep) on the surface
- Can be mitigated by improving surface finish and using higher viscosity lubricants
Scuffing and seizure
- Results from breakdown of the lubricant film and direct metal-to-metal contact
- Often occurs under high loads, high speeds, or inadequate lubrication conditions
- Characterized by rapid adhesive wear and material transfer between surfaces
- Prevention strategies include using extreme pressure (EP) additives and optimizing surface textures
Fatigue and spalling
- Subsurface fatigue caused by repeated stress cycles in EHL contacts
- Leads to formation of cracks that propagate to the surface, resulting in material removal (spalls)
- Influenced by factors such as material cleanliness, residual stresses, and lubrication conditions
- Life prediction models (e.g., ISO 281) incorporate EHL effects on fatigue life
Advanced concepts
- Advanced EHL concepts address more complex scenarios and refine existing models
- Help improve accuracy of predictions and extend applicability to a wider range of conditions
Thermal elastohydrodynamic lubrication
- Incorporates temperature effects on lubricant properties and surface deformation
- Accounts for heat generation due to shearing and compression of the lubricant
- Requires coupled solution of Reynolds, energy, and elasticity equations
- Important for high-speed applications and those with significant sliding
Transient elastohydrodynamic lubrication
- Addresses time-dependent effects in EHL contacts
- Relevant for applications with varying loads, speeds, or geometries (cam-follower systems)
- Considers squeeze film effects and time-dependent rheological behavior
- Requires solution of time-dependent Reynolds equation and elasticity equations
Mixed lubrication regime
- Occurs when film thickness is insufficient to fully separate surface asperities
- Combines aspects of boundary lubrication and EHL
- Requires consideration of asperity interactions and load sharing between fluid film and asperity contacts
- Modeled using statistical approaches or deterministic methods for known surface topographies
Future trends
- Emerging trends in EHL research aim to address new challenges and improve understanding of lubrication phenomena
- Advancements in these areas will lead to more efficient and reliable machine elements
Nano-scale elastohydrodynamic lubrication
- Investigates EHL phenomena at the nanometer scale
- Relevant for micro/nanoelectromechanical systems (MEMS/NEMS) and ultra-thin film lubrication
- Considers effects of surface forces (van der Waals, electrostatic) and molecular-scale fluid behavior
- Requires new experimental techniques and molecular dynamics simulations
Bio-inspired lubricants
- Develops new lubricants based on principles found in nature (synovial joints)
- Explores use of water-based lubricants and biolubricants for environmentally friendly applications
- Investigates surface texturing and smart materials for improved lubrication performance
- Aims to achieve low friction and wear in challenging operating conditions
Computational advancements
- Utilizes machine learning and artificial intelligence to improve EHL modeling
- Develops multiscale modeling approaches to bridge nano, micro, and macro-scale phenomena
- Implements high-performance computing for more detailed and efficient EHL simulations
- Enables real-time monitoring and predictive maintenance of EHL systems through digital twin technology
Key Terms to Review (39)
Central film thickness: Central film thickness refers to the maximum lubricant film thickness that forms between two surfaces in contact during elastohydrodynamic lubrication. This parameter is crucial as it determines the load-carrying capacity of the lubricant film and helps prevent direct contact between surfaces, reducing wear and friction. The behavior of this thickness is influenced by factors like the speed of the surfaces, the viscosity of the lubricant, and the load applied, making it essential in understanding lubrication mechanisms.
Contact Mechanics: Contact mechanics is the study of the deformation of solids that touch each other at one or more points. This field investigates how materials interact under contact conditions, including forces, pressure distribution, and material behavior. Understanding contact mechanics is essential for predicting wear, friction, and lubrication performance in various applications.
EHL: EHL stands for Elastohydrodynamic Lubrication, which is a lubrication regime that occurs when there is a significant elastic deformation of the surfaces in contact, leading to a change in the fluid film thickness. In this state, the pressure generated by the lubricant can lead to higher load-carrying capacities due to the combination of hydrodynamic and elastostatic effects. This results in reduced friction and wear between surfaces, particularly in applications like gears and rolling elements where high loads and speeds are present.
Elastic modulus: Elastic modulus is a fundamental property of materials that measures their stiffness or resistance to deformation when subjected to stress. It quantifies the relationship between stress (force per unit area) and strain (deformation) in a material, and is crucial for understanding how materials behave under load. A higher elastic modulus indicates a stiffer material, while a lower value suggests a more flexible one, impacting performance in scenarios involving lubrication and deformation.
Elastohydrodynamic lubrication: Elastohydrodynamic lubrication (EHL) is a lubrication regime that occurs when the pressure in a lubricant film is sufficiently high to cause elastic deformation of the surfaces in contact. This process allows for improved load-carrying capacity and reduced wear, which is crucial in applications involving rolling or sliding contact between surfaces, such as bearings and gears. EHL plays a vital role in optimizing performance and longevity in mechanical systems by balancing friction, wear, and lubrication.
Fatigue and Spalling: Fatigue refers to the progressive and localized structural damage that occurs when a material is subjected to repeated loading cycles, leading to crack initiation and propagation. Spalling is a specific form of fatigue failure where flakes or fragments of material break away from the surface due to stress concentrations and cyclic loading. These phenomena are critical in the context of elastohydrodynamic lubrication, as they can significantly influence the performance and lifespan of lubricated contact surfaces.
Film rupture: Film rupture refers to the breaking or failure of a lubricating film that separates two surfaces in contact during motion. This phenomenon is crucial because it can lead to direct contact between surfaces, resulting in increased friction and wear. Understanding film rupture is important for assessing lubrication performance, especially under high pressure and shear conditions where elastohydrodynamic lubrication comes into play.
Film thickness measurement: Film thickness measurement refers to the determination of the thickness of a lubricating film that separates two contacting surfaces in a tribological system. This measurement is crucial for understanding lubrication performance, especially in elastohydrodynamic lubrication where the film thickness significantly influences friction, wear, and overall machine efficiency.
Friction measurement: Friction measurement refers to the process of quantifying the resistance encountered when two surfaces slide against each other. Understanding this term is crucial in assessing the performance of materials in various applications, as it directly influences wear rates, lubrication needs, and the overall efficiency of mechanical systems.
Fully flooded conditions: Fully flooded conditions refer to a lubrication state where the entire surface area of the interacting components is covered by a continuous film of lubricant, preventing direct contact between the surfaces. This scenario is crucial for ensuring that the load is supported by the lubricant film, reducing wear and friction, and enhancing the performance of elastohydrodynamic lubrication systems.
Gear systems: Gear systems are mechanical devices consisting of interconnected gears that transmit torque and rotational motion between shafts. These systems play a crucial role in machinery by altering speed, direction, and force of motion, thus enhancing the efficiency of mechanical operations. The interaction between the gears in these systems is fundamental to understanding concepts such as elastohydrodynamic lubrication, which is essential for reducing friction and wear in high-speed gear applications.
Hardness: Hardness refers to the ability of a material to resist deformation, particularly permanent deformation or scratching. This property is crucial for understanding how materials behave under mechanical stress and is closely related to wear resistance, making it essential in evaluating performance in various applications.
Hertzian Contact Theory: Hertzian contact theory describes the elastic contact between two curved surfaces under load, predicting how they deform and distribute pressure at their contact point. This theory is fundamental in understanding friction and wear, as it establishes the relationship between contact geometry, material properties, and the resulting contact stresses, which can influence lubrication regimes, surface interactions, and the performance of mechanical systems.
Hydrodynamic Pressure: Hydrodynamic pressure is the pressure generated in a fluid when it is in motion, particularly under conditions of flow between two surfaces. This pressure is crucial in the context of lubrication, where it plays a key role in separating surfaces to prevent direct contact, thus reducing wear and friction. Hydrodynamic pressure helps maintain a stable lubricant film between moving parts, ensuring optimal performance and extending the lifespan of mechanical components.
Lambda ratio: The lambda ratio is a dimensionless parameter that describes the film thickness in relation to the surface roughness of the contacting surfaces in lubrication systems. It plays a critical role in understanding elastohydrodynamic lubrication, indicating whether the lubrication regime is hydrodynamic or boundary. A higher lambda ratio signifies a thicker lubricant film, which typically reduces wear and enhances performance in mechanical systems.
Load: In engineering, load refers to the external force or weight applied to a component or material, which can influence its performance and behavior under different conditions. Understanding load is essential for analyzing how materials interact under stress, as it directly affects wear, friction, and the overall durability of mechanical systems. The type and magnitude of load can vary significantly based on application, influencing phenomena like material deformation and failure mechanisms.
Lubrication film thickness: Lubrication film thickness refers to the measure of the layer of lubricant that separates two surfaces in relative motion, preventing direct contact and reducing wear. This thickness is crucial in elastohydrodynamic lubrication, as it influences the effectiveness of lubrication and the ability to support load without failure. The film thickness is determined by factors like lubricant viscosity, operating conditions, and surface roughness, impacting overall performance in mechanical systems.
Micropitting: Micropitting is a form of surface wear that occurs in lubricated contact between components, characterized by the formation of small pits or surface defects. This phenomenon is typically associated with elastohydrodynamic lubrication, where the lubricating film becomes very thin under high loads, leading to localized contact and material removal. Micropitting can significantly affect the performance and lifespan of mechanical components, making it crucial to understand its causes and implications.
Minimum film thickness: Minimum film thickness refers to the smallest distance between two sliding surfaces in a lubricated contact where a film of lubricant exists, preventing direct contact between the surfaces. This concept is crucial in elastohydrodynamic lubrication, where the lubricant film can deform under pressure to maintain separation and reduce wear. Understanding minimum film thickness helps predict performance, longevity, and efficiency of mechanical systems.
Mixed lubrication regime: The mixed lubrication regime is a state in lubrication where both fluid film and surface contact occur between interacting surfaces. This regime typically arises under moderate loads or speeds, where the lubricant is not thick enough to fully separate the surfaces, leading to a combination of hydrodynamic and boundary lubrication effects. Understanding this regime is crucial for predicting wear and ensuring the longevity of mechanical components.
Non-hertzian contacts: Non-hertzian contacts refer to the contact situations between surfaces that do not follow the classical Hertzian contact theory, which assumes elliptical contact shapes and elastic deformation. Instead, non-hertzian contacts may involve more complex geometries and loading conditions, leading to different stress distributions and potential failure modes. Understanding these contacts is crucial for predicting the behavior of lubricated contacts, especially under high loads or varying conditions, where traditional models may not apply.
Pressure-viscosity coefficient: The pressure-viscosity coefficient is a measure of how the viscosity of a lubricant changes with pressure. This coefficient is particularly important in scenarios where the lubricant is subjected to high pressures, such as in elastohydrodynamic lubrication, where the lubrication film can experience significant pressure increases. Understanding this coefficient helps engineers design better lubrication systems by predicting how lubricants will perform under varying pressure conditions.
Pressure-Viscosity Relationship: The pressure-viscosity relationship describes how the viscosity of a lubricant changes in response to variations in pressure. This concept is crucial in understanding how lubricants behave under different load conditions, especially in elastohydrodynamic lubrication where the contact pressures can be extremely high, leading to significant alterations in viscosity.
Reynolds Equation: Reynolds Equation is a fundamental equation in fluid mechanics that describes the behavior of thin films of lubricant in the context of lubrication. It provides a relationship between the pressure distribution in a lubricant film and the geometry and motion of the surfaces involved, making it essential for understanding hydrodynamic and elastohydrodynamic lubrication regimes. The equation is pivotal in predicting how lubricants reduce friction and wear between moving parts.
Rolling Element Bearings: Rolling element bearings are mechanical devices that allow for smooth motion between moving parts by utilizing rolling elements, such as balls or rollers, to reduce friction. They play a crucial role in supporting loads while minimizing wear and tear, which is essential in applications where components undergo continuous motion and stress.
Scuffing: Scuffing is a type of surface damage that occurs when two sliding surfaces come into contact, leading to localized wear and material transfer. This phenomenon is often a result of excessive friction and can be exacerbated by insufficient lubrication, causing a significant impact on the performance and longevity of mechanical components. Scuffing can lead to the degradation of surfaces in critical applications, particularly where adhesion between the materials is strong and there is limited ability for fluid film to separate the contacting surfaces.
Scuffing and Seizure: Scuffing and seizure refer to forms of wear and damage that occur in lubricated contacts, especially in mechanical systems. Scuffing typically manifests as localized surface damage due to inadequate lubrication or excessive load, leading to the removal of material from one or both surfaces. Seizure is a more severe condition where surfaces come into direct contact due to failure of lubrication, resulting in significant frictional heating and eventual binding of components.
Shear stress: Shear stress is the force per unit area acting parallel to the surface of a material, which can cause deformation and slippage within the material. This concept is crucial in understanding how lubricants behave under different loading conditions, particularly in various lubrication regimes and elastohydrodynamic situations where fluid films are involved.
Speed: Speed refers to the rate at which an object moves, defined as the distance traveled per unit of time. In lubrication contexts, it plays a critical role in determining the behavior of lubricants under various conditions, influencing factors like film thickness, pressure distribution, and overall performance. Understanding speed is essential for analyzing how lubrication systems function and how wear and friction are minimized in mechanical systems.
Starvation: Starvation refers to a condition in lubrication where the lubricant is insufficient to fully separate the surfaces of interacting components, leading to increased friction and wear. In elastohydrodynamic lubrication, starvation can significantly impact film thickness and the pressure distribution within the lubricant, which are crucial for reducing wear and preventing surface contact between moving parts.
Surface roughness effects: Surface roughness effects refer to the influence that the microscopic irregularities on a surface have on the behavior of friction and wear between contacting surfaces. These effects play a crucial role in various mechanical interactions, impacting load distribution, contact area, and lubricant performance, which are essential for understanding deformation theory, lubrication regimes, and experimental methodologies.
Temperature: Temperature is a measure of the thermal energy of a system, reflecting how hot or cold something is. It plays a crucial role in various mechanical processes, influencing factors such as material properties, reaction rates, and lubrication effectiveness in engineering applications. Understanding temperature is vital for managing wear processes, lubrication regimes, and the interactions between contacting surfaces.
Thermal conductivity: Thermal conductivity is a material property that measures a substance's ability to conduct heat. It indicates how quickly heat can pass through a material, which is crucial in understanding how heat dissipates or accumulates in various applications, particularly in lubricated contacts where friction generates significant heat.
Traction coefficient determination: Traction coefficient determination refers to the process of quantifying the frictional forces between surfaces in contact, particularly in lubrication scenarios where elastohydrodynamic effects are significant. This coefficient is crucial for predicting how well two surfaces will grip each other under load, influencing performance and wear in mechanical systems. Understanding the traction coefficient is vital in applications like gears and bearings, where proper lubrication directly affects efficiency and longevity.
Transient elastohydrodynamic lubrication: Transient elastohydrodynamic lubrication refers to a lubrication regime that occurs during the initial contact between two surfaces under load, where the lubricant forms a film that changes rapidly with time and pressure. This type of lubrication is characterized by the significant deformation of the contacting surfaces and the ability of the lubricant film to adapt quickly to varying loads and speeds, which helps reduce wear and friction during the dynamic conditions of operation.
Tribometer: A tribometer is an instrument used to measure friction, wear, and lubrication characteristics of materials in tribological studies. It allows researchers and engineers to simulate and evaluate the performance of materials under various loading and environmental conditions, providing critical insights into material behavior in real-world applications.
Viscosity Dependence: Viscosity dependence refers to how the viscosity of a lubricant affects its performance and behavior in lubrication systems. This concept is crucial in understanding how fluids behave under different conditions, especially in elastohydrodynamic lubrication where the film thickness, load, and sliding speed can lead to variations in viscosity and influence friction and wear characteristics.
Viscosity Index: The viscosity index (VI) is a measure of how much a lubricant's viscosity changes with temperature. A high VI indicates that the lubricant maintains a more stable viscosity across temperature changes, which is crucial for efficient lubrication in various applications. This property directly affects performance in friction and wear scenarios, ensuring that machinery operates smoothly even under varying thermal conditions.
Wear rate: Wear rate is a measure of the amount of material removed from a surface due to wear processes over a specific period or under certain conditions. It helps quantify the durability and performance of materials in contact, especially in relation to friction and lubrication mechanisms, making it a crucial parameter in various engineering applications.