1) Elastohydrodynamic lubrication (lubrication)(better)
The remarkable efficiency of elastohydrodynamic lubrication in preventing solid to solid contact even under extreme contact stresses prevents the rapid destruction of many basic mechanical components such as rolling bearings or gears. EHL is, however, mostly confined to mineral or synthetic oils since it is essential that the lubricant is piezo-viscous. The mechanism of EHL involves a rapid change in the lubricant from a nearly ideal liquid state outside of the contact to an extremely viscous or semi-solid state within the contact. This transformation allows the lubricant to be drawn into the contact by viscous drag while generating sufficient contact stress within the contact to separate the opposing surfaces. If a simple solid, i.e. a fine powder, is supplied instead, there is no viscous drag to entrain the powder and consequently only poor lubrication results. A non-piezo-viscous lubricant simply does not achieve the required high viscosity within the contact necessary for the formation of the lubricating film. The formulae for the calculation of the EHL film thickness are relatively simple and are based on load, velocity, dimensions and elastic modulus of the contacting materials. As well as providing lubrication of concentrated contacts, the EHL mechanism can be used to generate traction, i.e. where frictional forces enable power transmission. A unique combination of high tractive force with minimal wear, reduced noise levels, infinitely variable output speed and an almost constant torque over the speed range can be obtained by this means.
Andrew W. Batchelor, Gwidon W. Stachowiak, ENGINEERING TRIBOLOGY, p.420
Elastohydrodynamic lubrication(old description)
The term elastohydrodynamic lubrication (EHL) is reserved for hydrodynamic lubrication applied to lubricantfilms between elastically deforming solids.The principles of EHL are readily applicable to such diverse objects as gears, rolling-element bearings, and human animal joints. In general bearingsthat are lubricated in EHL mode are of low geometric conformity, and, in the absence of lubricant film and of elastic deformation, the opposing surfaces would contact in a point (ball bearings) or along a line (gears or roller bearings).
If the solid surfaces that are lubricated in the EHL mode have large elastic modulus,the contact pressures will be largei perhaps of the order of 1GPa.The film thickness will be correspondingly small, of the order of 1μm. Under such conditions the material properties of the lubricant will be distinctly different from its properties in bulk.This change in lubricant properties, when coupled with the effects of elastic deformation of the solid surfaces, yields film thicknesses one or two orders of magnitude larger than those estimated from constant viscosity theory applied to nondeforming surfaces.
EHL theory may be viewed as a combination of hydrodynamic lubrication, allowance for the pressure dependence of viscosity, and elatic deformation of bounding surfaces.
(Andras Z. Szeri,Fluid Film Lubrication,p.37)
Elastohydrodynamic lubrication(old description)
The term elastohydrodynamic lubrication (EHL) is reserved for hydrodynamic lubrication applied to lubricantfilms between elastically deforming solids.The principles of EHL are readily applicable to such diverse objects as gears, rolling-element bearings, and human animal joints. In general bearingsthat are lubricated in EHL mode are of low geometric conformity, and, in the absence of lubricant film and of elastic deformation, the opposing surfaces would contact in a point (ball bearings) or along a line (gears or roller bearings).
If the solid surfaces that are lubricated in the EHL mode have large elastic modulus,the contact pressures will be largei perhaps of the order of 1GPa.The film thickness will be correspondingly small, of the order of 1μm. Under such conditions the material properties of the lubricant will be distinctly different from its properties in bulk.This change in lubricant properties, when coupled with the effects of elastic deformation of the solid surfaces, yields film thicknesses one or two orders of magnitude larger than those estimated from constant viscosity theory applied to nondeforming surfaces.
EHL theory may be viewed as a combination of hydrodynamic lubrication, allowance for the pressure dependence of viscosity, and elatic deformation of bounding surfaces.
(Andras Z. Szeri,Fluid Film Lubrication,p.37)
2) Hydrostatic lubrication(lubrication)(better)
Hydrostatic lubrication provides complete separation of sliding surfaces to ensure zero or negligible wear and very low friction. Hydrostatic lubrication is based on the same physical principles as hydrodynamic lubrication but has certain fundamental differences. There is no friction force at infinitesimal sliding speeds unlike hydrodynamic lubrication which is a uniquely useful characteristicin the design and operation of precision control systems. The disadvantage of hydrostatic lubrication is a complete reliance on an external pressurized supply of lubricant which means that the pump must be reliable and the supply lines free of dirt that might block the flow of lubricant. Hydrostatic lubrication with a gas, which is known as aerostatic lubrication, can provide very low friction even at extremely high sliding speeds because of the low viscosity of gases. Quasi-ideal characteristics of zero wear and friction are obtained with hydrostatic or aerostatic lubrication at low to medium contact stresses but a more complicated technology, e.g. the application of an external high pressure pump, is required in comparison to other forms of lubrication. Bearing stiffness in these bearings can also be manipulated more easily than with other types of bearings to suit specific design requirements.
Andrew W. Batchelor, Gwidon W. Stachowiak, ENGINEERING TRIBOLOGY, p.333
Hydrostatic Lubrication: (01:56 - 28.04.2011)(old description)
In hydrostatic lubrication of friction bodies, a pocket or recess is incorporated in one friction body’s loaded surface into which a fluid is forced from outside at constant pressure. A pump outside the bearing generages the lubricant pressure are the most important features of hydrostatic lubrication. The lubricating pocket is normally positioned opposite the external load. The load-carrying capacity of a contact with hydrostatic lubrication is also assured when surfaces are not moving. When the volumetric flow of lubricant into the lubricating pocket is constant, the minimum lubrication film thickness is proportional to the cube root of tne ratio of the average lubricant viscosity in the lubrication gap and the load, i.e, the minimum lubrication film thickness is less dependent on the viscosity and the load than is the case in hydrodynamic lubrication.
Hydrostatic lubrication is mainly used: where the friction partners’ surfaces do not have any metallic contact, i.e, wear may not occur, not even when ramping up and ramping down a machine or at low speed; where as low a friction coefficient as possible must be produced at low speeds; and where, as a result of less effective lubricant entraining velocities in the lubrication gap, the wedge effect cannot produce any bearing lubricating film hydrodinamically.
(Springer Handbook of Mechanical Engineering, 10. Volume, Karl-Heinrich Grote, Erik K. Antonsson, p.313)
3) Hydrodynamic lubrication(lubrication)(better)
The theory of hydrodynamic lubrication has been presented to demonstrate how a basic property of all liquids, such as viscosity, can be used to produce cheap, reliable bearings that operate with low friction and wear. Like many important scientific principles, chance observation played an important role in the recognition of hydrodynamic action as a basic mechanism of bearing lubrication. The complete separation of sliding surfaces by a liquid film under full hydrodynamic lubrication can allow bearings to operate indefinitely without any wear. Any liquid or gas can be used for this form of lubrication provided that no chemical attack of the bearing occurs. The disadvantage of hydrodynamic lubrication is that a non-zero sliding or 'squeeze' velocity is required before load capacity is obtained. Some damage to bearings during starting or stopping is inevitable because of this condition. There is also the risk of a large rise in friction and possible bearing seizure if the limits of hydrodynamic lubrication are exceeded by excessive load or insufficient speed. A further problem is that vibration induced by hydrodynamic instability may occur during operation at high speeds and this should always be carefully controlled. Despite these deficiencies, hydrodynamic lubrication is the preferred form of lubrication in most bearing systems.
Andrew W. Batchelor, Gwidon W. Stachowiak, ENGINEERING TRIBOLOGY, p.236
Hydrodynamic lubrication(old description)
Even in an exclusively hydrodynamic state, the complex physical,hydrodynamic,thermal,elastici and plastic phenomena described in 3.1 exist.Therefore, some hypoteses will have have to be introduced to simplify and permit further treatment of the problem.Two hydrodynamic states can be distinguished:
1.Real hydrodynamic lubrication , in whichall parameters are considered to be interdependent: e.g., variation viscosity with pressure, temperature,and velocity gradien; heat transfer; roughness and deformations.
2. Ideal hydrodynamic lubrication, in which viscosity is constant in time and space; surfaces are perfectly smooth, rigid and without deformations;and thermal equilibrium is established.Further external forces and velocities are often considered to be constant in time.
(Nicolae Tipei,Theory of lubrication: with applications to liquid- and gas-film lubrication, p.39)
Hydrodynamic lubrication(old description)
Even in an exclusively hydrodynamic state, the complex physical,hydrodynamic,thermal,elastici and plastic phenomena described in 3.1 exist.Therefore, some hypoteses will have have to be introduced to simplify and permit further treatment of the problem.Two hydrodynamic states can be distinguished:
1.Real hydrodynamic lubrication , in whichall parameters are considered to be interdependent: e.g., variation viscosity with pressure, temperature,and velocity gradien; heat transfer; roughness and deformations.
2. Ideal hydrodynamic lubrication, in which viscosity is constant in time and space; surfaces are perfectly smooth, rigid and without deformations;and thermal equilibrium is established.Further external forces and velocities are often considered to be constant in time.
(Nicolae Tipei,Theory of lubrication: with applications to liquid- and gas-film lubrication, p.39)
4) Adhesive wear(material)
Most solids will adhere on contact with another solid to some extent provided certain conditions are satisfied. Adhesion between two objects casually placed together is not observed because intervening contaminant layers of oxygen, water and oil are generally present. The earths atmosphere and terrestrial organic matter provide layers of surface contaminant on objects which suppress very effectively any adhesion between solids. Adhesion is also reduced with increasing surface roughness or hardness of the contacting bodies. Actual observation of adhesion became possible after the development of high vacuum systems which allowed surfaces free of contaminants to be prepared. Adhesion and sliding experiments performed under high vacuum showed a totally different tribological behaviour of many common materials from that observed. in open air. Metallic surfaces free of oxide films under high vacuum exhibited the most dramatic changes and partly for this reason have been widely studied.
A well disguised tendency for all materials to mutually adhere when brought into a close contact is the basic cause of adhesive wear. Although atmospheric contaminants and lubricants provide effective means of preventing adhesive wear they can never entirely eliminate it. Adhesion results in high coefficients of friction and serious damage to the contacting surfaces. In extreme cases, when adhesive wear is fully established, the friction and wear rate can be so high that it may be impossible for the contacting surfaces to continue sliding. Adhesive wear is the fundamental cause of failure of most metal sliding contacts and therefore its effective prevention is essential to proper functioning of engineering machinery.
Andrew W. Batchelor, Gwidon W. Stachowiak, ENGINEERING TRIBOLOGY, p.632
Adhesive Wear: (01:38 – 28.04.2011)(old description)((better)
Adhesive Wear: (01:38 – 28.04.2011)(old description)((better)
Adhesive wear originates from adhesion between two surfaces that are placed in contact. When two surfaces are brought into contacti asperities of the tho surfaces make physical contact. This “true” contact area is significantly smaller than the apparent surface area of the two contact surfaces. The contact area between the two surfaces is localized to the small regions known as asperities; these asperity-asperity contact regions are referred to as junction. The size of a junctio
n is usually in the range 1-100 µm; the typical size of a junction is 10 µm in diameter. The number of junctions isdependent on the surface roughness and the amount of load that is applied. Under load, bonding between asperities on the two contact surfaces may occur. The amount of deformation at these junctions is also dependent on the number of junctions and the size of the junctions. Under sliding motion, plastic deformation, cracking and fracture can occur in the “true” contact area. Adhesive wear is largely due to fracture of material and transfer of material at the asperity-asperity contact regions. Prior to fracture, plastic deformation and crack formation may cause damage to the contact surfaces.
n is usually in the range 1-100 µm; the typical size of a junction is 10 µm in diameter. The number of junctions isdependent on the surface roughness and the amount of load that is applied. Under load, bonding between asperities on the two contact surfaces may occur. The amount of deformation at these junctions is also dependent on the number of junctions and the size of the junctions. Under sliding motion, plastic deformation, cracking and fracture can occur in the “true” contact area. Adhesive wear is largely due to fracture of material and transfer of material at the asperity-asperity contact regions. Prior to fracture, plastic deformation and crack formation may cause damage to the contact surfaces.(Biomedical Materials, Roger Narayan, p.186)
5) Rheology(material)(better)
Rheology has been properly defined as the study of the flow and deformation of materials, with special emphasis being usually placed on the former. However, we might ask the simple question ‘what is flow?’. If we carry water carefully in a bucket, it is certainly moving, but it is not flowing, however if we pour out the water it is flowing. What is the difference? In flow, elements of the liquid are deforming, and adjacent points in the liquid are moving relative to one another. There are two basic kinds of flow with relative movement of adjacent particles of liquid; they are called shear and extensional flows. In shear flows liquid elements flow over or past each other, while in extensional flow, adjacent elements flow towards or away from each other, see figure 1 for illustrations of shear and extensional deformation and flow respectively
.
Howard A. Barnes ,A Handbook of Elementary Rheology, p.5
Rheology(old description)
Rheology is a physical method of charecterization of the structure of matter.Rheology gives unambiguous,physically meaningful,quantative parameters of metarials. These paremeters can be correlated with the structure of matter, either chemical(monecular structure of a compound,length and architecture of a molecule, and so on) or physical (physical intermolecular interactions, phase state, size and distrubution of components in multi-component systems, and so on) structure.Rheological parameters correlate with the structure of material and can be used for structure characterization.
(Aleksandr IAkovlevich Malkin,Alexander Ya Malkin,Avraam I. Isayev,Rheology: concepts, methods & applications,p.351)
Rheology(old description)
Rheology is a physical method of charecterization of the structure of matter.Rheology gives unambiguous,physically meaningful,quantative parameters of metarials. These paremeters can be correlated with the structure of matter, either chemical(monecular structure of a compound,length and architecture of a molecule, and so on) or physical (physical intermolecular interactions, phase state, size and distrubution of components in multi-component systems, and so on) structure.Rheological parameters correlate with the structure of material and can be used for structure characterization.
(Aleksandr IAkovlevich Malkin,Alexander Ya Malkin,Avraam I. Isayev,Rheology: concepts, methods & applications,p.351)
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