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4.2.2 mechanisms of wear
progress in wear control can be aided by a better understanding of the mechanisms by which it occurs. research workers have tended to isolate study specific wear mechanisms such as adhesion ,abrasion ,erosion and fatigue. such research has generally been directed towards the study of surfaces in relative motion, the changes brought about by their interaction and the effects of the lubricant and the environment present.
1. Scuffing
Under sliding conditions, the chief task of lubricant is to allow relative motion between surfaces with low friction and no damage. This can be achieved if the lubricant film is thick enough to keep the surfaces apart and hydrodynamic conditions prevail. If, however, ideal conditions cannot be maintained, the surfaces will come into contact and wear or damage in the form of scuffing will occur.
The complex mechanism of scuffing is difficult understand as the process, by cumulative action ,destroys evidence of its initial stages.
2. abrasive wear
From an economic point of view, abrasive wear caused by ploughing or groughing of a hard surface, hard particles or debris, against a relatively softer mating surface is probably the most serious single cause of wear in engineering practice. There are indications that abrasion, primarily a crude machining process, is related to indentation hardness and hence to static yield stress. A particle of hard brittle material may cause damage in a single pass through the area of minimum film thickness of a bearing. However, in so passing, this brittle particle may be rendered ineffective due either to breakdown into smaller particles of dimensions smaller than the minimum oil film thickness or by being completely embedded in the softer of the mating materials. On the other hand a soft material particle may work-harden on passage between relatively moving surfaces. in gouging the softer bearing material surface, the particle may, if the bearing material also work-hardens, be only partially embedded in an equally hard surface area and become a source of further damage to the mating surface.
If abrasive particles are conveyed by a fluid stream the impact the abrasive particle laden fluid will give rise to erosive wear of any interposed surface. The extent and type of wear depends upon the impinging angle of the particles and ductility of the surface.
3 fretting
Fretting is a specific form of wear, which occurs when there is a slight vibratory movement between loaded surfaces in contact and which manifests itself by the pitting of the surfaces and the accumulation of oxidized debris. The debris, being largely the oxide of the metals involved, occupies a greater volume than that of the metal destroyed and in a limited space, this can lead to a pressure built up and seized. The form and extent of fretting damage depends on the chemical nature of the environment and on whether or not the debris can escape or is built up between the surfaces.
The actual rate of wear may slow down if the debris acts as a buffer between the two surfaces. thus a process. which initiates as adhesive wear, may change to abrasion and then the wear rate may slow down as a result of debris keeping the surfaces apart. The final failure may then be caused by fatigue fracture, crack initiation being effected by the stress-raising role of fretting pits.
4. fluid and cavitation erosion
these wear mechanisms arise from the impact of fluids at high velocities. Fluid erosion damage caused by small drops of liquid can occur in steam turbines and fast flying aircraft through the impact of water droplets causing plastic depressions on the surface. As the fluid flows from the deformed zone it can cause shear deformation in peripheral areas and repeated deformation causes a fatigue type of damage by pitting and roughening of the surface. Cavitation erosion damage is caused by impact from the collapse of vapor or gas bubbles formed in contact with a rapidly moving or vibrating surface. The physical damage to metals is characterized by pitting suggestive of a fatigue origin. The ultimate resilience of material appears to be an important property of metals in cavitation resistance. This resilience is measured as the energy that can be dissipated before appreciable deformation and cracking occur.
5. rolling contact fatigue
the useful life of rolling elements is limited by surface disintegration pits or fracture being caused by a fatigue process dependent upon the properties of the material, the nature of the lubricant and the environment. This phenomenon is characterized by the sudden removal of surface material or fracture caused by repented alternating stresses. The process has three phases, preconditioning of the material prior to crack initiation, crack initiation and crack propagation.
Rolling contact fatigue cracks initiate either on the surface and propagate into the material, or start below the surface in the area of calculated maximum Hertzian stress and propagate towards the surface depending upon operating circumstances. The propagation of surface cracks may be controlled by the nature of the lubricant and he environment. If the environment is deleterious, for example, if it leads to hydrogen embrittlement the cracks may propagate rapidly, deep into the material, so that fracture ensues.
It appears that several different modes of rolling contact fatigue can cause cracks to nucleate and propagate independently at various rates. This phenomenon is greatly influenced by highly localized conditions. While the general properties of the bulk material are important, specific aspects such as the steelmaking process, gas content and cleanliness are also equally important. The nature of the lubricant and the environment have a dominant effect on failure.
Material combination and material lubricant combination require careful consideration to ensure careful consideration to ensure satisfactory performance.
In rolling contact without a lubricant, failure occurs only by excessive wear that limits the useful life because of vibration and noise.
4.3LUBRICATION
4.3.1 lubrication theory
4.3.1.1 liquid lubrication
The way in which liquids lubricate can be simply explained by the example of a plain journal bearing shown in Fig.4.2.
As the shaft (journal) rotates in the bearing, lubricating oil is dragged into the loaded zone and the pressure and volume of the loaded zone both increase. The pressure rises, and therefore the thickness of the oil film will depend on the shaft speed and the lubricant viscosity. The relationship between speed, viscosity, load, oil film thickness and friction can be understood by a graph such as the one in Fig.4.3.
In this graph the coefficient of friction is plotted against the expression YV/P where
There are three distinct zones in the graph, separated by the points A and B.
At B the coefficient of friction is at its minimum, and this is the point at which the oil film is just thick enough to ensure that there is no contact between asperities on the shaft and bearing surfaces. In Zone 3, to the right of B. the oil-film thickness is increasing because of increasing viscosity. Increasing speed or decreasing load, and the coefficient of friction increases as the oil-film thickness increases. Zone 3 is the zone of hydrodynamic lubrication.
AS conditions change from B towards A, the oil-film thickness decreases so that the asperities on the shaft and the bearing will rub against each other. The amount of rubbing and the friction increases as the oil-film thickness decreases. Zone 2, between A and B, is known as the zone of mixed lubrication. The shaft load was traditionally considered to be supported by a mixture of hydrodynamic and boundary lubrication. The term mixed lubrication may in fact be even more appropriate than was originally thought, because modern theories suggest that in different systems there may be a mixture of four or more different types of lubrication present in this zone.
At A the oil-film thickness has been reduced virtually to nil, and load between shaft and bearing is being carried entirely on asperity contacts. In Zone 1, to the left of A, the coefficient of friction is almost independent of load, viscosity and shaft speed. Zone 1is the zone of boundary lubrication.
The different lubrication zones also have an important influence on wear. wear is a very complicated subject with several different forms, more than one of which may be present at the same time. But, generally speaking, the amount of wear depends on the severity with which two surfaces rub against each other.
In Zone 3 there is no contact between the surfaces and therefore there is no wear. As the oil becomes thinner in moving through Zone 2 and Zone 1 there is increasingly severe contact between the surfaces which conceives a greater tendency to wear.