- The method begins with an examination of worn parts of the type to be improved. - The next step is an estimate of stresses, temperatures, and likely conditions of operation of the redesigned machinery. - 6.1.1 Types, Appearances, and Mechanisms of Wear. - Wear is a loss or redistribution of surface material from its intended location by def- inition of the ASTM. - The preceding three natural processes are too broad to be useful for material selection in light of the known properties of materials. - A more detailed list of mate- rial properties appropriate to the topic of wear is given in Table 6.1.. - of wear. - How- ever, if we define a mechanism as that which is capable of explanation by the laws of physics, chemistry, and derivative sciences, then mere contact becomes a statement of the condition in which surfaces exist and not a mechanism. - Terms of the former type include dry wear, metal-to-metal wear, hot wear, frictional wear, mechanical wear, and impact wear. - Finally, some of the terms that come from the lit- erature on "lubricated". - Valiant attempts are continuously being made to define wear terms in the pro- fessional societies, but progress is slow. - Researchers have attempted to classify most of the terms as either abrasive or adhesive mechanisms primarily, with a few terms classified as a fatigue mechanism. - Some authors attempt to escape such categories by separating wear into the mild and severe cate- gories, which introduces value judgments on wear rates not inherently found in the other terms. - Mechanisms of wear will be discussed at greater length below.. - The opposite philosophy must apply to drilling bits used in the oil-well industry. - The temperature environment of the nuclear reactor is moderate, lubricants are not permitted, and the result of wear is exceedingly detrimental to the function of the system. - One often heard but misleading simple method of reducing wear is to increase the hardness of the material. - They are hardened in the range of 50 Rockwell C instead of to the maximum available, which may be as high as 67 R c . - We see in the preceding examples the possibility of special circumstances requiring special materials. - Little real progress has been made in this effort, and very little is likely to be made in the near future. - Such guidelines will be more useful as our technology becomes more complex, but some guidelines are given in the next section.. - Designers make most of the decisions concerning material selection. - Most such components have been well tested in the marketplace, hav- ing been designed and developed by very experienced designers. - Their specific expertise is not available to the general designer except in the form of the shapes and dimensions of hardware, the materials selected, and the recommended practices for use of their product.. - If good information is available on similar items, a prediction of the wear life of a new product can be made with ± 20 percent accuracy unless the operating conditions of the new design are much beyond standard experience. - An assessment should be made of the modes or mechanisms of wear of each part of the product. - For this purpose, it is also useful to examine old lubricants, the contents of the lubricant sump, and other accumulations of residue.. - When a vendor simply suggests an existing item or material, the wear life of a new product may not be predictable to an accuracy of better than ± 50 percent of the desired life. - The final result of the use of handbook data is a design which will probably not perform to an accuracy of better than ±95 percent.. - Ideally, a wear equation would provide a numerical value for material loss or transfer for a wide range of materials and oper- ating conditions of the wearing parts.. - The strictly empirical equations usually contain very few variables and are of the form. - It is generally assumed that the results will not depend on the selection of the variables to hold constant, which therefore assumes that there is neither any limit to the range of valid variables nor any interdependence between variables, which ultimately means that there is no change of wearing mechanisms over any chosen range of the variables.. - they have limited ability to predict conditions beyond those of the tests from which they were derived, and they have little appli- cability to other sliding systems.. - The fact, then, that quantities such as hardness are found in the numerator of some equations and in the denominator of others leads to some confusion. - The parameters that may be included in the equation are of three types, as listed in Table 6.2. - It may be readily seen from Table 6.2 that many of the parameters are difficult to quantify, and yet these (and perhaps several more) are known to affect the wear rate. - Moreover, many of the mechanisms produce wear rates that are not linear in the simple parameters, such as applied load, sliding speed, surface finish, etc. - 6.4 STEPSINSELECTINGMATERIALS FOR WEAR RESISTANCE. - When designing for wear resistance, it is necessary to ascertain that wear will pro- ceed by the same mechanism throughout the substantial portion of the life of the product. - The acceptable modes of wear failure or surface damage. - These considerations are discussed in detail in the next several pages.. - Furthermore, there may be a limitation on the sur- face finish available or the skill of the personnel to manufacture or assemble the product. - Finally, there may be considerations of delivery or storage of the item before use, leading to corrosion, or false brinelling, or several other events that may befall a wear surface.. - Some specifications in the gear and sleeve bearing industries limit the aver- age contact pressures for bronzes to about 1.7 MPa, which is about 1 to 4 percent of the yield strength of bronze. - Likewise, in pump parts and valves made of tool steel, the contact pressures are limited to about 140 MPa, which is about 4 to 6 per- cent of the yield strength of the hardest state of tool steel.. - However, one example of high contact pressure is the sleeve bearings in the landing gear of modern commercial aircraft. - The influence of temperature may be its effect on the mechanical properties of the slid- ing parts. - For a quick view of the factors that influence temperature rise A T of asperities on rubbing surfaces, we may reproduce one simple equation:. - coefficient of friction, W = applied load, V = sliding speed, and ki and k 2 = thermal conductivities of the sliding materials. - (6.2) it may seem that thermal conductivity of the materials could be influential in controlling temperature rise in some cases, but a more important fac- tor is £ the coefficient of friction. - Maximum allowable loads and sliding speeds for mate- rials are often specified in catalogs in the form of PV limits. - In the PV product, P is the calculated average contact pressure (in psi) and V is the sliding speed (in ft/min). - Indeed, a PV limit indicates nothing about the actual rate of wear of materials. - Misalignment arises from manufacturing errors or from a deflection of the system-producing loading at one edge of the bearing, or it may arise from thermal distortion of the system, etc. - This may involve designing a flexible bearing mount, or several bearings along the length of a shaft, or a distribution of the applied loading, etc.. - A simple tapping of a ball bearing with a hammer to seat the race may constitute more severe service than occurs in the lifetime of the machine and often results in early failure.. - If abrasive species can enter a bearing, the fastest wear will occur at the point of entry of the dirt. - It is often useful to determine also what materials surround the sliding sys- tem, such as chemical or abrasive particles.. - The beryllium bronze has a strength only twice that of the phosphor bronze, but the difference between industrial and aircraft use includes different treatment of bearings in maintenance.. - There is scheduled maintenance, but there is also careful continuous observation of the part and supplies. - Thus it is easier for an error to be made in selection of the lubricant in industry than with aircraft, for example. - Industrial machinery must operate in the dirtiest and hottest of places and with the poorest care. - Another vital consideration in the selection of materials is to determine whether or not a break-in procedure is necessary or prohibited. - The incidence of failure of a population of such parts decreases with time of operation as the sliding surfaces change, and frequently the ability of the sys- tem to accommodate an overload or inadequate lubricant increases in the same time. - It is difficult to determine whether or not some of the present conservative indus- trial design practices result from the impracticality of effecting a run-in of some products. - 6.4.5 Modes of Wear Failure. - The fifth consideration is to determine acceptable modes of wear failure or surface damage of machinery. - Again, in metal-working dies, loss of material from the sys- tem is less catastrophic than is scratching of the product.. - The same design and materials in the wearing parts of the example device will per- form adequately in the redesign, in terms of function, cost, and all other attributes.. - A slight change in size, lubrication, or cooling of the example parts will be ade- quate for the design.. - A significant change in size, lubrication, or cooling of the example parts will be necessary for the redesign.. - A different material will be needed in the redesign.. - The action to be taken after reaching one of the preceding conclusions will vary.. - The first conclusion can reasonably be followed by production of a few copies of the redesign. - The second conclusion should be followed by cautious action, and the third conclusion should invoke the building and exhaustive testing of a prototype of the redesign. - namely, the wear mechanisms must be very similar in each of the production designs, the prototype test, the subcomponent test, and the bench test. - The wear rate of each test in the hierarchy should be similar, the worn surfaces must be nearly identical, and the transferred and loose wear debris should contain the same range of particle sizes, shapes, and composition. - Thus it is seen that the pro- totype, subcomponent, and bench tests must be designed to correlate with the wear results of the final product. - A clear indication of the problem with bench tests may be seen in some results with three test devices. - These results may be compared with linear laws of wear discussed frequently in the literature, which would be of the form. - The point of the preceding discussion is that wear testing of materials and com- ponents must be done, but it must be done carefully. - Guidelines for selecting wear-resisting materials and for indirectly selecting lubricants are given in the next section using the methods of product analysis.. - The previous sections have established the important point that selecting materi- als for wear resistance requires a study of the details of wear in order to determine which of the several conventional properties of material can best resist a particu- lar mode of wear. - The virtue of the SEM is that very rough surfaces can be seen without the high regions and low regions being out of focus, as occurs in optical microscopy. - Optical metallurgical microscopes may be useful as well, but usually not in the con- ventional bright-field, reflected-light mode. - These microscopes often have several special objectives for phase contrast, polarized light, interference, and dark field, all requiring skill in use and in the interpretation of results.. - Sometimes it is useful to obtain a topographic profile of worn surfaces. - It is the surface shape rather than a numerical value of surface finish that is most useful. - A major precaution to observe in analysis of the strip-chart data is that the represen- tation of the height of surface shapes is often magnified from 10 to 1000 times greater than is the "horizontal". - Verify that the code listing is an adequate description of the worn surface. - In Section C, find the detailed description of the capitalized term from Section B and note which material-loss mechanism is applicable and confirm from the nature or description of wear debris.. - The worn surfaces in Section A may be described in terms of the three scales. - Where a scale of geometry is not given, that scale may not be of consequence in the description of the worn surface.. - ^Rigorous connection cannot always be made between the terms in the two columns in Section A because of the wide diversity of use and meaning of terms.. - Abrasion with coarse particles, including carbide and other hard inclusions in the sliding materials that are removed by sliding action as the wear of matrix progresses, or by. - Abrasion: Involves particles (or acute angular shapes but mostly obtuse) that produce wear debris, some of which forms ahead of the abrasive particle, which mechanism is called cutting, but most of which is material that has been plowed aside repeatedly by passing particles, and breaks off by low-cycle fatigue.. - Newly formed chemical compound, usually agglomerated and sometimes mixed with fragments of the original surface material Long, often curly chips or strings. - The mode of fracture will depend on the property of the material, involving various amounts of energy loss or ductility to fracture, that is, Low energy and ductility. - ductile fracture Fatigue: Due to cyclic strains, usually at stress levels below the yield strength of the material, also called high-cycle fatigue. - Tribology Transactions of the Society of Tribologists and Lubrication Engineers (formerly Transactions of the American Society of Lubrication Engineers).
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