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TABLE 34.2 Parameters Often Seen in Wear Equations
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no complete first principles or models available to use in selecting materials for wear resistance. However, there are good procedures to follow in selecting materials for wear resistance.
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34.4 STEPS IN SELECTING MATERIALS FOR WEAR RESISTANCE
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When designing for wear resistance, it is necessary to ascertain that wear will proceed by the same mechanism throughout the substantial portion of the life of the product. Only then is some reasonable prediction of life possible.
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Certain considerations are vital in selecting materials, and these may be more important than selecting a material for the best wear resistance. These considerations are 1. 2. 3. 4. 5. 6. The restriction on material use Whether the sliding surface can withstand the expected static load Whether the materials can withstand the sliding severity Whether a break-in procedure is necessary or prohibited The acceptable modes of wear failure or surface damage The possibility of testing candidate materials in bench tests or in prototype machines
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These considerations are discussed in detail in the next several pages.
34.4.1 Restrictions on Material Use The first step in selecting materials for wear resistance is to determine whether there are any restrictions on material use. In some industries it is necessary for economic and other purposes to use, for example, a gray cast iron, or a material that is compatible with the human body, or a material with no cobalt in it such as is required in a nuclear reactor, or a material with high friction, or a selected surface treatment applied to a low-cost substrate. Furthermore, there may be a limitation on the surface 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.
34.4.2 Static Load The second step is to determine whether the sliding surface can withstand the expected static load without indentation or excessive distortion. Generally, this would involve a simple stress analysis.
34.4.3 Sliding Severity The materials used must be able to withstand the severity of sliding. Factors involved in determining sliding severity include the contact pressure or stress, the temperature due to ambient heating and frictional temperature rise, the sliding speed, misalignment, duty cycle, and type of maintenance the designed item will receive. These factors are explained as follows. Contact Stress. Industrial standards for allowable contact pressure vary considerably. Some specifications in the gear and sleeve bearing industries limit the average 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 percent of the yield strength of the hardest state of tool steel.
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However, one example of high contact pressure is the sleeve bearings in the landing gear of modern commercial aircraft. These materials again are bronzes and have yield strengths up to 760 MPa. The design bearing stress is 415 MPa but with expectations of peak stressing up to 620 MPa. Another example is the use of tool steel in lubricated sheet-metal drawing. Dies may be expected to be used for 500 000 parts with contact pressures of about 860 MPa, which is half the yield strength. Temperature. The life of some sliding systems is strongly influenced by temperature. Handbooks often specify a material for wear conditions without stating a range of temperature within which the wear-resistance behavior is satisfactory. The influence of temperature may be its effect on the mechanical properties of the sliding parts. High temperatures soften most materials and low temperatures embrittle some. High temperature will produce degradation of most lubricants, but low temperature will solidify a liquid lubricant. Ambient temperature is often easy to measure, but the temperature rise due to sliding may have a larger influence. For a quick view of the factors that influence temperature rise T of asperities on rubbing surfaces, we may reproduce one simple equation: T = fW V 2a(k1 + k2 )J (34.2)
where f = coefficient of friction, W = applied load, V = sliding speed, and k1 and k2 = thermal conductivities of the sliding materials. The quantity a is related to junction size, that is, the few, widely scattered points of contact between sliding parts. From Eq. (34.2) it may seem that thermal conductivity of the materials could be influential in controlling temperature rise in some cases, but a more important factor is f, the coefficient of friction. If a temperature-sensitive wear mechanism is operative in a particular case, then high friction may contribute to a high wear rate, if not cause it. There is at least a quantitative connection between wear rate and the coefficient of friction when one compares dry sliding with adequately lubricated sliding, but there is no formal way to connect the coefficient of friction with the temperature rise. Sliding Speed. Both the sliding speed and the PV limits are involved in determining the sliding severity. Maximum allowable loads and sliding speeds for materials 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). Plastics to be used in sleeve bearings and bronze bushings are the most common material to have PV limits assigned to them. A common range of PV limits for plastics is from 500 to 10 000, and these data are usually taken from simple laboratory test devices. The quantity P is calculated from W/A, where W = applied load and A = projected load-carrying area between sliding members. Thus PV could be written as WV/A. Returning to Eq. (34.2) for the temperature rise, it may be seen that the product WV influences T directly, and it would seem that a PV limit might essentially be a limit on surface-temperature rise. This is approximately true, but not useful. That is, wear resistance of materials cannot be related in a simple way to the melting point or softening temperature of materials. The wide ranges of f, k, and other properties of materials prevent formulating a general rule on the relationship between PV limits and melting temperature. Indeed, a PV limit indicates nothing about the actual rate of wear of materials; it indicates only that
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