30079_21d - (Hmin)0 = ^ = 3.63/-68G-49W-073 (1 - e-6*k) R...

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min ) 0 = ^^ = 3.63£/°- 68 G°- 49 W-°- 073 (1 - e-°- 6 * k °) R X ,o = 3.63 X 2.087 X IQ- 7 X 65.29 X 1.785 X 0.9919 (21.156) - 0.876 x 10~ 4 Thus (h min ) 0 = 0.876 X 10- 4 R^ = 0.665 Aim In this case, the lubrication factor A is given by A ° = [(0.175) 2 + (0.0625)*r x 10-' = 3 ' 58 (2U57) Once again, it is evident that the smaller minimum film thickness occurs between the most heavily loaded ball and the inner race. However, in this case the minimum elastohydrodynamic film thickness is about three times the composite surface roughness, and the bearing lubrication can be deemed to be entirely satisfactory. Indeed, it is clear from Fig. 21.97 that very little improvement in the lubri- cation factor F and thus in the fatigue life of the bearing could be achieved by further improving the minimum film thickness and hence A. 21.4 BOUNDARYLUBRICATION If the pressures in fluid-film-lubricated machine elements are too high, the running speeds are too low, or the surface roughness is too great, penetration of the lubricant film will occur. Contact will take place between asperities, leading to a rise in friction and wear rate. Figure 21.99 (obtained from Bowden and Tabor 56 ) shows the behavior of the coefficient of friction in the different lubrication regimes. It is to be noted in this figure that in boundary lubrication, although the friction is much higher than in the hydrodynamic regime, it is still much lower than for unlubricated surfaces. As the running conditions are made more severe, the amount of lubricant breakdown increases, until the system scores or seizes so badly that the machine element can no longer operate successfully. Figure 21.100 shows the wear rate in the different lubrication regimes as determined by the operating load. In the hydrodynamic and elastohydrodynamic lubrication regimes, since there is no asperity contact, there is little or no wear. In the boundary lubrication regime the degree of asperity interaction and wear rate increases as the load increases. The transition from boundary lubrication to an unlubricated condition is marked by a drastic change in wear rate. Machine elements cannot operate successfully in the unlubricated region. Together Figs. 21.99 and 21.100 show that both friction and wear can be greatly decreased by providing a boundary lubricant to unlubricated surfaces. Understanding boundary lubrication depends first on recognizing that bearing surfaces have as- perities that are large compared with molecular dimensions. On the smoothest machined surfaces these asperities may be 25 nm (0.025 /nn) high; on rougher surfaces they may be ten to several hundred times higher. Figure 21.101 illustrates typical surface roughness as a random distribution of Fig. 21.99 Schematic drawing showing how type of lubrication shifts from hydrodynamic to elastohydrodynamic to boundary lubrication as the severity of running conditions is increased. (From Ref. 56.)
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This note was uploaded on 05/02/2010 for the course ME 100 taught by Professor Any during the Spring '10 term at Purdue.

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30079_21d - (Hmin)0 = ^ = 3.63/-68G-49W-073 (1 - e-6*k) R...

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