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Mechanical Engineer´s Handbook P30

(Hmin)0 = ^^ = 3.63£/°-68G°-49W-°-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 - 0.876 x 10~4 Thus (hmin)0 = 0.876 X 10-4 R^ = 0.665 Aim In this case, the lubrication factor A is given by A (21.156) ° = [(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. | 616 LUBRICATION OF MACHINE ELEMENTS Hmin 3.63U -68G -49W- 073 1 - e-06 3.63 X 2.087 x IO 7 x 65.29 X 1.785 X 0.9919 21.156 0.876 X 10 4 Thus min o 0.876 X IO-4 0.665 jum In this case the lubrication factor A is given by 0.665 x 10-6 0.175 2 0.0625 2 1 2 X 10-6 k 7 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 lubrication 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 BOUNDARY LUBRICATION 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 Tabor56 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 .