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30079_21b - Table 21.4 Pressure-Viscosity Coefficients for...

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2 f f p(x,y) dx.dy, ^>° ^JI [(,-W-W" (2U1) where /1 - v\ 1 - I/A' 1 E' = 2 ——+ —— (21.12) V ^ ^b I and v = Poisson's ratio E = modulus of elasticity, N/m 2 Therefore, Eq. (21.6) is normally involved in hydrodynamic lubrication situations, while Eqs. (21.7)-(21.11) are normally involved in elastohydrodynamic lubrication situations. 21.2 HYDRODYNAMIC AND HYDROSTATIC LUBRICATION Surfaces lubricated hydrodynamically are normally conformal as pointed out in Section 21.1.1. The conformal nature of the surfaces can take its form either as a thrust bearing or as a journal bearing, both of which will be considered in this section. Three features must exist for hydrodynamic lubri- cation to occur: 1. A viscous fluid must separate the lubricated surfaces. 2. There must be relative motion between the surfaces. 3. The geometry of the film shape must be larger in the inlet than at the outlet so that a convergent wedge of lubricant is formed. If feature 2 is absent, lubrication can still be achieved by establishing relative motion between the fluid and the surfaces through external pressurization. This is discussed further in Section 21.2.3. In hydrodynamic lubrication the entire friction arises from the shearing of the lubricant film so that it is determined by the viscosity of the oil: the thinner (or less viscous) the oil, the lower the friction. The great advantages of hydrodynamic lubrication are that the friction can be very low (IJL =* 0.001) and, in the ideal case, there is no wear of the moving parts. The main problems in hydrodynamic lubrication are associated with starting or stopping since the oil film thickness theo- retically is zero when the speed is zero. The emphasis in this section is on hydrodynamic and hydrostatic lubrication. This section is not intended to be all inclusive but rather to typify the situations existing in hydrodynamic and hydrostatic lubrication. For additional information the reader is recommended to investigate Gross et al., 19 Reiger, 20 Pinkus and Sternlicht, 21 and Rippel. 22 Table 21.4 Pressure-Viscosity Coefficients for Test Fluids at Three Temperatures (From Ref. 17) Test Fluid Advanced ester Formulated advanced ester Polyalkyl aromatic Polyalkyl aromatic + 10 wt % heavy resin Synthetic paraffinic oil (lot 3) Synthetic paraffinic oil (lot 4) Synthetic paraffinic oil (lot 4) + antiwear additive Synthetic paraffinic oil (lot 2) + antiwear additive C-ether Superrefined naphthenic mineral oil Synthetic hydrocarbon (traction fluid) Fluorinated polyether Temperature, 0 C 38 99 149 Pressure-viscosity Coefficient, f, m 2 /N 1.28 X 10~ 8 0.987 X 10~ 8 0.851 X IO" 8 1.37 1.00 .874 1.58 1.25 1.01 1.70 1.28 1.06 1.77 1.51 1.09 1.99 1.51 1.29 1.96 1.55 1.25 1.81 1.37 1.13 1.80 .980 .795 2.51 1.54 1.27 3.12 1.71 .939 4.17 3.24 3.02
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21.2.1 Liquid-Lubricated Hydrodynamic Journal Bearings Journal bearings, as shown in Fig. 21.8, are used to support shafts and to carry radial loads with minimum power loss and minimum wear. The bearing can be represented by a plain cylindrical bush wrapped around the shaft, but practical bearings can adopt a variety of forms. The lubricant is supplied at some convenient point through a hole or a groove. If the bearing extends around the full 360° of the shaft, the
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