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LUBRICATION OF MACHINE ELEMENTS P3

Fig. 21.50 Chart for determining optimal film thickness. (From Ref. 28.) (a) Grooved member rotating, (b) Smooth member rotating. 6. Calculate R1 = { AcP0 }112 hr [3T7K - Co0)[I - (K2/*,)2]] If R1Ih,. 10,000 (or whatever preassigned radius-to-clearance ratio), a larger bearing or higher speed is required. Return to step 2. If these changes cannot be made, an externally pressurized bearing must be used. 7. Having established what ar and Ac should be, obtain values of K00, Q, and T from Figs. 21.62, 21.63, and 21.64, respectively. From Eqs. (21.29), (21.30), and (21.31) calculate Kpt Q, and Tr. 8 | 556 LUBRICATION OF MACHINE ELEMENTS Fig. 21.50 Chart for determining optimal film thickness. From Ref. 28. a Grooved member rotating b Smooth member rotating. 6. Calculate i r2 hr 3 - û J l - R2 R 2 J If RJhr 10 000 or whatever preassigned radius-to-clearance ratio a larger bearing or higher speed is required. Return to step 2. If these changes cannot be made an externally pressurized bearing must be used. 7. Having established what ar and Ac should be obtain values of K . Q and T from Figs. 21.62 21.63 and 21.64 respectively. From Eqs. 21.29 21.30 and 21.31 calculate Kp Q and Tr- 8. From Fig. 21.65 obtain groove geometry b 3a and Ha and from Fig. 21.66 obtain Rg. 21.3 ELASTOHYDRODYNAMIC LUBRICATION Downson31 defines elastohydrodynamic lubrication EHL as the study of situations in which elastic deformation of the surrounding solids plays a significant role in the hydrodynamic lubrication process. Elastohydrodynamic lubrication implies complete fluid-film lubrication and no asperity interaction of the surfaces. There are two distinct forms of elastohydrodynamic lubrication. 1. Hard EHL. Hard EHL relates to materials of high elastic modulus such as metals. In this form of lubrication not only are the elastic deformation effects important but the pressure-viscosity 21.3 ELASTOHYDRODYNAMIC LUBRICATION 557 Fig. 21.51 Chart for determining optimal groove width ratio. From Ref. 28. a Grooved member rotating b Smooth member rotating. effects are equally as important. Engineering applications in which this form of lubrication is dominant include gears and rolling-element bearings. 2. Soft EHL. Soft EHL relates to materials of low elastic modulus such as rubber. For these materials that elastic distortions are large even with light loads. Another feature is the negligible pressure-viscosity effect on the lubricating film. Engineering applications in which soft EHL is important include seals human joints tires and a number of lubricated elastomeric material machine elements.