30079_21c

30079_21c - Fig. 21.50 Chart for determining optimal film...

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Fig. 21.50 Chart for determining optimal film thickness. (From Ref. 28.) (a) Grooved member rotating, (b) Smooth member rotating. 6. Calculate R 1 = { AcP 0 } 112 h r [3T 7 K - Co 0 )[I - (K 2 /*,) 2 ]] If R 1 Ih,. > 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 a r and A c should be, obtain values of K 00 , Q, and T from Figs. 21.62, 21.63, and 21.64, respectively. From Eqs. (21.29), (21.30), and (21.31) calculate K pt Q, and T r . 8. From Fig. 21.65 obtain groove geometry (b, /3 a , and H 0 ) and from Fig. 21.66 obtain R g . 21.3 ELASTOHYDRODYNAMICLUBRICATION Downson 31 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 pro- cess." Elastohydrodynamic lubrication implies complete fluid-film lubrication and no asperity inter- action 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
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Fig. 21.51 Chart for determining optimal groove width ratio. (From Ref. 28.) (a) Grooved mem- ber rotating, (b) Smooth member rotating. effects are equally as important. Engineering applications in which this form of lubrication is dom- inant 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. The recognition and understanding of elastohydrodynamic lubrication presents one of the major developments in the field of tribology in this century. The revelation of a previously unsuspected regime of lubrication is clearly an event of importance in tribology. Elastohydrodynamic lubrication not only explained the remarkable physical action responsible for the effective lubrication of many machine elements, but it also brought order to the understanding of the complete spectrum of lubri- cation regimes, ranging from boundary to hydrodynamic. A way of coming to an understanding of elastohydrodynamic lubrication is to compare it to hydrodynamic lubrication. The major developments that have led to our present understanding of hydrodynamic lubrication 13 predate the major developments of elastohydrodynamic lubrication 32 ' 33
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Fig. 21.52 Chart for determining optimal groove length ratio. (From Ref. 28.) (a) Grooved mem- ber rotating, (b) Smooth member rotating.
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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_21c - Fig. 21.50 Chart for determining optimal film...

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