Ch12 - bud21932_ch12_597-652 09/11/2006 10:28 PM Page 597...

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Lubrication and Journal Bearings Chapter Outline 12–1 Types of Lubrication 598 12–2 Viscosity 599 12–3 Petroff’s Equation 601 12–4 Stable Lubrication 603 12–5 Thick-Film Lubrication 604 12–6 Hydrodynamic Theory 605 12–7 Design Considerations 609 12–8 The Relations of the Variables 611 12–9 Steady-State Conditions in Self-Contained Bearings 625 12–10 Clearance 628 12–11 Pressure-Fed Bearings 630 12–12 Loads and Materials 636 12–13 Bearing Types 638 12–14 Thrust Bearings 639 12–15 Boundary-Lubricated Bearings 640 12 597
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598 Mechanical Engineering Design The object of lubrication is to reduce friction, wear, and heating of machine parts that move relative to each other. A lubricant is any substance that, when inserted between the moving surfaces, accomplishes these purposes. In a sleeve bearing, a shaft, or jour- nal, rotates or oscillates within a sleeve, or bushing, and the relative motion is sliding. In an antifriction bearing, the main relative motion is rolling. A follower may either roll or slide on the cam. Gear teeth mate with each other by a combination of rolling and sliding. Pistons slide within their cylinders. All these applications require lubrication to reduce friction, wear, and heating. The field of application for journal bearings is immense. The crankshaft and connecting-rod bearings of an automotive engine must operate for thousands of miles at high temperatures and under varying load conditions. The journal bearings used in the steam turbines of power-generating stations are said to have reliabilities approaching 100 percent. At the other extreme there are thousands of applications in which the loads are light and the service relatively unimportant; a simple, easily installed bearing is required, using little or no lubrication. In such cases an antifriction bearing might be a poor answer because of the cost, the elaborate enclosures, the close tolerances, the radial space required, the high speeds, or the increased inertial effects. Instead, a nylon bearing requiring no lubrication, a powder-metallurgy bearing with the lubrication “built in,” or a bronze bearing with ring oiling, wick feeding, or solid-lubricant film or grease lubri- cation might be a very satisfactory solution. Recent metallurgy developments in bearing materials, combined with increased knowledge of the lubrication process, now make it possible to design journal bearings with satisfactory lives and very good reliabilities. Much of the material we have studied thus far in this book has been based on fun- damental engineering studies, such as statics, dynamics, the mechanics of solids, metal processing, mathematics, and metallurgy. In the study of lubrication and journal bear- ings, additional fundamental studies, such as chemistry, fluid mechanics, thermody- namics, and heat transfer, must be utilized in developing the material. While we shall not utilize all of them in the material to be included here, you can now begin to appre- ciate better how the study of mechanical engineering design is really an integration
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This note was uploaded on 03/22/2010 for the course MEEN ISEN 302 taught by Professor Kim during the Spring '10 term at Texas A&M University–Commerce.

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Ch12 - bud21932_ch12_597-652 09/11/2006 10:28 PM Page 597...

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