598 Budynas-Nisbett:Shigley's Ill.Design of Mechanical 12.Lubrication and Journal T©The McGraw-Hill Mechanical Engineering Elements Bearings Companies,2008 Design,Eighth Edition 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 of most of your previous studies and a directing of this total background toward the resolution of a single objective. 12-1 Types of Lubrication Five distinct forms of lubrication may be identified: 1 Hydrodynamic Hydrostatic Elastohydrodynamic Boundary 5 Solid film Hydrodynamic lubrication means that the load-carrying surfaces of the bearing are separated by a relatively thick film of lubricant,so as to prevent metal-to-metal contact, and that the stability thus obtained can be explained by the laws of fluid mechanics. Hydrodynamic lubrication does not depend upon the introduction of the lubricant under pressure,though that may occur;but it does require the existence of an adequate sup- ply at all times.The film pressure is created by the moving surface itself pulling the lubricant into a wedge-shaped zone at a velocity sufficiently high to create the pressure necessary to separate the surfaces against the load on the bearing.Hydrodynamic lubri- cation is also called full-film,or fluid.lubrication.Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 12. Lubrication and Journal Bearings 598 © The McGraw−Hill Companies, 2008 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 of most of your previous studies and a directing of this total background toward the resolution of a single objective. 12–1 Types of Lubrication Five distinct forms of lubrication may be identified: 1 Hydrodynamic 2 Hydrostatic 3 Elastohydrodynamic 4 Boundary 5 Solid film Hydrodynamic lubrication means that the load-carrying surfaces of the bearing are separated by a relatively thick film of lubricant, so as to prevent metal-to-metal contact, and that the stability thus obtained can be explained by the laws of fluid mechanics. Hydrodynamic lubrication does not depend upon the introduction of the lubricant under pressure, though that may occur; but it does require the existence of an adequate sup￾ply at all times. The film pressure is created by the moving surface itself pulling the lubricant into a wedge-shaped zone at a velocity sufficiently high to create the pressure necessary to separate the surfaces against the load on the bearing. Hydrodynamic lubri￾cation is also called full-film, or fluid, lubrication