上海交通大学：《Design and Manufacturing》课程教学资源（参考文献）Shigley's Mechanical Engineering Design ch12 journal bearing.pdf

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 journal, 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 lubrication 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 fundamental engineering studies, such as statics, dynamics, the mechanics of solids, metal processing, mathematics, and metallurgy. In the study of lubrication and journal bearings, additional fundamental studies, such as chemistry, fluid mechanics, thermodynamics, 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 appreciate 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 supply 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 lubrication is also called full-film, or fluid, lubrication

Budynas-Nisbett:Shigley's Ill.Design of Mechanical 12.Lubrication and Journal T©The McGraw-Hill 59 Mechanical Engineering Elements Bearings Companies,2008 Design,Eighth Edition Lubrication and Journal Bearings 599 Hydrostatic lubrication is obtained by introducing the lubricant,which is some- times air or water,into the load-bearing area at a pressure high enough to separate the surfaces with a relatively thick film of lubricant.So,unlike hydrodynamic lubrication, this kind of lubrication does not require motion of one surface relative to another.We shall not deal with hydrostatic lubrication in this book,but the subject should be con- sidered in designing bearings where the velocities are small or zero and where the frictional resistance is to be an absolute minimum. Elastohydrodynamic lubrication is the phenomenon that occurs when a lubricant is introduced between surfaces that are in rolling contact,such as mating gears or rolling bearings.The mathematical explanation requires the Hertzian theory of contact stress and fluid mechanics. Insufficient surface area,a drop in the velocity of the moving surface,a lessening in the quantity of lubricant delivered to a bearing,an increase in the bearing load,or an increase in lubricant temperature resulting in a decrease in viscosity-any one of these-may prevent the buildup of a film thick enough for full-film lubrication.When this happens,the highest asperities may be separated by lubricant films only several molecular dimensions in thickness.This is called boundary lubrication.The change from hydrodynamic to boundary lubrication is not at all a sudden or abrupt one.It is probable that a mixed hydrodynamic-and boundary-type lubrication occurs first,and as the surfaces move closer together,the boundary-type lubrication becomes predominant. The viscosity of the lubricant is not of as much importance with boundary lubrication as is the chemical composition. When bearings must be operated at extreme temperatures,a solid-film lubricant such as graphite or molybdenum disulfide must be used because the ordinary mineral oils are not satisfactory.Much research is currently being carried out in an effort,too, to find composite bearing materials with low wear rates as well as small frictional coefficients. 12-2 Viscosity In Fig.12-1 let a plate A be moving with a velocity U on a film of lubricant of thickness h. We imagine the film as composed of a series of horizontal layers and the force F causing these layers to deform or slide on one another just like a deck of cards.The layers in con- tact with the moving plate are assumed to have a velocity U;those in contact with the stationary surface are assumed to have a zero velocity.Intermediate layers have velocities that depend upon their distances y from the stationary surface.Newton's viscous effect states that the shear stress in the fluid is proportional to the rate of change of velocity with respect to y.Thus F du T A=H (12-11 dy I Figure 12-1

Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 12. Lubrication and Journal Bearings © The McGraw−Hill 599 Companies, 2008 Lubrication and Journal Bearings 599 Figure 12–1 F u h y U A Hydrostatic lubrication is obtained by introducing the lubricant, which is sometimes air or water, into the load-bearing area at a pressure high enough to separate the surfaces with a relatively thick film of lubricant. So, unlike hydrodynamic lubrication, this kind of lubrication does not require motion of one surface relative to another. We shall not deal with hydrostatic lubrication in this book, but the subject should be considered in designing bearings where the velocities are small or zero and where the frictional resistance is to be an absolute minimum. Elastohydrodynamic lubrication is the phenomenon that occurs when a lubricant is introduced between surfaces that are in rolling contact, such as mating gears or rolling bearings. The mathematical explanation requires the Hertzian theory of contact stress and fluid mechanics. Insufficient surface area, a drop in the velocity of the moving surface, a lessening in the quantity of lubricant delivered to a bearing, an increase in the bearing load, or an increase in lubricant temperature resulting in a decrease in viscosity—any one of these—may prevent the buildup of a film thick enough for full-film lubrication. When this happens, the highest asperities may be separated by lubricant films only several molecular dimensions in thickness. This is called boundary lubrication. The change from hydrodynamic to boundary lubrication is not at all a sudden or abrupt one. It is probable that a mixed hydrodynamic- and boundary-type lubrication occurs first, and as the surfaces move closer together, the boundary-type lubrication becomes predominant. The viscosity of the lubricant is not of as much importance with boundary lubrication as is the chemical composition. When bearings must be operated at extreme temperatures, a solid-film lubricant such as graphite or molybdenum disulfide must be used because the ordinary mineral oils are not satisfactory. Much research is currently being carried out in an effort, too, to find composite bearing materials with low wear rates as well as small frictional coefficients. 12–2 Viscosity In Fig. 12–1 let a plate A be moving with a velocity U on a film of lubricant of thickness h. We imagine the film as composed of a series of horizontal layers and the force F causing these layers to deform or slide on one another just like a deck of cards. The layers in contact with the moving plate are assumed to have a velocity U; those in contact with the stationary surface are assumed to have a zero velocity. Intermediate layers have velocities that depend upon their distances y from the stationary surface. Newton’s viscous effect states that the shear stress in the fluid is proportional to the rate of change of velocity with respect to y. Thus τ = F A = μ du dy (12–1)

00 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 600 Mechanical Engineering Design where u is the constant of proportionality and defines absolute viscosity,also called dynamic viscosity.The derivative du/dy is the rate of change of velocity with distance and may be called the rate of shear,or the velocity gradient.The viscosity u is thus a measure of the internal frictional resistance of the fluid.For most lubricating fluids,the rate of shear is constant,and du/dy =U/h.Thus,from Eq.(12-1), 【=A= (12-2 Fluids exhibiting this characteristic are said to be Newtonian fluids.The unit of vis- cosity in the ips system is seen to be the pound-force-second per square inch;this is the same as stress or pressure multiplied by time.The ips unit is called the reyn,in honor of Sir Osborne Reynolds. The absolute viscosity is measured by the pascal-second(Pa.s)in SI;this is the same as a Newton-second per square meter.The conversion from ips units to SI is the same as for stress.For example,multiply the absolute viscosity in reyns by 6890 to convert to units of Pa.s. The American Society of Mechanical Engineers (ASME)has published a list of cgs units that are not to be used in ASME documents.'This list results from a recom- mendation by the International Committee of Weights and Measures(CIPM)that the use of cgs units with special names be discouraged.Included in this list is a unit of force called the dyne (dyn),a unit of dynamic viscosity called the poise (P),and a unit of kinematic viscosity called the stoke (St).All of these units have been,and still are,used extensively in lubrication studies. The poise is the cgs unit of dynamic or absolute viscosity,and its unit is the dyne- second per square centimeter (dyn.s/cm-).It has been customary to use the centipoise (cP)in analysis,because its value is more convenient.When the viscosity is expressed in centipoises,it is designated by Z.The conversion from cgs units to SI and ips units is as follows: u(Pa·s)=(10)-3Z(cP) Z(cP) u(reyn)= 6.8910)6 u(mPa.s)=6.89 u'(ureyn) In using ips units,the microreyn (ureyn)is often more convenient.The symbol u'will be used to designate viscosity in ureyn such that u=u/(10). The ASTM standard method for determining viscosity uses an instrument called the Saybolt Universal Viscosimeter.The method consists of measuring the time in seconds for 60 mL of lubricant at a specified temperature to run through a tube 17.6 mm in diameter and 12.25 mm long.The result is called the kinematic viscosity,and in the past the unit of the square centimeter per second has been used.One square centimeter per sec- ond is defined as a stoke.By the use of the Hagen-Poiseuille law,the kinematic viscosity based upon seconds Saybolt,also called Saybolt Universal viscosity (SUV)in seconds,is Z=(0.22- 180 (12-3) where Zk is in centistokes(cSt)and t is the number of seconds Saybolt. ASME Orientation and Guide for Use of Metric Units,2nd ed.,American Society of Mechanical Engineers. 1972.p.13

Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 12. Lubrication and Journal Bearings 600 © The McGraw−Hill Companies, 2008 600 Mechanical Engineering Design where μ is the constant of proportionality and defines absolute viscosity, also called dynamic viscosity. The derivative du/dy is the rate of change of velocity with distance and may be called the rate of shear, or the velocity gradient. The viscosity μ is thus a measure of the internal frictional resistance of the fluid. For most lubricating fluids, the rate of shear is constant, and du/dy = U/h. Thus, from Eq. (12–1), τ = F A = μ U h (12–2) Fluids exhibiting this characteristic are said to be Newtonian fluids. The unit of viscosity in the ips system is seen to be the pound-force-second per square inch; this is the same as stress or pressure multiplied by time. The ips unit is called the reyn, in honor of Sir Osborne Reynolds. The absolute viscosity is measured by the pascal-second (Pa · s) in SI; this is the same as a Newton-second per square meter. The conversion from ips units to SI is the same as for stress. For example, multiply the absolute viscosity in reyns by 6890 to convert to units of Pa · s. The American Society of Mechanical Engineers (ASME) has published a list of cgs units that are not to be used in ASME documents.1 This list results from a recommendation by the International Committee of Weights and Measures (CIPM) that the use of cgs units with special names be discouraged. Included in this list is a unit of force called the dyne (dyn), a unit of dynamic viscosity called the poise (P), and a unit of kinematic viscosity called the stoke (St). All of these units have been, and still are, used extensively in lubrication studies. The poise is the cgs unit of dynamic or absolute viscosity, and its unit is the dynesecond per square centimeter (dyn · s/cm2). It has been customary to use the centipoise (cP) in analysis, because its value is more convenient. When the viscosity is expressed in centipoises, it is designated by Z. The conversion from cgs units to SI and ips units is as follows: μ(Pa · s) = (10) −3Z (cP) μ(reyn) = Z (cP) 6.89(10)6 μ(mPa · s) = 6.89 μ (μreyn) In using ips units, the microreyn (μreyn) is often more convenient. The symbol μ will be used to designate viscosity in μreyn such that μ = μ /(106). The ASTM standard method for determining viscosity uses an instrument called the Saybolt Universal Viscosimeter. The method consists of measuring the time in seconds for 60 mL of lubricant at a specified temperature to run through a tube 17.6 mm in diameter and 12.25 mm long. The result is called the kinematic viscosity, and in the past the unit of the square centimeter per second has been used. One square centimeter per second is defined as a stoke. By the use of the Hagen-Poiseuille law, the kinematic viscosity based upon seconds Saybolt, also called Saybolt Universal viscosity (SUV) in seconds, is Zk = 0.22t − 180 t (12–3) where Zk is in centistokes (cSt) and t is the number of seconds Saybolt. 1 ASME Orientation and Guide for Use of Metric Units, 2nd ed., American Society of Mechanical Engineers, 1972, p. 13.