Chapter05-Sliding-Bearings-机械零件设计英文全套教案课件.ppt
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- Chapter05 Sliding Bearings 机械零件 设计 英文 全套 教案 课件
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1、 5.1 Types of Bearings 5.2 Uses and Characteristics of Sliding Bearings 5.3 Potential Failure ModesChapter 5 Sliding Bearings Fig.5.1 Various types of sliding bearings reciprocating,rotating or oscillating cylindrical members sliding in annular sleeves,disks sliding on mating disks.Candidate materia
2、l should have:adequate strength,low elastic modulus,good ductility and low hardness,high thermal conductivity,and compatible thermal expansion coefficientsMaterials typically used:bronze bearing alloys,babbitt metal,sintered porous metals,and self-lubricating nonmetallic materials(teflon,nylon,aceta
3、l,phenolic,or polycarbonate,thin silver layer plated on a higher-strength substrate,fluted rubber or other elastomers5.4 Sliding Bearing Materials 5.5 Boundary-Lubricated Bearing Design P=W/(dL)MPa P=load per unit of projected bearing area,V=surface velocity of journal relative to bearing surface,m/
4、s TA=ambient air temperature,TB=bearing bore temperature,fM=coefficient of mixed-film friction()BAMk TTPVf1.Calculate the journal(shaft)diameter d 2.Find the resultant radial bearing load W 3.Select tentative materials for journal and sleeve4.Calculate sliding velocity V,and compare to Vmax from Tab
5、le 5.1.5.For selected materials find limiting value(PV)max from Table 5.16.Using results of steps 4 and 5,calculate P and compare to Pmax 7.Using the result of step 6,and known values of W and d,calculate L8.Check if length/diame-ter ratios of the bearing configurations lie in the range122LdPrelimin
6、ary Design Procedure for Boundary-Lubricated Sliding Bearing 5.6 Hydrodynamic Bearing Design5.6.1 Lubricant PropertiesAbsolute viscosity is a measure of the internal shear resistance of a fluid.The derivative du/dy is the rate of shear,or the velocity gradint.In SI the units of are Pas.FduUAdyhAnoth
7、er unit of dynamic viscosity poise(P)is the cgs unit of viscosity and is in dyne-seconds per square centimeter(dyns/cm2).When the viscosity is expressed in centipoises,it is designated by Z.The conversion from cgs units to SI units is 1 Pas=(10)-3 Z(cP)=10 P=103 cP kinematic viscosity To convert dyn
8、amic viscosity to kinematic viscosity:v=/One square centimeter per second is defined as a stoke(St).In SI the kinematic viscosity v has the unit of square meter per second(m2/s)and the conversion is v(m2/s)=10-6Zk(cSt)or 1 m2/s=104 St=106 cSt Viscosities as a function of temperature for several SAE
9、numbered oils5.6.2 Petroffs Friction Analysis2AdLrL2 L UrFc 12FcUrDefined F1=F/L the tangential friction force per unit bearing length Assumptions used by Reynolds:1.The lubricant is a Newtonian fluid2.Inertia forces produced by the moving fluid are negligible3.The lubricant is an incompressible flu
10、id4.The lubricant viscosity is constant throughout the fluid film5.There is no pressure variation in the axial direction6.There is no lubricant flow in the axial direction 7.The film pressure is constant in the y-direction8.Lubricant particle velocity is a function of x and y only5.6.3 Hydrodynamic
11、Lubrication Theory -Reynolds EquationSchematic sketch showing relative position change of shaft in the sleeve,starting from rest and increasing to steady-state rotating speeddpdxy Reynolds Equation36dh dpdhUdxdxdx 336hphphUxxzzx 2rrNfccP One-dimensional Two-dimensional Sommerfeld solution The viscos
12、ity The load per unit of projected bearing area,PThe speed NThe bearing dimensions d,c,and L The coefficient of friction fThe temperature rise The flow of oil QThe minimum film thickness h05.6.4 Design considerations in hydrodynamic journal bearings Two groups of variables in the design To choose th
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