Chapter05-Sliding-Bearings-机械零件设计英文PPT全套教案课件

5.1 Types of Bearings 5.2 Uses and Characteristics of Sliding Bearings 5.3 Potential Failure ModesChapter 5 Sliding Bearings 5.1 Types of BearingsC1Fig.5.1 Various types of sliding bearings reciprocating,rotating or oscillating cylindrical members sliding in annular sleeves,disks sliding on mating disks.Fig.5.1 Various types of slidi2Chapter05-Sliding-Bearings-机械零件设计英文PPT全套教案课件3Chapter05-Sliding-Bearings-机械零件设计英文PPT全套教案课件4Chapter05-Sliding-Bearings-机械零件设计英文PPT全套教案课件5Candidate material 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 selflubricating nonmetallic materials(teflon,nylon,acetal,phenolic,or polycarbonate,thin silver layer plated on a higherstrength substrate,fluted rubber or other elastomers5.4 Sliding Bearing MaterialsCandidate material should have6 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/s TA=ambient air temperature,TB=bearing bore temperature,fM=coefficient of mixedfilm friction 5.5 Boundary-Lubricated 71.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 Table 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/diameter ratios of the bearing configurations lie in the rangePreliminary Design Procedure for BoundaryLubricated Sliding Bearing 1.Calculate the journal(shaf85.6 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.5.6 Hydrodynamic Bearing Desi9Another unit of dynamic viscosity poise(P)is the cgs unit of viscosity and is in dyneseconds 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 dynamic 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)=106Zk(cSt)or 1 m2/s=104 St=106 cStAnother unit of dynamic viscos10 Viscosities as a function of temperature for several SAE numbered oils Viscosities as a function of115.6.2 Petroffs Friction AnalysisDefined F1=F/L the tangential friction force per unit bearing length 5.6.2 Petroffs Friction Analy12Assumptions used by Reynolds:1.The lubricant is a Newtonian fluid2.Inertia forces produced by the moving fluid are negligible3.The lubricant is an incompressible fluid4.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 ydirection8.Lubricant particle velocity is a function of x and y only5.6.3 Hydrodynamic Lubrication Theory Reynolds EquationAssumptions used by Reynolds:13Schematic sketch showing relative position change of shaft in the sleeve,starting from rest and increasing to steadystate rotating speedSchematic sketch showing rel14 Reynolds Equation Onedimensional Twodimensional Sommerfeld solution Reynolds Equation One-d15 The viscosity 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 The viscosity 5.6.4 D16 To choose the bearing diameter and length,specify the surface roughness and clearance requirements between journal and sleeve,and determine acceptable lubricant properties and flow rates that will assure support of specified design loads and minimize frictional drag.The final design specifications must also produce an acceptable design life without premature failure,while meeting cost,weight,and space envelope requirements.Basic objectives of the design To choose the bearing diame17 5.6.5 Bearing PerformanceA design example for full journal bearings(=360)Bearing Characteristic NumberTemperature risedimensionless temperaturerise 5.6.5 Bearing Performance-A 18Example:For a full journal bearing:SAE 30 lubricant at T1=60C N=30 rev/s W=2.5 kNr=20 mmc=0.032 mmL=40mm The unit load is kPa Make a wild guess at the average temperature,say 70C.Entering Fig.5.3 to the SAE 30 line,we find:=20 mPa s.From Eq.(514)for SExample:For a full journal be19Now enter Fig.5.6 with this value of S.The temperature rise variable is found to be T(var)=16.Thus the temperature rise is Renter Fig.5.3=18 mPasFollowing the same procedure,we get S=0.135,T(var)=15.5,=18 mPas=16C Tav=68CThen S=0.135=16C,and Tav=68CNow enter Fig.5.6 with this v20 Minimum Film ThicknessSince L/d=40/40=1From Fig.5.7 with S=0.135 and L/d=1,we get and Since c=0.032 mm,thenh0=0.42(0.032)=0.0134 mm Minimum Film ThicknessSince L21Relationship between minimum film thickness and eccentricity ratioThe eccentricity ratio=e/c=0.58,Then,the eccentricity ise=0.58(0.032)=0.0186 mmSince h0=c-e,then if,e=0,h0=c;when h0=0,e=c;Relationship between minimum f22Determination of the position of the minimum film thicknessWe can find the angular location of the minimum film thickness from the chart of Fig.5.9.=53.Entering with S=0.135 and L/d=1,we findDetermination of the position 23 Coefficient of FrictionBy entering Fig.5.10 with S=0.135 and L/d=1,we can find the friction variable to be Therefore the coefficient of friction isThe torque required to overcome friction isT=fWr=0.0056(2.5)(20)=0.28NmThe power lost in the bearing isW Coefficient of Fricti24 Lubricant FlowUsing the same data as before,by entering Fig.5.11 with S=0.135 and l/d=1,we findTherefore,the total flow isQ=4.28rcNl=4.28(20)(0.032)(30)(40)=3287 mm3/s Lubricant FlowUsing the sa25Side Leakage FlowFrom Fig.5.12 we find the flow ratio to be Qs/Q=0.655.Qs=0.655Q=0.655(3287)=2153 mm3/sTherefore the side leakage is Side Leakage FlowFrom Fig.5.1265.6.6 Optimization Techniques(Design considerations)Select:grade of oil to be used,together with suitable values for P,N,r,c,and L.Bear in mind:the radial clearance c is difficult to hold accurate during manufacture,and it may increase because of wear.What is the effect of an entire range of radial clearances,expected in manufacture,and what will happen to the bearing performance if c increases because of wear?Answerby plotting curves of the performance as functions of the quantities over which the designer has control5.6.6 Optimization Techniques(27If the clearance is too tight,the temperature will be too high and the minimum film thickness too low.High temperatures may cause the bearing to fail by fatigue.If the oil film is too thin,dirt particles may not pass without scoring or may embed themselves in the bearing.In either event,there will be excessive wear and friction,resulting in high temperatures and possible seizing.If the clearance is too tight,28A large clearance will permit the dirt to pass through and also will permit a large flow of oil.This lowers the temperature and increases the life of the bearing.However,if the clearance becomes too large,the bearing becomes noisy and the minimum film thickness begins to decrease again.Considering the production tolerance and the future wear,the best compromise is a clearance range slightly to the left of the top of the minimumfilmthickness curve.In this way,future operating point will move to the right and increase the film thickness and decrease the operating temperature.A large clearance will permit 29The end of Chapter 5The end of Chapter 530