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
13、e 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 pro
14、duce an acceptable design life without premature failure,while meeting cost,weight,and space envelope requirements.Basic objectives of the design 5.6.5 Bearing Performance-A design example for full journal bearings(=360)Bearing Characteristic Number2rNScPTemperature rise12avTTTdimensionless temperat
15、ure-rise(var)HCTTP Example: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 62.5(10)156022(20)(40)WPrlMake 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.(5-14)for S223
16、32020(10)(30)0.1500.0321560(10)rNScPNow enter Fig.5.6 with this value of S.The temperature rise variable is found to be T(var)=16.Thus the temperature rise is31560(10)161616.5 C861(1760)HPTC116.56068.2 C22avTTT Renter Fig.5.3=18 mPasFollowing the same procedure,we get S=0.135,T(var)=15.5,T=18 mPasT=
17、16C Tav=68CThen S=0.135=16C,and Tav=68C Minimum Film ThicknessSince L/d=40/40=1From Fig.5.7 with S=0.135 and L/d=1,we get00.42hc and 0.58 Since c=0.032 mm,thenh0=0.42(0.032)=0.0134 mmRelationship between minimum film thickness and eccentricity ratioThe eccentricity ratio=e/c=0.58,Then,the eccentrici
18、ty ise=0.58(0.032)=0.0186 mmSince h0=c-e,then if,e=0,h0=c;when h0=0,e=c;01hc Determination 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 find Coefficient of FrictionBy enter
19、ing Fig.5.10 with S=0.135 and L/d=1,we can find the friction variable to be 3.50rfcTherefore the coefficient of friction is0.0323.503.500.005620cfrThe torque required to overcome friction isT=fWr=0.0056(2.5)(20)=0.28NmThe power lost in the bearing is22(0.28)(30)52.8HTNW Lubricant FlowUsing the same
20、data as before,by entering Fig.5.11 with S=0.135 and l/d=1,we find4.28QrcNlTherefore,the total flow isQ=4.28rcNl=4.28(20)(0.032)(30)(40)=3287 mm3/sSide 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 5.6.6 Optimization T
21、echniques(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
22、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 controlIf the clearance is too tight,the temperature will be too high and the minimum film thickness to
23、o 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.A large cleara
24、nce 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 minimum-film-thickness curve.In this way,future operating point will move to the right and increase the film thickness and decrease the operating temperature.The end of Chapter 5
