1、Chapter 6 Rolling Bearings 6.1 Characteristics of Rolling Bearings6.2 Types of Rolling Element Bearings6.3 Potential failure modes6.4 Bearing Materials6.5 Bearing Selection6.6 Mounting and Enclosure Example6.1 Characteristics of Rolling Element BearingsBearing specialists must consider such matters
2、as fatigue loading,friction,heat,corrosion resistance,kinematic problems,material properties,lubrication,machining tolerances,assembly,use,and cost.Most bearing manufacturers publish engineering manuals containing extensive descriptions of the various types and sizes available,and procedures for mak
3、ing appropriate selections.Designer of machines or bearing users should at least be able to choose the appropriate type and specifications of the bearing they need.Advantages of rolling element bearings High reliability with minimum maintenanceMinimum lubrication required.Lubricant can often be seal
4、ed in for the lifetime of the bearingGood for low-speed operationLow starting friction and low power loss due to frictional dragCan readily support radial,thrust,or combined radial and thrust loadsSmall axial space requiredNearly universal interchange ability among manufacturers due to industry-wide
5、 standardized sizes and closely controlled tolerancesCan be preloaded to eliminate internal clearances,improve fatigue life,or increase bearing stiffness Increase in operational noise level warns of impending failure6.2 Types of Rolling Element BearingRolling element bearings may be broadly classifi
6、ed as either ball bearings or roller bearings.They are manufactured to take pure radial loads,pure thrust loads,or a combination of these two.Rolling element bearings are almost universally standardized.Within each of these broad categories there are a host of geometrical configurations commercially
7、 available.Bearing nomenclature and geometry A variety of commercially available ball bearing configurations Types of roller bearings(a)Straight roller bearings(b)spherical-roller thrust bearing(d)Needle bearings(c)Taped roller thrust bearing(e)Tapered roller bearings Characteristics of taped roller
8、 bearingsTapered roller bearings combine the advantages of ball and straight roller bearings,since they can take either radial or thrust loads or any combination of the two,and in addition,they have the high load-carrying capacity of straight roller bearings.Because of the contacts of tapered roller
9、 and the conically angled raceways,when a purely radial external load is applied to a tapered roller bearing,a thrust(axial)load is induced within the bearing because of the taper.Hence,as for the case of angular contact ball bearings,tapered roller bearings are usually used in pairs to resist the t
10、hrust reaction or to provide desired stiffness.6.3 Potential Failure ModesUnder typical operating conditions,surface fatigue failure is the most likely failure mode.The cyclic subsurface Hertz shear stresses produced by the curved surfaces in rolling contact may initiate and propagate minute cracks
11、that ultimately dislodge particles and generate surface pits.In some cases,static loads on a bearing during idle non-rotating segments of a duty cycle may cause brinelling of the races,resulting in subsequent generation of noise,vibration,and heat as the rolling elements pass the local discontinuiti
12、es in the raceway.Design from a perspective of failure preventionWhen selecting bearings,designers must:Routinely examine both the ability of a bearing to resist surface fatigue failure(basic dynamic load rating)and the ability of a bearing to resist brinelling failure(basic static load rating).6.4
13、Bearing MaterialsVacuum-degassed high-carbon chrome steel,hardened and tempered for optimum strength and toughness,sometimes stabilized for precise dimensional control.Stainless-steel alloys are sometimes used when corrosion resistance is needed or moderately elevated temperatures must be accommodat
14、ed.For higher-temperature applications,cobalt base alloys may be used.GCr15 steel is widely used for both races and balls,and is typically through-hardened to Rockwell C61-C64.Separator materials include phenolic,bronze,phosphor bronze,or alloy steels.In the case of roller bearings,races and rollers
15、 are usually manufactured from low-carbon carburizing-grade,electric furnace,vacuum-degassed alloy steels.They are then case-carburized and heat-treated to produce a hard,fatigue-resistant case encompassing a tough ductile core.Case-hardened 12CrNi3A are often used.In selecting an appropriate bearin
16、g for any given application,design decisions must be made about bearing type,size,space allocation,mounting methods,and other details.Designer experience plays an important role in the selection process.Table 6.1 provides overall guidance in selecting an appropriate bearing type,but to use the table
17、 a designer must first summarize the specific design requirements at the bearing site in terms of radial loading and thrust loading spectrum,operating speeds,design life requirements,stiffness requirements,and operating environment.6.5 Bearing Selection Before detailing bearing selection procedures,
18、it is important to understand the concepts of basic dynamic load rating,Cd,and basic static load rating,Cs,universally used by bearing manufacturers in characterizing the load-carrying capacity of all types of rolling element bearings.Also of interest are the data-based relationship between the load
19、 and number of rotations to failure,and the influence on the selection process of reliability requirements higher than the standard 90 percent.Essentials for bearing selection 1)Basic load ratingsBasic load ratings have been standardized by the bearing industry as a uniform means of describing the a
20、bility of any given rolling element bearing to resist failure by(1)surface fatigue and(2)brinnelling.The basic dynamic load rating,Cd,is a measure of resistance to surface fatigue failure.The basic static load rating,Cs,is a measure of resistance to failure by brinnelling.Basic dynamic radial load r
21、ating Cd(90)It is defined to be the largest constant stationary radial load that 90 percent of a group of apparently identical bearings will survive for 1 million revolutions(inner race rotating,outer race fixed)with no evidence of failure by surface fatigue.The relationship between radial bearing l
22、oad P and bearing life L(revolutions to failure)for any given bearing iswhere a=3 for ball bearings and a=10/3 for roller bearings.610adCLPBasic static radial load rating CsIt is defined to be the largest stationary radial load that will produce significant brinnelling evidence at the most heavily l
23、oaded rolling element contact site.Significant brinnelling evidence is defined to be any plastic deformation di-ameter(or width)greater than 0.0001 of the diameter of the rolling element.2)Reliability specificationsValues of dynamic load rating corresponding to a reliability of R=90 percent,Cd(90),a
24、re routinely published by all bearing manufacturers.While 90 percent reliability is acceptable for a wide variety of industrial applications,higher reliabilities are sometimes desired by a designer.Reliability-adjustment factors,based on actual failure rate data,allow a designer to select bearings f
25、or reliability levels higher than 90 percent.Current practice is to use a reliability life-adjustment factor,KR.Thus the basic bearing rating life,L10=106 revolutions(corresponds to P=10 or R=90),may be adjusted to any chosen higher re-liability R using LP=KRL10 The value of KR is read from Table 6.
26、2.LP is called the reliability-adjusted rating life(revolutions).3)Suggested selection procedure for steady loadsFirst,design the shaft by taking it as a free body,perform a complete force analysis,and employ appropriate shaft design equations to determine a tentative strength-based shaft diameter a
27、t the bearing site.From the force analysis,calculate the radial load Fr and axial thrust load Fa to be supported by the proposed bearing.Determine the design life requirement,Ld,for the bearing.Determine the reliability R appropriate to the application,and select the corresponding life-adjustment fa
28、ctor,KR.Assess the severity of any shock or impact associated with the application so that an impact factor,IF,may be determined(see Table 6.3).Tentatively select the type of bearing to be used(see Table 6.1).Calculate a dynamic equivalent radial load Pe from the following empirical relationship Pe=
29、XdFr+YdFa Refer to Table 6.4 for related parameters.Calculate the basic dynamic load rating requirement corresponding to the reliability level selected in step 4,as Ld=design life,revolutions KR=reliabiliy adjustment factor from Table 6.2 IF=application impact factor from Table 6.3 Pe=equivalent rad
30、ial load a exponent equal to 3 for ball bearings or 10/3 for roller bearings 1/6()()(10)addreqeRLCRIF PKWith the obtained result,enter a basic load rating table for the type of bearing selected in step 6 and tentatively select the smallest bearing with a basic radial load rating Cd of at least Cd(R)
31、req.The ABMA has formulated a plan for standardized boundary dimensions for bearings.For a given bore,an assortment of standard widths and outside diameters are available,giving great flexibility in choosing a bearing geometry suitable to most applications.Bearings are identified by a two-digit numb
32、er called the dimension-series code.The first digit is from the width series(0,1,2,3,4,5,or 6).The second digit is from the(outside)diameter series(8,9,0,1,2,3,or 4).Check to make sure operating speed lies below limiting speed for the tentatively selected bearing.If not,select another bearing that m
33、eets both the dynamic load rating requirement and the limiting speed requirement.Calculate a static equivalent radial load rating Pse from the following empirical relationship Pse=XsFsr+YsFsa Refer to Table 6.4 for related parameters.Check bearing selection tables to make sure that Pse does not exce
34、ed the basic static radial load rating Cs for the bearing tentatively selected.Check to make sure the bore of the tentatively selected bearing will fit over the strength-based shaft diameter.If not,select a larger bearing bore that will just fit over the shaft.Recheck Cd,Cs,and limiting speed for th
35、e newly selected larger bearing.Using the final bearing selection,increase the shaft diameter at the bearing site to the nominal bearing bore size,using proper mounting dimensions and tolerances,as specified by the bearing manufacturer.6.6 Mounting and Enclosure1)MountingMounting methods of antifric
36、tion bearings in each new design test the ingenuity of the designer.The designer,in ferreting out a trouble-free and low-cost mounting,is faced with a difficult and important problem.The various bearing manufacturers handbooks give many mounting details in almost every design area.The most frequentl
37、y encountered mounting problem is that which requires one bearing at each end of a shaft.One of the bearings usually has the added function of positioning or axially locating the shaft.In above Figure,the inner rings are backed up against the shaft shoulders and are held in position by round nuts th
38、readed onto the shaft.The outer ring of the left-hand bearing is backed up against a housing shoulder and is held in position by a device which is not shown.The outer ring of the right-hand bearing floats in the housing.Figure 6.5The inner races are backed up against the shaft shoulders as before bu
39、t no retaining devices are required.The outer races are completely retained.This eliminates the grooves or threads,which cause stress concentration on the over-hanging end,but it requires accurate dimensions in an axial direction or the employment of adjusting means.This method has the disadvantage
40、that,if the distance between the bearings is great,the temperature rise during operation may expand the shaft enough to wreck the bearings.Figure 6.6Variations possible to the methods aboveThe function of the shaft shoulder may be performed by retaining rings,by the hub of a gear or pulley,or by spa
41、cing tubes or rings.The round nuts may be replaced by retaining rings or by washers locked in position by screws,cotters,or taper pins.The housing shoulder may be replaced by a retaining ring;the outer ring of the bearing may be grooved for a retaining ring,or a flanged outer ring may be used.The fo
42、rce against the outer ring of the left-hand bearing is usually applied by the cover plate,but if no thrust is present,the ring may be held in place by retaining ringsFigure 6.7 The retaining methods of the inner race of bearings Figure 6.8 The retaining methods of the outer inner race of bearings It
43、 is frequently necessary to use two or more bearings at one end of a shaft.For example,two bearings could be used to obtain additional rigidity or increased load capacity or to cantilever a shaft.These may be used with tapered roller bearings,as shown,or with ball bearings.In either case it should b
44、e noted that the effect of the mounting is to preload the bearings in an axial direction.When maximum stiffness and resistance to shaft misalignment is desired,pairs of angular-contact ball bearings(Fig.6.2)are often used in an arrangement called duplexing.The face-to-face mounting,called DF,will ta
45、ke heavy radial loads and thrust loads from either direction.The DB mounting(back to back)has the greatest aligning stiffness and is also good for heavy radial loads and thrust loads from either direction.The DT mounting,is used where the thrust is always in the same direction;since the two bearings
46、 have their thrust functions in the same direction,a preload,if required,must be obtained in some other manner.Bearings are usually mounted with the rotating ring a press fit,whether it be the inner or outer ring.The stationary ring is then mounted with a push fit.This permits the stationary ring to
47、 creep in its mounting slightly,bringing new portions of the ring into the load-bearing zone to equalize wear.2)PreloadingThe object of preloading is to remove the internal clearance usually found in bearings,to increase the fatigue life,and to decrease the shaft slope at the bearing.Preloading of s
48、traight roller bearings may be obtained by:(1)Mounting the bearing on a tapered shaft or sleeve to expand the inner ring(2)Using an interference fit for the outer ring(3)Purchasing a bearing with the outer ring preshrunk over the rollersBall bearings are usually preloaded by the axial load built in
49、during assembly.3)AlignmentThe life of the bearing decreases significantly when the misalignment exceeds the allowable limits.The permissible misalignment in cylindrical and tapered roller bearings is limited to 0.001 rad.For spherical ball bearings the misalignment should not exceed 0.0087 rad.But
50、for deep-groove ball bearings,the allowable range of misalignment is 0.0035 to 0.0047 rad.Shaft and housing shoulder diameters should be adequately to assure good bearing support.4)EnclosuresTo exclude dirt and foreign matter and to retain the lubricant,the bearing mountings must include a seal.The
