资源描述
,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 6 Rolling Bearings,6.1 Characteristics of Rolling Bearings,6.2 Types of Rolling Element Bearings,6.3 Potential failure modes,6.4 Bearing Materials,6.5 Bearing Selection,6.6 Mounting and Enclosure,Example,6.1 Characteristics of Rolling Element Bearings,Bearing specialists must consider such matters 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 making 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 maintenance,Minimum lubrication required.Lubricant can often be sealed in for the lifetime of the bearing,Good for low-speed operation,Low starting friction and low power loss due to frictional drag,Can readily support radial,thrust,or combined radial and thrust loads,Small axial space required,Nearly universal interchange ability among manufacturers due to industry-wide standardized sizes and closely controlled tolerances,Can be preloaded to eliminate internal clearances,improve fatigue life,or increase bearing stiffness,Increase in operational noise level warns of impending failure,6.2 Types of Rolling Element Bearing,Rolling element bearings may be broadly classified 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 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 bearings,Tapered 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 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 thrust reaction or to provide desired stiffness,.,6.3 Potential Failure Modes,Under 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 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 discontinuities in the raceway.,Design from a perspective of failure prevention,When 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,Bearing Materials,Vacuum-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 accommodated.,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 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 bearing 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 a designer must first summarize the,specific design requirements,at the bearing site in terms of radial
展开阅读全文