机械制造及其自动化专业毕业设计（论文）外文翻译.doc

1 河河南南农农业业大大学学 本本科科生生毕毕业业论论文文 设设计计 英英文文翻翻译译 学 院 机电工程学院 专业班级 机制 07 级 2 班 学生姓名 王悦茏 指导教师 李慧琴 撰写日期 2010 年 5 月 12 日 2 EXTENDING BEARING LIFE Abstract Nature works hard to destroy bearings but their chances of survival can be improved by following a few simple guidelines Extreme neglect in a bearing leads to overheating and possibly seizure or at worst an explosion But even a failed bearing leaves clues as to what went wrong After a little detective work action can be taken to avoid a repeat performance Keywords bearings failures life Bearings fail for a number of reasons but the most common are misapplication contamination improper lubricant shipping or handling damage and misalignment The problem is often not difficult to diagnose because a failed bearing usually leaves telltale signs about what went wrong However while a postmortem yields good information it is better to avoid the process altogether by specifying the bearing correctly in The first place To do this it is useful to review the manufacturers sizing guidelines and operating characteristics for the selected bearing Equally critical is a study of requirements for noise torque and runout as well as possible exposure to contaminants hostile liquids and temperature extremes This can provide further clues as to whether a bearing is right for a job 1 Why bearings fail About 40 of ball bearing failures are caused by contamination from dust dirt shavings and corrosion Contamination also causes torque and noise problems and is often the result of improper handling or the application environment Fortunately a bearing failure caused by environment or handling contamination is preventable and a simple visual examination can easily identify the cause Conducting a postmortem il1ustrates what to look for on a failed or failing bearing Then understanding the mechanism behind the failure such as brinelling or fatigue helps eliminate the source of the problem Brinelling is one type of bearing failure easily avoided by proper handing and assembly It is characterized by indentations in the bearing raceway caused by shock loading such as when a bearing is dropped or incorrect assembly Brinelling usually occurs when loads exceed the material yield point 350 000 psi in SAE 52100 chrome steel It may also be caused by improper assembly Which places a load across the 3 races Raceway dents also produce noise vibration and increased torque A similar defect is a pattern of elliptical dents caused by balls vibrating between raceways while the bearing is not turning This problem is called false brinelling It occurs on equipment in transit or that vibrates when not in operation In addition debris created by false brinelling acts like an abrasive further contaminating the bearing Unlike brinelling false binelling is often indicated by a reddish color from fretting corrosion in the lubricant False brinelling is prevented by eliminating vibration sources and keeping the bearing well lubricated Isolation pads on the equipment or a separate foundation may be required to reduce environmental vibration Also a light preload on the bearing helps keep the balls and raceway in tight contact Preloading also helps prevent false brinelling during transit Seizures can be caused by a lack of internal clearance improper lubrication or excessive loading Before seizing excessive friction and heat softens the bearing steel Overheated bearings often change color usually to blue black or straw colored Friction also causes stress in the retainer which can break and hasten bearing failure Premature material fatigue is caused by a high load or excessive preload When these conditions are unavoidable bearing life should be carefully calculated so that a maintenance scheme can be worked out Another solution for fighting premature fatigue is changing material When standard bearing materials such as 440C or SAE 52100 do not guarantee sufficient life specialty materials can be recommended In addition when the problem is traced back to excessive loading a higher capacity bearing or different configuration may be used Creep is less common than premature fatigue In bearings it is caused by excessive clearance between bore and shaft that allows the bore to rotate on the shaft Creep can be expensive because it causes damage to other components in addition to the bearing 0ther more likely creep indicators are scratches scuff marks or discoloration to shaft and bore To prevent creep damage the bearing housing and shaft fittings should be visually checked Misalignment is related to creep in that it is mounting related If races are misaligned or cocked The balls track in a noncircumferencial path The problem is incorrect mounting or tolerancing or insufficient squareness of the bearing mounting site Misalignment of more than 1 4 can cause an early failure Contaminated lubricant is often more difficult to detect than misalignment or creep Contamination shows as premature wear Solid contaminants become an abrasive in the lubricant In addition insufficient lubrication between ball and retainer wears and 4 weakens the retainer In this situation lubrication is critical if the retainer is a fully machined type Ribbon or crown retainers in contrast allow lubricants to more easily reach all surfaces Rust is a form of moisture contamination and often indicates the wrong material for the application If the material checks out for the job the easiest way to prevent rust is to keep bearings in their packaging until just before installation 2 Avoiding failures The best way to handle bearing failures is to avoid them This can be done in the selection process by recognizing critical performance characteristics These include noise starting and running torque stiffness nonrepetitive runout and radial and axial play In some applications these items are so critical that specifying an ABEC level alone is not sufficient Torque requirements are determined by the lubricant retainer raceway quality roundness cross curvature and surface finish and whether seals or shields are used Lubricant viscosity must be selected carefully because inappropriate lubricant especially in miniature bearings causes excessive torque Also different lubricants have varying noise characteristics that should be matched to the application For example greases produce more noise than oil Nonrepetitive runout NRR occurs during rotation as a random eccentricity between the inner and outer races much like a cam action NRR can be caused by retainer tolerance or eccentricities of the raceways and balls Unlike repetitive runout no compensation can be made for NRR NRR is reflected in the cost of the bearing It is common in the industry to provide different bearing types and grades for specific applications For example a bearing with an NRR of less than 0 3um is used when minimal runout is needed such as in disk drive spindle motors Similarly machine tool spindles tolerate only minimal deflections to maintain precision cuts Consequently bearings are manufactured with low NRR just for machine tool applications Contamination is unavoidable in many industrial products and shields and seals are commonly used to protect bearings from dust and dirt However a perfect bearing seal is not possible because of the movement between inner and outer races Consequently lubrication migration and contamination are always problems Once a bearing is contaminated its lubricant deteriorates and operation becomes noisier If it overheats the bearing can seize At the very least contamination causes wear as it works between balls and the raceway becoming imbedded in the races and 5 acting as an abrasive between metal surfaces Fending off dirt with seals and shields illustrates some methods for controlling contamination Noise is as an indicator of bearing quality Various noise grades have been developed to classify bearing performance capabilities Noise analysis is done with an Anderonmeter which is used for quality control in bearing production and also when failed bearings are returned for analysis A transducer is attached to the outer ring and the inner race is turned at 1 800rpm on an air spindle Noise is measured in andirons which represent ball displacement in m rad With experience inspectors can identify the smallest flaw from their sound Dust for example makes an irregular crackling Ball scratches make a consistent popping and are the most difficult to identify Inner race damage is normally a constant high pitched noise while a damaged outer race makes an intermittent sound as it rotates Bearing defects are further identified by their frequencies Generally defects are separated into low medium and high wavelengths Defects are also referenced to the number of irregularities per revolution Low band noise is the effect of long wavelength irregularities that occur about 1 6 to 10 times per revolution These are caused by a variety of inconsistencies such as pockets in the race Detectable pockets are manufacturing flaws and result when the race is mounted too tightly in multiplejaw chucks Medium hand noise is characterized by irregularities that occur 10 to 60 times per revolution It is caused by vibration in the grinding operation that produces balls and raceways High hand irregularities occur at 60 to 300 times per revolution and indicate closely spaced chatter marks or widely spaced rough irregularities Classifying bearings by their noise characteristics allows users to specify a noise grade in addition to the ABEC standards used by most manufacturers ABEC defines physical tolerances such as bore outer diameter and runout As the ABEC class number increase from 3 to 9 tolerances are tightened ABEC class however does not specify other bearing characteristics such as raceway quality finish or noise Hence a noise classification helps improve on the industry standard GEAR AND SHAFT INTRODUCTION Abstract The important position of the wheel gear and shaft can t falter in traditional machine and modern machines The wheel gear and shafts mainly install the direction that delivers the dint at the principal axis box The passing to process to make them can is divided into many model numbers useding for many situations respectively So we must be the 6 multilayers to the understanding of the wheel gear and shaft in many ways Key words Wheel gear Shaft In the force analysis of spur gears the forces are assumed to act in a single plane We shall study gears in which the forces have three dimensions The reason for this in the case of helical gears is that the teeth are not parallel to the axis of rotation And in the case of bevel gears the rotational axes are not parallel to each other There are also other reasons as we shall learn Helical gears are used to transmit motion between parallel shafts The helix angle is the same on each gear but one gear must have a right hand helix and the other a left hand helix The shape of the tooth is an involute helicoid If a piece of paper cut in the shape of a parallelogram is wrapped around a cylinder the angular edge of the paper becomes a helix If we unwind this paper each point on the angular edge generates an involute curve The surface obtained when every point on the edge generates an involute is called an involute helicoid The initial contact of spur gear teeth is a line extending all the way across the face of the tooth The initial contact of helical gear teeth is a point which changes into a line as the teeth come into more engagement In spur gears the line of contact is parallel to the axis of the rotation in helical gears the line is diagonal across the face of the tooth It is this gradual of the teeth and the smooth transfer of load from one tooth to another which give helical gears the ability to transmit heavy loads at high speeds Helical gears subject the shaft bearings to both radial and thrust loads When the thrust loads become high or are objectionable for other reasons it may be desirable to use double helical gears A double helical gear herringbone is equivalent to two helical gears of opposite hand mounted side by side on the same shaft They develop opposite thrust reactions and thus cancel out the thrust load When two or more single helical gears are mounted on the same shaft the hand of the gears should be selected so as to produce the minimum thrust load Crossed helical or spiral gears are those in which the shaft centerlines are neither parallel nor intersecting The teeth of crossed helical fears have point contact with each other which changes to line contact as the gears wear in For this reason they will carry out very small loads and are mainly for instrumental applications and are definitely not recommended for use in the transmission of power There is on difference between a crossed helical gear and a helical gear until they are mounted in mesh with each other They are manufactured in the same way A pair of meshed crossed helical gears usually have the same hand that is a right hand driver goes with a right hand driven In the design of crossed helical gears the minimum sliding velocity is obtained when the helix angle are equal However when the helix angle are not equal the gear with the larger helix angle 7 should be used as the driver if both gears have the same hand Worm gears are similar to crossed helical gears The pinion or worm has a small number of teeth usually one to four and since they completely wrap around the pitch cylinder they are called threads Its mating gear is called a worm gear which is not a true helical gear A worm and worm gear are used to provide a high angular velocity reduction between nonintersecting shafts which are usually at right angle The worm gear is not a helical gear because its face is made concave to fit the curvature of the worm in order to provide line contact instead of point contact However a disadvantage of worm gearing is the high sliding velocities across the teeth the same as with crossed helical gears Worm gearing are either single or double enveloping A single enveloping gearing is one in which the gear wraps around or partially encloses the worm A gearing in which each element partially encloses the other is of course a double enveloping worm gearing The important difference between the two is that area contact exists between the teeth of double enveloping gears while only line contact between those of single enveloping gears The worm and worm gear of a set have the same hand of helix as for crossed helical gears but the helix angles are usually quite different The helix angle on the worm is generally quite large and that on the gear very small Because of this it is usual to specify the lead angle on the worm which is the complement of the worm helix angle and the helix angle on the gear the two angles are equal for a 90 deg Shaft angle When gears are to be used to transmit motion between intersecting shaft some of bevel gear is required Although bevel gear are usually made for a shaft angle of 90 deg They may be produced for almost any shaft angle The teeth may be cast milled or generated Only the generated teeth may be classed as accurate In a typical bevel gear mounting one of the gear is often mounted outboard of the bearing This means that shaft deflection can be more pronounced and have a greater effect on the contact of teeth Another difficulty which occurs in predicting the stress in bevel gear teeth is the fact the teeth are tapered Straight bevel gears are easy to design and simple to manufacture and give very good results in service if they are mounted accurately and positively As in the case of squr gears however they become noisy at higher values of the pitch line velocity In these cases it is often good design practice to go to the spiral bevel gear which is the bevel counterpart of the helical gear As in the case of helical gears spiral bevel gears give a much smoother tooth action than straight bevel gears and hence are useful where high speed are encountered It is frequently desirable as in the case of automotive differential applications to have gearing similar to bevel gears but with the shaft offset Such gears are called hypoid gears 8 because their pitch surfaces are hyperboloids of revolution The tooth action between such gears is a combination of rolling and sliding along a straight line and has much in common with that of worm gears A shaft is a rotating or stationary member usually of circular cross section having mounted upon it such elementsas gears pulleys flywheels cranks sprockets and other power transmission elements Shaft may be subjected to bending tension compression or torsional loads acting singly or in combination with one another When they are combined one may expect to find both static and fatigue strength to be important design considerations since a single shaft may be subjected to static stresses completely reversed and repeated stresses all acting at the same time The word shaft covers numerous variations such as axles and spindles Anaxle is a shaft wither stationary or rotating nor subjected to torsion load A shirt rotating shaft is often called a spindle When either the lateral or the torsional deflection of a shaft must be held to close limits the shaft must be sized on the basis of deflection before analyzing the stresses The reason for this is that if the shaft is made stiff enough so that the deflection is not too large it is probable that the resulting stresses will be safe But by no means should the designer assume that they are safe it is almost always necessary to calculate them so that he knows they are within acceptable limits Whenever possible the power transmission elements such as gears or pullets should be located close to the supporting bearings This reduces the bending moment and hence the deflection and bending stress Although the von Mises Hencky Goodman method is difficult to use in design of shaft it probably comes closest to predicting actual failure Thus it is a good way of checkin