机械外文翻译中英文[共15页]

1、翻译：英文原文Definitions and Terminology of VibrationvibrationAll matter-solid, liquid and gaseous-is capable of vibration, e.g. vibration ofgases occurs in tail ducts of jet engines causing troublesome noise and sometimesfatigue cracks in the metal. Vibration in liquids is almost always longitudinal and

2、cancause large forces because of the low compressibility of liquids, e.g. popes conveyingwater can be subjected to high inertia forces (or“water hammer”) when a valve or tapis suddenly closed. Excitation forces caused, say by changes in flow of fluids orout-of-balance rotating or reciprocating parts

3、, can often be reduced by attention todesign and manufacturing details. Atypical machine has many moving parts, each ofwhich is a potential source of vibration or shock-excitation. Designers face theproblem of compromising between an acceptable amount of vibration and noise, andcosts involved in red

4、ucing excitation.The mechanical vibrations dealt with are either excited by steady harmonicforces ( i. e. obeying sine and cosine laws in cases of forced vibrations ) or, after aninitial disturbance, by no external force apart from gravitational force called weight ( i.e. in cases of natural or free

5、 vibrations). Harmonic vibrations are said to b“esimple”ifthere is only one frequency as represented diagrammatically by a sine or cosine waveof displacement against time.Vibration of a body or material is periodic change in position or displacementfrom a static equilibrium position. Associated with

6、 vibration are the interrelatedphysical quantities of acceleration, velocity and displacement-e. g. an unbalancedforce causes acceleration (a = F/m ) in a system which, by resisting, induces vibrationas a response. We shall see that vibratory or oscillatory motion may be classifiedbroadly as (a) tra

7、nsient; (b) continuing or steady-state; and (c) random.Transient Vibrations die away and are usually associated with irregulardisturbances, e. g. shock or impact forces, rolling loads over bridges, cars driven overpot holes-i. e. forces which do not repeat at regular intervals. Although transients a

8、retemporary components of vibrational motion, they can cause large amplitudes initiallyand consequent high stress but, in many cases, they are of short duration and can beignored leaving only steady-state vibrations to be considered.Steady-State Vibrations are often associated with the continuous op

9、erationof machinery and, although periodic, are not necessarily harmonic or sinusoidal. Sincevibrations require energy to produce them, they reduce the efficiency of machines andmechanisms because of dissipation of energy, e. g. by friction and consequentheat-transfer to surroundings, sound waves an

10、d noise, stress waves through framesand foundations, etc. Thus, steady-state vibrations always require a continuous energyinput to maintain them.Random Vibration is the term used for vibration which is not periodic, i. e.has no made clear-several of which are probably known to science students alrea

11、dy.Period, Cycle, Frequency and Amplitude A steady-state mechanicalvibration is the motion of a system repeated after an interval of time known as theperiod. The motion completed in any one period of time is called a cycle. The numberof cycles per unit of time is called the frequency. The maximum di

12、splacement of anypart of the system from its static-equilibrium position is the amplitude of the vibrationof that part-the total travel being twice the amplitude. Thus,“amplitude”is notsynonymous with “displacement”but is the maximum value of the displacement fromthe static-equilibrium position.Natu

13、ral and Forced Vibration A natural vibration occurs without anyexternal force except gravity, and normally arises when an elastic system is displacedfrom a position of stable equilibrium and released, i. e. natural vibration occurs underthe action of restoring forces inherent in an elastic system, a

14、nd natural frequency is aproperty of he system.A forced vibration takes place under the excitation of an external force (orexternally applied oscillatory disturbance) which is usually a function of time, e. g.in unbalanced rotating parts, imperfections in manufacture of gears and drives. Thefrequenc

15、y of forced vibration is that of the exciting or impressed force, i. e. theforcing frequency is an arbitrary quantity independent of the natural frequency of thesystem.Resonance Resonance describes the condition of maximum amplitude. Itoccurs when the frequency of an impressed force coincides with,

16、or is near to anatural frequency of the system. In this critical condition, dangerously largeamplitudes and stresses may occur in mechanical systems but, electrically, radio andtelevision receivers are designed to respond to resonant frequencies. The calculationor estimation of natural frequencies i

17、s, therefore, of great importance in all types ofvibrating and oscillating systems. When resonance occurs in rotating shafts andspindles, the speed of rotation is known as the critical speed. Hence, the predictionand correction or avoidance3 of a resonant condition in mechanisms is of vitalimportanc

18、e since, in the absence of damping or other amplitude-limiting devices,resonance is the condition at which a system gives an infinite responseto a finiteexcitation.Damping Damping is the dissipation of energy from a vibrating system, andthus prevents excessive response. It is observed that a natural

19、 vibration diminishes inamplitude with time and, hence, eventually ceases owing to some restraining ordamping influence. Thus if a vibration is to be sustained,the energy dissipated bydamping must be replaced from an external source.The dissipation is related in some way to the relative motion betwe

20、en thecomponents or elements of the system, and is caused by frictional resistance of somesort, e.g. in structures, internal friction in material, and external friction caused by airor fluid resistance called “viscous”damping if the drag force is assumed proportionalto the relative velocity between

21、moving parts. One device assumedto give viscousdamping is the “dashpot”which is a loosely fitting piston in a cylinder so that fluidcan flow from one side of the piston to the other through the annular clearance space.A dashpot cannot store energy but can only dissipate it.Basic Machining Operations

22、 and Machine ToolsBasic Machining OperationsMachine tools have evolved from the early foot-powered lathes of the Egyptians andJohn Wilkinsons boring mill. They are designed to provide rigid support for both theworkpiece and the cutting tool and can precisely control their relative positions andthe v

23、elocity of the tool with respect to the workpiece. Basically, in metal cutting, asharpened wedge-shaped tool removes a rather narrow strip of metal from the surfaceof a ductile workpiece in the form of a severely deformed chip. The chip is a wasteproduct that is comsiderably shorter than the workpie

24、ce from which it came but wotha corresponding increase in thickness of the uncut chip. The geometrical shape of themachine surface depednson the shape of the tool and its path during the machinigoperation.Most machining operations produce parts of differing geometry. If a rough cylindricalworkpiece

25、revolves about a central axis and the tool penetrates beneath its surface andtravels parallel to the center of rotation, a surface of revolution is producedandtheoperation is called turning. If a hollow tube is machined on the inside in a similarmanner, the operation is called boring. Producing an e

26、xternal conical surface ofuniformly varying diameter is called taper turning. If the tool point travels in a pathof varying radius,a contoured surface like that of a bowling pin a can be produced; or,if the piece is short enough and the support is sufficiently rigid, a contoured surfacecould be prod

27、uced by feeding a shaped tool normal to the axis of rotation. Shorttapered or cylindrical surfaces could also be contour formed.Flat or plane surfaces are frequently required. The can be generated by adial turningor facing, in which the tool point moves normal to the axis of rotation. In other cases

28、,it is more convenient to hold the workpiece steady and reciprocate the tool across it ina series of straight-line cuts with a crosswise feed increment before each cuttingstroke. This operation is called planing and is carried out on a shaper. For largerpieces it is easier to keep the tool stationar

29、y and draw the workpiece under it as inplaning. The tool is fed at each reciprocation. Contoured surfaces can be produced byusing shaped tools.Multiple-edged tools can also be used. Drilling uses a twin-edged fluted tool for holeswith depths up to 5 10times the drill diameter. Whether the dril turns

30、 or the workpiecerotates, relative motion between the cutting edge and the workpiece is the importantfactor. In milling operations a rotary cutter with a number of cutting edges engages theworkpiecem which moves slowly with respect to the cutter. Plane or contouredsurfaces may be produced, depending

31、 on the geometry of the cutter and the type offeed. Horizontal or vertical axes of rotation ma be used, and the feed of the workpiecemay be in any of the three coordinate directions.Basic Machine ToolsMachine tools are used to produce a part of a specified geometrical shape and precisesize by removi

32、ng metal from a ductile materila in the form of chips. The latter are awaste product and vary from long continuous ribbons of a ductile material such assteel, which are undesirable from a disposal point of view, to easily handledwell-broken chips resulting from cast iron. Machine tools perform five

33、basicmetal-removal processes:turning, planing, drilling, milling, and frinding. All othermetal-removal processes are modifications of these five basic processes. For example,boring is internal turning;reaming,tapping, and counterboring modify drilled holes andare related to drilling; hobbing and gea

34、r cutting are fundamentally milling operations;hack sawong and broaching are a form of planing and honing; lapping, superfinishing,polishing, and buffing are avariants of grinding or abrasive removal operations.Therefore, there are only four types of basic machine tools, which use cutting tools ofsp

35、ecific controllable feometry: 1.lathes, 2.planers, 3.drilling machines, and 4.millingmachines. The frinding process forms chips, but the geometry of the barasive grain isuncontrollable.The amount and rate of material removed by the various machining processes may belarge, as in heavy truning operati

36、ons, or extremely small, as in lapping orsuperfinishing operations where only the high spots of a surface are removed.A machine tool performs three major functions: 1.it rigidly supports the workpiece orits holder and the cutting tool; 2. it provedes relative motion between the workpieceand the cutt

37、ing tools; 3. it provides a range of feeds and speeds usually ranging from 4to 32 choices in each case.Speed and Feeds in MachiningSpeeds feeds, and depth of cut are the three major variables for economical machining.Other variables are the work and tool materials, coolant and geometry of the cuttin

38、gtool. The rate of metal removal and power required for machining depend upon thesevariables.The depth of cut, feed, and cutting speed are machine settings that must be establishedin any metal-cutting operation. They all affect the forces, the power, and the rate ofmetal removal. They can be defined

39、 by comparing them to the needle and record of aphonograph. The cutting speed is representedby the velocity of the record surfacerelative to the needle in the tone arm at any instant. Feed is represented by theadvance the needle radially inward per revolution, or is the difference in positionbetween

40、 two adjacent grooves.Turning on Lathe CentersThe basic operations performed on an engine lathe are illustrated in Fig. Thoseoperations performed on extemal surfaces with a single point cutting tool are calledturning. Except for drilling, reaming, and tapping, the operations on intermal surfacesare

41、also performed by a single point cutting tool.All machining operations, including turning and boring, can be classified as roughing,finishing, or semi-finishing. The objective of a roughing ooperation is to remove thebulk of the material sa repidly and as efficiently as possible, while leaving a sma

42、llamount of material on the work-piece for the finishing operation. Finishing operationsare performed to btain the final size, shape, and surface finish on the workpiece.Sometimes a semi-finishing operation will precede the finishing operation to leave asmall predetermined and uniform amount of stox

43、d on the work-piece to be removedby the finishing operation.Generally, longer workpieces are turned while supported on one or two lathe centers.Cone shaped holes, called center holes, which fit the lathe centers are drilled in theends of the workpiece-usually along the axis of the cylindrical part.

44、The end of theworkpiece adjacent to the tailstock is always supported by a tailstock center, while theend near the headstock may be supported by a headstock cener or held in a chuck. Theheadstock end of the workpiece may be held in a four-jar chuck, or in a collet typechuck. This method holds the wo

45、rkpiece firmly and transfers the power to theworkpiece smoothly; the additional support to the workpiece priovided by the chucklessens the tendency for chatter to occur when cutting. Precise results can be obtainedwith this method if care is taken to hold the workpiece accurately in the chuck.Very p

46、recise results can be obtained by supporting the workpiece between two centers.A lathe dog is clamped to the workpiece; together they are driven by a driver p;atemounted on the spindle nose. One end of the workpiece is machined; then theworkpiece can be turned around in the lathe to machine the othe

47、r end. The centerholes in the workpiece serve as precise locating surfaces as well as bearing surfaces tocarry the weight of the workpiece and to resist the xutting forces. After the workpiecehas been removed from the lathe for any reason, the center holes will accurately alignthe workpiece back in

48、the lathe or in another lathe,or in a cylindrical grinding machine.The workpiece must never be held at the headstock end by both a chuck and a lathecenter. While at first thought this seems like a quick method of aligning the workpiecein the chuck, this must not be done because it is not possible to

49、 press evenly with thejaws against the workpiece while it is also supported by the center. The alignmentprovided by the center will not be maintained and the pressure of the jaws maydamage the center hole, the lathe center,and prehaps even the lathe spindle.Compensatng or floating jaw chucks used al

50、most exclusively on high production workprovice an exception to the statementsmade above. These chucks are really workdrivers and cannot be used for the same purpose as ordinary three or four=jaw chucks.While very large diameter workpieces are sometimes mounted on two centers, theyare preferably hel

51、d at the headstock end by faceplate jaes to obtain the smooth powertransmission; moreover, large lathe dogs that are adequate to transmit the power notgenerally available, although they can be maed as a special. Faceplate jaws are likechuck jaws except that thet are mounted on a faceplate, which has

52、 less overhang fromthe spindle bearings than a large chuck would have.BoringThe boring operation is generally performed in two steps; namely, rough boring andfinish boring. The objective of the rough-boring operation is to remove the excessmetal rapidly and efficiently, and the objective of the fini

53、sh-boring operation is toobtain the desired size, surface finish, and location of the hole. The size of the hole isobtained by using the trial-cut procedure. The diameter of the hole can be measuredwith inside calipers and outside micrometer calipers. Basic Measuring Insteruments,or inside micromete

54、r calipers can be used to measure the diameter directly.Cored holes and drilled holes are sometimes eccentric wwith respect to the rotation ofthe lathe. When the boring tool enters the work, the boring bar will take a deeper cuton one side of the hole than on the other, and will deflect more when ta

55、king thisdeeper cut,with the result that the bored hole will not be concentric with the rotationof the work. This effect is corrected by taking several cuts through the hole using ashallow depth of cut. Each succeedingshallow cut causesthe resulting hole to bemore concentric than it was with the pre

56、vious cut. Before the final, finish cut is taken,the hole should be concentric with the rotation of the work in order to make certainthat the finished hole will be accurately located.Shoulders, grooves, contours, tapers, and threads are bored inside of holes. Internalgrooves are cut using a tool tha

57、t is similar to an external grooving tool. The procedurefor boring internal shoulders is very similar to the procedure for turningshoulders.large shoulders are faced with the boring tool positioned with the noseleading, and using the cross slide to feed the tool. Internal contours can be machinedusi

58、ng a tracing attachment on a lathe. The tracing attachment is mounted on the crossslide and the stylus follows the outline of the master profile plate. This causesthecutting tool to move in a path corresponding to the profile of the master profile plate.Thus, the profile on the master profile plate is reproduced inside the bore. The masterprofile plate is accurately mounted on a speci