機(jī)械外文翻譯中英文

1、精品文檔 翻譯： 英文原文 Definitions and Terminology of Vibration vibration All matter-solid, liquid and gaseous-is capable of vibration, e.g. vibration of gases occurs in tail ducts of jet engines causing troublesome noise and sometimes fatigue cracks in the metal. Vibration in liquids is almost always longit

2、udinal and can cause large forces because of the low compressibility of liquids, e.g. popes conveying water can be subjected to high inertia forces (or “water hamme”r) when a valve or tap is suddenly closed. Excitation forces caused, say by changes in flow of fluids or out-of-balance rotating or rec

3、iprocating parts, can often be reduced by attention to design and manufacturing details. Atypical machine has many moving parts, each of which is a potential source of vibration or shock-excitation. Designers face the problem of compromising between an acceptable amount of vibration and noise, and c

4、osts involved in reducing excitation. The mechanical vibrations dealt with are either excited by steady harmonic forces ( i. e. obeying sine and cosine laws in cases of forced vibrations ) or, after an initial disturbance, by no external force apart from gravitational force called weight ( i. e. in

5、cases of natural or free vibrations). Harmonic vibrations are said to be “simple ”if there is only one frequency as represented diagrammatically by a sine or cosine wave of displacement against time. Vibration of a body or material is periodic change in position or displacement from a static equilib

6、rium position. Associated with vibration are the interrelated physical quantities of acceleration, velocity and displacement-e. g. an unbalanced force causes acceleration (a = F/m ) in a system which, by resisting, induces vibration as a response. We shall see that vibratory or oscillatory motion ma

7、y be classified broadly as (a) transient; (b) continuing or steady-state; and (c) random. Transient Vibrationsdie away and are usually associated with irregular disturbances, e. g. shock or impact forces, rolling loads over bridges, cars driven over pot holes-i. e. forces which do not repeat at regu

8、lar intervals. Although transients are temporary components of vibrational motion, they can cause large amplitudes initially and consequent high stress but, in manycases, they are of short duration and can be ignored leaving only steady-state vibrations to be considered. Steady-State Vibrationsare o

9、ften associated with the continuous operation of machinery and, although periodic, are not necessarily harmonic or sinusoidal. Since vibrations require energy to produce them, they reduce the efficiency of machines and mechanisms because of dissipation of energy, e. g. by friction and consequent hea

10、t-transfer to surroundings, sound waves and noise, stress waves through frames and foundations, etc. Thus, steady-state vibrations always require a continuous energy input to maintain them. Random Vibration is the term used for vibration which is not periodic, i. e. has no made clear-severalof which

11、 are probably known to science students already. Period, Cycle, Frequency and AmplitudeA steady-state mechanical vibration is the motion of a system repeated after an interval of time known as the period. The motion completed in any one period of time is called a cycle. The number of cycles per unit

12、 of time is called the frequency. The maximum displacement of any part of the system from its static-equilibrium position is the amplitude of the vibration of that part-the total travel being twice the amplitude. Thus, “amplitude ”is not synonymous with “displacement ”but is the maximum value of the

13、 displacement from the static-equilibrium position. Natural and Forced Vibration A natural vibration occurs without any external force except gravity, and normally arises whenan elastic system is displaced from a position of stable equilibrium and released, i. e. natural vibration occurs under the a

14、ction of restoring forces inherent in an elastic system, and natural frequency is a property of he system. A forced vibration takes place under the excitation of an external force (or externally applied oscillatory disturbance) which is usually a function of time,e. g. in unbalanced rotating parts,

15、imperfections in manufacture of gears and drives. The frequency of forced vibration is that of the exciting or impressed force, i. e. the forcing frequency is an arbitrary quantity independent of the natural frequency of the system. Resonance Resonance describes the condition of maximum amplitude. I

16、t occurs when the frequency of an impressed force coincides with, or is near to a natural frequency of the system. In this critical condition, dangerously large amplitudes and stresses may occur in mechanical systems but, electrically, radio and television receivers are designed to respond to resona

17、nt frequencies. The calculation or estimation of natural frequencies is, therefore, of great importance in all types of vibrating and oscillating systems. Whenresonance occurs in rotating shafts and spindles, the speed of rotation is known as the critical speed. Hence, the prediction and correction

18、or avoidance3 of a resonant condition in mechanisms is of vital importance since, in the absence of damping or other amplitude-limiting devices, resonance is the condition at which a system gives an infinite response to a finite excitation. Damping Damping is the dissipation of energy from a vibrati

19、ng system, and thus prevents excessive response. It is observed that a natural vibration diminishes in amplitude with time and, hence, eventually ceases owing to some restraining or damping influence. Thus if a vibration is to be sustained, the energy dissipated by damping must be replaced from an e

20、xternal source. The dissipation is related in someway to the relative motion between the components 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 air or fluid resistance called “viscou

21、s ” damping if the drag force is assumedproportional to the relative velocity between moving parts. One device assumed to give viscous damping is the “dashpot ”which is a loosely fitting piston in a cylinder so that fluid can flow from one side of the piston to the other through the annular clearanc

22、e space. A dashpot cannot store energy but can only dissipate it. Basic Machining Operations and Machine Tools Basic Machining Operations Machine tools have evolved from the early foot-powered lathes of the Egyptians and John Wilkinson s boring mill. They are designed to provide rigid support for bo

23、th the workpiece and the cutting tool and can precisely control their relative positions and the velocity of the tool with respect to the workpiece. Basically, in metal cutting, a sharpened wedge-shaped tool removes a rather narrow strip of metal from the surface of a ductile workpiece in the form o

24、f a severely deformed chip. The chip is a waste product that is comsiderably shorter than the workpiece from which it came but woth a corresponding increase in thickness of the uncut chip. The geometrical shape of the machine surface depedns on the shape of the tool and its path during the machinig

25、operation. Most machining operations produce parts of differing geometry. If a rough cylindrical workpiece revolves about a central axis and the tool penetrates beneath its surface and travels parallel to the center of rotation, a surface of revolution is producedand the operation is called turning.

26、 If a hollow tube is machined on the inside in a similar manner, the operation is called boring. Producing an external conical surface of uniformly varying diameter is called taper turning. If the tool point travels in a path of varying radius,a contoured surface like that of a bowling pin a can be

27、produced; or, if the piece is short enough and the support is sufficiently rigid, a contoured surface could be produced by feeding a shaped tool normal to the axis of rotation. Short tapered or cylindrical surfaces could also be contour formed. Flat or plane surfaces are frequently required. The can

28、 be generated by adial turning or facing, in which the tool point moves normal to the axis of rotation. In other cases, it is more convenient to hold the workpiece steady and reciprocate the tool across it in a series of straight-line cuts with a crosswise feed increment before each cutting stroke.

29、This operation is called planing and is carried out on a shaper. For larger pieces it is easier to keep the tool stationary and draw the workpiece under it as in planing. The tool is fed at each reciprocation. Contoured surfaces can be produced by using shaped tools. Multiple-edged tools can also be

30、 used. Drilling uses a twin-edged fluted tool for holes with depths up to 5 10times the drill diameter. Whether the dril turns or the workpiece rotates, relative motion between the cutting edge and the workpiece is the important factor. In milling operations a rotary cutter with a number of cutting

31、edges engages the workpiecem which moves slowly with respect to the cutter. Plane or contoured surfaces may be produced, depending on the geometry of the cutter and the type of feed. Horizontal or vertical axes of rotation ma be used, and the feed of the workpiece maybe in any of the three coordinat

32、e directions. Basic Machine Tools Machine tools are used to produce a part of a specified geometrical shape and precise size by removing metal from a ductile materila in the form of chips. The latter are a waste product and vary from long continuous ribbons of a ductile material such as steel, which

33、 are undesirable from a disposal point of view, to easily handled well-broken chips resulting from cast iron. Machine tools perform five basic metal-removal processes: turning, planing, drilling, milling, and frinding. All other metal-removal processes are modifications of these five basic processes

34、. For example, boring is internal turning;reaming,tapping, and counterboring modify drilled holes and are related to drilling; hobbing and gear cutting are fundamentally milling operations; hack sawong and broaching are a form of planing and honing; lapping, superfinishing, polishing, and buffing ar

35、e avariants of grinding or abrasive removal operations. Therefore, there are only four types of basic machine tools, which use cutting tools of specific controllable feometry: 1.lathes, 2.planers, 3.drilling machines, and 4.milling machines. The frinding process forms chips, but the geometry of the

36、barasive grain is uncontrollable. The amount and rate of material removed by the various machining processes may be large, as in heavy truning operations, or extremely small, as in lapping or superfinishing operations where only the high spots of a surface are removed. A machine tool performs three

37、major functions: 1.it rigidly supports the workpiece or its holder and the cutting tool; 2. it provedes relative motion between the workpiece and the cutting tools; 3. it provides a range of feeds and speeds usually ranging from 4 to 32 choices in each case. Speed and Feeds in Machining Speeds feeds

38、, 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 cutting tool. The rate of metal removal and power required for machining depend upon these variables. The depth of cut, feed, and cutting speed are

39、machine settingsthat must be established in any metal-cutting operation. They all affect the forces, the power, and the rate of metal removal. They can be defined by comparing them to the needle and record of a phonograph. The cutting speed is represented by the velocity of the record surface relati

40、ve to the needle in the tone arm at any instant. Feed is represented by the advance the needle radially inward per revolution, or is the difference in position between two adjacent grooves. Turning on Lathe Centers The basic operations performed on an engine lathe are illustrated in Fig. Those opera

41、tions performed on extemal surfaces with a single point cutting tool are called turning. Except for drilling, reaming, and tapping, the operations on intermal surfaces are also performed by a single point cutting tool. All machining operations, including turning and boring, can be classified as roug

42、hing, finishing, or semi-finishing. The objective of a roughing ooperation is to remove the bulk of the material sa repidly and as efficiently aspossible, while leaving a small amount of material on the work-piece for the finishing operation. Finishing operations are performed to btain the final siz

43、e, shape, and surface finish on the workpiece. Sometimes a semi-finishing operation will precede the finishing operation to leave a small predetermined and uniform amount of stoxd on the work-piece to be removed by the finishing operation. Generally, longer workpieces are turned while supported on o

44、ne or two lathe centers. Cone shaped holes, called center holes, which fit the lathe centers are drilled in the ends of the workpiece-usually along the axis of the cylindrical part. The end of the workpiece adjacent to the tailstock is always supported by a tailstock center, while the end near the h

45、eadstock may be supported by a headstock cener or held in a chuck. The headstock end of the workpiece may be held in a four-jar chuck,or in a collet type chuck. This method holds the workpiece firmly and transfersthe power to the workpiece smoothly; the additional support to the workpiece priovided

46、by the chuck lessens the tendency for chatter to occur when cutting. Precise results can be obtained with this method if care is taken to hold the workpiece accurately in the chuck. Very precise results can be obtained by supporting the workpiece between two centers. A lathe dog is clamped to the wo

47、rkpiece; together they are driven by a driver p;ate mounted on the spindle nose. One end of the workpiece is machined; then the workpiece can be turned around in the lathe to machine the other end. The center holes in the workpiece serve as precise locating surfaces as well as bearing surfaces to ca

48、rry the weight of the workpiece and to resist the xutting forces. After the workpiece has been removed from the lathe for any reason, the center holes will accurately align the workpiece back in the lathe or in another lathe,or in a cylindrical grinding machine. The workpiece must never be held at t

49、he headstock end by both a chuck and a lathe center. While at first thought this seems like a quick method of aligning the workpiece in the chuck, this must not be done because it is not possible to press evenly with the jaws against the workpiece while it is also supported by the center. The alignm

50、ent provided by the center will not be maintained and the pressure of the jaws maydamagethe center hole, the lathe center,and prehaps even the lathe spindle. Compensatng or floating jaw chucks used almost exclusively on high production work provice an exception to the statements made above. These ch

51、ucks are really work drivers 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, they are preferably held at the headstock end by faceplate jaes to obtain the smooth power transmission; moreover, lar

52、ge lathe dogs that are adequate to transmit the power not generally available, although they can be maedas a special. Faceplate jaws are like chuck jaws except that thet are mounted on a faceplate, which has less overhang from the spindle bearings than a large chuck would have. Boring The boring ope

53、ration is generally performed in two steps; namely, rough boring and finish boring. The objective of the rough-boring operation is to remove the excess metal rapidly and efficiently, and the objective of the finish-boring operation is to obtain the desired size, surface finish, and location of the h

54、ole. The size of the hole is obtained by using the trial-cut procedure. The diameter of the hole can be measured with inside calipers and outsidemicrometer calipers. Basic Measuring Insteruments, or inside micrometer calipers can be used to measure the diameter directly. Cored holes and drilled hole

55、s are sometimes eccentric wwith respect to the rotation of the lathe. Whenthe boring tool enters the work, the boring bar will take a deeper cut on one side of the hole than on the other, and will deflect more when taking this deeper cut,with the result that the bored hole will not be concentric wit

56、h the rotation of the work. This effect is corrected by taking several cuts through the hole using a shallow depth of cut. Each succeeding shallow cut causes the resulting hole to be more concentric than it was with the previous cut. Before the final, finish cut is taken, the hole should be concentr

57、ic with the rotation of the work in order to makecertain that the finished hole will be accurately located. Shoulders, grooves, contours, tapers, and threads are bored inside of holes. Internal grooves are cut using a tool that is similar to an external grooving tool. The procedure for boring intern

58、al shoulders is very similar to the procedure for turning shoulders.large shoulders are faced with the boring tool positioned with the nose leading, and using the cross slide to feed the tool. Internal contours can be machined using a tracing attachment on a lathe. The tracing attachment is mounted

59、on the cross slide and the stylus follows the outline of the master profile plate. This causes the cutting 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 master profile plate is accurat

60、ely mounted on a special slide which can be precisely adjusted in two dirctions, in two directionsm, in order to align the cutting tool in the correct relationship to the work. This lathe has a cam-lick type of spindle nose which permits it to take a cut when rotating in either direction. Normal tur