1、word 文档 可自由复制编辑 附录 附录 1 英文原文 Rolling Contact Bearings The concern of a machine designer with ball and roller bearings is fivefold as follows:(a) life in relation to load; (b) stiffness,i.e.deflections under load; (c) friction; (d) wear; (e) noise. For moderate loads and speeds the correct selection
2、of a standard bearing on the basis of a load rating willbecome important where loads are high,although this is usually of less magnitude than that of the shafts or other components associated with the bearing. Where speeds are high special cooling arrangements become necessary whichmay increase fric
3、itionaldrag. Wear is primarily associated with the introduction of contaminants,and sealing arrangements must be chosen with regard to the hostility of the environment. Because the high quality and low price of balland roller bearing depends on quantity production,the task of the machine designer be
4、comes one of selection rather than design. Rolling-contactbearings are generally made withsteel which is through-hardened to about 900HV ,although in many mechanisms special races are not provided and the interacting surfaces are hardened to about 600H . V It is not surprising that,owing to the high
5、 stresses involved,a predominant form of failure should be metal fatigue, and a good deal of work isbased on acceptvalues of life and it is general practice in bearing industry to define the load capacity of the bearing as that value below which 90 percent of a batch will exceed life of one million
6、revolutions. Notwithstanding the fact that responsibility for basic design of ball and roller bearings rests with he bearing manufacturer, the machine designer must form a correct appreciation of the duty to be performed by the bearing and be concerned not only with bearing selection but with the co
7、nditions for correct installation. The fit of the bearing races onto the shaft or onto the housings is of critical importance because of their combined effect on the internal clearance of the bearing as well as preserving the desired degree of interference fit. Inadequateinterference can induce seri
8、ous trouble from fretting corrosion. The inner race is frequently located axially by against a shoulder. A radius at this point is essential for the avoidance of word 文档 可自由复制编辑 stressconcentration and ball racesare provided with a radius or chamfer to follow space for this. Where life is not the de
9、termining factor in design, it is usual to determine maximum loading by the amount to which a bearing will deflect under load. Thus the concept of static load-carrying capacity is understood to mean the load that can be applied to a bearing, which is either stationary or subject to slight swiveling
10、motions, without impairing its running qualities for subsequent rotational motion. This has been determined by practical experience as the load which when applied to a bearing results in a total deformation of 0.0025mm for a ball 25mm in diameter. The successful functioning of many bearings depends
11、upon providing them with adequate protection against theirenvironment, and in some circumstances the environment must be protected from lubricants or products of deterioration of the bearing design. Moreover, seals which are applied to moving parts for any purpose are of interest to tribologists bec
12、ausethey are components of bearing systems and can only be designed satisfactorily on basis of the appropriate bearing theory. Notwithstanding their importance, the amount of researcheffort that has been devoted to the understanding of the behavior of seals has been small when compared with that dev
13、oted to other aspects of bearing technology. Lathes Lathes are widely used in industry to produce all kinds of machined parts. Some are general purpose machines, and others are used to perform highly specialized operations. Engine lathes Engine lathes, of course, are general-purposemachine used in p
14、roduction and maintenanceshop all over the the world. Sized ranger from small bench models to huge heavy duty pieces of equipment. Many of the larger lathes come equipped with attachments not commonly found in the ordinary shop, such as automatic shop for the carriage. Tracer or Duplicating Lathes T
15、he tracer or duplicating lathe is designed o produce irregularly shaped parts automatically. The basic operation of this lathe is as fallows. A template of either a flat or three-dimensional shape is placed in a holder. A guide or pointer then moves along this shape and its movement controls that of
16、 the cutting tool. The duplication may include a squareor tapered shoulder, grooves, tapers, and contours. Work such as word 文档 可自由复制编辑 motor shafts, spindles, pistons, rods, car axles, turbine shafts, and a variety of other objects can be turned using this type of lathe. Turret Lathes When machinin
17、g a complex workpiece on a general-purpose lathe, a great deal of time is spent changing and adjusting the several tools that are needed to complete the work. One of the firstadaptations of the engine lathe which made it suitable to mass production was the addition of multi-tool in place of the tail
18、stock. Although most turrets have six stations, some have as many as eight. High-production turret lathes are very complicated machines with a wide variety of power accessories. The principal feature of all turret lathes, however, is that the tools can perform a consecutive serials of operations in
19、proper sequence. Once the tools have been set and adjusted, little skill is require to run out duplicate parts. Automatic Screw Machine Screw machines are similar in construction to turret lathes, except that their heads are designed to hold and feed long bars of stock. Otherwise, their is little di
20、fferent between them. Both are designed for multiple tooling, and both have adaptations for identical work. Originally, the turret lathe was designed as a chucking lathe for machining small casting, forgings, and irregularly shaped workpieces. The first screw machines were designed to feed bar stock
21、 and wire used in making small screw parts. Today, however, the turret lathe is frequently used with a collect attachment, and the automatic screw machine can be equipped with a chuck to hold castings. The single-spindle automatic screw machine, as its name implies, machines work on only one bar of
22、stock at a time. A bar 16 to 20 feet long is feed through the headstock spindle and is held firmly by a collect. The machining operations are done by cutting tools mounted on the cross slide. When the machine is in operation, the spindle and the stock are rotated at selected speeds for different ope
23、rations. If required, rapid reversal of spindle direction is also possible. In the single-spindle automatic screw machine, a specific length of stock is automatically fed through the spindle to a machining area.At this point, the turret and cross slide move into position and automatically perform wh
24、atever operations are required. After the machined piece is cut off, stock is again fed into the machining area and the entire cycle is repeated. Multiple-spindle automatic screw machines have from four to eight spindles word 文档 可自由复制编辑 located around a spindle carrier. Long bars of stock, supported
25、 at the rear of the machine,passthough these hollow spindles and are gripped by collects. With the single spindle machines, the turret indexes around the spindle. When one tool on the turret is working, the others are not. With a multiple spindle machine, however, the spindle itself index. Thus the
26、bars of stock are carried to the various end working and side working tools. Each tool operates in only one position, but tolls operate simultaneously. Therefore, four to eight workpieces can be machined at the same time. Vertical Turret Lathes A vertical turret is basically a turret lathe that has
27、been stood on its headstock end. It is designed to perform a variety of turning operations. It consists of a turret, a revolving table, and a side head with a square turret for holding additional tools. Operations performed by any of the tools mounted on the turret or side head can be controlled thr
28、ough the use of stops. Machining Centers Many of todays more sophisticatedlathes are called machining centers since they are capable of performing, in addition to the normal turning operations, certain milling and drilling operations. Basically, a machining center can be thought of as being a combin
29、ation turret lathe and millingmachine. Additionalfeatures are sometimes included by the versatility of their machines. Numerical Control One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control(NC). Prior to the advent of NC, all machine tools were
30、 manually operated and controlled. Among the many limitations associated with manual control machine tools, perhapsnone is more prominent than limitationof operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical con
31、trol represents the first major step away from human control of machine tools. Numericalcontrolmeans thecontrolofmachinetoolsandother manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician tool to be numerically con
32、trolled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a reader. Numerical control was developed to overcome the limitationof human word 文档 可自由复制编辑 operators, and it has done so. Numerical control machines are more accurate than manually operate
33、d machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology: 1. Electrical discharge machining. 2. Laser cutting. 3. Electron beam welding. Numerica
34、l control has also made machines tools more versatile than their manually operated predecessors.An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertak
35、e the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes. Like so many advanced technologies, NC was born in the laboratories of the Masschusetts Institute of Technology. The concept of NC was developed in early
36、 1950s with funding provided by the U.S.Air force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively. However,curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a cu
37、rve. The shorter is straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated. This problem led to the development in1959 of the Automatically Programmed Tools(APT) language. This is a special programming language for NC that uses statements s
38、imilar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the further development of NC technology. The original NC systems were vastly different from those used
39、today. The machines had hardwired logic circuits. This instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant st
40、ep forward in the control of machine tools. However, there were a number of problems with NC at this point in its development. A major problem wad the fragility of the punched paper tape medium. It word 文档 可自由复制编辑 was common for the paper tape containing the programmed instructions to break or tear
41、during a machining process. This problem was exacerbated by the fact that each programmed instructions had to be return through the reader. If it was necessary to produce 100 copies of a given part,it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes
42、 simply could not withstand the rigors of a shop floor environment and this kind of repeated use. This led to the development of a special magnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instruct
43、ions as a series of magnetic dots. The plastic tape was much stronger than the paper taps, which solved the problem of frequent tearing and breakage. However, it still left two other problems. The most important of these was that it was difficult or impossible to change the instructions entered on t
44、he tape. To make even the most minor adjustments in a program of instructions, it necessaryto interrupt machining operations and make a new tape. It was also still necessary to run the tape through the reader as many times as there were parts to be produced. Fortunately, computer technology became a
45、 reality and soon solved the problem of NC associated with punched paper and plastic tape. The development of a concept known as direct numerical control(DNC)solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the program
46、med instructions. In direct numerical control machine tools are tied, via a data transmission link, to a host computer. Programs for operating the machine tools are stored in the host computer and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represent
47、ed a major step forward over punched tape and plastic tape. However, it is subject to the same limitations as all technologies that depend o a host computer. When the lost computer goes down, the machine tools also experience downtime. This problem led to the development of computer numerical contro
48、l. The development of the microprocessor allowed for the development of programmable logic controllers(PNC)and microcomputer. These two technologies allowed for the development of computer numerical control(CNC). With CNC, each machine tool has a PLC or a microcomputer that serves the same purpose.
49、This allows programs to be input and stored at each individual machine tool. It also allows programs to be developed off-line and download at the individual machine tool. CNC solved the problems associated with downtime of the host computer, but it introduced word 文档 可自由复制编辑 another known as data ma
50、nagement.The same program might be loaded on ten different being solved by local area networks that connect microcomputer for better data management. CNC machine tool feed motion systems CNC machine tool feed motion systems, especially to the outline of the control of movement into the system, must
