DF-300多层纸复合机组裁切部分设计【说明书+CAD】
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毕业设计文献翻译 院（系）名称工学院机械系 专业名称机械设计制造及其自动化 学生姓名张旭东 指导教师邹景超2012年 03 月 10 日Speed control of A、C motorsS.P.Kolluri and A.A.TsengDepartment of manufacturing engineering and management City university of Hong Kong Key Words:Speed control;single-phase parallel inverter;general principle Most of a、c motors operate at constant speed and speed control can be obtained by varying the frequency of rhe applied voltage.In many cases the magnitude of the applied voltage will also be varied in direct proportion to the frequency in order to maintain the flux in the machine at a constant value.In general,a static power converter producing a variable frequency , variable magnitude polyphase outer voltage from fixed polyphase a、c mains is required and this can be achieved in one of two ways. Firstly ,a direct conversion (a、c to a、c)can be made using the so called cycloconverter principle. The second an more common way is to convert the fixed a、c to variable d、c and to then second reconvert this d、c voltage to required variable a、c system; such a method uses a d、c link in this case negative anode-cathode voltage does not occur naturally in the d、c to a、c .thyristor converter and the process of forced commutation must be used .two basic methods of forced commutation inverters,the so-called parallel inverter and the pulse-width modulated inverter are in common use. A simplified diagram representing a single-phase parallel inverter is given in Fig.8-8 in which the inductance L between the source and the thyristor acts as a current limiter.with thyristor TH1 in Fig.8-8 conducting ,the supply voltage V appears across one half of the primary of the output transformer T and load current flows. The voltage across the whole of the transformer primary winding is then 2V and the capacitor C is charged to the voltage 2V.when thyristor TH2 is fired,the capacitor dis charges through the two thyristor state and the TH1 is reverse biased until it turns off . thyristor TH2 is then in a conducting state and the supply voltage V appears across the other half of the transformer primary in an opposite sense. The output voltage across the secondary then reverses and is therefore an approximate square wave whose frequency is controlled by the firing pulse applied to the two thyristor. Filtering can be introduced at the output if a sinusoidal output voltage is required. The general principle of pulse-width modulation are illustrated by the circuit of Fig.8-9 . T Thyristor TH1 is the main circuit device and thyristor TH2, capacitor C and resistor R form the commutating circuit for the main thyristor .the presence of the diode D is now essential in that it provided a path for load current and allows the capacitor to discharge. Initially neither thyristor is conducting so that both points A and B in Fig.8-9 are at the potential of the negative rail.if TH1 is fired the point B in Fig.8-9 will now be at the potential of the positive rail .current will build up in the load on an exponential of time constant L/R and ,at the same time ,the capacitor C will be charged to a voltage +V , through the resistor R,with the potential at point B positive with respect to that at point A.if after some convenient time, thyristor TH2 is fired the potential at point A will rise to +v volts, the presence of the charged capacitor will result in the potential at point B in Fig.8-9,rising to +2V volts. The main thyristor Th1 will then be reverse biased and can turn off .load current must still be flowing , because of the presence of the load inductance and this current is supplied by the charge stored in the capacitor . the capacitor will thus be discharged resonantly until point B goes to a negative potential ,at which point the diode D will conduct ,thereby clamping the load voltage at approximately zero and also the providing a flywheel path for the load current .capacitor C will now have a voltage of +V across it with point A positive and the thyristor TH1 will be conducting with a small current passing through the resister R. the load current will fall exponentially until thyristor TH2 is fired again . this will cause the potential at point B to rise to +V so that the potential at point A rises to +2V.the thyristor TH1 is thus extinguished and the capacitor C recharges through R with the point B again at the positive potential,Ready for the next cycle of operation. More elegant forms of the basic circuit of Fig.8-9 can be generate a sinusoid of output voltage with a superimposed high frequency ripple . the ripple frequency is controlled by the maximum permissible switching rate thyristor and the allowable commutation loss which takes place as switching occurs.The frequency of the output voltage is ready controlled by control of the pulse repetition rate of the gate signal。Automatic Control SystemsT.S.HuangDepartment of manufacturing engineering and management City university of Hong KongKey words: open system, closed system,functionsAutomatic control systems are divided into two classes, open and closed. In the open system,the output of the process has no effect upon the input to the process. In the closed system ,the energy input to the system is some function of the output itself .This effect is commonly termed feedback;hense the system is a feedback control system, a closed system does not exist,if the closed ring is broken anywhere.Open systems are generally unsuitable for most process control systems,become the output of the system is not of a function of itself. The closed system is used in every completely automatic control application .It is the result of the working peoples boundless creative power that we have automatic control today .automatic controller have performed great functions in industry .they have made it possible to produce a more uniform quality of product . because they are on the job 100percent of the time, whereas a human operator finds it necessary to take rest and finds it difficult to remain alert on repetitious jobs. therefore with the development of industry we have replaced human operator with automatic devices in many industrial processes. The workers and technicians of our country have designed and made various automatic devices,which have made it possible to increase production costs,and to secure the safety of operating workers. Furthermore,with the wide-spread application of automatic controllers,more and more workers are free from routine operations and they can concentrate on production in breadth and depth.Look at Fig.18-1,it is a drawing of the control of temperature.it is an automatic control system. there is an operator in the drawing .he is operating the electric switch. his eyes focus on the temperature .one of his hands is on the switch. Now the switch is not closed, it is open. In the drawing,you can see a heater. In this example the objective is to maintain a consrant temperature in the closed space. if we replace the operator with a siple device, the system will become automatic. Fig.18-2 shows an automatic control system.The components of automatic control systems:Any automatic conreol system has two prime components:the process and the automatic controller. Each has important characteristics which influence the control abilities of the system.The process is the part of the system which performs some desired function;it may be mechanical,or a combination of these.The automatic controller is the mechanism which measures the value of the variable and corrects the deviation from the value.3 Ball and roller bearingG.Wubken and H.BangertDepartment of Manufacturing Engineering and ManagementCity University of Hong KongKey words: present state; Construction;Type; Roller Bearings;Contact stressBall and roller bearings have brought to their present state of perfection only after a long period of research and development .the benefits of such specialized can be obtained when it is possible to use a standardized bearing of the proper size and type. Ball bearing are used in almost every kind of machine and device with rotating part .However bearing cannot be used indiscriminately without a careful study of the load and operating conditions. in addition, the bearing must be provided with adequate mounting, lubrication, and sealing.Pillow blocks, incorporating ball or roll bearings, are available from stock, and are very convenient as the mounting and sealing of the bearing has already been done .Construction and type of ball bearing A ball bearing usually consists of four parts :an inner ring ,an outer ring, the balls and the cage or separator. To increase the contact area and permit larger loads to be carried, the ball run in curvilinear grooves in the rings. The radius of the grooves is slightly larger than the radius of the ball ,and a very slight amount of radial play must be provided. The bearing is thus permitted to adjust itself to small amounts of angular misalignments in the assembled shaft and mounting. The separator keeps the ball evenly spaced and prevents them from touching each other on the sides，where their relative velocities are the greatest. Ball bearings are made in wide variety of types and size. Single-row bearings are made in four serious,” extra light ,light ,medium ,and heavy ,”for each bore, as illustrated in Fig(a)(b )and (c).The heavy series of bearings is designated by 400.Most manufactures use a numbering system so devised that if the last two digits are multiplied by 5,the result will be the bore in millimeters. the digits in the third place from the right indicates the series number. Thus ,bearing 307 signifies a medium-series bearing of 30 mm bore. Additional digits, which may be present in the catalog number of a bearing ,refer to the manufacturers details .some makers list deep groove bearings and bearings with two rows of balls. The radials bearing is able to carry a considerable amount of axial thrust. However, when the load is directed entirely along the axis, the thrust should be used. the angular contact bearing will take care of both radial and axial load .the self aligning ball bearing will take care of large amount of angular misalignment. An increase in radial capacity may be secured by using rings with deep grooves, or by employing a double-row radial bearing. Radial bearing are divided into two general classes depending on the method of assembly. These are the Conrad ,or nonfilling-notch type, and the maximum, or filling-notch type. In the Conrad bearing, the ball are placed between the rings as shown in Fig. then they are evenly spaced and the separator is riveted in place. In the maximum-type bearing, the balls are inserted through a filling notch ground into each rings as shown in fig (b ) because more balls can be placed in such bearings their load capacity is greater than that of the Conrad type. However, the present of the notches limits the load-carrying abilities of those bearing in the axial direction. High-carbon chromium steel 52100 is used for balls and rings, It is heated to high strength and hardness ,and the surface are smoothly ground . the dimensional tolerances and very small ;the ball must be very uniform in size. The stresses are extremely high because of the small contact areas, and the yield point of the materials may be exceeded at certain point. Because of the high value of the fluctuating stress ,antifriction bearings are not designed for unlimited life, but for some finite period of service determined by the fatigue strength of the materials. A specified speed and number of hours of expected service must therefore accompany the given load value for these bearings.Roller bearings When shock and impact loads are present, or when a large bearing is needed ,cylindrical and tapered roller bearing are usually used. A roller bearing ,in general consists of the same for element as a ball bearing :the two rings ,the cage ,and the roller. Some typical examples of roller bearings are shown in Fig 3 In the cylindrical roller bearing, the flanges on the rings serve to guide the rollers in the proper direction. when the flanges are omitted from one of the rings .as shown in Fig 3 .The rings can then be displaced axially with respect to each other, and no thrust component can be carried . In addition to the radial load ,the tapered roller bearing can carry a large axial component whose magnitude depend on the angularity of the rollers. The radial load will also produce a thrust component the outer ring is separable from the remainder of the bearing. in this type of bearing ,it is possible to make adjustment for the radial clearance :two bearings are usually mounted opposed to each other, and the clearance is controlled by adjusting one bearing against the other. doublerow tapered roller bearing are also available. Roller bearings, in general ,can be applied only where the angular misalignment caused by shaft deflection is very slight. This deficiency is not present in the spherical roller bearing. It has excellent load capacity and can carry a thrust component in either direction . In the flexible roller bearing, the rollers are wound from strips of spring steel, and afterwards are hardened and ground to size. If desired the rollers can bear directly on the shaft without an inner ring, particularly if the shaft surface has been locally hardened. This bearing has been successfully applied under conditions of dirty environment. The needle bearings has rollers that are very long as compared to their diameters. Cages are frequently not used, and the inner ring may or may not be present. The outer ring may consists of hardened thin-walled metal as shown in Fig(3) ;the housing in which the bearing is mounted must haves sufficient thickness to give adequate support. The friction of needle bearing is several times as great as for ordinary cylindrical roller bearings. Because of the tendency of the unguided rollers to skew, needle bearings are particularly adapted to oscillating loads as in wrist pins , rocker arms, and universal joints. For continuous rotation, needle bearings are usually suitable where the loading is intermittent and variable so that the needles will be frequently unloaded and thus tend to return their proper locations. When the application involves angular misalignment of the shaft, two short bearings end to end are usually better than one bearing with long rollers. The needle bearing is lowpriced and requires very little radial space. Roller bearings are selected by a process similar to that used for ball bearings. They must be chosen, however, in accordance with the recommendations given in the catalog of the particular type of bearing under consideration . Contact stress between spheres and cylinders When two bodies having surfaces of different curvature are pressed together, an area of contact develops because there is no such thing as line or point contact. This area is usually very small, and the resulting stress attain a very high value In addition to rolling contact bearing t
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