毕业设计--圆振动筛设计的外文翻译

毕业设计--圆振动筛设计的外文翻译辽宁科技大学本科生毕业设计第1页Screening Vibrating screensPrinciples --Vibrating screens save space and weight and operate on little power because the screening surface may be actuated by vibrating, gyrating or pulsatingmovement of small amplitude, but at frequencies that normally exceed 3,000/min.Selection of Proper Vibration Sereen --Be sure the screen supplier knows all details ofthe application. The centrifugal force factor, or combination of frequency of vibration(speed) and amplitude (throw), may affect performance of anyvibrating screen. Also,a correct combination of slope and direction of mechanism rotationis vital forinclined screens. Usually, the larger the opening, the greater the amplitude needed fora screen.If the throw is too small, the material may clog or wedge in the openings. Increasingthe throw beyond what is required to prevent blinding or plugging does notnecessarily increase the life of the bearings and reduce screening efficiency.Increased rate of travel permits more tonnage to be passed over the screen per unit oftime. For a given tonnage, a faster rate of travel results in a thinner bed of materialand high screening efficiency.Maximum slope is reached when the material travels too fast for the fines to penetratethe ribbon of material and reach the apertures in the screen cloth. At this point anexcessive amount of fine material passes over the screen with the oversize, resultingin poor efficiency.When an existing screen is to be used for an application other than that for which itwas originally intended, check with the Supplier to see if any of the operatingcharacteristics need modification and if the the screen is structurally suitable for thenew application.The operator can get the correct vibrating screen by providing the supplier with thefollowing information:, Maximum tons per hour to be screened, including any circulating load or anysurges in the feed rate., A complete size consist or sieve analysis of the material or, if available, anestimated analysis., Type of material and weight per cubic foot in broken state., Separations desired on each deck., Surface moisture carried by the material if screening is to be dry or amount ofwater with feed if wet., Special operating requirements or conditions such as temperature, abrasiveness, corrosiveness or other physical characteristics of the feed,辽宁科技大学本科生毕业设计第2页efficiency or product requirements which determine selection of screeningsurface, or installation problems which affect screen size selection or capacity.General Types --Vibrating screens may be divided into two main classes: mechanically-vibrated and electrially-vibrated. The former can be subdivided into classes based on how the vibration is produced--by eccentrics; by unbalanced weight; by cams or bumpers. They can also be subdivided as inclined and horizontal. Sizes --Vibrating screens aremade in standard sizes of from 12 in. to 10 ft wide and from 2 1/2 ft to 28 ft long. Common practice dictates that the length of the screen should be 2.5 times the width for dry screening. For wet screening, wider and shorter screen is best. Screens for scalping ahead of primary crushers, operate at a slope of from 12 degrees to 18 degrees and have openings as large as 11-in. square. The eccentric throw for openings from 5 to 11 in. is usually 1/2 in.; for openings from 3 to 5 in. about 3/8 in.; and for smaller openings 1/4 in. The screening surface consists of a heavy cast desk, perforated steel with or without skid bars welded between the holes, rod deck, etc.The mechanical shaking screen comprises a rectangular frame, with perforated steel or wire cloth screening medium. It is usually inclined and suspended on loose rods or cables. These screens now are used mainly for special tasks of coarse screening, having given way to vibrating screens.Electrically-Heated Screens --Electrically heated screen cloth decks have affordedbetter screening and less dust. Modern or updated screening operations, now handling clays, limestone, potash, salts, phosphates and various hydroscopic materials, report minimum dust loss when equipped with heated screens. The controlling factor in this improvement is the electrically-heated screen doth deck. Any vibratory screen with fine opening (less than 1/2 in.) can be equipped with low volt-age-high amperage resistance heating.The principle of electric heating is based on the fact that small diameter wire of screen cloth (especially stainless steel) serves as a conductor, but offers resistance to a high-amperage current. This resistance causes heating of the wire when powerful transformer and specially designed bus bars connected to screen doth decks push up to 6,000 amps current into a circuit. The current is safe and shock-proof because voltage is low, ranging from approximately 1 1/2 to 16 volts. Workers can do their jobs around electrically-heated screens without special precautions.Heated screens are effective in preventing moisture content of material from causing buildup and blinding. The screen wire is kept at 100 degrees to 130 degrees F depending on character and tonnage of the material processed. This temperature is not high enough to weaken the wire cloth or screen structure, nor does it suffice to drive off moisture held in material. The warm wire stays dry, breaking the surface tension that otherwise would bind damp material to cold, damp metal. This differential or辽宁科技大学本科生毕业设计第3页warm, dry screen wire versus cold, damp material can be maintained economically through transformer control settings.With every opening in the heated screen mesh protected against blinding, there is no guesswork about what size particle will be delivered. A uniform, unvarying quality to meet tough specificationscomes through day after day, no matter what the weather. (High humidity makes trouble with unheated screens.) Plants operating heated screens will have no trouble with excessive amounts of fine, dusty material that formerly sifted through reduced meshes on clogged screens.Service life of screen cloth is greatly increased when electric heating ends the punishment of old-fashioned cleaning methods, such as rough pounding or brushing. Blowtorch flames put too much heat in one spot and bouncing chains added to wear and tear. But, screens kept clean and open with electric heating reportedly are seldom mistreated and last up to eight times as long. Heated decks end the threat of pile ups and strains that can break the mesh.Economical operation of electric heating for screens requires one transformer for installations of up to three panels of screen cloth (maximum area 4- x 12-ft). Two transformers are used on longer decks. Any deck (top, center or bottom) can be heated. Electric heating is most desirable of all where a screen surface is hard to get at. Changing heated screens does not involve more unbolting than needed for ordinary screens.In specifying screens to deliver a uniform particle through heated mesh, the first thing to consider is weight per square foot of the wire cloth needed to set up appropriate resistance. Most calculations are based on square openings. Slotted openings must be identified as to width of clear opening, diameter of wire and number of wires per inch before weight per square foot can be found in any screen clothmanufacturer's catalog. Having determined the usable weight per square foot of screen cloth, the next step is to select the size of clear opening needed, making no allowance for reduction in this size as formerly was the case when material stuck to the wires. Heated wireswill maintain the clear opening at all times, making it possible to screen finer without blinding.From a wire catalog, select a mesh weighing no more per square foot (can weigh less) than determined using the method above with the clear opening characteristics desired. This often turns out to be a more efficient screen because the wire diameter will be smaller and the percentage of the open area will be greater.For example, where a 1/8-in. clear opening was desired (but often blinded) on unheated screens, the wire diameter was 0.63 and the open area was 44 percent. The cloth weighed 1.43 lb/ft.2. For a heated 3- x 12-ft screen, the weight must not exceed辽宁科技大学本科生毕业设计第5页21.1 lb/ft. Two options were found in cloth with 1/8-in. clear opening: wire2diameter .054 weighing 1.09 lb/ft providing 48.8 percent open area; and wire2diameter .047 weighing 0.85 lb/ft with a 52.8 percent open area. Load conditions may make fine diameters of carbon steel wire inadequate. In suchcases, stainless steel wire of larger diameter with greater load-carrying capacity willhave the necessary resistance for good heating.Lighter wire with more open area yields higher tonnage and heated mesh stays open100 percent. Two screen cloths connected in a series may be of different mesh sizes orclear openings, as long as they both weigh the same per ft2 and donot exceed theweight allowed for that particular size of screen.With heated screens, major savings in pollution control are possible. Also moisturecontent of material is reportedly increased in a range of 5 to 8 percent. This moistureis added during or after crushing-grinding operations and holds down dust duringtransfer of material and passage over heated screens. Warm wire handles 5 to 8percent moisture in stride. The cost of heating equipment and mist spraying reportedlyis less than the cost of having bag towers and precipitators.辽宁科技大学本科生毕业设计第5页振动筛的选择振动筛的选择原则：选择振动筛的原则是所选择的振动筛要节省空间、重量并且驱动的功率要小，因为筛选表面可以驱动并且发生振动。