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Large Vibrating Screen Design amp; Maintenance

Vibrating screen structures are subjected to nearly 250 million fatigue cycles in an operating year. The design and construction of these structures are critical in achieving reliable screen performance. Regardless of screen size, the maxims for design continue to be: 1. Structural Endurance 2. Minimum Weight 3. Serviceability A screen design meeting these criteria yields the lowest cost per ton performance. Large screen technology is evolving more scientifically than did the development of small screen technology. As vibrating screen designs increase beyond six foot widths, reliable designs result from sophisticated engineering methods and manufacturing techniques. In addition, large screen technology amplifies the direct relationship of production cost and reliability. Static Stresses:At rest, motionless, a vibrating screen structure is subjected to the force of gravity, at a minimum. A vibrating screen must first support its own weight. Other motionless stresses are present i...

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Reliable vibrating screen designs are dependent upon the proper marriage of a firms manufacturing capabilities and the requirements of the design. It is not reasonable to expect that closely toleranced airframes will be successfully produced in a metal bending job shop. As design safety factors narrow on larger screens, manufacturing techniques evolve which minimize production variables. Design tolerancing is necessarily compatible with manufacturing accuracy. Residual metal working stress is the left over stress in metal when melted or formed into a shape. It is a result of a materials resistance to change shape. Stress concentration sites are more commonly termed notches or stress risers. These areas are not stresses, but sharp geometric transitions or reversals in a structure. Stress loads focus their effect on a structure at these sites. Experience has proven that the methods and procedures of structural assembly can result in preloading screen bodies with excessive static s...

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Anticipated operating and maintenance costs over the productive life of a processing plant design significantly influence the go or no go decision to build the plant. Large vibrating screens can both add to and reduce the magnitude of these costs. Plant designers must examine the serviceability of these large units. This includes the complexity of installation, start up, routine maintenance, major repairs, and operating instrumentation. In assessing these costs, the likely condition exists somewhere between the extreme of a screen leaping momentarily out of position long enough to repair itself and swarms of mechanics covering the unit like bees on honey over several production robbing shifts. As larger vibrating screens are used, their size will exceed cost effective shipping limits fully assembled. Screen manufacturers will join the ranks of other major equipment suppliers in on site assembly and testing of these units. The incremental costs associated with these efforts must be c...

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The use of larger vibrating screens results in the dependence of a larger percentage of total plant production on each unit. It is imperative that plant operators maximize the production availability of large screens. This effort is enhanced by carefully planned operating and maintenance procedures. Since volumes have been published on efficient and successful preventative maintenance programs, this discussion will not deal with that topic. There are several suggestions that can be made to help potential big screen users better position themselves to react to the service requirements of these units. As trite as it sounds, talk to potential screen suppliers specifically about the service requirements of their screens. Determine how recently a manufacturer has enter┬Čed the wide screen market. Was this entry preceded by years of research and testing? There are generally two major shortfalls in a hastily planned new product introduction. Invariably, replacement parts availability is a p...

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The development of vibrating screens over the last century has seen many variations to suit the exacting requirements of industry. Indeed, as each year passes, industry has presented the challenge to screen manufacturers of supplying larger machines than those used in the past and the question is often posed what is the maximum limit? Innovations introduced such as bouncing ball decks, heated decks, tri sloped and bi sloped decks and pool washing features have all sought to achieve improved anti blinding results and improved capacity for a given screening efficiency. Although the benefits achieved by the inclusion of these features were shown in some cases to be beneficial, the application of good throw in conjunction with the required G force in the operation of the screen has proven in screen performance today, to provide maximum screening efficiency and capacity. The importance of good throw is often overlooked and should be the first consideration when wishing to maximiz...

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Throw is the term used by vibrating screen designers denoting the distance that a vibrating screen travels during its operating motion. The term is also referred to as stroke. For a straight line motion screen the throw is the distance between the extremities of motion. For a circular motion screen, the throw is measured across the diameter of motion but if the screen has an oval motion, throw is measured by taking the mean of the major and minor axes. The throw which is specified for a particular application is determined on a screen body eccentric weight basis and normally does not take into allowance the load of material which will be handled by the vibrating screen. Therefore it is imperative that the live weight of the vibrating screen is sufficient to maintain, within reason, the throw which has been originally specified so as to effectively handle the loads being fed to the screen. The above comments relate essentially to a dry screening application but in we...

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The inclusion of large vibrating screens in the design of new plants by planning engineers and metallurgists responsible for such work, particularly where large associated equipment is available, is inevitable and is in fact a progression of size we have witnessed over the years. We should remind ourselves that size progression could not proceed without the accumulation of experience in screen body design, in application knowledge, improved quality of manufacture and refinements of mechanism design with regard to achieving improved bearing life which allows the use of a good G force. As referenced previously G force and throw are interrelated and therefore with the good G forces available today in the modern vibrating screens, the way is clear to taking full opportunity of increasing throw to handle the high tonnages which can be expected and are currently experienced on large vibrating screens. Where abrasion of the screen deck surface is severe as in most metalliferous m...

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The screen cyclone system relies on the blinding tendency of the screen deck apertures for its success, using either stainless steel wedgewire or polyurethane deck panels in conjunction with the use of cross dams spaced every 120 cm along the deck surface. When considering the screen cyclone system it is important to appreciate that the screen function is not one of separation at a given aperture size but bleeding of water through the restricted deck apertures caused by the semi blinding condition. That is, if the deck apertures were to remain completely free of blinding, which is not the case, practically all of the tailings would pass through the apertures in the first pass and would not allow the system to function. The underflow from the primary cyclones should be deposited on the horizontal section of the screen deck at the feed end where the maximum of water should be removed with the assistance of an additional section of wedgewire located on a 45┬░ inclined back plate to re...

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The actual results from the initial test run taken on the pilot plant installed at Philex Mining Corporation, Philippines in March, 1980 are as follows using a gravitated flow of tailing slurry from the concentrator.

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The problems involved in installing, maintaining, and operating large vibrating screens have been summarized and discussed, based on a survey of current use of such screens in selected North American mineral processing applications. Practical, effective solutions for the more serious common problems are described, along with some recommendations on design practice for specifying, selecting, and installing large screens. In order to properly assess the information gathered through the survey questionnaire, the results pertaining to each group of applications will be presented and discussed separately in the following section. The small number of installations actually surveyed makes any rigorous statistical interpretation of the data difficult, therefore the information is presented in a generalized fashion. Notwithstanding the small sample of operations as compared to the total number of such large screen installations around the world, the results are felt to fairly represent typic...

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the efficiency generally increases with length. The efficiency also depends, to a lesser degree, on the inclination angle, with efficiency declining as the angle gets steeper. Efficiency could reach up to 95% using a screen with large dimensions and the best technologies for everything. This rarely has been attained and requires dedicated and costly tests. In practice,...

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Vibrating screens are characterized by dynamic motions mostly in the vertical plane . The acceleration of such movements typically ranges from 3 to 6 g or even more. The lifting and dropping effect expands the effective screening bed. Pure vertical motion gives individual particles opportunity for finding and passing through an opening. Added horizontal motion can resolve this issue and spread materials over the screen. On the other hand, the vertical force component acts to eject near size particles stuck in the openings, thus resisting progressive blinding, and the turbulent expansion of the material bed prevents packing. Therefore, effective screening usually requires a proper combination of vertical and horizontal vibration. The performance of a vibrating screen can be optimized for any application by changing the vibration amplitude and frequency. The screening rate and performance respond more to...

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The basic operation of a vibrating screen is simple. The screen presents a barrier to the passage of oversize material but passes undersize material. Vibration facilitates this process by creating the movement needed to ensure that each particle has opportunities to reach the screen. In fact, each particle should receive several opportunities to pass through the screen. However, the actual screening process isnt quite that simple. It involves many side effects and complicated sub activities; all of these require very careful review and consideration. For instance, the motion imparted to solid particles during screening can generate dust. It also can result in the particles becoming electrostatically charged. Static charges can lead to screen blinding and significantly reduce the efficiency of the screen. In general, blinding is a major issue with any screen. In addition, static charges can act as an ignition source, which can be a major risk for some applications. All the metal com...

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