When the smaller rock has to be classified a vibrating screen will be used.The simplest Vibrating Screen Working Principle can be explained using the single deck screen and put it onto an inclined frame. The frame is mounted on springs. The vibration is generated from an unbalanced flywheel. A very erratic motion is developed when this wheel is rotated. You will find these simple screens in smaller operations and rock quarries where sizing isnt as critical. As the performance of this type of screen isnt good enough to meet the requirements of most mining operations two variations of this screen have been developed.
In the majority of cases, the types of screen decks that you will be operating will be either the horizontal screen or the inclined vibrating screen. The names of these screens do not reflect the angle that the screens are on, they reflect the direction of the motion that is creating the vibration.
An eccentric shaft is used in the inclined vibrating screen. There is an advantage of using this method of vibration generation over the unbalanced flywheel method first mentioned. The vibration of an unbalanced flywheel is very violent. This causes mechanical failure and structural damage to occur. The four-bearing system greatly reduces this problem. Why these screens are vibrated is to ensure that the ore comes into contact will the screen. By vibrating the screen the rock will be bounced around on top of it. This means, that by the time that the rock has traveled the length of the screen, it will have had the opportunity of hitting the screen mesh at just the right angle to be able to penetrate through it. If the rock is small enough it will be removed from the circuit. The large rock will, of course, be taken to the next stage in the process. Depending upon the tonnage and the size of the feed, there may be two sets of screens for each machine.
The reason for using two decks is to increase the surface area that the ore has to come into contact with. The top deck will have bigger holes in the grid of the screen. The size of the ore that it will be removed will be larger than that on the bottom. Only the small rock that is able to pass through the bottom screen will be removed from the circuit. In most cases the large rock that was on top of each screen will be mixed back together again.
The main cause of mechanical failure in screen decks is vibration. Even the frame, body, and bearings are affected by this. The larger the screen the bigger the effect. The vibration will crystallize the molecular structure of the metal causing what is known as METAL FATIGUE to develop. The first sign that an operator has indicated that the fatigue in the body of the screen deck is almost at a critical stage in its development are the hairline cracks that will appear around the vibrations point of origin. The bearings on the bigger screens have to be watched closer than most as they tend to fail suddenly. This is due to the vibration as well.
In plant design, it is usual to install a screen ahead of the secondary crusher to bypass any ore which has already been crushed small enough, and so to relieve it of unnecessary work. Very close screening is not required and some sort of moving bar or ring grizzly can well be used, but the modern method is to employ for the purpose a heavy-duty vibrating screen of the Hummer type which has no external moving parts to wear out ; the vibrator is totally enclosed and the only part subjected to wear is the surface of the screen.
The Hummer Screen, illustrated in Fig. 6, is the machine usually employed for the work, being designed for heavy and rough duty. It consists of a fixed frame, set on the slope, across which is tightly stretched a woven-wire screen composed of large diameter wires, or rods, of a special, hard-wearing alloy. A metal strip, bent over to the required angle, is fitted along the length of each side of the screen so that it can be secured to the frame at the correct tension by means of spring-loaded hook bolts. A vibrating mechanism attached to the middle of the screen imparts rapid vibrations of small amplitude to its surface, making the ore, which enters at the top, pass down it in an even mobile stream. The spring-loaded bolts, which can be seen in section in Fig. 7, movewith a hinge action, allowing unrestricted movement of the entire screening surface without transmitting the vibrations to the frame.
One, two, or three vibrators, depending on the length of the screen, are mounted across the frame and are connected through their armatures with a steel strip securely fixed down the middle of the screen. The powerful Type 50 Vibrator, used for heavy work, is shown in Fig. 7. The movement of the armature is directly controlled by the solenoid coil, which is connected by an external cable with a supply of 15-cycle single-phase alternating current ; this produces the alternating field in the coil that causes the up-and-down movement of the armature at the rate of thirty vibrations per second. At the end of every return stroke it hits a striking block and imparts to the screen a jerk which throws the larger pieces of ore to the top of the bed and gives the fine particles a better chance of passing through the meshes during the rest of the cycle. The motion can be regulated by spiral springs controlled by a handwheel, thus enabling the intensity of the vibrations to be adjusted within close limits. No lubrication is required either for the vibrating mechanism or for any other part of the screen, and the 15-cycle alternating current is usually supplied by a special motor-generator set placed somewhere where dust cannot reach it.
The Type 70 Screen is usually made 4 ft. wide and from 5 to 10 ft. in length. For the rough work described above it can be relied upon to give a capacity of 4 to 5 tons per square foot when screening to about in. and set at a slope of 25 to 30 degrees to the horizontal. The Type 50 Vibrator requires about 2 h.p. for its operation.
The determination of screen capacity is a very complex subject. There is a lot of theory on the subject that has been developed over many years of the manufacture of screens and much study of the results of their use. However, it is still necessary to test the results of a new installation to be reasonably certain of the screen capacity.
A general rule of thumb for good screening is that: The bed depth of material at the discharge end of a screen should never be over four times the size opening in the screen surface for material weighing 100 pounds per cubic foot or three times for material weighing 50 pounds per cubic foot. The feed end depth can be greater, particularly if the feed contains a large percentage of fines. Other interrelated factors are:
Vibration is produced on inclined screens by circular motion in a plane perpendicular to the screen with one-eighth to -in. amplitude at 700-1000 cycles per minute. The vibration lifts the material producing stratification. And with the screen on an incline, the material will cascade down the slope, introducing the probability that the particles will either pass through the screen openings or over their surface.
Screen capacity is dependent on the type, available area, and cleanliness of the screen and screenability of the aggregate. Belowis a general guide for determining screen capacity. The values may be used for dried aggregate where blinding (plugged screen openings), moisture build-up or other screening problems will not be encountered. In this table it is assumed that approximately 25% of the screen load is retained, for example, if the capacity of a screen is 100 tons/hr (tph) the approximate load on the screen would be 133 tph.
It is possible to not have enough material on a screen for it to be effective. For very small feed rates, the efficiency of a screen increases with increasing tonnage on the screen. The bed of oversize material on top of the marginal particlesstratification prevents them from bouncing around excessively, increases their number of attempts to get through the screen, and helps push them through. However, beyond an optimum point increasing tonnage on the screen causes a rather rapid decrease in the efficiency of the screen to serve its purpose.
Two common methods for calculating screen efficiency depend on whether the desired product is overs or throughs from the screen deck. If the oversize is considered to be the product, the screen operation should remove as much as possible of the undersize material. In that case, screen performance is based on the efficiency of undersize removal. When the throughs are considered to be the product, the operation should recover as much of the undersize material as possible. In that case, screen performance is based on the efficiency of undersize recovery.
These efficiency determinations necessitate taking a sample of the feed to the screen deck and one of the material that passes over the deck, that is, does not pass through it. These samples are subjected to sieve analysis tests to find the gradation of the materials. The results of these tests lead to the efficiencies. The equations for the screen efficiencies are as follows:
In both cases the amount of undersize material, which is included in the material that goes over the screen is relatively small. In Case 1 the undersize going over the screen is 19 10 = 9 tph, whereas in Case 2 the undersize going over is 55 50 = 5 tph. That would suggest that the efficiency of the screen in removing undersize material is nearly the same. However, it is the proportion of undersize material that is in the material going over the screen, that is, not passed through the screen, that determines the efficiency of the screen.
In the first cases the product is the oversize material fed to the screen and passed over it. And screen efficiency is based on how well the undersize material is removed from the overs. In other cases the undersize material fed to the screen, that is, the throughs, is considered the product. And the efficiency is dependent on how much of the undersize material is recovered in the throughs. This screen efficiency is determined by the Equation B above.An example using the case 1 situation for the throughs as the product gives a new case to consider for screen efficiency.
Generally, manufacturers of screening units of one, two, or three decks specify the many dimensions that may be of concern to the user, including the total headroom required for screen angles of 10-25 from the horizontal. Very few manufacturers show in their screen specifications the capacity to expect in tph per square foot of screen area. If they do indicate capacities for different screen openings, the bases are that the feed be granular free-flowing material with a unit weight of 100 lb/cu ft. Also the screen cloth will have 50% or more open area, 25% of total feed passing over the deck, 40% is half size, and screen efficiency is 90%. And all of those stipulations are for a one-deck unit with the deck at an 18 to 20 slope.
As was discussed with screen efficiencies, there will be some overs on the first passes that will contain undersize material but will not go through the screen. This material will continue recirculating until it passes through the screen. This is called the circulating load. By definition, circulating load equals the total feed to the crusher system with screens minus the new feed to the crusher. It is stated as a percentage of the new feed to the crusher. The equation for circulating load percentage is:
To help understand this determination and the equation use, take the example of 200 tph original or new material to the crusher. Assume 100% screen efficiency and 30% oversize in the crusher input. For the successive cycles of the circulating load:
The values for the circulating load percentages can be tabulated for various typical screen efficiencies and percents of oversize in the crusher product from one to 99%. This will expedite the determination for the circulating load in a closed Circuit crusher and screening system.
Among the key factors that have to be taken into account in determining the screen area required is the deck correction. A top deck should have a capacity as determined by trial and testing of the product output, but the capacity of each succeeding lower deck will be reduced by 10% because of the lower amount of oversize for stratification on the following decks. For example, the third deck would be 80% as effective as the top deck. Wash water or spray will increase the effectiveness of the screens with openings of less than 1 in. in size. In fact, a deck with water spray on 3/16 in. openings will be more than three times as effective as the same size without the water spray.
For efficient wet or dry screeningHi-capacity, 2-bearing design. Flywheel weights counterbalance eccentric shaft giving a true-circle motion to screen. Spring suspensions carry the weight. Bearings support only weight of shaft. Screen is free to float and follow positive screening motion without power-consuming friction losses. Saves up to 50% HP over4- bearing types. Sizes 1 x 2 to 6 x 14, single or double deck types, suspended or floor mounted units.Also Revolving (Trommel) Screens. For sizing, desliming or scrubbing. Sizes from 30 x 60 to 120.
TheVibrating Screen has rapidly come to the front as a leader in the sizing and dewatering of mining and industrial products. Its almost unlimited uses vary from the screening for size of crusher products to the accurate sizing of medicinal pellets. The Vibrating Screen is also used for wet sizing by operating the screen on an uphill slope, the lower end being under the surface of the liquid.
The main feature of the Vibrating Screen is the patented mechanism. In operation, the screen shaft rotates on two eccentrically mounted bearings, and this eccentric motion is transmitted into the screen body, causing a true circular throw motion, the radius of which is equivalent to the radius of eccentricity on the eccentric portion of the shaft. The simplicity of this construction allows the screen to be manufactured with a light weight but sturdy mechanism which is low in initial cost, low in maintenance and power costs, and yet has a high, positive capacity.
The Vibrating Screen is available in single and multiple deck units for floor mounting or suspension. The side panels are equipped with flanges containing precision punched bolt holes so that an additional deck may be added in the future by merely bolting the new deck either on the top or the bottom of the original deck. The advantage of this feature is that added capacity is gained without purchasing a separate mechanism, since the mechanisms originally furnished are designed for this feature. A positivemethod of maintaining proper screen tension is employed, the method depending on the wire diameter involved. Screen cloths are mounted on rubber covered camber bars, slightly arched for even distribution.
Standard screens are furnished with suspension rod or cable assemblies, or floor mounting brackets. Initial covering of standard steel screen cloth is included for separations down to 20 mesh. Suspension frame, fine mesh wire, and dust enclosure are furnished at a slight additional cost. Motor driven units include totally-enclosed, ball-bearing motors. The Vibrating Screen can be driven from either side. The driven sheave is included on units furnished without the drive.
The following table shows the many sizes available. Standard screens listed below are available in single and double deck units. The triple and quadruple deck units consist of double deck units with an additional deck or decks flanged to the original deck. Please consult our experienced staff of screening engineers for additional information and recommendations on your screening problems.
An extremely simple, positive method of imparting uniform vibration to the screen body. Using only two bearings and with no dead weight supported by them, the shaft is in effect floating on the two heavy-duty bearings.
The unit consists of the freely suspended screen body and a shaft assembly carried by the screen body. Near each end of the shaft, an eccentric portion is turned. The shaft is counterbalanced, by weighted fly-wheels, against the weight of the screen and loads that may be superimposed on it. When the shaft rotates, eccentric motion is transmitted from the eccentric portions, through the two bearings, to the screen frame.
The patented design of Dillon Vibrating Screens requires just two bearings instead of the four used in ordinary mechanical screens, resulting in simplicity of construction which cuts power cost in half for any screening job; reduces operating and maintenance costs.
With this simplified, lighter weight construction all power is put to useful work thus, the screen can operate at higher speeds when desired, giving greater screening capacity at lower power cost. The sting of the positive, high speed vibration eliminates blinding of screen openings.
The sketches below demonstrate the four standard methods of fastening a screen cloth to the Dillon Screen. The choice of method is generally dependent on screen wire diameters. It is recommended that the following guide be followed:
Before Separation can take place we need to get the fine particles to the bottom of the pile next to the screen deck openings and the coarse particles to the top. Without this phenomenon, we would have all the big particles blocking the openings with the fines resting atop of them and never going through.
We need to state that 100% efficiency, that is, putting every undersize particle through and every oversize particle over, is impossible. If you put 95% of the undersize pieces through we in the screen business call that commercially perfect.
Large vibrating screens represent a unique challenge for Manufacturers, Plant Designers, and Plant Operators. The inherent mode of operation for vibrating screens is self-destructive. More often than Manufacturers admit, Designers plan for, or Operators staff for, a vibrating screen succeeds and self-destructs. This is a problem. It can magnify with larger vibrating screens.
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:
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 in the structure as a result of cutting, bending, welding, burning, drilling, assembly, tolerancing, and manufacturing variances. Quite simply, these stresses exist whether or not the screen is operating.
The second step in FEA can be considered the construction of structural loads. These include the imposition of static, dynamic, material, and fatigue conditions on the mathematical model, which approximates the load conditions. An example would be to describe a structural misalignment and the forces input co bolt up this structure through the misalignment.
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 stresses. The scope of this discussion is limited to the discussion of welding, forming, and bolting as they relate to conditions described above.
The side plate of a vibrating screen literally bristles with fasteners. Multi-shift production facilities, as well as maintenance crews, quickly realize the merits of this system. Unlike conventional threaded fasteners, swaged bolts exhibit a distinctly different physical appearance when installed versus loosely installed. The guess-work and wasted efforts to repeatedly insure all bolts are properly torqued are eliminated. A second-shift assembler need not consult with his first-shift counter-part regarding loose or torqued bolts . Sound maintenance practice precludes the reuse of major structural fasteners. A huck-type fastener is destroyed during removal. Normal threaded fasteners depend on proper installation torques to achieve the optimum clamping force. Registered torque wrench values may not be indicative of the true values due to the effects of thread lubrication and frictional force of the fastener face on the bolting surface. Swaged fasteners are installed strictly in tension at an optimum preset tensile load. The positive clamping values are reliably consistent. Installation error is minimal. Replaceable, non-structural components may be installed with conventional fasteners.
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 considered in evaluating the plant construction and start-up costs.
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 entered 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 problem. Second is the frustrating response to a frantic maintenance question, The only guy who knows that unit is on an island in Indonesia. Solidly planned programs will have organizational depth.
The labor pains, which have normally accompanied the birth of new vibrating screen designs, have been no less severe with the gradual introduction of large, high-capacity screens. More difficulty would have been encountered without the aid of advanced engineering and manufacturing techniques.
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 maximize screen capacity.
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 wet applications where metalliferous pulp is received on the screen, the benefits of a large throw in terms of increased screen capacity have been demonstrated in commercial practice. The ideal machine for receiving pulp for wet screening or desliming, dewatering etc. is a horizontal screen. Among other reasons, the horizontal screen provides the benefit of long retention time for handling the pulp. Also the straight line motion provided with good throw imparts a positive breaking of surface tension present between the pulp and the screen deck within the apertures.
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 mining applications, and the separation sizes are in the order of mm to 50 mm aperture sizes, polyurethane screen panels are now in common use because of their excellent resistance to wear. The trend in the use of polyurethane panels in the metalliferous mining industry is quite definite and in fact in the major mining operations in Australia at least, the use of polyurethane screening panels is firmly established.
With reference to metalliferous tailings the need for dewatering presents a new dimension. The amount of tailings produced is very much greater since some 98-99% of mined ore is rejected in tailings form compared with varying amount of 3 to 5% rejected in a coal washing operation. Furthermore with dewatering of metalliferous tailings, using equipment as mostly used in coal washing would present maintenance problems because of the more abrasive nature of the tailings and therefore for that reason it is customary to discharge all metalliferous tailings slurry to a dam.
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 remove free water that has accumulated on top of the bed of slurry most solids having stratified to the deck surface. The underflow should be evenly distributed across the width of the screen at minimum velocity, so as to allow the full benefit of stratification provided by the screen.
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.
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 typical operating, maintenance and installation problems and practices in the sectors of the mineral processing industry the survey covered.
The results reported in this section refer to inclined vibrating screens used in conventional crushing and screening plants. Four operations replied to the survey questionnaire, all four are medium sized producers, primarily of copper concentrate, some with significant by-product production of Mo or Ag. Daily throughputs range from 5,300 tons to 38,000 tons.
The major problem areas reported by the users of these screens were bearing failure and replacement and side plate cracking. The minor problems reported were loose bolts, seals and routine wear items such as cloth and liner changes. Reported availability of the screens ranged from 92-96%. At one operation, the crushing and screening plant is oversized and operates only one shift per day, therefore downtime for maintenance is readily available and actual availability was not reported.
The maintenance of large vibrating screens in conventional crushing applications would normally consist of the regular replacement of wear parts, such as liners and screen cloths, as well as regular lubrication of the bearings and other moving parts as recommended by the manufacturer of the particular screens in use.
The operations with large horizontal vibrating screen installations replying to the survey questionnaire were Syncrude Canada Ltd., Climax Molybdenun (Henderson Operations), Quintana Minerals and Fording Coal Ltd. As previously noted, the screen applications at these operations are all basically very similar, involving wet screening of relatively large tonnages of slurry feed.
The major problem areas with these screen installations once again include bearing failure and side plate cracking in three out of the four installations. The fourth installation, Henderson, reported major problems with the mounting springs and feed lip both of which have presently been rectified to the point where only minimal unscheduled downtime occurs.
The major problems associated with the horizontal screens were with bearings and side plate cracking, and were evident soon after commissioning. Major efforts were undertaken at all the operations to correct the serious problems.
Large vibrating screens are normally selected for applications where multiple screens would be more costly to purchase and install. There have been a considerable number of large screen installations in a variety of mineral processing applications, therefore a considerable amount of operating data with respect to the screen components and performance has been gathered. From the plant designers viewpoint the design of a screen installation should consider the following areas:
The design of a large vibrating screen installation requires close attention to not only the screen itself, but also to the ancillary structures, maintenance procedures and personnel comfort and protection.
Large vibrating screens represent a considerable investment in equipment alone. In addition the loss due to interrupted production should one of these units go out of service can be economically much more severe. As plant tonnages have risen and larger equipment has been utilized in single trains or a small number of multiple trains, the risk of having a single large screen down for any length of time has become too great to ignore.
Direct drive screens offer versatility and high-performance screening in small, value-engineered packages. Screens are available during single motor design for dual motor design for elliptical motion or linear screening. Each style comes in standard sizes, but is often customized with a variety of deck systems, including wire mesh, steel, rod decks, or polyurethane panels.
Linear screens include reliable performance, low usage, low noise, long service life, high screening efficiency, etc. Linear vibrating screen is a plastic, grinding material, so it has been widely used in the industries of vibrating screen.
High screening accuracy, adjustable granularity, great handling capacity, high screening efficiency and long service life; Totally-closed structure and automatic material dissolution make it more suitable for product line work.
We manufacture the latest and most suitable products for different application in different industries. In order to manufacture these products for our customers, we have got a tremendous structural facility, divided into several units to ensure that all processes are managed most efficiently. We have also assigned well-qualified professionals to operate these units properly. Moreover, our team of quality controllers ensures that the quality of the products is never compromised.
We guarantee that only high-quality materials are employed by our modern professionals. In addition, we check on the spread in the field before finally shipping it to our customers destination. Highly acclaimed in the industry for their specificity, these are presented to our customers in standard forms. To add, only maximum class content is employed in their production.
We have built a good and functional infrastructure unit that will play a crucial role in the development of our company. We offer these products at reasonable rates and deliver within the promised timeframe. We have gained a huge asylum across the country.
We accept strength, high performance, corrosion resistance, low maintenance and long service life of our products and services. These products are widely used in the textile and wood industry to cut and shape objects as desired. Further, these products are available in various technical specifications and are specialized as per the requirement of our customers at industry enabled prices.
We are able to produce various magnetic devices supporting the need of customers. We have a group of skilled workers working for the timely completion of projects undertaken without compromising on quality. Our magnetic devices have been used successfully in various industries. In addition to the range offered, we provide installation, forging and lifting services according to the needs of our valued customers. We place great emphasis on equivalence during the manufacturing process by using the highest quality raw materials and adhering to industrial industrial standards and norms and we are very concerned with that standard.
The offered screen is available in different specifications as per the specific requirements of the customers. The screens offered by us are manufactured in accordance with international standards using best quality raw materials and sophisticated technology under the guidance of our experts. In addition, customers can avail this screen from us at customized options and market leading prices.
These products are manufactured in full compliance with the prescribed industry standards using high grade core obtained by reliable and trustworthy vendors of the industry. Our complete product array finds wide application in various industrial fields. Developed by skilled engineers and technicians, our products meet the stresses of industries such as heavy duty and mechanical. To test the quality of our products,
These products are manufactured in full compliance with the prescribed industry standards using high grade core obtained by reliable and trustworthy vendors of the industry. Our complete product array finds wide application in various industrial fields. Developed by skilled engineers and technicians, our products meet the stresses of industries such as heavy duty and mechanical. To test the quality of our products,
Vibrating screens are specially designed motor devices used in applications on the spectrum of industries, be they vibrating screens, ceramics, sand and associated bristles or chemicals. Circular vibrating screens are medium amplitude devices that accompany a standard motor and support the devices particle, amplitude, and frequency characteristics. Supported by springs, the round vibrating screen comes with a shaft on the axis of the deck so keep the device working with any errors.
Orbit Intelligent Engineering is a well-known manufacturer, exporter of almost qualitative assortment automatic fly ash brick plants, automatic ash brick plant. Integrated in the year 2004, at Mehsana (Gujarat, India)
Uma Engineering is a one of the leading Vibrating Screen Manufacturers in India offers a range of high quality Vibrating Screen and Industrial Vibrating Screen. We offer our products in different sizes and dimensions as well as based on customized needs of our customers. As a renowned manufacturer of Vibrating Screen in India, we manufacture products with utmost quality and standards using high grade raw materials. Our products are widely applicable in different industries for their assorted applications.
Normally, Vibrating screen is used in different industries to perform different processes such as to perform operation of grading, de-dusting, classification, oversize removal, fiber recovery as well as de-watering, filtration, and for size based separation. Vibrating Screens are used in different industries that includes
We also supply machines based on the requirement of our customers for their process needs of material separation. Our Vibrating Screens, Vibrating Screen separator, crusher screen, wire screen, industrial woven wire screen are widely applicable for different industries offer best performance in extreme conditions.
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Manuel Moncada M., Cristian G. Rodrguez, "Dynamic Modeling of a Vibrating Screen Considering the Ore Inertia and Force of the Ore over the Screen Calculated with Discrete Element Method", Shock and Vibration, vol. 2018, Article ID 1714738, 13 pages, 2018. https://doi.org/10.1155/2018/1714738
Vibrating screens are critical machines used for size classification in mineral processing. Their proper operation, including accurate vibration movement and slope angle, can provide the benefits of energy savings and cost reductions in the screening process and the whole mining process. Dynamic models of the vibrating screen movement available in the literature do not simulate ore motion or its interaction with screen decks. The discrete element method (DEM) allows for the calculation of the dynamic of the ore. In this paper, two 2D three-degrees-of-freedom dynamic models for a vibrating screen are tested, using linear and nonlinear approaches for angular displacement. These models consider the inertia of the ore and the ore force calculated with DEM. A double-deck linear motion vibrating screen is simulated using the DEM software LIGGGTHS. DEM is used to obtain the ore parameters in the steady state and the force on the screen decks. Two cases are compared: Case 1 considers the ore as moving together with the vibrating screen, and Case 2 considers the ore force on the screen deck as calculated by DEM. Simulations are carried out with data for an industrial vibrating screen used in copper mining. The force over the screen is significantly different between the cases. Case 1 produces a force that is unrealistic because the ore cannot produce a high-amplitude adhesion force over the screen decks. In Case 2, no adhesion force acts between the ore and deck. It is concluded that the linear dynamic model used in Case 2 is adequate to evaluate the influence of the ore on the movement of the vibrating screen. The linear dynamic model considering the force as in Case 1 can be used to simulate a vibrating screen, as long as a correct calibration parameter is included to obtain an accurate motion amplitude.
Size classification of particulate materials is an important process in mineral processing , particularly in the copper industry. Vibrating screens are frequently used to separate granulated ore materials based on particle size for particles with diameters greater than 0.5mm . In the copper industry, the most commonly used vibrating screens have linear motion and horizontal, sloped, or multisloped (banana) screens . A vibrating screen has one or more screening surfaces (decks) with square or rectangular openings, and its vibratory movement depends on a system of unbalanced masses. The appropriate type of vibration allows for better movement of ore and stratification of minerals .
Vibrating screens are critical machines prone to successive failures that can result in huge economic losses  and must be constantly improved in order to meet the requirements of the mining industry . Proper operation of this machine has important benefits for the whole mineral process , and for this reason, many studies have been conducted to investigate the behavior and operational parameters for successful operation. For example, a change in the slope of 1 can cause a decrease of 1-2% in the screening efficiency [1, 8, 9], whereas a change of 1mm in the vibration amplitude can cause a loss of between 5% and 10% of the screening efficiency [1, 9, 10]. These efficiency losses depend on the vibrating screen design, ore characteristics, and operational conditions. Inadequate classification produces undersize particles in an oversize stream (overflow) entering a comminution process, resulting in extra energy expenditure and obstruction of the grinding by packing of fines . Inadequate classification in a closed circuit produces a short circuit of the undersize stream and results in recirculating a constant percentage of fine ore in the circuit. To ensure proper operation and high performance, it is necessary to have a dynamic model that is able to predict movement of the deck to maintain or increase throughput.
The literature presents several models and studies covering different aspects of vibrating screens. Models of screens using a probabilistic approach [12, 13], stratification and passage in the screening process , particle movement , screen blocking , finite elements [5, 2124], crushing plants [7, 25], and phenomenological models  can be found in the literature. Dynamic models of vibrating screens can simulate motion of the vibrating screen structure and show good agreement with experimental measurements  and finite element method (FEM) results . The linear model proposed by He and Liu  considers three degrees of freedom. The excitation force is circular, and the vibrating screen structure is supported by symmetrical damping springs with equal stiffness. Liu et al.  developed a linear model with three degrees of freedom in 3D that has an excitation force in the vertical direction and different damping spring stiffnesses in each support position. Their study focused on vibrating screen fault diagnosis by performing a dynamical simulation on when the supports lose stiffness. Liu et al.  proposed a linear model similar to that developed by He and Liu  by incorporating a quadruple excitation mechanism into the model. The model proposed by Baragetti and Villa  considers motion in a plane with three degrees of freedom. Their model was used to calculate the dynamical response and natural frequencies, and the optimization used FEM and experimental measurements. Using that model, Baragetti  patented a new design for a vibrating screen. For optimum screening performance, the angular displacement was set equal to zero and held constant over time, and the load eccentricity was also made null. Thus, the resulting equations have two degrees of freedom. Slepyan and Slepyan  and Zahedi and Babitsky  analyzed a vibrating screen operating with parametric resonance, using a linear model with damping and a tensile force . They employed a nonlinear dynamic model with a system of autoresonant control . Peng et al.  developed a model with a single degree of freedom for a large vibrating screen that considered the bending and random vibration in the design of these machines, and in which the ore was simulated as a random force. Jiang et al.  proposed a linear dynamic model of a single-deck equal-thickness vibrating screen. In their results, the simulated amplitudes of the vertical and horizontal motion deviated by less than 5% from the corresponding experimental results. Wang et al.  developed a nonlinear dynamic model of a planar reciprocating vibrating screen and employed a matrix method to derive the equation of motion in order to analyze the motion of a particle on the screen. With dynamic simulation software, Jiang et al. proposed a new design of a vibrating screen, where its screen decks are composed by rigid-flexible rods .
These linear models  assume that the angular motion of the vibrating screen is low and implement linearization as and , which is useful under nominal operating conditions where angular displacement is not significant. However, in practice, significant angular motion occurs in vibrating screens during startup , shutdown, and under off-design operating conditions . The nonlinear models [19, 33] have been developed for particular types of vibrating screens and thus are not always applicable to the vibrating screens normally used in the copper industry, due to its deck material or type of movement.
Rodrguez et al.  developed a 2D nonlinear dynamic model of a vibrating screen with three degrees of freedom that allows for significant angular motion and damping in which the nonlinearity is geometric due to angular displacement. They proposed a range of admissible loss of stiffness as a percentage of nominal stiffness in order to ensure proper operation using orbital analysis. The calculated stiffness range was found to be 38% on the feed side and 46% on the discharge side. Moncada and Rodrguez  used a nonlinear model  to calculate the effects of loss of stiffness in the supporting positions on the steady and transient responses of a vibrating screen. For the steady response, the change in orbital direction was analyzed, while the transient response analyzed the change in the natural frequencies due to loss of stiffness in the supporting positions.
Simulations carried out with full-ore loads [3, 19, 36] assume that the ore has the same movement as the vibrating screen, i.e., the ore is added to the decks, approaching this force to the ore inertial force , where is the mass of the ore and is the acceleration of the center of mass of the ore. This produces an unrealistic attractive force when the ore is in its highest position (ore cannot pull the vibrating screen deck) or in the free-fall phase or when throwing index is equal to zero [15, 17].
Because the force generated by the ore material over the decks is inaccurate owing to the impossibility of traction or a pulling force over the deck, this study aims to calculate the ore force over a vibrating screen deck by means of simulations using the discrete element method (DEM). This method considers the interaction of each particle with each other and with decks and allows forces for different simulation conditions to be obtained. Furthermore, this method is widely used in the literature. In the mining field, several machines and processes have been simulated using DEM, including cone crushers , mills , hopper discharge , jaw crushers , feed boxes , and vibrating screens [1, 810, 4251]. Cleary et al. [42, 43] performed a DEM analysis of an industrial double-deck banana screen for a range of peak accelerations and two feed size distributions. Dong et al.  conducted a numerical analysis of the particle flow on a banana screen and demonstrated the importance of operational parameters like the slope angle of each deck, vibration amplitude, and frequency. Zhao et al.  carried out a numerical study of the motion particulates follow along a circularly vibrating screen deck using 3D-DEM. They studied the effects of vibration amplitude, throwing index, and screen deck inclination angle on the screening process. Fernandez et al.  used a one-way coupled model of smoothed particle hydrodynamics (SPH) and DEM to simulate large banana screens. DEM was used for the coarse particulate flow, while SPH was employed to model the transport of the fine particle slurry. Delaney et al.  used DEM to investigate the flow of a granular material over a horizontal vibrating screen, and performed a quantitative comparison between laboratory scale experiments and the simulation results. Liu et al.  simulated the particle flow on a banana screen deck using DEM and investigated the effects of slope angle and deck length on the screening process. Dong et al.  simulated the screening of particles for different vibration modes (linear, circular, and elliptical) and studied the resulting motion and penetration of the particles on the screen deck. Li et al.  used DEM to optimize the design and operational parameters of a linear vibrating screen. Jahani et al.  investigated the screening performance of banana screens using the DEM solver LIGGGHTS. An industrial double-deck banana screen with five panels and two laboratory single-deck banana screens with three and five panels were simulated, and the effects of design parameters such as the slope angle of decks, vibration amplitude, and frequency were analyzed. Their results were validated with partition numbers obtained from the literature . Zhao et al.  quantitatively compared DEM results and experimental data for a specially designed circularly vibrating screening model under a range of operating conditions. Jafari and Nezhad  studied the effects of different parameters on process efficiency and mesh wear using LIGGGHTS. Dong et al.  conducted a numerical analysis of the effects of aperture shape, length, and orientation on particle flow and separation in a vibrating screen process. Zhao et al.  analyzed the combined effects of vibration parameters on a circularly screening processes. With a DEM simulation, the effects of various design and operating variables on the efficiency of screen were investigated using open-source LIGGGHTS solver with spheral and irregularly shaped particles . Particle velocity, mass of oversized material, screening efficiency, and impact force were studied to reflect the performance of vibrating screen .
From this review of DEM simulations, we can summarize the following: (i) there is significant evidence that vibratory parameters are important for proper screening performance [1, 810, 4244, 47, 52, 53]; (ii) DEM simulations do not consider the effects of ore mass flow on amplitude movement although mass flow affects amplitude [54, 55] and efficiency depends on amplitude [1, 9, 10, 42, 43, 52, 53]; and (iii) DEM simulations do not focus on vibration. Most models use circular motion even when there are experimental data showing that the motion is not circular. In addition, rotation is neglected , which significantly affects the movement described for the vibrating screen supports.
The dynamic simulations in this study make two different assumptions or considerations. In Case 1 [3, 36], the ore moves together with the screen deck, which means that physically the ore adheres to the screen and there is no relative displacement between them. For modeling, parameters such as the mass of the vibrating screen with the load and the position of center of mass must be calculated. In Case 2, interaction between the ore and the screen deck is represented by a time vector force for each degree of freedom (). Movement of the ore is simulated with DEM to obtain the force over the screen deck, and this force is then used in the calculation of the dynamic model.
In this study, a 2D linear model with three degrees of freedom that considers ore inertia and the ore force over the screen calculated using discrete element method is developed in order to determine the influence of the ore on the movement of a vibrating screen. For cases 1 and 2, DEM is used to simulate a double-deck vibrating screen. The DEM simulation is carried out in LIGGGHTS and is used to obtain ore parameters and the ore force on the screen over time. Finally, the two dynamical responses are compared.
The dynamic model for a linear motion vibrating screen has three degrees of freedom in the plane , as shown in Figure 1(a) in its equilibrium position. The excitation force of the system is time-dependent on amplitude, , has a constant direction, , with respect to the line linking points A and B, is applied at a distance, , from the center of mass, and has excitation frequency. The structure of the vibrating screen and all its components is defined as a rigid body inclined at an angle, , with a center of mass located at a distance, , perpendicular to segment . Supports are located at points A and B and consist of a horizontal and vertical spring and damper, each at a distance, , from a projection of the center of mass, and with a stiffness, , and damping, . Figure 1(b) presents the degrees of freedom of the modelhorizontal displacement, , vertical displacement, , and angular displacement, .
For the simulation cases in this study, two approaches for modeling the ore are used. In Case 1, the ore is modeled as a rigid body and vibrate together with the screen deck. Therefore, the mass, , inertia, , and position of the center of mass of the vibrating screen with a load are the sum of the empty vibrating screen plus the ore. In the second case, the ore movement is calculated by DEM simulation. The load is represented in the dynamic model by a time vector force with three components for each degree of freedom: , , and , applied at the center of mass of the vibrating screen.
Parameters for the ore particles in the DEM simulation must be known in order to perform the dynamical simulations in Case 1. For that purpose, the mass, , radius, , inertia, , and position of every particle in the particles system are used in equations (1a)(1f) to obtain the load parameters. These load parameters include the total mass, , inertia, , position of the ore center of mass relative to the axes , and position relative to the vibrating screen ( and ). Figure 2 shows a vibrating screen modeled as a rigid body, along with the upper and lower deck, feeder chute, particles, and variables used in these equations. It also illustrates the geometry to allow comparison of the dynamic modeling and DEM, and it should be noted that the variable in the dynamic modeling and in the DEM modeling have different directions. The subscript represents the center of mass, represents the ore load over the screen decks, represents the empty vibrating screen, represents the screen deck, and and represent the lower and upper decks, respectively.
With these load data, the parameters for the vibrating screen with a load can be obtained using equations (2a)(2g). Figure 2 also shows this situation, illustrating the position of vibrating screen center of mass without a load or empty, , with a ore load, , and the new position of the center of mass of the vibrating screen with a load, represented without subscripts. The average values of in steady state are used in the model.
The force calculated with DEM, , is the force of all the particles over the screen decks. Applying Newtons second law to the particles system results in the following:where is the force of the screen decks over the particles, is the mass of a particle, is the gravitational acceleration, and is the acceleration of each particle. Therefore, using Newtons third law, the force of the particles over the screen decks is
As includes the inertial force of the particles, Case 2 also represents the dynamic characteristics of the particles. This force is applied to the vibration screen center of mass and parameters for the vibrating screen in empty conditions are used.
The equations of motion are developed using Lagranges equations. Applying the linearization and , the resulting linear equations of motion are given in equations (5)(7). These three equations correspond to a second-order linear system of equations, and can be solved using the Newmark method with a constant time step. The nonlinear model used for simulations where the angular displacement is significant is presented in Appendix . In these equations, nonlinearity is present in the terms , , and , among others. For the force direction, nonlinearity is present in terms that consider a change in , as can be seen in the term .
The equations of motion are developed in an equilibrium position with an inclination angle, . If the position of the center of mass or amount of mass changes, the vibrating screen will experience a variation in its equilibrium position, the slope . A free-body diagram calculation allows the new slope of to be determined before these changes occur. This is useful when a loss of stiffness exists in the support system .
The discrete element method was originally developed by Cundall and Strack in 1979  and it has proven to be an effective numerical technique for calculating particle movement in granular material flows. It is also useful for obtaining the force acting on each particle, which is difficult to obtain on a similar scale with physical experimentation . Since it is a Lagrangian method, Newtons second law is applied for each particle in the simulation domain. This explicitly determines the trajectories and kinematics of each particle at every time step by accounting for the interaction between particles and their environment with contact or field forces.
Using Newtons second law, the equations of motion for the translational and rotational motion of each particle, , in contact with a particle or wall, , are as follows:where is the particle mass, is its velocity, is the gravitational acceleration, is the contact force exerted by particle on , is the particle inertia, is its angular velocity, and is the contact moment.
By assuming that the particles are rigid, spherical, and of uniform material, the contact force, , for particle-particle and particle-wall interactions can be calculated with the HertzMindlin contact model . This force is composed of an elastic and viscous component and is expressed by equations (9a)(9c) in the normal and tangential directions, being and the respective unit vectors. The tangential overlap is truncated to fulfil Coulomb friction criterion in tangential force. Overlap distance is composed of a normal and tangential component, as shown in equation (9d). The choice of a contact model depends primarily on the experimental comparison and type of results expected. A simple model, such as the Hooke linear model, can also produce good agreement in some situation .
A constant directional torque (CDT) model is used to calculate the rolling resistance [59, 60]. A constant moment, , is applied to a particle to represent the rolling friction, where and are the angular velocities of the particles in contact, is the rolling friction coefficient, is the effective radius, and is the contact normal force. The torque is always in the opposite direction to the relative rotation between the two contact particles.
Figure 3 shows the schematic of the contact model and rolling resistance model, illustrating two particles, and , with an overlap, , and with contact stiffness, , and damping, . Each particle is characterized by its Youngs modulus, , mass, , and radius, .
Model coefficients for equation (9) are calculated using equations given in the LIGGGHTS manual . These coefficients depend on the material parameters, such as the restitution coefficient, , Youngs modulus, , and Poisson coefficient, .
To numerically solve the motion equations for each particle, the simulation is divided into time steps, , and the equations are solved to obtain a solution at the end of each time step. The velocity Verlet algorithm is used for these calculations, which can be derived by an approximation of the Taylor series .
DEM simulations are implemented in LIGGGHTS , an open source software based on LAMMPS used to perform massive granular simulation in parallel using MPI . This software makes it possible to import CAD files for the wall geometries; assign different movements to these geometries, such as a 3D vibration with a different amplitude, frequency, and phase; use nonspherical particles ; perform stress and wear analysis ; and develop smoothed particle hydrodynamics (SHP) models  and CFD-DEM coupling simulations . For these reasons, it has been used in the literature for vibrating screen DEM simulations [8, 10]. LIGGGHTS 3.6.0 was used to perform the simulations on a desktop computer.
The simulations in this study make use of data obtained from a vibrating screen in use at a copper mine . This screen is a double-deck linear motion vibrating screen with a size of 3.66 7.32m and a 10.5mm nominal stroke. The CAD model of this vibrating screen is shown in Figure 4(a), where the upper and lower screen decks, feed chute, and lateral and rear walls that serve as boundaries of the particles are detailed. The lateral wall on the left side is not presented in this figure, but it is included in the simulation. Only the features necessary for DEM are geometrically modeled, as the details of the structure of the machine are not relevant; the screen decks are correctly modeled.
The screen deck is composed of 27 square modules in the longitudinal direction, with a side length of each square module of 305mm. Each deck has rectangular openings, slotted with the flow direction, of 60 20mm (Figure 4(b)) and 47 11mm (Figure 4(c)) for the upper and lower decks, respectively. The opening area represents 30% and 36 of the upper and lower decks, respectively.
To reduce the computational cost of the DEM simulation, the simulated vibrating screen is a 1/12th scale model in the cross direction  of the real machine, using the symmetry in the -plane corresponding to a row of modules, as shown in Figure 4. This simplification implies that the intensive variables, such as force, mass, and inertia, must be multiplied by 12. In spite of the influence of the particle shape on stratification and passing , spherical particles are adopted because of their lower computational cost. The particle size distribution corresponds to nominal data provided by the manufacturer, and a diameter threshold of 6mm is used in order to reduce the simulation time . Youngs modulus is for the particles, and for the boundary walls. The common simplification to reduce  is not applied because it influences the contact force , and thus, the force between the particles and decks. With the period of vibration, , and steps per cycle, a time step is , or approximately . That time step is acceptable for this simulation  because it is smaller than the Rayleigh time step. Details of the geometrical conditions and material parameters are listed in Table 1 and are based on nominal data and previous studies [17, 37, 46]. These parameters correspond to the simplified vibrating screen, and thus, the mass flow and width are 1/12th the real value. The subscript refers to the particle.
To promote stratification and provide movement to the ore, a vibrating screen vibrates with a particular frequency, direction, and amplitude. In the DEM simulations, motion is imposed on both decks, with vibration in the x-axis and y-axis, as well as angularly an amount with respect to its center of mass. The vibration direction is ensured by the phase difference between the vibrations in x and y. The vibrational parameters are listed in Table 2. These parameters are obtained from dynamical simulations, and thus the frequency, , of the deck vibration movement is the same as that in the dynamic simulations. To the comparison of the cases be valid, both cases consider the same value of .
Table 3 lists the parameters used in the dynamical simulations for Cases 1 and 2 that correspond to the nominal data without load and geometrical conditions. The excitation frequency is calculated by .
Figure 5 presents snapshots of the DEM simulation of the vibrating screen in the steady state. The ore entering through the chute, distribution on the decks, stratification, and passing can be observed. In this figure, the red spheres have a larger diameter and the blue spheres have smaller diameters. The steady state is defined as when the ore mass on the vibrating screen remains approximately constant with time.
The total force and moment over the screen exerted by the ore are presented in Figure 6 for the directions. The vertical component, , has a greater amplitude than the horizontal component , and its peak amplitude is 7 times greater. Both amplitudes match when they have null values, as that corresponds to when the ore is detached from the screen. The value of exhibits an amplitude change every cycle, i.e., between peaks (1) and (2), that is related to the ore movement. On average, the first is equal to 122.4kN and the second is equal to 111.5kN. The moment, , has the same tendency as components x and y, and its maximum amplitude occurs when ore comes into contact with the screen deck.
The overall screening performance of both decks can be investigated with the partition curve of the overflow, which is defined as the ratio of the number of residue particles in the overflow to that of fed particles , and is shown in Figure 7. In terms of mass flow, the partition number is equal tofor each particle size, where refers to overflow stream and to feed. These results agree with the nominal screening efficiency of the vibrating screen, which is approximately 90%.
Figure 8 compares the forces in Case 1 and Case 2 with a frequency spectrum and orbit. Figure 8(a) shows the frequency spectrum of the signal in the vertical direction, y. For Case 1, only the 1X component is observed. For Case 2, a constant component at 0Hz is observed, which represents the mean time value, along with harmonics from 1X4X. The logarithmic spectrum shown in Figure 8(c) exhibits, in addition to these harmonics, nonsynchronous components at 0.5X and 1.5X, which are highlighted in the grass spectrum.
Figure 8(b) shows both forces in the -plane, allowing for a graphical understanding of the direction of each force. In Case 1, the orbit is elliptical with a nearly negligible semiminor axis, and thus, it approximately corresponds to a line. Case 2 has an ore force that is always negative in the direction of y because the ore cannot produce an adhesion force that lifts up the screen decks.
Arrows indicate the variation in each force over time. In Case 2, the orbit is a cycle, from O to O in clockwise direction, whereas the orbit of Case 1 moves between A and B. While the Case 1 orbit moves from B to A, the Case 2 orbit stays close to zero.
In analyzing these results, it can be concluded that Case 1 does not accurately physically represent the effect of the ore on the vibrating screen, because this force must be vertical and repulsive, as in the DEM simulation.
Movement in the two cases is compared by means of orbit analysis of the movement of support A. Figure 9 presents the frequency spectrum and displacement orbit for support A. Figure 9(a) shows the frequency spectrum of vertical movement and indicates that Case 1 only exhibits a 1X component, which differs by 12% from that of Case 2. Case 2 exhibits a low amplitude harmonic at 2X. For Case 2, owing to the transient condition of , a resonant zone appears near 1.875Hz. This corresponds closely to the values of the natural frequencies, and , which are 1.982Hz and 2.067Hz, respectively.
Figure 9(b) shows the orbits. In Case 1, an elliptical orbit is clearly observed, while in Case 2, the transient condition of the excitation force, , results in an elliptical trajectory that changes position in the vertical direction by an amount = 1.2mm. A difference in stroke is also observed, equal to = 9.230mm in Case 1 and = 10.592mm in case 2 on average. These values agree with commonly found in experimental measurements and in the literature [3, 10]. Furthermore, Figure 9(b) presents the nominal stroke length equal to = 10.5mm. Comparing nominal stroke with simulated stroke, case 2 provides a better agreement.
Angular displacement of the vibrating screen is obtained, and it is presented in Figure 10. Both cases present a low peak amplitude, less than 0.025. Figure 10(a) presents frequency spectrum showing that both have a 1X component that differs on 0.003. Case 2 presents a low amplitude 2X component and, as well as vertical movement, a resonant zone. Figure 10(b) shows waveform of angular displacement. It should be noted that Case 2 has nonzero mean value equal to . This value depends of , which is calculated based on and and the position of the center of mass of the ore with respect to the center of mass of vibrating screen.
Conclusions obtained in this study can be summarized as follows:(i)The proposed dynamic model allows for prediction of the behavior of the vibrating screen operating at both the nominal condition and high angular displacement with a nonlinear model . This model simulated ore movement along with the screen deck (Case 1), as well as the ore force over the screen decks calculated with DEM (Case 2). In both cases, the movement of the vibrating screen supports was also obtained.(ii)DEM simulation of a double-deck vibrating screen was carried out using the open-source LIGGGHTS software. Movement of the ore center of mass and the force exerted by the ore over the screen deck were obtained. The force has the same frequency as the excitation force; high oscillations in the ore do not produce significant changes in the force exerted over the screen.(iii)The partition curve and stroke of the vibrating screen motion have very good agreement with the nominal data, validating the model results.(iv)In a comparison of the results for the proposed cases, where the ore is represented as moving together with the vibrating screen (Case 1), or the ore force is obtained from DEM (Case 2), it is observed that:(a)The force over the screen deck is completely different in both cases, both in terms of the magnitude (the peak-to-peak amplitude in Case 1 is more than twice that of Case 2) and the shape on the -plane. Case 1 produces an unrealistic force, because it includes a contact force of adhesion between the ore and screen deck. However, this is the hypothesis used in most of the existing dynamic models available in the literature. The DEM model allows calculation of a force closer to reality, because it calculates the interaction of each individual particle with the screen deck.(b)Case 1 available in the literature was successfully implemented. The computation of Case 1 is faster than Case 2, because only a linear ordinary differential of three-degrees-of-freedom equation is solved, in contrast to DEM simulation that simulates the movement of 42000 particles. This is the reason why Case 1 is commonly used in the mining industry.(c)Case 2 is a new simulation approach that allows the coupling of simulation results in DEM in dynamic models. This model is able to evaluate how the ore affects the movement of the vibrating screen.(d)Notwithstanding the clear inequality in the force calculated for cases 1 and 2, the approach used in Case 1 can be used to predict the movement of vibrating screen if a correct calibration parameter is included.(e)Comparing with the nominal data and the results of Case 2, frequency, direction, and inclination calculated with Case 1 are accurate. The amplitude obtained using Case 1 is not accurate and must be corrected. In this case, the parameter of mass of ore must be adjusted with experimental data, decreasing its value so that the amplitude increases. To decrease the value of is physically correct, since the model of Case 1 considers that all the ore is in contact with the screen decks, while there is ore in free fall that is not in contact.(v)Under nominal operating conditions, the angular response, , has low amplitude (0.014), whereas the steady responses obtained for the linear model, equations (5)(7), and the nonlinear model, equations (12)(14), have negligible differences. Consequently, for simplification and lower computational cost, the linear model can be used for the steady case or without deterioration in the supports. For transient signals or when the amplitude of the angular response is high, the linear model is not recommended.(vi)The proposed dynamic model allows for greater accuracy and validity in different operating conditions, which is useful for predicting the angle of operation and the vibratory amplitude, parameters that affect the screening efficiency.
The DEM raw data used to support the findings of this study have been deposited in the Raw data of DEM simulation of linear motion double deck vibrating screen repository (DOI: 10.17632/cc798dvdbn.2).
Copyright 2018 Manuel Moncada M. and Cristian G. Rodrguez. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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