A simple definition of a screen is a machine with surface(s) used to classify materials by size. Screening is defined as The mechanical process which accomplishes a division of particles on the basis of size and their acceptance or rejection by a screening surface.
Knowledge of screening comes mainly from experience. However, through experiment, test facilities and compilation of field data, reliable criteria have been developed by screen manufacturers. This factual data is now tabulated for use in selecting the type and size of screen best suited for the job.
The most common application of a vibrating screen is to separate an unconfined conglomerate of materials into different size fractions. Other popular uses of screens are scalping, washing, dewatering and dedusting. A review of the duty is essential to know the type of screen to recommend. When this is established, the capacity chart is then used to determine the size of unit required.
Nordberg-Lokomo supplies different types of screens, each designed for a specific range of duty. Occasionally there is a choice between the types we offer. In these cases when there is doubt, you can rely on Nordberg-Lokomo experience to help you make the selection.
COARSE FRACTION Particles which pass over the screen deck, FINE FRACTION Particles which pass through the screen deck. SEPARATION SIZE/ SPLIT SIZE Particle size at which feed separates into two products (coarse fraction and fine fraction). OVERSIZE Material larger than the hole size. UNDERSIZE Material smaller than the hole size. HALF SIZE Material smaller than half of the hole size. SCREENING CAPACITY (Q)Amount of material passing through the screen deck in tonnes/hour FEEDING CAPACITY Amount of material fed to the screen deck in tonnes/hour EFFICIENCY OF SCREENING (EFFICIENCY OF UNDERSIZE RECOVERY)Amount of material smaller than the hole size in undersize compared to the total amount of material smaller than the hole size in the feed.
The particle distribution of the feed has an essential impact on purity. See three examples in figure 1. In each one of them the efficiency is 90 %, but the undersize proportion of the coarse fraction varies (3.2 %, 9.1 %, 23 %).
Factors effecting the screening can roughly be divided into three groups: characteristics of material (B, C, F, K, L) characteristics of screen (D, E, AF) characteristics of screening element (A, G, H, J)
[t/h] (passing through) A = Nominal capacity [(m/h)/m] (passing through) B = Oversize factor C = Halfsize factor D = Deck location factor E = Wet screening factor F = Material weight [t/m] (bulk density) G = Efficiency factor H = Shape factor for mesh holes J = Factor for proportion of holes in the mesh K = Factor for crushed stone and gravel L = Factor for humidity content AF = Effective screening area [m]
The factors are obtained from diagrams based on relationships observed empirically. Since these factors are known, it is consequently possible to calculate the specific capacity of the screen in tons/h per square meter.
The amount of oversize describes the amount of the particles of the limit size. Particles which are considerably larger than the hole do not make screening difficult. Large stones push stones of limit size through the screening element.
The quantity of the half size is used to inform / present the quantity of the fine material. Material smaller than half the hole size passes through the screening element very easily. If a feed contains a lot of fine material, it can be fed in large quantities onto a screen. If there is little fine material the screening capacity falls. This is due to the fact that there are a lot of particles of limit/critical size. The throughput of particles of limit size (0.5 1.0 x hole) is very poor.
Flaky (thickness is small, relative to the other two dimensions) and elongated (length is larger than other two dimensions) stones are the most difficult to screen. They pass over the screen deck laying on their widest side. At worst they become wedged in holes and thus block the whole screen deck.
When the humidity is under 3 % it has no significant importance. The problems start at 4 5 %. At 9 30 % screening is very difficult. When there is more water the screening gets easier, and it is close to separation of water screening.
Stroke length, rotation speed, stroke angle, and screen inclination form together parameters which affect the operation of the screen. These fundamental factors have to be in proportion to each other. Stroke length and material amplitude have an effect on:
how the holes of the element stay unblocked. If the stroke length is too small also the material amplitude stays too small and the element gets blocked. The problem arises when the hole size is large (50 mm or more).
Acceleration of the screen box can be calculated by the stroke and rotation speed. When stroke angle and inclination are taken into the calculation, the vertical acceleration can be found. Vertical acceleration has an effect on the screening efficiency and the rate of travel.
Acceleration should be 4.5-5.5 x G (G=9.81m/s) with horizontal screens to reach a good screening result. To avoid structural damage for the screening unit, no acceleration greater than 6-7 times G are allowed.
Stroke angle has an effect on the material amplitude and the rate of travel. The most suitable stroke angle for horizontal screens is 55-60 degrees. Too upright a position can reduce the rate of travel. Horizontal stroke angle can improve the rate of travel but reduce screening efficiency. It also increases the wear rate of the mesh.
Speed of travel can be increased by inclining the screening surface. If the surface is greatly inclined, the stroke must be short to prevent material sliding over the mesh too quickly. Inclination of the surface can keep the mesh holes open more easily.
The bed of material may not exceed a height more than 3-5 times the size of the mesh hole on the discharge side of the screening surface. A higher bed of material will reduce the screening efficiency. Feeding capacity for each mesh size depends on the width of the screen. To get efficient screening results the depth of material bed must be at least 2 times the mesh hole diameter on the end side of the surface. Then volume of oversize will determine the width of the screen.
The depth of material bed should be within allowable limits on the beginning and the end of the surface when choosing the screen.Screening area is not theonly dominant parameter while choosing the screen. In practice the length is 2- 3 times the width.
A deck factor should be used when calculating lower decks in muitideck screens. In lower decks the feed drops not only at the beginning of the deck, but also later in the direction of the flow. That is why material close to separation size will not be screened out.
Effective screening area is the area where material can drop down through the surface. Effective surface area is about 0.7-0.9 times the whole area. The whole area is determined by the inside parameters of the screening unit: length times width.
Figure 3. Schematic diagram showing how the screening effect varies along the screen deck. Stratification takes place within zone 1, screening of fine undersize particles (75% of the size of the screen apertures) takes place within zone 2 and screening of critical undersize particles, i.e. particles of a size close to the size of the screen apertures, takes place within zone 3.
The amount of loading influences screening efficiency. In practice it is impossible to reach 100% efficiency. Maximum efficiency is about 95%. In most of the cases 90% is achieved and the screen can be said to be under 100% loading.
The greater the open area of the mesh, the more effective is the throughput. When determining the open area of the mesh, the diameter of the wire between holes in different meshes differs, and has to be taken into consideration.
The type of the mesh will have an effect on screening efficiency. The most significant difference will be in special screening cases. For example while screening elongated material, mesh should be of the vibrating type (rubber or harpmesh)
By scalping it is meant screening of coarse material in order to remove the undersize, typically before a primary crusher. Because of the coarse feed the top deck, which may be the only one, is often of a grizzly type. i.e. grizzly bars as opposed to mesh. This type of screening calls for a robust construction whilst there is no requirement for screening efficiency.
Leaving out the ancient trommel screens, stationary grids, and similar types, the following means are used to make the screen vibrate. All screens today are vibrated by various methods to pass the undersize through the apertures of the screen mesh or grizzly bars.
By freely vibrating screens one means screens that are supported on springs, and the box is vibrated by a vibrating mechanism (also called an exciter) which vibrates the screen box in various ways, depending on the type of vibrating unit.
Screens with a circular motion are the most common type. The vibration is circular because of a single eccentric shaft mechanism. This movement would not move the material forward, unless the screen is inclined in the direction of the material flow. This in turn means that the screening efficiency is not quite as good as a horizontal screen. The capacity as such is often higher as this screen is able to transport the material more quickly. The higher the inclination, the greater the transport ability. Inclination is typically 12 20. The inclination also helps to prevent pegging.
The depth of this material layer is more critical with a circular motion screen than with a horizontal screen. The inclination reduces screening efficiency. This type of screen may be used for almost any application. They are also cheaper to produce.
The vibration of this type screen is created by two eccentric shafts, rotating in opposite directions. This gives the box a linear motion. The stroke angle would depend on the relation of the eccentric weights of the two shafts to each other. Because of the linear stroke the material is moved in the direction of the stroke and the screen may be installed horizontally. That is why they are often called horizontal screens. The inclination would be typically 0 5.
The horizontal screen gives high screening efficiency, and they are often used for final and fine screening. Another advantage over inclined screens is their lower profile and therefore, horizontal screens mean lower structures and buildings, and shorter conveyors.
Elliptical motion can be achieved by various means. One method is by using three eccentric shafts, two of which would create the long axis and the third, the short one. These screens are used in special applications where the aim is to gain advantages of both circular and linear motion screens. It is a compromise however, there would also be a measure of disadvantages. These screens are typically installed at an 0 5 angle.
The eccentric shaft(s) of this screen type are connected both to the screen box and the foundation. The two shaft type would give a circular motion whilst the single shaft type would give this near the vibrating unit, and differ with the loading, depending on the action at each end.
These screens are used mainly for screening coarse material. The screens become heavy, and the dynamic forces which the foundation has to absorb, are a disadvantage. Brute force screens are installed 12 20 inclined.
The vibration of this type of screen is created by the resonance between the under-frame and the screen box or decks, and because of the resonance little energy is needed to vibrate the box. These screens are always installed horizontally.
The advantage of this type is the high efficiency as the screen can be very long, and therefore are mainly used for fine screening. They also have a low profile which can be advantageous. However they have very heavy and expensive structures.
Sizers are generally small and equipped with multi decks to assist screening. The products of two or more decks are often blended in the chute work of the screen. They have very high capacity because of the inclination. The apertures of the meshes need to be considerably larger than the cut, and thus affect the efficiency. This is compensated for by the blending. The advantage of this type is that it can be used for difficult material with less blinding than with other types.
The steep angle at the feed end gives the material a high velocity, some 3 4 m/s. Later the angle levels out and slows down the material to 1 1.5 m/s in the middle and 0.5 0.8 m/s at the discharge end. This is where the screening efficiency is achieved. These screens are generally large and used in high tonnage plants, particularly in mining where fewer fractions are separated.
There are a number of special screens, of which the flip flop is an example. The special narrow rubber mesh strips are installed perpendicularly between two separate frames. The meshes being attached to one frame on one side and to the other at the other side the bulk receives extremely high accelerations. This helps screening of wet, dirty and other difficult materials.
This table is a guide only to the parameters of a horizontal screen. When solving screening problems, take also into account the size parameters of the material, screen cloths and physical screening conditions.
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.
The reliability is a key factor for the design and manufacture of large vibrating screen. In the paper, we presented a new large vibrating screen with hyperstatic net-beam structure. Dynamic characteristic of the vibrating screen was researched and dynamic simulation method of large screening machines was explored. We used finite element method (FEM) to analyze dynamic characteristic of large vibrating screen with hyperstatic net-beam structure. Multi natural frequency, natural modes of vibration and dynamic response of the vibrating screen were calculated. The structural size of stiffeners on the side plate was optimized under multiple frequencies constraints and an adaptive optimization criterion was given. The results show that the vibrating screens structural strength is increased and the natural frequency of bending deformation is enhanced. The modal frequencies are far from working frequency, and thus the structure is able to avoid resonance effectively and reduce the destructiveness. The maximum transverse displacement of the vibrating screen is 0.13 mm, the maximum difference in vibration amplitude of corresponding points is 0.44mm and the maximum dynamic stress is 16.63MPa. The structural optimization shows that the mass of the side plate is decreased by 194.50kg, the second and third modal frequency is increased by 1.73% and 2.91% respectively and a better optimal effect is received.