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double deck vibrating screen layout

vibrating screen working principle

vibrating screen working principle

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.

vibratory screens | mclanahan

vibratory screens | mclanahan

Screens can be considered the cashbox of the operation, because while crushersmake the gradation, screens make the specification. Material must go through or over a specified size to end up in the right pile. Unlike the crushers, Vibratory Screens cannot produce material; they can only size material that is already reduced to the product sizes. Vibratory Screens allow crushers to achieve maximum performance by sizing the material feed to the crusher and efficiently removing the finishing product from the circuit as it is produced.

Vibratory Screens can be subdivided into Inclined and Horizontal style screens. Today's screens come in widths from 4-12' wide and from 8-32' long. Screens are normally sized so that the length is 2.5 times the width. The width of a screen will determine the maximum carrying capacity of the screen deck, while the length of the screen will determine the overall efficiency of the deck.

McLanahan Vibratory Screens are engineered with ASTM A572 Grade 50 steel side plates. With a tensile strength of 65,300psi (65ksi), these side plates have a 45% higher yield strength than A-36 steel, which can withstand up to 36,000 psi (36ksi) of stress before it begins to drastically deform. A fully bolted construction reduces/eliminates cracking due to stress risers in the steel caused by welding. Worn components can be quickly replaced without cutting.

McLanahan Vibratory Screens are built with an integrated feed box and are able to withstand heavier loading and larger material in the feed end without worrying about bolts loosening or structural failure.

McLanahan Vibratory Screens feature robust side plate stiffening. Formed plates are bolted to side plates to form a rigid support grid along the length of the side sheet. Independent cross members can be pulled individually and relined in a clean work bay versus on a screen tower, and reduce the need for heavy and wear prone X-bracing. Cross members are on 4' centers to allow more clearance for personnel to access the decks. Replacement cross members come shorter in length and with machined/matched shims to allow for easy installation in areas with limited clearance.

Structural tubing gives the producer a variety of size options and allows you to replace only the worn tubes, not the complete deck frame. A sacrificial weld plate installed on top of the tubes allows stringers and bucker bars to be welded in without welding directly to the tube.

The performance of a screen is affected by four variables: eccentric throw, frequency (rpm), angle of adjustment and throw direction. By manipulating these variables, the operator can dial in the screen to match the application and material.

Eccentric ThrowEccentric throw is the radius of the screen box. Generally, the greater the throw, the more aggressive the screen action will be. Consequently, the smaller the throw, the less aggressive the screening action. Keeping this in mind, the operator can set up the operation with a heavy throw for heavier or larger materials, or a smaller throw to create a sifting action more suited for finer separations.

FrequencyThe frequency of the screen is measured in the number of revolutions per minute the screen makes. In conjunction with the eccentric throw, a lower frequency allows for a more aggressive screen action for larger material and cuts, while a higher frequency is used for smaller material and cuts.

Angle of AdjustmentThe angle of the screen plays a large factor in its overall performance as well. A flatter screen angle will provide a longer retention time of material on the deck and more probability that a particle will fall through the opening. As the angle is increased, the retention time is decreased.

ThrowIt may be advantageous to run the throw of the screen uphill. The goal is to increase the retention time on the screen, as well as change the orientation of the particles to the screen opening. The reverse action does not hurt the screen and is usually used in finer screening application, but be cautious not to increase the bed depth too much.

Stratification and SeparationTwo main operations have to occur for material to be screened: stratification and separation. Stratification is the process of larger sized material rising to the top of the bed, while smaller particles go to the bottom of the bed. Separation is the process by which particles introduced to the screen opening either fall through the opening or do not. Stratification must occur before separation can take place.

The separation probability is a function of the ratio between the size of the screen opening and the size of the particle. If the ratio is large in other words, the particle is much smaller than the opening there is a high probability the particle will fall through. If the ratio is small the particle is close in size to the opening then the probability is low that it will fall through.

Motion on a Vibratory Screen is produced with a combination of amplitude (stroke) and frequency (speed). The goal is to allow the particle to see as many openings as possible as it travels down the screen, but never see the same opening twice. Large screen openings for large cuts can be achieved with high amplitude and low speed. For small screen openings for finer cuts, the opposite is true: low amplitude and high speed.

Many producers have experienced a variety of problems that point to a screen deck that was improperly selected. It's wearing too fast. Its plugging (material getting stuck in the screen opening) or blinding (screen opening clogged by sticky material). The noise level is too high.

Many factors affect the overall efficiency of the screening process. Selecting the proper media for the application will be a big factor toward success. Wire cloth is the most widely used screen surface. Technological advances make it easier to consider other types of screen media.

The type of media chosen will depend on material abrasiveness, impact, material size, moisture content, cost-effectiveness and noise level. Wire cloth may be the lowest initial cost media, but the most cost-effective for anoperation will be the one that meets the specific application.

Rubber screens are a good choice for scalping decks in a dry, high-impact application. Rubber is very durable and can withstand the impact of the larger feed material hitting the deck. In a dry secondary application, a rubber screen can provide a long life, even in abrasive feed material.

vibrating screens

vibrating screens

Whilst the Parker Rapide is the workhorse of the companys screen range and is adaptable to most applications there are specialist uses for screens that are catered for within bespoke ranges that incorporate many of the standard features but add additional specific items for instance:

Parker ScreenRangers are a series of static / stationary and mobile screening units featuring the Rapide in a number of different sizes. These can have 1, 2 ,3 or 4 decks with discharge chutes tailored to individual preferences, including product blending when required. Every ScreenRanger is an independent unit. It can separate up to four sizes plus oversize, according to model.

Optionally, multi-deck screens can be fitted with discharge blending chutes offering even greater product flexibility by directing graded material and oversize to differing discharge points to facilitate proportional blending of the final graded products.

The electric control panel containing the starters for the screen and associated conveyors is mounted on the screen chassis for transport, but when operating, is removed to ground level for vibration-free performance.

All mobiles are equipped with pneumatic tyred running gear to highway specification, with dual line air brakes and a hand parking brake on the rear wheels. Standard machine semi-trailer; Optional full trailer.

The single activating shaft is eccentric and runs the width of the screen inside a protective steel tube. This tube also acts a substantial structural brace, being set in machined positions in the side plates to ensure the vibration is transmitted positively to the main frame. With eccentric weights the vibrator unit produces optimum amplitude for the majority of duties. Simple addition/subtraction of bolt-on adjustment weights to both sides of the shaft will increase/decrease amplitude for specific requirements.

The Rapide is a free floating screen, effectively isolated by its springs from the support structure. For transit, the screen is restrained in its angled travelling position by reusable clamp plates (travel brackets) simply bolted across the spring units.

Several different plant mounted chute arrangements are offered for directing graded material and oversize to separate discharge points. These can include the facility for proportional blending of products, for wet screening duties, the fine chute under the screen is replaced by a fluming chute.

Every feature of the Rapide is designed to ensure long, efficient service with minimum maintenance with its characteristic parallelogram shape giving more screening area compared to other multi-deck models. Feed and discharge trays are fitted as standard.

The eccentric weights on the vibrator units produce optimum amplitude for the majority of duties but the simple addition or subtraction of bolt-on weights on both sides of the shaft will increase/ decrease amplitude for specific requirements.

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