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.
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.
Browse our large selection of new and used screening plants, not to be confused with static grizzly screens, for contractors in the road building, rock mining, aggregate producing, demolition, and recycling industries. Looking for trommel screens? Read MoreScreeners range from small and compact equipment to heavy-duty, high-output equipment including tracked mobile screening plants with scalping screens, grading screens, double deck and triple deck vibrating screens. Different deck bar spacing allows the separation of material into different sized products. Material to be separated and filtered includes topsoil, compost, rock, mulch, wood chips, sand, gravel, coal, loam, and more. Read Less
Screening is the separation of aggregate materials into different sized products. Material is separated by passing through a vibrating screen box which has a number of different sized screens, or meshes, which the material falls through like a sieve.
Power Equipment Company has the largest aggregate equipment product availability in the Rocky Mountain region. All of our aggregate equipment is available to buy, rent, or rent-to-purchase.With over 30 years of aggregate industry experience, no other dealership can match the solutions that we deliver to your operation.
Despite our 70 years of successful history in providing equipment solutions for large and small industrial leaders, our commitment to taking an individual, hands-on approach to working with our customers remains essential to SMICOs DNA as a company.
With this information in hand, we can move on to discuss a specific equipment line or a custom unit. One durable but low-cost aggregate equipment system for cement, aggregate, sand, feed pellets and asphalt components is the SMICO Dual Deck Vibrating Screener. This dependable, low-maintenance screener combines excellent direct flow, high-volume production and product classification with a side-parallel conveying motion that produces up to 5 Gs of acceleration at its optimized operating speed of 986 RMP.
This unit can be manufactured in variable sizes from three to eight feet wide and from six to 20 feet long. However, it has a low profile that fits easily between your existing plant structure, reducing the need to relocate other equipment and loading positions. A wide choice of options are available including support legs or cable suspension, spouts or chutes, piano or woven wire, dust reduction enclosures, wear liners and high temperature features. SMICO also has a variety of standardized screeners in stock and ready for direct shipment if you require immediate delivery. Please call us directly to discuss your specific needs in detail or fill our quotation request form on our website.
The types of aggregate vibrating screens include linear vibrating screens, multi-layer vibrating screens, circular vibrating screens, etc. Vibrating screen manufacturers are selling direct sales, allowing customers to spend less money and buy satisfactory products. [Consultation price]
Linear motion vibrating screen and linear screen are rectangular vibrating screening machines designed for large granular materials or coarse-grained powder materials. They are mainly used for the production process of products with large processing capacity and high added value of products, and high cost performance.
The multi-layer linear vibrating screen uses the vibrating motor as the excitation source. The vibrating force of the vibrating motor causes the material to be thrown up on the screen surface, and the jump forward linear motion to match the screen to achieve the purpose of screening. Screen or filter any material at 0.074-5mm. The maximum feed size is recommended to be no more than 10mm, and the single-hour throughput is generally not more than 15-20 tons of material screening.
It is often used in the screening, de-mixing and grading of small-grain dry powdery or granular materials such as abrasives, building materials, chemicals, food, food, pharmaceuticals, engineering plastics, carbon, carbon black, metallurgy, and chemical fertilizers.
In recent years, the development of mining vibration equipment has been very rapid, and a aggregate vibrating screen has been widely used in stone yards. Its running trajectory is approximately circular, and we can also call it a circular vibrating screen. The sandstone vibrating screen can be used in a single layer or in multiple layers to screen different sizes of granular and small solid materials. It can be said to be specially designed around the work of the stone yard and the concentrator.
When the aggregate vibrating screen is working, the vibrator placed on the side plate of the screen box relies on the cylindrical eccentric shaft exciter and the partial block to convert the amplitude, and the eccentric block of the exciter starts to rotate at a high speed through the V-belt. When a large centrifugal force is emitted, the screen box can perform a circular motion of a certain amplitude, and the impact force is transmitted to the material inclined on the screen surface to continuously generate a throwing motion. When the material falls on the screen surface, it is smaller than the sieve hole. The granules are sifted through the screen so that grading can be achieved.
The excitation force, vibration frequency and screening effect of the dewatering screen are closely related. Too small excitation force will cause poor screening effect, and excessive excitation force may causeGet in Touch with Mechanic