Often just referred to as screens, vibrating screens work within a physical framework that vibrates so as to classify and separate materials by particle size during industrial material processing applications. The basic principle behind vibrating screen efficiency involves centrifugal force caused by an inbuilt imbalance in the driveshaft, which in turn separates materials according to particle size during processing. Consisting of a sieve or mesh surface, a vibrating mechanism and a damping device to regulate vibrations, the basic physics is similar to what occurs in landslides, in which solids flow like liquids.
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A motor with an unbalanced weight on its driveshaft initiates the vibrations, generating centrifugal force. Using a mechanism attached to the screen's side plate, this unbalanced weight connected to the driveshaft's base causes the screen to vibrate. Screen efficiency depends on a number of factors, which include the type of material being processed and the application, along with the screen's design. To fully understand the importance of vibrating screen efficiency in industrial applications, it's important to first review how these screens operate.
An Overview of Vibrating Screen Efficiency
The concept of vibrating screen efficiency is measured by the total amount of material that filters through the screen's holes, divided by the total amount of material processed. While screens made of fixed mesh have been used in material processing, their effectiveness is greater when they're made to oscillate. These vibrations cause particles to hop upwards without sliding along the screen's surface, with each jump providing a chance for material to pass through the screen's holes. The vibratory screens cause particles to separate, classifying them by size against the screen's mesh. Smaller particles fall through the mesh, while larger particles are discharged only once they reach the screen's edge.
Different varieties of vibratory screens include:
- Banana: Named for its distinctive shape, the banana screener is utilized by coal and mineral processing for high throughput processing with a larger proportion of fine particles.
- Circular: Using a circular motion and providing consistent screening across the entire surface of the screen, circular vibrating screens work well for classifying both larger and smaller particles; these screens are used for processing aggregates as well as in quarrying and mining.
- Dewatering: Used to remove moisture from bulk materials, these screens help handle tailings in mining operations or in the processing of wet sand.
- Elliptical: Used to achieve better vibrating screen efficiency, elliptical screens are used by the mining and recycling sectors for more viscous or otherwise difficult-to-process materials.
- Grizzly: This type of screen removes oversized particles from materials prior to crushing; it's used primarily in the recycling and mining sectors to augment crushing.
High-Frequency: For finer particle separation, high-frequency screens are used in dewatering, mineral processing and similar applications.
Inclined: Typically featuring an inclination between 15 to 30 degrees, these screens are often used for dry or wet screening for handling materials like gravel or sand.
Linear: These use linear motion to convey material along a production line; linear vibratory screens are used in industries like mining, metallurgy and construction for materials with fine to mid-sized particles.
Regardless of how the machine is designed, there are basic principles behind how a vibratory screen works. Fine particles ' often referred to as 'undersized particles' or simply 'fines' ' fall through the screen's mesh, while oversized particles slide to the edge of the screen. Vibrating screen efficiency is particularly important for the mining and mineral processing industries.
Factors That Influence Vibrating Screen Efficiency
A number of elements affect vibrating screen efficiency. Not only do factors like the angle of inclination, length and opening size of the screen influence the effectiveness of a screen during processing, but the condition of the material being processed also matters as well. Measuring vibrating screen efficiency is relatively straightforward, however, and simply involves calculating the proportion of particles that go through the screen against the total material being processed.
Factors affecting vibrating screen efficiency include:
- Feed rate: The rate at which material is fed into a material processing system affects the efficiency of vibratory screens. Lower feed rates lead to longer screening times, resulting in near complete particle separation as material passes through the screen, whereas higher feed rates cover screens quickly and in turn reduce efficiency.
- Moisture: Damp material can affect vibrating screen efficiency markedly, as surface moisture causes materials to agglomerate and plug the screen's openings, which can easily occur with materials like clay or those that are similarly viscous.
- Open area: The proportion of open area in the screen's material affects vibrating screen efficiency, with the ratio of openings within the screen compared to the total screening surface. Thus, the more areas through which material can pass, the greater the chance particles will have of passing through the screen.
- Particle shape: While materials with spherical particles require no special orientation, most coarser materials are irregularly shaped so have difficulty passing through a vibrating screen. Efficiency is hindered when too many irregularly shaped particles occur in a material.
- Particle size: Those particles that are considerably smaller than a screen's aperture will easily flow through the screen, whereas those particles that are nearly the size of the screen will take more time to pass through. The larger the particles, the longer it takes for them to pass through the vibrating screen; efficiency of processing suffers when too many near size particles clog the screen's openings and reduce the surface through which material can pass.
- Screen angle: Depending upon the positioning of a vibrating screen, efficiency can be affected by the angle at which particles meet the screen's surface. This largely depends on the angle at which the screen lies, as the more inclined a screen is, the narrower the opening and thus the fewer particles pass through. Horizontal screens are as a result generally a more efficient option.
- Vibratory intensity: When materials meet the surface of a screen, vibrations should be enough to ensure apertures aren't blinded or pegged, though not excessive enough to throw particles too far away from the screen's surface. Generally, the higher the feed rate, the higher the vibration rate should be.
Normally, separating coarser particles will involve using lower speeds that cause less oscillation, while finer materials require greater speed and more oscillation to screen. To help determine vibrating screen efficiency, it's important to look at the best aperture size at both the feed and discharge ends, along with the depth for the material bed.
Tips to Maximize Vibrating Screen Efficiency for Industrial Applications
While vibrating screen efficiency is affected by the above factors, it's also important to understand how exactly to deal with them. Though 100 percent efficiency is rather illusive in material handling applications, for many industries a 90 percent efficiency rating is normally considered outstanding. However, it often requires a careful balance to maximize screening efficiency while also producing as little contamination as possible within the material being processed.
Aperture & Screen Size
Depending on the material being processed, the size of the apertures may matter more. It's essential as well to consider congestion along the screen's surface to ensure that particles will pass through without clogging the holes. Additionally, the size of the screen may be a factor, with longer screens providing increased residence time through which material can pass through the screen.
Setting Configurations
If vibrations are too fast or slow there's a greater likelihood material won't pass through properly. The angle at which the screen's surface inclines should also complement the focus and frequency of these vibrations.
Feeding Tips
How material is fed will affect a vibrating screen. Efficiency will suffer if particles get through that aren't properly sized or separated. This may indicate the flow of the feed is too fast, or else the distribution of material is inappropriate for the application. As material is fed into the machine, it's a good idea to watch whether any product is expelled from the screen or is otherwise thrown upwards while feeding, which indicates the amount of material being fed is too high. For the most part, vibratory screens should feed into the impact area or feed box and should spread all the way across the surface to maximize the screening area.
Maintenance
A key aspect of vibrating screen efficiency involves ensuring screens are maintained properly. Any machine requires regular inspection and maintenance, so it's imperative to have a preventive maintenance plan. This will help avoid unplanned downtime that interrupts production. It's also important to regularly clean the screen to remove contaminants or product that may be blocking the screen's apertures so material can get through. Screens should also be replaced as per the manufacturer's recommendations, or if damaged to the point that they no longer work as they should. Properly tensioned screens are also important to ensure maximum energy is imparted to the particles.
Benefits of Optimizing Vibrating Screen Efficiency
The benefits of ensuring vibrating screen efficiency are worth the effort. Vibratory screens are an integral part of many industrial systems for mineral processing, mining, pelleting wood, recycling and solid waste management, as well as for the chemical, food, pharmaceutical and other industries. As such a versatile piece of equipment, ensures their optimal operation comes with multiple benefits.
Some benefits that result from pursuing vibrating screen efficiency include:
- Benefiting sustainability programs through reduced emissions, and energy consumption waste.
- Cutting operational costs through the simple and streamlined design of modern vibratory screens, which consume less energy.
- Enhancing the quality of materials produced during processing, especially in the mining and mineral processing sectors.
- Increasing screening efficiency, especially for higher throughputs often results in backflows and blinding of screen openings with other screening methods.
- Lowering upfront investment when compared to other types of industrial screening.
- Reducing maintenance expenses and resulting downtime, as modern designs have fewer parts and typically only the screen needs to be replaced.
To ensure the highest vibrating screen efficiency, material handling solutions should always follow best practices when working with vibratory screens, particularly when it comes to maintaining them.
Best Practices to Follow When Using a Vibrating Screen
By paying careful attention to best practices regarding maintenance, a manufacturer can guarantee a long and productive life for any vibratory screens within its material handling system.
To ensure the highest vibrating screen efficiency, best practices should include:
Maintenance is critical for achieving optimal vibrating screen efficiency. As with any industrial equipment, vibratory screens should be installed correctly, inspected consistently, kept clean and lubricated properly, with all maintenance activities logged properly.
Common Mistakes to Avoid
While it's important to follow best practices when dealing with a vibrating screen, efficiency can also be affected by improper use of a vibrating screen. Efficiency can be affected negatively by simply neglecting to keep the screen clear of material buildup, for example.
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Other common mistakes to avoid to maintain optimal vibrating screen efficiency involve:
- Allowing too much vibration results in damage to the vibrating screen; efficiency suffers when screens are improperly installed, misaligned or unbalanced.
- Failing to or incorrectly adjusting settings like frequency or amplitude of vibrations can lead to decreased vibrating screen efficiency and performance.
- Failing to safeguard against abrasive materials can damage the screen unnecessarily.
- Ignoring safety provisions poses a real danger to operators, as the risk of injury or worse accidents could result from improper grounding or failure to wear personal protective equipment.
- Neglecting to replace missing or damaged parts like bearings or springs will result in malfunctions that can lead to equipment failure.
- Not sufficiently training operators of vibrating screens may affect efficiency, as operators without proper training often don't understand best practices for operating and maintaining vibratory screens properly.
- Overloading too much material onto the screen causes stress to the screen that can cause a premature breakdown.
- Utilizing an unsuitable screen for an application could affect production, as it might not have the ability to handle the throughput necessary or its apertures may allow oversized particles through.
It's also important not to operate vibrating screens without their media, as this puts unnecessary wear on the support frames and bars.
Future Trends and Developments in the Field of Enhancing Vibration Screen Performance
While screening has been used for a variety of applications since ancient times, the vibratory screen only came into existence in the late 19th century. Even though the technology isn't new, it's still being improved upon. Innovations in screen technology continue to be made in a multitude of different industries. Here are just a few trends and developments seeking to enhance vibrating screen efficiency.
Structural Solution for Heavy Loads of Asphalt
The asphalt sector requires vibrating screens that can handle heavy loads. The screens used in the industry need to separate and select either an aggregate of stones and pebbles or the constituent parts of powders by particle size. This innovative design doesn't change the basic geometry seen in conventional screens but rather allows a screen that's dimensionally similar to provide greater structural resistance at load amplitudes and frequencies higher than those to which it's normally exposed.
Vibration Analysis for the Construction Industry
The idea of analyzing vibrations may seem like a throwback to 20th-century vibes, but tools that monitor and then analyze vibrations in real time offer a means by which to fine-tune vibrating screen efficiency in the construction industry. Using smart devices and analytics software, data can be sent from sensors monitoring vibrations to smartphones, tablets, computers or even cloud-based servers. These systems both calculate and communicate acceleration, deviations, orbit and other vibratory data in real time. Some more advanced systems can even store and utilize historical data to predict performance and make recommendations on when to schedule maintenance tasks.
Solving the Problem of Grain Loss
The processing of crops requires high-quality equipment that often involves using a vibrating screen. Efficiency in the agricultural sector seeks to reduce waste, and a device that combines a vibratory screener with a centrifugal fan helps with grain recovery. While vibrating screens have been used for separating grains from husks, conventional screens find it difficult to separate trapped grains within the husk. Though there are other methods and machines used for separating grain from husks, vibrating screens are less likely to accrue impurities in the raw product. The combination of the fan and the screen helps prevent grain loss.
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Screen Types
Inclined Screen
Inclined screens are the most basic type of screen, fixed to an inclined frame at an angle of between 15° and 30°. The entire body of the screen vibrates on helical springs circularly with the same characteristics and material flow is supplied by gravitational acceleration.
The slope of the inclined screen is fixed, although the stroke can be adjusted to the required level. The general working stroke of an inclined screen is between 8 and 12 mm, and design of inclined screens permits changes to this stroke through the addition or removal of additional eccentric masses on the exciter.
The exciters of inclined screens are generally driven by an external electric motor that transmits the power via a belt and pulley mechanism. Vibromotor applications are not preferred for inclined screens due to their fixed condition working principle.
Inclined screens are generally designed with multi deck arrangements, which allows material to be classified the between 2'5 grade types. Additionally, these screens are generally fitted with an impact area directly before the screening section begins that breaks up the material and causes any long pieces to lie flat during screening.
Inclined screens are comprised of eight basic components, being the side walls, decks, screening media, exciter, electric motor, motor console, spring and spring supports. The most critical factor in inclined screens is their weld-free design. All of the parts mentioned here are assembled with bolt and nut connections to prevent the cracking and failure to the screen body associated with welding.
The advantage of the circular vibrating exciting mechanism used in inclined screens relates to its simplicity, its low maintenance need and its cost-effective design.
Horizontal Screen
A horizontal screen is a non-conventional design that has unique properties that differentiate it from other types of conventional screen. Its most obvious advantage is its working angle. While conventional screens work at an angle of between 15° and 30°, horizontal screens work in parallel to the ground, or at a slight incline of between 0° and 5° degrees.
Horizontal are equipped with triple drive mechanisms that produce an elliptical vibration motion at the required stroke and slope. As mentioned at the exciter mechanism section above, the triple drive mechanism combines the linear and circular vibration types in elliptic vibration. In this way, the advantages of both vibration types are combined in a horizontal , so that while the material is being transporting horizontally at a determined velocity through the linear vibration motion, plunging is prevented due to the circular vibration characteristic of the elliptical motion.
The stroke of horizontal may be adjusted in a range of 14'20mm, although they generally operate with a stroke of between 16 and 18 mm and with a 750 rpm rotational speed. The material flow rate in the media varies between 0.2 m/s and 0.25 m/s. The mass flow rate depends on the differences of the phase angle between the eccentric masses.
The triple drive mechanism is driven by an electric motor, with power transmitted by a belt and pulley mechanism. There is a possibility that the belt may come away itself during operation as a result of the high stroke working conditions associated with horizontal A new belt stretching mechanism has been designed to overcome this problem, and all the power transmitting mechanisms in horizontal have been equipped with belt stretching mechanisms.
Have brought many advantages to operations, although there are some disadvantages, the most critical being the complexity of the triple drive mechanism. In fact, triple drive mechanisms have a sufficiently robust design to overcome any condition, however maintenance procedures are not short, although the robustness of the design means that frequent maintenance is unnecessary.
Banana Screen
Banana or multi-slope are capable of achieving exceptional throughput per area. These have high capacity, low bed depth and high velocity, and may include any number of deck slopes, from two to as many as six, varying from 45º through to horizontal on the final slope.
Banana are excited by a vibromotor located on the top of the . As mentioned, the exciter section of banana has a segmented deck structure, causing the linear motion created by the vibromotor to accelerate the material differently at each deck surface due to the geometry of the , ensuring effective operation is achieved.
The feed section (highly inclined) of a banana permits high velocity material flow, which serves for the quick removal of fine material. Midway along the banana , the resultant thinner bed stratifies quickly, and the remaining fine material (below the cut point) is d out more effectively than would be possible with a slower and thicker bed. The lower slope (see diagram) slows the material down, allowing more efficient of near size material at this point. The advantage of this is the quicker stratification provided due to the high velocity imparted by the banana shape.
The various slopes may also incorporate deck media with different apertures to meet the particular processing requirements. The are commonly designed to fit modular rubber or polyurethane deck panels, although woven wire or punched plates may also be used, depending on requirements.
Dewatering Screen
De-watering have been designed to allow the drainage of slurry material water and to reduce the moisture rate of the material. These systems comprise a vibromotor pair, a vibromotor console, media and a ody. The surface is slightly inclined (between 0° and 5°) to facilitate drainage and the working speed is between ' RPM.
The counter-rotated vibromotors create a linear vibration that causes the body to shake together with slurry material. The water from the slurry materials is drained under the effect of vibration and flows through bottom side of the surface as the material moves forward. In this way, a pool of water forms in the valley as sand builds up on the inclined surface. The uphill slope of the along with a discharge weir creates a deep bed that acts as a filter medium, allowing the retention of material much finer than the openings.
Different to dry material , dewatering work at G-forces in excess of '5g', ensuring a perfect drainage operation. Generally operating within a range of 5g and 6g this pre-condition is necessary for a good dewatering operation.
The fabricated vibromotor console frame, as a vital component in vibration transmission, is stress relived due to the high G-force working conditions. The side plate and the vibromotor side plates of the console must be machined to exacting tolerances to ensure a precise fit, and consequently, a long operating life. The steel frame should be constructed with bolted connections to avoid the cracking associated with weld failures.
Due to the wet and corrosive environment of the dewatering , rubber springs should be used on the support legs to absorb the live frame vibrating loads, which will extend life and maintenance periods.
High Frequency Screen
High-frequency are engineered to provide higher production capacities and more efficient sizing than conventional High-frequency operate with aggressive vibration applied directly to the , which allows for the highest capacity in the market for the removal of fine material, as well as chip sizing, dry-manufactured sand and more.1
Different to other type the vibromotors of high-frequency are mounted on each deck instead of on the body. Aggressive vibration is applied within a range of ' RPM directly to themedia, allowing for higher capacity and more efficient sizing when compared with conventional . Under the effect of high-frequency vibration, a smaller bed depth is obtained, which allows for stratification and greater efficiency.
The high-frequency and low-amplitude operation ensure a faster material travel speed without loss of efficiency. This combination of high frequency and know amplitude is ideal for fine material in which coarse material particles are lifted higher while the finer particles stay closer to , and as a result, the probability of separation is increased with high-frequency .
Grizzly Screen
Very coarse materials are usuallyusing an inclined called a grizzly Grizzlies are characterized by parallel steel bars or rails set at a fixed distance apart and installed in line with the flow of the material. The gap between the grizzly bars is usually greater than 50 mm, and can be as large as 300 mm, with a feed size of up to 1 m. Vibrating grizzlies are usually inclined at an angle of around 20° and have a circular or linear throw mechanism.
The bars are typically made from wear-resistant manganese steel and are usually tapered to create gaps that become wider towards the discharge end of the sto prevent material from wedging between the bars. Grizzly generally used as a feeder prior to the to supply the flow of correct-sized material through the
Designed to convey material while separating fines, Vibrating Feeders utilize smooth, controlled feed rates to maximize capacity. The grizzly bars are tapered to self-relieve, and feature adjustable spacing for bypass sizing. The feeder construction includes a heavy-duty deck plate with optional AR plate liners, while the heavy-duty spring suspension withstands loading impact and assists vibration.
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