Vibrating screens are well-known to play an important role in industrial screening. While there are several options for material handling systems in the waste management, recycling, and aggregate industries, vibrating screens have several advantages that make them the preferred method.
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If you want to know how a vibrating screen works and why it's better than more conventional screening equipment, this blog is the right piece of information for you!
Vibrating screens, as their name suggests, sort things by size by transferring vibrations to a screen surface.
Vibrations lower the surface tension between particles, which makes them move apart no matter how the machine is made. Fines are particles that are too small to pass past the screening surface. Oversized particles, meanwhile, are transported across the screen. Mineral processing is only one of several fields where this is put to use.
More and more industries are using vibratory screeners because they are good at separating materials and controlling their sizes. Screens come with either a single motor for elliptical motion or two motors for straight motion, and they can be set up horizontally or at an angle. Each part of the vibrating sieve that makes it move makes sure that small particles moving down the screen have a chance to fall through the screen mesh between the beginning and end of the deck. Decking can be made of wire mesh, steel, rod decks, or polyurethane panels, among other things. Each design comes with standard sizes.
Utilization, cleanliness, and the aggregate being screened can all affect screen capacity and lifespan. To address these issues, DPH Engineering offers a wide range of vibrating screen sizes to choose from so that you may get the one that's just right for your operation. DPH Engineering screens also have a simple design and only the most important moving parts. This makes them a low-cost and low-maintenance option.
To understand better what a vibrating screen is, it's important to know about the vibrating screen working principle. All screens are made with holes of different sizes so that they can do their main job, which is to separate things based on how big they are. In crushing plants, a mining vibrating screen is used to separate the stones into different sizes based on what the final product needs.
The pebble needs to be small enough to pass through the vibrating feeder's opening and into the area below the screen. As a result, the rock that was too big to fit through the screen did go through. Modern vibrating displays generally feature numerous layers. Each level has its own set of displays, which are carried on decks. If a material is too large to fit through the material handling system's screen at the end of a layer, it is dropped to the layer below, and otherwise it is released from the system at the end of the layer where the screen is located. At last, the elements from each layer are dispersed independently via the product outlets. So, the rocks are sorted according to their sizes.
The simplest kind of screen is the inclined screen, which is mounted at an inclination of 15° to 30° on an inclined frame. The flow of material is caused by the acceleration of gravity, and the whole body of the screen shakes on circular helical springs with the same properties.
Although the stroke can be changed to meet specific needs, the inclined screen's slope is always the same. Most inclined screens have a working stroke of between 8 and 12 mm. This stroke can be changed by adding extra eccentric masses to the exciter. Most inclined screens are powered by an electric motor outside of the screen that is linked to the screen with a belt and pulley system. Due to their fixed-state working concept, vibrator applications are not suited for inclined screens.
In general, inclined screens have many deck configurations, allowing for the classification of materials into two to five different grade categories. The screening phase of these machines often begins with an impact region, which is designed to break up the material and cause any lengthy pieces to lie flat during screening.
Walls, decks, screening material, an exciter, an electric motor, a motor console, a spring, and a spring support are the eight main parts of inclined screens. The absence of welds is the single most important feature of inclined screens. All of the aforementioned components are fastened together with bolts and nuts to avoid the screen body cracking and failure that can occur when welding is used.
The circular vibrating mechanism that makes inclined screens work is easy to use, doesn't need much maintenance and is cheap to build.
Unlike more common vertical screens, the horizontal variety has certain distinct advantages. The most noticeable benefit is the convenient working angle. Unlike traditional screens, horizontal screens work either flat on the ground or at an angle of between 0 and 5 degrees.
Horizontal screens with triple-drive mechanisms make an elliptical vibration with the right stroke and slope for the job. The triple drive mechanism, which we talked about above in the section on the exciter mechanism, combines the linear and circular vibration types to make elliptic vibration. So, even though the linear vibration motion moves the material horizontally at a certain speed, plunging is prevented by the circular vibrations of the elliptical motion. This means that the benefits of both vibration types are combined in a horizontal screen.
Horizontal screens have a stroke range of 14'20 mm and typically run at a stroke of 16'18 mm and a spinning speed of 750 rpm. Between 0.2 and 0.25 meters per second, materials move through the screening medium. As the phase angle between the eccentric masses varies, so does the mass flow rate.
The electric motor powers the triple drive mechanism via a belt and pulley system. The belt may detach from the pulleys and come loose during operation due to the high stroke operating conditions of horizontal screens. Engineers developed a belt stretching mechanism to address this issue, which is now standard on all power-transferring systems in horizontal screens.
Even though horizontal screens have helped screening operations in many ways, the most significant problem is that the triple drive mechanism is hard to understand. Even though the design of triple drive mechanisms is strong enough to handle any situation, the maintenance steps aren't as quick as one might like.
With a multi-slope or banana screen, you can get a lot more done per square foot of screening area. These screens can have anywhere from two to six deck slopes, with inclinations ranging from 45 degrees at the first slope to level at the sixth. They have a high capacity, a low bed depth, and a high velocity.
The vibrations that make banana screens work come from a motor that is attached to the top of the screen. Banana screens work well because their exciter sections have segmented decks. This means that the linear motion from the vibromotor speeds up the material in a different way at each deck surface.
Banana screens are used to separate large pieces of debris from smaller ones. This is done well because the steeply inclined design of the feed section allows for fast material flow. The faster stratification of the narrower bed in the middle of the banana screen makes it easier to separate the finer material (below the cut point) from the coarser material (above the cut point). Because the slope of the screen is less steep, particles move more slowly (see diagram). This makes it easier to screen material that is close to the right size. The high speed that the banana screen formation requires makes it possible for layers to form quickly.
Different deck media with different-sized holes can be used on the different incline configurations to better meet the needs of each processing application. Most of the time, the screens are made of modular rubber or polyurethane deck panels, but they can also be made of woven wire or punched plates.
The goal of dewatering screens is to reduce the amount of water in the slurry by letting water drain through them. Each of these units consists of a set of vibrators, a control unit, some sort of screening material, and a screen frame. The screen's surface is slanted at an angle of between 0 and 5 degrees to improve drainage, and the rotational speed of the machine is between and revolutions per minute (RPM).
The counter-rotating vibrators make linear vibrations that shake the whole body of the screen and the slurry inside it. Vibration helps drain the water from the slurry materials, which then flow out of the bottom of the screening surface. By gradually accumulating sand on the sloping surface, water collects in the valley. Material that is smaller than the holes in the screen can be kept because the screen slopes up and the discharge weir makes a deep bed that acts as a filter medium.
Dewatering screens, unlike their dry-material counterparts, operate at G-forces greater than '5g,' guaranteeing flawless drainage. As a rule, dewatering works best when the gravity is between 5g and 6g, typically falling within that range.
The high G-force operating conditions mean that the structure of the Vibro motor console, which is a key part of vibration transmission, needs to be able to handle stress. Careful machining is needed to make sure that the screen side plate and the Vibro motor side plates of the console fit together well and will last a long time. The steel frame could crack if the welds fail, so it's important to use bolted connections when building. The support legs of the dewatering screen should have rubber springs put on them to take the stress of the vibrations of the live frame. This will extend the life of the screen and cut down on how often it needs to be checked for problems.
High-frequency screens are made to be better than traditional screens in terms of their size and their ability to make things. High-frequency screens have the highest capacity in the industry because they use strong vibrations to remove fine material, chip size, dry manufactured sand, and other things.
High-frequency screens are distinguished from other types of screens by the fact that the vibrators are not attached to the screen body but rather to each deck. When compared to traditional screens, the ability to directly vibrate the screen medium at speeds between 3,600 and 5,000 RPM and increase its capacity and size efficiency is remarkable. High-frequency vibration causes the bed depth to decrease, facilitating stratification and improving screening performance.
The high-frequency, low-amplitude operation lets more material pass through without lowering the quality of the screening. Since coarse material particles are lifted higher while the finer particles stay closer to the screen with high-frequency screens, the likelihood of separation is greater with these types of screens.
Grizzly screens are used to separate very coarse materials from finer ones. The opening of a grizzly is guided by steel bars or rails that are placed at regular intervals and run parallel to each other. A grizzly's feed size can be up to 1 m, and the distance between its bars is often bigger than 50 mm (and up to 300 mm). A vibrating grizzly's throw mechanism is usually either circular or linear, and it is usually tilted at an angle of about 20 degrees.
The bars, which are typically made of manganese steel because it does not corrode easily, are frequently tapered so that the gaps grow larger as you move toward the end of the screen where the material exits. Screens are often used as feeders in front of crushers to make sure that the right-sized material flows through the crusher.
Vibrating feeders are made to move material while separating particles. They do this by using smooth, controlled feed rates that allow for the most work to be done. The grizzly bars can be made to fit a bypass because their spacing can be changed and they have a taper that makes them easier to use. The structure of the feeder has a heavy-duty deck plate with optional AR plate liners and a heavy-duty spring suspension that can handle loading force and help with vibration.
Vibratory screeners are used in a wide range of industries. Some of them are:
Certain materials need different kinds of screening equipment, but a vibrating screen machine has a lot of benefits when it comes to processing aggregate and mining materials. Vibrating screens are particularly well-suited to meet these industries' fineness requirements. Because of the way they are made, they can provide the force needed to separate large particles from fines. This makes it easier to get the product back, even when working with a material that needs much less size separation. This results in better yields and an enhanced final product.
The construction of vibrating screens is notoriously sturdy. Most of the time, vibrating screens are less expensive than other types of screeners because they work in a simple way and their modern designs are simple. This means you'll spend less money initially.
Vibrating screens are more cost-effective to run than other, more sophisticated gear since they require less power to do their job. Finally, because higher frequency vibrations are communicated directly to the screening surface, only the screen medium needs to be replaced on a regular basis. So, you might be able to avoid spending a lot of money on keeping and fixing other screening equipment.
Productivity and profits can take a serious hit when there is downtime for repairs or maintenance. Screens of all shapes and sizes get a lot of wear and tear, especially when they are used for hard jobs like processing aggregate. Vibrating screens, on the other hand, require comparatively less attention and upkeep. Because of how they're built, just the screening surface receives the vibratory forces.
Quality vibrating screening equipment is also built and designed so that the vibration forces barely affect other parts of the machine. Machines are getting more complicated so that vibrations outside of the screen media can be taken up and slowed down. As a result, there are fewer opportunities for failure and less overall wear and strain on the machine, which in turn reduces both downtime and maintenance.
Vibrating screens are great when an operation needs a straightforward answer that can manage a greater feed flow, and there is a vast variety of screening equipment to choose from. In fact, vibrating screens perform most efficiently with a constantly higher flow rate. The same amount of material going through different screening equipment could cause blinding of the screen or backflow of materials, so the feed rate would have to be lowered.
Since their inception, vibratory screeners have been utilized by every significant sector for the purpose of refining and separating raw materials. The efficiency benefits brought by this instrument are in great demand in sectors as different as the food industry and the mining industry. Thus, productivity rises while expenses fall. Among the available material handling systems, this is by far the superior choice.
DPH Engineering is the leading vibrating screen manufacturer, so industrialists have been turning to them to outfit their facilities with high-quality equipment. The team works hard for a long time to make sure that the final product is strong and will work well in any environment. Our vibrating feeders have been used successfully in the industry for many years. Our research and development (R&D) experts are always making changes to the way vibrating screens are made so that these tough machines can handle a wider range of tough screening situations in a more efficient and cost-effective way. We have worked hard over the years to gain a reputation as the go-to provider of top-tier magnetic separation and refining machinery in the business, and it shows in the quality of our products.
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Basic Terms
Stratification
This phenomenon occurs as vibration is passed through a bed of material. The vibration of
the machine sifts the material bed so that finer material passes through the coarser material.
This enables particles smaller than the mesh of the screen to pass through it.
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Bed Depth
It is the depth of the material on any given screen deck. The depth of the material on the
screen affects the ability of the screen to stratify the material and allow the fines to pass
through the deck
Travel Rate
The speed at which material travels down the screen.
Open Area
Open area of a screen is defined as the ratio of the net area of the apertures (holes) to the
whole area of the screening surface. Many times it is also expressed as a percentage of the
screening surface area. It helps to determine the screen capacity.
Screen Efficiency
The percentage of material that falls through a screen compared in relation to the total
amount of material in the product stream of that size.
Screen Stroke
The shape and amplitude of the motion of a screen. Usually screen stokes are circular, oval
or strait line strokes.
Frequency
- Measured in hertz (Hz) or revolutions per minute (RPM)
Frequency is the number of times the screen cloth peaks and troughs within a second. For a
gyratory screening motion, it is the number of revolutions the screen deck takes in a time
interval, such as revolution per minute (RPM).
G-Force of a Screen
The force at which a screen moves to the top and returns to bottom of its respective stroke.
Plugging
happens when near-size particles become lodged, blocking the openings.
Blinding
occurs when moisture causes fine particles to stick to the surface media and
gradually cover the openings.
Vibrating Screen Terms
Above figure defined terms like feed end, material flow direction, left side, right side, etc.
The hand of drive is determined by looking from feed end to discharge end (i.e. material flow
direction). The screen shown in above figure is right handed.
Factors Affecting Screen Performance
There has been no universally accepted method of defining screen performance and a
number of methods are employed. The most common screen performance criteria used is
efficiency. The efficiency of screening is determined by the degree of perfection of
separation of the material into size fractions above or below the aperture size. It is a
measure of how much of the feed material should have gone through the hole versus how
much really did go through the hole.
As per VSMA (Vibrating Screen Manufactures Association) Handbook,
It is also referred to as 'Efficiency of Undersize Recovery'.
Screen effectiveness must always be coupled with capacity as it is often possible by the use
of a low feed rate and a very long screening time to effect an almost complete (100%)
separation. However, most screening applications do not require 100% size separation and
the lower the requirements in that regard the higher the capacity of a given system. Hence,
for calculated capacities many manufacturers use an efficiency of 90% or 95%, when not
otherwise specified.
Following factors affect screen performance / efficiency
Particle Size
The process of screening is a series of probabilistic events, where particles are presented to
a screening surface many times, and on each presentation there exists a given probability
that a particle of a given size will pass.
As the particle size approaches that of the aperture, the chance of passage falls off very
rapidly ('Half Size' and smaller particles pass/go fairly easily whereas 'Near Size' takes a lot
more time). Hence the overall screening efficiency is markedly reduced by the proportion of
these near-mesh particles. The effect of near-mesh particles is compounded because these
particles tend to "peg" or "plug" the apertures, reducing the available open area.
Particle Shape
Most granular materials processed on screens are non-spherical. While spherical particles
pass with equal probability in any orientation, irregular-shaped near-mesh particles must
orient themselves in an attitude that permits them to pass. Elongated and flat particles will
present a small cross-section for passage in some orientations and a large cross-section in
others. Hence elongated or flat shapes particles does not pass easily. Even they hinder
passing of other particles also. The extreme particle shapes therefore have a low screening
efficiency.
Feed Rate
Feed rate affects material bed depth. Sieve sizing analysis use a low feed rate and a very
long screening time to effect an almost complete separation. In industrial screening practice,
economics dictate that relatively high feed rates and short particle dwell times on the screen
should be used. At these high feed rates, a thick bed of material is presented to the screen,
and fines must travel to the bottom of the particle bed before they have an opportunity to
pass through the screen surface. The net effect is reduced efficiency. High capacity and high
efficiency are often opposing requirements for any given separation, and a compromise is
necessary to achieve the optimum result.
For efficient screening, recommended height of the material bed depth (for dry screening of
material weighing 100 pounds per cubic foot) is as under.
Feed End: Maximum = 10 × aperture size
Discharge End: Less than 4 × aperture size (Example: For a ½' aperture, a bed depth of no
more than 2') [Less than 3 × aperture size for material weighing 50 pounds per cubic foot].
Minimum bed depth is 1 × aperture size. If bed depth is too thin, material can bounce, stay
suspended and thus reduce accuracy.
Screen Angle
The screen angle changes the size of the aperture relative to what the particle sees. That is,
when viewing a screen opening from above, the more horizontal the screen deck lays, the
larger the opening appears (size of opening seen will be largest when screen is horizontal).
As shown in above figure, if a particle approaches an inclined screen, it will "see" a narrower
effective aperture dimension (11.9 mm instead of 12.7 mm) and near-mesh size particles are
less likely to pass.
In view of this, horizontal screens are selected where screening efficiency is important.
The screen angle also affects the speed at which particles are conveyed along the screen,
and therefore the dwell time on the screen and the number of opportunities particles have of
passing the screen surface.
Open Area
The chance of particle passing through the aperture is proportional to the percentage of
open area in the screen material which is defined as the ratio of the net area of the apertures
to the whole area of the screening surface. The smaller the area occupied by the screen
deck construction material, the greater the chance of a particle reaching an aperture.
Open area generally decreases with the fineness of the screen aperture. In order to increase
the open area of a fine screen, very thin and fragile wires or deck construction must be used.
This fragility and the low throughput capacity are the main reasons for classifiers replacing
screens at fine aperture sizes.
When considering maximum open area, it is important to understand that the percentages of
open area listed in conventional wire cloth media catalogs are based on all the openings in a
section of the screen. Yet, a good portion of those openings are blocked by tension plates,
support bars. capping rubber and center hold downs.
Moisture
The amount of surface moisture present in the feed has a marked effect on screening
efficiency, as does the presence of clays and other sticky materials. Damp feeds screen very
poorly as they tend to agglomerate and "blind" the screen apertures.
Vibration
Screens are vibrated in order to throw particles off the screening surface so that they can
again be presented to the screen, and to convey the particles along the screen. The
vibration also induces stratification of the feed material.
Generally, coarse separation (screening with larger apertures) is performed using larger
amplitudes (strokes) and lower frequencies (speeds); whereas for fine separation (small/fine
apertures), small amplitudes and high frequencies are preferred.
The vibration must be sufficient to prevent pegging and blinding. However, excessive
vibration intensity (vibration G-force) will cause particles to bounce from the screen deck and
be thrown so far from the surface that there are very few effective presentations to the
screen surface. Higher vibration rates can, in general, be used with higher feed rates, as the
deeper bed of material has a "cushioning" effect which inhibits particle bounce.
Vibration intensity can be characterized by the vibration frequency, f cycles per second and
amplitude, a metres. The term "stroke" is commonly used and refers to the peak-to-peak
amplitude, or 2a.
The intensity of vibration is defined by the vibration G-force as under:
It can be seen from above formula that vibration frequency (rotation speed) and amplitude
(a) or stroke (2a) affects the vibration G-force. Hence, G-force of a vibrating screen can be
altered/changed by changing speed and/or changing amplitude of the vibrating screen.
Vibration frequency (f) can be changed by changing drive's V-belt pulleys or inverter
parameters. The change in rotation speed will not have any effect to the amplitude (stroke
length). The amplitude can be changed by altering (adding or removing) the counterweights.
Above figure shows relationship between G-force, rotation speed (stroke frequency) and
stroke length.
Speed and Stroke are selected based on the application. Vibrating screens typically operate
with a vibration G-force between 3G to 7G (3 to 7 times the force due to gravitational
acceleration).
Following are the typical G-force levels as per one of the leading vibrating screen
manufacturer.
Inclined Screen: 3.3 to 4.0
Above table may be used to select stroke and speed for horizontal vibrating screens.
Since high G-force level leads to higher stress, higher wear and reduction in bearing life, it is
recommended that G-force should be as kept as small as possible. It also reduces energy
costs.
Doubling of speed quadruples stress.
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