P&Q University Lesson 8: Screening

By |  August 10, 2015
Photo by Kevin Yanik

Photo by Kevin Yanik

Aggregate material is separated into sizes through the use of screens. In most crushed-stone operations, this process occurs after the shotrock has been processed by a primary crusher. The role of screening in the processing flow is to size and separate material ahead of secondary and tertiary crushing circuits, and/or to size and separate material in preparation for final product stockpiling. The bottom line is that crushers produce the material; screens separate the material; and screening efficiency affects the operation’s overall performance.

Screening is both art and science. The art of screening lies in the meticulous fine tuning, tweaking and synchronizing of screen setups within a near-limitless number of applications. Its science is stratification. In other words, the vibration of the screen deck agitates the material causing it to stratify, allowing the larger particles to remain on the top deck and the smaller particles to fall through the openings of the screening surface. Screening efficiency is calculated as the percentage of the undersize materials passing through the openings divided by the percentage of undersize in the feed. For example, if a screen is only 75 percent efficient, then 25 percent of the material within the desired product range is being rejected with the oversize material.

Vibrating screens must be properly selected and designed, or they will be the biggest bottleneck within an operation. Today’s trend is toward larger screens to increase capacity within larger plants. While most producers want more tons per hour across the screen, the key to optimum screening is maximizing capacity without losing efficiency. This may involve a good amount of trial and error, as there are many operating parameters to consider.

OPERATING PARAMETERS

Maximum screening efficiency results from proper adjustments in speed, stroke, rotation (or throw) direction and angle of inclination. Each of these parameters affects one of the most important facets in screening – proper depth of bed.

As feed material is a mixture of varying sizes, oversize material will restrict the passage of undersize material, which results in a build-up, or bed depth, of material on the screen surface. Bed depth diminishes as the undersize material passes through the screen openings. For efficient screening, the material bed should not reach a depth that prevents undersize from stratifying before it is discharged. The industry rule of thumb is this: Depth of bed (in dry screening) should not exceed four times the opening size at the discharge end of the screen. Consequently, with a ½-in. opening, the depth of bed at the discharge end should not exceed 2 in.

Loading screens too heavily is a common practice, and one that leads to a carryover problem and less screening efficiency. Operators should consider these four parameters to fine tune screening performance.

Photo by Kevin Yanik

Photo by Kevin Yanik

Increasing speed has its trade-offs. Greater speed may decrease depth of bed, but also increases the G-force, which decreases bearing life. Using the proper opening size for the desired particle separation, along with increased speed, will leave a minimal percentage of desired product size in the oversize. Alternatively, combining increased speed with a slightly larger opening size may allow a percentage of oversize in the desired product specification.

Increasing stroke delivers a higher carrying capacity and travel rate, while reducing plugging, blinding and enhancing stratification. However, it can create some inefficiency when lightly loaded decks lead to material bouncing. Generally, coarse separation requires increased stroke and less speed, while fines separation needs less stroke and higher speed.

Rotation direction can dramatically impact incline screen performance. Running counter flow, or uphill, increases material retention time and action on the screen, potentially giving the particles more opportunity to find an opening – and ultimately increasing efficiency. Direction of rotation has little effect on a linear-type horizontal screen.

Increasing the angle of inclination causes faster material travel, which can be advantageous in certain dry screening applications. Although, there may be a point where too much incline will hinder efficiency as fines may roll over the media rather than pass through. Consider adjusting both linear and triple-shaft horizontal screens for inclination as well. One can realize some gain in capacity, rate of travel and productivity by adding some incline to the horizontal screen.

INCLINE VS. HORIZONTAL SCREENS

There are a limited number of applications where a horizontal screen is more suitable than an incline screen. These may include portable applications or plants where proper clearance for an incline is not available or applications with heavy water use, such as a dredge-fed screen.

An incline model is less prone to plugging and uses gravity to reduce its energy and horsepower requirements. There are differences in rate of travel between an incline and horizontal unit. At 45 to 50 ft. per minute (and at a specific tonnage), a horizontal screen will experience diminished capacity due to a greater depth of bed. Alternatively, on a 20-degree incline and at 70 to 75 ft. per minute travel rate, an incline screen will deliver up to 25 percent more capacity than a linear-stroke horizontal machine. Unlike the latter, the circular motion of an incline screen results in less stress to the vibrating frame.

DEWATERING SCREENS

Most of the processes for separation and classification consume large amounts of water. Different types of machinery and equipment have been developed to recover the water used for processing and to produce a final product that is easy to transport and store. One such device is a dewatering screen.

The purpose of the dewatering screen is to remove the water content down to 14 percent or less so the material can be conveyed and stacked. Dewatering on a vibrating screen produces a dense, compact filter cake that moves to the screen deck. Polyurethane and profile wire are the best media options for dewatering screening.

Typically, the screen deck is minus-3 degrees (negative slope). The filter cake traps smaller particles and allows water to pass through to the screen deck openings. Dewatering in mineral processing is normally a combination of the sedimentation and filtration methods. The bulk of the water is removed in the first one-third of the machine by sedimentation. This thickening of the material produces a pulp of 55 to 65 percent solids by weight. Up to 80 percent of the water can be separated at this stage. Filtration of the thickened pulp then produces a moist filter cake of between 80 and 90 percent solids. Filtration is the process of separating solids from liquid by means of the porous filter cake that retains the solid but allows the liquid to pass.

Photo by Kevin Yanik

Photo by Kevin Yanik

APPLICATION PROBLEMS AND SOLUTIONS

Specifying the right screen involves making sure the manufacturer understands the production goals and is supplied with complete application data, which includes information such as tons per hour, material type, feed gradation and top particle size, particle shape, application type (wet or dry), type of screen media and deck opening, and the method of material feed. Armed with accurate information, the manufacturer can customize the screen setup for maximum performance. For example, with a known feed gradation, the manufacturer can analyze the loading on each deck. If a deck has a heavier depth-of-bed ratio relative to the opening, that deck may be specified at a steeper angle than an accompanying deck. Therefore, one might have an incline screen at 20 degrees on the top deck, and up to 24 degrees on the bottom deck where it’s more heavily loaded.

The main obstacles to efficient screening are plugging, blinding and carryover. Each can be minimized with a variety of solutions.

Plugging happens when near-size particles become lodged, blocking the openings. Solutions may include increasing stroke, changing media wire diameter or opening shape, using urethane or rubber media, and adjusting crusher settings.

Blinding occurs when moisture causes fine particles to stick to the surface media and gradually cover the openings. In this case, changing stroke and increasing speed may help. Also, if changing the screen media does not improve the situation, consider ball trays or heated decks. Ball trays incorporate rubber balls into pockets beneath the screen cloth. As the machine vibrates, the balls strike the media to free collected material. Heated decks have an electric current in the wire that heats and dries material, so that it easily knocks itself loose as the screen vibrates.

Carryover occurs when excessive undersize particles fail to pass through the openings. Solutions may involve changing stroke, speed or reversing screen rotation; changing wire diameter or the shape of the opening to increase open area; changing the angle of inclination; changing feed tonnage; controlling feed segregation; and centering feed on the screen.

ENSURING SCREEN PERFORMANCE

Vibration analysis, the acquisition and analysis of data regarding the vibrational characteristics of the machine, is one of the tools for ensuring optimum vibrating screen performance. Vibration analysis collects data on parameters such as natural frequencies, displacements and stroke amplitude, and the operation of bearings and gears. It typically involves using a hand-held analyzer connected to a series of accelerometers. The analyzer electronically records the vibrational data. This data can be immediately examined on the analyzer or downloaded onto a computer for a more detailed analysis.

Tests are conducted both at the factory and in the field. Baseline readings are taken at the factory on every machine while they are on the test stand for quality control. More readings should be taken shortly after start-up once the machines are operational in the field. Readings should be taken while the machine is empty and when it is fully under load. They should also be taken any time a speed or stroke change is made, when significant screen media changes occur, when applications change, and importantly, when and if there are any major support tower upgrades or rebuilds.

Vibration analysis benefits from the additional technologies of impact testing and operating deflection shape (ODS) analysis. Impact testing is used to determine natural frequencies that could cause issues at run speeds, or would require structural changes. A baseline reading is taken on each machine at the factory and is used to confirm the accuracy of engineering models. ODS analysis is used to animate and check new equipment and new concepts, while also confirming engineering models for accuracy. ODS identifies how a machine moves in actual operation and at specific frequencies. The analysis compares mode shapes to determine the most effective structural modifications to the machine.

Photo by Kevin Yanik

Photo by Kevin Yanik

Screen choices

At the primary stage, large scalping screens remove fine material before the feed enters the primary crusher, helping to protect the crusher’s wear parts from abrasive stone or sand material that has already been sized. Without scalping, the primary crusher’s liners wear faster, requiring more frequent changes and maintenance downtime.

Following the primary-crushing stage, screens with two or three decks and different opening sizes separate the aggregate material into different size categories – with conveyors transporting the sized material for further crushing or stockpiling as a saleable product. Usually this screening is accomplished through dry screens. Wet screens may help to remove debris from material before stockpiling, as clean stone is often required for concrete and asphalt specifications.

Depending on the process stage, the material to be screened is fed to the screen from an intermittent-feed loading device like a wheel loader or from a continuous-feed device like a hopper or a conveyor. The screen box uses shafts with counterweights or exciters to cause the material bed to vibrate. Through the vibration, larger particles work their way to the top of the material bed, while the smaller particles make contact with the screening surface.

Because they are inclined, circular-motion screens provide a high travel rate. They generally accept a continuous feed very well. Screens using circular motion are best suited for larger material, as finer material tends to blind on this style of screen. Also, wet, sticky material does not screen well with this type of screen, unless water spray is also used.

Linear-motion horizontal screens typically generate less blinding and pegging of material on screen media because their straight-line motion, with high G-forces, can both dislodge material and convey it forward across the screen. This motion can be more effective than circular- or elliptical-motion screens, resulting in a high-efficiency screen that also operates at a fairly high speed. The operator is able to better control the material travel rate across the screen, further improving screening efficiency. Linear-motion screens also benefit producers through a lower installed cost because they require less headroom than circular- or elliptical-motion screens.

Elliptical-motion horizontal screens offer some of the efficiency of linear-motion screens and the tumbling effect generated by inclined circular-motion screens. They also work to speed material travel rate at the feed end, while slowing it at the discharge end. However, this type of screen does not exert the high G-forces that linear-motion screens do.

There are formulas to help select screens based on many factors, including feed tonnage, screening area and desired efficiency. There are enough variables involved in the formula that it is best to work with manufacturers who understand the complete parameters of the application.

It is important that the manufacturer knows the feed method, size, gradation, moisture content and rate. Existing equipment and mounting structure, total plant production needs and efficiency requirements are also part of the equation. Manufacturers can help to specify not only the best screen unit for the application, but also the best screen media.

SCREEN MEDIA

Choosing the proper screen media for a given application is the key to delivering screen-sizing accuracy and maximum throughput, which also greatly impacts the performance of upstream and downstream equipment. In its most basic definition, screen media can be described as a surface with openings on a vibrating screen deck that allows undersized particles to pass through, and oversized particles to carry over. A vibrating screen can have anywhere from one to four decks, with each deck having a different sized opening, or mesh, for the separation of various particle fractions. Every application is a unique screening challenge, and thus the type of screen media selected is critical for success.

Screen media is a replaceable wear surface that can be made up of one or more removable panel sections on a single deck. There are a vast number of screen media configurations based on material types, aperture sizes and styles, fixing systems and surface features, to name a few. As a result, manufacturers are constantly striving to differentiate their products by varying these specifications to dial in a functional and often customized solution for producers.

To get the best possible screen media solution, it is imperative that the producer supplies the manufacturer with complete and accurate application data up front. Vibrating screen inside box dimensions, a particle-size distribution, moisture content and desired final products are some of the minimum requirements to properly select screen media. Further questions that should be asked of the producer include:

■ Is it a wet or dry screening process?
■ Will blinding or plugging be a problem?
■ How abrasive is the material?
■ Will there be much impact on the screening surface?
■ What is the top size and the bottom size feed to the screen deck?
■ How much screening area is there?
■ Does the material need to be washed?
■ Is noise a concern?

The two most important factors for screen media selection are the screen panel life expectancy and open area. Producers should examine the issue of maximum open area versus maximum wear life – there has to be a tradeoff between the two in designing the configuration of screen panel openings. In general, wire cloth will provide the maximum open area with a sacrifice to wear life, and the reverse is true for polymer screen media. However, recent and ongoing developments in material compounds and hybrid solutions (such as urethane-encapsulated wire) have helped to expand the spectrum of this sweet spot and enable producers to enjoy more of the best of both worlds.

Ultimately when making a decision on screen media, the producer needs to consider the benefits realized and the overall costs over the life of the media panel. A panel with a higher upfront cost may provide significant wear life or throughput benefits, compared with one offered at a fraction of the cost. Therefore, cost per ton of material processed is a more accurate gauge of the cost of screen media.

SCREEN MEDIA SELECTION

Screen media originated with the steel options of wire and plate. Now, the choices include wire, perforated and flame-cut plate, polymers (polyurethane and rubber), and hybrid media. Here’s a closer look at each of those options.

Wire cloth is the best option for an operation with frequent media change outs as a result of varying product specifications. The most common wire cloth options are high-carbon, oil-tempered and stainless steel wire, each with its own application benefits. Stainless steel, for example, is beneficial for corrosion prevention and is effective as an anti-blinding solution.

Perforated and flame-cut plate screens are a good alternative for secondary screening and are available in various steel types and hardness. Plate screens are ideal on top- and middle-deck applications for impact and abrasion resistance. Steel plates have seen recent improvements in quality with options available all the way up to the 400- to 500-Brinell range (a measurement of the hardness of the steel plate), providing for longer wear life and durability.

Polyurethane is available in different durometers and more frequently applied in wet applications where water is added or the feed is in slurry form. Urethane is also the best choice for dewatering screens.

Polyurethane does have its place in dry applications as well, with the development and improvement of material compounds and chemical formulations. Open-cast thermoset polyurethanes have superior wear-life performance over injection-molded urethanes, primarily due to the slow-curing manufacturing process, which creates stronger molecular bonds in the material. Polyurethane panels are often found in a modular configuration for ease of installation and replacement. However, there are large cable-tensioned polymer screens that are better suited for aggressive, high-impact applications.

Rubber media is ideal in dry, high-impact applications and can often be offered in place of plate screens, depending on the nature of the feed. Modular rubber systems combine the benefits of modular screen panels with the durability of rubber impact screens in a high, open-area design. Rubber screen media may also be recommended in a wet-screening application such as where a plant is processing only natural sand and gravel. As well, self-cleaning rubber screens are used in fine, sticky or near-size material applications to prevent blinding from fines buildup, and to gain greater sizing accuracy.

Rubber generally offers the longest wear life of any screen media in the most difficult and aggressive scalping applications. Rubber panels are effective in reducing noise levels by up to 9 decibels when compared with steel media, which is about a 50 percent reduction as recorded by the human ear.

Hybrid screens come in several different types that maximize open area and wear life. Urethane-encapsulated wire offers the advantage of urethane screen media (wear life and noise reduction) without the need to convert to a modular deck and without great sacrifice to open area. Another common hybrid screen combines wire held in place with rubber or urethane strips for greater wear life and an optimal flexing action during screening to prevent plugging or blinding.

Typically, premium rubber compounds are used in hybrid screens, as they are most effective in high-volume applications; and they are particularly ideal in hot and humid environments.

Photo by Kevin Yanik

Photo by Kevin Yanik

SCREEN MEDIA INSTALLATION

Screen media is attached to a deck frame in any number of ways. Proper installation, which includes tightening or tensioning the screen surface against the supporting frame, is integral in prolonging the life of the screen. This is applicable both for modular screen panels that are hammered into place on some types of stringer systems and tensioned panels that are tightened against a clamp rail with rubber pads beneath the screen creating a tensioned crown. Improper screen installation is the biggest cause of premature failure on a deck, and therefore it’s important to check the installation at each shift to ensure the screens are secure and in place. One check at start-up and one at shutdown will be far less costly than unplanned downtime.

Modular polymer screens (stringer system and individual panels) generally have a higher initial cost per square foot compared with wire screens. However, in addition to the wear life benefits, modular panels are smaller and safer for operators to handle. They allow for selective change out of individual worn panels, as opposed to a complete wire cloth panel that would need to be changed out if one section was worn. Modular systems offer greater ease of installation (without any pins or bushings), and are better engineered for retrofitting applications.

MEDIA WEAR LIFE

Wear life for any type of media is largely determined by its mass – the diameter of the wire or the thickness of the urethane. The media must be heavy enough to handle a given top-size material and peak feed rate. Synthetic screens (rubber or urethane) will wear far longer – often more than 10 times longer – than wire cloth or plate screens.

When working with wire cloth, workers typically detect excess wear when a hole is blown through the cloth, allowing oversize material to contaminate product stockpiles. Consequently, it is common to assume that the same wear pattern and result will happen with synthetic media – but that is not so. Operators tend to look for a hole to weld or repair rather than looking at the actual gradations. Frequent quality-control sampling to detect sudden or gradual specification changes is the most effective method to monitor the wear life and condition of synthetic screen panels.

With modular synthetic panels, the maintenance crew can catch any wear issues early by conducting a sieve analysis. This involves examining the particle distribution of a representative sample of material, which is expressed in the percentages of a particle size group passing through or being retained on standard testing sieves. For example, if production is slightly off on a number-one sieve, the crew should start gauging its screens and checking for any wear. After this routine maintenance, they simply take a few minutes to change out a modular panel or two, and they are up and running again.

Note that polyurethane and rubber panels are available in different durometers, which is a measure of surface resistivity or the resistance of plastics toward indentation. Media manufacturers may use the Shore-A scale in selecting plastic and rubber compounds for screen panels – the higher the number, the harder the material.

SCREEN APERTURES AND CONFIGURATIONS

An aperture is an individual opening in the screening surface. Synthetic media panels are manufactured in a wide range of opening types and sizes. Both polyurethane and rubber media panels are offered with either square (the most common type), slotted, zigzag, slotted zigzag and round openings. For example, zigzag openings reduce or eliminate plugging or pegging, which is a condition where near-size particles wedge or jam into the screen openings, preventing the passage of undersize material. Round openings are highly effective in primary scalping operations to minimize plugging or pegging.

Depending upon specification requirements, decks can be composed of panels with varying opening sizes and/or types. Note that solid (with no openings) rubber or polyurethane panels can be installed at the feed end of a screen deck where heavy wear is experienced. Or, solid panels can be used as a discharge lip.

Special surface features, such as dams, skid bars and deflectors can be used to enhance performance. When produced by an injection-molding process, these features can be molded into the surface as part of the original panel construction. This seamless integration of surface feature to panel allows greater strength and longer life versus that of a laminated-on feature.

For example, dams are used in wet applications to slow material and increase washing efficiency. Skid bars are effective in scalping applications to keep oversize material off the screen panel surface, while reducing wear. And, deflectors help redirect material toward the middle of panels.

When considering screen media options, keep these four media applications in mind: dry sizing, wet sizing, wash or rinse, and dewatering.

Sorting aggregate to specification piles requires accurate screen openings and high open area for optimum production capacity. Synthetic polyurethane or rubber media panels offer these characteristics, while increasing wear life over that of conventional wire cloth media. Note that for damp material typically prone to blinding, natural rubber panels are often recommended as they retain open area even in very sticky materials.

Wet sizing (usually with sprays) often increases a screen’s efficiency. Polyurethane media panels deliver greater wear life in this application. Rinse screens are part of the final wash to clean aggregate products prior to sale. Polyurethane media panels are a good fit for rinsing applications as they offer long service life and are available in a wide range of opening characteristics and sizes.

Dewatering involves draining the maximum amount of moisture out of a sand product or waste fines, while retaining as much solid material as possible. Manufacturers offer dewatering panels in a variety of openings from 0.1 mm (about 140 mesh) to 2 mm. Typically the panels have a heavier steel skeleton structure to withstand the very heavy bed depths and high G-forces of the application.

Efficiency is gauged by product throughput or product yield. It is the ratio of the percentage of material passing through the screen surface to the percentage of undersize material in the feed that is available to pass through.

Some assume that wire cloth offers greater open area versus synthetic media. However, 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 bucker bars, crown rubber, clamp rails and center hold-downs, causing actual open area to be compromised by as much as 40 percent.

In the case of synthetic media, the open area is sometimes calculated by ignoring the border. In many cases, the traditional synthetic screen panel has a large border or dead area around the perimeter that often is not taken into account, and thus the open area percentage is overstated. To avoid the specification of undersized vibrating screens, open area needs to be calculated by taking the total number of openings in the screen panel, and determining the percentage of actual open holes versus the complete surface of the panel itself. End users should compare the open area between two different screen panel brands of the same aperture by merely counting the number of holes on each screen panel.

While the use of synthetic screen media definitely reduces maintenance labor, it does not eliminate it. Producers may wish to specify and stock certain modular synthetic screen panels that can be used in multiple applications – as operations may be able to get a useful life out of a panel in one location, and then move it to another application where it will function for a period of time.

If the media supplier has provided a diagram of the deck layout, post it as a reference tool for the maintenance crew. This is especially important if the deck layout is made up of different panel types and opening sizes. This will ensure that the correct layout is maintained as panels are replaced – and will ensure that the deck design remains accurate for the given application.

PORTABLE SCREENING

Portable screening plants are a major part of the business for aggregate producers, road builders and contractors. Any of these operators can tell you how important quality screeners are to a business, but what’s right for one operator may lead to production issues for the next.

From small, highly customized design modifications to the overall type and size, there are a multitude of factors to sift through. Selecting the right screener takes time, research and clearly outlined goals for the operation. Here are six key considerations.

Analyze everything from output capacities to business goals before buying. The first thing to do is size the equipment to match the operation. This is not an option. Understanding the application and materials will help determine the ideal production, capacity and number of end-size products. The screen must be aligned with the goals of the operation.

Next, fully understand the company’s goals and projected sales to determine what size screen is needed. For example, if an operation can sell 500,000 tons per year, its screens need to sort nearly 42,000 tons per month. If the screen is in operation two days each week (about eight days each month), 10 hours each day, the operation will require a machine capable of screening around 525 tons per hour. A screen that processes 300 tons per hour would limit profits and cap growth potential. A machine with a potential output of 900 tons per hour would come with extra expenses and no added value.

Scalping and screening have several main differences. Standard screens are often considered finishing screens because they’re capable of producing specific-sized end products. Operators can adjust the speed of the feeder belt to help produce a clean, sized, finished product. These units typically have two or three screen decks and are ideal for use in sand and gravel pits, on asphalt jobs and in quarries.

Scalping screening plants are built to handle the toughest materials but are not as precise as standard screening plants. Material is fed directly onto the screen. Scalpers are ideal for sorting materials before crushing, processing scrap metals and recyclables, and to extract rock from dirt on construction sites.

Hopper size is typically 12-ft. wide with an option to upgrade to a 14-ft. wide. Those extra 2 ft. can capture more product and prevent spillage. The size of the hopper is perhaps most pertinent when pairing the screener with the loading machine, especially when using a large wheel loader.

A tipping grid or live head can be added to a screener above the hopper for additional sizing. While they perform a similar duty, they are very different. A tipping grid is essentially a hinged grid that blocks larger materials from entering the hopper. This is an affordable option but can become a chore, particularly in wet or dirty applications where the tipping grid may become plugged frequently.

A live head is essentially a vibrating screen that attaches to the hopper and is ideal for heavy-duty, dirty, wet and sticky applications. The unit can be used for two purposes: to scalp dirty material off and eliminate the need for manual cleaning, or to size material going into the machine so operators can produce an additional sized product.

While these are generally very efficient, operators should know that screeners with 14-ft. hoppers would not be used to the full potential. A typical live head measures 12 ft., making 2 ft. of the hopper unusable.

Apron feeder versus belt feeder is another key element to evaluate, as different products vary in durability. The standard belt feeder is perfect for sand and gravel operations, but is likely to tear or break when working with metal, large rock or extremely abrasive material. An apron feeder, which is essentially a belt made of metal, is durable and can handle nearly anything an operator throws at it.

Stockpiling offers little mystery. The higher the stockpile, the less time it will take operators because they’ll be able to run for longer periods without having to move material. Even an additional 8 to 10 in. of stockpile height can make a significant difference.

Photo by Kevin Yanik

Photo by Kevin Yanik

SCREEN MAINTENANCE

Aside from all the proper adjustments and operating parameters required to gain the most in screening efficiency, the need for good preventative maintenance practices is a must for longer-lasting screens and reliable performance. Here are eight key components to a solid maintenance program.

Establish an oil-sampling program. Although a commonly overlooked practice, a regularly scheduled oil sampling is an operator’s best insurance against catastrophic component failure and costly downtime. The valuable insights provided by samplings – such as detecting a worn bearing – allow operations to schedule maintenance downtime around periods of prime production. Scheduled sampling and analysis establishes a baseline of normal wear and can help indicate when abnormal wear or contamination is occurring. Oil that has been inside any moving mechanical apparatus for a period of time reflects the exact condition of that assembly. That’s because oil is in contact with mechanical components — as they wear, trace metallic particles enter the oil. These particles are so small that they remain in suspension. Particles caused by normal wear and operation will mix with the oil. Any externally caused contamination also enters the oil. Identifying and measuring these impurities, indicates the rate of wear as well as any excessive contamination. Importantly, an oil analysis will also suggest methods to reduce accelerated wear and contamination.

Employ recommended lubrication practices. Always consult the owner’s manual for the manufacturer’s recommended lubrication practices. Install the correct amount of oil, and use the recommended type of oil. Change the oil at the proper intervals, making sure that the oil in storage is clean and that clean containers are used to transport the oil. Make sure that the machine is completely level so that oil does not pool at the low side of the machine.

Maintain proper belt tension. Belt tensioning must be right on target for optimum screen performance – not too loose and not too tight. Ideally, the belts should only be tight enough so as to not slip during start-up. If necessary, use a belt gauge to set the correct tension. If belts squeal during start-up or operation – or whip excessively – this may indicate insufficient belt tension.

Over-tightened belts can cause serious damage such as pulling the vibrating frame out of square with the support frame. Operating in this twisted position introduces stresses that may lead to spring failure, metal fatigue, or cracking and broken welds. This twisting affects stroke amplitude and character, which then affects material flow and screening efficiency. Over-tightened belts also put an extra load on the mechanism bearings and may tear up motors and motor bases. Additionally, to prevent drive belts from slipping, flopping or coming off, keep belts and sheaves clean and properly aligned. Inspect sheaves for wear, and if the grooves are worn, replace the sheave.

Prevent material buildup. Accumulation of dust and stone around moving parts is one of the largest single causes of part failures, particularly for pivot motor bases, support springs, roller bearings and the vibrating frame. Impact between the vibrating frame and accumulated material may lead to tower vibrations as well as potential side sheet and support deck cracking. Note that sheaves and belts are susceptible to material jumping over the side sheets and causing damage. Where possible, use stationary skirt plates or rubber flaps to deflect airborne material. It’s also important to avoid material buildup in bins, hoppers and transfer points.

Maintain proper screen media support and tensioning. Uniform tension must be maintained on the screen surface to prevent whipping and to maintain contact between the screen surface and the bucker-up rubber on the longitudinal support bars. Improper tensioning may cause severe damage to costly screen media. Also, do not operate a vibrating screen with screen cloth or other screen media sections removed as this will accelerate wear on the support frames and the longitudinal support bars.

Inspect for wear. Inspect cross members for signs of premature wear – especially in wet-screen applications where wear is accelerated. Cover and protect the cross members, decking and housing tubes with rubber or urethane liners to extend their life. Prior to installing screen media sections, make sure they are appropriately square and flat so that they will seat properly on the longitudinal support bars.

Monitor spray systems. Use the required number of spray nozzles and make sure they are open and fully operational. Maintain the proper water volume and pressure. Avoid spraying perpendicular (at 90 degrees) to the screen surface – as this may result in a rapid deterioration of the screening surface. The spray should strike the screening surface at approximately 45 degrees. Nozzles can be positioned to spray against or with the flow of material. This choice depends upon the desired washing/rinsing efficiency and material properties. For most applications, a pressure of approximately 40 lbs. per square in. at the nozzles is desired.

Operate with proper clearances. Maintain adequate clearances around stationary structures, and never allow vibrating frames to hit stationary structures. Wherever possible, provide a minimum of 24-in. side clearance on each side of the machine. This enables the operator to adjust screen-cloth tension and check the unit’s condition and operation. Allow sufficient clearance in front of the screen at the discharge end, or in the rear at the feed end, for replacing screen sections. Set the clearance at least 1 ft. longer than the longest screen panel. Maintain a minimum vertical clearance of at least 5 in. between the vibrating frame and any stationary structures such as the feed hopper or discharge chutes and bins. Avoid providing places for dust and stones to accumulate and interfere with the movement of the vibrating frame.


 

Sources:

Contributors to this chapter include the following, in alphabetical order:

Deister Machine Co.

Sean Donaghy
National Sales Manager
IRock Crushers

Linatex Corp. of America
(a Weir Minerals company)

Polydeck Screen Corp.

Terex Minerals Processing Systems

W.S. Tyler

 

 


Lesson 8 Quiz

1. What is the role of screening in the processing flow?

2. What four things create maximum screening efficiency?

3. Why are incline screens typically preferred over horizontal screens?

4. What is the purpose of a dewatering screen?

5. When does carryover happen in dewatering screens?

6. What is screen media?

7. What are five options of screen media?

8. Why are dams sometimes used in wet applications in the screening process?

 

Click here for the quiz answers.

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