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Vibrating Screen Performance in Heavy Industries (Quarry, Mining)

How to design an efficient vibrating screen?
What operational parameters impact screening performance (amplitude, frequency, inclination)?
How to detect and fix common vibrating screen issues?

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Section summary
1. Core Principles of Effective Vibrating Screening
2. Calculation Methods for Screen Capacity and Area
3. Key Factors Influencing Screening Performance
4. Operational Best Practices, Maintenance, and Troubleshooting
5. Process integration

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Vibrating screens play a crucial role in heavy industries, particularly in mining and aggregate processing, for achieving efficient material separation and sizing. These industries rely heavily on the accurate classification of raw materials into specific fractions, which is essential for downstream processes and the production of marketable goods.

This webpage aims to synthesize the fundamental design principles, key operational factors, and calculation methods for vibrating screens specifically within the context of heavy industrial applications.

Figure 1 : Industrial vibrating screen for mining industries

Please note that while the core focus of the webpage is on the aggregate and mining industries processing materials like rock, gravel, sand, and coal, the fundamental design principles of stratification, carrying capacity, and the calculation methods for screen area and capacity could potentially be adapted for other industries that involve separating dry or wet bulk materials by size, such as some segments of the chemical processing, food processing, or recycling industries, although sources on which are based the page are more related to mining.

1. Core Principles of Effective Vibrating Screening

How is working bulk solids separation on vibrating screens ?

The fundamental goal of utilizing vibrating screens in heavy industries is to prepare a sized product. The ultimate aim is to achieve final product sizing, which involves separating a feed material into distinct size fractions based on specific requirements. This is achieved through a combination of mechanical actions and the characteristics of the screening media.

To effectively prepare a sized product, a vibrating screen must perform several essential operational functions. Firstly, it needs to stratify the material. Stratification is the process where particles within the feed material arrange themselves based on size, with larger particles typically moving to the top of the material bed and smaller particles migrating towards the screening surface. Secondly, the screen's operation should aim to prevent pegging (where material gets lodged in the screen openings) and prevent blinding (where fine material adheres to and obstructs the screen openings). Thirdly, the vibrating screen must separate the material into two or more fractions, typically an oversize fraction (particles larger than the screen openings) and an undersize fraction (particles smaller than the screen openings). Finally, it is crucial for the screen to transport the material along the screening deck to achieve its carrying capacity.

Carrying Capacity is defined as the amount of material a screening machine can carry over the decks before the momentum of the screen body is overcome by the weight of the material. Essentially, it is the amount of material a vibratory screen can carry without a significant reduction in screening efficiency due to overloading.

Achieving the desired accuracy in separating material according to aggregate specifications is a key objective. The operation of the screen, after the correct size is selected, should be optimized to yield the best combination of variables such as speed, stroke, and slope. Maintaining the correct bed depth of material is also crucial for accuracy at the discharge end; a bed depth that is too thick decreases the probability of sized aggregate properly stratifying and passing through an opening, while a bed depth that is too thin can cause material to bounce and not find an opening, thus reducing accuracy. Therefore, optimal performance relies on appropriate operation after the correct screen size has been chosen.

2. Calculation Methods for Screen Capacity and Area

How to determine the size required for a vibrating screen ?

The calculation is based on a formula given by VSMA, this is based on correlations determined by an association of vibrating screen manufacturers :

Screening Area (ft²) = U / (A x B x C x D x E x F x G x H x J)

Where:

• U = Amount in STPH of material in feed to deck that is smaller than a specified aperture
• A = Basic capacity (STPH)
• B = Oversize
• C = Halfsize
• D = Deck location
• E = Wet screening
• F = Material weight
• G = Open area of media
• H = Shape of opening
• J = Efficiency

Factor	Explanation
A	Predetermined rate of material in STPH through a square foot of a specified opening when feed to deck contains 25% oversize (factor B) and 40% halfsize (factor C) Basic Operation conditions : - feed to deck contains 25% oversize and 40% halfsize - Feed is granular free flowing material - Material weighs 100 lb/cu.ft - Operating slope of the screen inclined screen 18-20° with flow rotation ; horizontal screen 0° - Objective screening efficiency = 95%
B	Actual % of material in feed to deck that is larger than a specified aperture (adjusts factor A to suit actual conditions)
C	Actual % of material in feed to deck that is one half the size of a specified aperture (adjusts factor A to suit actual conditions)
D	Applies for multiple deck screens. Total screening area is available for top deck separation. Time delay for material to pass to deck and 2nd or 3rd deck leaves less effective area available. This factor is expressed in a % of the top deck effective area.
E	Applies when the water is sprayed on the material as it moves down the screening deck (typically 5-7 GPM per STPH of solids).
F	Applies for weights other than 100 lb/cu.ft, it is calculated as lb/cu.ft (actual) / 100
G	Applies when the open area of the screening surface is less than the reference used for factor A
H	applies when rectangular opening are used. Slotted or oblong openings will pass more material per square foot than square opening.
J	Applies when the target screening efficiency is < 95%

Figure 2 : water spraying on infeed to vibrating screen

3. Key Factors Influencing Screening Performance

What are the key process parameters to consider ?

Achieving effective vibrating screening hinges on a multitude of interconnected factors that can be broadly categorized into material characteristics, screen setup, feed management, and the properties of the screening media. Understanding and optimizing these factors is crucial for a safer, more cost-efficient operation and for attaining the desired final product sizing.

A. Material Characteristics: The properties of the material being screened significantly impact the screen's performance. Key aspects include:

• Type of material: The nature of the material, whether it's wet or dry, and the presence of slivers can affect screening.
• Amount of near-size material: A higher percentage of near-size material (particles close to the screen opening size) can reduce capacity and efficiency.
• Shape of material: Elongated particle shapes can hinder the screening process, reducing both capacity and efficiency. Cubical, slabby, flaky, or round particle shapes are also characteristics to consider.
• Wetness and Moisture content: Higher moisture content in the feed material generally reduces capacity and efficiency. However, wet screening, when implemented correctly, can sometimes increase capacity. Wet material can also become easily fluid, aiding stratification.
• Bulk density: The weight of the material per unit volume influences the load on the screen and thus its capacity.
• Feed curve: The size distribution of the material in the feed (feed curve) is a crucial factor.

B. Screen Setup: The operational parameters of the vibrating screen itself play a vital role:

• Speed: The rotational speed of the screen (RPM) affects the rate of material travel and stratification. Generally, larger openings pair with slower speeds, while smaller openings benefit from higher speeds. The speed helps create sufficient material travel for a shallow bed depth, allowing fines to sift through.
• Stroke: The stroke length (diameter of circular motion) influences material agitation and the probability of particles passing through the openings. The stroke should be sufficient to prevent plugging but not excessive enough to damage the screen or interfere with screening.
• Slope: The angle of the screen deck affects the material travel rate and bed depth. Steeper slopes increase travel speed, while shallower slopes increase retention time. For small openings, higher speeds and more aggressive strokes are generally required, often combined with flatter screen slopes (up to 10 degrees) to aid in overcoming plugging problems.
• Direction of rotation: The direction of the vibrating motion relative to the material flow (with-flow or counter-flow) can impact both capacity and accuracy. With-flow rotation is generally preferred for capacity, while counter-flow can offer higher accuracy but may limit capacity due to slower travel speed and higher bed depth.
• Natural frequency vs. operating frequency: The relationship between the screen body's natural frequency and its operating frequency is critical for avoiding "off-motion" and ensuring screen life. Operating too close to the natural frequency can lead to stress and reduce screen life.

C. Feed Rate and Material Bed Depth: How material is fed onto the screen and the resulting bed depth are critical for performance.

• Feed rate (stph): The amount of material fed onto the screen per unit time directly affects the capacity required of the screen. Overloading the screen can overcome its carrying capacity.
• Material bed depth: The thickness of the material layer on the screening media influences stratification and the probability of particles finding an opening. At the discharge end, the bed depth ideally should not exceed four times the deck opening to maintain accuracy. A bed depth that is too thick reduces the chance of proper stratification, while a bed depth that is too thin can cause material to bounce, preventing separation. Screen width plays a role in controlling material bed depth, which is essential for stratification.

D. Screening Media: The type and characteristics of the screen surface are fundamental to the separation process.

• Media type: A wide variety of media are available, including woven wire cloth, plastic, piano wire, rod decks, grizzly bars, and perforated plates, each suited for different applications and material types.
• Open area: The ratio of the open space to the total surface area of the media significantly affects capacity and efficiency. A finer wire diameter generally leads to a greater open area, increasing capacity and efficiency.
• Wire diameter: The thickness of the wires in woven wire cloth influences the open area and the durability of the media.
• Opening shape and size: The shape (square, long slot, short slot, etc.) and size of the openings determine the separation size and can influence capacity, especially with elongated particles. Slotted openings can increase capacity.
• Weave: For woven wire cloth, the type of weave (plain, crimped, etc.) affects the stability and opening size of the media.
• Media material: The material from which the screen media is made (e.g., steel, polyurethane, rubber) affects its wear resistance, flexibility, and suitability for wet or dry screening.

E. Maintenance and Installation: Proper maintenance and installation are crucial for consistent and optimal screening performance.

• Unit not installed level: An unleveled screen can lead to unequal spring deflection and off-motion.
• Broken/worn springs or rubber mounting units: Damaged suspension components can cause improper vibration and reduce screening efficiency.
• Loose fasteners: Ensure all fastening hardware is properly torqued to prevent issues.
• Material build-up: Accumulation of material on the deck can impede screening and cause off-motion. Cross dams can also affect performance.
• V-belt tension: Maintaining uniform and correct tension on V-belts is essential for proper drive and speed.
• Correct installation of motor base(s): Proper motor mounting ensures efficient power transmission.
• Alignment of sheaves and shafts: Sheaves must be aligned and shafts parallel for optimal belt life and performance.

4. Operational Best Practices, Maintenance, and Troubleshooting

Problem	Possible Causes	Recommended Actions
Plugging	Near-size or elongated material	Increase the stroke of the screen to help kick out the material. Consult with the factory before making any changes to speed or stroke.
Blinding	Fine material sticking to the screen media	Increase the speed of the screen. Sometimes, increasing the stroke can also be beneficial. Note if it started as a plugging problem. Consult with the factory before making any changes to speed or stroke.
Off-Motion	Unit not installed level (unequal corner spring deflection)	Ensure the unit is installed level.
	Broken / worn springs or rubber mounting units	Replace broken or worn springs or rubber mounting units.
	Loose fasteners	Check and tighten all loose fasteners.
	Material build-up on deck or decks	Remove any material build-up on the screen deck(s). Check the screening surface for material build-up before starting the screen.
	Side loading	Address any side loading issues.
	Overloading	Reduce the feed rate to avoid overloading.
	Plugging and blinding	Address plugging and blinding issues as described above.
	Incorrect speed	Verify and adjust the speed to the correct setting. Always consult with the factory before making any changes to speed or stroke.
	Inadequate support structure	Ensure the support structure is adequate for the screen.
	Inadequate body design	Consult with the manufacturer regarding the body design.
	Improper V-belt tension	Check and adjust the V-belt tension to the correct level. Ensure uniform belt tension.
	Vibration dampening adjustments	Review and adjust vibration dampening settings.
	Operational frequency too close to natural frequency (critical speed)	Consult with the manufacturer to address the relationship between operating and natural frequencies.
General Initial Checks	Broken springs, belts too tight, broken cross members, loose bolting hardware	Always look for the obvious problems first. Inspect these components. Check all bolted connections for proper torque.
Screen not starting	Power failure, starter inoperative, motor does not operate, material interference with screen body or motor base	Check power supply, fuses, breakers, heater. Refer to motor section. Clear build up from screen body or motor base.
Motor does not operate	Fuse or circuit breaker blown, defective power cable	Replace or reset. Check cable for broken conductors and replace if defective.
Motor hums but does not start	Defective motor, bearing lubricant too heavy	Replace defective motor. Clean bearings and relubricate with proper lubricant.
Motor overheats	Motor wired incorrectly, motor too small, bearing failing, defective motor, power circuit wire too small, power circuit overloaded	Correct wiring - consult MFG. for proper size. Replace damaged bearing. Install correct size motor. Install proper power circuit with correct wire size or reduce load.
Overheating of Vibrator	Trouble with vibrator assembly, too little lubricant, too much lubricant, improper lubricant, improper bearing clearance, material build-up on bearing housings, insufficient clearance on labyrinth seals, motor inoperative, bearing or seal components frozen or damaged, lubricant too heavy, drive belts too tight	Refer to vibrator sections. Check for leakage, damaged seals; relubricate. Remove lubricant to proper level, allow lubricant to purge from system if so designed. Replace with proper high temperature lubricant, ventilate area, use high temperature lubricant, consult screen manufacturer. Replace bearing and check for contamination. Remove build-up. Check seal clearance. Refer to motor section. Replace bearings or seals. Relubricate with recommended lubricant. Tighten V-Belts.
Vibrator will not rotate	Motor inoperative, bearing or seal components frozen or damaged, lubricant too heavy, drive belts too tight	Refer to motor section. Replace damaged bearings or seals. Remove lubricant, relubricate with recommended lubricant. Tighten V-Belts.
Lubricant leakage	Vibrator assembled incorrectly, excessive operating temperature, excessive lubricant, drain plugs omitted, damaged or worn seals, bearing failing, loose in the bearing housing	Review assembly procedure. Use high temperature lubricant. Consult screen manufacturer. Restore lubricant to proper level. Install drain plugs. Inspect seals and replace. See bearing trouble. Replace bearing, bolts and properly torque, check bearing, ensure that damage to the housing or fastener holes hasn't occurred, consult screen manufacturer.
Noisy bearing	Improper bearing clearance, normal fatigue failure, overloading, lack of lubricant, excessive lubricant, spalling from dirt or water entering bearing, brinelling from storage, improper float or allowance for expansion in vibrator assembly	Consult screen manufacturer. Replace bearing according to manufacturer's assembly instructions. Return screen to original operating mode and replace bearing. Restore correct lubricant level; use lubricant recommended for the ambient temperature, replace bearing, use correct lubricant. Flush housings and lubrication system, replace bearing and clean or replace seals. Replace bearing and correct cause. Replace bearing; reassemble per the manufacturer's instructions.

This table provides a starting point for troubleshooting common vibrating screen issues based on the information provided. Remember to always prioritize safety and consult the manufacturer before making any significant changes to the equipment.

5. Process integration

Where are used vibrating screen in industrial processes ?

Vibrating screens are critical components when integrated into heavy industry circuits.

Vibrating screens are essential in many industrial applications where it is necessary to remove undersized material before crushing, or to classify crushed products into specific size ranges. Their integration into heavy industry circuits, particularly crushing circuits, is fundamental for optimizing efficiency and achieving the desired product sizing.

Here are some key aspects of integrating vibrating screens in these circuits:

• Scalping Screens: Vibrating screens are often used at the front end of crushing circuits as scalping screens. These screens remove the finer material from the feed before it enters the primary crusher. This serves several purposes:

  • It reduces the load on the primary crusher, allowing it to operate more efficiently on the coarser material.
  • It bypasses the undersized material around the crusher, reducing unnecessary wear and energy consumption.
  • It can provide an initial separation of the material stream.

• Sizing Screens: After the material has been through one or more stages of crushing (primary, secondary, tertiary, etc.), vibrating screens are used as sizing screens. These screens separate the crushed material into different size fractions based on the requirements of the downstream processes or the final product specifications.

  • The oversize material from a sizing screen in a crushing circuit is often returned to a subsequent crusher for further reduction. This creates a closed circuit that ensures the entire product stream meets the desired size criteria.
  • The undersize material that meets the required specifications is then directed to the next stage of processing or to storage as the final product.

• Multiple Screening Stages: Complex heavy industry circuits often incorporate multiple vibrating screens at different points in the process. These screens may perform different functions (scalping, coarse sizing, fine sizing, etc.) to achieve the overall processing objectives. The arrangement of screens and crushers (e.g., open or closed circuits) depends on factors such as the feed material characteristics, the desired product size distribution, and the capacity requirements of the plant.

• Crusher Circuit Configurations: The sources illustrate different configurations of crusher circuits where vibrating screens play a crucial role:

  • Primary Crushing: A vibrating grizzly or screen may precede the primary crusher to remove fines.
  • Secondary Crushing: Screens are used after the secondary crusher to classify the output, with oversize being recirculated.
  • Tertiary and Quaternary Crushing: Further screening stages are integrated to achieve finer product sizing and to manage circulating loads within the crushing circuit.

In essence, vibrating screens are indispensable for efficient material flow and size control within heavy industry circuits involving crushing and sizing. They enable the production of accurately sized materials, optimize the performance of crushers, and contribute to the overall productivity and cost-effectiveness of these operations.

To know more about vibrating sieves

Vibrating sieves are a key components in a powder process to ensure reliability, safety of the installation and safety of the product.

Please follow the link to get access to vibrating sieves design details : All you need to know on industrial vibrating sieves for powder checking

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Sources

VSMA

Principles of screening and sizing (quarry academy)

Meka