Why McLanahan Vibratory Screens?
McLanahan Vibratory Screens are engineered with ASTM A572 Grade 50 steel side plates. With a tensile strength of 65,300psi (65ksi), these side plates have a 45% higher yield strength than A-36 steel, which can withstand up to 36,000 psi (36ksi) of stress before it begins to drastically deform. A fully bolted construction reduces/eliminates cracking due to stress risers in the steel caused by welding. Worn components can be quickly replaced without cutting.
McLanahan Vibratory Screens are built with an integrated feed box and are able to withstand heavier loading and larger material in the feed end without worrying about bolts loosening or structural failure.
McLanahan Vibratory Screens feature robust side plate stiffening. Formed plates are bolted to side plates to form a rigid support grid along the length of the side sheet. Independent cross members can be pulled individually and relined in a clean work bay versus on a screen tower, and reduce the need for heavy and wear prone X-bracing. Cross members are on 4' centers to allow more clearance for personnel to access the decks. Replacement cross members come shorter in length and with machined/matched shims to allow for easy installation in areas with limited clearance.
Structural tubing gives the producer a variety of size options and allows you to replace only the worn tubes, not the complete deck frame. A sacrificial weld plate installed on top of the tubes allows stringers and bucker bars to be welded in without welding directly to the tube.
How Vibratory Screens Work
Screening can very easily be viewed as the most important operation in the processing plant. If the screen is only operating at 75% efficiency, then 25% of the desired product is going somewhere else.
The performance of a screen is affected by four variables: eccentric throw, frequency (rpm), angle of adjustment and throw direction. By manipulating these variables, the operator can dial in the screen to match the application and material.
Eccentric throw is the radius of the screen box. Generally, the greater the throw, the more aggressive the screen action will be. Consequently, the smaller the throw, the less aggressive the screening action. Keeping this in mind, the operator can set up the operation with a heavy throw for heavier or larger materials, or a smaller throw to create a sifting action more suited for finer separations.
The frequency of the screen is measured in the number of revolutions per minute the screen makes. In conjunction with the eccentric throw, a lower frequency allows for a more aggressive screen action for larger material and cuts, while a higher frequency is used for smaller material and cuts.
Angle of Adjustment
The angle of the screen plays a large factor in its overall performance as well. A flatter screen angle will provide a longer retention time of material on the deck and more probability that a particle will fall through the opening. As the angle is increased, the retention time is decreased.
It may be advantageous to run the throw of the screen uphill. The goal is to increase the retention time on the screen, as well as change the orientation of the particles to the screen opening. The reverse action does not hurt the screen and is usually used in finer screening application, but be cautious not to increase the bed depth too much.
Stratification and Separation
Two main operations have to occur for material to be screened: stratification and separation. Stratification is the process of larger sized material rising to the top of the bed, while smaller particles go to the bottom of the bed. Separation is the process by which particles introduced to the screen opening either fall through the opening or do not. Stratification must occur before separation can take place.
Factors that affect stratification include:
- The material's travel flow rate down the screen, which is a function of bed thickness, stroke characteristics and screen slope. Generally the steeper the slope of the screen, the faster the rate of travel.
- Stroke characteristics, which include amplitude, direction, rotation, types of motion and frequency.
- Surface particle moisture; high surface moisture makes stratification difficult.
The separation probability is a function of the ratio between the size of the screen opening and the size of the particle. If the ratio is large — in other words, the particle is much smaller than the opening — there is a high probability the particle will fall through. If the ratio is small — the particle is close in size to the opening — then the probability is low that it will fall through.
Motion on a Vibratory Screen is produced with a combination of amplitude (stroke) and frequency (speed). The goal is to allow the particle to see as many openings as possible as it travels down the screen, but never see the same opening twice. Large screen openings for large cuts can be achieved with high amplitude and low speed. For small screen openings for finer cuts, the opposite is true: low amplitude and high speed.
Benefits of McLanahan Vibratory Screens
- Large spacing between the decks for ease of operation
- Individual cross tubes form the deck for ease of changing for wear
- Can be customized to fit almost any current screen installation without changing discharge chutes or feed points
- Direct drive system
- Integral feed box
- Higher strength steel for side sheets
- Quick-change spring kits
Frequently Asked Questions
What are important factors to consider when selecting the proper type and size of screen for my application?
To select the correct type and size of vibratory screen the following information is desired:
- Maximum tons per hour to be handled, including any surge loading.
- A sieve analysis or gradation of the feed material, including any recirculating load.
- Type of material and weight of material in broken state.
- Size of separation, or desired output gradation.
- Surface moisture of material if dry screening, or amount of water with feed if wet screening.
- Special operation requirements or conditions, such as temperature, abrasiveness, corrosiveness, or other physical characteristics of the feed, efficiency, or product requirements that may determine selection of screening surface or installation issues.
How do I calculate my screen efficiency?
Screen efficiency can be calculated by comparing the percent of undersize in the feed that passes an opening and the percent of undersize in the feed.
What are some best practices for operating a screen?
Screening best practices:
- 133% of required capacity for size of screen
- All specified for wet and dry installations
- Widths in excess of 6’ should be double crowned
- Double crowns are hard and the center bar is sometimes too wide, causing material to back up and reduce capacity
- Maximum bearing sizes should only be considered in primary and secondary applications
- Shock mounts or snubbers should be used where possible
- Back plates are required
- Feed boxes should be purchased with screens
- Flowing materials should be turned by a shelf that retains the material since this reduces wear
- Feed must be spread evenly across the screen to maximize screen length
- Discharge and chute designs must not restrict screen performance — allow clearance
- Springs and throw must be reviewed regularly
- Bed depth must not exceed four times screen opening
- Wet applications should have a minimum water pressure of 20 psi
- Spray bars should be independent of the screen body and should have cleanout caps and valves
- Fine sizing and high rpm screens should be specified by the manufacturer
What affects screen performance?
- Capacity, or tons per hour - capacity is proportional to screen width, length, open area, stroke and rpm
- Efficiency, or the percent of undersized material passing screen deck - 90-95% is attainable
- Bed depth - maximum depth should not exceed four times the screen opening size (For example: ¼” screen openings should have a maximum bed depth of 1”; 1” screen openings should have a maximum bed depth of 4”
What type of screen media should I use for my application?
Many producers have experienced a variety of problems that point to a screen deck that was improperly selected. It's wearing too fast. It’s plugging (material getting stuck in the screen opening) or blinding (screen opening clogged by sticky material). The noise level is too high.
Many factors affect the overall efficiency of the screening process. Selecting the proper media for the application will be a big factor toward success. Wire cloth is the most widely used screen surface. Technological advances make it easier to consider other types of screen media.
There are a few basics involved in the selection of screen media. A short list of media to choose from includes:
- Wire cloth
- Long slotted wire cloth
- Perforated or punch plate
- Profile wire
- Self-cleaning materials
The type of media chosen will depend on material abrasiveness, impact, material size, moisture content, cost-effectiveness and noise level. Wire cloth may be the lowest initial cost media, but the most cost-effective for an operation will be the one that meets the specific application.
Rubber screens are a good choice for scalping decks in a dry, high-impact application. Rubber is very durable and can withstand the impact of the larger feed material hitting the deck. In a dry secondary application, a rubber screen can provide a long life, even in abrasive feed material.
How do I calculate G-force on my screen?
G-force can be calculated by taking half the stroke setting times the speed (square), i.e. 1/2" stroke at 800 rpm.
• .250 x 800 x 800 / 32500 = 4.9 G’s
Generally, Inclined Screens can operated up to 4g, while Horizontal Screens can operate up to 6g.