Good Vibrations

There are many factors that need to be taken into consideration to ensure quality, safety and efficiency in screening recycled aggregates. Taking the proper steps in setup, operation and maintenance can improve a screen’s ability to produce a quality product, operate at maximum production efficiency and offer protection from wear and hazards.

The process of ensuring quality begins before the material even hits the screen.

LaVerge Pabian, customer support for Screen Machine Industries, Pataskala, Ohio, says, “It is always smart to decide placement of mobile equipment at a job site before it arrives.”

Pabian recommends producing a scale drawing of the site that will allow workers to visualize traffic flow, equipment location and material piles during setup.

Once placement has been determined, a level setup will allow the screen to operate optimally.

Mike Garrison, a technical specialist in vibrating equipment for Metso Minerals, Waukesha, Wis., says the screen’s supports should be placed on well-drained ground with compacted stone as the foundation.

In an instruction manual, Metso Minerals also advises, “During the screening process, make sure the unit is in horizontal position,” and “the maximum inclination is one degree in lateral and longitudinal direction.”

“With any screening plant, whether incline or horizontal, it needs to be set up level front to back and left to right, so the material spreads across the screen properly and you get the full effective use of the screen itself,” advises Gary Smith, president of Construction Equipment Co. (CEC), Tualatin, Ore.

He also suggests giving consideration to how the material will be stockpiled on the site and how much room will be needed in relation to the available space. The equipment should be set up in a way that allows for the material to be handled as little as possible before it is loaded for transport.

Many screening plants, including the ones that CEC offers, have the ability to run additional stockpiling conveyors. Some jobs may benefit from using additional conveyors to handle finished products.
 

DECISIONS, DECISIONS, DECISIONS
At a fixed crushing site, Gary Smith recommends that contractors first decide the smallest product that will be produced.

"With any screening plant, whether incline or horizontal, it needs to be set up level front to back and left to right, so the material spreads across the screen properly and you get the full effective use of the screen itself." – Gary Smith

“That is how you size your screens,” he says. “You have got to make sure they are large enough to give you adequate square footage of screen cloth surface area for the finest screening. Everything else becomes easier from there as a rule.”

Once the screen cloth has been selected, Gary Smith says it then is a function of amplitude—how far up and down a screen shakes—and frequency—how often a screen shakes. Depending on the application, a screen with a higher amplitude may be a better choice, he says.

In wetter seasons with stickier material, higher amplitude screens are more effective because they are designed to shake harder to get better separation as screening gets more difficult, Gary Smith says.

Gerry Mangrich of Universal Engineering Corp., Cedar Rapids, Iowa, says that in stationary plants, inclined screens may be preferential to horizontal screens. Inclined screens require longer conveyors to reach the elevated feed box, and structures supporting inclined screens tend to be high. However, he says inclined screens typically require less horsepower than horizontal screens and a lower initial investment.

When selecting screening media, Gary Pederson, vice president of sales at Major Wire, Norridge, Ill., says there are two main factors to consider: the abrasiveness of the material and how dirty it is.

Woven wire is typically used for the least abrasive and cleanest applications, Pederson says. “Any time an abrasive or dirty material is being screened, self-cleaning screen media could prove to produce more throughput by reducing blinding, clogging and/or pegging while lasting up to three times longer than traditional woven wire.”
 

BE ON THE LOOKOUT
Plant operators can look for signals indicating that a screen is not operating effectively. Pederson suggests checking for screen media breakage. “Screen media,” he says “should wear out, not break.” If it is breaking, there is either a problem with the screen media choice, screen box or material flow, Pederson adds.

He says blinding and pegging will cut down on screen deck capacity and could throw material out of spec.

“Signs are extremely important to investigate since they can often have a significant impact on the loss of production, higher costs and extended completion times.”

Bill Royce of Galion, Ohio-based Eagle Crusher says one of the biggest and most expensive mistakes operators make is using the wrong size screen. “If you have a 500-ton-an-hour crusher and you are putting a screen behind it that is only handles 200 tons [per hour], you are asking for trouble.”
 

MAINTENANCE RECOMMENDATIONS
Many screening equipment manufacturers recommend daily inspections to ensure the screens are tight and that the screening surface is in good repair as well as following manufacturer recommended scheduled maintenance.

Operators also can check to make sure the screen is shaking as hard as it was originally, according to Gary Smith. A gauge on the screen ensures the amplitude is correct. Springs should be checked monthly to make sure amplitude is correct as well, he adds.

According to Paul Smith, crushing and screening product manager for Eugene, Ore.-based Johnson Crushers International (part of the Astec company KPI-JCI), excessive wear to structural cross members, if left unattended, can cause decks to fail prematurely, resulting in expensive repairs. He says this can be common with highly abrasive materials.

Any extra mass that is added to screen decks, whether from high material bed depths, excessive feed sizes or heavy screen media, can cause wear and failures of the structural components as well as shortening bearing life, he says.

“Screens are inherently out of balance and generate a lot of energy at high frequency, so adhering to the manufacturer’s guideline regarding maximum allowable loads on a deck, as well as maintaining less than a 4:1 reduction ratio will definitely help boost longevity,” says Paul Smith.
 

SAFETY FACTORS
Pederson cautions, “When performing maintenance, always make sure that the screen box is properly locked out.” This consists of turning the power lever off and securing it with a padlock.

Stu Gamble, who leads the crushing and screening team for Sandvik in the U.S. and Canada, says removing guards from equipment is a major safety hazard.

Paul Smith says fall protection is generally required and always recommended when changing screen media.

As with any machinery, he says, always respect and stay clear of moving parts, such as eccentric weights, v-belts and belt conveyors, and “always wear hard hats, as rocks can and do bounce off vibrating equipment and can cause severe trauma.”