In many applications, the screening process is the key ingredient in taking a waste material and transforming it into a highly sought after, marketable material. In researching portable screening equipment you will find numerous styles of machines including: shaker screens, trommel screens, severe shake shaker screens, star screeners, and box screens. To better understand which of the above screening technologies are best, we have solicited the assistance of Rick Cohen, president of Screen USA Inc., the only U.S. manufacturer who has produced all of the above mentioned screening technologies.
“Screening is simply the segregation of material by size. Although all screeners are designed with such a purpose, different screening applications demand different screening technologies.” explains Cohen. “In the market place, many commissioned salesmen try to sell the customer what they have in stock, not necessarily what the customer needs.” …and this article should help educate the customer into finding the correct machine for their specific job.
Traditional shaker screens have been around for many years and are commonly found in screening aggregates and topsoil. The key to any screener is to have a nice consistent feed rate. The shaker screen is no exception. This consistent feed allows the material to evenly spread across the shaker screen to achieve the highest efficiency in screening. Many sand and gravel customers add spray bars to rinse the gravel during the screening operation. It is important to know that adding of spray bars will rinse your rock; however, it will not clean your sand; A fine material washer (sand screw) is needed in addition to the portable shaker screen to yield a washed sand product. To produce a sand to meet a certain specification, many customers may need the assistance of a classifier which will segregate different sizes of sand and blend them back together. Before you purchase any equipment at all, take a few samples and send them out for a full sieve analysis. The results of your sieve analysis will dictate what additional screening accessories will be needed.
In regards to screening topsoil, clumpy soils may need screeners with high-speed shredders (hammermills) which will break up the clumps before screening. This shredder is very desirable when screening clay-based soils, but is not needed in sandy, loam topsoil.
Trommel Screens vary a great deal from shaker screens and utilize a spinning drum to screen the material. Although both trommel screens and shaker screens do an excellent job processing topsoil, trommel screens have a distinct advantage when screening topsoil contaminated with a great deal of organic debris. Trommel screens normally have a considerably larger screening area and a cleaning brush that assists when screening higher moistures. Trommel screens are also very popular in screening ground wood such as mulch and compost. As one might expect, these applications require a fine screen size such as 3/8” and this complicates the screening process. The small screen opening equates to needing more screen area for efficient screening. Additionally, in high moisture materials (with moisture levels of 45% to 50%), a star screener will perform better than a trommel screen.
Star Screeners have been around for years screening peat and potatoes; however, a newer version has been designed to screen high moisture organics such as compost. As Cohen states, “Over the years, the desired particle size of compost has changed from 1” to approximately 3/8”. Due to moistures and the smaller required sizes, Screen USA was forced to find a solution, as our prior technologies were less effective due to the moistures, but we have perfected star screeners to screen high moisture compost”. Our unique continuous cleaning system has definitely changed the way the industry views star screeners.” Although star screens are excellent for high moisture, they are not recommended when the material is contaminated with twine, banding wire, or any other string like material.
Shaker Screens or Box Screens are simply vibrating screens (hence, "shaker") mounted on a portable chassis. Instead of having a feeder, the loader bucket must sprinkle the material onto to shaker screen. The screened material either falls through the screen onto the ground or onto a built-on conveyor. The oversized material falls directly off the end of the screener. These machines are excellent for coarser sizing; however, are fair to poor on precision screening. Since the loader bucket is being used as the feeder, it is difficult to provide a consistent feed. As a result, the machine is either overfed or is processing nothing at all. Many box screens have been introduced into the screening market. Most are extremely light duty and are marketed strictly on the internet. The manufacturers do not demonstrate them and they are solely selling on price. For light duty screening though, these machines work; for heavier duty applications however, choose a heavy-duty box screen. Hint: You will need to pay 2-3 times the amount for a heavy rather than light box screener. Many heavy-duty box screen manufacturers have also come out with heavy-duty, severe shaker portable screeners.
As the name implies, severe shake shaker screens (also known as C&D screens) are used in heavy-duty applications such as construction & demolition. Instead of just having a shaker screen, the machines are equipped with an extremely stout feeder. This feeder utilizes a feeder belt with impact beds or a steel apron. This allows the material to be fed consistently to achieve the most production possible. Slabs of concrete, asphalt, quarry stone, etc. can be fed into these robust screeners and they will typically screen into three fractions. Many of these machines are track mounted allowing for complete mobility. Log yard clean-up, screening root mat, and other tough applications find these severe shake shaker screens to be most efficient. Many customers who own crushers purchase these screeners as they can either screen crushed material into 3 sizes or use the same machine to scalp off the heavier material to minimize unnecessary crushing.
In conclusion, there are a variety of decisions to make when purchasing screening equipment. You must decide which of the screening machines fits your applications, and you must decide what size machine will fill your production needs. Normally the size of your bucket loader dictates the size of machine you will need.