Aside from the speed and throw of a sifter, nothing effects its performance more than the screening media and its condition. It was always assumed that tight screens were important, but no tension standards or test instruments were used.
There was limited information regarding proper tensioning of the screens and its effect on screen life and throughput. The "tighter the better" philosophy was the general rule. But one study by Kansas State University's Grain Science Department indicated that too much tension might actually reduce screen throughput and extreme tensions were found to cause premature screen failures.
With nylon and glued-on screens that ensure uniform tightness of every strand becoming more common, the overall life of the screens was an issue. With a glued-on screen, retensioning is not possible, and the screen cleaners cannot be replaced without ruining the screen. It is difficult to tell what causes seemingly premature screen failures, whether they are due to excessive tension, aggressive screen cleaners, poor wear characteristics of nylon, or poor quality of material.
EVOLVING SIEVE MEDIA. There are two main types of filaments used in sieve fabric construction, monofilaments and multifilaments. Silk is a multifilament strand made up of many individual fibers intertwined together. Synthetic fibers, such as nylon and polyester, are monofilament strands.
Other fabric parameters include the length of the fabric, which weavers term the warp, and the width, which is known as the weft. Key dimensional criteria include the opening of the fabric measured in microns (where 1 micron is equal to 1,000 millimeters); the thread diameter, also measured in microns; and the percent open area.
There are two types of weaves used in the flour milling industry, plain or square weave and taffeta weave. Plain weave is simply an over-under weave, with the warp and weft strands spaced evenly apart. Taffeta weave, on the other hand, is woven with two strands next to each other, and spaced alternatively to a single fiber in the warp direction only. Each of the strands making up the double warp fiber are smaller in diameter, allowing the opening to retain percent open area.
Prior to 1940, only natural silk was used in flour mills. However, as mill production loads increased, the life of the fabric started to diminish. In addition, the availability of silk was decreasing, causing higher prices. This led to the development of synthetic fibers designed to improve screen life and reduce costs.
The taffeta weave double X fabric proved to be a good replacement to natural silk, as millers noticed enhanced throughput efficiency. As mill loads increased, and more aggressive sieve cleaners were used at high-tension levels, the life of the double X fabric diminished.
Sefar Nytal, a complete package of five different flour milling screens, was developed from Sefar's weaving mills in Switzerland. The package includes the double X taffeta weave, milling forte, the triple X heavy-quality fabric and two grit gauze fabrics designed for purifiers.
The double X taffeta weave has the largest open area percentage, making it the most efficient fabric. Because of the weave, the fabric provides excellent surface exposure and particle bounce. This sifter cloth is used most often to produce wheat flour.
The milling forte fabric is a plain square weave that combines the efficiency characteristics of the double X fabric with the strength qualities of the triple X fabric. Due to its larger thread diameter, it has a decreased percent open area and is most effective on high abrasive stocks under heavy loads at high tensions.
The triple X heavy quality fabric provides the highest strength due to the increased diameter of the individual strands. This construction drastically reduces the throughput efficiency, but this fabric is well suited for highly abrasive stocks, such as durum wheat.
The Polyester GG and Nytal GG grit gauze cloths also resemble natural silk used years ago. The Nytal GG is used for coarse or scalping sieves and the Polyester GG is commonly used in purifier sieves due to its rougher surface and because it is unaffected by product humidity or climatic conditions.
By comparing the fabrics based on tension, abrasion, efficiency and cleaner type, the triple X has the longest life, the double X is the most efficient and milling forte combines strength and efficiency. It is important to consider thread diameter, percent open area and weave type factors when selecting a fabric.
STRETCHING RECOMMENDATIONS. To obtain optimum performance from the nylon screening media, uniform, repeatable stretching to a proper tension is important. These optimum tensions can only be obtained with a mechanical stretcher, which may stretch the media beyond its elastic limit.
Mechanical stretching and attachment of screens is not as simple as it might seem. The results are a function of the type of screen media, the temperature and relative humidity during stretching, having the proper tension and elongation measuring tools available and the proper training of personnel.
Optimum screen tension produces proper sifting performance inside the sifter after weeks of operation. How to relate that tension to the tension at the time of stretching and attachment is an unknown.
Hand-stretched nylon screens are generally tensioned by limiting their elongation. Screens were made smaller than the tacking frame and then stretched to the edge of the frame. When hand stretching, obtaining elongations on fine mesh screens greater than 2% is difficult.
Each nylon fabric has a certain elongation limit. When stretched beyond that limit, screen life can be reduced. Because the elongation and tension are not in a direct relationship — due to humidity or the moisture content of the fabric — the tension necessary to obtain a certain elongation may vary. For example, on a humid day, fabric elongates without gaining proper tension. On a less humid day, cloth elongates much less before reaching proper tension.
The following chart is a guide to accurate tensioning.
By focusing on the fabric's design, the miller has the ability to choose the correct media to meet production goals and maximize screen life. And by following proper installation techniques and monitoring tension, one can use the equipment to its full potential.
Jim Schroeder and Scott Jordan presented this paper at the 1999 A.O.M. Technical Conference in Fort Worth, Texas, U.S. Mr. Schroeder is president of Great Western Manufacturing in Leaven-worth, Kansas, U.S. Mr. Jordan is inside sales and parts manager for Thompson-Hill & Associates, Lenexa, Kansas, U.S.