Millers have sieved grain and mill products for more than 3,000 years. The early sieves were generally operated by hand shaking some type of wooden frame to which a screening material had been attached. Various screening materials, including papyrus reeds, horse hair, linens, flax and wool, were used in these early sieves. By 1820, the Swiss were making bolting cloth, producing it regularly after 1835.
Hand sieves were replaced with human-powered reels that later ran off the power of wind and water driven mills. In 1888 C. Haggenmacher, a noted Hungarian miller, patented a gyratory motion plansifter that avoided the excessive vibration associated with earlier sifter designs. It was the first true counterbalance that met the challenge of balancing the opposing forces associated with a mechanical drive system. The unit was suspended from reeds and was driven from a floor-mounted drive. Free-swinging sifter drives were developed in Europe 20 to 30 years later. The transition to free-swing drive configurations for sifters in the United States was a little slower.
The long sieve frame of the plansifter had sieve transport flights and used a mechanical sieve brush for cleaning flour clothing while beans and wheat were commonly used as cleaning devices in the U.S. In addition to several drawer type frame units that quickly fell by the wayside, the square sieve was developed in the early 1900s replacing the elongated frame of the plansifter and becoming the basic model of the plansifters that are still used today. The square sieve frame configuration allows maximum flexibility with minimal amount of floor space.
Modern sifter designs incorporate a counterweight and drive system built into the center of the machine. On each side of the drive frame component is a sifter box. Each box contains multiple sets, sections or stacks of sieve frames. The number of sections varies from two to 10, as does the number of frames within each section — ranging from 25 to 35 frames. Product flows into each section through flexible connectors attached to both rigid spouting and inlet thimbles on the sifter. The flexible connectors are sometimes called stockings or socks. Frames equipped with cloth of differing aperture sizes grade the incoming stock based primarily on size. (Differences in stock characteristics are not simply a function of particle size but are influenced by the grain’s prior treatment — tempering or steaming — in conjunction with anatomical differences that cause the grain to separate into different fractions of varying size during the grinding process.)
The separated fractions from each section flow into a sifter knee or spout through a similar type of flexible connector. The flexible connectors into and out of the box are essential as the sifter itself gyrates in a circular pattern around its axis. The speed and throw of the machine can range from 180 to 240 revolutions per minute and from 4.5 to 16.0 centimeters, respectively.
SIFTER DESIGN IMPROVEMENTS
The drive systems and counterweight stems have evolved over the years. Once driven from the floor or line shaft in a complicated system of quarter-turn belts, pulleys and oil pots, today’s sifters have self contained drive systems that improve sanitation, safety, maintenance and energy consumption.
Free swinging sifters can be supported by wooden sifter reeds or fiberglass type rods. In some regions, the support system also includes the addition of one or more cables to the wooden reeds or fiberglass rods as a safety device.
General sifter design and construction has evolved to reduce potential insect harborages and to improve cleaning ease. Wooden sieve boxes and sieve frames are now lined with a variety of materials ideal for food contact surfaces while preventing wear of wooden parts. Except in the U.S., many mill equipment manufactures construct their sifter boxes and supports using metal instead of wood. In either case, the outside coating presents an aesthetically pleasing look protecting the construction material and provides a smooth surface to eliminate dust buildup. For further sanitation, insulated doors, sieve boxes and airflow designs have been designed (or redesigned) as methods to combat the potential for mold development inside the sifter.
New modular sifter designs separating sifter boxes and drives allow easier movement, shipping and installation of larger sifting machines.
To "crank" down the press top, many traditional sifter designs use a screw and ratchet or hand wheel system. The pressure applied to the stacked sieve frames by the press top hold the stack together forming a complete sifting unit.
Some manufactures have replaced this mechanism with a pneumatic cylinder system. This makes the process of removing the sieve frames faster and eliminates safety risks associated with employees standing on ladders at the top of sifter boxes, working to loosen or tighten the press top. (See Illustration 1a and 1b on page 30.)
The demountable tray (whether of wood or aluminum) has afforded the opportunity to improve on the basic attachment of the bolting cloth.
Prior to the past decade or so, sieve cloths were nailed, tacked or stapled onto a wooden sieve frame. The attachment of bolting cloth was completely dependant on the ability, training and attention of the person attaching the replacement cloth. A key issue, of course, was the proper tensioning of the cloth on the frame. In addition to eventually destroying the tacking area and shortening the frame life, staples, nails and tacks have the potential to become a product safety and plant operation hazard. The demountable tray affords the miller an opportunity to mechanically stretch a cloth to a known tension or elongation and glue the bolting cloth onto the frame.
Sieve frame design improvements are also providing efficiency opportunities for millers. Use of standard depth sieve frames and blank inserts to increase space under the frame for material moving across the sieve below improves flexibility. Where additional sieving space is required, the combination of standard depth frames and variable depth spacers does not have the limitations imposed by using customized height sieve frames because a standard frame and properly sized, inexpensive spacer can be selected and moved to easily fit the millers changing needs.
A second major frame design improvement is the sieve frame without backwire. This design, eliminates the use of backwire to hold sieve cloth cleaners in place. Instead product falls through the screen onto the pan where it can be properly directed. The sieve and pan cleaners used in the earlier sieves are replaced with a single combination cleaner that rests on the pan. This style of cleaner keeps the pan clean and sieve cloth cleaner, with sieve apertures open and stock free.
Improvements in sieve clothing have been a benefit to the miller from a cost and productivity standpoint. One only needs to look at the openings of a silk bolting cloth and compare them to the openings of a synthetic cloth or wire to see the improvement in aperture uniformity. In addition to generally lower costs, synthetics and fine woven wire clothing has improved uniformity and durability over the silk bolting cloth of years past.
The design of rebolt sifters has followed the same general design and implementation as sifters employed in the production process. Rebolting or resifting of flour is often done as the flour is transported from the mill to the warehouse and from the warehouse into the packing system or bulk load out system. The purpose of rebolting is to allow millers to check for potential contamination of their product.
Rebolt system installation, design and maintenance are becoming more important issues as food security and safety have seen increased emphasis in the milling industry. Until the development of the in-line sifter, it was necessary to install equipment to remove the flour from the airflow stream, sift the flour and reintroduce the flour into the pneumatic conveying system. The in-line sifter can be installed in a pneumatic conveying line without much of the ancillary equipment previously described. The in-line units relieve the conveying line pressure, preventing pressurized air from driving impurities through the screen.
Sifter and sieve design improvement for milling purposes has continued to progress at a steady pace.
Yet it always takes some time for equipment improvements or process flow changes to make their way into general usage in the milling industry. Fortunately or unfortunately (depending on your perspective) milling equipment has a considerable lifespan — which means that unless the improved piece of equipment presents a significant and obvious payback, it is not likely to be adapted until the existing piece of equipment is worn out and has to be replaced in order to continue business.
In some instances the improvement can be adapted to existing pieces of equipment on a piecemeal basis. For example many conversion kits have been sold for roller mills allowing the miller to replace Babbitt bearings with roller bearings. These types of modifications can be made one stand at a time over a period of years, if necessary, without interrupting flour production. Similar analysis could be made for using improved sieve frame construction improvement.
Milling organizations worldwide have their own personality just like people or societies; what is appropriate for one may be totally inappropriate for another. In the case of turn-key milling operations designed and proposed by a milling engineering company, the best possible and most up-to-date equipment, flow designs and innovations are expected and incorporated.
The important consideration determining how widespread a particular technology or product improvement becomes is the fact that millers never stop looking for techniques, practices or equipment that provide them with a competitive advantage.
Particle size separation technology is employed in many industries outside of cereal milling, but it is doubtful that any industry uses it to the extent applied in the milling industry. No doubt millers and mill equipment engineers will continue to look for new ways to improve sifting productivity and reduce cost. The desire and creativity of millers to seek out new and improved solutions has never changed. That mind set brought us to where we are today and will takes us even further tomorrow.
Jeff Gwirtz is c.e.o. of JAG Services Inc., a consulting company serving the grain and milling industries. He was formerly an assistant professor in the Grain Sciences department of Kansas State University. He can be reached at firstname.lastname@example.org.