Sifters may be the most misunderstood and underappreciated machine in the milling system. Roller mills are considered by many to be the workhorse and the heart of any milling system. However, the production of flour, or nearly any ground product, depends on the proper selection and maintenance of the right sifter and sieving material.
Sifters come in many shapes and sizes. Multiple-section, gyratory sifters swinging from the ceiling are the most common sifter in most modern flour mills. While one product goes into a sifter, two or more products come out. A sifter can be used as part of the quality assurance program or as an integral part of the production process. It can be used to remove coarse material, separate fines or grade material into multiple streams. To keep a sifter working as it was designed requires maintenance both inside and out.
Typical applications for sifters include scalping, grading and the removal of fine material. Scapling, or the removal of course, over-sized material, is the sifter’s most common quality assurance application. The scalped material can be lumps in materials such as flour mixes as well as foreign materials such as insects, scale from a bin wall or transport lines and flakes, moldy material or tramp metal.
When removing fines, the sifter removes undersized material to assure a granular product meets final specifications. This is useful for friable materials that produce fines during transport. Grading is the most common application of a sifter in the milling process. The sifter separates and distributes material in multiple granulations for further processing.
Keeping sifters in Good Repair
Because a sifter rotates or gyrates at high speed, it needs regular maintenance to keep it operating and performing the job it is designed to do. Regular inspection and service of the sifter drive and reeds is necessary to address changes due to wear and environment.
It is inevitable that the sifter drive mechanism will wear out, requiring replacement or repair of critical parts. Material backing up from the top of one or more sifter sections can indicate the sifter is running slower than required as a result of slipping drive belts or a bearing beginning to freeze up.
The reeds used to suspend the sifter from the ceiling or support beams required regular checks as well. As the mill environment changes throughout the year, the brackets holding the reeds of the sifter must be checked to assure they remain tight. However, consult the manufacturer’s suggested torque rating for these brackets. Applying too much tension to the brackets could cause them to cut into the reeds, causing them to break under the swinging load of the sifter box.
Inspecting the Outputs
Regardless of the application of the sifter, regular monitoring of its separations provides important information about the sifter’s operation and internal condition. If the sifter is performing a quality assurance application, scheduled inspection of the tailings container is a standard food safety check. Examine the tailings to determine if contaminants or unusual material is present which may indicate a problem. Never discard unexamined or unusual tailings as waste without determining the source of the contamination.
If the sifter is part of the flour milling system, a regular check of the streams flowing from the sifter can indicate changes in the wheat moisture. These changes may affect flour quality, or any number of possibilities. It is important to know the expected quality and visual appearance of the sifter streams to enable the miller or mill operator to identify when a sieve inside the sifter may have a hole in the screening material or alternatively, a sieve may have blinded over or choked up.
The need for regular inspection of either a food safety sifter or a process sifter requires the proper sifter location and training of the employees. The sifter should be installed in an area that is easily accessible for the employees who inspect the sifter. A floor level location with sufficient space to maneuver around the machine enables employees to have easy access to the sifter.
Cleaning the Sifter Interior
The maintenance and inspection frequency of a sifter depends on many variables. The application, quality assurance or processing, the material it handles, and the environment, specifically the relative humidity in which it operates, all impact the durability of the sieving material inside the sifter. For example, a sifter used as a food safety check point in a bulk loading system may require the inspection of the tailing before and after each load and should be opened on a weekly schedule to inspect for wear of the sieving material and sieve gaskets. Whereas, a sifter in the milling system may require regular, around-the-clock checks to identify problems before product quality is impacted.
When a sifter is opened for required or regular maintenance, a good practice is to have a checklist of items for the operator to examine during each inspection. The sieving material, screen frames, screen cleaners, sieve tension, felt or gaskets between screen decks, the head press clamping mechanism, and sleeves connecting the top and bottom of the sifter to product spouts are all critical parts of effective sifting. The checklist should allow a space after each item for the operators to report problems to bring reoccurring problems to the attention of their supervisor. A space for the operator to initial and date the inspection is critical for follow-up and future reference.
After opening the sifter section and releasing the pressure applied by the head press, it is time to remove and inspect each sieve. Carefully brush each sieve inspecting each sieve component, the sieving material, screen frame, screen cleaners, felt or other sealing material on the top and bottom of each sieve frame. Wear on some components are obvious, such holes and tears in the sieving material or felt that leak material. Other parts may have less obvious wear problems which require a closer look.
An example is screen cleaners which may have worn to the size some have passed through the sieves back wire. Insufficient screen cleaners can result in screen blinding, but if the operator is unaware of the appropriate number, they may not get replaced.
Another less obvious problem is sieve tension. A sagging screen that does not have the proper tension may cause several problems impacting efficient sifter operation. Material doesn’t stratify on sagging sieves which prevents the material from moving easily across it. This may cause the sifter to choke and prevent the flow of material downstream. The sagging sieve material may prevent the screen cleaners from doing an effective job. Cleaners are less effective when bouncing against loose screens. This may cause the screen to blind over and cause undersized product to carry over with the tailings.
The sifter checklist will also help to prevent incorrect sieve installation. Material may back up if a smaller micron size sieve is installed incorrectly or if sieves are placed out of order in the sifter section. The operator must also be careful not to accidentally turn a sieve 90 or 180 degrees from the proper position. In all of these scenarios, the sifter may work for a while, but eventually the incorrect separations will cause the section to fill or send product to the wrong location downstream in the process. The operator must check and double-check that the sieves are placed in the correct order and are properly oriented.
Another variable to be aware of is how replacing one sieve clothing type with another may affect the performance of the sifter. The sieve size and material originally specified for the sifter matched the stock application requirements, with the correct type (wire or synthetic), open area and mesh size. Replacing the original screen with one that has less open area will restrict the sifter, causing a change in the product to exit with the tailings. Be sure the proper changes are made. Consult the sifter or sieve clothing manufacturer to help determine which sieve clothing is most appropriate to meet the required changes.
Finally, when you inspect the sifter, also check the head press mechanism to ensure that the sieves will be properly secured during operation and the sleeves connecting the top and bottom of the sifter to product spouts. A worn head press clamp may result in leakage around the sieve, impacting product quality. Dirty or worn sleeves should be replaced as a good sanitation practice to avoid stock from dusting out onto the floor.
Keeping the Separations
When a sifter is properly designed and maintained, it can be the most overlooked machine in the mill. The rolls need adjusted regularly, but a sifter simply gyrates slowly and does its job. However, it is critical not to overlook the importance of checking the performance of the sifter regularly as well.
The sieves, sieving material and the moving parts of a sifter wear just like any other machine and require a preventive maintenance plan to ensure they continue to operate as designed. The sifters are relied upon to grade and distribute the ground material throughout the mill to maximize flour extraction and to maintain high quality product.