Plansifter

Overview

The plansifter is a high-capacity gyratory sieve used throughout the grain and flour milling industry to classify mill streams into uniform size fractions. Unlike simple single-deck static sieves, a plansifter uses multiple stacked decks, each with a different mesh size, in a machine that oscillates in a controlled elliptical pattern. This motion keeps fines flowing across mesh surfaces and prevents bridging or blinding (clogging) that would reduce efficiency.

The plansifter is indispensable for separating flour into distinct grades (e.g., patent flour, clear flour, shorts, bran) following a Flour Roller Mill break and reduction sequence. The machine excels at handling products with varying moisture and density, automatically classifying them without operator intervention or manual screening.

How it works

Raw mill product (flour or middlings) enters the Stacked Sieve Box Assembly through a hopper onto the top Top Sieve Frame, which typically holds a coarse mesh (60–100 mesh for flour). The gyratory motion, generated by the Eccentric Drive Shaft rotating inside the Gyratory Drive Unit, imparts an elliptical or circular orbit to the entire sieve stack at 300–500 rpm.

As the sieve bounces and orbits, material on the top mesh is continuously thrown upward and forward. Fine particles (those passing through 60 mesh) fall through the mesh, while coarser particles roll across the deck and drop off the end toward a Discharge Material Chute.

The undersize from the top deck cascades onto the Middle Sieve Frame, which holds a finer mesh (e.g., 100–140 mesh). This repeat of the process separates another size fraction. Multiple decks (typically three to five) each remove a size grade, with the finest material (passing the bottom deck) collecting as the ultimate undersize fraction.

The Suspension and Isolation System uses Suspension Coil Spring elements at the four corners to suspend the sieve stack. These springs allow the Eccentric Drive Shaft crank arm to drive the stack in its gyratory motion while isolating vibration from the supporting Supporting Steel Frame.

Mesh selection and grading

Mesh sizes are chosen to separate flour into commercial grades:

• Coarse mesh (40–60): Removes bran and large middlings
• Medium mesh (100–140): Separates flour from fine shorts
• Fine mesh (150–200): Separates premium patent flour from bread flour

The number of decks and mesh selections determine how many product grades the machine yields. A four-deck plansifter might produce five saleable products (four through undersize each deck, plus one oversize).

Vibration and isolation

The Vibration Damper mounted in the suspension limits oscillation amplitude after the motor stops, improving operator safety and reducing shock loads to the frame. The Vibration Isolation Foot under the machine prevent vibration from radiating to the mill floor and affecting other equipment.

The Vibration Transducer continuously monitors sieve motion and can trigger alarms if amplitude exceeds limits, indicating mesh clogging or motor issues.

Mesh life and maintenance

Woven Mesh Screen screens experience wear from abrasion and impact as products slide across them. Mesh life depends on product hardness, moisture, and mesh material; stainless steel lasts longer than brass but is more expensive. Typical replacement intervals are 500–2000 tons of throughput.

When a mesh becomes partially blind (clogged with fine particles), grading efficiency falls dramatically. Regular cleaning, sometimes with brushes or low-pressure air during operation, can extend mesh life. Gaskets (Chute Connection Gasket) must also be inspected and replaced to prevent product bypass, which ruins grading accuracy.

Motor control and operation

Modern plansifters use variable-frequency drives (VFDs) to adjust gyratory speed, allowing operators to fine-tune separation by changing orbit frequency. Lower speeds suit delicate products, while higher speeds increase throughput for robust materials.

The Control and Vibration Monitoring PLC can trigger automatic shutdowns if vibration exceeds safe levels or if product level sensors indicate overflow, preventing spillage and maintaining consistent product grades.

Throughput and product quality

A typical flour plansifter handling 20 ton/h of mill product separates it into four distinct fractions within 30–60 seconds total residence time. The machine's ability to handle variable input (moisture, density) and still maintain grading accuracy (>95% of each size fraction falls within the target mesh range) makes it far superior to manual or batch sieving.

High-quality flour mills often use two plansifters in series: one for primary grading and a second for regrading shorts or intermediate fractions to improve overall extraction rate.