cleaning sieve
The lower sieve in a combine harvester's cleaning shoe assembly, designed for the final separation of clean grain from finer debris.
cleaning sieve: the final grain sorter in a combine
The cleaning sieve is the lower, fixed screen in a combine harvester's cleaning shoe, positioned beneath the chaffer sieve. It performs the final separation stage, allowing cleaned grain to drop through while retaining finer material: chaff, dust, broken kernels, and light weed seeds. The sieve itself is a flat or gently curved steel frame stretched with perforated metal or woven wire mesh, typically with hole diameters between 6 and 10 millimeters depending on crop and variety.
Grain that has already passed through the chaffer sieve above arrives at the cleaning sieve with most large chaff removed. The sieve's mesh is coarser than the chaffer's, allowing sound grain kernels through while catching smaller debris that would otherwise contaminate the final product. Material that fails to pass through, called tailings, is either returned to the threshing mechanism or directed to a collection auger for re-processing.
Cleaning sieves wear primarily along high-traffic zones where grain flow concentrates, typically in the center third of the mesh. Perforated metal sieves (common in high-throughput machines) dull at the hole edges and can develop cracks radiating from perforation points under vibration stress. Woven wire sieves clog easily when harvesting damp crops or fields with excessive fines, restricting airflow and reducing cleaning efficiency. Partial blockage is often invisible until grain loss spikes downfield.
The sieve must vibrate at 300 to 600 cycles per minute to maintain grain movement and prevent bridging. Its angle, typically 6 to 12 degrees, allows cleaned grain to slide forward under gravity while the oscillating motion sorts material by size and density. Air from below the shoe helps carry lighter chaff particles rearward and out of the machine, so any gap or seal failure in the shoe floor dramatically reduces performance.
Most cleaning sieves are user-replaceable as wear items, with service intervals of 80 to 120 operating hours in field conditions with high trash crops. Proper mesh selection at replacement is critical: too fine a mesh catches good grain; too coarse a mesh allows trash through. Regional dealer stock often favors sieves matched to local dominant crops, so ordering cross-model replacements requires careful dimension and hole-size verification.