Last reviewed 1 Oct 2026 · 7 min read
Threshing — principles
Threshing is the separation of grains from the panicles, pods or ears. After harvesting and field drying, the crop is threshed, then winnowed (cleaned).
Mechanisms of detachment
| Mechanism | Action | Example |
|---|---|---|
| Impact | A blow from a high-speed bar or tooth knocks the grain off | Spike-tooth drum thresher |
| Rubbing (shearing) | The crop is rubbed between a drum and a stationary concave | Rasp-bar drum |
| Stripping (combing) | The grain is combed off the standing crop | Stripper header |
| Vibration, tension | Used in some pulse and legume threshers | Soybean |
| Treading (animals/tractor wheels) | Traditional | Obsolete |
| Flailing, beating | Hand threshing by beating on a floor or a log | Small farms |
Resistance depends on crop type and variety, moisture content, maturity, and grain–straw ratio: dry crop (12–14 % grain moisture) threshes easily and gives clean straw; wet crops thresh poorly and damage the grain.
Traditional threshing
- Trampling by bullocks (dainy), beating on the floor, stick flailing, tractor or roller treading — low capacity, high losses (about 3–5 %), cleaning is separate by winnowing (wind).
Power thresher
Construction
FormulaMain parts of a thresher
- Feeding hopper (chute and feed table) — the crop is fed head-first (or in some whole crop) by hand or a conveyor.
- Threshing drum (cylinder) — a rotating drum with spikes (peg teeth), rasp bars, wire loops, or angle bars; speed 400–1100 rpm (peripheral speed 15–30 m/s).
- Concave (stationary grate) — a curved sieve of rods or perforated plate under the drum; the clearance (10–25 mm) is adjustable; the grain falls through it.
- Beater / straw walkers (or rotor) — separate the remaining grain from the straw.
- Cleaning unit — blower (fan) and oscillating sieves (top sieve (chaffer) and bottom sieve) to separate chaff and straw bits from the grain; aspirator.
- Grain outlet — bagging spout; straw outlet (blower or discharge) — straw is thrown out as bhusa (chaffed) or whole.
- Power source — electric motor (5–15 hp), diesel engine or tractor PTO; belt or PTO drive; frame, wheels, safety guards.
Types of power threshers
| Type | Features | Crops |
|---|---|---|
| Spike-tooth (peg-tooth) drum thresher | Spikes on the drum strike the crop against concave pegs; impact + rubbing; breaks straw into bhusa | Wheat, barley, millets; common in India for wheat (chaffed straw is valued as fodder) |
| Rasp-bar (rub-bar) thresher | Rasp bars rub the crop against the concave; gentler, straw remains whole | Rice (paddy), sorghum, soybean, legumes |
| Axial-flow thresher | A rotor moves the crop along its axis while threshing and separating; high capacity, good cleaning, less damage | Multi-crop, rice, wheat; modern |
| Wire-loop drum thresher | Loops of wire thresh gently | Paddy |
| Multi-crop thresher (changeable drum, concave and sieves) | Replaceable drum and sieves for different crops | Wheat, paddy, sorghum, soybean, gram, sunflower, groundnut |
| Hold-on (head-feed) thresher | The ear/panicle end is fed while the stalk is held; straw not damaged | Rice for paddy straw and thatch |
| Throw-in (whole-crop) thresher | Entire crop thrown in | Large capacity |
| Pedal / hand-operated paddy thresher | Foot treadle drum | Small paddy farms; 20–40 kg/h |
Capacity
- Power thresher capacity: 0.5–3 t/h of grain for small to medium machines (for wheat, about 400–1000 kg/h with an 8–10 hp motor); throughput is expressed as kg of crop per hour or kg of grain per hour.
- Power requirement: roughly 7–15 hp for a medium wheat thresher; rises with feed rate and crop moisture.
Performance measures
| Measure | Definition | Typical value |
|---|---|---|
| Threshing efficiency | 98–99.5 % | |
| Cleaning efficiency | (or removal of impurities) | 95–99 % |
| Grain damage (breakage) | Percentage of broken or cracked grains | < 1–2 % (higher in dry grain) |
| Total loss | Unthreshed + scattered + blown-out grain as a % of total | < 2–3 % |
| Output/throughput | kg/h |
Worked ExampleExample — thresher performance
A wheat thresher receives 200 kg of crop containing 40 % grain by mass (80 kg grain). The grain outlet gives 77.5 kg (including 0.7 kg of broken grain); the straw outlet carries 1.2 kg of unthreshed grain and the blower throws out 0.6 kg of grain with the chaff (the remaining 0.2 kg is scattered or unaccounted for).
Threshing efficiency . Blow-out loss ; total grain loss . Broken grain (within the limit).
Factors affecting performance
- Crop moisture and variety; feed rate (uniform feeding is important); drum speed and concave clearance (high speed → damage; low → unthreshed); blower and sieve settings; spike wear; straw length; crop maturity.