Agricultural Engineering · Farm Machinery & Power · also Farm Machinery & Equipment-II, Plant Protection and Harvesting Equipment
Sprayers, weeders, mowers, reapers, combines and threshers: how they work, how to calibrate them and where losses come from.
In the exams: GATE AG Section 2 (sprayer problems in 6 of 8 papers 2019–26); ICAR AIEEA-PG Unit V; ICAR AICE subject 39 Unit 2; PRC ABE
Farm Machinery and Equipment II covers the machines that protect a crop while it grows and the machines that take it from the field: sprayers, dusters, weeders, mowers, reapers, combine harvesters, threshers, chaff cutters, root-crop diggers, cotton and sugarcane harvesters, and the equipment that deals with straw afterwards. It follows the tillage and sowing machines of Farm Machinery I and draws on the same basic ideas of field capacity, efficiency and cost, but its engineering centre is different: the physics of turning liquid into droplets and getting them onto a leaf, the mechanics of cutting a plant stem, and the separation of grain from straw and chaff.
The subject matters in practice because these machines decide how much of a chemical does its job and how much ends up in water or on neighbours, and how much of the grain a farmer grew actually reaches the store. A calibration error of twenty per cent is not unusual, and it either wastes money and harms the environment or fails to control the pest. A combine that is set badly can leave several per cent of the crop on the ground. Most of the practical work of an agricultural engineer in this area is calibration, setting, loss measurement and safety, and these are the parts this manual treats in detail.
The subject is examined in similar ways across the world. India's GATE Agricultural Engineering paper has set a sprayer calculation (swath from nozzle angle and height, application rate, field capacity, pump power) in several recent papers, together with thresher and combine performance. ICAR's PG and PhD entrance examinations ask for machine parts and functions, performance indices and design principles of plant protection and harvesting equipment. The Philippine ABE licensure, the US PE Agricultural and Biological Engineering examination and national licensing and civil-service examinations elsewhere test calibration, field capacity and loss arithmetic. Rules on pesticide application, buffer zones, aerial spraying and machine safety differ by country, and the manual says so wherever they matter.
Crop protection chemicals (herbicides, insecticides, fungicides) only work when the right dose reaches the right target: a weed leaf, the underside of a cotton leaf, the inside of a fruit-tree canopy. A sprayer is the machine that turns a tank of liquid into droplets, carries those droplets to the target and meters how much goes on each square metre. Much of what leaves a nozzle lands on soil, drifts or runs off; the engineer's task is to raise the useful fraction, keep the dose uniform, and keep drift and operator exposure low. Three things decide the result: how the liquid is atomised (the nozzle and energy source), how the droplets are transported (gravity, the droplet's own momentum, an air stream, or electrostatic attraction), and how the machine is set (pressure, speed, height, spacing). Sprayers are classified by the energy used to form and carry droplets. Hydraulic sprayers use liquid pressure to atomise through a nozzle. Air-assisted sprayers add a fan air stream to carry droplets into a canopy; in a mist blower the air stream itself may also shear the liquid into droplets. Rotary (centrifugal) atomisers throw liquid off a spinning disc or cage and give a narrow droplet size range; this is the basis of controlled droplet application (CDA). Electrostatic sprayers put an electric charge on the droplets so that they are attracted to the earthed plant.
A hydraulic sprayer has a tank, an agitator, strainers, a pump, a pressure regulator or relief valve, a pressure gauge, a delivery line or boom, and nozzles. The agitator keeps wettable powders and emulsions uniformly mixed; it is either mechanical (paddles) or hydraulic (a jet of liquid returned from the pump). Strainers sit at the filling opening, the pump suction, the line and each nozzle, becoming finer towards the nozzle so that the smallest orifice never blocks. Pump choice follows pressure and chemical. The piston (reciprocating) pump is positive-displacement and reaches high pressure, so it is used in knapsack sprayers, orchard guns and high-pressure power sprayers; because it delivers a fixed volume per stroke it must have a relief or unloader valve to return excess flow. The diaphragm pump is also positive-displacement, tolerates abrasive wettable powders well because the liquid does not contact sliding seals, and works at low to moderate pressure. The roller pump is cheap and compact but wears quickly with abrasive suspensions. The centrifugal pump gives high flow at low pressure, handles abrasives well, and needs no relief valve because flow falls as pressure rises. Manual forms include the lever-operated knapsack sprayer (a lever-driven piston or diaphragm pump feeds an air chamber that smooths pressure), the hand compression sprayer (pumped up before spraying; pressure falls as it empties unless a constant-flow valve is fitted), and foot and rocker sprayers. Powered forms range from engine-driven portable units to tractor and self-propelled boom sprayers. Pulse-width-modulation (PWM) nozzles change flow by duty cycle, not pressure, so droplet size stays constant.
Air-assisted sprayers use a fan to replace the air inside a canopy with droplet-laden air and to shake leaves so that both sides are exposed. The orchard air-blast sprayer uses an axial fan blowing radially from behind the tank through hydraulic nozzles arranged around an arc; tower and cross-flow designs deliver the air more horizontally so the top of tall trees is reached without blowing spray above the canopy. The motorised knapsack mist blower uses a small engine-driven fan; liquid fed into a high-speed air stream is sheared into fine droplets, giving low volume rates and good coverage but high drift risk. On field-crop booms an air sleeve (air curtain) blows air downward behind the nozzles to help droplets penetrate dense crops and reduce drift. The rule for air-assisted spraying is to match air volume and speed to the canopy: too little air leaves the inside unsprayed, too much blows spray through and over the target. Electrostatic sprayers charge droplets by induction (an electrode near the point of atomisation induces charge on the forming droplets), by corona discharge, or by direct contact charging of a conducting liquid. A charged droplet near an earthed plant induces an opposite charge on the plant surface and is drawn to it, so droplets can curve round onto the underside of leaves (wrap-around) and deposit more efficiently with less drift. The effect is strongest for small droplets, which carry a high charge-to-mass ratio. It is weaker inside dense canopies because charge builds up on outer leaves and shields the interior (a space-charge or Faraday-cage effect), and it depends on the liquid's conductivity. Rules for drone (unmanned aerial) spraying differ widely by country.
A hydraulic nozzle meters flow, breaks the liquid into droplets and shapes the pattern. Flat-fan nozzles produce a fan-shaped sheet from an elliptical orifice; the sheet thins and breaks into droplets. The standard flat fan has a tapered edge pattern, heavier in the middle and lighter at the edges, so adjacent fans must overlap to give a uniform deposit across a boom; it is the usual nozzle for herbicides on broadcast booms. The even flat fan puts a uniform deposit across its whole width and is used for band spraying over a row, where no overlap occurs. Pre-orifice nozzles meter flow through a first orifice and form the pattern through a second, lowering exit pressure and giving coarser droplets. Air-induction (venturi) nozzles draw air into the liquid, forming large air-filled droplets that drift less and shatter on impact. Hollow-cone nozzles use a swirl plate or core to spin the liquid before a round orifice, giving a ring-shaped pattern of fine to medium droplets that is good for covering foliage with insecticides and fungicides. Full-cone nozzles fill the centre of the cone and give coarser drops, used for soil-applied chemicals. Nozzle flow rate varies with the square root of pressure, Q2 = Q1 times the square root of (P2/P1), so doubling flow needs four times the pressure. Pressure is therefore a poor tool for large rate changes; change nozzle size or travel speed instead. Nozzle materials wear at different rates: brass wears fastest, stainless steel and hardened stainless better, ceramic best. Worn nozzles deliver more flow and a distorted pattern; a common practical rule is to replace a set when its flow exceeds that of a new nozzle by about ten per cent. ISO 10625 fixes a colour code for flat-fan nozzle sizes by flow rate so that nozzles of the same size from any maker look alike.
A nozzle does not produce one size of droplet but a spectrum, from a few micrometres to several hundred. The spectrum is described by cumulative volume fractions. Dv0.5, the volume median diameter (VMD), is the diameter such that half the spray volume is in droplets smaller than it. Dv0.1 and Dv0.9 are the diameters below which ten and ninety per cent of the volume lie. The number median diameter (NMD) splits the number of droplets in half; because a few large droplets hold most of the volume, NMD is always smaller than VMD, and the ratio VMD/NMD is an index of uniformity (one would mean all droplets are the same size). Relative span, (Dv0.9 minus Dv0.1) divided by Dv0.5, is the more widely used spread index today; smaller is more uniform. Droplet volume goes as diameter cubed, so halving diameter gives eight times as many droplets from the same liquid. That is the trade-off at the heart of spraying: small droplets give many deposits and good coverage, needed for contact insecticides and protectant fungicides, but they fall slowly, evaporate quickly and drift. Large droplets resist drift but give few deposits and can bounce or run off. ASABE S572 classifies sprays by comparison with reference nozzles into categories from very fine through fine, medium, coarse, very coarse, extremely coarse to ultra coarse, and pesticide labels in several countries name the category required. Droplet size falls with higher pressure and with wider fan angle for the same flow, and rises with larger orifice size. Field deposits are checked with water-sensitive paper, counted as droplets per square centimetre. The fraction of volume in droplets smaller than about 100 micrometres is often reported as a drift-risk index.
Drift is the movement of spray away from the target during application, either as droplets carried by wind (particle drift) or as vapour after deposit (vapour drift, from volatile products). Particle drift rises with wind speed, boom height, the fraction of fine droplets, travel speed and high temperature with low humidity, which shrinks droplets by evaporation. Temperature inversions, with cool air lying beneath warmer air on calm evenings and early mornings, keep a cloud of fine droplets suspended and move it long distances, so calm is not always safe. Control measures are, in rough order of effect: choose a coarser spray quality, such as air-induction or pre-orifice nozzles; lower the boom to the least height that still gives full overlap; spray within the wind speed limits on the label; avoid inversions; use shields, air-assist or drift-reducing adjuvants; and leave buffer zones next to water and sensitive crops. Buffer zones, drift-reduction classes and wind limits are set by national regulators and labels and differ between countries.
Application rate is the volume of spray liquid applied per hectare. For field crops a widely used classification by Matthews is high volume above about 600 L/ha, medium volume 200 to 600, low volume 50 to 200, very low volume 5 to 50 and ultra-low volume below 5 L/ha; bush and tree crops use higher bands because the canopy is larger. The dose of active ingredient is fixed by the label; the water volume carries it. A single flat-fan nozzle at height h above the target with fan angle theta covers a width of 2h tan(theta/2). On a boom, nozzles are spaced so that patterns overlap enough for a uniform deposit, and the swath of the boom is the number of nozzles times their spacing. Field capacity follows the same rule as for any implement: theoretical field capacity (ha/h) equals swath (m) times speed (km/h) divided by ten; effective field capacity multiplies this by field efficiency and, for problems that give it, by (1 minus wheel slip) when the stated speed is the no-slip speed. Refilling and turning time lower sprayer field efficiency. Distribution uniformity across the boom is checked on a patternator, a tray of channels that collect spray into measuring tubes, and expressed as a coefficient of variation; in-use inspection standards such as ISO 16122 commonly require a CV of not more than about ten per cent.
The basic calibration relation for a boom is: application rate q (L/ha) = 600 times nozzle flow Q (L/min) divided by (nozzle spacing w in m times speed v in km/h). The 600 comes from units: Q times 60 gives litres per hour, and a strip w metres wide at v km/h covers w times v divided by ten hectares per hour. Rearranged, the required nozzle flow is Q = q w v / 600. For a whole boom use the total flow and the full swath. A calibration therefore needs three measured things: speed over the ground in the actual field (time a measured distance, because wheel slip and soft ground change it), nozzle flow at the working pressure (catch each nozzle for one minute in a measuring jug and reject any nozzle more than about five to ten per cent from the mean, a common tolerance), and nozzle spacing. For a knapsack sprayer or spot spraying, the simplest method is to spray a measured area with water at normal walking speed and measure what was used: litres used divided by area sprayed in hectares gives L/ha. Once the rate is known, the area one tankful covers is tank volume divided by q, and the amount of product per tank is the label dose per hectare times that area. When a label gives a concentration instead, such as a spray of 0.1 per cent active ingredient from a 50 per cent formulation, the formulation needed per 100 L is 100 times 0.1 divided by 50, which is 0.2 L. The pump must supply the nozzles plus agitation and bypass flow. Hydraulic power is pressure times flow; shaft power is that divided by pump efficiency. For example, 24.4 L/min at 400 kPa is 400,000 Pa times 0.000407 m3/s, about 163 W hydraulic, and at 60 per cent efficiency about 271 W at the shaft. Leaving out the bypass flow is the most common error in exam answers.
Constant-pressure systems deliver a constant rate only at constant speed. Rate controllers measure speed (wheel sensor, radar or GNSS) and either flow or pressure and adjust a regulating valve so that litres per hectare stay constant as speed changes; because flow goes with the square root of pressure, a speed increase of 40 per cent needs about double the pressure, which also shifts droplet size, so the usable speed range of a nozzle is limited unless PWM or multi-nozzle bodies are used. Automatic section control switches boom sections off where the field has already been sprayed or at headlands and field edges, using GNSS maps, reducing overlap. Direct injection meters chemical into carrier water in the line, and camera-based spot spraying opens nozzles only over detected weeds. Every one of these still depends on correct nozzle choice, clean strainers and a verified calibration; electronics cannot correct a worn or blocked nozzle.
A duster applies a pesticide formulated as a dry powder, carried by an air stream. Dusts need no water, which made them useful where water is scarce, but the fine particles drift far, deposit poorly on dry leaves and expose the operator, so in many countries dusts have been replaced by granules or sprays and some dust formulations are no longer registered. Where they are still used, dusting is usually advised when dew is on the leaves, early in the morning, so that particles stick, and never in wind. A duster has a hopper, an agitator to keep the powder loose, a metering device (an adjustable orifice or a feed rotor), a fan or bellows to make the air stream, and a delivery tube ending in a deflector or fishtail outlet to spread the dust. The plunger duster is a hand pump with a piston that blows a puff of dust per stroke, suited to small areas and spot treatment. The bellows duster uses a bellows, often worn on the body, squeezed by hand. The rotary (hand-crank) duster, carried on the chest or shoulder, uses a crank and gear train to spin a fan at high speed, with the agitator driven from the same gear; it gives a continuous stream and can cover field crops. Power dusters use an engine-driven fan; the motorised knapsack mist blower can be converted to a duster by changing the metering and outlet. Tractor-mounted dusters take drive from the PTO. Calibration follows the same logic as a sprayer: output (kg/min) = dose (kg/ha) times area covered per minute, where area per hour is swath times speed divided by ten.
Granules are larger particles, usually a fraction of a millimetre to a few millimetres, that fall under gravity and hardly drift. They are placed in furrows at sowing, broadcast on soil or into standing water in rice, or dropped into the whorl of maize against stem borers. Applicators use a hopper and a metering unit, such as a fluted roller, a rotating plate or an adjustable orifice with an agitator, delivering to a tube, a spreader plate or a spinning disc. Because output depends on granule size, density and flowability, a granule applicator is calibrated with the actual product, not with sand or fertiliser, by collecting output over a measured distance or time. Some granular insecticides are highly toxic and restricted in many countries; the dose always comes from the national label.
Inter-cultivation is soil work done between crop rows after the crop has emerged: killing weeds, breaking surface crust, loosening soil for aeration and infiltration, mixing top-dressed fertiliser, and earthing up soil against the stems of crops such as potato, sugarcane and maize. Weeds are killed mechanically by being cut below the surface, uprooted, buried or dried out. The tool that cuts is usually a sweep or duck-foot shovel, a V-shaped blade running flat just below the surface that severs roots with little soil movement, so it conserves moisture. Narrow shovels and spring tines stir soil and uproot weeds. Blade harrows and bar blades, the traditional bakhar-type implements of South Asia, slice horizontally through the topsoil. Rolling cultivators have gangs of star-shaped spider wheels set at an angle so that they throw soil towards or away from the row. Ridgers and earthing-up bodies are small double mouldboards that heap soil onto the row. Rotary cultivators drive tines or blades on a horizontal shaft between rows. Intra-row weeds, those in the crop row between plants, are hardest to reach; finger weeders (rubber fingers driven by ground wheels that reach into the row), torsion weeders (spring tines that bend round stems), and flexible-tine harrows used across the whole field while the crop is anchored and weeds are small are the main mechanical answers. Camera-guided hoes and robotic weeders steer tools around crop plants.
Hand hoes and push-type wheel hoes remain the commonest weeders in smallholder farming. Power weeders are small engine-driven rotary units, walk-behind, with rotors sized to the row gap; power tillers and tractors carry multi-row cultivators. In transplanted and line-sown rice, weeding is done in saturated soil. Rotary paddy weeders carry toothed or star wheels that churn and bury weeds between rows; the cono weeder uses two cone-shaped rotors with serrated strips, set in tandem with opposite orientation, plus a float to prevent sinking, and is pushed and pulled along rows. It is associated with the system of rice intensification, which uses wider square spacing that allows weeding both ways. Performance of a weeder is judged by weeding efficiency, plant damage, field capacity and operator effort. Weeding efficiency (per cent) = (W1 minus W2) divided by W1 times 100, where W1 is the weed count (or weed mass) per unit area before weeding and W2 after. Plant damage (per cent) is the number of crop plants damaged divided by the number in the length examined, times 100. Some test procedures combine these into a performance index, field capacity times (100 minus plant damage) times weeding efficiency, divided by power input, to compare weeders of different sizes. Timing matters more than machine choice: mechanical weeding works best when weeds are at the two- to four-leaf stage and the soil is friable; wet soil lets uprooted weeds re-root, and large weeds escape sweeps. Manual weeders are judged ergonomically by pull force and heart rate.
A plant stem can be cut in two ways. In supported (counter-shear) cutting, a moving blade presses the stem against a fixed edge, the ledger plate or shear bar, and the stem fails in shear between the two edges, as in scissors; this works at low blade speed. In unsupported (impact) cutting, a blade strikes a free-standing stem, and the only support is the stem's own inertia and bending stiffness; the blade must move fast enough that the stem fails locally before it bends away, so impact cutters run at high speed, typically several tens of metres per second at the blade tip. Cutting force and energy depend on the crop, its moisture and maturity, the stem diameter, the sharpness of the edge (thin edge radius), the bevel angle of the blade, the clearance between knife and ledger, and the angle of the edge to its direction of motion. A sharp, thin edge concentrates stress; a blunt edge crushes the stem before it cuts it and multiplies energy. Too much clearance between knife and ledger lets stems wedge in the gap and bend rather than shear. Oblique cutting, where the blade slides along its own edge as it moves (a slicing action), reduces the force needed, which is why curved and inclined knives are used. Serrated edges grip smooth stems such as mature grain straw and stop them sliding out of the V between knife and guard. Specific cutting energy varies widely with species and moisture.
The reciprocating mower, the basis of cutter bars on reapers and combine headers, is a supported cutter. Its parts are the knife (a back bar carrying riveted triangular sections), the guards (fingers bolted to the cutter bar that carry the ledger plates, the fixed shearing edges), knife clips that hold the sections down on the ledgers, wearing plates that support the knife back, and a drive, usually a crank and pitman (connecting rod) or, on combines, a wobble (swash-plate) box. On the standard cutter bar the knife stroke, section spacing and guard spacing are all 76.2 mm (3 inches). Two adjustments matter. Alignment means the guard points and ledger edges lie in a straight line and the knife moves parallel to them. Registration means that at each end of its stroke the centre line of each knife section lies on the centre line of a guard; if registration is wrong, stems are cut unevenly and some are left uncut. The knife speed varies through each stroke, zero at the ends and maximum mid-stroke. For a crank of radius r turning at angular velocity omega, the knife velocity is close to r omega sin(omega t) (ignoring pitman obliquity), so its maximum is r omega, while its mean is stroke times strokes per second. Each crank revolution gives two strokes. The machine moves forward while the knife moves across, so each section sweeps a region whose length along the travel direction is the forward advance per stroke; a large advance per stroke leaves uneven stubble and uncut stems, a small one wastes energy. Advance per stroke equals forward speed divided by the number of strokes per second. Double-knife bars with two opposed knives and no guards reduce vibration and clog less in heavy, wet crops. Knife speed is limited by inertia forces and vibration.
Rotary mowers use impact cutting. The disc mower has a row of small, often oval, discs on a low gear-driven cutter bar, each carrying two free-swinging knives; adjacent discs turn in opposite directions in pairs to move cut crop into swaths. The drum mower has a few large drums driven from above, each with knives at its base, and delivers crop between drum pairs. Both cut fast and are much less prone to blockage than cutter bars, at the cost of higher power per metre of width. Blade tip speed is pi times diameter times revolutions per second; a 0.5 m blade circle at 3000 rev/min gives about 78.5 m/s. The flail mower's horizontal rotor carries swinging flails that cut and lacerate forage, grass or residue. Broken knives can be thrown, so guards and curtains are essential. A mower-conditioner combines cutting with crushing or abrading the stems to speed drying. Roll conditioners pass the crop between intermeshing rubber or steel rolls that crimp the stems and are preferred for legumes such as lucerne (alfalfa), whose leaves shatter easily. Flail or tine (impeller) conditioners strip the waxy cuticle and suit grasses.
A reaper cuts the crop and lays it in an orderly row for later collection and threshing; it does not thresh. Scarce, costly harvest labour drove reaper use across South and South-East Asia. The most common small reaper is the vertical conveyor reaper (VCR). A reciprocating cutter bar cuts the crop, crop dividers at the ends separate the cut strip from the standing crop, star wheels or star-shaped feeding wheels hold the stems upright against the knife, and lugged flat belts carry the upright stems across to one side, where they fall in a windrow at right angles to travel. It is built as a walk-behind self-propelled machine with its own small engine, as an attachment on a power tiller, or as a front-mounted attachment on a tractor with a wider cutter bar. The reaper-binder adds a binding mechanism: stems are gathered by packers into a bundle against a trip, and a knotter with a needle, twine holder (twine disc) and billhook ties the bundle with twine, the same knotting principle used in square balers; bundles are ejected and can be stacked and carried to a thresher. Reapers need a crop that stands; lodged or tangled crop feeds badly, as does very short or very tall crop outside the design range. Field capacity is calculated as for other machines: width times speed divided by ten times field efficiency. For example, a 1.2 m reaper at 3 km/h with 80 per cent field efficiency covers 0.288 ha/h, so 2 ha takes about 6.9 hours.
On wider reapers, windrowers and combine headers a reel pushes the crop onto the cutter bar and then onto the platform. The reel carries bats or, for lodged crops, spring tines (a pick-up reel) on a rotating frame. The ratio of reel peripheral speed to forward speed, the reel index, must be greater than one so that the bats move rearward relative to the crop and push it onto the knife; values of about 1.25 to 1.5 are commonly recommended, higher for lodged crop. Too low an index lets crop fall forward and be lost; too high an index beats the heads and causes shatter loss. The reel is set so that the bats enter the crop just above the cut height needed to support the stems and slightly ahead of the knife, and reel height is raised in tall crops. The windrower (swather) cuts the crop and lays it in a continuous swath on the stubble to dry and ripen before a combine with a pick-up header collects it. Swathing is used for crops that ripen unevenly or shatter easily, such as canola and some pulses, and in cool, wet harvest climates; draper platforms carry the crop to the delivery opening on flat canvas or rubber belts rather than an auger, handling it gently. Stripper headers strip heads from standing straw and send far less straw into the combine.
A combine harvester cuts (or picks up), threshes, separates, cleans and stores grain in one pass. Its functions follow the flow of material. The header gathers and cuts the crop and delivers it to the feeder house. The feeder house, a slat-and-chain conveyor, carries the crop up to the threshing unit. The threshing cylinder, rotating against a concave, detaches grain from the heads. Most free grain passes through the concave grate at this point; the straw and the grain still held in it pass to the separating unit, straw walkers in a conventional combine or rotor sections in an axial combine, which shakes out the remaining free grain. Grain and chaff from the concave and separator fall onto the grain pan and cleaning shoe, where sieves and a fan remove chaff and light material. Clean grain is augered to an elevator and into the grain tank; unthreshed heads (tailings) are returned for re-threshing; straw and chaff leave at the rear, spread or windrowed. The performance of the whole machine is described by its capacity, the feed rate at which losses stay within an acceptable level. Because most power and most losses depend on the straw and chaff, capacity is expressed as the feed rate of material other than grain (MOG), in tonnes per hour, and the MOG to grain ratio of the crop decides how much grain that capacity represents. Harvest throughput can be estimated from field capacity: a 4.2 m header at 4 km/h and 75 per cent field efficiency covers 1.26 ha/h; with 5 t/ha of grain that is 6.3 t/h of grain, and at a MOG to grain ratio of 1.2 it is about 7.6 t/h of MOG.
The grain platform (cutter-bar header) has a reciprocating knife, a reel, crop dividers and a cross auger with retractable fingers at its centre that pushes the cut crop into the feeder house. Flexible cutter bars follow the ground to cut low in crops such as soybean whose lowest pods are near the soil. Draper headers replace the auger with belts. Row-crop headers for maize have gathering chains and snapping rolls: two counter-rotating rolls pull the stalk downward between stripper plates, so the ear is snapped off at the plates and carried to the auger while the stalk passes down to the ground. Cutting height is chosen as a compromise: cutting low picks up more low heads and lodged stems but sends more straw through the machine, lowering capacity; cutting high leaves tall stubble that must be managed afterwards.
Grain is detached by impact, by rubbing, and by combing or stripping. The rasp-bar cylinder carries corrugated bars on its periphery; it threshes mainly by rubbing crop against the concave, with some impact, and is the standard cylinder in combines for small grains. The spike-tooth cylinder carries pegs that pass between pegs on the concave, threshing by impact and tearing; it is aggressive, handles tough and wet crop and is common in threshers for rice and in some combines. The wire-loop (loop-type) drum, with wire loops on a drum, strips grains by impact and combing and is used in pedal-operated and small power paddy threshers, where heads are held against the drum. The concave is an open grate, usually transverse bars with wires between them, wrapped round part of the cylinder. Concave clearance is set larger at the front, where the crop mat is thick, than at the rear. The two main settings are cylinder speed (peripheral speed, pi times diameter times revolutions per second) and concave clearance. Higher speed and smaller clearance thresh more completely, lowering unthreshed loss, but crack and bruise more grain, lower seed germination, break straw into short pieces that overload the cleaning shoe, and take more power. Lower speeds and wider clearances are used for easily damaged crops such as beans, pulses and seed crops; cereals need higher speeds. Recommended settings are given in operator's manuals and vary by crop, variety and moisture, so the practical rule is to start from the manual setting and adjust while checking the tank sample for damage and the straw for unthreshed heads. In axial-flow combines the crop moves helically round a long rotor inside a cage for several turns; each impact can be gentler because residence time is longer, grain damage is often lower, and centrifugal force separates grain through the cage, replacing straw walkers.
After the concave, straw still carries free grain that must be shaken out. In a conventional (tangential) combine, a beater behind the cylinder slows the straw and throws it onto the straw walkers, and a hanging curtain slows it further so that it does not fly off the walkers too quickly. Straw walkers are several long, narrow, stepped sections side by side mounted on a crankshaft, each out of phase with its neighbours. As the crankshaft turns, each section lifts the straw and throws it back and up, so the mat is repeatedly loosened and moved rearward while free grain falls through the openings in the walker decks and slides forward on the bottoms to the grain pan. Walker separation depends on the thickness of the straw mat: at low feed rates almost all free grain falls out, but as MOG feed rate rises the mat thickens and the fraction of grain carried out over the back rises sharply. Walker loss therefore rises roughly exponentially with MOG feed rate, and walker loss often sets the capacity of conventional combines. Rotary separators use centrifugal force, many times gravity, giving high capacity in dry straw but needing more power in tough, green straw.
The cleaning shoe receives grain, chaff, short straw and unthreshed heads from the concave and separator. On the grain pan, an oscillating tray, the mixture stratifies: heavy grain moves to the bottom and light chaff rises, so that when it drops onto the sieves the grain is already underneath. The upper sieve, the chaffer, has adjustable louvres (lips) that let grain and small heads through and pass chaff and straw rearwards. The lower sieve, adjustable or with fixed round holes, passes clean grain and returns anything too large, mainly unthreshed heads, as tailings, which an auger carries back to the cylinder or to a separate re-thresher. A fan blows air up through both sieves. Air cleaning works because grain and chaff have different terminal velocities, the air speed at which a particle is held suspended. Air speed through the sieves must be above the terminal velocity of chaff and light material but below that of grain, so chaff is lifted and blown out the back while grain falls. Too little air leaves chaff blanketing the sieves, so grain rides over the back and dockage in the tank rises. Too much air blows grain out. Chaffer and sieve openings are set to the grain size, and the fan speed and direction adjusted with them. Shoe loss rises with the chaff and broken-straw load, which is why over-aggressive threshing, which chops straw, can raise shoe loss. On side slopes material piles on the downhill side of the shoe; hillside and self-levelling combines and levelling shoes counter this. Piezoelectric loss monitors at the walker and shoe exits give relative readings that must be calibrated against measured loss.
Harvest losses are grain that was produced but does not reach the grain tank or store in usable form. Separating them by source is the basis of both measurement and correction. Pre-harvest losses are grain on the ground before the machine arrives, from shattering of over-ripe crop, lodging, birds and weather; they rise quickly when harvest is delayed after the crop is ready, which is why timeliness has a real cost. Header (gathering) losses are grain shattered by the reel, dividers and knife, heads on stems cut too high or not cut at all, and heads dropped at the front of the platform. Threshing (cylinder) losses are grain still attached to heads that pass out with the straw or chaff, recorded as unthreshed grain. Separation (walker or rotor) losses are free grain carried out in the straw. Cleaning (shoe) losses are free grain blown or carried out with the chaff. Leakage losses escape through gaps and damaged seals. Grain damage (cracked and bruised kernels) is a loss of quality and, for seed, of germination; manual and reaper systems also lose grain in bundle handling and on the threshing floor. Loss levels are strongly crop, variety, moisture, machine and operator dependent, and published targets vary, so the aim of measurement is to find where losses are high and which setting reduces them.
Losses are measured by counting or weighing grain in known areas. A frame of known area, often one square metre or a frame as wide as the header, is placed on the ground at the chosen positions. Pre-harvest loss is measured in the standing crop ahead of the machine. Header loss is measured behind the header but in an area where straw and chaff from the rear have not fallen, or by stopping the combine and backing it away. Total machine loss is measured behind the machine, and machine (threshing, separating and cleaning) loss is the total minus header and pre-harvest loss. Grain counts are converted using thousand-grain weight (TGW): grains per square metre times TGW in grams divided by 1000 gives grams per square metre, and grams per square metre times 10 gives kilograms per hectare, so loss in kg/ha equals grains per square metre times TGW divided by 100. For example, 50 grains per square metre of wheat with a TGW of 40 g is 20 kg/ha, which is 0.4 per cent of a 5 t/ha crop. Grain damage is measured by hand-sorting a sample from the tank for visibly broken kernels, and for seed by germination tests, since cracks too small to see still reduce germination.
Each source has its own remedies. Pre-harvest loss: harvest at the right moisture and plan capacity so that the harvest window is not exceeded; varieties with shatter resistance and lodging resistance help. Header loss: set the reel index and height correctly, keep the knife sharp and registered, match forward speed to the crop, lower the cutting height in lodged or short crop, and use crop lifters on the cutter bar. Threshing loss: raise cylinder speed or close the concave in steps until unthreshed heads disappear, while checking damage. Separation loss: reduce forward speed to reduce MOG feed rate, raise the cutting height to send less straw through, and avoid over-threshing that breaks straw. Cleaning loss: adjust fan speed, chaffer and sieve openings, and reduce the chaff load. Damage: lower cylinder speed, open the concave, and avoid harvesting very dry seed crops in the heat of the day. In rice, grain moisture at harvest affects both shattering and cracking, and cracks formed by rapid re-wetting and drying of dry grain lower head-rice yield at milling, so both harvest timing and prompt drying matter.
A thresher is a stationary machine that detaches grain from harvested crop and cleans it, powered by an electric motor, engine or tractor PTO. The main types are named after the threshing drum. Spike-tooth threshers use pegs on the drum passing pegs on the concave and are common for rice and for multi-crop work. Rasp-bar threshers rub the crop. Hammer-mill or beater (drummy) threshers, widely used for wheat in South Asia, have flat beaters on a drum that break the straw into short pieces (bhusa) valued as animal feed while threshing. Axial-flow threshers move the crop along the drum axis in a helical path, threshing and separating over a long length; multi-crop axial-flow threshers handle wheat, rice, sorghum, pulses and others by changing drum speed, concave and sieves. Wire-loop threshers, pedal-operated or powered, are used for paddy by holding bundles against the drum. A power thresher has a feeding chute or hopper, the threshing drum and concave, an aspirator or blower, a sieve shaker, and outlets for grain and straw. Performance is described by output capacity (kg of grain per hour), threshing efficiency, cleaning efficiency, broken grain percentage and losses at each outlet, as unthreshed, blown and spilled grain. Threshing efficiency (per cent) = 100 minus the percentage of unthreshed grain relative to total grain input. Cleaning efficiency (per cent) = clean grain at the main outlet divided by total material at the main outlet times 100. Power threshers have caused many serious hand and arm injuries at feeding, particularly with long working hours and tired operators; safety features include a long covered feed chute so that hands cannot reach the drum, guards over belts and pulleys, and stopping the machine before clearing blockages. India's Dangerous Machines (Regulation) Act of 1983 was enacted with power threshers among its main concerns; safety rules and standards differ between countries.
A chaff cutter chops straw and green fodder into short lengths for livestock, reducing wastage at the manger and improving intake. The hand-operated type has knives on a spoked flywheel turned by a handle; fodder is pushed or fed by feed rolls through a mouthpiece, and each knife shears it against a fixed shear plate. Power chaff cutters use an electric motor or engine and have two or more serrated feed rolls driven through a gearbox that sets the length of cut. Cylinder types carry knives on a cylinder rather than a flywheel. Theoretical length of cut equals the feed speed of the material divided by the number of knife cuts per second, that is, feed roll surface speed divided by (number of knives times revolutions per second). For example, feed rolls moving fodder at 0.1 m/s with two knives at 300 rev/min give 0.1 divided by 10 cuts per second, a 10 mm cut. The same relation governs forage harvesters used for silage, whose cutterheads of flywheel or cylinder type chop against a shear bar. Knife-to-shear-plate clearance and sharpness decide power and cut quality. Injuries happen when a hand follows fodder into the feed rolls, so knife and gear guards, a long feed chute and a quick-reverse or stop on the feed rolls are essential.
Root and tuber crops such as potato, groundnut, sugar beet, onion, carrot and cassava must be lifted from the soil and separated from it without cutting or bruising. The basic potato digger has a share (blade) running below the tuber zone that lifts a slab of soil and tubers onto a rising elevator of steel rods linked by chains; the rods are spaced so that loose soil falls through, and agitators (eccentric sprockets or knockers) shake the elevator to sieve more soil out. The tubers are dropped in a row on the surface for hand picking. A harvester adds haulm (vine) separation, clod and stone separation and a picking table or cross conveyor that delivers tubers into a trailer. Haulms are killed or removed before digging so that skins set and separation is easier. Damage comes from blade cuts when depth is too shallow, bruising from drops and from the tubers riding bare on an over-agitated elevator, so elevators should be kept full of soil as a cushion, drop heights kept low, and elevator speed matched to forward speed. Soil condition matters: wet, sticky clay does not sieve, and dry soil forms clods that are hard to separate from tubers. Groundnut diggers use a blade to cut the taproot below the pods and lift the plants; digger-shakers shake the soil from the pods and windrow the plants, and digger-inverters turn them pods-up to dry before a separate thresher or combine picks them.
Mechanical cotton harvesting uses two principles. The spindle picker has rows of rotating barbed spindles on drums that enter the plant, wind lint from open bolls, and are stripped (doffed) by rubber doffers; the spindles are moistened so lint winds and releases cleanly. Spindle pickers are selective, taking open bolls and leaving green ones, and deliver cleaner seed cotton. The stripper pulls all bolls, open and unopened, together with burs and leaves from the plant using brush rolls or finger combs, suits short, compact plants, costs less, and delivers more trash, so a field cleaner or burr extractor and more cleaning at the gin are needed. Both need defoliated plants, since leaves add trash and stain lint, and plant architecture, variety and uniform boll opening determine how well machines work. Sugarcane is cut either as whole stalks or as billets. Whole-stalk harvesters cut the cane at the base, top it and lay it for loading. Chopper (billet) harvesters, now dominant where cane harvesting is mechanised, have crop dividers that lift and separate the rows, a topper that cuts off the green top, base cutters (two counter-rotating discs with blades at ground level), feed rollers that carry the stalks back, a chopper that cuts them into billets, a primary extractor fan that blows leaves and trash out, an elevator, and a secondary extractor. Billets deteriorate much faster than whole stalks after cutting, losing sugar, so harvest and crushing must be closely coordinated. Machine harvest needs row spacing matched to the harvester and a field layout that limits stool damage, which lowers ratoon yields.
After combining, loose straw and standing stubble must be managed. In north-west India the practice of burning rice straw to clear the field quickly for wheat is a major cause of seasonal air pollution, and machines were developed and subsidised to manage the straw instead. In-situ options keep residue in the field. A straw chopper-spreader on the combine chops straw and spreads it evenly across the header width; an attachment of this kind behind a combine is known in India as a super straw management system. The happy seeder combines a flail rotor that clears straw from in front of each furrow opener with a seed drill, so wheat is sown directly into standing rice stubble and loose straw, which remains as mulch. Mulchers, straw choppers, rotary-tiller-based seeders and reversible mouldboard ploughs are other in-situ options. Ex-situ options remove straw for use elsewhere: rakes gather it into windrows (side-delivery, wheel and rotary rakes), and balers compress it. Small rectangular balers use a reciprocating plunger to pack slices into a bale chamber and tie bales with knotters. Round balers roll the material into a cylinder in a fixed chamber (a soft core forms) or a variable chamber (belts keep pressure from the start, giving a denser core). Bale density is mass divided by volume; a 20 kg bale measuring 0.36 by 0.46 by 0.9 m has a density of about 134 kg per cubic metre. The straw reaper, used in parts of India after combine harvest of wheat, cuts the standing stubble, threshes out remaining grain and chops the straw into bhusa for feed.
A student should be able to calibrate a boom and a knapsack sprayer with water and a jug, choose nozzles from a catalogue for a given rate and spray quality, and do the GATE-type sprayer, mower and thresher calculations without hesitation. In the field course, set a reel index, check cutter-bar registration, measure combine losses with a frame and convert grain counts to kg/ha. Learn the parts of a combine by following the crop through it, and learn the thresher and chaff cutter safety features, because you will be asked about them and you will one day stand next to one running.
With five years' work, the engineer runs sprayer checks and operator training, sets combines at the start of each harvest and teaches operators to diagnose losses behind the machine, tests threshers and reapers to national codes at a testing centre, or advises custom-hiring services on which machines to buy and how to price them. Decisions are practical: which nozzle meets a label's drift requirement, whether a thresher's broken-grain figure is acceptable, how many combines a district needs to finish in the harvest window, and which residue machine suits a two-week gap between rice and wheat.
A senior engineer with twenty-five years shapes the systems around the machines: national test codes and subsidy lists, sprayer inspection schemes, safety rules for threshers and chaff cutters, and mechanisation plans for crops still harvested by hand, such as cotton and sugarcane in many regions. They balance manufacturers, farmers, regulators and environmental agencies, decide which evidence is good enough to change a recommendation, and judge when a new technology such as drone spraying or camera spot spraying is ready for smallholders and under what rules.
A researcher at this level works on the physics and models that underlie the machines: droplet formation and drift modelling, electrostatic deposition in canopies, cutting mechanics of stems, threshing and separation models, and loss functions that link settings to throughput. They design experiments with proper replication, build sensors and control systems for automatic combine setting and targeted spraying, supervise students, and write the reviews and standards others use. The questions worth asking are those where losses or chemical use are still high and the mechanism is not yet understood.
This manual does not give numeric boundaries for the ASABE droplet-size categories (they are defined by reference nozzles and revised over time), crop-specific cylinder speeds and concave clearances (take these from operator's manuals), terminal velocities of particular grains, or national test-code numbers for threshers, reapers and sprayers. Aerial and drone spraying rules, buffer-zone widths and pesticide registrations are country-specific and are only flagged, not detailed. Forage harvester and baler design, tree-fruit and vegetable harvesters, and detailed combine power budgets are treated only briefly.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.