Unit 6: Harvesting and threshing machines

I. Orientation — Crop Separation as the Governing Principle

Harvesting removes the economically useful crop from the field at suitable maturity, while threshing separates grain or seed from harvested plant material. Machine selection and adjustment must maximize recoverable, undamaged produce while minimizing field loss, grain damage, labour, time, energy use, and operating cost.

A. Governing principles and characteristics

The performance of harvesting and threshing machinery depends on crop condition, machine adjustment, operating speed, and timeliness.

• Crop maturity: Harvesting should begin when grain has reached physiological maturity and moisture content is suitable for cutting, threshing, and storage.
• Material separation: A machine progressively divides crop material into useful grain and material other than grain (MOG), such as straw, chaff, leaves, and weeds.
• Machine–crop interaction: Cutting requires shear force; threshing uses impact, rubbing, stripping, or a combination of these actions.
• Loss control: Total harvesting loss includes pre-harvest, header, threshing, separation, cleaning, and handling losses.
• Capacity: Machine output may be expressed as area covered in ha/h or material processed in kg/h or t/h.
• Power matching: Tractor or engine power must satisfy draft, PTO, hydraulic, and auxiliary power requirements without excessive wheel slip or overload.
• Safety: Guards, shields, stable adjustment, and complete power shutdown are essential around knives, belts, chains, cylinders, and PTO shafts.

II. Harvesting — Removal of the Mature Crop

A. Fundamentals of harvesting

Harvesting is the timely cutting, gathering, picking, digging, or stripping of the marketable crop from the field.

• Objectives: Harvesting seeks to recover the maximum yield at acceptable quality while completing field work within the available harvesting period.
• Harvesting operations:
  • Cutting: Severing stems with a sickle, mower, reaper, or combine cutter bar.
  • Gathering and conveying: Bringing cut crop to a platform, windrow, elevator, or threshing unit.
  • Bundling or windrowing: Arranging material for drying, transport, or later threshing.
  • Picking or digging: Removing fruits, cobs, pods, roots, or tubers without necessarily cutting the entire plant.
• Maturity indicators: Grain hardness, straw colour, pod colour, leaf drying, seed moisture, and days after sowing help determine harvest time.
• Moisture effect: Excessively wet crops thresh poorly and may clog machinery; over-dry crops suffer shattering, cracking, and greater field loss.
• Harvesting losses:
  • Pre-harvest loss: Natural shattering, lodging, bird damage, or weather damage before machine entry.
  • Machine loss: Uncut plants, shattered grain, dropped heads, incomplete threshing, and grain discharged with straw or chaff.
• Timeliness: Delay can reduce quality and yield even when machine cost is lower; therefore, effective capacity must match field area and the available harvest window.
• Performance measurement: Percentage loss is commonly estimated by collecting lost grain from a known ground area.

Loss (%) = (mass of lost grain / total potential grain mass) × 100

B. Types of harvesting

Harvesting systems are classified according to the amount of human labour, degree of mechanization, and sequence of operations.

1. Manual and semi-mechanical harvesting

   • Manual harvesting: Sickles, knives, or hand tools are used for cutting or picking. It suits small, irregular fields and crops requiring selective harvest, but labour demand is high.
   • Semi-mechanical harvesting: A reaper or mower cuts the crop, after which gathering, bundling, transport, and threshing are performed separately.
   • Advantages: Low initial investment, selective operation, and adaptability to lodged or mixed crops.
   • Limitations: Low capacity, high labour requirement, and increased handling and transport loss.

2. Mechanical harvesting

   • Reaper harvesting: A cutter bar severs standing crop and places it in a swath; a reaper-binder additionally ties bundles.
   • Windrow harvesting: Crop is cut and laid in rows for field drying before pickup and threshing.
   • Combine harvesting: Cutting, feeding, threshing, separation, cleaning, and grain collection occur in one field pass.
   • Specialized harvesting: Maize pickers, cotton pickers, sugarcane harvesters, potato diggers, and groundnut diggers are designed around crop geometry and the harvested plant part.
   • Selection factors: Crop type, field size, terrain, lodging, moisture, labour availability, expected yield, machine capacity, and ownership cost determine the suitable system.

III. Thresher — Grain Detachment and Separation

A thresher detaches grain from ears, heads, panicles, cobs, or pods and separates it from straw and chaff through controlled mechanical action.

A. Thresher: types

Threshers are classified by crop suitability, threshing mechanism, feeding arrangement, and power source.

• By threshing cylinder:
  • Spike-tooth type: Spikes on the rotating cylinder produce impact and combing action; commonly used for wheat and similar crops.
  • Rasp-bar type: Corrugated bars rub and impact crop against the concave; widely used in combines.
  • Wire-loop type: Wire loops provide rubbing and stripping action, often suited to paddy.
  • Hammer-mill type: Swinging hammers give severe impact and may chop straw; power demand and grain damage can be higher.
  • Axial-flow type: Crop moves helically along the rotor, allowing repeated threshing and separation in a compact unit.
• By crop: Wheat, paddy, maize, soybean, groundnut, millet, and multi-crop threshers use crop-specific screens, concaves, and cylinder settings.
• By feeding: Hold-on threshers admit mainly ear heads, whereas throw-in threshers receive the whole crop.
• By power source: Units may be pedal-operated, engine-operated, motor-operated, or PTO-driven.
• By output: Small portable, medium-capacity, and high-capacity stationary machines are selected according to required throughput.

B. Thresher: working

A thresher processes crop through feeding, threshing, separation, cleaning, and discharge in a continuous sequence.

• Feeding: Crop enters uniformly through the feed chute; uneven feeding causes overload, blockage, poor threshing, and fluctuating power demand.
• Threshing: The cylinder or rotor accelerates the crop against the concave, detaching grain by impact, rubbing, and stripping.
• Cylinder speed: Peripheral speed determines impact intensity and is calculated as:

V = πDN / 60

• V = cylinder peripheral speed, m/s
• D = cylinder diameter, m
• N = cylinder rotational speed, rev/min

• Concave action: Grain and small material pass through concave openings; larger straw continues toward the outlet.
• Separation: Straw walkers, racks, or rotor grates recover grain remaining in the straw.
• Cleaning: A blower and oscillating sieves separate grain from light chaff and undersized impurities.
• Adjustment: Higher cylinder speed or smaller concave clearance improves detachment but can increase grain cracking and straw pulverization.
• Output evaluation: Threshing efficiency compares grain actually detached with the total grain entering the machine.

Threshing efficiency (%) =
[threshed grain / (threshed grain + unthreshed grain)] × 100

C. Thresher: components

Each thresher component performs a distinct function in crop movement, grain detachment, separation, or power transmission.

• Main frame: Supports the cylinder, concave, blower, sieves, bearings, and drive system while resisting vibration.
• Feed chute or hopper: Guides crop safely and uniformly into the threshing chamber.
• Threshing cylinder or rotor: Carries spikes, rasp bars, loops, or hammers and supplies the principal threshing action.
• Concave: A curved perforated grate below the cylinder; its clearance and opening size influence threshing severity and grain passage.
• Straw separation unit: Straw walkers, oscillating racks, or rotor grates recover loose grain before straw discharge.
• Cleaning system: The fan produces an air stream, while upper and lower sieves grade material according to size and aerodynamic behaviour.
• Outlets: Separate outlets discharge clean grain, chaff, and straw; some machines use a bagging attachment or grain elevator.
• Power transmission: PTO shaft, pulleys, belts, chains, gears, and bearings transmit and regulate speed.
• Safety devices: Belt guards, PTO shields, feed-chute extensions, and emergency stopping arrangements reduce entanglement risk.

IV. Combine Harvester — Integrated Field Harvesting

A. Combine harvester and its working

A combine harvester combines reaping, threshing, separation, cleaning, and grain collection in a single mobile machine.

• Header operation: Crop dividers separate the standing swath; the reel guides stems toward the reciprocating cutter bar, and the auger conveys cut material centrally.
• Feeding: The feeder house uses chains and slats to carry crop from the header to the threshing unit.
• Threshing: A transverse cylinder–concave system or longitudinal axial-flow rotor detaches grain through impact and rubbing.
• Separation: Straw walkers or rotor grates separate residual grain from straw, which leaves through the rear.
• Cleaning shoe: A fan directs air through oscillating sieves; chaff is blown away while clean grain falls into the grain auger.
• Grain handling: An elevator carries clean grain to the grain tank, from which an unloading auger transfers it to a trailer.
• Returns system: Unthreshed heads and large tailings are routed back for reprocessing.
• Propulsion and control: Self-propelled combines integrate engine, transmission, hydraulics, steering, and operator controls; tractor-drawn combines receive traction or PTO power from a tractor.
• Critical adjustments: Header height, reel speed, forward speed, cylinder speed, concave clearance, fan speed, and sieve opening must suit crop moisture and yield.
• Loss diagnosis: Grain ahead of the combine is pre-harvest loss; grain behind the header indicates header loss; grain in straw or chaff indicates separation or cleaning loss.
• Limitations: High capital cost, skilled adjustment, fragmented fields, wet soils, and unsuitable crop conditions can reduce economic and field performance.

V. Farm Power and Machinery Calculations

A. Numerical problems based on farm power and machinery

Machinery calculations relate width, speed, time, draft, power, fuel, and efficiency to field performance.

• Theoretical field capacity:

TFC = W × S / 10

• TFC = theoretical field capacity, ha/h
• W = rated machine width, m
• S = travel speed, km/h

• Effective field capacity and efficiency:

EFC = A / T
FE = (EFC / TFC) × 100

• EFC = effective field capacity, ha/h
• A = area completed, ha
• T = total field time, h
• FE = field efficiency, %

• Draft and drawbar power:

Draft = unit draft × width
DBP = Draft × v

• Draft = horizontal pull, kN
• unit draft = pull per unit width, kN/m
• v = forward speed, m/s
• DBP = drawbar power, kW, because 1 kN·m/s = 1 kW

• Worked example: A 3 m combine travels at 5 km/h, operates at 75% field efficiency, and harvests 18 ha.

TFC = (3 × 5) / 10 = 1.50 ha/h
EFC = 1.50 × 0.75 = 1.125 ha/h
Time = 18 / 1.125 = 16 h

• Fuel estimation: If fuel consumption is 12 L/h, the same operation requires:

Fuel used = 12 × 16 = 192 L

• Interpretation: Actual capacity is lower than theoretical capacity because of turning, unloading, adjustment, overlap, blockage, and other non-productive time.