Unit 6: Harvesting and threshing machines - Subjective Questions

Harvesting is the process of gathering a mature crop from the field at the proper stage and preparing it for subsequent handling, threshing, cleaning, storage, or processing.

The fundamental operations are:

• Cutting: The crop stems are severed close to the ground using a cutter bar, knife, or rotary mechanism.
• Gathering: The standing or lodged crop is collected and guided toward the conveying mechanism.
• Conveying: The cut crop is transported to the threshing or discharge unit using belts, augers, or elevators.
• Threshing: Grain is detached from ears, pods, or panicles by impact, rubbing, or stripping action.
• Separation: Threshed grain is separated from straw and other coarse materials.
• Cleaning: Chaff, dust, and light impurities are removed using sieves and an air blast.
• Collection and unloading: Clean grain is collected in bags or a grain tank and subsequently unloaded.

Timely harvesting minimizes shattering, lodging, weather, and quality losses.

Harvesting machines may be classified according to the operations they perform:

• Mower: Cuts forage or grass and leaves it in the field, usually in a swath.
• Reaper: Cuts standing cereal crops and places them in a continuous windrow.
• Reaper-binder: Cuts the crop, forms bundles, and ties them with twine.
• Windrower or swather: Cuts and lays the crop in uniform rows for drying before threshing or combining.
• Forage harvester: Cuts and chops fodder crops into small pieces for feeding or silage preparation.
• Root and tuber harvester: Lifts crops such as potato, groundnut, or sugar beet and separates soil from the produce.
• Thresher: Detaches grain from harvested crop and performs separation and cleaning.
• Combine harvester: Combines cutting, feeding, threshing, separation, cleaning, grain collection, and residue discharge in one machine.

Selection depends on crop type, field condition, farm size, available power, and the desired level of mechanization.

The correct harvesting time and method are determined by the following factors:

• Stage of maturity: Grain should have reached physiological maturity with suitable moisture content.
• Grain moisture: Excess moisture increases drying cost and threshing difficulty, while very low moisture increases shattering and cracking.
• Crop condition: Crop height, density, lodging, weed infestation, and uniformity affect machine selection and adjustment.
• Weather conditions: Rain, wind, dew, and high humidity may delay harvesting or increase losses.
• End use of produce: Seed crops require gentler handling than grain intended for processing.
• Availability of machines and labour: Harvesting capacity must match the area and available harvesting period.
• Field conditions: Field slope, soil strength, plot size, and obstacles influence mobility and machine performance.
• Expected losses: The selected method should minimize pre-harvest, header, threshing, separation, and cleaning losses.

Therefore, harvesting should begin when crop maturity, moisture content, field conditions, and machine availability collectively permit minimum loss and acceptable grain quality.

Additional differences include:

• A mower has the simplest construction and lowest power requirement.
• A reaper includes crop dividers, a reel, a cutter bar, and a conveying arrangement.
• A reaper-binder additionally uses a knotting and bundle-forming mechanism.
• A combine is the most complex machine and contains a header, threshing cylinder or rotor, separator, cleaning shoe, grain tank, and unloading system.

Threshing is the mechanical process of detaching grain or seed from ears, pods, cobs, or panicles by impact, rubbing, stripping, or a combination of these actions.

Classification based on threshing mechanism:

• Spike-tooth thresher: Uses projecting spikes on the cylinder and concave; threshing occurs mainly by impact and combing.
• Rasp-bar thresher: Uses corrugated rasp bars; threshing takes place by rubbing and impact.
• Wire-loop thresher: Uses wire loops mounted on the cylinder and is commonly suitable for paddy.
• Hammer-mill thresher: Uses swinging hammers and produces intensive impact and chopping.
• Peg-tooth thresher: Uses pegs on the drum and concave to detach grain.

Classification based on crop flow:

• Tangential-flow thresher: Crop moves tangentially across a portion of the cylinder circumference.
• Axial-flow thresher: Crop travels helically along the axis of the rotating drum or rotor.

Threshers may also be classified as manual, animal-powered, tractor-operated, or engine-operated according to their power source.

A power-operated thresher generally consists of the following components:

• Feeding chute or conveyor: Receives the crop and supplies it uniformly to the threshing unit while keeping the operator away from moving parts.
• Threshing cylinder or drum: Carries spikes, pegs, rasp bars, or wire loops and produces impact and rubbing action.
• Concave: A perforated curved surface around part of the cylinder; it provides resistance and allows detached grain to pass through.
• Blower or aspirator: Produces an air stream to remove chaff, dust, and light impurities.
• Sieves: Separate clean grain from broken straw and impurities according to size.
• Straw rack or separator: Recovers grain remaining in the straw and conveys straw toward the outlet.
• Grain outlet: Delivers cleaned grain for collection.
• Straw outlet: Discharges straw after threshing and separation.
• Frame and wheels: Support the components and facilitate transportation.
• Power transmission system: Includes pulleys, belts, chains, shafts, bearings, and guards to transmit power at the required speed.

Correct coordination of these parts determines output, grain cleanliness, and threshing loss.

The working of a spike-tooth thresher involves the following sequence:

1. The harvested crop is fed uniformly through the feeding chute.
2. The rotating cylinder carries rows of projecting spikes that strike and comb the crop against the stationary concave.
3. Impact, rubbing, and combing detach grain from the earheads or panicles.
4. Grain, chaff, and small pieces of straw pass through the concave openings.
5. Larger straw particles move toward the straw outlet due to cylinder action and airflow.
6. The mixture passing through the concave falls onto oscillating sieves.
7. The blower sends an air stream through the falling material, carrying away light chaff and dust.
8. The sieves retain oversized impurities while clean grain passes to the grain outlet.

The principal adjustments are cylinder speed, cylinder-concave clearance, feed rate, sieve opening, and air velocity. Excessive cylinder speed or insufficient clearance may crack grain, whereas low speed or excessive clearance may cause unthreshed grain loss.

An axial-flow machine offers prolonged threshing and separation, but its performance depends strongly on rotor speed, concave clearance, crop moisture, and feed uniformity.

Important thresher adjustments include:

• Cylinder or drum speed: It must be high enough to detach grain. Excessive speed causes grain cracking and excessive straw breakage; low speed causes unthreshed grain.
• Cylinder-concave clearance: A small clearance increases threshing intensity but may damage grain. A large clearance reduces grain damage but may leave grain attached to earheads.
• Feed rate: Uniform feeding maintains stable performance. Overfeeding causes choking, belt slip, and separation loss, while underfeeding reduces capacity.
• Blower speed: Excessive airflow carries sound grain out with chaff. Insufficient airflow produces dirty grain.
• Sieve opening and inclination: Large openings allow impurities to enter the grain, whereas small openings may restrict grain flow and increase loss.
• Belt tension: Loose belts slip and lower cylinder speed; excessive tension overloads shafts and bearings.

Adjustments should be made gradually after examining grain damage, unthreshed material, cleanliness, and outlet losses.

Threshing efficiency indicates the proportion of total grain successfully detached from the crop:

where is the mass of threshed grain and is the mass of unthreshed grain.

Cleaning efficiency is the percentage of clean grain in the total material collected at the grain outlet:

where is the mass of clean grain and is the mass of impurities in the grain sample.

Grain damage percentage is:

where is the mass of visibly cracked or broken grain and is the total mass of the grain sample.

Evaluation is carried out by collecting representative samples from the grain, straw, and chaff outlets, separating each fraction, weighing it, and applying the equations. A good thresher should provide high threshing and cleaning efficiencies with minimum grain damage and loss.

Major sources of grain loss are:

• Pre-harvest loss: Natural shattering, lodging, bird damage, and weather damage before machine operation.
• Cutter-bar loss: Uncut stalks and dropped heads caused by incorrect cutting height or poor knife condition.
• Reel loss: Grain shattered by excessive reel speed or improper reel position.
• Threshing loss: Grain remaining attached to earheads because of low cylinder speed, wide concave clearance, or overfeeding.
• Separation loss: Free grain discharged with straw due to high feed rate or inadequate separation.
• Cleaning loss: Sound grain blown out with chaff because of excessive fan speed or improper sieve adjustment.
• Handling loss: Leakage from elevators, augers, joints, grain tanks, or bags.

Losses can be reduced by harvesting at suitable moisture content, maintaining sharp cutter sections, matching reel speed with forward speed, feeding uniformly, adjusting cylinder speed and concave clearance, setting sieves and airflow correctly, sealing leakage points, and regularly checking material discharged from each outlet.

The main components of a self-propelled combine harvester are:

• Crop dividers: Separate the standing crop strip entering the header from the remaining crop.
• Reel: Guides standing or lodged crop toward the cutter bar.
• Cutter bar: Cuts the crop stems at the selected height.
• Platform auger: Consolidates cut material and delivers it to the feeder house.
• Feeder house: Conveys crop uniformly to the threshing mechanism.
• Threshing cylinder and concave or axial rotor: Detaches grain through impact and rubbing.
• Beater and straw walkers or separator rotor: Separate remaining grain from straw.
• Cleaning shoe: Uses chaffer, sieves, and airflow to separate grain from chaff.
• Grain elevator: Transfers clean grain to the grain tank.
• Return elevator: Returns unthreshed material for reprocessing.
• Grain tank and unloading auger: Store and unload clean grain.
• Residue management unit: Windrows or chops and spreads straw and chaff.
• Engine, transmission, and controls: Provide propulsion, operating power, adjustment, and monitoring.

A combine harvester performs several operations continuously:

1. Dividing and gathering: Crop dividers separate the swath, while the reel guides crop toward the header.
2. Cutting: The reciprocating cutter bar severs the stems.
3. Conveying: The platform auger moves cut crop to the feeder house, which delivers it to the threshing unit.
4. Threshing: The cylinder-concave or axial rotor system detaches grain by impact and rubbing.
5. Primary separation: Most grain passes through the concave with chaff and small straw pieces.
6. Secondary separation: Straw walkers or a rotary separator recover grain remaining in the straw.
7. Cleaning: The chaffer and sieves, assisted by fan airflow, remove chaff and impurities.
8. Recirculation: Incompletely threshed material is returned through the tailings elevator.
9. Collection: Clean grain is elevated into the grain tank.
10. Residue discharge: Straw is windrowed or chopped and spread over the field.
11. Unloading: An unloading auger transfers grain from the tank to a trailer or transport vehicle.

Continuous monitoring and adjustment are required to minimize header, threshing, separation, and cleaning losses.

Efficient combine operation requires coordinated adjustments:

• Reel speed: Reel peripheral speed is generally set slightly higher than forward speed. Excessive speed causes shattering, while low speed gives poor crop feeding.
• Reel position: It should contact the crop near the center of gravity. For lodged crop, the reel is moved forward and lowered carefully.
• Cutter-bar height: It should be high enough to reduce unnecessary straw intake but low enough to collect all earheads or pods.
• Cylinder or rotor speed: It is increased for difficult threshing and reduced for fragile or dry grain to prevent cracking.
• Concave clearance: It is reduced when threshing is incomplete and increased when grain damage or excessive straw breakage occurs.
• Fan speed: It should remove light chaff without blowing sound grain from the cleaning shoe.
• Sieve openings: The chaffer controls coarse material, while the lower sieve controls final grain cleanliness.

Only one adjustment should normally be changed at a time, followed by inspection of the grain tank sample and material discharged behind the combine.

Important safety precautions include:

• Fit guards over belts, pulleys, chains, gears, shafts, and power take-off components.
• Use a feeding chute of adequate length and never push crop into a thresher with hands or feet.
• Avoid loose clothing, scarves, jewellery, or uncovered long hair near moving parts.
• Stop the engine, disengage the power take-off, remove the key, and wait for all parts to stop before removing blockages.
• Keep children and unauthorized persons away from the machine.
• Maintain safe distance from straw, chaff, and grain discharge outlets.
• Operate the machine on stable ground and apply parking brakes before stationary work.
• Keep a suitable fire extinguisher nearby because dry crop residue and hot bearings create fire risk.
• Clean dust and straw from the engine and exhaust system regularly.
• Use hearing, eye, respiratory, and foot protection where required.
• Never allow riders on the combine except in a manufacturer-provided seat.
• Inspect guards, brakes, steering, lights, bearings, and emergency controls before operation.

Given:

• Width,
• Speed,
• Field efficiency,
• Area,

Theoretical field capacity is:

Effective field capacity is:

Time required is:

Therefore:

• Theoretical field capacity:
• Effective field capacity: approximately
• Time required: approximately

Given:

• Area harvested,
• Time,
• Width,
• Speed,

Effective field capacity is:

Theoretical field capacity is:

Field efficiency is:

Therefore:

• Effective field capacity:
• Field efficiency: approximately

Given:

• Draft,
• Speed,
• Transmission efficiency,

Convert speed to metres per second:

Drawbar power is:

Alternatively, when draft is in kilonewtons and speed is in kilometres per hour:

Engine power required is:

Therefore:

• Drawbar power required:
• Engine power required: approximately

Given:

• Torque,
• Rotational speed,

Angular velocity is:

Power is:

Substituting the values:

Converting to kilowatts:

Therefore, the power transmitted by the power take-off is approximately .

This is the mechanical power available at the shaft; the useful power at the threshing cylinder will be slightly lower because of belt, bearing, and transmission losses.

1. Threshing efficiency

Given and :

2. Cleaning efficiency

Given and :

3. Grain damage percentage

Given damaged grain and sample mass :

Therefore:

• Threshing efficiency:
• Cleaning efficiency:
• Grain damage: