Grain Harvesting

Grain Harvesting

Purpose: To recover grains from then field and separate them from the rest of the crop material in a timely manner with minimum grain loss while maintaining the highest quality of grain.

Major grain crops include; Barley, edible beans, maize, rice, sorghum, wheat.

The harvesting method and the choice of equipment will depend on;

·  Type of grain crop

·  Planting method

·  Climatic conditions

Harvesting requiring human or animal power

Operation will entail;

·  Cutting of the crop to a central location

·  Threshing of crop to detach grains

·  Separation of grains from the rest of the crop material

Grain Harvesting Operations

·  Cutting

·  Threshing

·  Separation

·  Cleaning

·  Handling

These operations can be carried out;

(i) By different machines

(ii) Combined in a machine

Direct Harvesting

·  All functions are performed by machine called the combine harvester

·  Combines may be conventional or rotary depending on the threshing and separation employed

·  Combine – self-propelled or pulled by tractor or powered by PTO drive

Combine Harvester

A combine harvester is a complex harvester is complex harvesting machine, which performs varied functions in single operations.

The following are the functions;

Cutting

·  the harvester cuts the standing crop, snaps ears of maize or pick up windrows. Windrows are rows of cut crops that are put for the combine to pick up and process.

Conveying and feeding

·  this involves the transportation and the passing of the crop into the machine.

Threshing

·  this is the removal of grain from the heads, pods or ears.

Separation.

·  this implies the separation of the grain from the straw, husks or stalk pieces.

Cleaning

·  the chaff and debris are removed from the grain.

Handling

·  the grain is finally put into grain tank.

Combine Harvester parts and function identification

The combine harvester can be divided into 4 main sections namely cutting and feeding, threshing, separating and cleaning

Refer to the diagram provided.

Cutting and Feeding

Combine Header

The reel(1) which rotates at predetermined speed pushes or picks up the uncut stalks against the cutterbar(2) and the material is delivered onto the header platform by the platform auger. The cutterbar performs the cutting action. The header consists of the reel, cutterbar and dividers The dividers separate the crop so that there is no overloading of the cutting mechanism. The reel speed can be adjusted for proper harvesting.

Floating cutterbar(2)

This performs the cutting process. It is suspended beneath the header. It flexes to follow the ground surface. This ensures that the crop is cut at a particular set height and reduces the risk of missing some heads.

Feeding auger(3) /Header cross conveyor

This is an auger which delivers the material to the feeder conveyor.

Feeding conveyor(4)

This moves the material upwards to the cylinder and concave assembly for threshing.

Factors affecting Header Losses

(i)  Cutting height

(ii)  Reel position with respect to the cutter bar

(iii)  Reel speed with respect to the forward ground speed

Threshing

Cylinder(5) and concave(6)

These constitute what the threshing section of the combine harvester. The cylinder and concave performs the threshing action. The cylinder rotates at pre-set speed while the concave is stationary. The crop passes through a clearance between the two components. The clearance can be adjusted for the type and condition. The cylinder can be rasp bar or spike tooth. The concave can be grated or non-grated. This means the concave may have openings through which the grain can pass or it may be closed. If grated, the separation of the grain and chaff occurs at this stage. About 80% of the grain is separated at this stage. The separated grain falls into the channels and moves to the front of the combine into the oscillating grain pan. From the grain pan the grain-chaff mixture moves to the rear and then onto the cleaning the shoe.

Cylinder beater(8)

Cylinder beater slows down the speed of the material entering the clearance. Havested material may want to enter the cylinder and concave clearance in quantities and speed which the threshing mechanism may not cope with.

Threshing Mechanism Performance

Measured by;

(i)  Threshing efficiency

(ii)  Separation Efficiency

(iii)  Grain damage

(iv)  Amount of straw break up

Threshing Efficiency

The % of the grains thrashed as calculated on the basis of the total grains entering the threshing mechanism.

Separation Efficiency (of the threshing cylinder)

The % of the grains separated at the concave (conveyor) or at threshing part of the rotary combine as compared to the total grains in the crop entering the threshing mechanism.

Grain Damage

Mechanical damage- includes grain with broken kernels, kernels with skin damage, kernels with the damage resulting in poor germination, poor storability, poor processing characteristics of the grain.

Methods of measuring grain damage include;

(i)  visual inspection of the sample of grain

(ii)  sieving through the standard sieve

(iii)  germination rate

Excessive straw break-up

(i)  increases with the load in the cleaning

(ii)  increases with cleaning losses

(iii)  result in increase in power requirements in the threshing cylinder

Factors Affecting threshing Performance Parameters

1.  Design- cylinder diameter, concave and the number of rasps

2.  Operation- cylinder speed, material feed rate

3.  Crop condition- moisture content, crop maturity, crop type

Separation

Check flaps(11)

These are suspended above the strawwalkers. They regulate the amount of material flowing and maintain a layer of uniform thickness of the material to be separated.

Straw walkers/Straw pack (10)

The move in a cascading fashion and they assist in separating grain. Unthreshed heads that fall through the walker openings are collected by the grain return parts. The parts return the unthreshed heads for rethreshing by cylinder and concave assembly.

Separation Performance

Measured by

(i)  Walker efficiency (as % of grain loss)

(ii)  Walker capacity (t/hr)

Walker Efficiency – this is found by dividing the amount of grains separated by the amount of grains entering the separator.

Separation loss- amount of grain still in the straw as it leaves the combine

Factors Affecting Separation Performance Parameters

1.  Design factors

·  Walker length

·  Grain Throw and speed

2.Operating Parameters

·  Material feed rate

·  Walker slope

2.  Crop conditions

·  Grain to material other grain (mog) ratio

·  Physical and Mechanical properties of the crop

Grain cleaning

Cleaning shoe

This consist of the chaffer(top) sieve and the sieve(15) and the blowing fan(16). These do the cleaning of the grain .An air blast directed upwards through the bottom of the sieve towards the rear of the combine aid in the separation of threshed and unthreshed seed. The chaffer openings are adjustable. The unthreshed heads are called tailings that are too heavy to be blown ride over the chaffer sieve and are collected by the tailing auger(19). The tailings are taken back to the cylinder and concave assembly for rethreshing. A clean grain auger(18) receives grain from the sieve and delivers it to the grain tank.

Cleaning Performance

Measured by ;

(i)  Grain loss or cleaning efficiency

(ii)  Cleaner capacity

(iii)  Grain dockage

Grain loss- calculated by determining the % of lost grain on the basis of total grain entering the cleaning shoe

Cleaning Efficiency- % of grain recovered by the shoe.

Cleaner capacity- determined by first plotting a curve of grain loss against material other grain (mog) feed rate passing through the cleaning shoe

Grain Dockage - mount of chaff that is separated with grain. It is determined by taking a sample of grain from the grain tank and sieving the grain to determine the % of chaff in the sample,