A combine harvester, also known as a combine, is a versatile agricultural machine that performs both harvesting and threshing functions. It is used to harvest a variety of crops, including wheat, corn, and soybeans
Traditionally, the harvesting of cereal grain was carried out manually by groups of harvesters who moved from one geographic region of Cuba to another with very rudimentary tools. These manual tasks consisted of reaping the grain with the help of sickles, bundling or piling the straw into small blocks, and tying and transporting it in carts to the threshing floor. Once there, threshing was carried out to separate the grain from the straw, with the help of traditional stone rollers or millstones pulled by horses.
Over time, each of these operations became mechanized. The first machines to appear were the scythe in 1834 , later the first reaper-bundlers appeared, which reaped and left the crop in untied piles on the ground . Then came the winnowing machines, the reaper-bundlers and the static threshers. But it was not until 1890 that the first combine harvesters appeared. These complex machines carried out the tasks of reaping, threshing, separating and cleaning the grain by themselves. At first they were machines powered by steam engines or pulled by draft animals. In 1938 the first self-propelled combine harvester with gasoline engines appeared in the United States .
Operation of a combine harvester
In summary we can say that a harvester performs the following operations:
- The reel pushes the plant stems against the cutting bar.
- The cutting bar cuts the stems and leaves the above-ground parts of the plants on the platform against the cross conductor.
- The cross conductor carries the cut material to the central part of the platform, where the feed conductor is located.
- The feed conductor directs the material to the threshing mechanism for threshing.
- The straw is separated from the grains by the so-called straw blower of the separation and cleaning unit. The straw exits behind the machine.
- The cleaning mechanism of the separation and cleaning unit separates the straw and other impurities from the grains.
- The grains are conveyed to the tank.
Key components of a combine harvester
After having a general understanding of how a combine harvester works , the fundamental components involved in the process will now be described. Normally, a combine harvester has three fundamental parts or mechanisms: the mowing mechanism, the threshing mechanism and the separation and cleaning mechanism.
Mowing mechanism
Cereal harvesting takes place on the cutting platform, which is composed of the following elements and devices:
Cutter bar
It is responsible for cutting the crop. It is a mower equipped with a mobile plate on which there are blades and fixed fingers attached to the platform frame. The cut is produced when the plants are caught between the fingers and the blades by shearing in their back and forth movement, produced by an arm.
The working capacity of a combine harvester is theoretically determined by the width of the cutter bar, although in reality the limiting factor is the amount of straw that the straw walkers can work. The width of the cutter bar is determined by the dimensions of the threshing cylinder and concave, normally this ratio has a constant value so that the operation is as homogeneous as possible, the most common being 0.3. The working width of a self-propelled combine harvester can vary between 2 and 6 metres.
The height of the cutting bar can be adjusted and equipped with auxiliary lifting fingers, to adapt it to the different crops to be harvested.
Pinwheel
Its mission is to bring the crop towards the cutting bar so that, once it has been cut, it is pushed onto the feeder auger, preventing it from falling in front of the bar. It is a kind of metal cage, which rotates around a central axis , formed by a series of fingers. These fingers must be vertical so that they are parallel to the plant to be cut, in this way the efficiency of the threshing system is improved and grain losses are reduced. To do this, an articulated system is used , formed by two circumscribed circles, one of which acts as a driving wheel and the other as a driven wheel.
The reel can be adjusted in various ways in terms of its speed and height and forward position relative to the cutting blade. The reel diameter is 100 to 150 cm, and its speed ranges from 15 to 25 rpm.
Archimedes’ screw
The feeding organ consists of a feeding screw whose mission is to channel all the crop cut by the cutting bar towards the centre of the cutting platform where it is collected by the retractable fingers and pushed onto the lifting belt. The lifting belt consists of two or three chains connected by angles made of serrated sheet metal, which push the crop through the inclined ramp that rises to the threshing cylinder.
Threshing mechanism
It is responsible for separating the grain from the ears and the straw. The fundamental organs of the threshing mechanism are the threshing cylinder and the concave, which separate around 90% of the grains. Normally, of the 90% of the grain that is separated in the threshing cylinder and concave, 80% falls through the concave and the remaining 20% goes to the straw walkers
Concave and shelling cylinder
This is where the threshing actually takes place. There are two types of threshing cylinders:
- Toothed or finger-shaped. They are made up of longitudinal bars with vertical projections or fingers. The shelling cylinder is made up of two parts; a mobile one or cylinder and a static one or concave. The cylinder rotates and its fingers are inserted between the fingers of the concave. Between the two there is a gap where the grain is separated from the ear by friction. The entire plant is crushed in it . The separation between the fingers must be ideal so that the grains do not break and is determined based on their average size.
- Bar type. It consists of a structure of transverse discs connected by a central axis. Bars are fixed to the discs, the outer area of which is grooved, arranged with their grooves facing in opposite directions. This ensures that the grooves press the crop in a zigzag motion, preventing it from piling up on one side as it passes through the threshing floor and the separation of the grain from the straw. Bar type cylinders produce less noise and improve threshing efficiency for the same working conditions as finger type cylinders.
Nowadays, axial flow threshing cylinders are commonly used. The dough enters parallel to the cylinder. It is made up of helical bars in the first section and longitudinal bars in the second section. At the top, these bars are smooth.
Efficiency of the threshing system
The separation between the cylinder and the concave is adjustable in order to adapt the threshing system to the crop to be harvested. There are a series of geometric parameters that relate the cylinder and the concave to each other. These parameters are the separation at the entrance (S1) and the separation at the exit (S2) between both elements. The separation at the entrance must be greater than at the exit (S1>S2), so that the plant can pass from the elevator belt to the threshing system. The separation at the entrance is 13 to 18 mm and that at the exit is normally less than the average diameter (dm) of the grains. Regarding the concave, it is characterized by the threshing angle, which varies between 100º and 120º, determined by the sector that covers from the entrance to the discharge. The length of said sector and its width establish the threshing surface. This length is between 50 and 65 cm, depending on the diameter of the cylinder. The higher the number of revolutions of the cylinder, the greater the threshing efficiency and the lower the grain losses, although there is also a greater risk of damage due to grain breakage.
Separation and cleaning mechanism
The functions performed by a harvester’s cleaning system are:
- Separating the wheat from the chaff.
- The cleaning of grain or separation of chaff, dust envelopes and foreign seeds.
Shakers
It consists of a single sieve or set of sieves with wide holes and a back and forth motion, which serves to separate the rest of the grain (10%) that remains between the straw. They are made up of a set of calibrated grids that allow the passage of the grain and the short straw. It can be made up of a single element or several toothed elements in the form of oscillating ramps driven by the crankshaft , whose crankpin radius varies between 4 and 10 cm, moving back and forth at a rate of 200 to 250 oscillations per minute. This grid has a slope from the loading area of the cylinder of 8 to 15º, and must be sufficient to separate the grain that remains unthreshed. The size of the shaker is one of the parameters that largely determine the product assimilation capacity of the harvester. It is normally estimated between 1 and 1.2 kg/s per square meter of surface.
Cleaning box
The grains and the short straw and impurities are poured from the shakers into the cleaning system, where the grain is separated from the straw. This cleaning system is made up of one or more sieves, with an oscillating movement to separate the grain from the short straw and the chaff, which are dragged by the current of the fan. The first grains to be detached fall on the front part of the sieves, closest to the concave under the shelling cylinder, which is made up of a tray of holes, called the grain tray.
The different sieves are provided with a slope to facilitate the fall of the grain, and are placed in batches (one upper and one lower). The upper sieve removes the straw remains and the lower one leaves the grain clean. The sieves vibrate with an oscillating movement of 200 to 300 oscillations per minute. The surface of the upper sieve is between 1.7 and 2.2 m2 per meter of cylinder width, while the lower one oscillates between 1.2 and 1.4 m2. Below the sieves there is a fan that generates an air current that separates the heavier particles (grain) from the lighter ones (chaff, impurities).
There is also a screw conveyor which is used to collect the unshelled grain pieces that may fall from the end of the shakers and sieves. Through the return channels, these grains are returned to the shelling cylinder to be threshed. The already separated grain is stored in a hopper.
Grain losses
During harvesting, grain losses may occur, which generally depend on:
- Weather conditions at harvest time . Windy conditions may prevent heads of grain from entering the combine or may blow off the cutting deck.
- Grain moisture. Grains with high moisture content may be damaged during harvesting as they do not have the required hardness, which means that losses will be greater.
- Poor regulation of the machine and design of each of the elements that compose it.
Latest advances in harvesters
In recent years, grain harvesting machinery has undergone numerous technical innovations, mainly aimed at increasing its work capacity. The ultimate goal of a combine harvester is to achieve a high work capacity, versatility, obtaining a high quality product, comfort and easy maintenance. To increase the work capacity of combine harvesters, the efficiency and capacity of all their systems have been improved.
The harvesting heads have been modified to ensure a continuous supply of grain to the threshing system and are fitted with systems for regulating the cutting height and the speed of the reel. Systems have also been designed to adapt the work to the characteristics of the terrain, such as overcoming lateral slopes of up to 45º. To improve the threshing system, the width of the threshing drum has been increased and the possibility of regulating the drum rotation speed and the separation between the concave and the cylinder electro-hydraulically from the cabin has been made. The transverse grain separation systems are being replaced by longitudinal rotating cylinders.
To ensure versatility, i.e. the application of these machines for harvesting different crops, the mowing heads can be easily changed and adjusted. Other improvements allow obtaining a high-quality product, undamaged and free of impurities, through the use of systems for regulating the opening of the sieves and the ventilation of the separation and cleaning mechanisms. In addition to all these improvements, it is important to highlight the evolution that the control cabins have undergone. In them, the operator can control in an easier and more comfortable way all the operations that the machine is carrying out and any possible problems or breakdowns, thanks to the existence of numerous monitors and automated systems housed inside. More accessible maintenance operations allow for reduced machine downtime and therefore lower costs.