How does a combine harvester work?
The genious mechanism of corn ear separation 
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Combine harvesters are the miracle machines of mechanical engineering. You may observe a magical technology in this corn combine harvester. The combine allows only the corn ears inside the machine and discards the remaining plant.
If we open the cover of this corn header, we can see a genius mechanism. As the combine moves forward the header cones help the plants to reach in the region of the mechanism.
We can see a pair of spinning square bars as shown. Can you tell what they will do to the plant? Initially the gap between the squares is wide. As they rotate, the gap becomes small and the plant is pushed down.
After a small break, one more pushing down. However, after a few cycles the plant is stuck. The corn ear is not able to pass through the narrow gap of deck plates. However, the square bars are still trying to push the plant down. This means eventually the corn ears should get removed from the plant stalk as shown in above animation.
Once the corn ears are removed guiding chains come into action. They will make sure that the corn ears will reach the auger. The cutting blades below the mechanism will make sure that the whole operation is much easier. The augers make sure that the corn ears collected from a large area get concentrated toward the center of the machine.
The beauty of a combine harvester is that, with a different header, the same machine can harvest any crop. For more details, please check our article How does a combine harvester save time and labor in modern farming?
How do combine harvesters cut crops?
As we know we can use the same combine harvester machine to do the harvesting but with a different header. In the case of wheat, the combine cuts the whole crop and sends it forward for threshing.
Is the machine cutting like a human holding the plant stationary and slicing? No. Machine uses a smarter method. I will demonstrate with one plant. In my experiment I supported the plant and cut it quickly. wow, what a perfect cut!
The high speed oscillating blades perform the same cutting action. Look at the hidden supporters of the cutter head below. Because of the high speed and support, the plant cannot flex and is cut cleanly.
Flexibility of the combine harvester
What if the ground is irregular? Don’t worry, modern combine cutter heads are flexible as shown. After the cut, the reel pushes the crops into the auger. In fact, even before starting the cut the reel gently pushes the wheat crop toward the machine, holding it steady for the cutting action.
The augers collect the crops towards the centre of the combine. From the auger, the crops reach a feeding conveyor, a chain with cast-iron slats.
The real hero of a combine - thresher
The feeding conveyor makes sure that the crops reach inside the machine to meet the most important part of the combine harvester - the thresher. At one end, the thresher is fitted with a spiral vane. This allows for the smooth entry of the straw. The threshing cylinder is fitted eccentric to this concave grid plate.
This means when the cylinder rotates, the straw gets threshed between the cylinder and the rasp bars. But wait! What if the straw gets trapped between these rasp bars and escapes moving spirally?
The engineers are tricky. They provided vanes in the direction opposite to the spiral of the rasp bars so the escape motion we discussed is not possible.
The crop has to get crushed under the threshing cylinder. This is an axial type threshing unit. From the main threshing region, the majority of the grain gets separated and falls down. Let me explain how the grain removal process is completed.
The physics of grain removal
1.1 Impact threshing
The physics of the thresher is exciting. The first mechanism is impact threshing. Can we run the thresher at maximum speed for optimal threshing? No, this would lead to crop damage. There is a critical velocity for the thresher, beyond that, we cannot increase the speed.
1.2 Rubbing threshing
The next mechanism is rubbing threshing. The rasp bars rub the grain ear against the concave grates. Sometimes two or more grains can undergo the rubbing action. The only goal here is to knock the kernels out of the seed head.
1.3 Grinding threshing
The new mechanism is called grinding threshing, this causes severe crop damage because the thresher applies a normal force to the grains. The grains undergo a compressive force, and the chance of them getting damaged is high. Engineers always try to avoid such situations.
Rubbing clearance
Tighter clearance increases rubbing and impact, which is needed for tough to thresh crops to dislodge all kernels from the head. The risk is that a gap that's too tight will lead to grinding threshing and damage the grain kernels.
A wider gap is gentler on the crop. It's used for delicate, easy to thresh crops like dry beans, soybeans or corn, where you just need to shell the kernels off the cob. Too wide gap is also risky; it may lead to unthreshed crop.
The real question is how does the combine know about all these settings? The modern combine’s internal cameras and sensors monitor the grain quality in real time. If the loss sensors detect too much grain going out the back, the system will automatically tighten the concave clearance to thresh more aggressively. For more details, please check our article - Combine harvesters technology and control systems.
Separation region
Along with flowing straw, some grains are separated but trapped inside the straw. This region of the thresher is called the separation region, the second stage of grain removal. Here, due to the centrifugal force effect, the grains get separated from the straw and fall down. The grains coming out of the thresher and separator region are obviously not pure. They are full of short material, and sometimes the grains have unremoved husks. Let me explain how the grain cleaning process is implemented.
Straw walkers
Grain and fine material falling through the concave are collected by augers and pushed forward to a series of straw walkers—long, oscillating trays with holes. The straw walkers move back and forth and up and down to separate material from the separation area.
Grain transfer mechanism
The oscillating motion makes it difficult for the short material to pass through the holes, especially when air flows over it. However, the grains can easily pass through. The high speed air is supplied by a blower.
Grain collection
The airflow has one more crucial job. Consider two grains falling down, one is perfectly separated from the chaff, and the other still has some chaff on it. When the air blows, which one will move farther away? Obviously, the second one. This way, the clean grains are collected at one collection pan, and the impure grains get collected in another. What to do with these impure grains? Simple trick: re-thresh them. The grains that are clean are pushed by the auger at the bottom and collected in one area.
Grain Unloading
The clean grains finally reach the storage tank. The grain tank is fitted on the top of the combine. The grain tank is equipped with sensors monitoring the grain level. When the tank is full or near-full, an alert is sent to the operator's cab so they know when it's time to unload.
An automatic way the combine unloads the grain when the tank is full. This is how modern combine harvester technology efficiently saperate the grains crops. I hope you understand the working mechanism of the modern combine harvester. Thanks for reading.
