Threshing and Shelling Questions and Answers

Answer: C

Correct answer: Only three

Three pairs are right. A thresher separates grain from the head and straw, a maize sheller separates kernels from the cob, and a groundnut decorticator separates kernels from the shell.

The fourth is wrong. Straw walkers separate grain from straw, shaking the straw so that trapped grain falls through. Separating grain from chaff is the cleaning shoe's work, using sieves and an air blast - a different mixture handled at a different stage.

Answer: D

Correct answer: Only three

Three pairs are right. Threshing efficiency is the proportion of grain detached, cleaning efficiency the proportion of clean grain at the outlet, and specific energy the energy consumed per tonne of grain.

The fourth is wrong. Capacity is the throughput of the machine, in tonnes per hour. Cylinder speed in metres per second is a setting, not a measure of output, and two machines running at the same peripheral speed may have quite different capacities.

Answer: A

Correct answer: Winnowing

Winnowing is a cleaning action, not a threshing one. It uses an air stream to carry away chaff and light material after the grain has already been detached, and in a thresher it is the cleaning shoe's business.

The other three are the actions by which grain is separated from the head: impact, as a spike-tooth cylinder strikes the crop; rubbing, as a rasp-bar drags the mat against the concave; and stripping, as remaining grain is pulled from the heads while the mat is carried round.

Answer: B

Correct answer: Opening the concave clearance

Opening the concave gives the mat more room, so it is compressed and rubbed less severely and fewer grains are cracked. It is the standard first move when damage is too high.

The other three all raise damage: a higher cylinder speed increases impact energy with the square of the speed, closing the concave works the mat harder, and very dry grain is brittle and cracks under treatment it would tolerate at a higher moisture.

Answer: C

Correct answer: Cleaning efficiency

Cleaning efficiency is a performance measure - the proportion of the delivered stream that is clean grain. A high figure is good, whereas every loss term is something to be minimised.

The other three are genuine losses: unthreshed grain still attached to the head, separation loss leaving with the straw, and blown loss carried out by the cleaning fan. Together with breakage they account for everything that fails to arrive in the grain tank sound and whole.

Answer: D

Correct answer: Cleaning shoe

The cleaning shoe handles grain and chaff. By the time material reaches it the straw has been removed by the walkers, so what remains is grain mixed with chaff, short straw and dust, which sieves and an air blast then separate.

The other three all work on grain and straw together: the cylinder and concave thresh the whole crop mat, and the straw walkers shake the straw to release the grain caught in it. Each stage handles a progressively cleaner mixture.

Answer: A

Correct answer: The grain tank capacity

Grain tank capacity is fixed by the machine's construction and has nothing to do with how well a crop threshes. It determines how often the tank must be emptied, not what happens in the cylinder.

The other three are the operator's working adjustments: cylinder speed and concave clearance together set how hard the crop is worked, and feed rate determines the thickness of the mat. Between them they account for almost every threshing problem that can be corrected in the field.

Answer: B

A clean sample together with grain in the chaff is the signature of an over-strong blast. Slowing the fan keeps the grain in the machine, at the cost of a slightly dirtier sample that the sieve setting can then address.

Answer: C

Correct answer: Reduce speed and open the concave

Failing germination means the embryo is being damaged, and both principal adjustments should be backed off together: a lower cylinder speed reduces the impact energy, which falls with the square of the speed, and a wider concave works the mat less severely.

Some grain will be left unthreshed as a result, and that is the right trade for seed - unthreshed grain can be recovered by a second pass, whereas a damaged embryo cannot be repaired. Closing the concave or raising speed would make matters considerably worse.

Answer: D

Correct answer: The mat has become too thick for grain to reach the concave or shake free

Both losses rising together is the signature of overloading. A thick mat means grain in its middle never reaches the concave to fall through, so unthreshed loss rises, and the same thick mat on the walkers holds grain in the straw, so separation loss rises too.

Nothing was adjusted except the feed rate, so the cylinder speed and concave clearance are unchanged. This is why capacity claims must always be read alongside loss figures - a machine can be pushed to any throughput if the grain recovered is not counted.