Threshing and Shelling Questions and Answers

Answer: C

Correct answer: Closing the concave clearance

Closing the concave squeezes the mat harder against the grate, so more grain is crushed. It is one of the two levers that raise damage, not reduce it.

The other three all help: opening the clearance works the mat less severely, fitting a wire-loop cylinder replaces impact with gentle rubbing, and reducing cylinder speed cuts the impact energy, which falls with the square of the speed. Each buys lower damage at the price of some unthreshed grain.

Answer: D

Correct answer: It accounts for grain broken inside the machine

This is the false statement. Threshing efficiency measures only detachment - what proportion of the grain left the head. A machine can score 99.7 per cent while cracking six per cent of the sample.

The other three are true: it does measure the proportion detached, and it rises both as the concave is closed and as cylinder speed increases. Breakage is a separate figure precisely because the two move in opposite directions, and quoting one without the other is misleading.

Answer: A

Correct answer: It handles damp and tough crops better

Damp and tough crops are the spike-tooth's territory, not the rasp-bar's. Impact is what gets grain out of material that rubbing alone will not free, which is why spike-tooth cylinders remain in use despite their drawbacks.

The other three are genuine advantages of the rasp-bar: it breaks less straw, cracks less grain, and consequently loads the cleaning shoe far less heavily, since it is broken straw that arrives at the shoe and has to be sieved out.

Answer: B

Correct answer: Misdirected: this is a separation problem

Grain left in the straw was detached from the head - it simply failed to shake free on the walkers. That is a separation loss, and raising cylinder speed does nothing about it while cracking more of the grain that is being recovered.

The right responses are a lower feed rate to thin the mat, or attention to the walkers themselves. Distinguishing unthreshed loss from separation loss is exactly what tells the operator which end of the machine to adjust, which is why they are measured separately.

Answer: C

Correct answer: Axial-flow machines generally show higher values than tangential ones

An axial-flow machine carries the mat along the whole length of the rotor over many passes, and keeping it moving costs power - typically around 8 kWh per tonne against 6.5 for a tangential machine.

What that buys is throughput and thorough threshing, which is the right trade on a large combine. Specific energy also depends strongly on the crop, its moisture and its straw-to-grain ratio, and it is quoted per tonne of grain rather than per hour so that machines can be compared.

Answer: D

Correct answer: A fall in unthreshed loss

Overloading raises unthreshed loss; it does not reduce it. A thick mat means grain in the middle never reaches the concave, so more leaves still attached to the head.

The other three are all expected: separation loss rises as the same thick mat holds grain in the straw over the walkers, and total loss rises as the sum of the two. Both symptoms from one cause is what makes a simultaneous rise diagnostic of overloading rather than of a setting error.

Answer: A

Correct answer: Very dry kernels shell with the least damage

This is the false statement, and it inverts the position. Very dry kernels are brittle and crack under the shearing action - damage runs to about 6 per cent at 12 per cent moisture against 2.4 per cent at 16.

The other three are true: wet kernels cling to the cob, shelling percentage falls at high moisture, and there is a window around 16 to 18 per cent where shelling is nearly complete and damage still low. That window is why maize is dried to it and not past it.

Answer: B

Correct answer: A cylinder speed set too low

A low cylinder speed leaves grain unthreshed on the heads - a quantity loss. It does not make the sample dirty; if anything it puts less broken straw into the stream, since less work is being done on the crop.

The other three all produce a dirty sample: a spike-tooth cylinder shatters straw into fragments that reach the shoe, a sieve opening set too wide lets chaff through with the grain, and a fan speed set too low fails to lift the light material away.

Answer: C

Correct answer: A tighter beater cracks more and breaks more

The beater clearance is the decorticator's principal adjustment, and it behaves exactly as a thresher's concave does: tightening it cracks more shells - fewer pods pass through whole - but breaks more kernels in the process.

Since broken groundnut kernels are worth appreciably less than whole ones, the setting is a direct trade of one loss against another. The machine does not grade by size, does not work on terminal velocity, and does not detach pods from the plant, which is a separate harvesting operation.

Answer: D

Correct answer: Losses climb steeply once the mat becomes too thick to work

A thresher has no natural capacity limit - it will accept more crop almost indefinitely, simply delivering less and less of the grain. There is no feed rate at which it stalls or refuses material.

What does happen is that losses rise slowly and then steeply as the mat thickens beyond what the cylinder and walkers can work. Capacity must therefore be defined at a chosen loss figure, which is why a capacity quoted without its accompanying loss percentage tells you nothing at all.