Threshing and Shelling Questions and Answers

Answer: C

Correct answer: Wire-loop

The table gives the wire-loop the lowest straw breakage and names paddy and seed crops as its typical use, which is exactly the requirement here: mechanical damage to the embryo destroys germination, and a seed lot that will not germinate is worthless whatever else is true of it.

A spike-tooth cylinder works by impact and would be the worst choice. The rasp-bar and axial rotor are gentler but still work the crop harder than flexible loops do, and some unthreshed grain is an acceptable price here.

Answer: D

Correct answer: 16 per cent

At 12 per cent the kernels are brittle and damage reaches 6.2 per cent; at 26 per cent they cling to the cob and shelling falls to 91.2 per cent, leaving nearly a tenth unshelled.

Sixteen per cent gives 99.1 per cent shelling with only 2.4 per cent damage - nearly all the kernels removed and few of them broken. Twenty per cent halves the damage again but loses 1.5 points of shelling, so 16 is the better balance. This window is why maize is dried to that range before shelling rather than after.

Answer: A

Correct answer: It has the highest output but pays for it in energy per tonne

The axial-flow machine produces 1.8 t/h against 1.1 to 1.3 for the tangential ones, but consumes 8.1 kWh per tonne against about 6.5.

The reason is inherent to the design: keeping the mat travelling the length of the rotor over many passes takes more power than working it once in a short tangential pass. What is bought with that energy is throughput and thorough threshing, which is the right trade on a large combine and the wrong one on a small stationary machine.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains the convention. It is the surface speed that determines the energy of the blow the crop receives, and that depends on the diameter as well as the revolutions.

A 0.6 m cylinder at 700 rev/min and a 0.4 m cylinder at 1050 rev/min both give 22 m/s and thresh alike. Quoting revolutions alone would make settings meaningless between machines, which is why manuals give peripheral speed and leave the operator to convert.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason is decisive. A seed lot is bought for its germination, and mechanical damage to the embryo cannot be repaired - a cracked seed will not grow however sound it looks.

The wire-loop's flexible loops deliver the gentlest action of any cylinder type, so it is chosen despite leaving more grain unthreshed. That unthreshed grain can be recovered by a second pass, which is why the trade is worth making for seed and not for a milling crop.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains why they rise together. A thick mat has the same effect at two different stations: grain in the middle of it cannot reach the concave to fall through, and it cannot shake free of the straw on the walkers.

One cause therefore produces two symptoms. That is diagnostically useful - when both losses climb at once the answer is almost always to reduce the feed rate rather than to adjust the cylinder or the shoe.

Answer: A

Correct answer: A is false but R is true

The assertion is false. Raising fan speed carries a definite penalty: a stronger blast takes grain out with the chaff, so blown loss rises.

The reason is true - the blast does remove more of the light fraction, which is why the sample looks cleaner. The two effects are inseparable, and setting the shoe means choosing a point on that trade rather than escaping it. Judging by the appearance of the sample alone will always push the fan too high.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains the practice. Very dry kernels are brittle, and the shearing action that frees them from the cob cracks them - damage reaches about 6 per cent at 12 per cent moisture against 2.4 per cent at 16.

There is a limit at the other end too: above about 20 per cent the kernels are soft and cling to the cob, so shelling percentage falls away. The window between is where both figures are acceptable, which is why maize is dried to it rather than past it.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the second explains the first. A rasp-bar works by rubbing the mat against the concave, whereas a spike-tooth strikes it; impact shatters straw and rubbing does not.

Short straw fragments pass through the concave with the grain and arrive at the cleaning shoe, where they must be sieved and blown out. Less broken straw therefore means a lighter load on the shoe and a cleaner sample for the same fan setting - a benefit realised two stations downstream of the cause.

Answer: D

The definition given is correct and it is exactly why the assertion fails: a machine may detach nearly all the grain while cracking it, losing it in the straw or blowing it away. Four measures at least are needed together.