Threshing and Shelling Questions and Answers

Answer: A

Correct answer: 97.5 per cent

Threshing efficiency is the proportion of the grain that was actually detached:

\[ \eta = \frac{320 - 8}{320} \times 100 = \frac{312}{320} \times 100 = 97.5\ \mathrm{per\ cent} \]

The 800 kg of crop is not part of the calculation - the measure concerns grain, not total material. The 8 kg still on the heads is the unthreshed loss, and reducing it means a faster cylinder or a tighter concave, both of which would raise damage.

Answer: B

Correct answer: 97.0 per cent

Cleaning efficiency is the proportion of the delivered stream that is clean grain:

\[ \eta = \frac{291}{300} \times 100 = 97.0\ \mathrm{per\ cent} \]

The remaining 9 kg is chaff, straw fragments and dust that the shoe failed to remove. Note that this is a measure of purity at the outlet and says nothing about how much grain was lost - a shoe could deliver a perfectly clean sample while blowing a great deal of grain out with the chaff.

Answer: C

The circumference is \( \pi \times 0.55 = 1.728 \) m and the speed is \( 1.728 \times 600/60 = 17.3 \) m/s. Peripheral speed rather than rotational speed is the meaningful setting, since it is what the crop actually experiences.

Answer: D

Correct answer: 7.5 kWh per tonne

Divide the power drawn by the throughput:

\[ \frac{9.0\ \mathrm{kW}}{1.2\ \mathrm{t/h}} = 7.5\ \mathrm{kWh\ per\ tonne} \]

The units work out directly, since kilowatts divided by tonnes per hour give kilowatt-hours per tonne. Expressed this way the figure can be compared between machines of quite different size, and it can be set against the value of the grain to judge whether the fuel or electricity cost is significant.

Answer: A

Correct answer: 2.0 per cent

Add the two losses and express them against the grain entering:

\[ \frac{6 + 4}{500} \times 100 = \frac{10}{500} \times 100 = 2.0\ \mathrm{per\ cent} \]

The two have different causes and different remedies: grain leaving with the straw is a separation loss, cured by slowing the feed or improving the walkers, while grain blown out with the chaff is a fan setting that is too high. Measuring them separately is what tells the operator which adjustment to make.

Answer: B

Correct answer: 2.5 t/h

A straw-to-grain ratio of 1.5 means every tonne of grain arrives with 1.5 tonnes of straw, so the total material is 2.5 times the grain:

\[ 1.0 \times (1 + 1.5) = 2.5\ \mathrm{t/h} \]

This is why the ratio matters so much to machine sizing. The cylinder, the walkers and the shoe must all handle the whole of the material fed, not merely the grain, so a crop with a high straw-to-grain ratio loads a thresher far more heavily for the same grain output.

Answer: C

Correct answer: 80.0 per cent

The shelling percentage is the kernel mass as a fraction of the cob mass fed:

\[ \frac{720}{900} \times 100 = 80.0\ \mathrm{per\ cent} \]

The remaining 20 per cent is the cob itself, which is discarded or used as fuel. The figure is a property of the crop more than of the machine - variety, maturity and moisture all affect it - so it is used as a check on the crop rather than as a measure of how well the sheller is performing.

Answer: D

Correct answer: 2.98 per cent

Breakage is expressed against the kernel output, not against the pods fed:

\[ \frac{5}{168} \times 100 = 2.98\ \mathrm{per\ cent} \]

The distractor of 2.00 per cent divides by the 250 kg of pods instead, which understates the damage by mixing the shell mass into the denominator. Broken groundnut kernels are worth appreciably less than whole ones, so this figure translates directly into money and is the measure a decorticator is set by.

Answer: A

Correct answer: 1.78

Kinetic energy per unit mass varies with the square of the speed:

\[ \frac{E_2}{E_1} = \left(\frac{20}{15}\right)^2 = 1.333^2 = 1.78 \]

The distractor 1.33 is the speed ratio itself, before squaring. The square law is the whole reason cylinder speed is such a sensitive adjustment: a 33 per cent increase in speed delivers a 78 per cent increase in impact energy, which is why damage climbs so much faster than threshing improves.

Answer: B

Correct answer: 2 mm per 100 mm of wrap

The clearance narrows by 6 mm over 300 mm of wrap:

\[ \frac{12 - 6}{300} = \frac{6}{300} = 0.02\ \mathrm{mm/mm} = 2\ \mathrm{mm\ per\ 100\ mm} \]

The taper is deliberate. The mat enters thick and is progressively compressed as it is carried round, so the gap narrows to keep working it as its bulk falls. A parallel concave would either be too tight at entry or too slack at exit.