Threshing and Shelling Questions and Answers

Answer: A

Correct answer: The crop is too wet to shear cleanly

At 24 per cent moisture the kernels are soft and pliable and their attachment to the cob is tough, so the shelling action deforms the material rather than shearing the kernels free. Maize is normally shelled below about 18 per cent for this reason.

Raising the sheller speed would damage the soft kernels without solving the attachment problem. The crop is plainly not too dry - dry maize shells easily and the difficulty then is breakage rather than kernels left attached - and cob size is not the issue.

Answer: B

Correct answer: A spike-tooth cylinder is breaking straw

Good threshing with a dirty sample points to the cylinder rather than the shoe. A spike-tooth cylinder works by impact, and impact shatters straw into short pieces that pass through the concave with the grain and overwhelm the cleaning stage.

A high fan speed would give a cleaner sample, at the cost of blown grain. A low cylinder speed or an open concave would leave grain unthreshed, which is the opposite of what is reported. A rasp-bar cylinder is the usual remedy.

Answer: C

Correct answer: The first, at 8.9 against 9.3 kWh per tonne

Divide power by throughput in each case:

\[ \frac{8.0}{0.9} = 8.9, \qquad \frac{13.0}{1.4} = 9.3\ \mathrm{kWh\ per\ tonne} \]

The first machine is the more efficient per tonne even though the second has half again the output. This is exactly why specific energy is quoted rather than raw power: throughput and consumption must be read together, and the larger machine is not automatically the better one.

Answer: D

Correct answer: concave

The concave is the curved perforated grate wrapping beneath the cylinder. The crop is worked in the gap between the two, detached grain falls through the openings, and the straw is carried on to the walkers.

Its clearance is one of the two principal adjustments on the machine. A chaffer is a sieve in the cleaning shoe, a walker shakes grain out of the straw further downstream, and the rotor is the cylinder itself in an axial-flow machine.

Answer: A

Correct answer: unthreshed

Unthreshed loss is grain that never left the head at all, still attached when the straw is discharged. It is the direct measure of how thoroughly the cylinder and concave did their work.

It is reduced by raising cylinder speed or closing the concave - both of which raise breakage, which is why the two are traded against each other. The other terms describe grain that was detached but went astray: with the straw, out with the chaff, or damaged in the process.

Answer: B

Correct answer: blown

Blown loss is grain carried out of the machine by the cleaning fan along with the chaff. It is controlled by fan speed and by the sieve settings.

The adjustment pulls directly against sample cleanliness: a stronger blast removes more light material and gives a cleaner sample, but takes more grain with it. Setting the shoe means finding the balance between blown loss and purity, and measuring both rather than judging by eye.

Answer: C

Correct answer: axial-flow

In an axial-flow machine the crop enters at one end and travels along the rotor on a helical path, passing over the concave many times before being discharged. A tangential machine, by contrast, carries the crop across the cylinder in a single short pass.

The repeated working gives thorough threshing with a gentler action per pass, which suits high-capacity combines. The cost is energy: keeping the mat moving along the whole rotor consumes more power per tonne than a single pass does.

Answer: D

Correct answer: decorticator

A decorticator cracks the groundnut shell and separates the kernels from it, usually with a beater working against a screen sized to the crop.

The beater clearance is the critical setting: tighter cracks more shells but breaks more kernels, and broken kernels are worth appreciably less. A sheller removes maize kernels from the cob, a thresher detaches grain from heads and straw, and a winnower separates grain from chaff by air - four different machines for four different separations.

Answer: A

Correct answer: 16 m/s

Read the two columns together. Going from 16 to 20 m/s buys only 1.5 points of threshing efficiency - from 0.978 to 0.993 - while damage rises from 1.2 to 2.9 per cent, more than doubling.

At 12 m/s the threshing is too poor at 0.941, leaving nearly 6 per cent of the grain on the heads. So 16 m/s is the compromise: nearly all the grain detached, and damage still close to one per cent. Broken grain is unsaleable at full price, so it is the column that governs.

Answer: B

Total loss passes two per cent between 2.0 and 2.5 tonnes per hour, and interpolating puts it near 2.4. Note that unthreshed and separation losses climb together while blown loss barely moves, which is the signature of overloading.