Threshing and Shelling Questions and Answers

Answer: A

Every threshing cylinder uses some combination of these four, and the proportions depend on the cylinder type and the crop. A spike-tooth cylinder relies mostly on impact while a rasp-bar works largely by rubbing the material against the concave.

Answer: B

Correct answer: The curved grate beneath the cylinder

The concave is the curved, perforated grate that wraps beneath the rotating cylinder, and the gap between the two is where threshing actually happens. The crop is worked in that gap, and the detached grain falls through the openings while the straw is carried on.

Its clearance is the single most important adjustment on the machine after cylinder speed: closing it threshes more thoroughly but damages more grain. The straw hood, the feeding chute and the fan housing are all elsewhere and take no part in detaching grain.

Answer: C

The teeth strike the material and knock grain loose, which makes the design tolerant of tough or damp crops. It is also more aggressive than a rasp-bar and tends to break more straw into short pieces the cleaning shoe must handle.

Answer: D

Correct answer: Rubbing the crop on the concave

A rasp-bar cylinder carries corrugated bars along its periphery, and these drag the crop mat against the concave so that the grain is rubbed free. The action is largely one of friction and shear rather than of blows.

That is why a rasp-bar breaks the straw far less than a spike-tooth cylinder, which relies on impact and shatters the straw as it works. Cutting plays no part - a thresher is not a chopper - and blowing is the business of the cleaning shoe further downstream.

Answer: A

Correct answer: Delicate crops needing gentle handling

A wire-loop cylinder carries flexible loops instead of rigid bars or teeth, so it works the crop gently and delivers a much softer blow than either alternative. It is used for pulses, oilseeds and seed crops where a cracked or bruised grain is a serious loss.

The price is capacity and thoroughness: tough cereal straw, maize cobs and groundnut pods all need far more energetic treatment than a wire loop can supply, and each has its own machine or cylinder type.

Answer: B

The helical path gives a much longer residence time, so threshing and separation happen together over the whole cylinder length. It handles damp and tough material well but consumes more power than a tangential machine.

Answer: C

Correct answer: Straw walkers or a separating drum

The cylinder and concave detach the grain, but a good deal of it leaves still mixed with the straw. Straw walkers - oscillating stepped racks - shake that straw so the remaining grain falls through, while the straw itself is walked to the discharge.

The concave separates only what falls through it at the cylinder, and the cleaning shoe works on the grain and chaff after the straw has been removed. Each stage handles a different mixture, which is why all three exist.

Answer: D

The chaffer and sieve stratify the material while the fan blows the light fraction off the back, so the two properties of size and terminal velocity are used together. Fan speed and sieve opening are the two adjustments available.

Answer: A

Correct answer: Increases threshing efficiency but also increases grain damage

Faster cylinder rotation means a harder blow and more work done on the crop, so more grain is detached - and more is cracked. The two move together, and the operator's task is to find the speed that threshes adequately while keeping damage acceptable.

The relationship is not linear: impact energy rises with the square of the peripheral speed, so raising the speed from 15 to 20 m/s increases the energy per unit mass by a factor of 1.78. Damage therefore climbs faster than threshing does.

Answer: B

Correct answer: Increases threshing and damage together

Closing the concave squeezes the crop mat harder against the grate, so more grain is rubbed free - and more is crushed in the process. Clearance behaves exactly as cylinder speed does: both levers raise threshing and damage in the same direction.

Because the two adjustments pull together, the usual practice is to set the clearance as wide as will still thresh the crop, and to use speed as the finer control. On a seed crop both are backed off deliberately, accepting a little unthreshed grain to protect germination.