Cleaning Grain: Post II - The Machines, the Adjustments, and the Limits
Post II of VI
A cleaner can be running just fine and still be doing the wrong job.
The screens are moving. The fan is pulling air. Grain comes out one spout and screenings out another. Everything looks busy.
But activity is not separation.
In Post I we looked at the physical differences that make cleaning possible: size, shape, density, and how a particle behaves in moving air. Now we follow those differences into the machines.
This is not an equipment catalog. Plenty of good companies build these machines, and there is a reference list at the end. Before anybody compares brands, we need to be clear on what we are asking each machine to do.
Screens and Air
Scalpers: the first rough cut
A scalper takes the big stuff out. Cob pieces, stalks, pods, and other oversized trash stay on top of the screen while the grain falls through.
At most country elevators that is a rotary drum or reel scalper. Grain enters a turning cylinder of perforated metal or wire screen, kernels drop through, and the trash rides out the end. Flat-deck scalpers do the same job with a shaking screen.
There is also the gravity screen cleaner, the kind you bolt into a spout or set under a leg discharge. It has no motor. In the zigzag style, grain tumbles down a series of reversing screen sections, and the tumbling does the work. Some of these are rated up to 50,000 bushels an hour. The older diamond style is a bigger box. Grain spreads over the outside of a diamond-shaped screen, fines drop through, and clean grain runs out the bottom. It takes more room, but you can get inside it to change screens.
Whatever the design, a scalper does not know a rock from a kernel. If a stone or a weed seed fits through the opening, it goes with the grain. Scalping is a first step, not a cleaning system.
The opening establishes the cut. Feed rate, and how the machine presents grain to that opening, decide how well the cut gets made. Making the hole smaller does not automatically solve a separation problem. It can also change which good kernels pass.
Screen cleaners: same principle, different motion
Screen cleaners take size separation further. One machine may scalp oversized material on the top deck, pull fines on the bottom deck, and split the product into several size fractions in between.
They do not all shake the same way . Some use straight-line horizontal motion. Others start with a circular motion at the feed end and work into a back-and-forth stroke toward the discharge. Sifters belong in this family too. The name on the machine matters less than which fraction passes each screen and which fraction stays above it.
The operator’s main lever is the screen itself: the size and shape of the openings. Round holes, slots, and wire mesh all make different cuts. Feed rate, and where it is adjustable, stroke and deck slope, decide how well the grain gets a fair look at those openings.
Keep two things straight. Changing a screen changes the cut the machine can make. Adjusting the machine helps it make that cut. Related, but not the same.
A screen does not recognize a weed seed, a damaged kernel, or a stone by name. If the dimensions let it follow the grain, that is where it goes. Screen selection starts with the material, not with whatever screen happens to be sitting in the shop.
Graders: width and thickness
Some jobs care less about trash and more about the kernel itself.
A slotted screen or slotted cylinder sorts kernels by width or thickness. Malting barley is the classic case. The maltster wants plump kernels, and the thin ones get pulled out on slots.
Food-corn buyers want kernels in a consistent size range, and graders deliver that.
A grader is not a trash remover. It is making a product spec out of grain that is already clean.
Aspirators: setting the air cut
Air separation asks a different question. Does this particle fall through the moving air, or does the air carry it off?
Aspirators and air legs separate by how material behaves in an air stream. The engineers call it terminal velocity. The rest of us call it what blows away. Weight is part of it, but not the whole story. Shape and surface decide how the particle answers the air.
In Post I, I told about dropping wheat through an old Kansas truck bay and letting the wind strip out the light trash. An aspirator puts that same draft inside a box where you can control it. Air legs and aspiration channels belong to the same family. The arrangements differ. The question does not: are we taking the light material without carrying too much good grain into the reject?
The operator sets the air cut. More air is not automatically better. Turn it up too far and light kernels go to the trash. That is grain the plant meant to sell.
A shriveled kernel and a piece of chaff can act about the same in the air. The aspirator separates on that behavior. It does not read the customer’s spec.
Air-screen cleaners: two jobs in one frame
An air-screen cleaner puts screens and aspiration in one machine. The screens make the size cuts. The air pulls out what is light. Air may act at more than one point, depending on the design. That combination earns its keep because incoming grain rarely brings only one kind of trash.
Putting both in one frame does not erase their limits. Material that matches the grain in size and in how it acts in the air will stay with the grain.
That is where the next set of machines comes in.
Density, Length, and Shape
Destoners: find the heavy stuff
A stone can be close enough to a kernel in size to go through the same screen hole. That is what destoners are for.
A destoner moves air up through a sloped, shaking deck. The air lifts and loosens the grain bed. Heavier particles stay in contact with the deck and get walked uphill to their own outlet, while the grain floats off the low end.
Pressure machines push air up through the deck from below. Vacuum machines pull it up from above. Same idea, different fan arrangement. Neither is sucking air down through the bed.
You want a product stream and a small heavy-reject stream. Check both. Stones in the product means the job is not getting done. Too much good grain in the reject means you are paying for clean grain with grain you could have sold.
Gravity tables: get the bed working
A gravity table uses air, vibration, and a tilted deck to grade material by density. Heavier particles settle against the deck. Lighter ones ride on top. Deck motion and the slopes carry those layers to different points along the discharge edge.
Gravity tables work best on grain that has already been cleaned and sized. They are a finishing tool, not a first pass.
There is no single knob. Oliver’s operating instructions list five main adjustments, and they work on each other: feed rate, airflow, side tilt, end raise, and deck speed. Feed sets the load the bed has to carry. Air lets the material loosen and stratify. Too little air and the bed never layers. Too much and it boils. Deck speed and the two slopes move that stratified bed across the deck. That sequence is Oliver’s, not a universal startup for every deck. The five controls are the idea that travels.
The dividers at the discharge edge set where you split the fractions. They do not make the separation.
That matters on the floor. You can move a divider and change what goes in each spout. If the bed has not layered out, moving the divider just splits a mixed stream a different way.
Get the bed working first. Then set the dividers and pull samples. We will get into the setup sequence in Post IV.
Indent cylinders and disc separators: length
Some trash is too close to the grain in width and thickness for any screen to catch. Length is the next test.
An indent cylinder is a turning drum lined with small pockets. Short particles sit down in the pockets, get lifted, and drop into a trough inside the drum. Long particles do not fit and slide out the end. Pocket size sets the potential cut.
The lifted fraction is not always trash. Depending on the job, the machine may lift the good grain away from longer material, or lift short weed seeds and broken kernels out of the grain. A disc separator works on the same principle, with pocketed discs turning through the grain instead of pockets inside a drum.
On an indent cylinder, cylinder speed and trough position decide where the lifted particles drop, and whether they land in the trough. Feed and cylinder pitch matter too. Do not assume the small spout is the reject. Find out what the machine was set up to collect.
Spirals: watch how it rolls
A spiral separator sorts by how things roll. Round seeds pick up speed going down the flights and swing outward. Flat, broken, or odd-shaped material stays toward the inside. The two paths get caught separately. Soybeans are the classic spiral crop. Great for removing corn from that bean stream.
A plain spiral has no motor. That does not mean nothing adjusts. Depending on the design, there may be feed control, adjustable fingers along the flights, a sliding plate, a separation gate, or brushes set where unwanted seed starts to swing out. Some of those adjustments work on the path itself. Others only divide the streams at the bottom.
We used to clip clothespins along the edges of the spiral flights to help the separation. Where you put them mattered. You watched the beans coming down, checked the streams, and moved the pins until the split looked right. That is a memory from running a line, not a recommendation. Nobody should be adding loose wooden parts to a cleaning line today.
The lesson holds. On a spiral, the separation can be tuned along the flights, not just at the discharge. Feed rate is part of that tune. Change one thing at a time, because the adjustments interact.
Texture separators: the seed-plant specialists
Seed plants sometimes need to separate seeds that match in size, density, length, and shape but differ in surface. A rough-coated weed seed and a smooth crop seed are the classic case.
Velvet-roll separators and inclined-belt separators handle that. Rough seeds catch on the velvet or the belt and get carried off. Smooth seeds roll or slide away. You will not find them at an elevator. They show how far a cleaning line can go when the spec gets tight.
Metal, Optics, and Surface Work
Magnets and metal detectors: know their job
Magnets do not care about size or density. They pull ferrous metal out of the stream to protect the people, the equipment, and the product downstream. Metal detectors pick up what a magnet cannot, such as stainless steel and aluminum, and kick it out.
We covered both in the tramp-metal series, so I will not repeat it here. The point for this post is simple. A magnet is not a destoner, and it does not catch every kind of metal. It belongs in the system. It does not replace the system.
Optical sorters: a different kind of difference
An optical sorter goes after what mechanical cleaners leave behind: off-color kernels, mold damage, stained or diseased kernels, and foreign material that matches the grain in size and weight.
Cameras and sensors look at particles as they pass. When one fails the test, a puff of air from a timed ejector knocks it out of the stream. Some newer machines use machine learning to help recognize defects. That does not remove the need for a detectable difference, a defined target, and settings that match the job.
The operator is not setting a screen hole or a density cut here. The operator is setting a decision: what counts as a reject, and how hard to look for it.
The cameras do not get you out of the basics. Feed presentation, clean lenses, and healthy ejectors all matter. A sorter is not an excuse to skip the cleaning ahead of it.
The question is not whether the machine has cameras or AI. It is three questions. Can it see this particular defect? Can it reject that defect consistently? And how much good grain goes out with the rejects?
Surface work: cleaning is more than separating
Some equipment does not separate particles at all. It works on the surface of the kernel.
In Post I, I told about a food-soybean line where we sized the beans and then polished them in a drum with ground corncob and a vinegar solution. It took off dirt and pod stain and brightened the seed coat. The customer reported more even soaking and better tofu yield. That was product preparation, not scalping.
Wheat scourers are another example. They use friction to loosen dirt and hairs stuck to the kernel, then aspiration to carry the loosened material off. Barley de-awners knock off awns that would otherwise ride along with the grain. In a rice mill, the cleaning and destoning up front are a different job from the husking, whitening, and polishing that follow.
Calling all of that cleaning makes it harder to explain what each step actually does. Name the job, then name the machine.
Same Machine, Different Job
The same gravity table can sit in a country elevator, a seed plant, and a food plant. What changes is the job it is asked to do, and that changes how hard you run it.
Where it runs | What they are cleaning for | How it runs |
Country elevator and terminal | Grade, foreign-material limits, storability | Fast. Volume is king, and good enough is the target. |
Feed mill | Protecting equipment, consistent ingredients | Moderate. Scalping and magnets carry most of the load. |
Food corn for snacks, cereal, and distilling | Stones, color, kernel size, mycotoxin risk | Tight specs. Destoners, graders, and sorters earn their keep. |
Malting barley | Plump kernels, germination, no thins | Slotted screens and de-awners, with sampling on every lot. |
Rice mill | Clean paddy ahead of husking and whitening | Cleaning and destoning are the front door of the mill. |
Soybean processing | Clean beans ahead of cracking and dehulling | Scalping, aspiration, and magnets to protect the plant. |
Seed plant | Purity and germination | Slow and precise. Every machine in this post may be in the line. |
The point of this table is not the rows. It is that nobody can tell you whether a cleaner is working until they know which row they are standing in.
We will come back to several of these with real stories in Post VI.
Capacity Needs a Job Description
Capacity numbers get thrown around, so look at a few.
One major screener maker advertises a grain cleaner rated up to 50,000 bushels an hour. Another documents a shaker scalping corn at 60,000 bushels an hour. A gravity-flow spout cleaner with no motor at all is rated up to 50,000.
None of those is an apples-to-apples comparison.
The same maker lists a shaker at 10,000 bushels an hour on dry corn: scalp the cobs, pull the fines and chips, and hit the foreign-material number the buyer already set. The bushel rate is how hard it was run. It is not the job.
Feed rate works the same way in the rest of the cleaning house. It is a knob, not a number on the side of the machine. On a gravity table you move the feed until it agrees with the air, the deck speed, and the slope. On a spiral you change the feed because the separation changes with it.
Before you accept a capacity number, put the job next to it. What crop? What incoming condition? How much trash? What has to come out? What does the finished product have to look like? And can the reject system handle what you are taking out?
Those are the selection and sizing questions we will work through in Post III.
The Bottom Line
Every machine in the line has a specific job, and every spout needs monitored. Screens, air, density decks, pockets, spirals, magnets, and cameras each see something different. None of them sees everything.
The operator’s job is to make those differences work for the plant.
Before you ask how big a cleaner you need, make sure you have defined what it needs to do. Are you asking the right machine to do the job?
Which adjustment made the biggest difference on a cleaner you ran, and what did the product and reject samples tell you? Drop it in the comments.
Thank you for reading.
Regards,
Grain Guy Fifty
Who Builds It: A Reference List
Please read this first. This list is not a recommendation or an endorsement, and it is not a complete list of manufacturers. Plenty of good companies are not on it. It is a reference to help you start your own search. Talk to the manufacturers, visit installations running your crop, and judge the equipment against your own job. Company names and ownership change, so check current details with each company.
Country shown is headquarters. Many of these companies build several kinds of equipment. GSI, Kepler Weber, and FAMSUN are included from published product lines for pre-cleaning and cleaning. Two Chinese color-sorter names still need a primary source page before they belong here.
Equipment | Manufacturers (country) |
Scalpers, screen cleaners, sifters | A.T. Ferrell / Clipper (US), AGI Milltec (Canada/India), Akyurek (Turkey), Alapala (Turkey), Allgaier (Germany), Alvan Blanch (UK), Bühler (Switzerland), Carter Day (US), Cimbria (Denmark), FAMSUN (China), Fowler Westrup (India), GSI (US), Kepler Weber (Brazil), Magik Kleener / GrainTech (US), PETKUS (Germany), Rotex (US), Sweco (US), Triple/S Dynamics (US), Westrup (Denmark) |
Air-screen cleaners | A.T. Ferrell / Clipper (US), Alvan Blanch (UK), Bühler (Switzerland), Carter Day (US), Cimbria (Denmark), Crippen (US), Damas (Denmark), FAMSUN (China), GSI (US), PETKUS (Germany), Westrup (Denmark) |
Aspirators | ArrowCorp (Canada), Bühler (Switzerland), Carter Day (US), Cimbria (Denmark), Kice (US) |
Width and thickness graders | Allgaier (Germany), Bühler (Switzerland), Carter Day (US), Cimbria (Denmark), PETKUS (Germany), Westrup (Denmark) |
Destoners | AGI Milltec (Canada/India), Bühler (Switzerland), Cimbria (Denmark), Forsberg (US), LMC (US), Oliver (US), Satake (Japan), Triple/S Dynamics (US) |
Gravity tables | ArrowCorp / Kipp-Kelly (Canada), Cimbria (Denmark), Forsberg (US), LMC (US), Oliver (US), PETKUS (Germany), Triple/S Dynamics (US), Westrup (Denmark) |
Indent cylinders and disc separators | Bühler (Switzerland), Carter Day (US), Cimbria (Denmark), Damas (Denmark), PETKUS (Germany), Westrup (Denmark) |
Spirals | PETKUS (Germany), Profile Industries (US) |
Scourers and wheat cleaning | Alapala (Turkey), Bühler (Switzerland), Golfetto Sangati (Italy), Ocrim (Italy), Satake (Japan) |
Rice cleaning and milling | AGI Milltec (Canada/India), Bühler (Switzerland), Satake (Japan), Schule (Germany), Yamamoto (Japan), Zaccaria (Brazil) |
Optical sorters | AGI Milltec (Canada/India), Bühler SORTEX (Switzerland/UK), Cimbria (Denmark), Daewon GSI (Korea), Hefei Meyer (China), Satake (Japan), TOMRA (Norway) |
Magnets and metal detectors | Bunting (US), Eriez (US), Goudsmit (Netherlands), Mesutronic (Germany), Sesotec (Germany) |
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