A field checklist for combine harvesters throwing clean grain out with the chaff

You finish a pass, walk behind the machine, and there's a trail of clean grain lying on the stubble. Open the fan inspection cover and the housing has grain in it too. Neither of those is a mystery. Grain is getting into the air box somewhere above the fan, and the fan is doing exactly the job it was built for: moving air, and whatever else is caught in it, out the back of the machine.

On most harvesters the cause is a seal, a sieve, or a setting. Less often it's the threshing section upstream sending more material into the cleaning shoe than the shoe can handle. Telling those apart takes about twenty minutes in the field, and the order you check them in matters more than the tools you carry.

What grain in the fan box actually means

On a conventional combine, the cleaning fan sits in a housing underneath the cleaning shoe and blows air up through the chaffer. Grain and chaff drop from the grain pan onto the chaffer; the air lifts the chaff and carries it out the back; the grain falls through the sieve to the clean grain auger. Once grain is inside the fan housing it is already below the sieves. There is no second chance for it. It leaves with the chaff.

That's what makes this fault confusing from the seat. The sample in the grain tank still looks clean, the loss monitor climbs, and the grain ends up scattered behind the machine instead of staying in a narrow band under the discharge.

Six places grain gets in

These are the ones that turn up most often on machines with a few thousand hectares behind them.

1. The seal strips at the grain pan and fan duct joint. The pan hands material to the chaffer directly above the fan intake, and the rubber or canvas strip along that joint takes the worst of the wear. Once it hardens, splits, or pulls away from its retainer, every pass drops a thin curtain of grain straight into the housing. On an older machine this is the first thing to check, and usually the cheapest to put right.
2. Sieve damage, or the wrong sieve for the crop. Holes open at the front of the chaffer first, because that's where the drop lands hardest. The other version of this is a sieve that was never right for the job, a rice sieve run in soybeans or a fine wheat sieve in maize, which plugs, overloads, and pushes grain over the edge instead of letting it through.
3. Side seals and divider plates. Grain escapes over the side of the shoe rather than through the sieve. When the trail in the housing runs down one side only, look for a bent side plate or a missing retainer before you touch the fan.
4. Too much air, or a deflector out of position. The fan can't tell a light kernel from a piece of chaff of the same weight. Open the air up far enough and light grain rides over the chaffer and out with the rest. On machines with adjustable louvers or deflectors, they also get knocked out of position during cleaning and stay that way for the rest of the season.
5. Feed rate above what the shoe can carry. Ground speed, yield, straw moisture, green weeds. A mat that's too thick never stratifies properly: grain sits on top of the chaff and gets carried back. If dropping a kilometre an hour clears the problem, you had a capacity issue, not a leak.
6. Tailings coming back too heavy. Worn flights on the tailings auger, or a return that dumps too much material back onto the shoe, send the same grain through the cleaning section two or three times. Unthreshed heads and half-threshed pods make every pass heavier than the one before it.

There is one more source that isn't the fan at all: leaks from the grain tank, the unloading auger, or the elevator boots. Grain falls from above, lands on the shoe cover or in the fan area, and reads as grain in the fan box. Check the tank seams and the elevator first, before you take the fan cover off.

Twenty minutes in the field

Shut the engine down and take the key out. The cleaning fan on a big machine keeps turning for a while after the engine stops, and it's heavy enough to take a hand with it. Wait until everything is still before you reach into the housing.

7. Clean it out first. Run the shoe empty, then stop and clear the fan housing. You can't read where grain is entering if there's already an inch of it sitting on the floor.
8. Look up into the intake with a torch. A narrow trail of grain points at a seal. Grain spread across the full width points at air volume or feed rate. Grain on one side points at a side seal or an unbalanced fan.
9. Bend every seal strip by hand. If it's hard, cracked, or not touching the surface it's meant to seal against, it isn't sealing. Replace it. Gluing a hard strip back on buys you one day.
10. Lay the sieves flat and look through them against a light. Worn holes cluster at the front edge. An adjuster already sitting at the end of its travel usually means the sieve is worn out, not badly set.
11. Check the fan itself: blade pitch on both sides, cracks at the blade root, bearing play. A fan with one bent blade still moves air, just unevenly, and one side of the shoe gets blown through.
12. Set the air in steps, not in one jump. Start below the figure in the operator's book, run a short pass, watch the discharge. Increase until the chaff leaves clean and the grain stays on the sieve. One step past that and you're throwing grain again. The window is narrow, and it moves with crop moisture.
13. Measure the loss instead of estimating it. Drag a tarp or a sheet of canvas behind the machine for ten to twenty metres and count the kernels. Eyes are bad at this; a tarp isn't.
14. Only then change ground speed and concave clearance. If the loss drops when you slow down, the shoe was overloaded. If the tailings are full of unthreshed heads, close the concave a notch or check the rasp bars before you blame the fan again.

Fix it in the right order

Seals and sieve condition first, because they're cheap, reversible, and they remove a leak instead of hiding it. Air setting next, since it costs nothing but time. Ground speed after that, because slowing down to cover a leak means paying for the fault with capacity on every hectare. Concave clearance and threshing parts last, and only when the tailings sample says the problem starts upstream.

The order matters for a second reason: if you open the air and slow down at the same time, you can't tell which change did what. Change one thing, run a pass, write it down.

Quick reference

Table 1. Symptom to cause, for grain in the cleaning fan housing

What to record before next season

Fan setting, sieve part numbers, and concave clearance for each crop. The same figures almost never work for wheat and rice, and a setting that was right in dry wheat will be too hot in a damp crop. A page per machine per season is worth more than it looks.

Two jobs at the end of the season: replace the seal strips rather than storing them for another year, and store sieves flat, dry, and off the ground. Sieves that spend the winter leaning against a wall come out of it slightly bowed, and a bowed sieve leaks along its edge all season.

About Golddafeng

Golddafeng has been building harvesters and tractors for 32 years, and roughly 85% of the core components, cleaning shoe assemblies, augers, sieves and fan housings among them, come out of our own workshops. For a fault like this, that has a practical value: a replacement sieve or seal kit is dimensioned off the same tooling the machine was built from, not a generic part that should fit. Every unit is tested before it leaves the plant, and our service engineers travel to dealers and farms in Africa, South Asia, Southeast Asia, Central Asia and South America to work on machines in the field rather than discuss them on the phone.

If you have been through the checks above and the loss is still there, send us the model, the crop, the moisture, and a photo of the sieve section. Our service team will come back with the sieve and setting combination we run on that machine.