SECTION 4
4-117
Cracked or damaged grain can be caused by a con-
cave setting that is too close to the drum. Increase the
concave clearance and then reduce the drum speed
to correct the problem.
Bunch feeding can cause grain damage in the straw
elevator. An incorrect straw elevator chain tension
can also cause grain damage.
Excessive returns will contribute to grain damage due
to the additional pass(es) the grain must take through
the drum/concave area. Plugged concaves can
cause grain damage because the free grain cannot
get out of the concave area.
Loose elevator chains can also cause grain damage.
It is important to maintain the proper tension on these
chains.
Unthreshed material in the grain tank can be caused
by underthreshing or the lower sieve that is opened
too wide. Increase the drum speed and reduce the
concave clearance as needed to correct the problem.
Also close the lower sieve slightly to obtain a cleaner
grain tank sample.
GRAIN PAN / UPPER SIEVE DISTRIBUTION
The distribution of material on the grain pan affects
the distribution of material on the sieve. The distribu-
tion on the grain pan should be level to slightly higher
directly under the drum and taper slightly towards
both sides.
The front third of the upper sieve should be com-
pletely clean. The middle third should have some
grain but mostly residue. The rear third of the upper
sieve should have only residue.
If there is some grain on the rear third of the upper
sieve, returns will be increased and there is a possibil-
ity that grain will ride out the back of the combine.
Open the upper sieve to allow more of the grain to fall
onto the lower sieve.
If the material on the upper sieve is broken into small
pieces, the crop is being overthreshed. This can plug
the upper sieve and cause high losses. Increase the
concave clearance and/or reduce drum speed to de-
crease the threshing action.
Unthreshed heads indicate the drum speed should be
increased and/or the concave clearance should be re-
duced. Unthreshed heads can also be caused by
worn rasp bars and worn concaves.
RETURNS SAMPLE
It is normal for the returns sample to contain a few un-
threshed heads. The purpose of the returns system is
to return unthreshed heads to the threshing area for
another pass. However , a large quantity of un-
threshed heads indicates the concave clearance is
too great.
If the returns sample contains a large quantity of
clean grain, the cleaning fan speed should be re-
duced and/or the lower sieve should be opened more.
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