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Combine Adjustments

Combine Adjustments

Combine Adjustments

by Paul R. Hoff, Cornell Extension Bulletin 591, March 1943

Most of the grain losses from combine operation can be prevented if the grain is ripe enough when it is cut and if the machine is correctly adjusted. The machine should be adjusted for each field that is cut and adjusted several times each day for changing weather conditions. Good operation of the combine is difficult where there is a large proportion of weeds in the crop, but correct adjustment reduces the trouble.

Grain loss may be at the following places: the grain platform (reel or cutter bar), the cylinder, the straw rack, and the cleaning shoe.

Time to Harvest

Grain should not be harvested until it is dead ripe. This may be a week or ten days after it normally would be ready to cut with a binder. The loss from green or damp grain is more than the loss from dry grain. Damp grain stored in bins or in bags that are piled close together heats and molds. The moisture content of the grain cannot exceed 14.5 per cent for safe storage. Grain that feels damp or that can be easily dented with the fingernail contains too much moisture for storage.

Power Take-Off

Correct, constant, power take-off speed is essential for efficient combine operation. With the power take-off speed too low, grain is lost at the cylinder, over the straw rack, and in the cleaning shoe. With the power take-off speed too high, grain coming from the cylinder may be cracked and grain may be wasted over the straw rack and the cleaning shoe. The instruction book that comes with each combine gives the correct power take-off speed and the method of determining this with a speed counter. No combine operator should attempt to run his combine without a speed counter. Do not guess at the speed; know it. When the correct speed has been found, the tractor-throttle quadrant should be marked for the throttle-lever setting and the tractor should be operated with the throttle in that notch. If the tractor motor slows down on the uphill pull, or for any other reason, the power take-off speed should be maintained by opening the throttle farther or, if necessary, by changing to a lower gear.

The tractor hitch and the drive-shaft support arms should be adjusted so that the drive shaft runs in as near a straight line as possible, to decrease wear on the universal joints. The slip joint must be lubricated so that it is free to telescope, and the jaws of the slip clutch must be lubricated occasionally to prevent rusting or sticking. The slip clutch should be adjusted just tight enough to handle the normal load without slipping.

Auxiliary Motor

On a combine operated by an auxiliary motor, the speed of the motor is as important as the speed of the power take-off. The instruction book tells how to determine combine speed with a speed counter. When the correct combine speed has been found, the throttle on the auxiliary motor should be set to maintain that speed.

Adjustment of the Reel

The reel is adjustable both horizontally and vertically, and the speed also is adjustable. In standing grain, the reel works best if the shaft is ahead of the cutter bar and if the slats strike the grain just under the heads. If weeds are present, it is best to cut a high stubble. Then the reel may need to be lowered and pushed back until the shaft is directly over the cutter bar, to prevent the unthreshed heads from dropping to the ground. In fallen and tangled grain, the reel is best set low and forward.

Usually, the reel should run at the slowest speed, unless some of the cut grain falls to the ground ahead of the cutter bar. When this happens, the reel speed may be increased or the number of slats increased. If cut grain is carried around the reel, or if grain is being shelled out of the heads and falls to the ground, the reel speed should be decreased.

Height of Cut

Combines are designed to operate best when the cutter bar is adjusted to cut stubble that is from 1/3 to 1/2 the height of the standing grain. Only when the straw is short or the crop is down and lodged, should the cutter bar be lowered to cut short stubble. Too much straw thru the combine at one time overloads the machine. In overripe or crimpled grain, bent-over heads may be missed if the cutter bar is too high.

If the weeds are not too high, the cutter bar may be raised enough to reduce the amount of weedy material going into the combine.

Cylinder and Concave Adjustment

The adjustment of the cylinder speed and of the concave clearance controls the threshing and affects the efficiency of the straw rack and cleaning shoe.

With the cylinder speed too high, or the concave clearance too small, or both, some grain may be cracked and the straw broken fine. Small pieces of straw overload the straw rack and the cleaning sieves and cause grain to be blown onto the ground. With the cylinder speed too low or the concave clearance too great, or both, unthreshed heads go thru the machine and out with the straw.

Cylinder speed must be correctly maintained. Much grain is lost when a power take-off outfit is slowed down by heavy grain or by closing the tractor throttle on a corner. If field conditions reduce the power take-off speed, the tractor should be put into a lower gear so that the motor speed and the power take-off speed remain constant.

Under most field conditions, the combine operator must choose between over threshing to get out every kernel, with resulting higher grain loss in the cleaning sieves, or leaving a grain or two in the heads and reduce the cleaning loss.

Combine Adjustments

Rasp or rubber-covered angle-bar cylinder and concave (figure 1)

Cylinder speed, concave clearance, and shelling-plate clearance are adjustable. In general, recommendations in the instruction book are a dependable guide for adjusting cylinder speed and concave and shelling-plate clearance. The slowest cylinder speed and the greatest clearance recommended for the grain being threshed will assure less breakage of straw and consequently less loss of grain.

The good combine operator adjusts the cylinder speed several times each day. He may start earlier in the morning if the cylinder is speeded up to handle the tougher straw that is still damp. Thank as the straw dries out, he reduces the cylinder speed to prevent overthreshing; towards sundown, he increases the speed again as the straw becomes damp and tough.

In weedy fields, high cylinder speed and close concave clearance are objectionable because the weeds are broken up and the moisture in the stems and leaves coats the kernels of grain, thus raising the moisture content of the grain. Weed breakage can be reduced to a minimum if the concave and shelling-plate clearances are as great, and if the cylinder speed is as low, as recommended in the instruction book. This is best even though an occasional kernel is left in the heads.

Combine Adjustments

Spike-tooth cylinder and concave (figure 2)

When the cylinder and concave teeth are properly adjusted, the spacing on both sides and at the ends of the teeth is uniform (figure 2, a). If the cylinder and concave teeth are not set close enough, unthreshed heads will result (figure 2, b). Too little clearance on one side (figure 2, c) causes cracked grain.

The numbers of rows of concave teeth can be reduced when easily cracked grains are harvested with a combine. If removing the concave teeth does not eliminate grain cracking, the concave may be moved away from the cylinder to reduce threshing (figure 2, b). This may be the best setting for grain that is easily threshed, and also will reduce straw breakage and may reduce grain loss at the straw rack. It is always advisable to run with the fewest number of concaves. Grain racking and grain loss at the straw rack will be reduced.

Combine Adjustments

Reducing Grain Loss at the Straw Rack

Loss of grain at the straw rack is indicated by loose grain coming over the end of the rack with the straw (figure 3). The amount of grain lost can be determined by holding a hat or a bag at the end of the straw rack while the machine is running.

Grain loss is due to one or more of the following causes:

– Incorrect speed of the straw rack. On combines with adjustable straw-rack pulleys, the adjustment may be set for the wrong speed. If the straw-rack speed is non-adjustable, belt slippage may cause slow straw-rack speed. The straw-rack speed may be determined by a speed counter, and the instruction book gives the correct speed and the method of testing speed.

– Incorrect position of deflectors. Just above the rack are one or more deflectors of either canvas or metal that can be raised or lowered. The purpose of these deflectors is to force the straw down onto the rack as soon as it leaves the cylinder. Grain loss at the straw rack my be reduced by lowering the deflectors. If the machine has a down beater just back of the cylinder, the deflector should not be lowered to where it causes the down beater to wrap with the straw.

– Overthreshing. Over threshing breaks the straw and weeds into small pieces that may clog the rack and prevent kernels from falling thru into the grain pan.

– Overloading the machine with straw. Generally, loss of grain at the rack can be prevented by cutting a high stubble; if this is not enough, it may be necessary to go into a lower gear to reduce ground speed in relation to the speed of the combine mechanism, or to cut a narrower swath.

Combine Adjustments

Reducing Grain Loss in the Shoe and Fan

The amount of grain loss at the gaffer and cleaning sieves (figure 4) can be found by catching in a hat or a bag the loose grain that comes off the sieves. Over threshing may cause loss of grain at this point. Incorrect adjustment of the wind blinds or deflectors, or incorrect adjustment of the sieves, or the use of the wrong non-adjustable sieves may cause loss of grain.

If grain loss is too high and the machine is doing a good job of cleaning, the sieves may be opened until the clean grain becomes slightly dirty. At that time, the wind can be adjusted to remove all of the chaff. Should the grain still blow over the end of the sieves, the trouble may be due to the overloading of sieves by overthreshing; it may be necessary to lower the cylinder speed to reduce the breakage of straw.

Instruction Book

The instruction book furnished for each combine has detailed information about the adjustments for different crops harvested. Complete familiarity with this book enables any combine operator to use his machine most efficiently.


An efficient operator will: run the combine with the correct and uniform power take-off speed; set the reel to feed the grain uniformly onto the grain platform without shattering the heads and throwing the grain; prevent overthreshing by running with the cylinder speed as low as possible and the concave clearance as great as possible and still shell the kernels; prevent overloading the straw rack by running the combine at a slow ground speed cutting a high stubble, or cutting a part swath in heavy grain; adjust the cleaning sieves and blower to get out as much dirt and chaff as is possible without causing excessive loss of grain.

Trouble Chart

  • Loss of grain by reel and platform:
    • Reel not set to throw grain well back on canvas
    • Reel set too high to pick up lodged grain
    • Reel slats too narrow
    • Platform backstop too low
    • Outer end of platform not screened
  • Cracked grain:
    • Cylinder speed too high
    • Concaves set too high or too many concave teeth
    • Concave, or cylinder, teeth bent or out of alignment (spike-tooth cylinder)
    • Concave not parallel to cylinder
    • Cylinder and concave clearance too small (rasp or bar)
    • Threshed grain returned to cylinder with tailings
    • End play in cylinder shaft (spike-tooth cylinder)
  • Cylinder clogs:
    • Green or damp straw
    • Cylinder speed too slow
    • Too much straw
  • Grain loss at the straw rack:
    • Unthreshed heads:
      • Grain too damp
      • Cylinder speed too slow
      • Concaves too low or not enough concave teeth
    • Threshed grain:
      • Incorrect straw-rack speed
      • Straw deflectors stop high
      • Cylinder speed too high or concave clearance too small or both (overthreshing)
      • Overloading the machine with straw
  • Grain loss at the cleaning shoe:
    • Sieve openings too small
    • Overloaded sieves
    • Too much wind or wind direction wrong
    • Cylinder speed too great or concave clearances too small
    • Tailboard missing or not properly set
  • Poor cleaning:
    • Sieve openings too large
    • Overloaded sieves
    • Cylinder speed too great or concave clearances too small
    • Not enough wind
    • Weeds

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