The present invention relates to combine harvesters and more particularly to the cleaning mechanism on such combine harvesters. 
In known combine harvesters, grain is threshed and separated in a threshing and separating mechanism and the separated grain, together with impurities of all sorts, such as chaff, dust, straw particles, tailings, etc.  is fed to a cleaning mechanism for cleaning.  Clean grain is collected therebelow and fed to a graintank for temporary storage.  The tailings are separated from the clean grain and impurities for reprocessing.  This reprocessing either means recycling the tailings through the threshing and separating mechanism or treating them in a separate tailings rethreshing means. 
Recent developments in combine harvesters have led to so called rotary combines wherein both threshing and separating are accomplished in mechanism comprising rotary components cooperable with stationary threshing, respectively separating concaves and grates.  In conventional combines separating is accomplished by straw walkers.  In rotary combines the crop material is subjected to a much more aggressive and positive separating action during a relatively prolonged period of time whereby the efficiency of a rotary combine usually is greater than the efficiency of a conventional combine. 
Several types of rotary combines have now already appeared on the market or will appear on the market soon.  In one such rotary combine a conventional transversely extending threshing mechanism having a threshing cylinder and a cooperable concave is combined with a rotary separating mechanism of a width greater than that of the threshing mechanism and which is disposed parallel thereto with its ends extending transversely past the respective ends of the threshing mechanism and being arranged spirally to convey the crop material received from the threshing mechanism towards each of its ends while submitting the crop to a separating action. 
Such a rotary separating mechanism comprises at least one separator rotor rotatable within a rotor housing comprising separator concaves over its entire circumference, except at its top where covers are provided.  Rearwardly of the separator housing a deflector member is provided which deflects grain separated in the rear, generally upright separator concave towards the cleaning device therebelow.  This deflector member together with said concave and a section of the combine chassis form a downwardly facing cavity. 
The rotor or rotors comprise crop treating and conveying elements which are disposed at an angle relative to the rotor axis.  In practice it has now been experienced that these crop treating and conveying elements create air streams around the or all rotors which are directed from centrally below and rearwardly of said rotor or rotors in an upward direction around and over top thereof towards locations in front of and below the outer sections of said rotor or rotors.  As a result thereof, light particles such as chaff, short straw, etc.  are lifted from the cleaning mechanism and sucked into said cavity which soon is filled up therewith and which thus results in a plugging of the separator concave partially defining said cavity. 
In the separating mechanism described above, the incoming layer of crop material should be divided in two substantially equally sized halves, each of which is then spirally conveyed from the center portion of the separating mechanism to one of the opposite ends.  Occasionally tough crops such as wet, green crops with long straw or heavily weed infested crops cause problems in as far as the above described division of the crop layer is not accomplished as smoothly and fluently as is desired. 
Furthermore, the separating efficiency may vary with varying crops and crop conditions and occasionally this efficiency is not fully satisfactory.  This appears to be due to a great extent to the fact that the layers of crop material occasionally do not move smoothly and regularly through the separator mechanism whereby grain separation is hampered. 
However, in general rotary combine harvesters have a substantially increased threshing and separating capacity.  This results in a much heavier loading of the cleaning mechanism which now frequently appears to have become the capacity limiting factor. ( On conventional combines, the straw walkers usually are the capacity limiting factor).  This is particularly true for still another reason.  Indeed, as is experienced, rotary combines produce more short straw particles, etc.  which means an additional loading of the cleaning mechanism.  Increasing the size, such as the width of a cleaning mechanism, has not solved the problem. 
Indeed, in wide cleaning devices, it has been experienced to be very difficult to provide a substantially constant cleaning air blast transversely of the cleaning mechanism.  Uneven transverse air blast pattern, necessarily results in an inferior operation of the cleaning mechanism. 
Also, because of the increased volumes of material to be handled in cleaning devices on rotary combines, it has become necessary to provide cleaning fans of a much greater capacity.  This has proven to be difficult to obtain so far, unless the size of the fan is made considerably larger.  However, preferably, cleaning fans should no longer be increased in size. 
Furthermore, the chaffer sieve of conventional cleaning devices easily gets overloaded with the increased volumes of material to be processed in the cleaning device, and thus sieve losses under these circumstances soon reach an unacceptable level. 
Finally, if no special precautions were taken, material separated in the separating mechanism would fall directly onto the chaffer sieve at a location rearwardly of its leading edge.  This may result in increased sieve losses as grain in this material does not get enough time to be separated from the impurities and to fall through the meshes in the sieve. 
The foregoing illustrated limitations of the known prior art.  Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations as set forth above.