Method for operating a grain cleaning system in a combine harvester

1. A method for optimizing the operation of a grain cleaning system in a combine harvester during harvesting, said grain cleaning system comprising an upper chaffer sieve and a lower grain sieve disposed the one above the other, the upper and lower sieves having openings to permit passage of crop material by gavity through the openings of the upper and lower sieves, wherein the size of the openings of the upper sieve are adjustable, said method comprising:

harvesting a crop and guiding said harvested crop to a threshing and separating system in order to separate desired crop material from undesired crop material;

guiding at least part of said separated crop material to said grain cleaning system;

cleaning the separated crop material in said grain cleaning system by passing the material over the upper and lower sieves, whereby material that is not discarded from the combine harvester and that does not pass through the opening of the lower grain sieve is returned to the front end of the grain cleaning system for repeating the cleaning action thereby creating a material return flow; and

adjusting the openings of the upper sieve to enlarge the size of the openings for a period of time in response to registering a deviance from a threshold value for crop material load on the upper sieve such that the material return flow is adjusted wherein the deviance from the threshold value for crop material load on the upper sieve is registered by a sieve loss sensor system provided adjacent the upper sieve.

2. The method according to claim 1 , wherein the step of adjusting the openings of the upper sieve comprises adjusting a setting of the upper sieve openings for a period of time.

3. The method according to claim 1 further comprising enlarging the openings of the upper sieve in the grain cleaning system when the threshold value for crop material load on the upper sieve is exceeded, such that the material return flow is increased, thereby constituting a temporary buffer for the excessive crop material and limiting the sieve loss.

4. The method according to claim 1 , wherein the deviance from the threshold value for material load on the upper sieve is further registered by a field slope sensor system.

5. The method according to claim 1 , wherein setting of the upper sieve openings in the grain cleaning system are automatically adjusted for a period of time upon registering the deviance from the threshold value for crop material load on the upper sieve.

6. The method according to claim 5 , wherein adjustment of the setting of the upper sieve openings is controlled by means of a sieve overload control algorithm.

7. The method according to claim 5 , wherein adjusting the position of the upper sieve comprises increasing the opening of the upper sieve for a period of time such that the material return flow is increased and thereafter reducing said opening such that the material return flow is reduced.

8. The method according to claim 7 , wherein the opening of the upper sieve is increased immediately upon registering crop material load on the upper sieve exceeding the threshold value.

9. The method according to claim 7 , wherein the opening of the upper sieve is reduced after a period of time not exceeding a transport delay of the material return flow.

10. The method according to claim 9 , wherein the opening of the upper sieve is reduced upon registering crop material load on the upper sieve that no longer exceeds the threshold value.

11. The method according to claim 1 , wherein a ground speed of the combine harvester is adjusted upon registering the deviance from a further threshold value for crop losses and/or for an intake crop load rate.