Grain cleaning system for a combine harvester

1. A drive linkage for reciprocating a sieve that is suspended within a frame of a combine harvester for movement in mutually orthogonal directions, the drive linkage comprising:

a cranking mechanism, the cranking mechanism imparting a reciprocating motion to the sieve in a fore and aft movement in the direction of travel of the harvester; and

a pivot arm pivotably connected at one end to one side of the sieve, the pivot arm configured to impart movement to the sieve such that the sieve moves in an arc centered on a pivot axis at the opposite end of the pivot arm as the sieve is reciprocated, wherein means are provided for displacing the pivot axis of the pivot arm relative to the frame in synchronism with the reciprocation of the sieve, further wherein the means displaces the pivot axis inwardly when the sieve is at the limits of the fore and aft movement and moved back to a central position when the sieve is at a midpoint in the fore and aft movement, in such a manner as to cause the sieve to follow a substantially rectilinear path relative to the frame.

2. A drive linkage according to claim 1 , wherein the pivot axis of the pivot arm is carried on a support lever that is pivotable relative to the frame by means connecting the support lever to the cranking mechanism.

3. A drive linkage according to claim 2 , wherein the means connecting the support lever to the cranking mechanism comprise a variable length actuator.

4. A drive linkage according to claim 1 , wherein the cranking mechanism comprises:

a rocker plate, the rocker plate being pivotably mounted on the frame of the combine harvester;

a cranking arm, the cranking arm configured to rock the plate; and

a connecting rod connecting the rocker plate to the sieve, such that a reciprocating motion is imparted to the sieve as the plate is rocked by the cranking arm.

5. A drive linkage according to claim 4 , wherein the cranking arm is an eccentric mechanism.

6. A drive linkage for reciprocating a sieve that is suspended within a frame of a combine harvester for movement in mutually orthogonal directions, the drive linkage comprising:

a cranking mechanism, the cranking mechanism imparting a reciprocating motion to the sieve in a fore and aft movement in the direction of travel of the harvester, the cranking mechanism comprising:

a rocker plate, the rocker plate being pivotably mounted on the frame of the combine harvester;

a cranking arm, the cranking arm configured to rock the plate; and

a connecting rod connecting the rocker plate to the sieve, such that a reciprocating motion is imparted to the sieve as the plate is rocked by the cranking arm; and

a pivot arm pivotably connected at one end to one side of the sieve, the pivot arm configured to impart movement to the sieve such that the sieve moves in an arc centered on a pivot axis at the opposite end of the pivot arm as the sieve is reciprocated, wherein means are provided for displacing the pivot axis of the pivot arm relative to the frame in synchronism with the reciprocation of the sieve, further wherein the means displaces the pivot axis inwardly when the sieve is at the limits of the fore and aft movement and moved back to a central position when the sieve is at a midpoint in the fore and aft movement, in such a manner as to cause the sieve to follow a substantially rectilinear path relative to the frame, and wherein the pivot axis of the pivot arm is carried on a support lever that is pivotable relative to the frame by a variable length actuator, the variable length actuator extending between the support lever and the rocker plate.

7. A combine harvester, the combine harvester including a drive linkage, the drive linkage configured to reciprocate a sieve that is suspended within a frame of a combine harvester for movement in mutually orthogonal directions, the drive linkage comprising:

a cranking mechanism for imparting a reciprocating motion to the sieve in the direction of travel of the harvester; and

a pivot arm pivotably connected at one end to one side of the sieve, the pivot arm configured to impart movement to the sieve such that the sieve moves in an arc centered on a pivot axis at the opposite end of the pivot arm as the sieve is reciprocated, wherein means are provided for displacing the pivot axis of the pivot arm relative to the frame in synchronism with the reciprocation of the sieve in such a manner as to cause the sieve to follow a substantially rectilinear path relative to the frame.

8. A combine harvester according to claim 7 , further comprising a control system for setting the mean position of the pivot axis of the pivot arm in dependence or ground slope, in order to take account of the density of the grain being harvested.