Controls for a grain cleaning system for a combine harvester

A drive linkage is described for reciprocating a sieve (10) that is suspended within a frame of a combine harvester for movement in mutually orthogonal directions. The drive linkage comprises a cranking mechanism (12,14,16,24) for imparting a reciprocating motion to the sieve in the direction of travel of the harvester and a pivot arm (36) pivotably connected at one end to one side of the sieve to cause the sieve to move in an arc centred on a pivot axis (34) at the opposite end of the pivot arm as the sieve is reciprocated. The pivot axis of the pivot arm is displaced 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. A control system monitors a variable speed fan and changes the lateral movement of the sieve in dependence of the ground slope and also of the fan speed signal to compensate for variation in grain density.