Cutting unit with double knife system and movable sliding surface

A cutting unit (2) for attachment to a combine harvester which comprises a frame (4), a knife bar (6), on which a cutting system is arranged, a depositing surface (8) for depositing cut stalk material, conveying apparatuses (10) for conveying away the cut stalk material, drive apparatuses for driving the cutting system with a mechanism (38) which is arranged at a location between side walls (36) of the cutting unit (2), the mechanism (38) transmitting drive power via output shafts (46) which are guided outwards out of a mechanism housing on an upper side of the mechanism (38) to drive rocker arms (34) which are connected fixedly to the output shafts (46) for conjoint rotation and are drive-connected to lifting rods which are driven in oscillating movements as part of the cutting system via the drive rocker arms (34), further including a double knife cutting system (11) that is configured as the cutting system on the knife bar (6), in the case of which double knife cutting system (11) a first part of knife blades (18a) present on the cutting unit (2) are fastened to a first knife back (20a), this first part of knife blades (18a) together with the first knife back (20a) forming an upper knife (22), a second part of the knife blades (18b) present on the cutting unit (2) being fastened to a second knife back (20b), this second part of the knife blades (18b) together with the second knife back (20b) forming the lower knife (24), the upper knife (22) and a lower knife (24) being driven in opposite directions, and, of the drive rocker arms (34), a first drive rocker arm (34) being drive-connected to the upper knife (22) and a second drive rocker arm (34) being drive-connected to the lower knife (24).