Dual sickle knife drive with variable phase angle

A crop cutting apparatus comprising:

• a frame structure arranged for forward travel over ground having a standing crop thereon;
• a cutter bar secured to the frame structure and extending transversely across a front end of said frame structure;
• a plurality of knife guards mounted in spaced relation along said cutter bar and projecting forwardly therefrom;
• each of said guards having a ledger surface with opposed side edges thereof arranged to provide first and second shearing edges;
• a first and a second sickle bar mounted in transversely extending position and being driven for reciprocating movement relative to said knife guards;
• the first and second sickle bars each having a plurality of knife blades mounted thereon for movement therewith with each of the knife blades having two side cutting edges which cooperate with said shearing edges of said knife guards;
• a drive system driving the first sickle bar through repeated cycles of reciprocating movement and driving the second sickle bar through repeated cycles of reciprocating movement;
• wherein the drive system is arranged such that the reciprocating movements of the first and second sickle bars occur at a phase angle difference;
• the drive system having a phase difference control component operable to change the phase angle difference between the first and second sickle bars from a first phase angle difference to a second phase angle difference which is different from the first phase angle difference.