that longitudinal axis 24 is not visible there because of the eccentric arrangement of shaft 11 in eccentric sleeve 19 upon rotation of eccentric sleeve 19 shaft 11 is subjected to a rotary motion around longitudinal axis 24 of eccentric sleeve 19 so that small bevel gear 14 can be brought into engagement with large bevel gear 15 or with large bevel gear 16 depending on the rotational state of eccentric sleeve 19 fig3 further shows that small bevel gear 14 is joined nonrotatably to shaft 11 eccentric sleeve 19 is nonrotatably joined to a shifting plate 25 that can be actuated by an operator shifting plate 25 and eccentric sleeve 19 are embodied in such a way that upon a rotation of shifting plate 25 through an angle of 180 degrees crank drive system 10 can be conveyed from a first operating state to a second operating state in the first operating state small bevel gear 14 is in engagement with large bevel gear 15 in the second operating state small bevel gear 14 is in engagement with large bevel gear 16 fig4 a 5 a and 6 a each show a plan view of crank 13 and of shifting plate 25 shifting plate 25 is shown in three different respective positions which correspond to the three operating states provided for crank drive system 10 according to the present invention fig4 b 5 b and 6 b arranged alongside respectively show side views of crank drive system 10 according to the present invention in the operating states that correspond respectively to the shift positions of shifting plate 25.