Abstract: In a shredding machine, oppositely rotating cutter shafts (11a, 11b) are driven by an electric motor (E) through an epicyclic gear mechanism (G) comprising a casing (17) with an internal gear formation (52) and a planet carrier (31) carrying planet gears (34) in mesh with the internal gear formation (52), the motor (E) driving a sun gear (35) in mesh with the planet gears (34), the drive to the cutter shafts (11a, 11b) being from the planet carrier (31). Preferably there are two planet carriers (31, 37), the planet gears (33) of the second carrier (37) being driven by a sun gear (32) driven by the first planet carrier (31), the cutter shafts (11a, 11b) being driven from the second planet carrier (37). One of the shafts (11b) is driven directly, and the other (11a) through further gearing (66, 38). The casing (17) is mounted for limited movement under high load, a switch (90) being provided to detect such movement, to stop or reverse the motor (E).