A material processing device has a rotor coupled to a rotatable shaft, and a rotor control system. The rotor control system comprises a rotor control mechanism with a first engagement member that is engagable with a second engagement member coupled to the shaft. The rotor control mechanism is operable into and out of an engaged state in which the first and second engagement members are engaged with one another. In the engaged state, the rotor control mechanism is operable in a locking mode in which the first engagement member is fixed in position to prevent rotation of the shaft, or in a rotation mode in which the first engagement member is movable to rotate the shaft. The material processing device may be a crusher, a shredder or a milling machine.