A rotor locking device (113) in a crusher (1) for locking a rotational position of a rotor shaft (6). A shaft engager (105) includes a female toothed mating surface (120) for coaxial engagement with a male mating surface (121) at an end of the rotor shaft (6), so they become rotationally interlocked. A fastening portion (122) with engager apertures (123) overlaps with a bearing mount (200) surrounding the rotor shaft (6) so it can be fastened therewith. Preferably, the engager apertures (123) are circumferentially elongated in order to accommodate alignment with fastening points (205) in the bearing mount (200), at least when a closest rotational mating position is chosen for alignment. This enables any rotational position of the rotor shaft (6) to be locked, for safety, before having to access the crusher chamber. This safety feature is desirable when the rotor of the crusher (1) has become blocked and requires maintenance.