Abstract: A robotic surgical instrument comprising: an articulated end effector; an instrument interface configured to engage with and being driven by a corresponding robot arm interface of a surgical robot arm; a casing housing a drive mechanism connected to the instrument interface, the drive mechanism comprising a drive gear; and a shaft pivotally connected to the casing at a proximal end and connected to the articulated end effector at a distal end, the shaft comprising a driveable shaft member configured to drive articulation of the articulated end effector, and a shaft gear attached to the driveable shaft member, the shaft gear meshing with the drive gear such that rotation of the drive gear drives the shaft gear to rotate which drives the driveable shaft member which drives articulation of the articulated end effector, wherein the shaft gear is pivotable with respect to the drive gear when the shaft gear and drive gear are meshed together.