A drive transfer element for transferring drive between a surgical robot arm and a surgical instrument, the drive transfer element comprising a drive transfer element recess being releasably engageable with an interface protrusion； and a drive transfer element protrusion being releasably engageable with an interface recess； the drive transfer element protrusion comprising a cavity in communication with the drive transfer element recess so as to enable the interface protrusion engageable with the drive transfer element recess to project into the cavity.
