Clutch mechanism

A surgical tool holding mechanism configured to be coupled to a linear drive device, the surgical tool holding mechanism comprising:

• a clutch mechanism;
• and
• a carrier configured to be fitted to a threaded shaft of the linear drive device by the clutch mechanism, the clutch mechanism comprising:
• a housing;
• a first gear coupled to the housing and having gear teeth configured to engage the threaded shaft of the linear drive device, such that rotation of the shaft causes the housing to move along the shaft in a first mode of operation;
• and
• a first clutch arrangement associated with the first gear and configured to prevent substantial rotation of the first gear with respect to the housing when a rotational force applied to the first gear is less than a threshold force and to permit rotation of the first gear with respect to the housing when a rotational force applied to the first gear is greater than the threshold force, such that the housing is manually moveable along a length of the shaft, in a second mode of operation, by exerting a manual force on the housing such that the rotational force applied to the first gear through the interaction of the threaded shaft and the gear teeth exceeds the first threshold, wherein:
• the threshold force is determined by a compressive force across two ends of the first gear,
• the first clutch arrangement further includes a resilient biasing element which is configured to apply the compressive force across the two ends of the first gear,
• the housing is formed of two parts which are coupled together, and
• the two parts of the housing in the region of the first gear are coupled together by engagement of respective hooked coupling members.