Power screw mechanism with rotation clutch

A method of operating a power screw mechanism, the power screw mechanism comprising a drive screw and a main shaft, the method comprising the steps of:

• providing a clutch between the drive screw and the main shaft, the clutch being configured to provide a frictional engagement between the drive screw and main shaft within a first range of torques applied to the drive screw, such that rotation of the drive screw causes rotation of the main shaft over the first range of torques, and the clutch being further configured to slip out of frictional engagement between the drive screw and main shaft within a second range of torques applied to the drive screw, such that rotation of the drive screw does not cause rotation of the main shaft over the second range of torques, the second range of torques being greater in magnitude than the first range of torques;
• and
• rotating the drive screw within one of the first and second ranges of torques.