Actuating device with double screw connecting means

An actuating device comprising:

• a non-rotating and translating longitudinal shaft,
• a rotating and translating internal sleeve positioned coaxially to and around the shaft,
• a rotating and non-translating external sleeve positioned coaxially to the shaft and around said internal sleeve,
• internal screw connecting means provided between the longitudinal shaft and the internal sleeve,
• external screw connecting means provided between the internal sleeve and the external sleeve, wherein
• the internal screw connecting means and the external screw connecting means are adapted to move the longitudinal shaft in the same longitudinal direction when the external sleeve is rotated in the same rotary direction;
• wherein
• the direct efficiency of the internal screw connecting means is higher than the direct efficiency of the external screw connecting means;
• and wherein
• terminal abutment means are provided for limiting the longitudinal motion of the internal sleeve with respect to the external sleeve at corresponding terminal longitudinal positions;
• second terminal abutment means are provided for limiting the longitudinal motion of the longitudinal shaft with respect to the internal sleeve at corresponding terminal longitudinal positions;
• and
• self-releasable coupling means for maintaining a coupling state between the internal sleeve and the longitudinal shaft are provided on one of the terminal abutment means provided for limiting the longitudinal motion of the longitudinal shaft with respect to the internal sleeve, such that the resistant torque induced by the self-releasable coupling means is higher than the driven torque of the external friction screw connecting means.