A tubular drive motor is located in an external tube (20) and rotates a drive ring (21) connected to a shaft (19) which carries the drive gear (13). An internal connector (22) is secured to the external tube, with a pair of pins (30,37) which may be selectively engaged with the drive ring and with a coaxial gear (14).The connector is actuated by a tangential screw (15) which is in turn actuated by a manual handle. A sleeve (39) coaxially surrounds the shaft while allowing it to rotate freely, and is engageable with the connector when moved axially by cooperation of a pin (44) on the coaxial gear and a helical slot (43) of the sleeve.
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