A screw-tightening spindle comprising a drive sleeve associated with a chuck comprising a cylindrical support member fitted with moving jaws, the cylindrical support member being provided externally with at least one stud extending radially outwards, the sleeve including at least one slot having a first segment opening out in a free edge of the sleeve and a second segment having an end portion forming an angle relative to the sleeve suitable for retaining the stud in the end portion of the slot when the stud is engaged therein, the sleeve being fitted with a ring having at least one stop extending radially inwards and received in the slot, the ring being mounted on the sleeve to slide between a locking position in which the stop extends in the first segment of the slot in register with the second segment, and an unlocking position in which the stop is retracted into a third segment of the slot.