An annular locking device (10) for the winding of reels comprises an inner ring (12) and an outer ring (13) coaxial with each other and radially superimposed so as to be rotatable relative to each other. Between inner ring (12) and outer ring (13) there are arranged engaging elements (14) which protrude radially from openings in the outer ring, the outer ring being rotatable on the inner ring between a first angular rest position and a second angular locking position. In the first angular rest position the engaging elements (14) are elastically retractable against the action of elastic means by a greater amount than in the second angular locking position. The inner ring has a circumferential groove (15) which houses engaging elements (14) and which has a bottom (16) shaped so as to vary the depth of the groove along its extension around the inner ring with an alternation of deeper zones and shallower zones arranged so that, upon rotation of the outer ring between the angular rest position and the angular lock
