action of the cam 11 is substantially radial with respect to the cylinder 12 a spring 14 stretched between a fixed pin 15 and the free end of the arm 8 maintains the contact between the roller 10 and cam 11 while a spring 16 stretched between a pin 17 of the support 6 and pawl 5 tends to maintain the latter in contact with the peripheral tooth formation 4 the annular cam 11 has a peripheral profile having in the direction of rotation a first outside sloping part 11a which extends over at least one quarter of a circle preferably over a half circle followed by a second outside sloping part 11b of appreciably steeper inclination angle which terminates at the topmost point 11c of the cam and is followed by an inside sloping part 11d the radial increment of each part 11a 11b is substantially equal to one half the difference between the topmost point 11c of the cam and the lowermost point 11e that difference being equal to a displacement of two teeth by the pawl 5 along the tooth formation 4 the downward part 11d may slope progressively downward to the lowermost point 11e thus exhibiting a slope angle which is substantially twice that of the part 11a shown in the drawings or alternatively may have a greater inclination angle and be for example connected to the start of the part 11a by a circular part or section the device according to the invention further comprises a control lever 18 journalled to the frame 3 along an axis parallel to the axis of the drum 1 and of substantially square bracket configuration an arm 18a of the lever 18 lies.