per se known structure by which clutch the position of the member can be released from the movement of the shaft application_number US 6515579 A description first referring to fig1 to 3 a device for shed formation in a loom according to the invention comprises a plurality of mechanical units such as the one shown in fig1 each formed of a double conjugate race cam 1 a rocking lever 2 having feeling rollers 2 39 to the angular rotation of cam 1 corresponds a different angular rotation of roller lever 2 and on rotation of lever 2 the pin 3 moves through a well defined arc and a connecting rod 4 by means of which the movement of pin 3 is transmitted through a clamp 5 to a registering lever 6 thus the pin 7 of lever 6 can be moved through any arc in the range of maximum and minimum values depending on whether said clamp 5 is closer to or farther from the center of oscillation 8 of the lever the devices uses double race cams of the single type for 34 taffeta 34 dobbies that is cams for providing the so called 34 taffeta movement 34 according to which the position of the heald frames is alternatively high and low for successive blows of the loom each of the double race cams 1 is mounted on the operating shaft 9 which for reasons to be better explained hereinafter is an intermittently rotating shaft by means of a sleeve or bush 10 keyed to said shaft and terminating at its end externally of the cam with a projecting flange 10 39 on which a splined profile 11 is formed said cam 1 which is freely rotable on sleeve or bush 10 has an.