position following an initial angular rotation of shaft 28 and is moved afterwards to an intermediate c position during a large part of the revolution cycle the curve of fig2 also illustrates the movement of the connecting rod 21 the movement obtained from cam wheel 29 by locking the lever 5 is illustrated in fig3 by curve 42 on the other hand the movement from cam wheel 30 by locking the lever 6 is illustrated on curve 43 quite obviously the curve of fig2 is a combination of both curves 42 and 43 whereby it assumes at each moment the profile of curves 42 or 43 having the largest instantaneous amplitude fig2 and 3 illustrate also very clearly the advantage of the use of the two levers 5 and 6 if by means of one lever a movement must be applied to a thread clamp 2 as illustrated by the curve of fig2 the cam wheel must be designed in such a way that the cam curve has the same shape as the curve of fig2 the lever end cooperating with the locking means comes only for a short while in its lowest a position in such a case whereby.