can be still further shortened by a second stationary brake jaw 72 positioned at the end of the pivoting path of the spindle when moved into the inoperative position a stop 8 is associated to the pivot arm 4 carrying the spindle 1 and limits the pivoting motion of the pivot arm 4 on pivoting the spindle 1 into the operating position at the point at which the spindle is located exactly underneath the bore 34 in the cover 30 this stop 8 acting as a counter force can also if necessary be replaced by other means for example by magnetic means when the spindle shown in the stopped position in fig1 and by dot dash lines in fig2 is to be brought into the operating position the support bolt 6 is rotated by means of the handle 62 and the pivot arm 4 is hence pivoted in the horizontal plane out of its position 4 39 in the direction towards the tangential belt 11 until it strikes against the stop 8 fig2 simultaneously with the pivot arm 4 the lever 7 located in the position 7 39 is also pivoted horizontally by the rotation of the support bolt 6 the belt lifting roller 70 arranged on the lever 7 and lying as seen in the direction towards the tangential belt behind the spindle or respectively its drive whorl pushes the belt away from the spindle 1 and its drive is prevented the pivoting motion of the pivot arm 4 and also for the time being of the lever 7 also is ended when the pivot arm 4 strikes against the stop 8 and the spindle 1 is.