referring now to the drawing and , first , particularly , to fig1 thereof , there is shown fragmentarily therein , in a side elevational view , a section of a rotary printing machine for processing sheet material 1 . the section of the printing machine that is shown is an offset printing unit 2 which includes a printing form cylinder 3 , a rubber blanket cylinder 4 , and an impression cylinder 5 having at least one gripper system 6 for holding the printing material 1 . the rubber blanket cylinder 4 is equipped with a rubber blanket that has good qr ( quick release ) characteristics ; however , it is usually impossible to prevent the printing material 1 from remaining stuck completely to the rubber blanket cylinder 4 after the printing material 1 has passed through a printing nip 7 at the side that has been printed by the offset printing unit and from being picked up thereafter by the impression cylinder 5 . the size of a tear angle α between the printing nip 7 and a tear line 8 of the printing material 1 depends upon the adhesiveness ( tack or tackiness value ) of the ink that is applied by the rubber blanket cylinder 4 , the paper quality of the printing material 1 , and the speed of the peripheral surface of the rubber blanket cylinder 4 , which corresponds to the speed of the printing machine . in the region of the tear line 8 , the ink that is located between the printing material 1 and the rubber blanket cylinder 4 forms so - called tear threads 9 ( note fig2 ), which are purposefully severed by a plurality of piezo - electric ultrasonic sources 10 , 11 and 12 . ultrasonic waves 13 , 14 and 15 of the ultrasonic sources 10 , 11 and 12 , respectively , are focused on the tear threads 9 , so that the ultrasonic waves 13 , 14 and 15 which impinge on the tear threads 9 serve to reduce 90 % or more of the tackiness of the ink locally in the tear threads 9 , and consequently sever the tear threads 9 more easily . each of the ultrasonic sources 10 , 11 and 12 is disposed , together with additional ultrasonic sources which are concealed thereby in fig1 in a wedge - shaped space 16 located between the printing material 1 and the rubber blanket cylinder 4 in a row extending over the whole width of the printing material 1 axially parallel to the rubber blanket cylinder 4 and the tear line 8 . the invention is based upon the recognition that , in a labile system of complexly interrelated influences such as the characteristics of the rubber blanket and the printing material 1 as well as the speed of the printing machine , in which system , the slightest variations of parameters , such as the speed of the printing machine , can change the result considerably , a strong , defined additional influence must be introduced , which superimposes the labile system on smaller influences and , in this manner , stabilizes it . with this recognition in mind , the characteristics of the ultrasonic waves 13 , 14 and 15 are exploited in that , in impinging upon the ink , they lower the adhesive power thereof to a value below 10 % of the starting situation . thus , a line of preliminary severing of the tear threads 9 is created directly behind or downline from the printing nip 7 by the purposeful and focused use of the ultrasonic waves 13 , 14 and 15 . should the ultrasonic waves 13 , 14 and 15 not have been used , the tear angle α would grow with increasing speed of the printing machine and would shrink with diminishing speed of the printing machine . due to the tearing of the tear threads 9 , which is controlled by the ultrasonic waves 13 , 14 and 15 , the tear line 8 is stabilized , so that the predetermined tear angle α remains constant , assuming variations in the speed of the printing machine . furthermore , the tear line 8 no longer wanders forward or backward along the circumferential line of the rubber blanket cylinder 4 if changes should occur in other parameters .