To prevent introduction of paper scraps or torn paper into the inker of a web printing machine, a web paper tear sensor (17, 18) controls a hydraulic or pneumatic cylinder-piston unit (25) which severs engagement between an ink supply roller (13) and an ink application roller (12) while maintaining contact of the ink application roller with the plate cylinder (4) of the printing machine; the greater quantity of ink on the ink application roller will cause paper to adhere to the ink application roller, and thus wrap itself about that roller, severance of contact with the ink supply roller (13) preventing introduction of torn paper into the inker.