Device for controlling the movement of a web through a printing machine

A printing machine comprising:

• a rotary printing cylinder having a printing speed for printing a web, with the possibility of changing the format, said machine operating in successive printing cycles and each cycle being defined as one rotation of the printing cylinder and involving the printing of one area of the web in the said format;
• a first endless traction device and another endless traction device both located downstream of the printing cylinder for engaging and conveying the web independently of one another after it has travelled past the printing cylinder, said first endless traction device conveying the web forwards at a take-up speed which is at most equal to the printing speed and comprising a freely rotating web engaging sprocket device having a stop and a rotor which is continuously driven at the take-up speed, said rotor having a stop which drives the sprocket device by the contact of its stop with the stop of the sprocket device whereby said sprocket device is free to intermittently rotate at a higher speed than the take-up speed but constrained to rotate at the take-up speed when its stop contacts the rotor stop, and said another endless traction device comprising a rotary member for intermittently conveying the web at the printing speed, in synchronisation with printing contact between said printing cylinder and the web;
• and
• a device upstream of the printing cylinder for pulling the web backwards in order to re-absorb, during each printing cycle, any excess web length resulting from a difference between conveying at the printing speed and conveying at the take-up speed.