Safety interlock for offset printing press

In an offset printing press having a mainframe, a printer head fixed to the mainframe, the printer head including a blanket cylinder and a plate cylinder, the plate cylinder and the blanket cylinder being rotatably supported by the printer head, the plate cylinder being movable into and out of an engagement position with the blanket cylinder wherein the plate cylinder is rotatably driven by a drive means at said engagement position, and a carriage mounted on the mainframe and linearly movable to and from the plate cylinder, the carriage including dampening and inking rollers, a safety interlock system for electrically deenergizing a motor mounted on the mainframe and driving the drive means, comprising:

• a pair of power lines across which the press motor is connected for electrical energization;
• a relay connected across the power lines, the relay being in electrically parallel relation with the press motor, the relay including at least one pair of relay contacts which are normally open when the relay is deenergized, the relay contacts being electrically series-interposed between the motor and one of the power lines;
• and
• a switch means electrically connected in series relation between the relay and one of the power lines, the switch means being responsive both to carriage movement to and from the printer head over a predetermined distance and to the plate cylinder movement into and out of its engagement position, wherein movement of the carriage away from the printer head for the predetermined distance, when the plate cylinder is in its engagement position, opens the switch means to deenergize the relay and electrically open the pair of relay contacts wherein the motor is electrically disconnected from one of the power lines, movement of the plate cylinder out of its engagement position closing the switch means and reenergizing the relay to electrically close the pair of relay contacts wherein the motor is reconnected to the power lines when the carriage is at the predetermined distance from the printer head.