1. Field of the Invention
The present invention pertains to sheet-fed, offset printing presses. In particular, the present invention pertains to an improved chain delivery assembly for an offset printing press which prevents contact of ink wet sheets with mechanisms of the delivery system.
2. Description of the Prior Art
In sheet-fed, offset printing presses, a sheet of paper is drawn between an ink coated blanket cylinder and an impression cylinder. An image is transferred to the paper sheets in the form of wet ink. It is necessary that the wet ink, freshly printed sheet be supported and guided during transfer of the sheet from the impression cylinder to a delivery station or another printing station for further printing thereon. This is accomplished, in most printing presses, by a delivery system which includes a chain conveyor carrying one or more gripper assemblies having sheet grippers which grip and pull the sheet from the impression cylinder.
The typical chain delivery system includes a pair of sprockets mounted on opposite ends of a shaft for rotation about an axis which is parallel to the axis of the impression cylinder. A chain extends around each of the sprockets and has horizontal upper and lower runs. The sheet grippers of the gripping assemblies carried by the chains grip the leading edge of each sheet after it passes through the impression cylinder.
A long-standing problem associated with offset printing presses of the prior art is avoiding contact of the wet ink on the sheets with the shaft which extends between the two sprockets. Any contact of the shaft with the ink will likely cause smearing. Most systems attempt to avoid such problems by placing small wheels, sometimes referred to as "star wheels" or "skeleton wheels", on the shaft to hold the sheets away from the shaft. The wheels are preferably positioned to contact the sheet of paper where no ink is deposited. This is not always possible, particularly in sheets with extensive amount of ink coverage.
A number of mechanisms have been proposed for eliminating the problem of smearing of ink by the shaft which extends between the two sprockets of the chain delivery assembly or by the star wheels or skeleton wheels mounted on the shaft. In U.S. Pat. No. 4,665,823 the chain delivery system is provided with hollow cylinder support bars of resilient, rigid plastic material which has triboelectric charge directing properties resistive to printing ink and printing ink solvents. These bars are supposed to prevent contact of the ink wet sheet with the sprocket shafts.
In U.S. Pat. No. 4,722,276, a horizontal tube is mounted adjacent to and parallel to the axis of the shaft connecting the sprockets. The tube is provided with circular openings through to which air is discharged against the sheets of paper preventing them from contacting the shaft.
In U.S. Pat. No. 4,977,828, a frame assembly is coupled to the sprocket shaft which is provided with elongated cylindrical rollers which are mounted for free rotation about axes extending parallel to the sprocket shaft for engagement with the wetted side of the sheet during transportation. The rollers are covered with a fabric impregnated with a liquid repellent substance and are supposed to reduce smearing.