Offset Printing Unit

Printing unit consists of inking and dampening units , the plate cylinder with the printing plate, the blanket cylinder with the blanket fixed to it, and the impression cylinder.

The plate cylinder with the inked printing plate rolls over the circumference of the blanket cylinder. The blanket cylinder, in turn, rolls over the circumference of the impression cylinder, on which the sheet of paper is held by grippers. The contact line between the blanket cylinder and the impression cylinder is called the printing nip.
The printing plate – an up to 0.3 mm thick sheet of metal or a foil – carries the image elements of the respective color separation. The blanket is an approximately 2 mm-thick, exchangeable cylinder cover made from flexible material and layers of fabric.
To ensure perfect transfer of the print image from plate to paper, all three cylinders must have the same circumferential speed at the contact line/printing nip.
The offset printing unit is driven by the impression cylinder via a gear train. The printing unit in turn transmits the necessary drive power for the inking unit.
An important aspect of gearwheel design is to provide for an adjustable spacing of cylinders, so that certain operating conditions can be met (engaging and disengaging of the cylinders).When setting the paper
thickness, the variable spacing between blanket and impression cylinder shafts must be given particular consideration when going to impression on.
For the proper transfer of ink between plate and blanket and between blanket and paper, a sufficiently high contact pressure between the cylinders as well as the same circumferential speed of the cylinders is necessary.
To adjust the contact pressure between plate cylinder and blanket cylinder, which move towards each other at a fixed axial clearance, the plate or the
blankets are under laid, a process which results in a 0.05–0.15 mm radial deformation of the blanket. The printing pressure between the blanket and the impression cylinder is set according to the nature of the substrate.
The gearwheel-drives produce slippage between the surfaces. The slippage means that, in addition to the radial deformation of the blanket in the contact zone, the cylinders also undergo tangential deformation. The forces cause the deformation of the blanket to die down abruptly during the rotation of the cylinders at the start of the gap and are just as abruptly built up again when the cylinders come into contact again. This fluctuation of the contact pressure between zero and the maximum value is also referred to as “cylinder gap shock.”
It causes vibrations in the printing unit which impair the printing result.
Bearer rings, also called “cylinder bearers”, made from hardened steel with high rolling strength are inserted on the sides of the plate and blanket cylinders to minimize the effect of this vibration. They are of approximately
the same diameters as the working pitch diameters of the gears and roll upon one another at high initial tension.
For the proper interaction of plate, blanket and impression cylinders it is essential that they be connected to each other so that no relative motion
takes place in a circumferential or radial direction, that is, as little vibration as possible is produced. Thus the cylinders have to be fixed to each other with a minimum of oscillation. Therefore, the structural design of cylinder bearing and printing unit frames takes on considerable importance.

For all three cylinders the bearing assemblies must be dimensioned for high forces. Moreover, to ensure an accurate ink transfer, the cylinders must be very rigid and rotate with zero backlashes.
Due to the diagonal register adjustment, the plate cylinder must also allow for an angular adjust-ability of the bearings and the blanket cylinder bearing must allow for complex switching motions.
The cylinders with their bearings are supported between two vertical side
frames which are mainly made from high-quality casting material. Alongside supporting the cylinders, the press frame has the task of absorbing the forces and torque that occur during operation of the printing unit.
There are many components that move against one another in a printing unit. In the interests of continuous functional performance and the service life of these components, adequate lubrication is essential.

The bearings and gears, in particular, place high demands on the lubricant. From a technical point of view, oil lubrication is the most suitable. However, leakage can be a problem. Therefore, for presses of small- to medium sized formats liquid grease is also used.