In web offset printing, a desired image is repeatedly printed on a continuous web or substrate such as paper. In a typical printing process, the continuous web is slit in the longitudinal direction (the direction of web movement) to produce a plurality of continuous ribbons. The ribbons are aligned one on top of the other, folded longitudinally, and cut laterally to produce a plurality of multi-page, approximately page-length segments, each of which is termed a signature. The term signature also encompasses a single printed sheet that has or has not been folded. Because more than one different signature can be printed at one time, it is often desirable to separate the different signatures by transporting successive signatures in different directions or paths.
One way to accomplish the sorting of a single stream of signatures is to use a diverter mechanism, such as a diverter wedge, to divert successive signatures to one of two paths. Once diverted, the signatures typically are transferred to a conveyor using rotating buckets (also known in the art as fans, fan wheels, paddle fans, or rotary flywheels).
Typically, two sets of rotating buckets assemblies are utilized, one set to deliver signatures traveling along a first path to a first conveyor and the other set to deliver signatures traveling along the second path to a second conveyor. Each set of bucket assemblies includes several individual buckets arranged at a spaced distance from one another along a common axis or shaft. Each bucket has multiple aligned blades that define pockets or slots between them for receiving signatures and transferring the signatures to the respective conveyor.
It is desirable to increase the operating speed of a printing press in order to increase the printed product output. However, as the rotational speed of the buckets is increased, it is more difficult to ensure the reliable operation of the buckets and to ensure that signatures are not damaged. For example, signature quality problems that can occur at higher press speeds include ink offset, dog-eared edges, and defects to both the leading and trailing edges of the signatures. These and other defects can lead to paper jams in the folder, resulting in press downtime and expense.
When the signatures are not snugly held between guide belts, the signatures may flutter or, when the signatures are folded signatures, the signatures may open partially. Signature fluttering and/or opening can result in damage to the signatures in the folder. Signatures may also “free fall ” (i.e. travel without any belt guidance whatsoever) as they move through the folder, resulting in the potential for the signatures to flutter and/or opening.