This invention relates generally to printing equipment.
More particularly, the invention relates to equipment for adjusting the gap between the ductor roller and the associated doctor blade of the ink supply system in a printing machine.
The purpose of the doctor blade is to strip excess ink off the surface of the ductor or ink pickup roller. To control the rpint quality it is necessary to be able to control the thickness of the ink film which remains on the roller; usually, a uniform thickness of this film over the entire length of the roller is not sufficient. Therefore, the width of the gap between the roller and the doctor blade is made separately adjustable in a plurality of ink zones which are arranged adjacent one another, in direct lengthwise of the roller and blade. The adjustment is effected by exerting pressure upon sections of the doctor blade edge which extends along the gap--and the pressure is applied by socalled ink-control keys (screws) which act upon the doctor blade either directly or via appropriate levers.
It is important to be able to reproduce any particular gap width at any particular point of adjustment--which of course means that the setting of the particular ink-control screw must be similarly reproducible. This is possible only if a well-defined zero position can be achieved for each of the screws, i.e., a position in which the gap is so adjusted that the ink is stripped almost completely off the ductor roller, leaving behind only the thinnest of ink films.
Since there are a great many of these ink-control screws on each machine (where they are not always in readily accessible positions), and since an operator often has to service more than one machine, it is not practicable to have to adjust the screws manually and in film. Remotely controlable ink-control screws have therefore been developed.
One of these has been proposed in GDR patent DL-PS No. 139,114. It has an axial abutment which is clamped onto the shaft of the screw and which, when it engages a cooperating counter-abutment, limits the extent to which the screw can move in the sense of reducing the width of the gap between doctor blade and ductor roller. To be able to effect gap adjustments with the desired accuracy, ink-control screws have threads with only a small pitch. In the context of this prior-art arrangement this small pitch is a disadvantage, because the engagement of the two abutments with one another--and the small pitch of the thread--produce a high force in axial direction of the screw which results in seizing of the thread and prevents, or at least hinders, subsequent backing-off of the screw. A reproducability of the setting--and of the ink-film thickness--is thereby prevented, with resultant misprints and machine down-time.
There are other advantages associated with prior-art remote-controlled ink screws. For example, to effect zero-setting adjustments of such screws it is necessary to advance the screws (and close the gap) until no ink at all is allowed onto the surface of the ductor roller. Such an adjustment is not reproducible. Moreover, it may result in damage to the doctor blade and/or the ductor roller, and is in any case inaccurate because it may result in flexing of the ductor roller.
Also, the prior art permits the screws to be adjusted only in the sense causing the gap between the blade and the roller to become narrower. However, it is often necessary to increase the gap width, rather than reduce it, and in that event the prior-art teaching is not useable.