I have communicated with printers from many countries regarding this issue, and heard horror stories of costs related to solving it, from completely re-gearing a press to installing a new press with a new Kompac! In several cases a printer had spent many times ( 4 & 5 times) the cost of a Kompac to no avail.
The primary cause of tone bars accross the sheet is a ‘miss-calibrated Kompac’.
A seizing metering roll bearing, a’very glazed’ form roller will cause it, and a very shiney (slick) metering roll will too, but the most common cause is the ‘lack of pressure’ between the metering and form roll!
Remember, the metering roll is powered by friction against the form roll.
If you do not have proper/adequate nip pressure, you will run too wet, which often causes pressmen to reduce ink tack, and the resulting condition is tone bars accross the sheet.
To re-calibrate your Kompacs’ nip pressure, clean it and loosen the excentric rings. Use two 1″ strips of 20-lb bond (or ideally two 1″ wide strips of a compareable gauge, .004″ plastic) to detect metering roll parallel to the form. Once you have achieved an even, snug pull at both sides of the Kompac with your feeler strips, lock the set screws to set roller parallel. Now, loosen set screws that set the indicator rings to the excentrics and move them to ‘O’. Now, move each ring to read ‘2.5’ and you are set at what I call the ‘midpoint’. From this setting (if your form roll durometer and surface finish is within specs) you can now run with the necessary moisture to print dense color with moderate tack inks.
If, you experience a ‘too wet’ condition at the 2.5 setting and you are ‘not’ using a high tack ink, you can reduce moisture by increasing nip pressure by moving indicator rings to a setting of 3.0. If your Kompac runs too dry at 2.5, chances are your form roller is too hard (above 62 Shore) or glazed. If ‘not’ glazed, replace it. If glazed, hand scrub it with a good/strong ink deglazer until its surface develops a dull (not shiny) surface.
As a preventive practice, I recommend re-calibrating your Kompac every year. Re-calibration will assure you that your Kompac is in proper adjustment and insure optimum performance.