One of several rollers found in the inking system or dampening system on a press used in offset lithography. There can be as many as five (or more) oscillator rollers on an offset lithographic printing press. In addition to occupying several other positions within the ink roller train, oscillators are also the rollers which are in direct contact with the ductor roller and the form rollers. Oscillators (also called vibrators or drums) are typically made of steel, covered with a lipophilic (oil-receptive) material such as copper or nylon to prevent a printing problem known as roller stripping. Oscillators, as their name implies, move laterally as they rotate, which has the effect of not only smoothing and working the ink, but also preventing the transfer of images from the plate to the rest of the inking system (see Mechanical Ghosting). The rate of oscillation can be adjusted, although it is generally preferable to set the rollers to the highest amount of oscillation; a lesser amount of oscillation can result in overinking, but in many cases a greater amount of oscillation can cause ink starvation. Oscillators rotate by means of chains and gears, and move adjacent intermediate rollers by friction. (See Inking System: Offset Lithography and Offset Lithography.)
In the offset press dampening system, only one oscillator is used. A dampening oscillator is in intermittent contact with a dampening ductor roller (which operates in a manner similar to an inking ductor roller) and in constant contact with the first dampening form roller, which contacts the plate. Dampening oscillators are not covered, and have a chrome- or aluminum-plated surface. They are motor-driven, and move the adjacent form rollers by surface pressure.