Why Use Bustle Wheels?
Bustle wheels improve print-to-print color registration by reducing the effects of paper fan-out on web offset presses. Fan-out is the physical lateral growth of the web (and printed image) as it goes through the printing process.
As the web grows in size laterally, so to does the image area. If the image area grows too large, there will be registration and fit issues when overprinting the next color on top of the image area.
In order to reduce the effect of fan-out, bustle wheels are installed between print units. There is typically a bustle wheel mounted in each alley. By moving the bustle wheels slightly into the paper, the paper will begin to bustle (dimple) causing the outer edges of the paper to move inward. The inward movement of the paper helps to fit the previous printed image to the next image that will be overprinted. This greatly improves the fit of the color-to-color register.
Fan-out is not linear. Depending on the amount of moisture being introduced into the paper separate adjustments are made to each bustle wheel.