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How does a roller’s shape influence a coated product and coating defects? The variation of a roller’s geometry can produce many web handling problems.
Roller geometry directly affects the way a web is handled through a machine. Common roll tolerances address roller diameter, taper, and total indicated runout (TIR). But what do these variations really mean to the end-user? Specifically, how does the roller shape influence a coated product and coating defects an operator may experience.
Most line operators and engineers have a general understanding of these properties but not how they affect their product, let alone the proper way to measure these variations. A simple dial indicator cannot reveal the true concentricity of a roller.
This article will discuss roller geometry variation, its affects on web handling, and new-to-the-industry techniques to determine the “real” values for these variations.
Let’s start with the most common tolerances that are used with rollers. This list includes but is not limited to:
* Diameter
* TIR
* Runout
* Profile
* Surface Finish
* Crown
In addition, concentricity is specified and in very rare cases, cylindricity is called out. These tolerances are used to describe the variations from the specified roller geometries. If the geometric variances are too great, then web handling problems, such as wrinkling, stretching, bagging, repeating patterns, poor steering, and tension variation, can occur.
Most of the time there are only two variations that are specified when making rollers. These are diameter and TIR. These variances are common to most rollers because they are the easiest measurements to take. Just because they are conveniently measured doesn’t mean they are the most critical when it comes to predicting roller success. Coating success depends on precision, and roller precision isn’t simple. If the gaps between a coating head and a roller surface are not predictable and maintainable, then the coating will be riddled with defects.
Figure 1. Camshafts are only round at their bearing points, not over their full length.
TIR or Total Indicated Runout, usually measured with a dial indicator, only tells the measurer how round one slice of the roller is at the location where the dial indicator is contacting the roller. Using TIR as a guiding tolerance implies that the entire roller surface will...