- March 02, 2012, Timothy J. Walker
WebHandlersQuiz 6: Rollers - What is a reasonable specification for roller alignment in mm/m, milli-radians, or mils/in? Answer...
Based on my experience in 0.5 mil (12 micron) polyester films and many converting processes operating at a 60-in (1.5m) width, I usually recommend 2 mil/ft or about 0.2 milli-radians or 0.2 mm/m (usually measured at the roller shaft or bearing mount width).
I understand that lower numbers are used on wide paper industry equipment. For webs with lower elastic modulus that run at higher elongations, I can imagine loosening this specification to 0.5-1.0 milli-radians.
There are certainly many rollers in any process that are much less sensitive than this specification, but it is difficult to predict in the design phase, which these alignment insensitive rollers will be. Alignment in the plane of the entering and exiting spans are most important (e.g. for a vertical entry and exit of a 180-degree wrap roller, the level is more important than tram to centerline).
More on this topic was covered in this archived Web Lines column:
If you missed it, here are all 20 questions in my Web Handler's Quiz.